Agonist, antagonist, and inverse agonist properties of antipsychotics at human recombinant 5-HT(1A) receptors expressed in HeLa cells

SerotoninAI: Serotonergic System Focused, Artificial Intelligence-Based Application for Drug Discovery

SerotoninAI is an innovative web application for scientific purposes focused on the serotonergic system. By leveraging SerotoninAI, researchers can assess the affinity (pKi value) of a molecule to all main serotonin receptors and serotonin transporters based on molecule structure introduced as SMILES. Additionally, the application provides essential insights into critical attributes of potential drugs such as blood-brain barrier penetration and human intestinal absorption. The complexity of the serotonergic system demands advanced tools for accurate predictions, which is a fundamental requirement in drug development. SerotoninAI addresses this need by providing an intuitive user interface that generates predictions of pKi values for the main serotonergic targets. The application is freely available on the Internet at https://serotoninai.streamlit.app/, implemented in Streamlit with all major web browsers supported. Currently, to the best of our knowledge, there is no tool that allows users to access affinity predictions for serotonergic targets without registration or financial obligations. SerotoninAI significantly increases the scope of drug development activities worldwide. The source code of the application is available at https://github.com/nczub/SerotoninAI_streamlit.

Functional characterization of 5-HT1A and 5-HT1B serotonin receptor signaling through G-protein-activated inwardly rectifying K+ channels in a fluorescence-based membrane potential assay

The 5-HT_1A and 5-HT_1B serotonin receptors are abundantly expressed in the CNS and constitute validated as well as putative drug targets in a variety of psychiatric and cognitive disorders, alcoholism/addiction, pain and migraine. In the present study we have characterized the functional properties of human 5-HT_1A and 5-HT_1B stably co-expressed with the human G-protein-activated inwardly rectifying K⁺ channel 2 (GIRK2) in HEK293 cells in the fluorescence-based FLIPR® Membrane Potential Blue (FMP) assay. Serotonin and other agonists induced robust decreases in fluorescence levels in the 5-HT_1A/GIRK2- and 5-HT_1B/GIRK2-HEK293 cells in a concentration-dependent manner in the assay, and these responses could be inhibited by selective 5-HT_1A/5-HT_1B antagonists and by the Gα_i/o-protein inhibitor pertussis toxin (PTX). Five additional stable HEK293 cell lines co-expressing 5-HT_1A or 5-HT_1B with GIRK2 and one of the PTX-insensitive Gα_i/o-subunit mutants Gα_i1^C351I, Gα_i2^C352I and Gα_o1^C351I were constructed, and 5-HT_1A/5-HT_1B-mediated responses through these specific Gα_i/o-subunits were measured in these cells pretreated with PTX in the FMP assay. The functional properties of 16 reference 5-HT₁ agonists were characterized at the seven cell lines, which constitutes the most detailed pharmacological profiling and comparison of 5-HT_1A and 5-HT_1B receptor signaling in the same assay published to date. We propose that this approach to assay 5-HT₁-mediated signaling through endogenous Gα_i/o-proteins in HEK293 cells or through specific Gα_i/o-subunits in a fairly high-throughput manner holds some advantages to other functional assays for Gα_i/o-coupled receptors. The assay will facilitate detailed profiling of the Gα_i/o- and Gβγ-mediated signaling of 5-HT_1A and 5-HT_1B at the molecular level, and it could also be used to identify novel modulators for the receptors.

Do in vitro assays in rat primary neurons predict drug-induced seizure liability in humans?

Drug-induced seizures contribute to the high attrition rate of pharmaceutical compounds in development. The assessment of drug-induced seizure liability generally occurs in later phases of development using low throughput and intensive in vivo assays. In the present study, we evaluated the potential of an in vitro assay for detecting drug-induced seizure risk compared to evaluation in rats in vivo. We investigated the effects of 8 reference drugs with a known seizurogenic risk using micro-electrode array (MEA) recordings from freshly-dissociated rat primary neurons cultured on 48-well dishes for 28 days, compared to their effects on the EEG in anesthetized rats. In addition, we evaluated functional responses and mRNA expression levels of different receptors in vitro to understand the potential mechanisms of drug-induced seizure risk. Combining the functional MEA in vitro data with concomitant gene expression allowed us to identify several potential molecular targets that might explain the drug-induced seizures occurring in both rats and humans. Our data 1) demonstrate the utility of a group of MEA parameters for detecting potential drug-induced seizure risk in vitro; 2) suggest that an in vitro MEA assay with rat primary neurons may have advantages over an in vivo rat model; and 3) identify potential mechanisms for the discordance between rat assays and human seizure risk for certain seizurogenic drugs.

Distinctive in vitro signal transduction profile of NLX-112, a potent and efficacious serotonin 5-HT1A receptor agonist

Objectives

NLX-112 (befiradol, F13640) is a selective serotonin 5-HT1A receptor agonist. Although it has been tested in vivo, little has been reported on its in vitro signal transduction profile.

