Role of cholinergic and GABAergic systems in the feedback inhibition of dorsal raphe 5-HT neurons

Central cholinergic transmission affects the compulsive-like behavior of mice in marble-burying test

The presence of the cholinergic system in the brain areas implicated in the precipitation of obsessive-compulsive behavior (OCB) has been reported but the exact role of the central cholinergic system therein is still unexplored. Therefore, the current study assessed the effect of cholinergic analogs on central administration on the marble-burying behavior (MBB) of mice, a behavior correlated with OCB. The result reveals that the enhancement of central cholinergic transmission in mice achieved by intracerebroventricular (i.c.v.) injection of acetylcholine (0.01 µg) (Subeffective: 0.1 and 0.5 µg), cholinesterase inhibitor, neostigmine (0.1, 0.3, 0.5 µg/mouse) and neuronal nicotinic acetylcholine receptor agonist, nicotine (0.1, 2 µg/mouse) significantly attenuated the number of marbles buried by mice in MBB test without affecting basal locomotor activity. Similarly, central injection of mAChR antagonist, atropine (0.1, 0.5, 5 µg/mouse), nAChR antagonist, mecamylamine (0.1, 0.5, 3 µg/mouse) per se also reduced the MBB in mice, indicative of anti-OCB like effect of all the tested cholinergic mAChR or nAChR agonist and antagonist. Surprisingly, i.c.v. injection of acetylcholine (0.01 µg), and neostigmine (0.1 µg) failed to elicit an anti-OCB-like effect in mice pre-treated (i.c.v.) with atropine (0.1 µg), or mecamylamine (0.1 µg). Thus, the findings of the present investigationdelineate the role of central cholinergic transmission in the compulsive-like behavior of mice probably via mAChR or nAChR stimulation.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Perinatal Nicotine Reduces Chemosensitivity of Medullary 5-HT Neurons after Maturation in Culture

Perinatal exposure to nicotine produces ventilatory and chemoreflex deficits in neonatal mammals. Medullary 5-HT neurons are putative central chemoreceptors that innervate respiratory nuclei and promote ventilation, receive cholinergic input and express nicotinic acetylcholine receptors (nAChRs). Perforated patch clamp recordings were made from cultured 5-HT neurons dissociated from the medullary raphé of 0-3 day old mice expressing enhanced yellow fluorescent protein driven by the enhancer region for PET1 (ePet-EYFP). The effect of exposure to low (6 mg kg^-1day^-1) or high (60 mg kg^-1day^-1) doses of nicotine in utero (prenatal), in culture (postnatal), or both and the effect of acute nicotine exposure (10 μM), were examined on baseline firing rate (FR at 5% CO₂, pH = 7.4) and the change in FR with acidosis (9% CO₂, pH 7.2) in young (12-21 days in vitro, DIV) and older (≥22 DIV) acidosis stimulated 5-HT neurons. Nicotine exposed neurons exhibited ∼67% of the response to acidosis recorded in neurons given vehicle (p = 0.005), with older neurons exposed to high dose prenatal and postnatal nicotine, exhibiting only 28% of that recorded in the vehicle neurons (p < 0.01). In neurons exposed to low or high dose prenatal and postnatal nicotine, acute nicotine exposure led to a smaller increase in FR (∼+51% vs +168%, p = 0.026) and response to acidosis (+6% vs +67%, p = 0.014) compared to vehicle. These data show that exposure to nicotine during development reduces chemosensitivity of 5-HT neurons as they mature, an effect that may be related to the abnormal chemoreflexes reported in rodents exposed to nicotine in utero, and may cause a greater risk for sudden infant death syndrome (SIDS).

Asenapine modulates mood-related behaviors and 5-HT1A/7 receptors-mediated neurotransmission

AIM

Asenapine is a new atypical antipsychotic prescribed for the treatment of psychosis/bipolar disorders that presents higher affinity for serotonergic than dopaminergic receptors. The objective of this study was to investigate its antidepressant-like and antimanic-like properties on relevant animal models of depression and mania and to assess the acute and chronic effect of Asenapine on dorsal raphe nucleus (DRN) 5-HT cell firing activity.

METHODS

We assessed the effects of Asenapine using in vivo electrophysiological and behavioral assays in rats.

