The primary difference between competitive and non-competitive is that competitive inhibition affects the substrate's ability to bind by binding an inhibitor in place of a substrate, which lowers the affinity of the enzyme for the substrate. In non-competitive inhibition, the inhibitor binds to an allosteric site and prevents the enzyme-substrate complex from performing a chemical reaction. This does not affect the Km (affinity) of the enzyme (for the substrate). Non-competitive inhibition differs from uncompetitive inhibition in that it still allows the substrate to bind to the enzyme-inhibitor complex and form an enzyme-substrate-inhibitor complex, this is not true in uncompetitive inhibition, it prevents the substrate from binding to the enzyme inhibitor through conformational change upon allosteric binding.


In the presence of a non-competitive inhibitor, the apparent enzyme affinity is equivalent to the actual affinity. In terms of  Michaelis-Menten kinetics ,  K m app  = K m . This can be seen as a consequence of  Le Chatelier's principle  because the inhibitor binds to both the enzyme and the enzyme-substrate complex equally so that the equilibrium is maintained. However, since some enzyme is always inhibited from converting the substrate to product, the effective enzyme concentration is lowered.


Mathematically,


Noncompetitive inhibitors of  CYP2C9   enzyme  include  nifedipine ,  tranylcypromine ,  phenethyl isothiocyanate , and 6-hydroxyflavone. Computer docking simulation and constructed mutants substituted indicate that the noncompetitive binding site of 6-hydroxyflavone is the reported allosteric binding site of  CYP2C9   enzyme . [ 9 ]


Off-target activity  is  biological activity  of a  drug  that is different from and not at that of its intended  biological target . It most commonly contributes to  side effects . However, in some cases, off-target activity can be taken advantage of for therapeutic purposes.


An example of this is the repurposing of the  antimineralocorticoid  and  diuretic   spironolactone , which was found to produce  feminization  and  gynecomastia  as side effects, for use as an  antiandrogen  in the treatment of  androgen-dependent conditions  like  acne  and  hirsutism  in women. [ 1 ]


Metformin  also causes off-target activity. [ citation needed ]


An  organ chamber ,  organ bath , or  isolated tissue bath , is a chamber in which isolated organs or tissues can be administered with  drugs , or stimulated electrically, in order to measure their function. The tissue in the organ bath is typically oxygenated with  carbogen  and kept in a solution such as  Tyrode's solution  or  lactated Ringer's solution . [ citation needed ]  Historically, they have also been called  gut baths . [ 1 ]


It is used in  pharmacology  research, particularly when studying the contraction of  smooth muscle  in tissues such as  ileum , [ 2 ]   colon , [ 3 ]   vas deferens , [ 4 ]   trachea , [ 5 ]   bladder , [ 6 ]   corpus cavernosum , [ 7 ]  and blood vessels such as  aortic  rings. The contraction of smooth muscle tissues can be readily measured with a  myograph ; this type of physiological response is more readily quantifiable than that of other tissues. Organ baths were originally developed to study the effects of  agonists  and  antagonists  on excitable tissues, such as  nervous tissue  and  muscle  though they have been adapted to study tissues such as  epithelium . [ 8 ]  Typical tissues and receptors studied with organ bath preparations include  nicotinic ,  muscarinic , and  histamine receptors  in the  ileum  or  beta adrenoceptors  in the  bladder . [ 8 ]  Tissues are typically taken from  rodents , such as  guinea pigs ,  mice , and  rats .


For studying the effects of  drugs  on receptors in  drug discovery  and  combinatorial chemistry , novel techniques such as  high throughput screening , ultrahigh throughput screening and high content screening,  pharmacogenomics ,
 proteomics , and array technology have largely superseded the use of organ baths. [ 9 ]  These techniques can allow more receptor specificity than organ bath preparations, as a single tissue sample can express many different receptor types. [ citation needed ]


The use of organ bath preparations for the measurement of physiological tissue responses to drug concentrations allows the generation of  dose response curves . This in turn allows the quantification of a drug's pharmacological profile in the tissue in question, such as the calculation of the drug's  EC50 ,  IC50 , and  Hill coefficient . [ citation needed ]


Examples of important contributions made using this technique include:


Physiological agonism  describes the action of a substance which ultimately produces the same effects in the body as another substance—as if they were both  agonists  at the same  receptor —without actually binding to the same receptor.  Physiological antagonism  describes the behavior of a substance that produces effects counteracting those of another substance (a result similar to that produced by an  antagonist  blocking the action of an agonist at the same receptor) using a mechanism that does not involve binding to the same receptor.


Porous silicon  (abbreviated as "PS" or "pSi") is a form of the chemical element  silicon  that has introduced  nanopores  in its  microstructure , rendering a large surface to volume ratio in the order of 500 m 2 /cm 3 .


Porous silicon was discovered by accident in 1956 by Arthur Uhlir Jr. and Ingeborg Uhlir at the  Bell Labs  in the U.S. At the time, the Uhlirs were in the process of developing a technique for polishing and shaping the surfaces of silicon and  germanium . However, it was found that under several conditions a crude product in the form of thick black, red or brown film were formed on the surface of the material. At the time, the findings were not taken further and were only mentioned in Bell Lab's technical notes. [ 1 ]


Despite the discovery of porous silicon in the 1950s, the scientific community was not interested in porous silicon until the late 1980s. At the time,  Leigh Canham  – while working at the  Defence Research Agency  in England – reasoned that the porous silicon may display quantum confinement effects. [ 2 ]  The intuition was followed by successful experimental results published in 1990. In the published experiment, it was revealed that silicon wafers can emit light if subjected to  electrochemical  and  chemical  dissolution.


The published result stimulated the interest of the scientific community in its  non-linear optical  and  electrical  properties. The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon. In an article published in 2000, it was found that the number of published work grew exponentially in between 1991 and 1995. [ 3 ]


In 2001, a team of scientists at the  Technical University of Munich  inadvertently discovered that  hydrogenated  porous silicon reacts explosively with  oxygen  at  cryogenic  temperatures, releasing several times as much energy as an equivalent amount of  TNT , at a much greater speed. (An abstract of the study can be found below.) Explosion occurs because the oxygen, which is in a liquid state at the necessary temperatures, is able to  oxidize  through the porous molecular structure of the silicon extremely rapidly, causing a very quick and efficient  detonation . Although hydrogenated porous silicon would probably not be effective as a weapon, due to its functioning only at low temperatures, other uses are being explored for its explosive properties, such as providing thrust for  satellites .


Anodization  and  stain-etching  are the two most common methods used for fabrication of porous silicon; however, there are almost twenty other methods to fabricate this material. [ citation needed ]   Drying and surface modification might be needed afterwards.  If anodization in an aqueous solution is used to form microporous silicon, the material is commonly treated in ethanol immediately after fabrication, to avoid damage to the structure that results due to the stresses of the capillary effect of the aqueous solution. [ 4 ]


One method of introducing pores in silicon is through the use of an anodization cell. A possible anodization cell is made of Teflon and employs a platinum cathode and a crystalline Si wafer anode immersed in  hydrogen fluoride  (HF) electrolyte. Recently, inert diamond cathodes were used to avoid metallic impurities in the electrolyte and inert diamond anodes form an improved electrical back plate contact to the silicon wafers.  Corrosion  of the  anode  is produced by running electric current through the cell. It is noted that etching with constant  DC  is usually implemented to ensure steady tip-concentration of HF resulting in a more  homogeneous  porous layer, while pulsed current is more appropriate for the formation of thick PS layers with thickness greater than 50 μm. [ 5 ]  Pore direction is governed by crystal orientation. In (100)-cut Si the pores are oriented perpendicular to the wafer's surface.


It was noted by Halimaoui that  hydrogen  evolution occurs during the formation of porous silicon.


When purely aqueous HF solutions are used for the PS formation, the hydrogen bubbles stick to the surface and induce lateral and in-depth inhomogeneity


The hydrogen evolution is normally treated with absolute  ethanol  in concentration exceeding 15%. It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores. Subsequently, uniform distribution of porosity and thickness is improved.


It is possible to obtain porous silicon through stain-etching with  hydrofluoric acid ,  nitric acid  and  water . A publication in 1957 revealed that stain films can be grown in dilute solutions of nitric acid in concentrated hydrofluoric acid. Porous silicon formation by stain-etching is particularly attractive because of its simplicity and the presence of readily available corrosive reagents; namely nitric acid (HNO 3 ) and hydrogen fluoride (HF). Furthermore, stain-etching is useful if one needs to produce a very thin porous Si films. [ 6 ]  A publication in 1960 by R. J. Archer revealed that it is possible to create stain films as thin as 25 Å through stain-etching with HF-HNO 3  solution. [ 7 ]


Porous silicon can be synthesized chemically from  silicon tetrachloride , using self-forming salt byproducts as templates for pore formation. The salt templates are later removed with water. [ 8 ]


Porous silicon is systematically prone to presence of cracks when the water is evaporated. The cracks are particularly evident in thick or highly porous silicon layers. [ 9 ]  The origin of the cracks has been attributed to the large  capillary  stress due to the minute size of the pores. In particular, it has been known that cracks will appear for porous silicon samples with thickness larger than a certain critical value. Bellet concluded that it was impossible to avoid cracking in thick porous silicon layers under normal evaporating conditions. Hence, several appropriate techniques have been developed to minimize the risk of cracks formed during drying.


Supercritical drying  is reputed to be the most efficient drying technique but is rather expensive and difficult to implement. It was first implemented by Canham in 1994 and involves  superheating  the liquid pore above the critical point to avoid interfacial tension. [ 10 ]


Freeze drying  procedure was first documented around 1996. [ 11 ]  After the formation of porous silicon, the sample is frozen at a temperature of about 200 K and sublimed under vacuum. [ 12 ]


The technique uses  pentane  as the drying liquid instead of water. In doing so the capillary stress is reduced because pentane has a lower surface tension than water. [ 13 ]


Slow evaporating  technique can be implemented following the water or ethanol rinsing. It was found that slow evaporation decreased the trap density.


Physical parameters describing PS are pore diameter, pore density and thickness of the porous layer. During formation of porous silicon layer by means of anodization of a Si wafer, these parameter can be controlled by the Si resistivity, HF concentration, current density and etching time. It is possible to create several porous layers with different pore densities and diameters of the pores on the same substrate by etching with different current densities.