Methods

NLX-112 was tested on G-protein activation, inhibition of adenylyl cyclase, ERK1/2 phosphorylation (pERK) and receptor internalization in recombinant cell lines. NLX-112 was also tested on G-protein activation in rat hippocampal membranes. Gα subunit mRNA expression in cell lines and rat brain tissue was quantified by quantitative PCR.

Key findings

For all signalling measures, NLX-112 exhibited agonist efficacy greater than for reference compounds ((±)8-OH-DPAT or buspirone), but similar to the endogenous agonist, serotonin, and was more potent for pERK than other responses. In rat hippocampal membranes, NLX-112 stimulated ‘total G-proteins' but, unlike (±)8-OH-DPAT and buspirone, was more potent for Gαo activation. Cell lines predominantly expressed Gαi1 and Gαi2 mRNA, with low levels of Gαo, whereas in rat brain Gαo subunits showed highest mRNA expression.

Conclusions

Unlike reference compounds, NLX-112 was a highly efficacious agonist in vitro, preferentially activating pERK in cell lines and Gαo proteins in rat hippocampal membranes. However, Gα subunit mRNA levels differ markedly between rat brain and cell lines, warranting caution when extrapolating from recombinant systems to native tissues.

Functional binding assays for estimation of the intrinsic efficacy of ligands at the 5-HT1A receptor: application for screening drug candidates

INTRODUCTION

Determination of the intrinsic efficacy of ligands at the 5-HT1A receptor is important for selecting drug candidates, e.g. in the case of schizophrenia where partial agonism is a favorable property shared by different atypical antipsychotics.

METHODS

Using seven ligands with different intrinsic efficacies and rat hippocampus synaptosomes, we compared critically three "functional" binding assays based on the ternary complex model that considers that the activated conformation of the receptor is the one coupled to G-protein.

RESULTS

The Ki ratio method, based on the difference of affinity of the competing drug when using an antagonist vs. an agonist as radioligand, discriminated the ligands according to their intrinsic efficacies, with values from 77 for the full agonist 5-hydroxytryptamine to 0.09 for the inverse agonist WAY 100,635. The GTP-shift method, based on the decrease of affinity observed with agonists when GTP is added to the competition binding assay, was equally effective in classifying the drugs according to their intrinsic efficacy. The lower sensibility of the GTP-shift assay was investigated and explained by the different ionic conditions used in the two assays and the way competition curves were analyzed. Albeit more direct, the assay based on agonist-stimulated [(35)S]-GTPγS binding to G proteins was more expensive and of greater variability in our hands.

DISCUSSION

We conclude that the GTP-shift procedure described herein for 5-HT1A receptors may expedite drug discovery efforts by predicting at the same time the affinity and intrinsic efficacy of ligands through a simple, rapid and economic ligand binding assay.

Withdrawal symptoms and rebound syndromes associated with switching and discontinuing atypical antipsychotics: theoretical background and practical recommendations

With the widespread use of atypical or second-generation antipsychotics, switching treatment has become current practice and more complicated, as the pharmacological profiles of these agents differ substantially despite their similarity in being 'atypical'. All share the ability to block dopamine D₂ receptors, and most of them also block serotonin 5-HT2A receptors. Apart from these common features, some atypical antipsychotics are also able to block or stimulate other dopamine or serotonin receptors, as well as histaminergic, muscarinergic or adrenergic receptors. As a result of the varying receptor affinities, in switching or discontinuing compounds several possible pitfalls have to be considered, including the occurrence of withdrawal and rebound syndromes. This article reviews the pharmacological background of functional blockade or stimulation of receptors of interest in regard to atypical antipsychotics and the implicated potential withdrawal and rebound phenomena. A MEDLINE search was carried out to identify information on withdrawal or rebound syndromes occurring after discontinuation of atypical antipsychotics. Using the resulting literature, we first discuss the theoretical background to the functional consequences of atypical antipsychotic-induced blockade or stimulation of neurotransmitter receptors and, secondly, we highlight the clinical consequences of this. We then review the available clinical literature on switching between atypical antipsychotics, with respect to the occurrence of withdrawal or rebound symptoms. Finally, we offer practical recommendations based on the reviewed findings. The systematic evaluation of withdrawal or rebound phenomena using randomized controlled trials is still understudied. Knowledge of pharmacological receptor-binding profiles may help clinicians in choosing adequate switching or discontinuation strategies for each agent. Results from large switching trials indicate that switching atypical antipsychotics can be performed in a safe manner. Treatment-emergent adverse events during or after switching are not always considered to be, at least in part, associated with the pre-switch antipsychotic. Further studies are needed to substantiate the evidence gained so far on different switching strategies. The use of concomitant medication, e.g., benzodiazepines or anticholinergic drugs, may help to minimize symptoms arising from the discontinuation or switching of antipsychotic treatment.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

Evaluation of the clinical efficacy of asenapine in schizophrenia

IMPORTANCE OF THE FIELD

Asenapine is a new atypical antipsychotic medication with high affinity for D(2) and 5HT(2A) receptors that has been approved by the FDA in adults for the acute treatment of schizophrenia in the USA. The purpose of this review is to describe the compound and examine whether it addresses some of the unmet clinical needs in treating schizophrenia.

