Differential profile of typical, atypical and third generation antipsychotics at human 5-HT7a receptors coupled to adenylyl cyclase: detection of agonist and inverse agonist properties

Cariprazine, A Broad-Spectrum Antipsychotic for the Treatment of Schizophrenia: Pharmacology, Efficacy, and Safety

Schizophrenia is characterized by positive, negative, cognitive, and affective symptoms. Antipsychotic medications, which work by blocking the dopamine D₂ receptor, are the foundation of pharmacotherapy for schizophrenia to control positive symptoms. Cariprazine is a dopamine D₃ receptor-preferring D₃/D₂ partial agonist antipsychotic that is approved for the treatment of schizophrenia (USA and European Union [EU]) and manic and depressive episodes associated with bipolar I disorder (USA). Partial agonist agents have a lower intrinsic activity at receptors than full agonists, so they act as either functional agonists or functional antagonists depending on the surrounding neurotransmitter environment. Beyond efficacy against positive symptoms, the unique D₃-preferring partial agonist pharmacology of cariprazine suggests potential advantages against negative symptoms, and cognitive and functional impairment, which are challenging to treat. The efficacy and safety of cariprazine in adult patients with schizophrenia have been demonstrated in four short-term randomized, double-blind, placebo-controlled clinical trials, two long-term open-label studies, one relapse prevention study, and one prospective negative symptom study versus the active comparator risperidone. Additional post hoc investigations have supported efficacy across individual symptoms and domains in schizophrenia, as well as in diverse areas of interest including cognition, functioning, negative symptoms, hostility, and global well-being. This comprehensive review of cariprazine summarizes its pharmacologic profile, clinical trial evidence, and post hoc investigations. Collective evidence suggests that the pharmacology of cariprazine may offer broad-spectrum efficacy advantages for patients with schizophrenia, including effects against difficult-to-treat negative and cognitive symptoms, as well as functional improvements. Cariprazine was generally safe and well tolerated in patients with short- and long-term exposure and no new safety concerns were associated with longer-duration treatment.

Trial registration ClinicalTrials.gov identifiers, NCT00404573, NCT00694707, NCT01104766, NCT01104779, NCT01412060, NCT00839852, NCT01104792.

Translating biased agonists from molecules to medications: Serotonin 5-HT1A receptor functional selectivity for CNS disorders

Biased agonism (or "functional selectivity") at G-protein-coupled receptors has attracted rapidly increasing interest as a means to improve discovery of more efficacious and safer pharmacotherapeutics. However, most studies are limited to in vitro tests of cellular signaling and few biased agonists have progressed to in vivo testing. As concerns 5-HT_1A receptors, which exert a major control of serotonergic signaling in diverse CNS regions, study of biased agonism has previously been limited by the poor target selectivity and/or partial agonism of classically available ligands. However, a new generation of highly selective, efficacious and druggable agonists has advanced the study of biased agonism at this receptor and created new therapeutic opportunities. These novel agonists show differential properties for G-protein signaling, cellular signaling (particularly pERK), electrophysiological effects, neurotransmitter release, neuroimaging by PET and pharmacoMRI, and behavioral tests of mood, motor activity and side effects. Overall, NLX-101 (a.k.a. F15599) exhibits preferential activation of cortical and brain stem 5-HT_1A receptors, whereas NLX-112 (a.k.a. befiradol or F13640) shows prominent activation of 5-HT_1A autoreceptors in Raphe nuclei and in regions associated with motor control. Accordingly, NLX-101 is potently active in rodent models of depression and respiratory control, whereas NLX-112 shows promising activity in models of Parkinson's disease across several species - rat, marmoset and macaque. Moreover, NLX-112 has also been labeled with ¹⁸F to produce the first agonist PET radiopharmaceutical (known as [¹⁸F]-F13640) for investigation of the active state of 5-HT_1A receptors in rodent, primate and human. The structure-functional activity relationships of biased agonists have been investigated by receptor modeling and novel compounds have been identified which exhibit increased affinity at 5-HT_1A receptors and new profiles of cellular signaling bias, notably for β-arrestin recruitment versus pERK. Taken together, the data suggest that 5-HT_1A receptor biased agonists constitute potentially superior pharmacological agents for treatment of CNS disorders involving serotonergic mechanisms.

