Theoretical study of the interaction of agonists with the 5-HT2A receptor

The 5-HT2A receptor (5-HT2AR) is a biogenic amine receptor that belongs to the class A of G protein coupled receptors. It is characterized by a low affinity for serotonin (5-HT) and for other primary amines. Introduction of an ortho-methoxybenzyl substituent at the amine nitrogen increases the partial agonistic activity by a factor of 40 to 1400 compared with 5-HT. The present study was to analyse the QSAR of a series of 51 5-HT2AR partial agonistic arylethylamines, tested in vascular in-vitro assays on rats, at a structure-based level and to suggest ligand binding sites. The compounds belong to three different structural classes, (1) indoles, (2) methoxybenzenes and (3) quinazolinediones. Following a hierarchical strategy, different methods have been applied which all contribute to the investigation of ligand-receptor interactions: fragment regression analysis (FRA), receptor modeling, docking studies and 3D QSAR approaches (comparative molecular field analysis, CoMFA, and comparative molecular similarity index analysis, CoMSIA). An initial FRA indicated that methoxy substituents at indole and phenyl derivatives increase the activity and may be involved in polar interactions with the 5-HT2AR. The large contribution of lipophilic substituents in p position of phenethylamines suggests fit to a specific hydrophobic pocket. Secondary benzylamines are more than one order of magnitude more active than their NH2 analogs. An ortho-OH or -OMe substituent at the benzyl moiety further increases activity. Homology models of the human and rat 5-HT2AR were generated using the crystal structure of bovine rhodopsin and of the beta2-adrenoceptor as templates. The derivation of the putative binding sites for the arylethylamines was based on the results from FRA and on mutagenesis data. Both templates led to 5-HT2AR models with similar topology of the binding pocket within the transmembrane domains TM3, TM5, TM6 and TM7. Docking studies with representative members of the three structural classes suggested that the aryl moieties and particularly para-substituents in phenyl derivatives fit into a hydrophobic pocket formed by Phe2435.47, Phe2445.48 and Phe3406.52. The 5-methoxy substituents in indole and phenyl compounds form H bonds with Ser2395.43. In each case, an additional H bond with Ser1593.36 may be assumed. The cationic amine interacts with the conserved Asp1553.32. The benzyl group of secondary arylethylamines is inserted into another hydrophobic pocket formed by Phe3396.51, Trp3677.40 and Tyr3707.43. In this region, the docking poses depend on the template used for model generation, leading to different interactions especially of ortho- substituents. The docking studies with the beta2-adrenoceptor based rat 5-HT2AR model provided templates for a structure-based alignment of the whole series which was used in 3D QSAR analyses of the partial agonistic activity. Both approaches, CoMFA and CoMSIA, led to highly predictive models with low complexity (cross-validated q2 of 0.72 and 0.81 at 4 and 3 components, respectively). The results were largely compatible with the binding site and confirm the docking studies and the suggested ligand-receptor interactions. Steric and hydrophobic field effects on the potency indicate a hydrophobic pocket around the aryl moiety and near the para position of phenyl derivatives and account for the increased activity of secondary benzylamines. The effects of electrostatic and H-bond acceptor fields suggest a favourable influence of negative charges around the aryl moiety, corresponding to the increase in potency caused by methoxy substituents in 2-, 4-, 5- and 6-position of phenethylamines and by the quinazolinedione oxygens. This is in accord with the role of Ser1593.36 and Ser2395.43 as H bond donors. At the benzyl moiety, the negative charge and the acceptor potential of 2-hydroxy and -methoxy substituents is of advantage. Agonists stabilize or induce active receptor states not reflected by the existing crystal structures. Based on models of different rhodopsin states, a homology modeling and ligand docking study on corresponding 5-HT2AR states suggested to be specific to agonist and partial agonist binding, respectively, was performed. The models indicate collective conformational changes of TM domains during activation. The different 5-HT2AR states are similar with respect to the amino acids interacting with the arylethylamines, but show individual topologies of the binding sites. The interconversion of states by TM movements may be accompanied by co-translations and rotations of the ligands. In the case of the secondary amines considered, the tight fit of the benzyl substituent into a hydrophobic pocket containing key residues in TM6 probably impedes the complete receptor activation due to inhibiting the rotation of this helix. High affinity of a partial agonist is therefore often at the expense of its ability to fully activate a receptor