AREAS COVERED IN THIS REVIEW

The development of asenapine is described with attention to its chemistry, pharmacodynamic and pharmacokinetic profile. Preclinical and clinical trials of safety and efficacy are reviewed. The advantages and disadvantages of asenapine relative to other antipsychotic medications are discussed.

WHAT THE READER WILL GAIN

Asenapine will be evaluated for whether it: i) causes a reduction in symptoms of schizophrenia; ii) has a side-effect profile minimizing extrapyramidal symptoms, weight gain and cardiac effects; and iii) affects negative and/or cognitive symptoms.

TAKE HOME MESSAGE

Asenapine is a recently approved agent with an acceptable cardiometabolic profile and exhibits similar efficacy as other antipsychotic medications, primarily on positive symptoms of schizophrenia. Relatively less weight gain compared with other agents may confer a notable advantage. Sublingual administration may have positive and negative effects on patient compliance. Potential 'pro-cognitive' effects of asenapine are preliminary and require more investigation.

Pharmacological, neurochemical, and behavioral profile of JB-788, a new 5-HT1A agonist

A novel pyridine derivative, 8-{4-[(6-methoxy-2,3-dihydro-[1,4]dioxino[2,3-b]pyridine-3-ylmethyl)-amino]-butyl}-8-aza-spiro[4.5]decane-7,9-dione hydrochloride, termed JB-788, was designed to selectively target 5-HT(1A) receptors. In the present study, the pharmacological profile of JB-788 was characterized in vitro using radioligands binding tests and in vivo using neurochemical and behavioural experiments. JB-788 bound tightly to human 5-HT(1A) receptor expressed in human embryonic kidney 293 (HEK-293) cells with a K(i) value of 0.8 nM. Its binding affinity is in the same range as that observed for the (+/-)8-OH-DPAT, a reference 5HT(1A) agonist compound. Notably, JB-788 only bound weakly to 5-HT(1B) or 5-HT(2A) receptors and moreover the drug displayed only weak or indetectable binding to muscarinic, alpha(2), beta(1) and beta(2) adrenergic receptors, or dopaminergic D(1) receptors. JB-788 was found to display substantial binding affinity for dopaminergic D(2) receptors and, to a lesser extend to alpha(1) adrenoreceptors. JB-788 dose-dependently decreased forskolin-induced cAMP accumulation in HEK cells expressing human 5-HT(1A), thus acting as a potent 5-HT(1A) receptor agonist (E(max.) 75%, EC(50) 3.5 nM). JB-788 did not exhibit any D(2) receptor agonism but progressively inhibited the effects of quinpirole, a D(2) receptor agonist, in the cAMP accumulation test with a K(i) value of 250 nM. JB-788 induced a weak change in cAMP levels in mouse brain but, like some antipsychotics, transiently increased glycogen contents in various brain regions. Behavioral effects were investigated in mice using the elevated plus-maze. JB-788 was found to increase the time duration spent by animals in anxiogenic situations. Locomotor hyperactivity induced by methamphetamine in mouse, a model of antipsychotic activity, was dose-dependently inhibited by JB-788. Altogether, these results suggest that JB-788 displays pharmacological properties, which could be of interest in the area of anxiolytic and antipsychotic drugs.

Role of serotonin-1A receptors in the action of antipsychotic drugs: comparison of prepulse inhibition studies in mice and rats and relevance for human pharmacology

This study aimed to explore strain and species differences in the involvement of 5-HT1A receptors in the action of antipsychotic drugs, using prepulse inhibition (PPI), a model of sensory processing which is deficient in schizophrenia patients. We used automated startle boxes to compare the effect of the 5-HT1A receptor agonist, (+/-)-8-hydroxy-dipropyl-amino-tetralin (8-OH-DPAT), on PPI in three mouse strains. Balb/c mice were then pretreated with antipsychotics, treated with 8-OH-DPAT or saline, and tested for PPI. 8-OH-DPAT treatment dose dependently increased PPI in Balb/c mice, but had less effect in 129Sv and C57Bl/6 mice. In Balb/c mice, the effect of 8-OH-DPAT was blocked by the typical antipsychotic and dopamine D2 receptor antagonist, haloperidol and the third generation antipsychotic, aripiprazole, which has activity at both 5-HT1A and dopamine D2 receptors. The atypical antipsychotics, clozapine, olanzapine and risperidone, had lesser effects. Similar to our earlier studies in rats, the present PPI results suggest that 5-HT1A receptors are involved in the action of some antipsychotic drugs in mice. Despite strain and species differences in the magnitude and direction of the effect of 8-OH-DPAT, downstream dopamine D2 receptor activation seems to be an important mediator. These comparative results allow a theoretical framework of receptor interactions, which may guide further studies on the involvement of 5-HT1A receptors in schizophrenia.