Porosity is defined as volume fraction of voids within the PS layer and can be determined easily by weight measurement. [ 5 ]  The porosity of PS may range from 4% for macroporous layers to 95% for mesoporous layers. A study by Canham in 1995 found that "a 1 μm thick layer of high porosity silicon completely dissolved within a day of in-vitro exposure to a  simulated body fluid ". [ 14 ]  It was also found that a silicon wafer with medium to low porosity displayed more stability. Hence, the porosity of PS is chosen according to its potential application areas. The porosity of PS is a macroscopic parameter and doesn’t yield any information regarding the microstructure of the layer. It is proposed that the properties of a sample are more accurately predicted if the pore size and pore distribution within the sample can be obtained.


PS demonstrates  optical properties  based on porosity and complex  refractive indices  of Si and the medium inside the pores. Effective refractive index of PS can be modelled by means of  effective medium approximations  (EMA). Usually generalised Bruggeman model is used. If the refractive index of the medium inside pores is high, the effective refractive index of PS will be high as well. This phenomenon causes the  spectrum  to shift towards longer  wavelength . [ 15 ]


Porous silicon is classified into three categories according to the size of its pores:  macroporous ,  mesoporous , and  microporous .


The surface of porous silicon may be modified to exhibit different properties. Often, freshly etched porous silicon may be unstable due to the rate of its  oxidation  by the atmosphere or unsuitable for cell attachment purposes. Therefore, it can be surface modified to improve stability and cell attachment


Following the formation of porous silicon, its surface is covered with  covalently  bonded hydrogen. Although the hydrogen coated surface is sufficiently stable when exposed to inert atmosphere for a short period of time, prolonged exposure render the surface prone to oxidation by atmospheric oxygen. The oxidation promotes instability in the surface and is undesirable for many applications. Thus, several methods were developed to promote the surface stability of porous silicon.


An approach that can be taken is through  thermal oxidation . The process involves heating the silicon to a temperature above 1000 C to promote full oxidation of silicon. The method reportedly produced samples with good stability to aging and electronic surface  passivation . [ 16 ]


Porous silicon exhibits a high degree of  biocompatibility . The large surface area enables  organic  molecules to adhere well. It degrades to  Orthosillicic acid (H 4 SiO 4 ) , [ 17 ]  which causes no harm to the body. This has opened potential applications in medicine such as a framework of the growth of  bone .


Surface modification can also affect properties that promote  cell adhesion . One particular research in 2005 studied the mammalian cell adhesion on the modified surfaces of porous silicon. The research used rat  PC12 cells  and  Human Lens Epithelial (HLE)  cells cultured for four hours on the surface modified porous silicon. Cells were then stained with vital dye FDA and observed under  fluorescence microscopy . The research concluded that "amino  silanisation  and coating the pSi surface with collagen enhanced cell attachment and spreading". [ 18 ]


Porous silicon studies conducted in 1995 showed that the behaviour of porous silicon can be altered in between "bio-inert", "bioactive" and "resorbable" by varying the porosity of the silicon sample. [ 14 ]  The in-vitro study used simulated body fluid containing ion concentration similar to the human blood and tested the activities of porous silicon sample when exposed to the fluids for prolonged period of time. It was found that high porosity mesoporous layers were completely removed by the simulated body fluids within a day. In contrast, low to medium porosity microporous layers displayed more stable configurations and induced hydroxyapatite growth.


The first sign of porous silicon as a  bioactive  material was found in 1995. In the conducted study, it was found that  hydroxyapatite  growth was occurring on porous silicon areas. It was then suggested that "hydrated microporous Si could be a bioactive form of the  semiconductor  and suggest that Si itself should be seriously considered for development as a material for widespread  in vivo  applications." [ 14 ]  Another paper published the finding that porous silicon may be used a substrate for hydroxyapatite growth either by simple soaking process or laser-liquid-solid interaction process. [ 19 ]


Since then,  in-vitro  studies have been conducted to evaluate the interaction of cells with porous silicon. A 1995 study of the interaction of  B50 rat hippocampal cells  with porous silicon found that B50 cells have clear preference for adhesion to porous silicon over untreated surface. The study indicated that porous silicon can be suitable for  cell culturing  purposes and can be used to control cell growth pattern. [ 20 ]


Another positive attribute of porous silicon is the degradation of porous silicon into  monomeric  silicic acid (SiOH4). Silicic acid is reputed to be the most natural form of element in the environment and is readily removed by  kidneys .


The human  blood plasma  contains monomeric silicic acid at levels of less than 1 mg Si/L, corresponding to the average dietary intake of 20–50 mg/day. It was proposed that the small thickness of silicon coatings presents minimal risk to a toxic concentration being reached. The proposal was supported by an experiment involving volunteers and silicic-acid drinks. It was found that concentration of the acid rose only briefly above the normal 1 mg Si/L level and was efficiently expelled by urine excretion. [ 21 ]


The simple adjustment of pore morphology and geometry of porous silicon also offers a convenient way to control its wetting behavior. Stable ultra- and  superhydrophobic  states on porous silicon can be fabricated and used in  lab-on-a-chip ,  microfluidic  devices for the improved surface-based bioanalysis. [ 22 ]


In  pharmacology ,  potency  or  biological potency [ 1 ]  is a measure of a drug's  biological activity  expressed in terms of the dose required to produce a pharmacological effect of given intensity. [ 2 ]  A highly potent drug (e.g.,  fentanyl ,  clonazepam ,  risperidone ,  benperidol ,  bumetanide ) evokes a given response at low concentrations, while a drug of lower potency (e.g.  morphine ,  alprazolam ,  ziprasidone ,  haloperidol ,  furosemide ) evokes the same response only at higher concentrations. Higher potency does not necessarily mean greater  effectiveness  nor more  side effects  nor less side effects.


The  International Union of Basic and Clinical Pharmacology  (IUPHAR) has stated that "potency is an imprecise term that should always be further defined", [ 2 ]  and lists of types of potency as follows:


https://www.addictioncenter.com/news/2019/08/15-most-dangerous-drugs/
 https://nida.nih.gov/research-topics/commonly-used-drugs-charts#top


In  pharmacology  and  pharmaceutics , a  prototype drug  is an individual  drug  that represents a  drug class  – group of medications having similar  chemical structures ,  mechanism of action  and  mode of action . Prototypes are the most important, and typically the first developed drugs within the class, and are used as a reference to which all other drugs are compared. [ 1 ] [ 2 ]


A  receptor antagonist  is a type of   receptor   ligand  or  drug  that blocks or dampens a biological response by binding to and blocking a  receptor  rather than activating it like an  agonist . Antagonist drugs interfere in the natural operation of receptor proteins. [ 1 ]  They are sometimes called  blockers ; examples include  alpha blockers ,  beta blockers , and  calcium channel blockers . In  pharmacology ,  antagonists  have  affinity  but no  efficacy  for their cognate receptors, and binding will disrupt the interaction and inhibit the function of an  agonist  or  inverse agonist  at receptors. Antagonists mediate their effects by binding to the  active site  or to the  allosteric site  on a receptor, or they may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity. Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on the nature of antagonist–receptor binding. The majority of drug antagonists achieve their potency by competing with endogenous ligands or substrates at structurally defined binding sites on receptors. [ 2 ]


The English word antagonist in pharmaceutical terms comes from the  Greek  ἀνταγωνιστής –  antagonistēs , "opponent, competitor, villain,  enemy, rival", which is derived from  anti-  ("against") and  agonizesthai  ("to contend for a prize"). Antagonists were discovered in the 20th century by American biologist Bailey Edgren. [ 3 ] [ 4 ]


Biochemical  receptors  are large  protein  molecules that can be activated by the binding of a  ligand  such as a  hormone  or a  drug . [ 5 ]  Receptors can be membrane-bound, as  cell surface receptors , or inside the cell as  intracellular receptors , such as  nuclear receptors  including those of the  mitochondrion . Binding occurs as a result of  non-covalent interactions  between the receptor and its ligand, at locations called the  binding site  on the receptor.  A receptor may contain one or more binding sites for different ligands. Binding to the active site on the receptor regulates receptor activation directly. [ 5 ]  The activity of receptors can also be  regulated  by the binding of a ligand to other sites on the receptor, as in  allosteric binding sites . [ 6 ]  Antagonists mediate their effects through receptor interactions by preventing agonist-induced responses. This may be accomplished by binding to the active site or the allosteric site. [ 7 ]  In addition, antagonists may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity to exert their effects. [ 7 ] [ 8 ] [ 9 ]


The term  antagonist  was originally coined to describe different profiles of drug effects. [ 10 ]  The biochemical definition of a receptor antagonist was introduced by Ariens [ 11 ]  and Stephenson [ 12 ]  in the 1950s. The current accepted definition of receptor antagonist is based on the  receptor occupancy model . It narrows the definition of antagonism to consider only those compounds with opposing activities at a single receptor. Agonists were thought to turn "on" a  single  cellular response by binding to the receptor, thus initiating a biochemical mechanism for change within a cell. Antagonists were thought to turn "off" that response by 'blocking' the receptor from the agonist. This definition also remains in use for  physiological antagonists , substances that have opposing physiological actions, but act at different receptors. For example,  histamine  lowers arterial pressure through  vasodilation  at the  histamine H 1  receptor , while  adrenaline  raises arterial pressure through vasoconstriction mediated by alpha -adrenergic receptor  activation.