RESULTS

Behavioral experiments showed that Asenapine had no significant effect on immobility time in the forced swim test (FST) in control rats. In the ACTH-treated rats, a model of antidepressant-resistance, Asenapine failed to alter immobility time in the FST. In contrast in the sleep deprivation (SD) model of mania, acute administration of Asenapine significantly decreased the hyperlocomotion of SD rats. In the DRN, acute administration of Asenapine reduced the suppressant effect of the selective 5-HT₇ receptor agonist LP-44 and of the prototypical 5-HT_1A receptor agonist 8-OH-DPAT on 5-HT neuronal firing activity. In addition, chronic treatment with Asenapine enhanced DRN 5-HT neuronal firing and this effect was associated with an alteration of the 5-HT₇ receptor responsiveness.

CONCLUSION

These results confirm that Asenapine displays robust antimanic property and effective in vivo antagonistic activity at 5-HT_1A/7 receptors.

Modulation of firing and synaptic transmission of serotonergic neurons by intrinsic G protein-coupled receptors and ion channels

Serotonergic neurons project to virtually all regions of the central nervous system and are consequently involved in many critical physiological functions such as mood, sexual behavior, feeding, sleep/wake cycle, memory, cognition, blood pressure regulation, breathing, and reproductive success. Therefore, serotonin release and serotonergic neuronal activity have to be precisely controlled and modulated by interacting brain circuits to adapt to specific emotional and environmental states. We will review the current knowledge about G protein-coupled receptors and ion channels involved in the regulation of serotonergic system, how their regulation is modulating the intrinsic activity of serotonergic neurons and its transmitter release and will discuss the latest methods for controlling the modulation of serotonin release and intracellular signaling in serotonergic neurons in vitro and in vivo.

Stress-hyperresponsive WKY rats demonstrate depressed dorsal raphe neuronal excitability and dysregulated CRF-mediated responses

Major depression is a debilitating psychiatric disease that may be precipitated by a dysregulation of stress neurocircuitry caused by chronic or severe stress exposure. Moreover, hyperresponsivity to stressors correlates with depressed mood and may contribute to the etiology of major depression. The serotonergic dorsal raphe nucleus (DRN) is an important site in the neurocircuitry underlying behavioral responses to stressors, and is tightly regulated, in part, by a combination of intrinsic cell properties, autoinhibition, and GABAergic synaptic transmission. The stress-related neurotransmitter corticotropin-releasing factor (CRF) modulates DRN neuronal excitability and subsequent 5-HT release in the forebrain. Wistar Kyoto (WKY) rats exhibit exaggerated behavioral responses to stressors, that is, stress hyperresponsivity, and are considered an animal model of depression. To better understand the neurobiological basis of the stress hyperresponsivity, we used a combination of mRNA analysis and whole-cell electrophysiological techniques to measure differences in intrinsic activity and receptor response, in 5-HT- and non-5-HT-containing neurons of the DRN in WKY rats compared with Sprague-Dawley controls. In the WKY rat, there was a decrease in the neuronal excitability of 5-HT neurons coupled with decreased TPH2 production. Additionally, we found that CRF did not increase GABAergic activity in 5-HT neurons as is normally seen in 5-HT neurons of Sprague-Dawley controls. The CRF modulation of 5-HT DRN neurotransmission at the single-cell level is selectively disrupted in the WKY animal model of depression and may be one of the cellular correlates underlying depression.

Estrogen receptor beta regulates the expression of tryptophan-hydroxylase 2 mRNA within serotonergic neurons of the rat dorsal raphe nuclei

Dysfunctions of the brain 5-HT system are often associated with affective disorders, such as depression. The raphe nuclei target the limbic system and most forebrain areas and constitute the main source of 5-HT in the brain. All 5-HT neurons express tryptophan hydroxylase-2 (TPH2), the brain specific, rate-limiting enzyme for 5-HT synthesis. Estrogen receptor (ER) beta agonists have been shown to attenuate anxiety- and despair-like behaviors in rodent models. Therefore, we tested the hypothesis that ER beta may contribute to the regulation of gene expression in 5-HT neurons of the dorsal raphe nuclei (DRN) by examining the effects of systemic and local application of the selective ER beta agonist diarylpropionitrile (DPN) on tph2 mRNA expression. Ovariectomized (OVX) female rats were injected s.c. with DPN or vehicle once daily for 8 days. In situ hybridization revealed that systemic DPN-treatment elevated basal tph2 mRNA expression in the caudal and mid-dorsal DRN. Behavioral testing of all animals in the open field (OF) and on the elevated plus maze (EPM) on days 6 and 7 of treatment confirmed the anxiolytic nature of ER beta activation. Another cohort of female OVX rats was stereotaxically implanted bilaterally with hormone-containing wax pellets flanking the DRN. Pellets contained 17-beta-estradiol (E), DPN, or no hormone. Both DPN and E significantly enhanced tph2 mRNA expression in the mid-dorsal DRN. DPN also increased tph2 mRNA in the caudal DRN. DPN- and E-treated rats displayed a more active stress-coping behavior in the forced-swim test (FST). No behavioral differences were found in the OF or on the EPM. These data indicate that ER beta acts at the level of the rat DRN to modulate tph2 mRNA expression and thereby influence 5-HT synthesis in DRN subregions. Our results also suggest that local activation of ER beta neurons in the DRN may be sufficient to decrease despair-like behavior, but not anxiolytic behaviors.