5-HT2B Receptor-mediated Cardiac Valvulopathy

5-HT2B receptor agonism causes cardiac valvulopathy, a condition characterized by thickening of the heart valves and as a result, regurgitation of blood within the heart. The anti-obesity drug fenfluramine, which was originally prescribed as an anorectic, was withdrawn from the market due to causing cardiac valvulopathy. Fenfluramine, after metabolism by N-dealkylation, produces the metabolite norfenfluramine, which acts as a more potent valvulopathogen. The same was seen with MDMA (ecstasy), a popular drug of abuse, which is metabolized by N-dealkylation to produce MDA, a more potent valvulopathogen. Glennon and co-workers. studied a series of 2,5-dimethoxy-4- substituted phenylisopropylamines (DOX type) hallucinogens and determined their affinities at the three types of 5-HT2 receptors. A high correlation was found between the affinities of these molecules at 5-HT2A and 5-HT2B receptors. Therefore, these hallucinogens have a high possibility of causing valvulopathy, which gives rise to a new class of valvulopathogens. Since certain hallucinogens have the common phenylisopropylamine structural scaffold as that of MDA and norfenfluramine, we conducted 3D-QSAR studies to identify the common structural features of these molecules that are responsible for their high affinities. We were unable to obtain a suitable CoMFA and CoMSIA model for 5-HT2B receptors, but we were able to obtain an internally and externally validated model for 5-HT2A receptor affinities which indicated the hydrophobicity of the substituent at the 4- position was essential for high affinity. Following up with this evidence, we conducted a correlation analysis for the hydrophobicity (π-value) of the 4-position substituent and found a positive correlation between the π-value and the affinity of the molecules. The same results were not observed for the volume of the substituents. We docked the molecules into the 5-HT2B receptor and successfully generated models of the putative interactions made by the DOX molecules and the receptor. In order to compare their binding modes with respect to known valvulopathogens, we also generated models for norfenfluramine and MDA. Our docking results revealed that DOX molecules bind in a more or less similar manner to valvulopathogens MDA and norfenfluramine. Ours is the first in silico model developed for the potent valvulopathogen MDA and the hallucinogenic DOX series of molecules

Effects of 5-HT2 receptor ligands on tail pinch-induced stress responding and open field behavior

Stress is known to exert an influence on neuroendocrine, autonomic, hormonal, and immune functioning. As a result of the debilitating effects of stress on numerous bodily systems, there exists a large body of research devoted to the etiology, physiological sequelae, and treatment of the condition. Further, the neurotransmitter serotonin (5-HT) has been implicated in stress responding. Presently, there is conflict in the literature as to the precise role serotonin plays in mediating the stress response. This study was an attempt to further elucidate the role of 5-HT in mediating an organism’s response to tail pinch stress and the open field. Previously, we have demonstrated that peripheral administration of the broad-spectrum 5-HT2 agonist, DOI, reduces stress responding in rats subjected to a tail pinch stressor (Hawkins, et al., 2002). This effect was fully blocked by peripheral coadministration of the broad spectrum 5-HT2 antagonist, ketanserin. The present study examined further the role of the 5-HT2 receptor subclass in mediating the stress response. We employed antagonists that selectively target either the 5-HT2A or 5-HT2C receptor in an effort to clarify the relative importance of each of these receptors in mediating the stress response. These compounds were injected subcutaneously in an effort to black the effects previously seen with DOI. DOI attenuated rearing and oral behavior directed at food, while increasing the frequency of head and body shakes in the open field. DOI-induced head shakes were blacked by the 5-HT2C antagonist, SDZ SER 082, as well as by a combination of SDZ SER 082 and the 5-HT2A antagonist, spiperone. Implications for the 5-HT2 receptor subclass in mediating stress responding are discussed

5-HT2 receptor binding, functional activity and selectivity in N-benzyltryptamines