Our understanding of the mechanism of drug-induced receptor activation and  receptor theory  and the biochemical definition of a receptor antagonist continues to evolve. The two-state model of receptor activation has given way to multistate models with intermediate conformational states. [ 13 ]  The discovery of  functional selectivity  and that ligand-specific receptor conformations occur and can affect interaction of receptors with different second messenger systems may mean that drugs can be designed to activate some of the downstream functions of a receptor but not others. [ 14 ]   This means efficacy may actually depend on where that receptor is expressed, altering the view that  efficacy  at a receptor is receptor-independent property of a drug. [ 14 ]


By definition, antagonists display no  efficacy [ 12 ]  to activate the receptors they bind. Antagonists do not maintain the ability to activate a receptor. Once bound, however, antagonists inhibit the function of  agonists ,  inverse agonists , and  partial agonists . In functional antagonist assays, a  dose-response curve  measures the effect of the ability of a range of concentrations of antagonists to reverse the activity of an agonist. [ 5 ]  The  potency  of an antagonist is usually defined by its  half maximal inhibitory concentration  (i.e.,  IC 50  value). This can be calculated for a given antagonist by determining the concentration of antagonist needed to elicit half inhibition of the maximum biological response of an agonist. Elucidating an IC 50  value is useful for comparing the potency of drugs with similar efficacies, however the dose-response curves produced by both drug antagonists must be similar. [ 16 ]  The lower the IC 50  the greater the potency of the antagonist, and the lower the concentration of drug that is required to inhibit the maximum biological response. Lower concentrations of drugs may be associated with fewer side-effects. [ 17 ]


The affinity of an antagonist for its  binding site  (K i ), i.e. its ability to bind to a receptor, will determine the duration of inhibition of agonist activity. The affinity of an antagonist can be determined experimentally using  Schild regression  or for competitive antagonists in radioligand binding studies using the  Cheng-Prusoff equation . Schild regression can be used to determine the nature of antagonism as beginning either competitive or non-competitive and K i  determination is independent of the affinity, efficacy or concentration of the agonist used. However, it is important that equilibrium has been reached. The effects of receptor desensitization on reaching equilibrium must also be taken into account. The affinity constant of antagonists exhibiting two or more effects, such as in competitive neuromuscular-blocking agents that also block ion channels as well as antagonising agonist binding, cannot be analyzed using Schild regression. [ 18 ] [ 19 ]  Schild regression involves comparing the change in the dose ratio, the ratio of the EC 50  of an agonist alone compared to the EC 50  in the presence of a competitive antagonist as determined on a dose response curve. Altering the amount of antagonist used in the assay can alter the dose ratio. In Schild regression, a plot is made of the log (dose ratio-1) versus the log concentration of antagonist for a range of antagonist concentrations. [ 20 ]  The affinity or K i  is where the line cuts the x-axis on the regression plot. Whereas, with Schild regression, antagonist concentration is varied in experiments used to derive K i  values from the Cheng-Prusoff equation, agonist concentrations are varied. Affinity for competitive agonists and antagonists is related by the Cheng-Prusoff factor used to calculate the K i  (affinity constant for an antagonist) from the shift in IC 50  that occurs during  competitive inhibition . [ 21 ]  The Cheng-Prusoff factor takes into account the effect of altering agonist concentration and agonist affinity for the receptor on inhibition produced by competitive antagonists. [ 17 ]


Competitive antagonists bind to receptors at the same  binding site  (active site) as the endogenous ligand or agonist, but without activating the receptor. Agonists and antagonists "compete" for the same binding site on the receptor. Once bound, an antagonist will block agonist binding. Sufficient concentrations of an antagonist will displace the agonist from the binding sites, resulting in a lower frequency of receptor activation.  The level of activity of the receptor will be determined by the relative  affinity  of each molecule for the site and their relative concentrations. High concentrations of a competitive agonist will increase the proportion of receptors that the agonist occupies, higher concentrations of the antagonist will be required to obtain the same degree of binding site occupancy. [ 17 ]  In functional assays using competitive antagonists, a parallel rightward shift of agonist dose–response curves with no alteration of the maximal response is observed. [ 23 ]


Competitive antagonists are used to prevent the activity of drugs, and to reverse the effects of drugs that have already been consumed.  Naloxone  (also known as Narcan) is used to reverse  opioid overdose  caused by drugs such as  heroin  or  morphine . Similarly,  Ro15-4513  is an antidote to  alcohol  and  flumazenil  is an antidote to  benzodiazepines .


Competitive antagonists are sub-classified as  reversible  ( surmountable ) or  irreversible  ( insurmountable ) competitive antagonists, depending on how they interact with their  receptor protein  targets. [ 22 ]  Reversible antagonists, which bind via noncovalent intermolecular forces, will eventually dissociate from the receptor, freeing the receptor to be bound again. [ 24 ]  Irreversible antagonists bind via covalent intermolecular forces. Because there is not enough  free energy  to break covalent bonds in the local environment, the bond is essentially "permanent", meaning the receptor-antagonist complex will never dissociate. The receptor will thereby remain permanently antagonized until it is  ubiquitinated  and thus destroyed.


A non-competitive antagonist is a type of insurmountable antagonist that may act in one of two ways: by binding to an  allosteric  site of the receptor, [ 25 ] [ 22 ]  or by irreversibly binding to the active site of the receptor. The former meaning has been standardised by the  IUPHAR , [ 22 ]  and is equivalent to the antagonist being called an  allosteric antagonist . [ 22 ]   While the mechanism of antagonism is different in both of these phenomena, they are both called "non-competitive" because the end-results of each are functionally very similar. Unlike competitive antagonists, which affect the amount of agonist necessary to achieve a maximal response but do not affect the magnitude of that maximal response, non-competitive antagonists reduce the magnitude of the maximum response that can be attained by any amount of agonist. This property earns them the name "non-competitive" because their effects cannot be negated, no matter how much agonist is present.  In functional assays of non-competitive antagonists,  depression  of the maximal response of agonist dose-response curves, and in some cases, rightward shifts, is produced. [ 23 ]  The rightward shift will occur as a result of a  receptor reserve  (also known as spare receptors) [ 12 ]  and inhibition of the agonist response will only occur when this reserve is depleted.


An antagonist that binds to the active site of a receptor is said to be "non-competitive" if the bond between the active site and the antagonist is irreversible or nearly so. [ 25 ]  This usage of the term "non-competitive" may not be ideal, however, since the term "irreversible competitive antagonism" may also be used to describe the same phenomenon without the potential for confusion with the second meaning of "non-competitive antagonism" discussed below.


The second form of "non-competitive antagonists" act at an  allosteric  site. [ 25 ]  These antagonists bind to a distinctly separate binding site from the agonist, exerting their action to that receptor via the other binding site. They  do not compete with agonists for binding at the active site. The bound antagonists may prevent conformational changes in the receptor required for receptor activation after the agonist binds. [ 26 ]   Cyclothiazide  has been shown to act as a reversible non-competitive antagonist of  mGluR1 receptor . [ 27 ]  Another example of a non-competitive is  phenoxybenzamine  which binds irreversibly (with  covalent bonds ) to alpha- adrenergic receptors , which in turn reduces the fraction of available receptors and reduces the maximal effect that can be produced by the  agonist . [ 28 ]


Uncompetitive antagonists differ from non-competitive antagonists in that they require receptor activation by an agonist before they can bind to a separate allosteric binding site. This type of antagonism produces a kinetic profile in which "the same amount of antagonist blocks higher concentrations of agonist better than lower concentrations of agonist". [ 29 ]   Memantine , used in the treatment of  Alzheimer's disease , is an uncompetitive antagonist of the  NMDA receptor . [ 30 ]


Silent antagonists are competitive receptor antagonists that have zero intrinsic activity for activating a receptor. They are true antagonists, so to speak. The term was created to distinguish fully inactive antagonists from weak partial agonists or inverse agonists. [ 31 ]


Partial agonists are defined as drugs that, at a given receptor, might differ in the amplitude of the functional response that they elicit after maximal receptor occupancy. Although they are agonists, partial agonists can act as a  competitive antagonist  in the presence of a  full agonist , as it competes with the full agonist for receptor occupancy, thereby producing a net decrease in the receptor activation as compared to that observed with the full agonist alone. [ 32 ] [ 33 ]  Clinically, their usefulness is derived from their ability to enhance deficient systems while simultaneously blocking excessive activity. Exposing a receptor to a high level of a partial agonist will ensure that it has a constant, weak level of activity, whether its normal agonist is present at high or low levels. In addition, it has been suggested that partial agonism prevents the adaptive regulatory mechanisms that frequently develop after repeated exposure to potent full agonists or antagonists. [ 34 ] [ 35 ]  E.g.  Buprenorphine , a partial agonist of the  μ-opioid receptor , binds with weak morphine-like activity and is used clinically as an  analgesic  in pain management and as an alternative to  methadone  in the treatment of opioid dependence. [ 36 ]


An  inverse agonist  can have effects similar to those of an antagonist, but causes a distinct set of downstream biological responses.  Constitutively active receptors  that exhibit intrinsic or basal activity can have inverse agonists, which not only block the effects of binding agonists like a classical antagonist but also inhibit the basal activity of the receptor. Many drugs previously classified as antagonists are now beginning to be reclassified as inverse agonists because of the discovery of constitutive active receptors. [ 37 ] [ 38 ]   Antihistamines , originally classified as antagonists of  histamine   H 1  receptors  have been reclassified as inverse agonists. [ 39 ]


Many antagonists are reversible antagonists that, like most agonists, will bind and unbind a receptor at rates determined by  receptor-ligand kinetics .


Irreversible antagonists  covalently  bind to the receptor target and, in general, cannot be removed; inactivating the receptor for the duration of the antagonist effects is determined by the rate of receptor turnover, the rate of synthesis of new receptors.  Phenoxybenzamine  is an example of an irreversible  alpha blocker —it permanently binds to  α   adrenergic receptors , preventing  adrenaline  and  noradrenaline  from binding. [ 40 ]  Inactivation of receptors normally results in a depression of the maximal response of agonist dose-response curves and a right shift in the curve occurs where there is a receptor reserve similar to non-competitive antagonists. A washout step in the assay will usually distinguish between non-competitive and irreversible antagonist drugs, as effects of non-competitive antagonists are reversible and activity of agonist will be restored. [ 23 ]


Irreversible competitive antagonists also involve competition between the agonist and antagonist of the receptor, but the rate of covalent bonding differs and depends on affinity and reactivity of the antagonist. [ 16 ]  For some antagonists, there may be a distinct period during which they behave competitively (regardless of basal efficacy), and freely associate to and dissociate from the receptor, determined by  receptor-ligand kinetics . But, once irreversible bonding has taken place, the receptor is deactivated and degraded. As for non-competitive antagonists and irreversible antagonists in functional assays with irreversible competitive antagonist drugs, there may be a shift in the log concentration–effect curve to the right, but, in general, both a decrease in slope and a reduced maximum are obtained. [ 16 ]


A  receptor modulator , or  receptor ligand , is a general term for a substance, endogenous or exogenous, that binds to and regulates the activity of chemical  receptors . They are  ligands  that can act on different parts of receptors and regulate activity in a positive, negative, or neutral direction with varying degrees of efficacy. Categories of these modulators include  receptor agonists  and  receptor antagonists , as well as receptor  partial agonists ,  inverse agonists , orthosteric modulators, and  allosteric modulators , [ 1 ]  Examples of receptor modulators in modern medicine include CFTR modulators, [ 2 ]  selective androgen receptor modulators (SARMs), and muscarinic ACh receptor modulators.