5-HT7 Receptor Inhibition Transiently Improves Respiratory Function Following Daily Acute Intermittent Hypercapnic-Hypoxia in Rats With Chronic Midcervical Spinal Cord Contusion

Background. Intermittent hypoxia can induce respiratory neuroplasticity to enhance respiratory motor outputs following hypoxic treatment. This type of respiratory neuroplasticity is primarily mediated by the activation of Gq-protein-coupled 5-HT2 receptors and constrained by Gs-protein-coupled 5-HT7 receptors. Objective. The present study hypothesized that the blockade of 5-HT7 receptors can potentiate the effect of intermittent hypercapnic-hypoxia on respiratory function after cervical spinal cord contusion injury. Methods. The ventilatory behaviors of unanesthetized rats with midcervical spinal cord contusions were measured before, during, and after daily acute intermittent hypercapnic-hypoxia (10 episodes of 5 minutes of hypoxia [10% O2, 4% CO2, 86% N2] with 5 minutes of normoxia intervals for 5 days) at 8 weeks postinjury. On a daily basis, 5 minutes before intermittent hypercapnic-hypoxia, rats received either a 5-HT7 receptor antagonist (SB269970, 4 mg/kg, intraperitoneal) or a vehicle (dimethyl sulfoxide). Results. Treatment with intermittent hypercapnic-hypoxia induced a similar increase in tidal volume between rats that received SB269970 and those that received dimethyl sulfoxide within 60 minutes post-hypoxia on the first day. However, after 2 to 3 days of daily acute intermittent hypercapnic-hypoxia, the baseline tidal volumes of rats treated with SB269970 increased significantly. Conclusions. These results suggest that inhibiting the 5-HT7 receptor can transiently improve daily intermittent hypercapnic-hypoxia–induced tidal volume increase in midcervical spinal contused animals. Therefore, combining pharmacological treatment with rehabilitative intermittent hypercapnic-hypoxia training may be an effective strategy for synergistically enhancing respiratory neuroplasticity to improve respiratory function following chronic cervical spinal cord injury.

Potentiation of capsaicin-induced neurogenic inflammation by 5-HT7 receptors in the rat hind paw: Involvement of calcitonin gen-related peptide

A decrease in the activation threshold of primary sensory neurons to transient receptor potential V1 (TRPV1) stimulation by serotonin 5-HT7 receptors has been reported but no confirmation if this might translate into facilitation of neurogenic inflammation has been provided. We analysed the modulation of capsaicin (CAP)-induced neurogenic inflammation in the rat hind paw by the selective 5-HT7 receptor agonist, LP-44, and the involvement of calcitonin gen-related peptide (CGRP) in this effect. Animals received intra-plantar injections (30 μL) of vehicle, CAP (0.05%, 0.1% and 0.2%), LP-44 (7.5 and 15 nmol) and the combination of LP-44 + CAP; then, the time course of the inflammatory responses was measured. The effect of the 5-HT7 receptor antagonist, SB-269970 (3 mg/kg, s.c.), on responses produced by LP-44 alone and combined with CAP was tested. As expected, CAP produced concentration- and time-dependent inflammatory responses in the hind paw. Interestingly, LP-44 by itself also produced inflammation in a concentration- and time-dependent manner, and magnified CAP-induced responses. Systemic pre-treatment with SB-269970 significantly blunted LP-44 (15 nmol)-induced inflammation as well as magnified inflammatory responses produced by the combination of LP-44 (7.5 and 15 nmol) + CAP (0.1%) thus confirming the involvement of 5-HT7 receptors. Finally, the non-peptide CGRP receptor antagonist, BIBN4096 (3 mg/kg, s.c.), strongly inhibited the potentiated inflammatory responses induced by LP-44 (7.5 and 15 nmol) + CAP (0.1%) thus substantiating their neurogenic nature. Thus, sensitization of CAP-sensitive primary sensory neurons by 5-HT7 receptors may result in facilitation of neurogenic inflammation involving CGRP in the rat hind paw.

