Molecular modeling of serotonin, ketanserin, ritanserin and their 5-HT2C receptor interactions

Dysfunctional serotonergic neuron-astrocyte signaling in depressive-like states

Astrocytes play crucial roles in brain homeostasis and are regulatory elements of neuronal and synaptic physiology. Astrocytic alterations have been found in Major Depressive Disorder (MDD) patients; however, the consequences of astrocyte Ca2+ signaling in MDD are poorly understood. Here, we found that corticosterone-treated juvenile mice (Cort-mice) showed altered astrocytic Ca2+ dynamics in mPFC both in resting conditions and during social interactions, in line with altered mice behavior. Additionally, Cort-mice displayed reduced serotonin (5-HT)-mediated Ca2+ signaling in mPFC astrocytes, and aberrant 5-HT-driven synaptic plasticity in layer 2/3 mPFC neurons. Downregulation of astrocyte Ca2+ signaling in naïve animals mimicked the synaptic deficits found in Cort-mice. Remarkably, boosting astrocyte Ca2+ signaling with Gq-DREADDS restored to the control levels mood and cognitive abilities in Cort-mice. This study highlights the important role of astrocyte Ca2+ signaling for homeostatic control of brain circuits and behavior, but also reveals its potential therapeutic value for depressive-like states.

Involvement of 5-HT2 serotonin receptors in cognitive defects induced by aristolochic acid I in mice

Aristolochic acids (AAs) are a natural bioactive substance found in Chinese herbs, which are widely used for treating diseases. Many studies have demonstrated that AAs have various pharmacological function, while increasing reports indicated its toxicity. However, the role AAs in cognition remains poorly understood. This study explored the neurotoxic effect of aristolochic acid I (AAI), the most toxic component of the AAs family, on hippocampal synaptic plasticity and spatial cognition in mice. C57BL/6 mice were exposed to 5 mg/kg AAI for 4 weeks. After chronic treatment, AAI considerably increased the level of anxiety and the degree of behavioral despair in mice. Working and reference error rates were higher in the AAI exposed mice than in the control. This was further validated by the molecular docking studies, which AAI might interact with 5-HT₂ serotonin receptor (5-HT_2AR). Mechanism investigation indicated that AAI triggered inflammation in the hippocampus of mice through increasing the activity of Tnf-α-NF-κB-IL-6 signaling pathway. Conclusively, chronic AAI administration causes inflammation, and it possibly also serves as a potential antagonist of 5-HT_2AR to influence the cognition function in C57BL/6 mice.

Behavioral Effects of Systemic, Infralimbic and Prelimbic Injections of a Serotonin 5-HT2A Antagonist in Carioca High- and Low-Conditioned Freezing Rats

The role of serotonin (5-hydroxytryptamine [5-HT]) and 5-HT2A receptors in anxiety has been extensively studied, mostly without considering individual differences in trait anxiety. Our laboratory developed two lines of animals that are bred for high and low freezing responses to contextual cues that are previously associated with footshock (Carioca High-conditioned Freezing [CHF] and Carioca Low-conditioned Freezing [CLF]). The present study investigated whether ketanserin, a preferential 5-HT2A receptor blocker, exerts distinct anxiety-like profiles in these two lines of animals. In the first experiment, the animals received a systemic injection of ketanserin and were exposed to the elevated plus maze (EPM). In the second experiment, these two lines of animals received microinjections of ketanserin in the infralimbic (IL) and prelimbic (PL) cortices and were exposed to either the EPM or a contextual fear conditioning paradigm. The two rat lines exhibited bidirectional effects on anxiety-like behavior in the EPM and opposite responses to ketanserin. Both systemic and intra-IL cortex injections of ketanserin exerted anxiolytic-like effects in CHF rats but anxiogenic-like effects in CLF rats. Microinjections of ketanserin in the PL cortex also exerted anxiolytic-like effects in CHF rats but had no effect in CLF rats. These results suggest that the behavioral effects of 5-HT2A receptor antagonism might depend on genetic variability associated with baseline reactions to threatening situations and 5-HT2A receptor expression in the IL and PL cortices. Highlights -CHF and CLF rats are two bidirectional lines that are based on contextual fear conditioning.-CHF rats have a more "anxious" phenotype than CLF rats in the EPM.-The 5-HT2A receptor antagonist ketanserin had opposite behavioral effects in CHF and CLF rats.-Systemic and IL injections either decreased (CHF) or increased (CLF) anxiety-like behavior.-PL injections either decreased (CHF) anxiety-like behavior or had no effect (CLF).

Distinct effect of 5-HT1A and 5-HT2A receptors in the medial nucleus of the amygdala on tonic immobility behavior

The tonic immobility (TI) response is an innate fear behavior associated with intensely dangerous situations, exhibited by many species of invertebrate and vertebrate animals. In humans, it is possible that TI predicts the severity of posttraumatic stress disorder symptoms. This behavioral response is initiated and sustained by the stimulation of various groups of neurons distributed in the telencephalon, diencephalon and brainstem. Previous research has found the highest Fos-IR in the posteroventral part of the medial nucleus of the amygdala (MEA) during TI behavior; however, the neurotransmission of this amygdaloid region involved in the modulation of this innate fear behavior still needs to be clarified. Considering that a major drug class used for the treatment of psychopathology is based on serotonin (5-HT) neurotransmission, we investigated the effects of serotonergic receptor activation in the MEA on the duration of TI. The results indicate that the activation of the 5HT1A receptors or the blocking of the 5HT2 receptors of the MEA can promote a reduction in fear and/or anxiety, consequently decreasing TI duration in guinea pigs. In contrast, blocking the 5HT1A receptors or activating the 5HT2 receptors in this amygdalar region increased the TI duration, suggesting an increase in fear and/or anxiety. These alterations do not appear to be due to a modification of spontaneous motor activity, which might non-specifically affect TI duration. Thus, these results suggest a distinct role of the 5HT receptors in the MEA in innate fear modulation.