Currently, receptor modulators are categorized in the Agonist, Partial Agonist, Selective Tissue Modulators, Antagonist, and Inverse Agonist categories in terms of the effect they cause. They are further divided into Orthosteric or Allosteric Modulators according to how they effect said result. Typically, a chemical acts in an agonist fashion whenever it instigates or else facilitates a particular reaction by binding to a particular receptor. In contract, a chemical acts as an antagonist whenever binding to a particular receptor blocks or inhibits a particular response. Between these endpoints exists a gradient defined by a number of variables. One example is Selective Tissue Modulators, which mean a given ligand can behave differently according to the tissue type it is in. As for orthosteric and allosteric modulation, this describes the manner in which the ligand binds to the receptor in question: if it binds directly to the prescribed binding site of a receptor, the ligand is orthosteric in this instance; if the ligand alters the receptor by interacting with it at any place other than a binding site, allosteric interaction occurred. Note that a drug's categorization does not dictate how another drug of the same family could be categorized or whether the same drug may also function in another category. An example is found in medications used to treat opioid addiction, with  methadone ,  buprenorphine ,  naloxone , and  naltrexone  all in separate categories or in more than one simultaneously. In addition, depending on the cell type, the specific effect, whether agonist, antagonist, inverse agonist, etc., could have a unique specific effect. An example is seen in  insulin , under "Receptor Agonists," as it interacts with multiple different cell types as an agonist, but incites multiple and different responses in both.


A  receptor agonist  is a chemical that binds to a receptor with the end result of directly inducing a  conformational change  in the bound receptor and activating a downstream effect. Some common examples are opium derivates, such as  heroin  and Toll-like receptor agonists. [ 3 ]  Heroin functions in this manner, along with other opioids, when bound to  μ-opioid receptors . [ 4 ]  Opioids' manner of action are both concentration- and receptor-dependent, which provides a key difference between agonists and partial agonists. Another example is  insulin , which activates cell receptors to instigate  blood glucose  uptake. [ 5 ]


Partial agonists  are any chemical that can bind to a receptor without eliciting the maximum downstream response as compared to the response from a full agonist. A given partial agonist's affinity for a given receptor is also irrelevant to the consequent effect. An example is  buprenorphine , a partial opioid receptor agonist used to treat opioid addictions by directly substituting for them without the same strength of effect.


A  receptor antagonist  is any given ligand that binds to a receptor in some way without causing any immediate or downstream response, essentially neutralizing the receptor until something with a stronger affinity removes the antagonist or the antagonist itself unbinds. Generally, antagonists can act one of two ways: 1) they can either block the receptors directly, preventing the usual ligand from binding, such as in the case of atropine when it blocks specific  acetylcholine  receptors to provide important medical benefits. This is competitive antagonism, as they are competing for the same binding sites on the receptor. [ 6 ]  The other is by binding to a receptor in a site other than the designated receptor site, inducing a conformational change to prevent the usual ligand(s) from binding and activating a downstream cascade. A commonly-seen and used receptor antagonist is  naloxone , another opioid competitive antagonist typically used to treat opioid overdoses by blocking receptors outright. [ 7 ]  Further elaboration can be found in "Orthosteric v. Allosteric Modulators."


Inverse agonists  differ from regular agonists in that they effect receptors to which a regular agonist binds such that the bound receptors demonstrate reduced activity compared to when they are normally inactive. [ 8 ]  In other words, inverse antagonists limit the efficacy of the bound receptor in some way. This is noted to be beneficial in instances wherein expression of receptors or up-regulated receptor sensitivity could be detrimental, thus making suppression of response the best recourse. A handful of examples of inverse agonist use in therapy include  β-blockers ,  antihistamines ,  ACP-103  to treat  Parkinson's disease ,  hemopressin , drugs to treat  obesity , and more besides. [ 9 ]


Reuptake inhibitors  ( RIs ) are a type of  reuptake modulators . It is a drug that inhibits the  plasmalemmal   transporter -mediated  reuptake  of a  neurotransmitter  from the  synapse  into the  pre-synaptic neuron . This leads to an increase in  extracellular   concentrations  of the neurotransmitter and  an increase in  neurotransmission . Various  drugs  exert their  psychological  and  physiological  effects through reuptake inhibition, including many  antidepressants  and  psychostimulants . [ 1 ]


Most known reuptake inhibitors affect the  monoamine neurotransmitters   serotonin ,  norepinephrine  (and  epinephrine ), and  dopamine . [ 1 ]  However, there are also a number of  pharmaceuticals  and  research chemicals  that act as reuptake inhibitors for other  neurotransmitters  such as  glutamate , [ 2 ]   γ-aminobutyric acid  (GABA), [ 3 ]   glycine , [ 4 ]   adenosine , [ 5 ]   choline  (the  precursor  of  acetylcholine ), [ 6 ]  and the  endocannabinoids , [ 7 ]  among others. [ 1 ]


Standard reuptake inhibitors are believed to act simply as  competitive   substrates  that work by  binding  directly to the  plasmalemma   transporter  of the  neurotransmitter  in question. [ 8 ] [ 9 ] [ 10 ] [ 11 ]  They  occupy  the  transporter  in place of the respective  neurotransmitter  and  competitively   block  it from being  transported  from the  nerve terminal  or  synapse  into the  pre-synaptic   neuron . With high enough doses,  occupation  becomes as much as 80–90%. At this level of inhibition, the  transporter  will be considerably less efficient at removing excess  neurotransmitter  from the  synapse  and this causes a substantial increase in the  extracellular   concentrations  of the  neurotransmitter  and therefore an increase in overall  neurotransmission .


Alternatively, some reuptake inhibitors  bind  to  allosteric sites  and inhibit  reuptake  indirectly and  noncompetitively .


Phencyclidine  and related  drugs  such as  benocyclidine ,  tenocyclidine ,  ketamine , and  dizocilpine  (MK-801), have been shown to inhibit the reuptake of the  monoamine neurotransmitters . [ 12 ] [ 13 ] [ 14 ]  They appear to exert their reuptake inhibition by binding to vaguely characterized allosteric sites on each of the respective  monoamine transporters . [ 15 ] [ 16 ] [ 17 ] [ 18 ] [ 19 ]   Benztropine ,  mazindol , and  vanoxerine  also bind to these sites and have similar properties. [ 15 ] [ 19 ] [ 20 ]  In addition to their high  affinity  for the  main site  of the monoamine transporters, several  competitive   transporter   substrates  such as  cocaine  and  indatraline  have lower affinity for these allosteric sites as well. [ 17 ] [ 19 ] [ 20 ]


A few of the  selective serotonin reuptake inhibitors  (SSRIs) such as the dextro- enantiomer  of  citalopram  appear to be allosteric reuptake inhibitors of serotonin. [ 21 ] [ 22 ]  Instead of binding to the active site on the  serotonin transporter , they bind to an allosteric site, which exerts its effects by causing  conformational changes  in the  transporter protein  and thereby modulating the affinity of substrates for the active site. [ 21 ]  As a result,  escitalopram  has been  marketed  as an  allosteric serotonin reuptake inhibitor . Notably, this allosteric site may be directly related to the above-mentioned  PCP   binding sites . [ 15 ] [ 20 ]


A second type of reuptake inhibition affects  vesicular   transport , and  blocks  the  intracellular   repackaging  of  neurotransmitters  into  cytoplasmic  vesicles. In contrast to  plasmalemmal  reuptake inhibitors, vesicular reuptake inhibitors do not increase the  synaptic   concentrations  of a  neurotransmitter , only the  cytoplasmic   concentrations ; unless, that is, they also act as  plasmalemmal   transporter   reversers  via  phosphorylation  of the  transporter protein , also known as a  releasing agent . Pure vesicular reuptake inhibitors tend to actually lower  synaptic   neurotransmitter   concentrations , as  blocking  the  repackaging  of, and  storage  of the  neurotransmitter  in question leaves them vulnerable to  degradation  via  enzymes  such as  monoamine oxidase  (MAO) that exist in the  cytoplasm . With vesicular  transport   blocked ,  neurotransmitter  stores quickly become depleted.


Reserpine  (Serpasil) is an  irreversible  inhibitor of the  vesicular monoamine transporter 2  (VMAT2), and is a prototypical example of a vesicular reuptake inhibitor.


Two of the primary  active constituents  of the  medicinal herb   Hypericum perforatum  (St. John's Wort) are  hyperforin  and  adhyperforin . [ 23 ] [ 24 ]  Hyperforin and adhyperforin are wide-spectrum inhibitors of the reuptake of serotonin, norepinephrine, dopamine, glutamate, GABA, glycine, [ 25 ]  and choline, [ 26 ]  and they exert these effects by binding to and  activating  the  transient receptor potential   cation channel   TRPC6 . [ 24 ] [ 27 ]  Activation of TRPC6 induces the entry of  calcium  (Ca 2+ ) and  sodium  (Na + ) into the  cell , which causes the effect through unknown mechanism. [ 27 ]


In  pharmacology ,  Schild regression analysis , based upon the  Schild equation , both named for  Heinz Otto Schild , are tools for studying the effects of  agonists  and  antagonists  on the  response  caused by the  receptor  or on ligand-receptor binding.


Dose-response curves  can be constructed to describe response or ligand-receptor complex formation as a function of the ligand concentration. Antagonists make it harder to form these complexes by inhibiting interactions of the ligand with its receptor. This is seen as a change in the dose response curve: typically a rightward shift or a lowered maximum. A reversible competitive antagonist should cause a rightward shift in the dose response curve, such that the new curve is parallel to the old one and the maximum is unchanged. This is because reversible competitive antagonists are surmountable antagonists. The magnitude of the rightward shift can be quantified with the dose ratio, r. The dose ratio r is the ratio of the dose of agonist required for half maximal response with the antagonist

 B

 {\displaystyle {\ce {B}}}

  present divided by the agonist required for half maximal response without antagonist ("control"). In other words, the ratio of the  EC50s  of the inhibited and un-inhibited curves. Thus, r represents both the strength of an antagonist and the concentration of the antagonist that was applied. An equation derived from the  Gaddum equation  can be used to relate r to

 [

 B

 ]

 {\displaystyle [{\ce {B}}]}

 , as follows:


A  Schild plot  is a double logarithmic plot, typically

 log

 10

 (
 r
 −
 1
 )

 {\displaystyle \log _{10}(r-1)}

  as the  ordinate  and

 log

 10

 [

 B

 ]

 {\displaystyle \log _{10}[{\ce {B}}]}

  as the  abscissa . This is done by taking the base-10 logarithm of both sides of the previous equation after subtracting 1:


This equation is linear with respect to

 log

 10

 [

 B

 ]

 {\displaystyle \log _{10}[{\ce {B}}]}

 , allowing for easy construction of graphs without computations. This was particular valuable before the use of computers in pharmacology became widespread. The y-intercept of the equation represents the negative logarithm of

 K

 B

 {\displaystyle K_{B}}

  and can be used to quantify the strength of the antagonist.