Asenapine increases dopamine, norepinephrine, and acetylcholine efflux in the rat medial prefrontal cortex and hippocampus

Atypical antipsychotic drugs, which are more potent direct acting antagonists of brain serotonin (5-HT)(2A) than dopamine (DA) D(2) receptors, preferentially enhance DA and acetylcholine (ACh) efflux in the rat medial prefrontal cortex (mPFC) and hippocampus (HIP), compared with the nucleus accumbens (NAc). These effects may contribute to their ability, albeit limited, to improve cognitive function and negative symptoms in patients with schizophrenia. Asenapine (ASE), a new multireceptor antagonist currently in development for the treatment of schizophrenia and bipolar disorder, has complex serotonergic properties based upon relatively high affinity for multiple serotonin (5-HT) receptors, particularly 5-HT(2A) and 5-HT(2C) receptors. In the current study, the effects of ASE on DA, norepinephrine (NE), 5-HT, ACh, glutamate, and gamma-aminobutyric acid (GABA) efflux in rat mPFC, HIP, and NAc were investigated with microdialysis in awake, freely moving rats. ASE at 0.05, 0.1, and 0.5 mg/kg (s.c.), but not 0.01 mg/kg, significantly increased DA efflux in the mPFC and HIP. Only the 0.5 mg/kg dose enhanced DA efflux in the NAc. ASE, at 0.1 and 0.5 mg/kg, significantly increased ACh efflux in the mPFC, but only the 0.5 mg/kg dose of ASE increased HIP ACh efflux. ASE did not increase ACh efflux in the NAc at any of the doses tested. The effect of ASE (0.1 mg/kg) on DA and ACh efflux was blocked by pretreatment with WAY100635, a 5-HT(1A) antagonist/D(4) agonist, suggesting involvement of indirect 5-HT(1A) agonism in both the actions. ASE, at 0.1 mg/kg, increased NE, but not 5-HT, efflux in the mPFC and HIP. ASE, at 0.1 mg/kg (s.c.), had no effect on glutamate and GABA efflux in either the mPFC or NAc. These findings indicate that ASE is similar to clozapine and other atypical antipsychotic drugs in preferentially increasing the efflux of DA, NE, and ACh in the mPFC and HIP compared with the NAC, and suggests that, like these agents, it may also improve cognitive function and negative symptoms in patients with schizophrenia.

Niacin-induced "flush" involves release of prostaglandin D2 from mast cells and serotonin from platelets: evidence from human cells in vitro and an animal model

Niacin lowers serum cholesterol, low-density lipoprotein, and triglycerides, and it raises high-density lipoprotein. However, most patients experience cutaneous warmth and vasodilation (flush). Acetylsalicylic acid (ASA) can reduce this flush, presumably by decreasing prostaglandin D(2) (PGD(2)) release from macrophages. Here, we show that methylnicotinate induces significant PGD(2) release from human mast cells and serotonin from human platelets. Intradermal injection of methylnicotinate induces rat skin vasodilation and vascular permeability. Niacin increases plasma PGD(2) and serotonin in a rat model of flush. The phenothiazine prochlorperazine, the H(1), serotonin receptor antagonist cyproheptadine, and the specific serotonin receptor-2A antagonist ketanserin inhibit niacin-induced temperature increase by 90% (n = 5, p < 0.05), 90 and 50% (n = 3, p < 0.05), and 85% (n = 6, p = 0.0008), respectively, in this animal model. These results indicate that niacin-induced flush involves both PGD(2) and serotonin, suggesting that drugs other than ASA are required to effectively inhibit niacin-induced flush.

The flavonoid luteolin inhibits niacin-induced flush

BACKGROUND AND PURPOSE

Sustained release niacin effectively lowers serum cholesterol, LDL and triglycerides, while raising HDL. However, 75% of patients experience cutaneous warmth and itching known as flush, leading to discontinuation. Acetylsalicylic acid (aspirin) reduces this flush only by about 30%, presumably through decreasing prostaglandin D2 (PGD2). We investigated whether niacin-induced flush in a rat model involves PGD2 and 5-HT, and the effect of certain flavonoids.

EXPERIMENTAL APPROACH

Three skin temperature measurements from each ear were recorded with an infrared pyrometer for each time point immediately before i.p. injection with either niacin or a flavonoid. The temperature was then measured every 10 min for 60 min.