The last fifteen years have seen the emergence and overflow into the drug scene of “superpotent” N-benzylated phenethylamines belonging to the “NBOMe” series, accompanied by numerous research articles. Although N-benzyl substitution of 5-methoxytryptamine is known to increase its affinity and potency at 5-HT2 receptors associated with psychedelic activity, N-benzylated tryptamines have been studied much less than their phenethylamine analogs. To further our knowledge of the activity of N-benzyltryptamines, we have synthesized a family of tryptamine derivatives and, for comparison, a few 5-methoxytryptamine analogs with many different substitution patterns on the benzyl moiety, and subjected them to in vitro affinity and functional activity assays vs. the human 5-HT2 receptor subtypes. In the binding (radioligand displacement) studies some of these compounds exhibited only modest selectivity for either 5-HT2A or 5-HT2C receptors suggesting that a few of them, with affinities in the 10–100 nanomolar range for 5-HT2A receptors, might presumably be psychedelic. Unexpectedly, their functional (calcium mobilization) assays reflected very different trends. All of these compounds proved to be 5-HT2C receptor full agonists while most of them showed low efficacy at the 5-HT2A subtype. Furthermore, several showed moderateto-strong preferences for activation of the 5-HT2C subtype at nanomolar concentrations. Thus, although some N-benzyltryptamines might be abuse-liable, others might represent new leads for the development of therapeutics for weight loss, erectile dysfunction, drug abuse, or schizophreniaThis work was supported by FONDECYT (Chile) regular research grants 1110146 and 1150868 to BKC and CONICYT doctoral grant 21140358 to MT-SS

Molecular modelling of interactions between antipsychotic drugs and receptors mediating antipsychotic effects and important side effects

Dopamine and serotonin are two neurotransmitters that exert their actions through mediation of dopaminergic and serotonergic receptors respectively. The receptors in focus in the current study, are the dopamine D2 and serotonin 5-HT2A receptors. Common for both receptors is that they are class A G-protein-coupled receptors consisting of seven transmembrane helices embedded in the lipid membrane of neurons. Imbalance and disruption of especially the dopamine system in the CNS may result in hallucinations, delusions, and lowered levels of motivation, which are treated with antipsychotic drugs that predominantly antagonize dopamine D2 and serotonin 5-HT2A receptors. The main aim of this thesis is to get a deeper understanding of the mechanisms of action and side effects of antipsychotics utilizing induced fit docking and molecular dynamic simulations. Our results suggest that there is a correlation between the binding affinities of the antipsychotic drugs to different aminergic receptors, and the most common side effect observed. Additionally, MD simulations revealed that antipsychotic drugs with different intrinsic activity, bind to the dopamine D2 receptor in distinct ways

Hallucinogens: mechanisms and medical complications

Thesis (M.A.)--Boston UniversityHallucinogens are drugs that alter consciousness by distorting primarily auditory and visual perception but they can affect any sensory system. Hallucinogens also affect judgment, orientation, memory, or emotion. Despite the profound alteration in perception, adverse effects are minimal and hallucinogens are not addictive. Hallucinogen use has its roots in shamanic practices of indigenous cultures and is even incorporated in today’s religions like the Native American Church. By putting a person in an altered state of consciousness, many religions believed that the user was able to see beyond the boundaries of reality and reach out to mythical beings. Hallucinogen use in scientific research was not popular until the 1950’s when Albert Hoffman discovered lysergic acid diethylamide (LSD). The discovery of drug encouraged further research into understanding its mechanisms and its relationship with mental diseases like schizophrenia. Unfortunately, the Comprehensive Drug Abuse Prevention and Control Act of 1970 significantly limited hallucinogenic research and human research for the last 42 years. However, animal research in the last 20 years has determined the importance of serotonergic mechanisms and more specifically the 5-HT2A receptors in mediating LSD’s hallucinogenic effects. Researchers continue to identify mechanisms of LSD action. In addition to serotonergic actions, LSD is active with dopaminergic and metabotropic glutamate receptors. PET scans and fMRI’s have also revealed the importance of the prefrontal cortical region and its interaction with other areas during a hallucinogenic state. The relationship between LSD and acute psychosis is also being explored via animal models. Although human clinical research is limited, recent research sees a much deeper relationship by linking LSD brain activity and neurotransmitter levels to psychotic behaviors. This further understanding of hallucinogens on a physiological and psychological level has led to possible psychotherapeutic areas of research in anxiety and substance abuse. This thesis describes a brief history of hallucinogenic research, the pharmacology and neuroanatomy of serotonergic hallucinogens, the acute and chronic adverse effects of serotonergic hallucinogens, the possible treatments for complications of hallucinogens, the epidemiology, the relationship between hallucinogens and schizophrenia, and possible therapeutic uses of serotonergic hallucinogens. With its minimal adverse effects in humans and its powerful influence on the human psyche, serotonergic hallucinogens are invaluable tools for understanding the human mind