Identification, functional characterization, and pharmacological profile of a serotonin type-2b receptor in the medically important insect, Rhodnius prolixus

In the Chagas disease vector, Rhodnius prolixus, two diuretic hormones act synergistically to dramatically increase fluid secretion by the Malpighian tubules (MTs) during the rapid diuresis that is initiated upon engorgement of vertebrate blood. One of these diuretic hormones is the biogenic amine, serotonin (5-hydroxytryptamine, 5-HT), which controls a variety of additional activities including cuticle plasticization, salivary gland secretion, anterior midgut absorption, cardioacceleratory activity, and myotropic activities on a number of visceral tissues. To better understand the regulatory mechanisms linked to these various physiological actions of serotonin, we have isolated and characterized a serotonin type 2b receptor in R. prolixus, Rhopr5HTR2b, which shares sequence similarity to the vertebrate serotonin type 2 receptors. Rhopr5HTR2b transcript is enriched in well-recognized physiological targets of serotonin, including the MTs, salivary glands and dorsal vessel (i.e., insect heart). Notably, Rhopr5HTR2b was not enriched in the anterior midgut where serotonin stimulates absorption and elicits myotropic control. Using a heterologous functional receptor assay, we examined Rhopr5HTR2b activation characteristics and its sensitivity to potential agonists, antagonists, and other biogenic amines. Rhopr5HTR2b is dose-dependently activated by serotonin with an EC50 in the nanomolar range. Rhopr5HTR2b is sensitive to alpha-methyl serotonin and is inhibited by a variety of serotonin receptor antagonists, including propranolol, spiperone, ketanserin, mianserin, and cyproheptadine. In contrast, the cardioacceleratory activity of serotonin revealed a unique pharmacological profile, with no significant response induced by alpha-methyl serotonin and insensitivity to ketanserin and mianserin. This distinct agonist/antagonist profile indicates that a separate serotonin receptor type may mediate cardiomodulatory effects controlled by serotonin in R. prolixus.

Aromatic interactions impact ligand binding and function at serotonin 5-HT2C G protein-coupled receptors: Receptor homology modeling, ligand docking, and molecular dynamics results validated by experimental studies

The serotonin (5-hydroxytryptamine, 5-HT) 5-HT2 G protein-coupled receptor (GPCR) family consists of types 2A, 2B, and 2C that share ~75% transmembrane (TM) sequence identity. Agonists for 5-HT2C receptors are under development for psychoses, whereas, at 5-HT2A receptors, antipsychotic effects are associated with antagonists-in fact, 5-HT2A agonists can cause hallucinations and 5-HT2B agonists cause cardiotoxicity. It is known that 5-HT2A TM6 residues W6.48, F6.51, and F6.52 impact ligand binding and function, however, ligand interactions with these residues at the 5-HT2C receptor has not been reported. To predict and validate molecular determinants for 5-HT2C-specific activation, results from receptor homology modeling, ligand docking, and molecular dynamics (MD) simulation studies were compared with experimental results for ligand binding and function at wild type and W6.48A, F6.51A, and F6.52A point-mutated 5-HT2C receptors.

Molecular interactions between general anesthetics and the 5HT2B receptor

BACKGROUND

Serotonin modulates many processes through a family of seven serotonin receptors. However, no studies have screened for interactions between general anesthetics currently in clinical use and serotonergic G-protein-coupled receptors (GPCRs). Given that both intravenous and inhalational anesthetics have been shown to target other classes of GPCRs, we hypothesized that general anesthetics might interact directly with some serotonin receptors and thus modify their function.

METHODS

Radioligand binding assays were performed to screen serotonin receptors for interactions with propofol and isoflurane as well as for affinity determinations. Docking calculations using the crystal structure of 5-HT2B were performed to computationally confirm the binding assay results and locate anesthetic binding sites.

RESULTS

The 5-HT2B class of receptors interacted significantly with both propofol and isoflurane in the primary screen. The affinities for isoflurane and propofol were determined to be 7.78 and .95 μM, respectively, which were at or below the clinical concentrations for both anesthetics. The estimated free energy derived from docking calculations for propofol (-6.70 kcal/mol) and isoflurane (-5.10 kcal/mol) correlated with affinities from the binding assay. The anesthetics were predicted to dock at a pharmacologically relevant binding site of 5HT2B.

CONCLUSIONS

The molecular interactions between propofol and isoflurane with the 5-HT2B class of receptors were discovered and characterized. This finding implicates the serotonergic GPCRs as potential anesthetic targets.

Molecular Determinants for Ligand Binding at Serotonin 5-HT2A and 5-HT2C GPCRs: Experimental Affinity Results Analyzed by Molecular Modeling and Ligand Docking Studies

Ligands that activate the serotonin 5-HT_2C G protein-coupled receptor (GPCR) may be therapeutic for psychoses, addiction, and other neuropsychiatric disorders. Ligands that are antagonists at the closely related 5-HT_2A GPCR also may treat neuropsychiatric disorders; in contrast, 5-HT_2A activation may cause hallucinations. 5-HT_2C-specific agonist drug design is challenging because 5-HT₂ GPCRs share 80% transmembrane (TM) homology, same second messenger signaling, and no crystal structures are reported. To help delineate molecular determinants underlying differential binding and activation of 5-HT₂ GPCRs, 5-HT_2A, and 5-HT_2C homology models were built from the β₂-adrenergic GPCR crystal structure and equilibrated in a lipid phosphatidyl choline bilayer performing molecular dynamics simulations. Ligand docking studies at the 5-HT₂ receptor models were conducted with the (2R, 4S)- and (2S, 4R)-enantiomers of the novel 5-HT_2C agonist/5-HT_2A/2B antagonist trans-4-phenyl-N,N-dimethyl-2-aminotetralin (PAT) and its 4'-chlorophenyl congners. Results indicate PAT-5-HT₂ molecular interactions especially in TM domain V are important for the (2R, 4S) enantiomer, whereas, TM domain VI and VII interactions are more important for the (2S, 4R) enantiomer.

Envolvimento de receptores 5-HT2C do hipocampo ventral em comportamentos de defesa de ratos no labirinto em cruz elevado

A ativação farmacológica dos receptores 5-HT2C induz comportamentos de defesa em modelos animais. O estudo busca investigar se o bloqueio seletivo de receptores 5-HT2C no hipocampo ventral (HV) previne comportamentos defensivos induzidos por um agonista de receptor 5-HT2C administrado perifericamente em ratos expostos ao labirinto em cruz elevado (LCE). Quinze minutos após injeções intraperitoniais (IP, 1ml/kg) do agonista 5-HT2C WAY-161503, ratos foram microinjetados bilateralmente no HV com o antagonista seletivo de receptores 5-HT2C SB-242084 (0, 0,1, 0,5 ou 1.5μg). Dez minutos após, cada animal foi exposto ao LCE para o registro de categorias de ansiedade. Injeções sistêmicas do WAY-161503 reduziram seletivamente as explorações nos braços abertos e aumentaram padrões de avaliação de risco. Esse efeito foi atenuado de maneira dose-dependente pela microinjeção de SB-242084 no HV, confirmando a ação ansiogênica de agonistas 5-HT2C e sugerindo que esse perfil comportamental seja mediado, pelo menos em parte, por receptores 5-HT2C do HV.