KEY RESULTS

Niacin (7.5 mg per rat, equivalent to a human dose of 1750 mg per 80 kg) maximally increased ear temperature to 1.9+/-0.2 degrees C at 45 min. Quercetin and luteolin (4.3 mg per rat; 1000 mg per human), administered i.p. 45 min prior to niacin, inhibited the niacin effect by 96 and 88%, respectively. Aspirin (1.22 mg per rat; 325 mg per human) inhibited the niacin effect by only 30%. Niacin almost doubled plasma PGD2 and 5-HT, but aspirin reduced only PGD2 by 86%. In contrast, luteolin inhibited both plasma PGD2 and 5-HT levels by 100 and 67%, respectively. CONCLUSIONS AND IMPLICATIONS. Niacin-induced skin temperature increase is associated with PGD2 and 5-HT elevations in rats; luteolin may be a better inhibitor of niacin-induced flush because it blocks the rise in both mediators.

Antipsychotics differ in their ability to internalise human dopamine D2S and human serotonin 5-HT1A receptors in HEK293 cells

Antipsychotic drugs act preferentially via dopamine D(2) receptor blockade, but interaction with serotonin 5-HT(1A) receptors has attracted interest as additional target for antipsychotic treatment. As receptor internalisation is considered crucial for drug action, we tested the propensity of antipsychotics to internalise human (h)D(2S) receptors and h5-HT(1A) receptors. Agonist-induced internalisation of hemaglutinin (HA)-tagged hD(2S) and HA-h5-HT(1A) receptors expressed in HEK293 cells was increased by coexpression of G-protein coupled receptor kinase 2 and beta-arrestin2. At the HA-hD(2S) receptor, dopamine, quinpirole and bromocriptine behaved as full agonists, while S(-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)-3PPP] and sarizotan were partial agonists. The typical antipsychotic, haloperidol, and the atypical compounds, olanzapine, nemonapride, ziprasidone and clozapine did not internalise HA-hD(2S) receptors, whereas aripiprazole potently internalised these receptors (>50% relative efficacy). Among antipsychotics with combined D(2)/5-HT(1A) properties, bifeprunox and (3-exo)-8-benzoyl-N-[[(2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl]methyl]-8-azabicyclo-[3.2.1]octane-3-methanamine (SSR181507) partially internalised HA-hD(2S) receptors, piperazine, 1-(2,3-dihydro-1,4-benzodioxin-5-yl)-4-[[5-(4-fluorophenyl)-3-pyridinyl]methyl (SLV313) and N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine (F15063) were inactive. At the HA-h5-HT(1A) receptor, serotonin, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)-8-OH-DPAT] and sarizotan were full agonists, buspirone acted as partial agonist. (-)-Pindolol showed little activity and no internalising properties were manifested for the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635). Most antipsychotics induced HA-h5-HT(1A) receptor internalisation, with an efficacy rank order: nemonapride>F15063>SSR181507>bifeprunox approximately SLV313 approximately ziprasidone>aripiprazole and potencies: SLV313>SSR181507 approximately F15063>bifeprunox approximately nemonapride approximately aripiprazole>ziprasidone. Interestingly, the internalisation induced by clozapine was only minimal, whereas aripirazole and bifeprunox were more potent for internalisation than for G-protein activation. These different profiles of antipsychotics for receptor internalisation may help to evaluate their potential therapeutic impact in the treatment of schizophrenia.

5-HT1A receptor agonist properties of antipsychotics determined by [35S]GTPgammaS binding in rat hippocampal membranes

1. 5-Hydroxytryptamine 1A (5-HT1A) receptors have attracted increasing attention as a promising target for antipsychotic therapy. Although many atypical antipsychotic drugs, including the prototype clozapine, have been reported to be partial agonists at 5-HT1A receptors, these results are often fragmental and derived mainly from experiments that used cultured cells. 2. In the present study, [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding assay in rat hippocampal membranes was applied to a series of antipsychotic drugs, especially atypical antipsychotics. 3. Most, but not all, of atypical antipsychotic drugs and the classical antipsychotic drug nemonapride behaved as partial agonists at 5-HT1A receptors with varied potencies and relative efficacies. The most potent compound was perospirone with a mean EC50 of 27 nmol/L, followed by aripiprazole (45 nmol/L) > ziprasidone (480 nmol/L) > nemonapride (790 nmol/L) > clozapine (3900 nmol/L) > quetiapine (26,000 nmol/L). The maximal percentage increases over the basal binding (%Emax) for these antipsychotic drugs were 30-50%, with the exception of perospirone (approximately 15%), whereas 5-HT stimulated the binding to a mean %Emax of 105%. 4. Increasing concentrations of the selective and neutral 5-HT1A antagonist WAY100635 shifted the concentration-response curve of nemonapride-stimulated [35S]GTPgammaS binding to the right and in parallel. 5. The relative efficacy or intrinsic activity of a compound was affected differently by the differing concentrations of guanosine diphosphate (GDP) in the assay buffer, which should be taken into consideration when determining the relative efficacies of these antipsychotics as 5-HT1A receptor agonists. 6. These results provide important information concerning the relevance of 5-HT1A receptor partial agonist properties in the treatment for schizophrenic patients with most, if not all, of atypical antipsychotic drugs.