Safety, tolerability, and risks associated with first- and second-generation antipsychotics: a state-of-the-art clinical review

Since the discovery of chlorpromazine (CPZ) in 1952, first-generation antipsychotics (FGAs) have revolutionized psychiatric care in terms of facilitating discharge from hospital and enabling large numbers of patients with severe mental illness (SMI) to be treated in the community. Second-generation antipsychotics (SGAs) ushered in a progressive shift from the paternalistic management of SMI symptoms to a patient-centered approach, which emphasized targets important to patients - psychosocial functioning, quality of life, and recovery. These drugs are no longer limited to specific Diagnostic and Statistical Manual of Mental Disorders (DSM) categories. Evidence indicates that SGAs show an improved safety and tolerability profile compared with FGAs. The incidence of treatment-emergent extrapyramidal side effects is lower, and there is less impairment of cognitive function and treatment-related negative symptoms. However, treatment with SGAs has been associated with a wide range of untoward effects, among which treatment-emergent weight gain and metabolic abnormalities are of notable concern. The present clinical review aims to summarize the safety and tolerability profile of selected FGAs and SGAs and to link treatment-related adverse effects to the pharmacodynamic profile of each drug. Evidence, predominantly derived from systematic reviews, meta-analyses, and clinical trials of the drugs amisulpride, aripiprazole, asenapine, brexpiprazole, cariprazine, clozapine, iloperidone, lurasidone, olanzapine, paliperidone, quetiapine, risperidone, sertindole, ziprasidone, CPZ, haloperidol, loxapine, and perphenazine, is summarized. In addition, the safety and tolerability profiles of antipsychotics are discussed in the context of the "behavioral toxicity" conceptual framework, which considers the longitudinal course and the clinical and therapeutic consequences of treatment-emergent side effects. In SMI, SGAs with safer metabolic profiles should ideally be prescribed first. However, alongside with safety, efficacy should also be considered on a patient-tailored basis.

Evaluation of the Relationship Between Pharmacokinetics and the Safety of Aripiprazole and Its Cardiovascular Effects in Healthy Volunteers

AIMS

The aim of this study was the evaluation of the possible relationship between pharmacokinetics and the safety of aripiprazole as well as its influence on blood pressure (BP), heart rate (HR), and corrected QT (QTc) interval.

METHODS

The study population comprised 157 healthy volunteers from 6 bioequivalence clinical trials. Subjects were administered a single 10-mg oral dose of each formulation separated by a 28-day washout period. Plasma concentrations were measured using high-performance liquid chromatography coupled to mass spectrometry. Blood pressure was measured at the following times: predose and 0.5, 2, 4, 6, and 8 hours postdose. An electrocardiogram was recorded at predose, 4, and 8 hours postdose.

RESULTS

Area under the curve (AUC), maximum plasma concentration, half-life, and distribution volume corrected for weight were higher in women. Aripiprazole treatment produced a decrease of BP (9.3 mm Hg on systolic and 6.2 mm Hg on diastolic pressure) and an increase in HR (12.1 beats per minute) and QTc interval (9.1 milliseconds). There were sex differences in BP, HR, and QTc interval. Women and subjects with higher AUC and maximum plasma concentration values were more prone to experience adverse drug reactions and gastrointestinal adverse reactions. The AUC was related with systolic BP and diastolic BP decrease and HR increase but there was no relationship between aripiprazole concentrations and QTc increase.

CONCLUSIONS

Aripiprazole decreases BP and increases HR and QTc interval. Pharmacokinetics, pharmacodynamics, and safety of aripiprazole are affected by sex. There is a directly proportional relationship between pharmacokinetic parameters and adverse drug reactions and effect on BP and HR.

Luciferase reporter gene assay on human 5-HT receptor: which response element should be chosen?