These experiments must be carried out on a very wide range (therefore the logarithmic scale) as the mechanisms differ over a large scale, such as at high concentration of drug. [ citation needed ]


The fitting of the Schild plot to observed data points can be done with  regression analysis .


Although most experiments use cellular response as a measure of the effect, the effect is, in essence, a result of the binding kinetics; so, in order to illustrate the mechanism,  ligand  binding is used. A ligand A will bind to a  receptor  R according to an equilibrium constant :


Although the equilibrium constant is more meaningful, texts often mention its inverse, the affinity constant (K aff  = k 1 /k −1 ): A better binding means an increase of binding affinity.


The equation for simple ligand binding to a single homogeneous receptor is


This is the Hill-Langmuir equation, which is practically the  Hill equation  described for the agonist binding. In chemistry, this relationship is called the  Langmuir equation , which describes the adsorption of molecules onto sites of a surface (see  adsorption ).


[
 R

 ]

 t

 {\displaystyle [R]_{t}}

  is the total number of binding sites, and when the equation is plotted it is the horizontal asymptote to which the plot tends; more binding sites will be occupied as the ligand concentration increases, but there will never be 100% occupancy.  The binding affinity is the concentration needed to occupy 50% of the sites; the lower this value is the easier it is for the ligand to occupy the binding site.


The binding of the ligand to the receptor at equilibrium follows the same kinetics as an enzyme at steady-state ( Michaelis–Menten equation ) without the conversion of the bound substrate to product.


Agonists and antagonists can have various effects on ligand binding.  They can change the maximum number of binding sites, the affinity of the ligand to the receptor, both effects together or even more bizarre effects when the system being studied is more intact, such as in tissue samples. (Tissue absorption, desensitization, and other non equilibrium steady-state can be a problem.)


A surmountable drug changes the binding affinity:


A nonsurmountable drug changes the maximum binding:


The Schild regression also can reveal if there are more than one type of receptor and it can show if the experiment was done wrong as the system has not reached equilibrium.


The first radio-receptor assay (RRA) was done in 1970 by Lefkowitz et al., [ dubious  –  discuss ]  using a radiolabeled hormone to determine the binding affinity for its receptor. [ 1 ]


A radio-receptor assay requires the separation of the bound from the free ligand.  This is done by  filtration ,  centrifugation  or  dialysis . [ 2 ]


A method that does not require separation is the  scintillation proximity assay  that relies on the fact that β-rays from  3 H travel extremely short distances.  The receptors are bound to beads coated with a polyhydroxy scintillator.  Only the bound ligands to be detected.


Today, the fluorescence method is preferred to radioactive materials due to a much lower cost, lower hazard, and the possibility of multiplexing the reactions in a high-throughput manner.  One problem is that fluorescent-labeled ligands have to bear a bulky fluorophore that may cause it to hinder the ligand binding.  Therefore, the fluorophore used, the length of the linker, and its position must be carefully selected.


An example is by using  FRET , where the ligand's fluorophore transfers its energy to the fluorophore of an antibody raised against the receptor.


Other detection methods such as  surface plasmon resonance  do not even require fluorophores.


Tachyphylaxis  ( Greek  ταχύς,  tachys , "rapid", and φύλαξις,  phylaxis , "protection") is a medical term describing an  acute , sudden decrease in response to a drug after its administration (i.e., a rapid and short-term onset of  drug tolerance ). [ 1 ]  It can occur after an initial dose or after a series of small doses. Increasing the dose of the drug may be able to restore the original response. [ 2 ]


Tachyphylaxis is characterized by the rate sensitivity: the response of the system depends on the rate with which a stimulus is presented. To be specific, a high-intensity prolonged stimulus or often-repeated stimulus may bring about a diminished response also known as desensitization.


In biological sciences, molecular interactions are the physical bases of the operation of the system. The control of the operation, in general, involves interaction of a stimulus molecule with a receptor/enzyme subsystem by, typically, binding to the macromolecule A and causing an activation or an inhibition of the subsystem by forming an activated form of the macromolecule B. The following schematic represents the activity:


A

 →

 p

 B

 {\displaystyle A{\xrightarrow {\ \ p\ \ }}B}


where p is the activation rate coefficient. It is customary that p is called a rate constant, but, since the p stands for measure of the intensity of the stimulus causing the activation, p may be variable (non-constant).


More complete is an open system, namely, in its simplest form,


R

 →

 A

 →

 p
 (
 S
 )

 B

 →

 q

 ,

 {\displaystyle R{\xrightarrow {}}A{\xrightarrow {\ \ p(S)\ \ }}B{\xrightarrow {\ \ q\ \ }},}


where  R  stands for the  rate of production  of  A ,  p(S)  is the  activation  rate coefficient explicitly expressing its dependence on the stimulus intensity  S  and  q  represents the rate coefficient of  removal  from the state  B . In this elementally open system the steady state of  B  always equal to  R/q .


The above scheme is only the necessary condition for the rate sensitivity phenomenon, and other pathways of deactivation of B may be considered, with the subsequent return to the inactive form of the receptor/enzyme A. Examples [ 3 ] [ 4 ] [ 5 ]  offer particular use of such (mathematical) models in endocrinology, physiology and pharmacology.


Psychedelics  such as  LSD , [ 6 ]  and  psilocybin -containing  mushrooms  demonstrate very rapid tachyphylaxis.


In a patient fully withdrawn from  opioids , going back to an intermittent schedule or maintenance dosing protocol, a fraction of the old tolerance level will rapidly develop, usually starting two days after therapy is resumed and, in general, leveling off after day 7. Whether this is caused directly by  opioid receptors  modified in the past or affecting a change in some metabolic set-point is unclear. Increasing the dose will usually restore efficacy; relatively rapid opioid rotation may also be of use if the increase in tolerance continues.


Inhalation of an agonist for the  beta-2 adrenergic receptor , such as  salbutamol  (albuterol —  USAN ), is the most common treatment for  asthma . Polymorphisms of the beta-2 receptor play a role in tachyphylaxis. Expression of the Gly-16 allele (glycine at position 16) results in greater receptor downregulation by endogenous catecholamines at baseline compared to Arg-16. This results in a greater single-use bronchodilator response in individuals homozygous for Arg-16 compared to Gly-16 homozygotes. [ 7 ]  However, with regular beta-2 agonist use, asthmatic Arg-16 individuals experience a significant decline in bronchodilator response. This decline does not occur in Gly-16 individuals. It has been proposed that the tachyphylactic effect of regular exposure to exogenous beta-2 agonists is more apparent in Arg-16 individuals because their receptors have not been downregulated prior to agonist administration. [ 8 ]


Nicotine  may also show tachyphylaxis over the course of a day, although the mechanism of this action is unclear. [ 9 ]


Acute tachyphylaxis which does not carry into the next day has been observed in children taking  methylphenidate , and inspired the development of  OROS  methylphenidate. [ 10 ]


Use of nasal decongestants (e.g.,  oxymetazoline ) for more than three days leads to tachyphylaxis of response and rebound  congestion , caused by  alpha-adrenergic receptor  downregulation and desensitization. The mechanism may specifically include receptor internalisation and resistance to endogenous vasoconstrictors causing worsening in symptoms post use of medication.  Oxymetazoline-induced tachyphylaxis and rebound congestion are reversed by intranasal  fluticasone . [ 15 ]


Toxicodynamics , termed  pharmacodynamics  in  pharmacology , describes the dynamic interactions of a toxicant with a biological target and its biological effects. [ 1 ]  A  biological target , also known as the site of action, can be binding proteins,  ion channels ,  DNA , or a variety of other receptors. When a  toxicant  enters an organism, it can interact with these receptors and produce structural or functional alterations. The  mechanism of action  of the toxicant, as determined by a toxicant’s chemical properties, will determine what receptors are targeted and the overall toxic effect at the cellular level and organismal level.


Toxicants have been grouped together according to their chemical properties by way of  quantitative structure-activity relationships  (QSARs), which allows prediction of toxic action based on these properties.   endocrine disrupting chemicals  (EDCs) and  carcinogens  are examples of classes of toxicants that can act as QSARs. EDCs mimic or block transcriptional activation normally caused by natural  steroid hormones .  These types of chemicals can act on  androgen receptors ,  estrogen receptors  and  thyroid hormone receptors .  This mechanism can include such toxicants as  dichlorodiphenyltrichloroethane  (DDE) and  polychlorinated biphenyls  (PCBs). Another class of chemicals, carcinogens, are substances that cause  cancer  and can be classified as  genotoxic  or nongenotoxic carcinogens.  These categories include toxicants such as  polycyclic aromatic hydrocarbon  (PAHs) and  carbon tetrachloride  (CCl 4 ).


The process of toxicodynamics can be useful for application in environmental  risk assessment  by implementing toxicokinetic-toxicodynamic (TKTD) models.  TKTD models include phenomenas such as time-varying exposure, carry-over  toxicity , organism recovery time, effects of mixtures, and extrapolation to untested  chemicals  and  species .  Due to their advantages, these types of models may be more applicable for risk assessment than traditional modeling approaches.


While  toxicokinetics  describes the changes in the concentrations of a toxicant over time due to the uptake,  biotransformation , distribution and elimination of toxicants, toxicodynamics involves the interactions of a toxicant with a  biological target  and the functional or structural alterations in a cell that can eventually lead to a toxic effect. Depending on the toxicant’s chemical reactivity and vicinity, the toxicant may be able to interact with the biological target. Interactions between a toxicant and the biological target may also be more specific, where high-affinity  binding sites  increase the selectivity of interactions. For this reason, toxicity may be expressed primarily in certain  tissues  or  organs . The targets are often receptors on the cell surface or in the  cytoplasm  and  nucleus . Toxicants can either induce an unnecessary response or inhibit a natural response, which can cause damage. If the biological target is critical and the damage is severe enough, irreversible injury can occur first at the molecular level, which will translate into effects at higher levels of organization. [ 1 ]


EDCs are generally considered to be toxicants that either mimic or block the transcriptional activation normally caused by natural steroid hormones. [ 2 ]  These chemicals include those acting on androgen receptors, estrogen receptors and thyroid hormone receptors. [ 2 ]


Endocrine disrupting chemicals can interfere with the  endocrine system  in a number of ways including hormone synthesis, storage/release, transport and clearance, receptor recognition and binding, and postreceptor activation. [ 3 ]


In wildlife, exposure to EDCs can result in altered fertility, reduced viability of offspring, impaired hormone secretion or activity and modified reproductive anatomy. [ 4 ]  The reproductive anatomy of offspring can particularly be affected if maternal exposure occurs. [ 5 ]  In females, this includes  mammary glands ,  fallopian tubes ,  uterus ,  cervix , and  vagina . In males, this includes the  prostate ,  seminal vesicles ,  epididymitis  and  testes . [ 5 ]  Exposure of fish to EDCs has also been associated with abnormal thyroid function, decreased fertility, decreased hatching success, de-feminization and masculinization of female fish and alteration of  immune function . [ 5 ]


Endocrine disruption as a  mode of action  for  xenobiotics  was brought into awareness by  Our Stolen Future  by Theo Colborn. [ 2 ]  Endocrine disrupting chemicals are known to accumulate in body tissue and are highly persistent in the environment. [ 6 ]  Many toxicants are known EDCs including  pesticides ,  phthalates ,  phytoestrogens , some industrial/commercial products, and  pharmaceuticals . [ 3 ]  These chemicals are known to cause endocrine disruption via a few different mechanisms. While the mechanism associated with the thyroid hormone receptor is not well understood, two more established mechanisms involve the inhibition of the androgen receptor and activation of the estrogen receptor.