Differential Effects of Antipsychotic Drugs on Serotonin-1A Receptor-Mediated Disruption of Prepulse Inhibition

Serotonin-1A (5-HT(1A)) receptors have been implicated in the symptoms of schizophrenia. However, there is limited in vivo evidence for an interaction of antipsychotic drugs with 5-HT(1A) receptor-mediated behavioral effects. We therefore investigated in rats the action of several antipsychotic drugs on prepulse inhibition (PPI), a measure of sensorimotor gating that is deficient in schizophrenia. Disruption of PPI at the 100-ms interstimulus interval (ISI), but not the 30-ms ISI, was induced by treatment with 0.5 mg/kg 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), the 5-HT(1A) receptor agonist. In rats pretreated with 0.25 mg/kg haloperidol (4-[-4-(p-chlorophenyl)-4-hydroxypiperidino]-4'-fluoro butyrophenone) or raclopride [3,5-dichloro-N-(1-ethylpyrrolidin-2-ylmethyl)-2-hydroxy-6-methoxybenzamide tartrate], the disruption of PPI was no longer significant. Of the atypical antipsychotic drugs clozapine (8-chloro-11-(4-methyl-1-piperazinyl)-5H-dibenzo[b,e][1,4]-diazepine), olanzapine (2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine), risperidone [3-[2-[-4-(6-fluoro-1,2-benzisoxazol-3-yl) piperidino] ethyl-6,7,8,9-tetrahydro-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one)], amisulpride (4-amino-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(ethylsulfonyl)-o-anisamide), and aripiprazole (7-[4-[-4[-(2,3-dichlorophenyl)-1-piperazinyl]butoxy]-3,4-dihydrocarbostyrilor 7-[4-[4-(2,3-dichlorophenyl) piperazin-1-yl]butoxy]-1,2,3,4,-tetrahydroquinolin-2-one), only aripiprazole significantly reduced the effect of 8-OH-DPAT on PPI. This effect was mimicked by pretreatment with the 5-HT(1A) receptor partial agonist, buspirone [N-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride]. On the other hand, some of the antipsychotic drugs and other pretreatments showed complex, prepulse-dependent effects on their own. These data show little in vivo interaction of several atypical antipsychotic drugs with the disruption of PPI mediated by 5-HT(1A) receptor stimulation. The action of haloperidol and raclopride suggests a major involvement of dopamine D(2) receptors in this effect, possibly downstream from the initial serotonergic stimulation. The action of aripiprazole could be mediated by its partial agonist properties at 5-HT(1A) receptors or its dopamine D(2)-blocking properties.

Effects of novel antipsychotics with mixed D(2) antagonist/5-HT(1A) agonist properties on PCP-induced social interaction deficits in the rat

Considerable interest has arisen in identifying antipsychotic agents with improved efficacy against negative symptoms, such as social withdrawal. In rats, a social interaction deficit can be induced by the NMDA antagonist phencyclidine (PCP). Here, we examined the effects of antipsychotics, reported to exert dual 5-HT(1A)/D(2) actions, on PCP-induced social interaction deficits. Drugs were administered daily for 3 days in combination with either vehicle or PCP (2.5mg/kg, SC) and social interaction was measured on the last day of drug treatment. Pairs of unfamiliar rats receiving the same treatment were placed in a large open field for 10 min and the number of social behaviors were scored. The results indicate that: (1) PCP significantly reduced social interaction by over 50% compared with vehicle-treated controls; (2) haloperidol (0.0025-0.16 mg/kg, SC) and clozapine (0.04-10mg/kg, IP) did not reverse PCP-induced social interaction deficits; (3) the substituted benzamide remoxipride reversed PCP-induced deficits at 0.63 and 2.5mg/kg (4) the 5-HT(1A) agonist 8-OH-DPAT was inactive (at 0.01-0.63 mg/kg, SC); (5) among compounds reported to exert dual 5-HT(1A)/D(2) actions, SSR181507 (at 0.16 mg/kg, SC) and aripiprazole (at 0.04 and 0.16 mg/kg, IP), but not ziprasidone (0.04-2.5mg/kg, IP), SLV313 (0.0025-0.16 mg/kg, SC) or bifeprunox (0.01-0.63 mg/kg, IP), significantly reversed PCP-induced social interaction deficits; and (6) the 5-HT(1A) receptor antagonist WAY100635 blocked the effects of SSR181507 and aripiprazole. These findings indicate that the balance of activity at 5-HT(1A) and D(2) receptors profoundly influences the activity of antipsychotics in this model of social withdrawal, and their potential benefit on at least some of the negative symptoms of schizophrenia.