Human Serotonin 5-HT2C G Protein-Coupled Receptor Homology Model from the β2 Adrenoceptor Structure: Ligand Docking and Mutagenesis Studies

Activation of the serotonin (5-hydroxytryptamine, 5-HT) 5HT_2C G protein-coupled receptor (GPCR) is proposed as novel pharmacotherapy for obesity and neuropsychiatric disorders. In contrast, activation of the 5-HT_2A and 5-HT_2B GPCRs is associated with untoward hallucinogenic and cardiopulmonary effects, respectively. There is no crystal structure available to guide design of 5-HT_2C receptor-specific ligands. For this reason, a homology model of the 5-HT_2C receptor was built based on the crystal structure of the human β₂ adrenoceptor GPCR to delineate molecular determinants of ligand-receptor interactions for drug design purposes. Computational and experimental studies were carried out to validate the model. Binding of N(CH₃)₂-PAT [(1R, 3S)-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene], a novel 5-HT_2C agonist/5-HT_2A/2B inverse agonist, and its secondary [NH(CH₃)-PAT] and primary (NH₂-PAT) amine analogs were studied at the 5-HT_2C wild type (WT) and D3.32A, S3.36A, and Y7.43A 5-HT_2C point-mutated receptors. Reference ligands included the tertiary amines lisuride and mesulergine and the primary amine 5-HT. Modeling results indicated that 5-HT_2C residues D3.32, S3.36, and Y7.43 play a role in ligand binding. Experimental ligand binding results with WT and point-mutated receptors confirmed the impact of D3.32, S3.36, and Y7.43 on ligand affinity.

Drug discovery targeting human 5-HT(2C) receptors: residues S3.36 and Y7.43 impact ligand-binding pocket structure via hydrogen bond formation

Specific activation of serotonin (5-HT) 5-HT(2C) G protein-coupled receptors may be therapeutic for obesity and neuropsychiatric disorders. Mutagenesis coupled with computational and molecular modeling experiments based on the human β₂ adrenergic receptor structure was employed to delineate the interactions of different ligands at human 5-HT(2C) residues D3.32, S3.36 and Y7.43. No binding of the tertiary amine radioligand ([³H]-mesulergine) could be detected when the 5-HT(2C) D3.32 residue was mutated to alanine (D3.32A). The S3.36A point-mutation greatly reduced affinity of primary amine ligands, modestly reduced affinity of a secondary amine, and except for the 5-HT(2C)-specific agonist N(CH₃)₂-PAT, affinity of tertiary amines was unaffected. Molecular modeling results indicated that the primary amines form hydrogen bonds with the S3.36 residue, whereas, with the exception of N(CH₃)₂-PAT, tertiary amines do not interact considerably with this residue. The Y7.43A point-mutation greatly reduced affinity of 5-HT, yet reduced to a lesser extent the affinity of tryptamine that lacks the 5-hydroxy moiety present in 5-HT; modeling results indicated that the 5-HT 5-hydroxy moiety hydrogen bonds with Y7.43 at the 5-HT(2C) receptor. Additional modeling results showed that 5-HT induced a hydrogen bond between Y7.43 and D3.32. Finally, modeling results revealed two low-energy binding modes for 5-HT in the 5-HT(2C) binding pocket, supporting the concept that multiple agonist binding modes may stabilize different receptor active conformations to influence signaling. Ligand potencies for modulating WT and point-mutated 5-HT(2C) receptor-mediated phospholipase C activity were in accordance with the affinity data. Ligand efficacies, however, were altered considerably by the S3.36A mutation only.

Pituitary-directed medical treatment of Cushing's disease

The treatment of Cushing's disease is very complex and represents a challenge for clinicians. Transphenoidal surgical excision of adrenocorticotropic hormone (ACTH)-secreting pituitary adenoma remains the treatment of choice but, unfortunately, the rate of cure at long-term follow-up is suboptimal and recurrences are high, even in the hands of skilled neurosurgeons. Other treatment options, such as bilateral adrenalectomy and pituitary radiotherapy, are currently in use but no treatment has proven fully satisfactory during the lengthy progress of this chronic and devastating disease. Nelson's syndrome and hypopituitarism are of particular concern as affected patients need lifelong hormone-replacement therapy and have notably increased mortality. Although medical treatment represents a second-line treatment option in patients with Cushing's disease, so far pharmacological therapy has been considered a transient and palliative treatment. Many drugs have been employed: they may act at the hypothalamic-pituitary level, decreasing ACTH secretion; at the adrenal level, inhibiting cortisol synthesis (steroidogenesis inhibitors); or at the peripheral level by competing with cortisol (glucocorticoid receptor antagonists). Recently, there has been renewed interest in the medical therapy of Cushing's disease and pituitary-directed drugs include old compounds commercially available for other diseases, such as cabergoline, and new promising compounds, such as pasireotide (SOM230) or retinoic acid. This review focuses on the tumor-directed pharmacological approaches for the management of Cushing's disease based on the recent identification of possibile targets at a pituitary level.

Design, Synthesis, and Pharmacology of Fluorescently Labeled Analogs of Serotonin: Application to Screening of the 5-HT2C Receptor

Novel fluorescent derivatives of serotonin have been synthesized and used as tracers for the development of a 5-HT2C fluorescence polarization assay. Serotonin analogs that feature a fluorescent probe attached through an ether linkage at the tryptamine 5-position have high affinity for the 5-HT2C receptor, and affinity is dependent on both linker length and pendent dye. These variables have been optimized to generate Cy3B derivative 5a, which has 10-fold higher 5-HT2C affinity relative to serotonin (Kd = 0.23 nM). In receptor activation experiments, 5a acts as a full agonist of 5-HT2C. Upon binding to 5-HT2C cell membranes, 5a shows a robust increase in fluorescence polarization (FP) signal. In an FP binding assay using 5a as a tracer ligand, Ki values for known 5-HT2C agonists and antagonists showed excellent agreement with Ki values from radioligand binding ( r2 = 0.93). The FP ligand assay is suitable for high-throughput drug screening applications with respect to speed of analysis, displaceable signal, precision, and sensitivity to various reagents. A 384-well-based high-throughput assay that is rapid, economical, and predictive of test compounds' ability to bind to the 5-HT2C receptor has been compiled and validated. ( Journal of Biomolecular Screening 2009:360-370)