Unilateral lesion of the nigrostriatal pathway induces an increase of neuronal firing of the midbrain raphe nuclei 5-HT neurons and a decrease of their response to 5-HT(1A) receptor stimulation in the rat

Several studies have shown that the 5-hydroxytryptamine (serotonin, 5-HT) system is severely affected after degeneration of nigrostriatal dopaminergic neurons. In the present study, we examined the changes in the firing rate and firing pattern of the dorsal and median raphe nuclei (DRN and MRN) 5-HT neurons, and the effect of the selective 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and antagonist (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridylcyclohexane carboxamide maleate salt (WAY-100635) on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta by using extracellular recording. The unilateral lesion of the nigrostriatal pathway significantly increased the mean firing rate of DRN and MRN 5-HT neurons compared with normal rats, and the firing pattern of these neurons also changed significantly towards a more bursty one. The lower dose of 8-OH-DPAT, 4 microg/kg (cumulative doses, i.v.), completely inhibited the firing activity of all DRN and MRN 5-HT neurons examined in normal and sham rats. In contrast to normal and sham rats, only the higher doses of 8-OH-DPAT, 128 and 64 microg/kg, completely inhibited the firing rate of DRN and MRN 5-HT neurons in 6-OHDA-lesioned rats, respectively. Furthermore, the local application of 8-OH-DPAT, 1.5 microg, in the DRN completely inhibited the firing rate of 5-HT neurons in normal and sham rats, while having no effect on firing rate in the lesioned rats. Altogether, these results indicate that lesion of the nigrostriatal pathway leads to hyperactivity of DRN and MRN 5-HT neurons, suggesting the implication of the DRN and MRN in the pathophysiology of Parkinson's disease, and the decreased response of these 5-HT neurons to 5-HT(1A) receptor stimulation, reflecting 5-HT(1A) receptor dysfunction in 6-OHDA-lesioned rats.

Serotonin-1A receptor imaging in recurrent depression: replication and literature review

INTRODUCTION

Serotonin-1A receptor (5-HT1AR) function appears to be decreased in major depressive disorder (MDD) based on physiological responses to 5-HT1AR agonists in vivo and to 5-HT1AR binding in brain tissues postmortem or antemortem. We have previously assessed 5-HT1AR binding potential (BP) in depression using positron emission tomography (PET) and [carbonyl-(11)C]WAY-100635, and we have demonstrated reduced 5-HT1AR BP in the mesiotemporal cortex (MTC) and raphe in depressives with primary recurrent familial mood disorders (n=12) versus controls (n=8) [Drevets WC, Frank E, Price JC, Kupfer DJ, Holt D, Greer PJ, Huang Y, Gautier C, Mathis C. PET imaging of serotonin 1A receptor binding in depression. Biol Psychiatry 1999;46(10):1375-87]. These findings were replicated by some, but not other, studies performed in depressed samples that were more generally selected using criteria for MDD. In the current study, we attempted to replicate our previous findings in an independent sample of subjects selected according to the criteria for primary recurrent depression applied in our prior study.

METHODS

Using PET and [carbonyl-(11)C]WAY-100635, 5-HT1AR BP was assessed in 16 depressed subjects and 8 healthy controls.

RESULTS

Mean 5-HT1AR BP was reduced by 26% in the MTC (P<.005) and by 43% in the raphe (P<.001) in depressives versus controls.