Binding mode of novel multimodal serotonin transporter compounds in 5-hydroxytryptamine receptors

Antidepressants are the most common treatment of depression, one of the leading causes of suicide and disability worldwide. Currently marketed antidepressants have certain limitations; they have a delayed response time, only about 1/3 of the patients respond to the first agent prescribed, and many of them produce side effects that reduce the quality of life. The need for more efficacious and faster-acting antidepressants with fewer side effects is thus apparent. Studies have shown that 5-HT receptors (5-HTRs) are involved in many of the adverse effects of antidepressants, and may be responsible for efficacy issues and the delayed onset of therapeutic action. Some novel multimodal (two or more pharmacological actions) antidepressants combine inhibition of the serotonin transporter (SERT) with agonist or antagonist activity at 5-HTRs, to counteract the activity responsible for the aforementioned problems with the present antidepressants. This study continues a previous virtual screening study, where we identified new compounds for SERT. Several of the compounds also showed affinity for one or more 5-HTRs. Although affinities are known, their ligand – 5-HTRs binding modes and their mode of action (agonist or antagonist action) for the target 5-HTRs have not been established. The aim of this study was to predict their mode of action, and to identify binding modes important for high affinity, by the use of computational methods. Homology modeling was used to construct models of 5-HT1AR, 5-HT2AR, 5-HT6R and 5-HT7R. The models were used for molecular docking and calculations of structural interaction fingerprints. Several residues important for affinity to the target receptors were identified, and preferable binding modes were determined. The mode of action of the compounds was predicted based on their preferences for agonist/antagonist-selective models, and on previous studies of agonists and antagonists showing that agonists form strong polar interactions transmembrane helix 5 (TM5). The results indicated that several of the compounds might have potential to be developed into new antidepressant drugs

Unraveling monoamine receptors involved in the action of typical and atypical antipsychotics on glutamatergic and serotonergic transmission in prefrontal cortex

El pdf del artículo es la versión post-print.The systemic administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists has been considered as a pharmacological model of schizophrenia. In the present work, we used in vivo microdialysis to examine: first, the effects of MK-801, on the efflux of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC) of the rat; second, whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by atypical (clozapine and olanzapine) and classical (haloperidol and chlorpromazine) antipsychotic drugs given intra-mPFC; and third, the role of local blockade of dopamine D2/D3/D4, serotonin 5-HT2A and α1-adrenergic receptors as well as agonism at dopamine D1/D5 and 5-HT1A receptors in the mPFC on the increased efflux of glutamate and 5-HT elicited by MK-801. The four antipsychotic drugs blocked the MK-801-induced increase in glutamate, whereas only clozapine and olanzapine were able to block the increased efflux of 5-HT. Furthermore, M100907 (5-HT2A antagonist), BAY x 3702 (5-HT1A agonist) and prazosin (α1-adrenergic antagonist) blocked the MK-801-induced increase of 5-HT and glutamate in the mPFC. In contrast, raclopride (D2/D3 antagonist) and L-745,870 (D4 antagonist) were able to prevent the increased efflux of glutamate (but not that of 5-HT) elicited by MK-801. SKF-38393 (dopamine D1/D5 agonist) also prevented the MK-801-induced increase of glutamate in the mPFC, but the same effect on cortical 5-HT was reached only at the highest concentration tested. We suggest that the blockade of an exacerbated 5-HT release in the mPFC induced by NMDA antagonists can be a characteristic of atypical antipsychotic drugs. Moreover, we propose that D 2/D3/D4 receptor antagonists would act predominantly on a subpopulation of GABAergic interneurons of the mPFC, thus enhancing cortical inhibition, which would prevent an excessive glutamatergic transmission. Dopamine D1/D5 agonists would further stimulate GABA release from other subpopulation of interneurons controlling cortical output to the dorsal raphe nucleus. Atypical antipsychotic drugs might further act upon 5-HT2A, 5-HT1A and α1- adrenoceptors present in pyramidal cells (including those projecting to the dorsal raphe nucleus), which would directly inhibit an excessive excitability of these cells. © 2010 Bentham Science Publishers Ltd.This work was supported by the Spanish Ministry of Health (FIS Grant PI070111 to A. A.), the Spanish Ministry of Education and Science (Grant SAF 2007-62378 to F.A.), and the Generalitat de Catalunya (SGR2005/00758). X.L.- G. is the recipient of a predoctoral fellowship from the Consejo Superior de Investigaciones Cientificas (CSIC).Peer Reviewe