Effects of chronic citalopram treatment on 5-HT1A and 5-HT2A receptors in group- and isolation-housed mice

Selective serotonin reuptake inhibitors (SSRI) are characterized by high clinical effectiveness and good tolerability. A 2-3 week delay in the onset of effects is caused by adaptive mechanisms, probably at the serotonergic (5-HT) receptor level. To analyze this in detail, we measured 5-HT(1A) and 5-HT(2A) receptor bindings in vitro after 3 weeks of citalopram treatment (20 mg/kg i.p. daily) in group-housed as well as isolation-housed mice, reflecting neurobiological aspects seen in psychiatric patients. Isolation housing increased somatodendritic (+52%) and postsynaptic (+30-95%) 5-HT(1A) as well as postsynaptic 5-HT(2A) receptor binding (+25-34%), which confirms previous findings. Chronic citalopram treatment did not induce alterations in raphe 5-HT(1A) autoreceptor binding, independent of housing conditions. Housing-dependent citalopram effects on postsynaptic 5-HT(1A) receptor binding were found with increases in group- (+11-42%) but decreases in isolation-housed (-11 to 35%) mice. Forebrain 5-HT(2A) receptor binding decreased between 11 and 38% after chronic citalopram administration, independent of housing conditions. Citalopram's long-term action comprises alterations at the postsynaptic 5-HT(1A) and 5-HT(2A) receptor binding levels. Housing conditions interact with citalopram effects, especially on 5-HT(1A) receptor binding, and should be more strongly considered in pharmacological studies. In general, SSRI-induced alterations were more pronounced and affected more brain regions in isolates, supporting the concept of a higher responsiveness in "stressed" animals. Isolation-induced receptor binding changes were partly normalized by chronic citalopram treatment, suggesting the isolation housing model for further analyses of SSRI effects, especially at the behavioral level.

Pharmacophore-directed homology modeling and molecular dynamics simulation of G protein-coupled receptor: study of possible binding modes of 5-HT2C receptor agonists

A new pharmacophore-based modeling procedure, including homology modeling, pharmacophore study, flexible molecular docking, and long-time molecular dynamics (MD) simulations, was employed to construct the structure of the human 5-HT(2C) receptor and determine the characteristics of binding modes of 5-HT(2C) receptor agonists. An agonist-receptor complex has been constructed based on homology modeling and a pharmacophore hypothesis model based on some high active compounds. Then MD simulations of the ligand-receptor complex in an explicit membrane environment were carried out. The conformation of the 5-HT(2C) receptor during MD simulation was explored, and the stable binding modes of the studied agonist were determined. Flexible molecular docking of several structurally diverse agonists of the human 5-HT(2C) receptor was carried out, and the general binding modes of these agonists were investigated. According to the models presented in this work and the results of Flexi-Dock, the involvement of the amino acid residues Asp134, Ser138, Asn210, Asn331, Tyr358, Ile131, Ser132, Val135,Thr139, Ile189, Val202, Val208, Leu209, Phe214, Val215, Gly218, Ser219, Phe223, Trp324, Phe327, and Phe328 in agonist recognition was studied. The obtained binding modes of the human 5-HT(2C) receptor agonists have good agreement with the site-directed mutagenesis data and other studies.

Behavioral effects of systemically administered MK-212 are prevented by ritanserin microinfusion into the basolateral amygdala of rats exposed to the elevated plus-maze

RATIONALE

Although 5-HT2 receptors seem to play an important role in anxiety, results from numerous studies are still highly variable. Moreover, little is known about the behavioral effects of centrally administered 5-HT2 compounds in animal models of anxiety.

OBJECTIVE

The current study was performed to: (1) further investigate the effects of 5-HT2 receptor activation in rats exposed to the elevated plus-maze (EPM) and the open-field arena, two widely used animal models for studying anxiety and locomotor activity; and (2) evaluate the involvement of the 5-HT2 receptors within the basolateral nucleus of the amygdala (BLA) in the modulation of such effects.

METHODS

In the first experiment, male Wistar rats were exposed for 5 min to the EPM 27 min following intraperitoneal (i.p.) (1.0 ml/kg) injections of the preferential 5-HT2C receptor agonist 6-chloro-2[1-piperazinyl]pyrazine (MK-212) at doses of 1.0, 2.0, or 4.0 mg/kg. Control animals were injected with saline. The percentage of open-arm entries and the percentage of time spent in these arms were employed as anxiety indexes, whereas the number of closed-arm entries was calculated as indicative of locomotor activity. In the second experiment, rats were exposed for 10 min in an open-field arena to further assess the interference of the same MK-212 doses upon locomotor activity. In Experiment 3, rats were microinjected (0.2 microl) either with the mixed 5-HT 2A/2C receptor antagonist ritanserin (0.5, 1.25, 2.5, and 5.0 microg) or its vehicle into the BLA 12 min following i.p. injections of saline or the intermediate dose of MK-212 (2.0 mg/kg). Fifteen minutes later, each animal was exposed to the EPM as before.

RESULTS

Whereas the highest dose of MK-212 (4.0 mg/kg) induced motor-suppressant effects in both EPM and open-field arena, the intermediate dose of the drug (2.0 mg/kg) reduced open-arm exploration without significantly affecting the number of closed-arm entries. This behavioral profile, consistent with selective anxiogenic effect in the EPM, was dose-dependently prevented by ritanserin microinfusion into the BLA. In saline-pretreated animals, however, ritanserin (all doses) was ineffective.

CONCLUSIONS

MK-212 increases anxiety and decreases locomotor activity. The anxiogenic-like profile of 5-HT2 receptor activation is prevented by the blockade of 5-HT2 receptors within the BLA, which does not have an effect by itself upon basal anxiety levels triggered by the EPM.