Certain toxicants act as endocrine disruptors by interacting with the androgen receptor. DDE is one example of a chemical that acts via this mechanism. DDE is a  metabolite  of  DDT  that is widespread in the environment. [ 1 ]  Although production of DDT has been banned in the Western world, this chemical is extremely persistent and is still commonly found in the environment along with its metabolite DDE. [ 1 ]  DDE is an  antiandrogen , which means it alters the expression of specific androgen-regulated genes, and is an androgen receptor (AR)-mediated mechanism. [ 1 ]  DDE is a  lipophilic  compound which diffuses into the cell and binds to the AR. [ 1 ]  Through binding, the receptor is inactivated and cannot bind to the androgen response element on DNA. [ 1 ]  This inhibits the transcription of androgen-responsive genes [ 1 ]  which can have serious consequences for exposed wildlife. In 1980, there was a spill in  Lake Apopka , Florida which released the pesticide dicofol and DDT along with its metabolites. [ 4 ]  The neonatal and juvenile alligators present in this lake have been extensively studied and observed to have altered  plasma  hormone concentrations, decreased clutch viability, increased juvenile mortality, and morphological abnormalities in the testis and ovary. [ 4 ]


Toxicants may also cause endocrine disruption through interacting with the estrogen receptor. This mechanism has been well-studied with PCBs. These chemicals have been used as coolants and lubricants in transformers and other electrical equipment due to their insulating properties. [ 7 ]  A purely  anthropogenic  substance, PCBs are no longer in production in the United States due to the adverse health effects associated with exposure, but they are highly persistent and are still widespread in the environment. [ 7 ]  PCBs are a  xenoestrogen , which elicit an enhancing (rather than inhibiting) response, and are mediated by the estrogen receptor. [ 1 ]  These are often referred to as estrogen mimics because they mimic the effects of estrogen. PCBs often build up in sediments and  bioaccumulate  in organisms. [ 1 ]  These chemicals diffuse into the nucleus and bind to the estrogen receptor. [ 1 ]  The estrogen receptor is kept in an inactive conformation through interactions with proteins such as  heat shock proteins  59, 70, and 90. [ 8 ]  After the toxicant binding occurs, the estrogen receptor is activated and forms a  homodimer  complex which seeks out estrogen response elements in the DNA. [ 8 ]  The binding of the complex to these elements causes a rearrangement of the  chromatin  and transcription of the gene, resulting in production of a specific protein. [ 8 ]  In doing this, PCBs elicit an estrogenic response which can affect numerous functions within the organism. [ 1 ]  These effects are observed in various aquatic species. The levels of PCBs in marine mammals are often very high as a result of bioaccumulation. [ 9 ]  Studies have demonstrated that PCBs are responsible for reproductive impairment in the  harbor seal  ( Phoca vitulina ). [ 9 ]  Similar effects have been found in the  grey seal  ( Halichoerus grypus ), the  ringed seal  ( Pusa hispida ) and the  California sea lion  ( Zalophys californianus ). [ 9 ]  In the grey seals and ringed seals, uterine occlusions and  stenosis  were found which led to  sterility . [ 9 ]  If exposed to a xenoestrogen such as PCBs, male fish have also been seen to produce  vitellogenin . [ 8 ]  Vitellogenin is an egg protein female fish normally produce but is not usually present in males except at very low concentrations. [ 8 ]  This is often used as a  biomarker  for EDCs. [ 8 ]


Carcinogens  are defined as any substance that causes  cancer . The toxicodynamics of carcinogens can be complex due to the varying mechanisms of action for different carcinogenic toxicants. Because of their complex nature, carcinogens are classified as either genotoxic or nongenotoxic carcinogens.


The effects of carcinogens are most often related to human exposures but mammals are not the only species that can be affected by cancer-causing toxicants. [ 10 ]   Many studies have shown that cancer can develop in fish species as well. [ 10 ]   Neoplasms  occurring in  epithelial tissue  such as the  liver , gastrointestinal tract, and the  pancreas  have been linked to various environmental toxicants. [ 10 ]  Carcinogens preferentially target the liver in fish and develop hepatocellular and biliary lesions. [ 10 ]


Genotoxic carcinogens interact directly with DNA and genetic material or indirectly by their reactive metabolites. [ 11 ]  Toxicants such as  PAHs  can be genotoxic carcinogens to aquatic organisms. [ 10 ] [ 12 ]   PAHs are widely spread throughout the environment through the incomplete burning of coal, wood, or petroleum products. [ 12 ]  Although PAHs do not bioaccumulate in vertebrate tissue, many studies have confirmed that certain PAH compounds such as  benzo(a)pyrene ,  benz(a)anthracene , and Benzofluoranthene, are  bioavailable  and responsible for liver  diseases  like cancer in wild fish populations. [ 12 ]  One mechanism of action for genotoxic carcinogens includes the formation of  DNA adducts . Once the PAH compound enters an organism, it becomes  metabolized  and available for biotransformation.


The  biotransformation  process can activate the PAH compound and transform it into a diol epoxide, [ citation needed ]  which is a very  reactive intermediate . These diol-epoxides  covalently bind  with DNA  base pairs , most often with  guanine  and  adenine  to form stable adducts within the DNA structure. [ citation needed ]   The binding of diol epoxides and DNA base pairs blocks  polymerase  replication activity. This blockage ultimately contributes to an increase in DNA damage by reducing repair activity. [ citation needed ]


Due to these processes, PAH compounds are thought to play a role in the initiation and early promotion stage of  carcinogenesis . Fish exposed to PAHs develop a range of liver  lesions , some of which are characteristic of  hepatocarcinogenicity . [ 12 ]


Nongenotoxic, or  epigenetic  carcinogens are different and slightly more ambiguous than genotoxic carcinogens since they are not directly carcinogenic. Nongenotoxic carcinogens act by secondary mechanisms that do not directly damage genes. This type of carcinogenesis does not change the sequence of DNA; instead it alters the expression or repression of certain genes by a wide variety of cellular processes. [ 11 ]  Since these toxicants do not directly act on DNA, little is known about the mechanistic pathway. [ 10 ]  It has been proposed that modification of gene expression from nongenotoxic carcinogens can occur by  oxidative stress , peroxisome proliferation, suppression of  apoptosis , alteration of intercellular communication, and modulation of metabolizing enzymes. [ 11 ]


Carbon tetrachloride   is an example of a probable nongenotoxic carcinogen to aquatic vertebrates. Historically, carbon tetrachloride has been used in pharmaceutical production, petroleum refining, and as an industrial solvent. [ 13 ]  Due to its widespread industrial use and release into the environment, carbon tetrachloride has been found in drinking water and therefore, has become a concern for aquatic organisms. [ 14 ]  Because of its high  hepatotoxic  properties, carbon tetrachloride could potentially be linked to liver cancer. Experimental cancer studies have shown that carbon tetrachloride may cause  benign  and  malignant  liver  tumors  to  rainbow trout . [ 13 ] [ 14 ]  carbon tetrachloride works as a nongenotoxic carcinogen by formulating  free radicals  which induce oxidative stress. [ 12 ]  It has been proposed that once carbon tetrachloride enters the organism, it is metabolized to trichloromethyl and trichloromethyl peroxy radicals by the  CYP2E1   enzyme . [ 12 ] [ 15 ]  The more reactive radical, trichloromethyl peroxy, can attack  polyunsaturated fatty acids  in the  cellular membrane  to form  fatty acid  free radicals and initiate  lipid peroxidation . [ 15 ]  The attack on the cellular membrane increases its permeability, causing a leakage of enzymes and disrupts cellular calcium  homeostasis . [ 15 ]  This loss of calcium homeostasis activates calcium dependent  degradative enzymes  and  cytotoxicity , causing hepatic damage. [ 15 ]  The regenerative and proliferative changes that occur in the liver during this time could increase the frequency of genetic damage, resulting in a possible increase of cancer. [ 15 ]


Toxicodynamics can be used in combination with  toxicokinetics  in  environmental risk assessment  to determine the potential effects of releasing a toxicant into the environment.  The most widely used method of incorporating this are TKTD models.