Serotonin (5-HT) receptors are valuable molecular targets for antipsychotic drug discovery. Current reported methods for detecting 5-HT receptors, such as cAMP accumulation and calcium influx assay, are often demanding specialized instruments and inconvenient. The luciferase reporter gene assay, based on the responsible-element-regulated expression of luciferase, has been widely applied in the high-throughput functional assay for many targets because of its high sensitivity and reliability. However, 5-HT receptors couple to multiple G-proteins regulate respective downstream signalling pathways and are usually detected using different response elements. Hence, finding a suitable response element to fulfil the detection of different 5-HT receptors and make the results of luciferase reporter gene assays generalizable is very useful for active compounds screening. Here, we conducted three luciferase reporter assays using CRE, NFAT, and SRE response elements attached to 5-HT to detect the activation of different 5-HT receptors in CHO-K1 cells. The potencies and efficacies of the reported ligands (agonists and antagonists) were determined and compared. Our results indicate that CRE-luciferase reporter gene is sensitive and reliable to detect the activities of G protein-coupled 5-HT receptors.

5-HT is a potent relaxant in rat superior mesenteric veins

Serotonin (5-HT, 5-hydroxytryptamine) reduces blood pressure of the conscious rat when administered chronically (1 week). 5-HT does not directly relax isolated arteries, and microsphere experiments in 5-HT-infused rats suggested that 5-HT increased flow to the splanchnic bed. We hypothesized that 5-HT increased splanchnic flow because of direct venous relaxation; our focus was thus on the superior mesenteric vein (SMV) as an important vein in splanchnic circulation. Real-time RT-PCR, immunohistochemistry and Western analyses supported the predominant expression of the 5-HT_2B and 5-HT₇ receptor in the SMV. The SMV was mounted in tissue baths for measurement of isometric contraction. 5-HT caused a concentration-dependent relaxation of the endothelin-1 (ET-1)-contracted vein. The threshold of 5-HT-induced venous relaxation was significantly lower than for 5-HT-induced venous contraction (∼2 vs. 700 nmol/L, respectively). A series of serotonergic agonists established in their use of receptor characterization was tested, and the following rank order of potency found for agonist-induced relaxation (receptor selectivity): 5-CT (5-HT₁/5-HT₇)>5-HT = LP-44 (5-HT₇)>PNU109291 (5-HT_1D) = BW723C86 (5-HT_2B). 8-OH-DPAT (5-HT_1A/7), CP93129 (5-HT_1B), mCPBG (5-HT_3/4), AS19 (5-HT₇) and TCB-2 (5-HT_2A) did not relax the isolated vein. Consistent with these findings, two different 5-HT₇ receptor antagonists SB 269970 and LY215840 but not the 5-HT_2B receptor antagonist LY272015 nor the nitric oxide synthase inhibitor LNNA abolished 5-CT-induced relaxation of the isolated SMV. 5-CT (1 μg kg⁻¹ min⁻¹, sc) also reduced blood pressure over 7 days. These findings suggest that 5-HT directly relaxes the SMV primarily through activation of the 5-HT₇ receptor.

Rational site-directed pharmacotherapy for major depressive disorder

It is now accepted that major depressive disorder (MDD) is not a single pathophysiological entity. It is therefore not surprising that remission rates to a first antidepressant trial are low. In addition, antidepressants may target various neuronal elements for which there are gene polymorphisms, such as the serotonin (5-HT) reuptake transporter, which may modulate response. Acting on a single monoaminergic target, such as inhibiting the 5-HT transporter, may confer efficacy in MDD, but other targets may be used and/or combined in treatment-resistant patients. These include the blockade of norepinephrine transporters, monoamine oxidase, 5-HT(2A), 5-HT(1B) and 5-HT7 receptors, and the activation of 5-HT(1A) and dopamine 2 receptors. While antidepressants may have more than one of these properties, so do atypical antipsychotics. When using the latter medications, however, their regimens should be below those effective in treating psychosis to avoid dopamine 2 antagonism, which could be counter-productive in MDD. In some patients, combining medications from treatment initiation may also provide additional therapeutic benefits.