The human polyomavirus, JCV, uses serotonin receptors to infect cells

The human polyomavirus, JCV, causes the fatal demyelinating disease progressive multifocal leukoencephalopathy in immunocompromised patients. We found that the serotonergic receptor 5HT2AR could act as the cellular receptor for JCV on human glial cells. The 5HT2A receptor antagonists inhibited JCV infection, and monoclonal antibodies directed at 5HT2A receptors blocked infection of glial cells by JCV, but not by SV40. Transfection of 5HT2A receptor-negative HeLa cells with a 5HT2A receptor rescued virus infection, and this infection was blocked by antibody to the 5HT2A receptor. A tagged 5HT2A receptor colocalized with labeled JCV in an endosomal compartment following internalization. Serotonin receptor antagonists may thus be useful in the treatment of progressive multifocal leukoencephalopathy.

New insights into the human 5-HT4 receptor binding site: exploration of a hydrophobic pocket

A body of evidences suggests that a hydrophobic pocket of the human 5-HT(4) receptor contributes to the high affinity of some bulky 5-HT(4) ligands. A thorough study of this pocket was performed using mutagenesis and molecular modeling. Ligand binding or competition studies with selected bulky ligands (RS39604, RS100235, [(3)H]GR113808 and ML11411) and small ligands (5-HT and ML10375) were carried out on wild-type and mutant receptors (W7.40A/F, Y7.43F, R3.28L) transiently transfected in COS-7 cells. The functional activity of the mutated receptors was evaluated by measuring the ability of 5-HT to stimulate adenylyl cyclase. For W7.40F mutation, no changes in the affinity of studied ligands and in the functional activity of the mutant receptor were observed, in contrary to W7.40A mutation, which abolished both binding of ligands and 5-HT-induced cAMP production. Mutation R3.28L revealed a totally silent receptor with a basal level of cAMP production similar to the mock control despite its ability to product cAMP in the presence of 5-HT. Moreover, a one order loss of affinity of RS39604 and a 45-fold increase of ML11411 affinity were observed. Mutation Y7.43F modified the affinity of GR113808, which displays a 13-fold lower affinity for the mutant than for the wild-type receptor. In conclusion, in the hydrophobic pocket, two polar amino acids are able to interact through hydrogen bonds with bulky ligands depending on their chemical properties. Moreover, these experimental data may validate the proposed new three-dimensional model of the human 5-HT(4) receptor.

A serotonergic (5-HT2) receptor mechanism in the laterodorsal tegmental nucleus participates in regulating the pattern of rapid-eye-movement sleep occurrence in the rat

Serotonin [5-hydroxytryptamine (5-HT)] plays an inhibitory role in rapid-eye-movement (REM) sleep although the exact mechanism(s) and site(s) of action are not known. It is commonly assumed that 5-HT exerts its influence on REM sleep via input from the dorsal raphe nucleus (DRN) directly onto cholinergic neurons involved in the generation of REM sleep. 5-HT(2) receptor sites have been found on cholinergic neurons in the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT). We locally microinjected the 5-HT(2) agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl) and the 5-HT(2) antagonist, ketanserin, in LDT in rats to determine whether these receptor sites are involved in the regulation of behavioral states. DOI and ketanserin primarily affected REM sleep, by significantly decreasing or increasing, respectively, the number, but not the duration, of REM sleep episodes. DOI specifically decreased the occurrence of clusters of REM sleep episodes appearing at intervals less than or equal to 3 min (sequential episodes) without affecting single episodes separated by more than 3 min. An opposite effect of ketanserin on REM sleep clusters, although not statistically significant, was observed.

Identification of the binding sites and selectivity of sarpogrelate, a novel 5-HT2 antagonist, to human 5-HT2A, 5-HT2B and 5-HT2C receptor subtypes by molecular modeling

The aim of the present study was to investigate the binding sites interactions and the selectivity of sarpogrelate to human 5-HT(2) receptor family (5-HT(2A), 5-HT(2B) and 5-HT(2C) receptor subtypes) using molecular modeling. Rhodopsin (RH) crystal structures were used as template to build structural models of the human serotonin-2A and -2C receptors (5-HT(2A)R, 5-HT(2C)R), whereas for 5-HT(2B)R, we used our previously published three-dimensional (3D) models based on bacteriorhodopsin (BR). Sarpogrelate, a novel 5-HT(2)R antagonist, was docked to the receptors. Molecular dynamics (MD) simulations produced the strongest interaction for 5-HT(2A)R/sarpogrelate complex. Upon binding, sarpogrelate constraints aromatic residues network (Trp(3.28), Phe(5.47), Trp(6.48), Phe(6.51), Phe(6.52) in 5-HT(2A)R; Phe(3.35), Phe(6.51), Trp(7.40) in 5-HT(2B)R; Trp(3.28), Phe(3.35), Phe(5.47), Trp(6.48), Phe(6.51), Phe(6.52) in 5-HT(2C)R) in a stacked configuration, preventing activation of the receptor. The models suggest that the structural origin of the selectivity of sarpogrelate to 5-HT(2A)R vs both 5-HT(2B)R and 5-HT(2C)R comes from the following results: (1) The tight interaction between the antagonist and the transmembrane domain (TMD) 3. Asp(3.32) neutralizes the cationic head and interacts simultaneously with carboxylic group hydrogen of the antagonist molecule. (2) Due to steric hindrance, Ser(5.46) (vs Ala(5.46) in 5HT(2B) and 5HT(2C)) prevents sarpogrelate to enter deeply inside the hydrophobic core of the helix bundle and to interact with Pro(5.50). (3) The side chain of Ile(4.56) (vs Ile(4.56) in 5HT(2B)R and Val(4.56) in 5HT(2C)R) constraints sarpogrelate to adjust its position by translating toward the strongly attractive Asp(3.32). These results are in good agreement with binding affinities (pKi) of sarpogrelate for 5-HT(2) receptor family expressed in transfected cell.

The serotonin binding site of human and murine 5-HT2B receptors: molecular modeling and site-directed mutagenesis

Bacteriorhodopsin and rhodopsin crystal structures were used as templates to build structural models of the mouse and human serotonin (5-HT)-2B receptors (5-HT(2B)Rs). Serotonin was docked to the receptors, and the amino acids predicted to participate to its binding were subjected to mutagenesis. 5-HT binding affinity and 5-HT-induced inositol triphosphate production were measured in LMTK(-) cells transfected with either wild-type or mutated receptor genes. According to these measurements, the bacteriorhodopsin-based models of the 5-HT(2B)Rs appear more confident than the rhodopsin-based ones. Residues belonging to the transmembrane domains 3 and 6, i.e. Asp(3.32), Ser(3.36), Phe(6.52), and Asn(6.55), make direct contacts with 5-HT. In addition, Trp(3.28), Phe(3.35), Phe(6.52), and Phe(7.38) form an aromatic box surrounding 5-HT. The specificity of human and mouse 5-HT(2B)Rs may be reflected by different rearrangements of the aromatic network upon 5-HT binding. Two amino acids close to Pro(5.50) in the human transmembrane domain 5 sequence were permuted to introduce a "mouse-like" sequence. This change was enough to confer the human 5-HT(2B)R properties similar to those of the mouse. Taken together, the computed models and the site-directed mutagenesis experiments give a structural explanation to (i) the different 5-HT pK(D) values measured with the human and mouse 5-HT(2B)Rs (7.9 and 5.8, respectively) and (ii) the specificity of 5-HT binding to 5-HT(2B)Rs as compared with other serotonergic G-protein coupled receptors.