Both toxicokinetics and toxicodynamics have now been described, and using these definitions models were formed, where the internal concentration (TK) and damage (TD) are  simulated  in response to exposure.  TK and TD are separated in the model to allow for the identification of properties of toxicants that determine TK and those that determine TD.  To use this type of model,  parameter  values for TK processes need to be obtained first.  Second, the TD parameters need to be estimated.  Both of these steps require a large database of toxicity information for  parameterization .  After establishing all the parameter values for the TKTD model, and using basic scientific precautions, the model can be used to predict toxic effects, calculate recovery times for organisms, or establish extrapolations from the model to toxicity of untested toxicants and species. [ 16 ] [ 17 ]


It has been argued that the current challenges facing risk assessments can be addressed with TKTD modeling. [ 16 ]   TKTD models were derived in response to a couple of factors.  One is the lack of time being considered as a factor in toxicity and risk assessment.  Some of the earliest developed TKTD models, such as the  Critical Body Residue  (CBR) model and Critical Target Occupation (CTO) model, have considered time as a factor but a criticism has been that they are for very specific circumstances such as reversibly acting toxicants or irreversibly acting toxicants.  Further extrapolation of the CTO and CBR models are  DEBtox , which can model sublethal endpoints, and hazard versions of the CTO, which takes into account  stochastic  death as opposed to individual  tolerance . [ 18 ]   Another significant step to developing TKTD models was the incorporation of a  state variable  for damage.  By using damage as a toxicodynamic state-variable, modeling intermediate recovery rates can be accomplished for toxicants that act reversibly with their targets, without the assumptions of instant recovery (CBR model) or irreversible interactions (CTO model). TKTD models that incorporate damage are the Damage Assessment Model (DAM) and the Threshold Damage Model (TDM). [ 16 ] [ 18 ]  For what may seem like straightforward  endpoints , a variety of different TKTD approaches exist.  A review of the assumptions and  hypotheses  of each was previously published in the creation of a general unified threshold model of survival (GUTS). [ 18 ]


As referenced above, TKTD models have several advantages to traditional models for risk assessments.  The principal advantages to using TKTD models are: [ 16 ]


Due to its advantages, TKTD models may be more powerful than the traditional  dose-response  models because of their incorporation of chemical  concentrations  as well as temporal dimensions. [ 16 ]   Toxicodynamic modeling (such as TKTD models) has been shown to be a useful tool for toxicological research, with increasing opportunities to use these results in risk assessment to permit a more scientifically based risk assessment that is less reliable on  animal testing . [ 21 ]   Overall, these types of models can formalize knowledge about the toxicity of toxicants and organism  sensitivity , create new hypotheses, and simulate temporal aspects of toxicity, making them useful tools for risk assessment. [ 16 ] [ 18 ]


Neuropharmacology  is the study of how drugs affect function in the nervous system, and the neural mechanisms through which they influence behavior. [ 1 ]  There are two main branches of neuropharmacology: behavioral and molecular. Behavioral neuropharmacology focuses on the study of how drugs affect human behavior ( neuropsychopharmacology ), including the study of how drug dependence and  addiction  affect the  human brain . [ 2 ]  Molecular neuropharmacology involves the study of neurons and their  neurochemical  interactions, with the overall goal of developing drugs that have beneficial effects on neurological function. Both of these fields are closely connected, since both are concerned with the interactions of  neurotransmitters ,  neuropeptides ,  neurohormones ,  neuromodulators ,  enzymes ,  second messengers ,  co-transporters ,  ion channels , and  receptor proteins  in the  central  and  peripheral nervous systems . Studying these interactions, researchers are developing drugs to treat many different neurological disorders, including  pain , neurodegenerative diseases such as  Parkinson's disease  and  Alzheimer's disease , psychological disorders, addiction, and many others.


Neuropharmacology did not appear in the scientific field until, in the early part of the 20th century, scientists were able to figure out a basic understanding of the nervous system and how nerves communicate between one another. Before this discovery, there were drugs that had been found that demonstrated some type of influence on the nervous system. In the 1930s, French scientists began working with a compound called phenothiazine in the hope of synthesizing a drug that would be able to combat malaria. Though this drug showed very little hope in the use against malaria-infected individuals, it was found to have sedative effects along with what appeared to be beneficial effects toward patients with Parkinson's disease. This black box method, wherein an investigator would administer a drug and examine the response without knowing how to relate drug action to patient response, was the main approach to this field, until, in the late 1940s and early 1950s, scientists were able to identify specific neurotransmitters, such as  norepinephrine  (involved in the constriction of blood vessels and the increase in heart rate and blood pressure),  dopamine  (the chemical whose shortage is involved in Parkinson's disease), and  serotonin  (soon to be recognized as deeply connected to depression [ citation needed ] ). In the 1950s, scientists also became better able to measure levels of specific neurochemicals in the body and thus correlate these levels with behavior. [ 3 ]  The invention of the  voltage clamp  in 1949 allowed for the study of ion channels and the nerve  action potential . These two major historical events in neuropharmacology allowed scientists not only to study how information is transferred from one neuron to another but also to study how a neuron processes this information within itself. [ citation needed ]


Neuropharmacology is a very broad region of science that encompasses many aspects of the nervous system from single neuron manipulation to entire areas of the brain, spinal cord, and peripheral nerves. To better understand the basis behind  drug development , one must first understand how neurons communicate with one another. [ citation needed ]


To understand the potential advances in medicine that neuropharmacology can bring, it is important to understand how human behavior and thought processes are transferred from neuron to neuron and how medications can alter the chemical foundations of these processes. [ citation needed ]


Neurons are known as excitable cells because on its surface membrane there are an abundance of proteins known as ion-channels that allow small charged particles to pass in and out of the cell. The structure of the neuron allows chemical information to be received by its  dendrites , propagated through the  perikaryon  (cell body) and down its  axon , and eventually passing on to other neurons through its  axon terminal . These  voltage-gated ion channels  allow for rapid  depolarization  throughout the cell. This depolarization, if it reaches a certain threshold, will cause an  action potential . Once the action potential reaches the axon terminal, it will cause an influx of calcium ions into the cell. The calcium ions will then cause vesicles, small packets filled with  neurotransmitters , to bind to the cell membrane and release its contents into the synapse. This cell is known as the pre-synaptic neuron, and the cell that interacts with the neurotransmitters released is known as the post-synaptic neuron. Once the neurotransmitter is released into the synapse, it can either bind to receptors on the post-synaptic cell, the pre-synaptic cell can re-uptake it and save it for later transmission, or it can be broken down by enzymes in the synapse specific to that certain neurotransmitter. These three different actions are major areas where drug action can affect communication between neurons. [ 3 ]


There are two types of receptors that neurotransmitters interact with on a post-synaptic neuron. The first types of receptors are ligand-gated ion channels or LGICs. LGIC receptors are the fastest types of transduction from chemical signal to electrical signal. Once the neurotransmitter binds to the receptor, it will cause a conformational change that will allow ions to directly flow into the cell. The second types are known as G-protein-coupled receptors or GPCRs. These are much slower than LGICs due to an increase in the amount of biochemical reactions that must take place intracellularly. Once the neurotransmitter binds to the GPCR protein, it causes a cascade of intracellular interactions that can lead to many different types of changes in cellular biochemistry, physiology, and gene expression. Neurotransmitter/receptor interactions in the field of neuropharmacology are extremely important because many drugs that are developed today have to do with disrupting this binding process. [ 4 ]


Molecular neuropharmacology involves the study of neurons and their neurochemical interactions, and receptors on neurons, with the goal of developing new drugs that will treat neurological disorders such as pain, neurodegenerative diseases, and psychological disorders (also known in this case as  neuropsychopharmacology ). There are a few technical words that must be defined when relating neurotransmission to receptor action: [ citation needed ]


The following neurotransmitter/receptor interactions can be affected by synthetic compounds that act as one of the three above. Sodium/potassium ion channels can also be manipulated throughout a neuron to induce inhibitory effects of action potentials. [ citation needed ]


The GABA neurotransmitter mediates the fast synaptic inhibition in the central nervous system. When GABA is released from its pre-synaptic cell, it will bind to a receptor (most likely the GABA A  receptor) that causes the post-synaptic cell to hyperpolarize (stay below its action potential threshold). This will counteract the effect of any excitatory manipulation from other neurotransmitter/receptor interactions. [ citation needed ]


This GABA A  receptor contains many binding sites that allow conformational changes and are the primary target for drug development. The most common of these binding sites, benzodiazepine, allows for both agonist and antagonist effects on the receptor. A common drug,  diazepam , acts as an allosteric enhancer at this binding site. [ 5 ]  Another receptor for GABA, known as GABA B , can be enhanced by a molecule called baclofen. This molecule acts as an agonist, therefore activating the receptor, and is known to help control and decrease spastic movement. [ citation needed ]


The  dopamine  neurotransmitter mediates synaptic transmission by binding to five specific GPCRs. These five receptor proteins are separated into two classes due to whether the response elicits an excitatory or inhibitory response on the post-synaptic cell. There are many types of drugs, legal and illegal, that affect dopamine and its interactions in the brain. With Parkinson's disease, a disease that decreases the amount of dopamine in the brain, the dopamine precursor Levodopa is given to the patient due to the fact that dopamine cannot cross the  blood–brain barrier  and L-dopa can. Some dopamine agonists are also given to Parkinson's patients that have a disorder known as restless leg syndrome or RLS. Some examples of these are  ropinirole  and  pramipexole . [ 6 ]


Psychological disorders like that of  attention deficit hyperactivity disorder  (ADHD) can be treated with drugs like  methylphenidate  (also known as Ritalin), which block the re-uptake of dopamine by the pre-synaptic cell, thereby providing an increase of dopamine left in the synaptic gap. This increase in synaptic dopamine will increase binding to receptors of the post-synaptic cell. This same mechanism is also used by other illegal and more potent stimulant drugs such as  cocaine .


The neurotransmitter  serotonin  has the ability to mediate synaptic transmission through either GPCR's or LGIC receptors. The excitatory or inhibitory post-synaptic effects of serotonin are determined by the type of receptor expressed in a given brain region. The most popular and widely used drugs for the regulation of serotonin during depression are known as  selective serotonin reuptake inhibitors  (SSRIs). These drugs inhibit the transport of serotonin back into the pre-synaptic neuron, leaving more serotonin in the synaptic gap. [ citation needed ]


Before the discovery of SSRIs, there were also drugs that inhibited the enzyme that breaks down serotonin.  Monoamine oxidase inhibitors  (MAOIs) increased the amount of serotonin in the synapse, but had many side-effects including intense migraines and high blood pressure. This was eventually linked to the drugs interacting with a common chemical known as  tyramine  found in many types of food. [ 7 ]


Ion channels located on the surface membrane of the neuron allows for an influx of sodium ions and outward movement of potassium ions during an action potential. Selectively blocking these ion channels will decrease the likelihood of an action potential to occur. The drug  riluzole  is a neuroprotective drug that blocks sodium ion channels. Since these channels cannot activate, there is no action potential, and the neuron does not perform any transduction of chemical signals into electrical signals and the signal does not move on. This drug is used as an anesthetic as well as a sedative. [ 8 ]


One form of behavioral neuropharmacology focuses on the study of drug dependence and how drug addiction affects the human mind. Most research has shown that the major part of the brain that reinforces addiction through neurochemical reward is the  nucleus accumbens . The image to the right shows how dopamine is projected into this area. Long-term excessive alcohol use can cause  dependence  and  addiction . How this addiction occurs is described below. [ citation needed ]