Predicted Structures and Dynamics for Agonists and Antagonists Bound to Serotonin 5-HT2B and 5-HT2C Receptors

Subtype 2 serotonin (5-hydroxytryptamine, 5-HT) receptors are major drug targets for schizophrenia, feeding disorders, perception, depression, migraines, hypertension, anxiety, hallucinogens, and gastrointestinal dysfunctions.' We report here the predicted structure of 5-HT2B and 5-HT2C receptors bound to highly potent and selective 5-HT2B antagonist PRX-08066 3, (pKi: 30 nM), including the key binding residues [V103 (2.53), L132 (3.29), V190 (4.60), and L347 (6.58)] determining the selectivity of binding to 5-HT2B over 5-HT2A. We also report structures of the endogenous agonist (5 HT) and a HT2B selective antagonist 2 (1-methyl-1-1,6,7,8-tetrahydro-pyrrolo [2,3-g]quinoline-5-carboxylic acid pyridine-3-ylamide). We examine the dynamics for the agonist-and antagonist-bound HT2B receptors in explicit membrane and water finding dramatically different patterns of water migration into the NPxxY motif and the binding site that correlates with the stability of ionic locks in the D(E)RY region

A Roadmap for Development of Novel Antipsychotic Agents Based on a Risperidone Scaffold

Schizophrenia is a chronic psychotic illness affecting ~21 million people globally. Currently available antipsychotic agents act through a dopamine D2 receptor mechanism, and produce extrapyramidal or metabolic side effects. Hence, there is a need for novel targets and agents. The mGlu2/5-HT2A receptor heteromer has been implicated in the action of antipsychotic agents, and represents a novel and attractive therapeutic target for the treatment of schizophrenia. A long-term goal of this project is to synthesize bivalent ligands where a 5-HT2A receptor antagonist is tethered to an mGlu2 PAM via a linker. The goals of the investigation were to study the SAR of risperidone (an atypical antipsychotic agent) at 5-HT2A receptors using a “deconstruction-reconstruction-elaboration” approach to determine the minimal structural features of risperidone that contribute to its 5-HT2A receptor affinity and antagonism, and to determine where on the “minimized risperidone” structure an mGlu2 PAM can be introduced. Additional goals included studying the binding modes of various mGlu2 PAMs and identifying where on an mGlu2 PAM a risperidone “partial” structure could be introduced. Biological studies of deconstructed/elaborated analogs of risperidone suggest that the entire structure of risperidone is not necessary for 5-HT2A receptor affinity and antagonism, and that a fluoro group contributes to 5-HT2A binding. 6-Fluoro-3-(4-piperidinyl)-1,2-benz[d]isoxazole that has only half the structural features of risperidone retains 5-HT2A receptor affinity and antagonist activity, and represents the “minimized risperidone” structure with the piperidine nitrogen atom representing a potential linker site for eventual construction of bivalent ligands. Molecular modeling studies at 5-HT2A receptors suggest that risperidone and its analogs have more than one binding mode. Modeling studies to evaluate binding modes of various PAMs at mGlu2 receptors, coupled with known SAR information, were used to identify a PAM (JNJ-40411813), and the pyridone nitrogen atom of JNJ-40411813 as a potential linker site. Additionally, potential synthetic routes for JNJ-40411813 were explored that might be of value in the synthesis of bivalent ligands. Based on the structural features of 6-fluoro-3-(4-piperidinyl)-1,2-benz[d]isoxazole, a new pharmacophore for 5-HT2A receptor antagonists, consisting of one aromatic region, a basic protonated amine and hydrogen bond acceptors, has been proposed