Residual symptoms in depression an emerging therapeutic concept

Cushing's syndrome is due to chronic glucocorticoid excess that may have various etiologies. The most common endogenous form is pituitary-dependent bilateral adrenal hyperplasia, which is termed Cushing's disease. Major depression occurs in more than half of the cases. The presence of depressive symptoms connotes severity of clinical presentation and, in patients with hypothalamic-pituitary forms, entails prognostic value. Medical treatment may be used while awaiting more definitive solutions for the illness by surgery. The inhibitors of steroid production (e.g., ketoconazole, metyrapone and aminoglutethimide), rather than antidepressant drugs, are generally successful in lifting depression as well as other disabling symptoms. Since central serotonergic regulation could have a role in the course of Cushing's disease, serotonin antagonists (e.g., cyproheptadine, ritanserin and ketanserin) have been employed. Findings related to the pharmacological response of depression in Cushing's disease were found to have implications for the pathophysiology of depression and the potential involvement of the hypothalamic-pituitary-adrenal axis (HPA axis) in resistance and tolerance to antidepressant drugs. The use of serotonergic drugs in Cushing's disease may yield important insights in the understanding of serotonergic regulation both in Cushing's disease and in the HPA axis in nonendocrine major depression.

Conserved helix 7 tyrosine functions as an activation relay in the serotonin 5HT(2C) receptor

The function of the helix VII Tyr in the conserved Asn-Pro-X-X-Tyr segment of rhodopsin-like G protein coupled receptors has been investigated in many receptors. Various effects of site-directed mutation of this locus have been found, including altered coupling, sequestration and agonist affinity. We report the first constitutively active mutations of this Tyr. In the serotonin 5HT(2C) receptor, substituting Ala or Cys for Tyr resulted in a marked increase in the basal level of inositol phosphate accumulation in transfected COS-1 cells. This constitutive signaling was abolished by the inverse agonist SB206553. Introducing Phe at this locus eliminated both basal and agonist-stimulated signaling. All three mutant receptors showed an increase in binding affinity for the structurally dissimilar agonists 5-hydroxytryptamine (5HT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and quipazine, suggesting that both the activating and inactivating mutations stabilize a high affinity state. These results implicate the conserved Tyr in the conformational rearrangements that occur during agonist complexing and receptor activation.

Differential modes of agonist binding to 5-hydroxytryptamine(2A) serotonin receptors revealed by mutation and molecular modeling of conserved residues in transmembrane region 5

Site-directed mutagenesis and molecular modeling were used to investigate the molecular interactions involved in ligand binding to, and activation of, the rat 5-hydroxytryptamine(2A) (5-HT(2A)) serotonin (5-HT) receptor. Based on previous modeling studies utilizing molecular mechanics energy calculations and molecular dynamics simulations, four sites (S239[5.43], F240[5.44], F243[5.47], and F244[5.48]) in transmembrane region V were selected, each predicted to contribute to agonist and/or antagonist binding. The F243A mutation increased the affinity of (+/-)4-iodo-2, 5-dimethoxyphenylisopropylamine, decreased the binding of alpha-methyl-5HT, N-omega-methyl-5HT, ketanserin, ritanserin, and spiperone and had no effect on the binding of 5-HT and 5-methyl-N, N-dimethyltryptamine. The F240A mutant had no effect on the binding of any of the ligands tested, whereas F244A caused an agonist-specific decrease in binding affinity (3- to 10-fold). S239A caused a 6- to 13-fold decrease in tryptamine-binding affinity and a 5-fold increase in affinity of 4-iodo-2, 5-dimethoxyphenylisopropylamine. A subset of the agonists used in binding studies were used to determine the efficacies and potencies of these mutants to activate phosphoinositide hydrolysis. The F243A and F244A mutations reduced agonist stimulated phosphoinositide hydrolysis, whereas the S239A and F240A mutations had no effect. There was little correlation between agonist binding and second messenger production. Furthermore, molecular dynamics simulations, considering these data, produced ligand-bound structures utilizing substantially different bonding interactions even among structurally similar ligands (differing by as little as one methyl group). Taken together, these results suggest that relatively minor changes in either receptor or ligand structure can produce drastic and unpredictable changes in both binding interactions and 5-HT(2A) receptor activation. Thus, our finding may have major implications for the future and feasibility of receptor structure-based drug design.

Effect of the serotonin antagonists ritanserin and ketanserin in Cushing's disease

Central serotonergic regulation could have a role in the course of pituitary-dependent Cushing's disease. We studied the effects of ritanserin and ketanserin, two related selective 5HT2 receptor antagonists, in 11 patients with Cushing's disease. Treatment lasted from 1 month to 1 year (up to 4 years in one patient). Daily doses were 10-15 mg for ritanserin, and 40-80 mg for ketanserin. Since the two drugs share the same mechanism of action and no qualitative or quantitative differences in response to their administration were observed, the results were pooled together. Patients were assessed by clinical and hormonal evaluation. Urinary cortisol and ACTH were considered the parameters of interest. Short-term response: after 1 month, there was a significant decrease of urinary cortisol from 781 (160) to 331 (215) nmol/d (P < 0.02) while ACTH was 9.8 (1.5) pmol/L baseline and again 8.8 (2.2) pmol/L at 1 month (P = NS). For 9 patients, hormonal parameters were available after 1 week of treatment. In this case, also ACTH levels were significantly decreased (from 9.6 (1.7) to 5.2 (1.3) pmol/L; P < 0.01) together with urinary cortisol (from 781 (194) to 372 (165) nmol/d; P < 0.01). Long-term response: in 3 patients, hormonal parameters failed to respond to serotonin receptor antagonists, which were thus discontinued. An improvement was recorded in the remaining 8 patients, that was prolonged in 3, and transient in 5. In 3 of these latter patients, a marked increase of ACTH was observed before treatment discontinuation. Ketanserin was given to 2 patients with Nelson's syndrome, with only transient ACTH decrease in one, and no changes in ACTH response to CRH after 1 month treatment in both cases. An inhibitory effect of ritanserin and ketanserin on ACTH and cortisol production in Cushing's disease appeared to be limited both in terms of duration of response and number of patients with a satisfactory outcome. However, the results may provide a better understanding of serotonergic modulation in Cushing's disease and lead to therapeutic developments.