Detailed pharmacological characterization of 5-HT1A-receptor-mediated [35S]GTP gamma S binding in rat hippocampal membranes

5-HT-stimulated [(35)S]GTPgammaS binding to rat hippocampal membranes was pharmacologically characterized. Signal/noise ratio or percent increase over basal was optimized with 100 microM GDP, 2-10 mM MgCl(2), and 150-200 mM NaCl. However, we preferred the standard condition (20 microM GDP, 5 mM MgCl(2), and 100 mM NaCl: Condition I) to the alternative one (100 microM GDP, 5 mM MgCl(2), and 150 mM NaCl: Condition II) because 1) absolute values of basal and 5-HT-sensitive bindings decreased with higher concentrations of GDP and NaCl; 2) EC(50) values determined under Condition II were 2 - 6 fold higher than those under Condition I; 3) some partial agonists had less intrinsic activities in the presence of higher concentrations of GDP; and 4) Inhibitory effects of WAY100635 were complete under Condition I, while incomplete under Condition II. Pharmacological profile of concentration-dependent stimulation by a series of 5-HT ligands and concentration-dependent inhibition of 5-HT-stimulated binding by several 5-HT-receptor antagonists clearly indicated that this response under Condition I was mediated solely through 5-HT(1A) receptors. Although caution should be paid especially to the apparent intrinsic activities susceptible to the assay conditions, this method appears useful to investigate functional coupling between 5-HT(1A) receptors and their coupled G proteins in native hippocampal membranes.

Clozapine, ziprasidone and aripiprazole but not haloperidol protect against kainic acid-induced lesion of the striatum in mice, in vivo: Role of 5-HT1A receptor activation

Excessive activation of non-NMDA receptors, AMPA and kainate, contributes to neuronal degeneration in acute and progressive pathologies, possibly including schizophrenia. Because 5-HT(1A) receptor agonists have neuroprotective properties (e.g., against NMDA-induced neurotoxicity), we compared the effects of the antipsychotics, clozapine, ziprasidone and aripiprazole, that are partial agonists at 5-HT(1A) receptor, with those of haloperidol, which is devoid of 5-HT(1A) agonist properties, on kainic acid (KA)-induced striatal lesion volumes, in C57Bl/6N mice. The involvement of 5-HT(1A) receptors was determined by antagonist studies with WAY100635, and data were compared with those obtained using the potent and high efficacy 5-HT(1A) receptor agonist, F13714. Intra-striatal KA lesioning and measurement of lesion volumes using cresyl violet staining were carried out at 48 h after surgery. F13714, antipsychotics or vehicle were administered ip twice, 30 min before and 3 1/2 h after KA injection. WAY100635 (0.63 mg/kg) or vehicle were given sc 30 min before each drug injection. Clozapine (2 x 10 mg/kg), ziprasidone (2 x 20 mg/kg) and aripiprazole (2 x 10 mg/kg) decreased lesion volume by 61%, 59% and 73%, respectively. WAY100635 antagonized the effect of ziprasidone and of aripiprazole but only slightly attenuated that of clozapine. In contrast, haloperidol (2 x 0.16 mg/kg) did not affect KA-induced lesion volume. F13714 dose-dependently decreased lesion volume. The 61% decrease of lesion volume obtained with F13714 (2 x 0.63 mg/kg) was antagonized by WAY100635. WAY100635 alone did not affect lesion volume. These results show that 5-HT(1A) receptor activation protects against KA-induced striatal lesions and indicate that some atypical antipsychotic agents with 5-HT(1A) agonist properties may protect against excitotoxic injury, in vivo.

Synthesis and structure-activity relationships of 1-aralkyl-4-benzylpiperidine and 1-aralkyl-4-benzylpiperazine derivatives as potent sigma ligands

In the attempt to define more accurately structure-affinity relationships for sigma(1) and sigma(2) ligands, we synthesized and tested on sigma subtype receptors a series of aralkyl derivatives of 4-benzylpiperidine, in which the effect of modifications on the aralkyl moiety was studied in a systematic way. The affinity of the compounds here described varied to a great extent, with a sigma(2)/sigma(1) selectivity ranging from 0.1 to 9. Thus, to confirm the ability of the piperazine derivative to bind to sigma(1) receptors in a different way than piperidines, we synthesized and tested a series of piperazine compounds; the comparison of their affinity with that of the corresponding piperidines strongly supports the possibility of a different binding mode. While the compounds here described are on the whole selective for sigma vs serotonin 5-HT(1A) and dopamine D(2) receptors, 9aa, 9ba and 9ab possess a remarkable affinity for both sigma and 5-HT(1A) receptors, with K(i) in the nanomolar range, and are selective with respect to D(2) receptors. They displayed also a partial agonist profile in a human 5-HT(1A) [(35)S]GTP gamma S binding assay, suggesting their potential use as atypical antipsychotic agents.