Clozapine effects on adenylyl cyclase activity and serotonin type 1A receptors in human brain post-mortem

Although the pharmacological profile of the atypical antipsychotic clozapine has been extensively studied in animal models, little information is available on its effects in the human brain. In particular, much interest is focused on the understanding of clozapine activity on serotonin (5-HT) neurotransmission, particularly on 5-HT receptor of type 1A (5-HT(1A)) that seems to play a pivotal role in the control of the 5-HT system. The present work, therefore, aimed at evaluating the effects of clozapine and its major metabolite, norclozapine, on the modulation of adenylyl cyclase (AC) velocity via 5-HT(1A) receptors in human post-mortem brain regions, in particular the prefrontal cortex, hippocampus and raphe nuclei. Concomitantly, the ability of the two compounds to displace the specific binding of the 5-HT(1A) receptor agonist [³H]-8-hydroxy-(2-di-N-propylamino) tetralin ([³H]-8-OH-DPAT) was evaluated in the same brain areas. The results showed that both clozapine and norclozapine, although with a 20-fold lower affinity, displaced [³H]8-OH-DPAT binding in all of the brain regions analysed, suggesting their interaction with 5-HT(1A) receptors. At the same time, clozapine and, to a lesser extent, norclozapine were found to inhibit the forskolin (FK)-stimulated AC system, while decreasing cyclic adenosine monophosphate (cAMP) concentrations in the hippocampus only. The receptor characterisation of the clozapine effect on AC observed in the hippocampus by the use of antagonists showed a mixed profile, involving not only the 5-HT(1A) receptor but also a muscarinic (M) receptor subtype, most likely the M₄ one. These findings, while considering all the limitations due to the use of post-mortem tissues, are strongly suggestive of a region-dependent pharmacological action of clozapine in the human brain that may explain its peculiar clinical effects and open up research towards novel targets for future antipsychotic drugs.

The effects of combining serotonin reuptake inhibition and 5-HT7 receptor blockade on circadian rhythm regulation in rodents

Disruption of circadian rhythms may lead to mood disorders. The present study investigated the potential therapeutic utility of combining a 5-HT7 antagonist with a selective serotonin (5-HT) reuptake inhibitor (SSRI), the standard of care in depression, on circadian rhythm regulation. In tissue explants of the suprachiasmatic nucleus (SCN) from PER2::LUC mice genetically modified to report changes in the expression of a key clock protein, the period length of PER2 bioluminescence was shortened in the presence of AS19, a 5-HT7 partial agonist. This reduction was blocked by SB269970, a selective 5-HT7 antagonist. The SSRI, escitalopram, had no effect alone on period length, but a combination with SB269970, yielded significant increases. Dosed in vivo, escitalopram had little impact on the occurrence of activity onsets in rats given access to running wheels, whether the drug was given acutely or sub-chronically. However, preceding the escitalopram treatment with a single acute dose of SB269970 produced robust phase delays, in keeping with the in vitro explant data. Taken together, these findings suggest that the combination of an SSRI and a 5-HT7 receptor antagonist has a greater impact on circadian rhythms than that observed with either agent alone, and that such a multimodal approach may be of therapeutic value in treating patients with poor clock function.

Interaction of mGlu2/3 agonism with clozapine and lurasidone to restore novel object recognition in subchronic phencyclidine-treated rats

RATIONALE

Subchronic administration to rodents of the N-methyl-D-aspartate non-competitive antagonist, phencyclidine (PCP), impairs novel object recognition (NOR). Atypical antipsychotic drugs (APDs) reverse the effects of subchronic PCP on NOR. The effect of metabotropic glutamate₂/₃ receptor (mGlu₂/₃) agonists upon NOR is unknown.

OBJECTIVES AND METHODS

We tested the hypotheses that the mGlu₂/₃ agonist, LY379268, by itself, or in combination with APDs or pimavanserin, a 5-HT(2A) inverse agonist, would reverse the deficit in NOR induced by subchronic treatment with PCP (2 mg/kg, b.i.d., for 7 days).

RESULTS

The mGlu₂/₃ agonist LY379268 (1 or 3 mg/kg) did not attenuate the PCP-induced NOR deficit. However, together with sub-effective dose of the atypical APDs, clozapine (0.1 mg/kg) or lurasidone (0.03 mg/kg), but not the typical APD, haloperidol (0.1 mg/kg), or pimavanserin (3 mg/kg), LY379268, 1 mg/kg, significantly reversed the PCP-induced NOR deficit. Moreover, the effect of clozapine was blocked by the mGlu₂/₃ antagonist, LY341495 (1 mg/kg).