Bufotenine: toward an understanding of possible psychoactive mechanisms

A review of the neuropharmacology of the alleged hallucinogen bufotenine is presented, including recent experimental results showing activity similar to LSD and other known hallucinogens (psilocin and 5-MeO-DMT) at the purported hallucinogenic serotonin (5-HT) receptors, 5-HT2A and 5-HT2C. In addition, current reports of computer modeling of the receptors and ligand binding sites give evidence of bufotenine's ability to bind and activate these receptors. While binding and activation of the purported hallucinogenic receptors are not the full extent of the hallucinogenic signature, this evidence shows support for the rationale that the reported lack of the drug's classic hallucinogenic response in human experiments is due to poor ability to cross the blood brain barrier (BBB), not lack of activation of the appropriate brain receptors. Further evidence is reviewed that in some physiological states, some drugs with characteristics similar to bufotenine which do not normally cross the BBB, cross it and enter the brain. While direct human experimental evidence of bufotenine's hallucinogenic activity seems lacking, the above combined factors are considered, and possible explanations of bufotenine's reported psychoactivity are suggested. Additionally, updated experimental models testing the possible nature of bufotenine's hallucinogenic potential are proposed.

Induction of a high-affinity ketanserin binding site at the 5-Hydroxytryptamine(1B) receptor by modification of its carboxy-terminal intracellular portion

Two chimeric 5-hydroxytryptamine (5-HT) receptors were constructed by exchanging the C-terminal portion of the human (h) 5-HT(1B) receptor with the equivalent domain of the h 5-HT(2A) receptor (5-HT(1B/2A)) or with this domain truncated from its last 44 amino acids (5-HT(1B/2ADelta44)). The equilibrium dissociation constant of the radioligand [(3)H]GR 125743 was similar for both chimera compared to the wild-type (wt) h 5-HT(1B) receptor upon transient expression in COS-7 cells. Ketanserin binding affinity was 21-fold increased from pK(i): 5.79 (wt h 5-HT(1B) receptor) to pK(i): 7.11 at the 5-HT(1B/2A) chimeric receptor, this latter value being close to that of the wt h 5-HT(1D) receptor (pK(i): 7.62). This enhanced ketanserin binding affinity was lost when the last 44 C-terminal amino acids of the 5-HT(2A) receptor were deleted in the chimera 5-HT(1B/2ADelta44) (pK(i): 5.80). The binding affinities of the 5-HT antagonists ritanserin, GR 125743, and SB-224289 were not modified at either chimeric 5-HT receptor. The agonists F 11356, 5-HT, zolmitriptan, and sumatriptan yielded slightly increased (2- to 6-fold) binding affinities at both chimera as compared to the wt h 5-HT(1B) receptor. The present data suggest a role for the C-terminal intracellular receptor domain in modifying ketanserin/5-HT(1B) receptor interactions.

Molecular dynamics of NPY Y1 receptor activation

A three-dimensional model of the human neuropeptide Y(NPY)Y1 receptor (hY1) was constructed, energy refined and used to simulate molecular receptor interactions of the peptide ligands NPY, [L31, P34]NPY, peptide YY (PYY) and pancreatic polypeptide (PP), and of the nonpeptide antagonist R-N2-(diphenylacetyl)-N-(4-hydroxyphenyl)methyl-argininamide (BIBP3226) and its S-enantiomer BIBP3435. The best complementarity in charges between the receptor and the peptides, and the best structural accordance with experimental studies, was obtained with amino acid 1-4 of the peptides interacting with Asp194, Asp200, Gln201, Phe202 and Trp288 in the receptor. Arg33 and Arg35 of the peptides formed salt bridges with Asp104 and Asp287, respectively, while Tyr36 interacted in a binding pocket formed by Phe41, Thr42, Tyr100, Asn297, His298 and Phe302. Calculated electrostatic potentials around NPY and hY1 molecules indicated that ligand binding is initiated by electrostatic interactions between a highly positive region in the N- and C-terminal parts of the peptides, and a negative region in the extracellular receptor domains. Molecular dynamics simulations of NPY and BIBP3226 interactions with the receptor indicated rigid body motions of TMH5 and TMH6 upon NPY binding as mechanisms of receptor activation, and that BIBP3226 may act as an antagonist by constraining these motions.

Identification of an amino acid residue important for binding of methiothepin and sumatriptan to the human 5-HT(1B) receptor

Site-directed mutagenesis of the human 5-HT1B receptor was performed to investigate the role of the amino acid residues cysteine 326 and tryptophan 327 in transmembrane region VI and aspartic acid 352 in transmembrane region VII in ligand binding. Binding studies were performed with the antagonist radioligand [3H]GR125743 on mutant and wild-type receptors stably expressed in Chinese hamster ovary cells (CHO)-K1 cells. Substitution of tryptophan 327 by alanine resulted in decreased affinities of all ligands tested. The most prominent changes in affinity were observed for the antagonist methiothepin and the antimigraine drug sumatriptan, which were reduced approximately 300- and 60-fold, respectively. Nevertheless, the affinity of 5-HT remained the same. Replacement of the aspartic acid 352 by alanine reduced high-affinity binding of 5-HT. Substitution of cysteine 326 by alanine had minor effects on ligand binding. Some of these results agree with the results from mutagenesis studies of the corresponding amino acids in other receptors. However, some notable differences also emerge showing that functional roles of individual amino acid residues must be tested experimentally in each receptor subtype.