Alcohol's rewarding and reinforcing (i.e.,  addictive ) properties are mediated through its effects on  dopamine  neurons in the  mesolimbic reward pathway , which connects the  ventral tegmental area  to the  nucleus accumbens  (NAcc). [ 9 ] [ 10 ]  One of alcohol's primary effects is the allosteric inhibition of  NMDA receptors  and facilitation of  GABA A  receptors  (e.g., enhanced GABA A  receptor-mediated  chloride  flux through  allosteric regulation  of the receptor). [ 11 ]  At high doses,  ethanol  inhibits most  ligand gated ion channels  and  voltage gated ion channels  in neurons as well. [ 11 ]  Alcohol inhibits  sodium–potassium pumps  in the  cerebellum  and this is likely how it impairs cerebellar computation and body co-ordination. [ 12 ] [ 13 ]


With acute alcohol consumption,  dopamine  is released in the  synapses  of the mesolimbic pathway, in turn heightening activation of postsynaptic  D1 receptors . [ 9 ] [ 10 ]  The activation of these receptors triggers postsynaptic internal signaling events through  protein kinase A  which ultimately  phosphorylate   cAMP response element binding protein  (CREB), inducing CREB-mediated changes in  gene expression . [ 9 ] [ 10 ]


With chronic alcohol intake, consumption of ethanol similarly induces CREB phosphorylation through the D1 receptor pathway, but it also alters  NMDA receptor  function through phosphorylation mechanisms; [ 9 ] [ 10 ]  an adaptive  downregulation  of the D1 receptor pathway and CREB function occurs as well. [ 9 ] [ 10 ]   Chronic consumption is also associated with an effect on CREB phosphorylation and function via postsynaptic NMDA receptor signaling cascades through a  MAPK/ERK pathway  and  CAMK -mediated pathway. [ 10 ]   These modifications to CREB function in the mesolimbic pathway  induce expression  (i.e., increase gene expression) of  ΔFosB  in the  NAcc , [ 10 ]  where ΔFosB is the "master control protein" that, when overexpressed in the NAcc, is  necessary and sufficient  for the development and maintenance of an addictive state (i.e., its overexpression in the nucleus accumbens produces and then directly modulates compulsive alcohol consumption). [ 10 ] [ 14 ] [ 15 ] [ 16 ]


Parkinson's disease  is a  neurodegenerative disease  described by the selective loss of  dopaminergic  neurons located in the  substantia nigra . Today, the most commonly used drug to combat this disease is levodopa or  L-DOPA . This precursor to dopamine can penetrate through the  blood–brain barrier , whereas the neurotransmitter dopamine cannot. There has been extensive research to determine whether L-dopa is a better treatment for Parkinson's disease rather than other dopamine agonists. Some believe that the long-term use of L-dopa will compromise neuroprotection and, thus, eventually lead to dopaminergic cell death. Though there has been no proof,  in-vivo  or  in-vitro , some still believe that the long-term use of dopamine agonists is better for the patient. [ 17 ]


While there are a variety of hypotheses that have been proposed for the cause of  Alzheimer's disease , the knowledge of this disease is far from complete to explain, making it difficult to develop methods for treatment. In the brain of Alzheimer's patients, both neuronal nicotinic acetylcholine (nACh) receptors and NMDA receptors are known to be down-regulated. Thus, four anticholinesterases, such as  Donepezil  and  Rivastigmine , have been developed and approved by the  U.S. Food and Drug Administration  (FDA) for the treatment in the U.S.A. However, these are not ideal drugs, considering their side-effects and limited effectiveness. The excessive stimulation of muscarinic and nicotinic receptors by acetylcholine may contribute to the side effects that anticholinesterases have. [ 18 ]


One promising drug,  nefiracetam , is being developed for the treatment of Alzheimer's and other patients with dementia, and has unique actions in potentiating the activity of both nACh receptors and NMDA receptors. [ 19 ]


With advances in technology and our understanding of the nervous system, the development of drugs will continue with increasing drug  sensitivity and specificity .  Structure–activity relationships  are a major area of research within neuropharmacology; an attempt to modify the effect or the potency (i.e., activity) of bioactive chemical compounds by modifying their chemical structures. [ 8 ]


In  pharmacology  and  biochemistry ,  allosteric modulators  are a group of substances that bind to a  receptor  to change that receptor's response to stimuli. Some of them, like  benzodiazepines  or  alcohol , function as  psychoactive  drugs. [ 1 ]  The site that an allosteric modulator binds to (i.e., an  allosteric site ) is not the same one to which an endogenous  agonist  of the receptor would bind (i.e., an  orthosteric site ). Modulators and agonists can both be called receptor  ligands . [ 2 ]


Allosteric modulators can be 1 of 3 types either: positive, negative or neutral. Positive types increase the response of the receptor by increasing the probability that an agonist will bind to a receptor (i.e.  affinity ), increasing its ability to activate the receptor (i.e.  efficacy ), or both. Negative types decrease the agonist affinity and/or efficacy. Neutral types don't affect agonist activity but can stop other modulators from binding to an allosteric site. Some modulators also work as allosteric agonists and yield an agonistic effect by themselves. [ 2 ]


The term "allosteric" derives from the Greek language.  Allos  means "other", and  stereos , "solid" or "shape". This can be translated to "other shape", which indicates the  conformational  changes within receptors caused by the modulators through which the modulators affect the receptor function. [ 3 ]


Allosteric modulators can alter the affinity and efficacy of other substances acting on a receptor. A modulator may also increase affinity and lower efficacy or vice versa. [ 4 ]   Affinity  is the ability of a substance to bind to a  receptor .  Efficacy  is the ability of a substance to activate a receptor, given as a percentage of the ability of the substance to activate the receptor as compared to the receptor's endogenous  agonist . If efficacy is zero, the substance is considered an  antagonist . [ 1 ]


The site to which endogenous agonists bind to is named the  orthosteric site . Modulators don't bind to this site. They bind to any other suitable sites, which are named  allosteric sites . [ 2 ]  Upon binding, modulators generally change the three-dimensional structure (i.e.  conformation ) of the receptor. This will often cause the orthosteric site to also change, which can alter the effect of an agonist binding. [ 4 ]  Allosteric modulators can also stabilize one of the normal configurations of a receptor. [ 5 ]


In practice, modulation can be complicated. A modulator may function as a  partial agonist , meaning it doesn't need the agonist it modulates to yield agonistic effects. [ 6 ]  Also, modulation may not affect the affinities or efficacies of different agonists equally. If a group of different agonists that should have the same action bind to the same receptor, the agonists might not be modulated the same by some modulators. [ 4 ]


A modulator can have 3 effects within a receptor. One is its capability or incapability to activate a receptor (2 possibilities). The other two are agonist affinity and efficacy. They may be increased, lowered or left unaffected (3 and 3 possibilities). This yields 17 possible modulator combinations. [ 4 ]  There are 18 (=2*3*3) if neutral modulator type is also included.


For all practical considerations, these combinations can be generalized only to 5 classes [ 4 ]  and 1 neutral:


Due to the variety of locations on receptors that can serve as sites for allosteric modulation, as well as the lack of regulatory sites surrounding them, allosteric modulators can act in a wide variety of mechanisms. [ citation needed ]


Some allosteric modulators induce a  conformational  change in their target receptor which increases the binding affinity and/or efficacy of the receptor agonist. [ 2 ]  Examples of such modulators include  benzodiazepines  and  barbiturates , which are  GABA A  receptor positive allosteric modulators . Benzodiazepines like  diazepam  bind between α and γ subunits of the GABA A  receptor  ion channels  and increase the channel opening frequency, but not the duration of each opening. Barbiturates like  phenobarbital  bind β domains and increase the duration of each opening, but not the frequency. [ 9 ]


Some modulators act to stabilize conformational changes associated with the agonist-bound state. This increases the probability that the receptor will be in the active conformation, but does not prevent the receptor from switching back to the inactive state. With a higher probability of remaining in the active state, the receptor will bind agonist for longer.  AMPA receptors  modulated by  aniracetam  and  CX614  will deactivate slower, and facilitate more overall cation transport. This is likely accomplished by aniracetam or CX614 binding to the back of the "clam shell" that contains the binding site for  glutamate , stabilizing the closed conformation associated with activation of the AMPA receptor. [ 5 ] [ 9 ]


Overall signal can be increased by preventing the  desensitization  of a receptor. Desensitization prevents a receptor from activating, despite the presence of an agonist. This is often caused by repeated or intense exposures to an agonist. Eliminating or reducing this phenomenon increases the receptor's overall activation. AMPA receptors are susceptible to desensitization via a disruption of a ligand-binding domain dimer interface.  Cyclothiazide  has been shown to stabilize this interface and slow desensitization, and is therefore considered a positive allosteric modulator. [ 5 ]


Modulators can directly regulate receptors rather than affecting the binding of the agonist. Similar to stabilizing the bound conformation of the receptor, a modulator that acts in this mechanism stabilizes a conformation associated with the active or inactive state. This increases the probability that the receptor will conform to the stabilized state, and modulate the receptor's activity accordingly.  Calcium-sensing receptors  can be modulated in this way by adjusting the  pH . Lower pH increases the stability of the inactive state, and thereby decreases the sensitivity of the receptor. It is speculated that the changes in charges associated with adjustments to pH cause a conformational change in the receptor favoring inactivation. [ 10 ]


Modulators that increase only the affinity of partial and full agonists allow their efficacy maximum to be reached sooner at lower agonist concentrations – i.e. the slope and plateau of a  dose-response curve  shift to lower concentrations. [ 4 ]


Efficacy increasing modulators increase maximum efficacy of partial agonists. Full agonists already activate receptors fully so modulators don't affect their maximum efficacy, but somewhat shift their response curves to lower agonist concentrations. [ 4 ]


Related receptors have orthosteric sites that are very similar in structure, as  mutations  within this site may especially lower receptor function. This can be harmful to organisms, so  evolution  doesn't often favor such changes. Allosteric sites are less important for receptor function, which is why they often have great variation between related receptors. This is why, in comparison to orthosteric drugs, allosteric drugs can be very  specific , i.e. target their effects only on a very limited set of receptor types. However, such allosteric site variability occurs also between species so the effects of allosteric drugs vary greatly between species. [ 11 ]


Modulators can't turn receptors fully on or off as modulator action depends on endogenous ligands like  neurotransmitters , which have limited and controlled production within body. This can lower  overdose  risk relative to similarly acting orthosteric drugs. It may also allow a strategy where doses large enough to saturate receptors can be taken safely to prolong the drug effects. [ 4 ]  This also allows receptors to activate at prescribed times (i.e. in response to a stimulus) instead of being activated constantly by an agonist, irrespective of timing or purpose. [ 12 ]


Modulators affect the existing responses within tissues and can allow tissue specific drug targeting. This is unlike orthosteric drugs, which tend to produce a less targeted effect within body on all of the receptors they can bind to. [ 4 ]