CONCLUSIONS

These results indicate that mGlu₂/₃ agonism can potentiate the ability of atypical, but not typical APDs, to ameliorate the effect of subchronic PCP on NOR, that mGlu₂/₃ agonism may contribute to the ability of atypical APDs to acutely reverse the effect of subchronic PCP on NOR, but that by itself, mGlu₂/₃ agonism, is ineffective in this model of cognitive impairment in schizophrenia. These results suggest that mGlu₂/₃ receptor agonism should be investigated as an adjunctive treatment of cognitive impairment in schizophrenia rather than as monotherapy, which may be effective for control of psychosis, but not for cognitive impairment.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

5-hydroxtryptamine receptors in systemic hypertension: an arterial focus

BACKGROUND

Serotonin (5-hydroxytryptamine [5-HT]) was named for its isolation from blood serum (sero-) and ability to contract smooth muscle (-tonin). Thus, its relationship with the cardiovascular system began with its discovery.

AIMS

This review will focus on the effects of 5-HT and its receptors in the vasculature, with a focus on their involvement in high blood pressure (hypertension). Two seemingly contradictory bodies of evidence exist that make it difficult to assign any one function to 5-HT in vascular control of blood pressure.

RESULTS

 In vitro, 5-HT is an established vasoconstrictor, the effects of which are amplified in hypertension. By contrast, 5-HT (or its precursor 5-hydroxytryptophan) lowers blood pressure when given chronically in vivo. We will discuss ideas that might help us understand these differences, discuss relatively new pharmacology parameters (e.g. biased, inverse agonism) as they pertain to 5-HT receptors, and pose questions that are vital to answer so as to understand the role played by 5-HT in control of blood pressure, especially as it pertains to vascular function.

CONCLUSIONS

Our goal is to understand if the actions of 5-HT in hypertension are physiologically and clinically relevant. The community understands 5-HT has complex cardiovascular effects, and clinical studies have proven equivocal in terms of the involvement of 5-HT. This article provides a balanced view of evidence/literature that illustrates involvement of 5-HT in hypertension as controversial. It contributes new pharmacological knowledge of 5-HT compounds, and poses timely questions as to how this field can move forward. The take home message is that the cardiovascular effects of 5-HT are markedly complex such that we have not yet answered the question of whether 5-HT is beneficial or detrimental to hypertension.

Actions of the prototypical 5-HT1A receptor agonist 8-OH-DPAT at human alpha2-adrenoceptors: (+)8-OH-DPAT, but not (-)8-OH-DPAT is an alpha2B subtype preferential agonist

8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] is the prototypical agonist at serotonin 5-HT1A receptors; however, activity at other targets contributes to the functional effects of the compound as well. We examined the properties of 8-OH-DPAT and its enantiomers at recombinant human (h)alpha2-adrenoceptor subtypes, using a panel of radioligand binding and functional tests. In competition binding experiments using [3H]-RX821002, about 10-fold selectivity of (+)8-OH-DPAT for the halpha2B subtype (pKi about 7) over halpha2A- and halpha2C-adrenoceptors was observed. In contrast, the S(-) enantiomer of 8-OH-DPAT showed similar weak affinities for the three receptor subtypes (pKis<6). The binding affinity of (+)8-OH-DPAT at the halpha2B- and the halpha2A-adrenoceptor was found sensitive to GTPgammaS, a receptor/G protein-uncoupling agent, indicating agonist properties of the drug. Furthermore, using [35S]GTPgammaS binding determination at CHO-halpha2B or CHO-halpha2A cell membranes and G protein coupled inwardly rectifying potassium (GIRK) current recordings in Xenopus oocytes expressing halpha2B, partial agonist activity of (+)8-OH-DPAT at the respective receptors was confirmed in these two different functional assays. Potency of (+)8-OH-DPAT for stimulation of [35S]GTPgammaS incorporation was lower at the halpha2A- than at the halpha2B-adrenoceptor, consistent with binding affinities. Thus, (+)8-OH-DPAT and, as a consequence, racemic (+/-)8-OH-DPAT are partial agonists at halpha2-adrenoceptors with selectivity for the halpha2B subtype, a property that might contribute to the effects of the compound described in native systems.