Chimeric receptor analysis of the ketanserin binding site in the human 5-Hydroxytryptamine1D receptor: importance of the second extracellular loop and fifth transmembrane domain in antagonist binding

The 5-hydroxytryptamine (5-HT)1B/1D receptor subtypes are involved in the regulation of 5-HT release and have gained particular interest because of their apparent role in migraine. Although selective antagonists for both receptor subtypes recently have been developed, the receptor domains involved in the pharmacological specificity of these antagonists are defined poorly. This was investigated with a chimeric 5-HT1B/1D receptor analysis and using ketanserin as a selective antagonist of h5-HT1D (h5-HT1D) Ki = 24-27 nM) as opposed to h5-HT1B (Ki = 2193-2902 nM) receptors. A domain of the h5-HT1D receptor encompassing the second extracellular loop and the fifth transmembrane domain is necessary and sufficient to promote higher affinity binding (Ki = 65-115 nM) for ketanserin to the h5-HT1B receptor. The same domain of the h5-HT1B receptor, when exchanged in the h5-HT1D receptor, abolished high affinity binding of ketanserin (Ki = 364-1265 nM). A similar observation was made with the antagonist ritanserin and seems specific because besides the unmodified binding affinities for 5-HT and zolmitriptan, only minor modifications (2-4-fold) were observed for the agonists L 694247 and sumatriptan and the antagonists GR 127935 and SB 224289. Generating point mutations of divergent amino acids compared with the h5-HT1B receptor did not demonstrate a smaller peptide region related to a significant modification of ketanserin binding. The antagonists ketanserin and ritanserin are likely to bind the h5-HT1D receptor by its second extracellular loop, near the exofacial surface of the fifth transmembrane domain, or both.

The molecular biology of serotonin receptors: therapeutic implications for the interface of mood and psychosis

This review summarizes the molecular biology of serotonin (5-HT; 5-hydroxytryptamine) receptors and indicates the potential relevance of this information for the treatment of mood and psychotic disorders. At least 15 separate subtypes of 5-HT receptors have been identified by molecular cloning techniques to be distinct genetic entities. Subtle differences in the primary amino acid sequences of these receptors can yield large differences in ligand selectivity. Additionally, it has recently been discovered that drugs such as atypical antipsychotic drugs and serotonin-selective reuptake inhibitors may interact with a large number of heretofore unknown 5-HT receptors. Thus clozapine, for instance, has high affinity for at least four separate 5-HT receptors, and it is unknown which of these receptors is essential for its unique therapeutic efficacy. One way to approach these questions is to test subtype-selective agents, although there are few of these currently available. Approaches to the design of subtype-selective ligands are described, including structure-based drug design and combinatorial approaches. Modes of regulation of 5-HT receptors are also summarized, and it is emphasized that antipsychotic drugs and antidepressants likely exert their effects via nontranscriptional and posttranslational means. Understanding the cellular mechanisms by which 5-HT receptors are regulated by psychopharmacologic agents is likely to yield novel insights into drug action.

The selective 5-HT1B receptor inverse agonist 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro- spiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289) potently blocks terminal 5-HT autoreceptor function both in vitro and in vivo

5-HT1 receptors are members of the G-protein-coupled receptor superfamily and are negatively linked to adenylyl cyclase activity. The human 5-HT1B and 5-HT1D receptors (previously known as 5-HT1Dbeta and 5-HT1Dalpha, respectively), although encoded by two distinct genes, are structurally very similar. Pharmacologically, these two receptors have been differentiated using nonselective chemical tools such as ketanserin and ritanserin, but the absence of truly selective agents has meant that the precise function of the 5-HT1B and 5-HT1D receptors has not been defined. In this paper we describe how, using computational chemistry models as a guide, the nonselective 5-HT1B/5-HT1D receptor antagonist 4 was structurally modified to produce the selective 5-HT1B receptor inverse agonist 5, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6, 7-tetrahydrospiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289). This compound is a potent antagonist of terminal 5-HT autoreceptor function both in vitro and in vivo.

Interaction of tryptamine and ergoline compounds with threonine 196 in the ligand binding site of the 5-hydroxytryptamine6 receptor

We examined the ligand-binding site of the 5-hydroxytryptamine6 (5-HT6) receptor using site-directed mutagenesis. Interactions with residues in two characteristic positions of trans-membrane region V are important for ligand binding in several bioamine receptors. In the 5-HT6 receptor, one of these residues is a threonine (Thr196), whereas in most other mammalian 5-HT receptors, the corresponding residue is alanine. After transient expression in human embryonic kidney 293 cells, we determined the effects of the mutation T196A on [3H]d-lysergic acid diethylamide (LSD) binding and adenylyl cyclase stimulation. This mutation produced a receptor with a 10-fold reduced affinity for [3H]LSD and a 6-fold reduced affinity for 5-HT. The potency of both LSD and 5-HT for stimulation of adenylyl cyclase was also reduced by 18- and 7-fold, respectively. The affinity of other N1-unsubstituted ergolines (e.g., ergotamine, lisuride) was reduced 10-30 fold, whereas the affinity of N1-methylated ergolines (e.g., metergoline, methysergide, mesulergine) and other ligands, such as methiothepine, clozapine, ritanserin, amitriptyline, and mainserin, changed very little or increased. This indicates that in wild-type 5-HT6 receptor, Thr196 interacts with the N1 of N1-unsubstituted ergolines and tryptamines, probably forming a hydrogen bond. Based on molecular modeling, a serine residue in transmembrane region IV of the 5-HT2A receptor has previously been proposed to interact with the N1-position of 5-HT. When the corresponding residue of the 5-HT6 receptor (Ala154) was converted to serine, no change in the affinity of twelve 5-HT6 receptor ligands or in the potency of 5-HT and LSD could be detected, suggesting that this position does not contribute to the ligand binding site of the 5-HT6 receptor.

Synthesis, biological activity, and molecular modeling of selective 5-HT(2C/2B) receptor antagonists

The synthesis and biological activity are reported for a series of analogues of the previously published indole urea 2 (SB-206553), designed to probe the 5-HT(2C) receptor binding site. Small molecule modeling studies have been used to define a region in space which is allowed at the 5-HT(2C) receptor but disallowed at the 5-HT(2A) receptor. In a complementary approach, docking of 2 into our model of the 5-HT(2C) receptor has allowed us to propose a novel primary binding interaction for this series of diaryl ureas, involving a potential double hydrogen-bonding interaction between the urea carbonyl oxygen of the ligand and two serine residues in the receptor. The difference of two valine residues in the 5-HT(2C) receptor for leucine residues in the 5-HT(2A) receptor is believed to account for the observed 5-HT(2C)/5-HT(2A) selectivity with 2.