Comparison of hippocampal G protein activation by 5-HT(1A) receptor agonists and the atypical antipsychotics clozapine and S16924

This study employed [(35)S]guanosine 5'- O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding to compare the actions of antipsychotic agents known to stimulate cloned, human 5-HT(1A) receptors with those of reference agonists at postsynaptic 5-HT(1A) receptors. In rat hippocampal membranes, the following order of efficacy was observed (maximum efficacy, E(max), values relative to 5-HT=100): (+)8-OH-DPAT (85), flesinoxan (62), eltoprazine (60), S14506 (59), S16924 (48), buspirone (41), S15535 (22), clozapine (22), ziprasidone (21), pindolol (7), p-MPPI (0), WAY100,635 (0), spiperone (0). Despite differences in species and tissue source, the efficacy and potency (pEC(50)) of agonists (with the exception of clozapine) correlated well with those determined previously at human 5-HT(1A) receptors expressed in Chinese hamster ovary (CHO) cells. In contrast, clozapine was more potent at hippocampal membranes. The selective antagonists p-MPPI and WAY100,635 abolished stimulation of binding by (+)8-OH-DPAT, clozapine and S16924 (p-MPPI), indicating that these actions were mediated specifically by 5-HT(1A) receptors. Clozapine and S16924 also attenuated 5-HT- and (+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding, consistent with partial agonist properties. In [(35)S]GTPgammaS autoradiographic studies, 5-HT-induced stimulation, mediated through 5-HT(1A) receptors, was more potent in the septum (pEC(50) approximately 6.5) than in the dentate gyrus of the hippocampus (pEC(50) approximately 5) suggesting potential differences in coupling efficiency or G protein expression. Though clozapine (30 and 100 microM) did not enhance [(35)S]GTPgammaS labelling in any structure, S16924 (10 micro M) modestly increased [(35)S]GTPgammaS labelling in the dentate gyrus. On the other hand, both these antipsychotic agents attenuated 5-HT (10 microM)-stimulated [(35)S]GTPgammaS binding in the dentate gyrus and septum. In conclusion, clozapine, S16924 and ziprasidone act as partial agonists for G protein activation at postsynaptic 5-HT(1A) receptors in the hippocampus. These data support a role of postsynaptic 5-HT(1A) receptors in the functional profiles of certain antipsychotic agents.

Antibody capture assay reveals bell-shaped concentration-response isotherms for h5-HT(1A) receptor-mediated Galpha(i3) activation: conformational selection by high-efficacy agonists, and relationship to trafficking of receptor signaling

Although serotonin 5-HT(1A) receptors couple to several Gi/o G-protein subtypes, little is known concerning their differential activation patterns. In this study, in membranes of Chinese hamster ovary cells expressing h5-hydroxytryptamine(1A) receptors (CHO-h5-HT(1A)), isotherms of 5-HT-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding were biphasic, suggesting coupling to multiple G-protein subtypes. The high potency component was abolished by preincubation with an antibody recognizing Galpha(i3) subunits and was resistant to induction of [(35)S]GTPgammaS dissociation by unlabeled GTPgammaS, thus yielding a bell-shaped concentration-response isotherm. To directly investigate Galpha(i3) activation, we adopted an antibody-capture/scintillation proximity assay. 5-HT and other high-efficacy agonists yielded bell-shaped [(35)S]GTPgammaS binding isotherms, with peaks at nanomolar concentrations. As drug concentrations increased, Galpha(i3) stimulation progressively returned to basal values. In contrast, the partial agonists (-)-pindolol and 4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) displayed sigmoidal stimulation isotherms, whereas spiperone and other inverse agonists sigmoidally inhibited [(35)S]GTPgammaS binding. Agonist-induced stimulation and inverse agonist-induced inhibition of Galpha(i3) activation were i) abolished by pretreatment of CHO-h5-HT(1A) cells with pertussis toxin; ii) reversed by the selective 5-HT(1A) antagonist (N-[2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclohexane-carboxamide) fumarate (WAY100,635), and iii) absent in nontransfected CHO cell membranes. 5-HT isotherms could be modified by altering sodium concentration; only stimulatory actions were observed at 300mM NaCl, whereas only inhibitory actions were seen at 10 mM NaCl. Furthermore, bell-shaped isotherms were not detected at short incubation times, suggesting time-dependent changes in receptor/Galpha(i3) coupling. Taken together, these data show that low but not high concentrations of high-efficacy 5-HT(1A) agonists direct receptor signaling to Galpha(i3). In contrast, partial agonists favor h5-HT(1A) receptor signaling to Galpha(i3) over a wide concentration range, whereas inverse agonists inhibit constitutive Galpha(i3) activation.