CONCLUSIONS

These data replicate our original findings, which showed that BP was reduced by 27% in the MTC (P<.025) and by 42% in the raphe (P<.02) in depression. The magnitudes of these reductions in 5-HT1AR binding were similar to those found postmortem in 5-HT1AR mRNA concentrations in the hippocampus in MDD [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73] and in 5-HT1AR-binding capacity in the raphe in depressed suicide victims [Arango V, Underwood MD, Boldrini M, Tamir H, Kassir SA, Hsiung S, Chen JJ, Mann JJ. Serotonin 1A receptors, serotonin transporter binding and serotonin transporter mRNA expression in the brainstem of depressed suicide victims. Neuropsychopharmacology 2001;25(6):892-903]. There exists disagreement within the literature, however, regarding the presence and direction of 5-HT1AR-binding abnormalities in depression, which may be explained in some cases by differences in anatomical location (e.g., [Stockmeier CA, Shapiro LA, Dilley GE, Kolli TN, Friedman L, Rajkowska G. Increase in serotonin-1A autoreceptors in the midbrain of suicide victims with major depression--postmortem evidence for decreased serotonin activity. J Neurosci 1998;18(18):7394-401]) and in other cases by pathophysiological heterogeneity within MDD (e.g., some depressives hypersecrete cortisol, which would be expected to down-regulate 5-HT1AR expression [López JF, Chalmers DT, Little KY, Watson SJ. Regulation of serotonin 1A, glucocorticoid, and mineralocorticoid receptor in rat and human hippocampus: implications for neurobiology of depression. Biol Psychiatry 1998;43:547-73]). Antidepressant drug treatment does not alter these abnormalities in 5-HT1AR binding [Sargent PA, Kjaer KH, Bench CJ, Rabiner EA, Messa C, Meyer J, Gunn RN, Grasby PM, Cowen PJ. Brain serotonin1A receptor binding measured by positron emission tomography with [11C]WAY-100635: effects of depression and antidepressant treatment. Arch Gen Psychiatry 2000;57(2):174-80; Moses-Kolko EL, Price JC, Thase ME, Meltzer CC, Kupfer DJ, Mathis CA, Bogers WD, Berman SR, Houck PR, Schneider TN, Drevets WC. Measurement of 5-HT1A receptor binding in depressed adults before and after antidepressant drug treatment using positron emission tomography and [11C]WAY-100635. Synapse 2007;61(7):523-30] but may compensate for blunted 5-HT1AR function by increasing post-synaptic 5-HT1AR transmission [Chaput Y, de Montigny C, Blier P. Presynaptic and postsynaptic modifications of the serotonin system by long-term administration of antidepressant treatments. An in vivo electrophysiologic study in the rat. Neuropsychopharmacology 1991;5(4):219-29].

A positron emission tomography study of 5-hydroxytryptamine-1A receptors in Alzheimer disease

OBJECTIVE

The important role of serotonin-1A (5-hydroxytryptamine-1A [5-HT(1A)]) receptors in cognition, behavior, and drug response is increasingly being recognized. Postmortem studies suggest decreased 5-HT(1A) receptors in patients with Alzheimer disease (AD), but this has not been confirmed in vivo. Our primary objective was to assess the extent of 5-HT(1A) receptor losses in mild to moderate AD.

METHODS

The authors examined 5-HT(1A) receptors in 10 patients with mild to moderate AD and 10 healthy volunteers with the same sex and similar age using positron emission tomography imaging with the selective 5-HT(1A) receptor radioligand, [(11)C]WAY-100635. Regions of interest (ROIs) were manually drawn on coregistered magnetic resonance images for the frontal, lateral temporal, medial temporal (MTC), parietal, and cerebellar cortices. Using the simplified reference tissue model, 5-HT(1A) binding potentials (BPs) were calculated relative to the cerebellum.

RESULTS

After adjusting for partial volume effects, ROI analysis showed a significant group effect (AD versus comparison group) on BP. Analysis of between-subjects factors showed significantly decreased 5-HT(1A) BP in the right MTC, but not in the other ROIs.

CONCLUSION

Given the strategic role of these receptors, loss of right medial temporal 5-HT(1A) receptors might play an important role in AD symptomatology.

Atropine, a muscarinic cholinergic receptor antagonist increases serotonin, but not dopamine levels in discrete brain regions of mice

We investigated the effects of atropine, a muscarinic acetylcholine (ACh) receptor antagonist, on the level of serotonin in discrete brain regions, the nucleus raphe dorsalis (NRD), nucleus caudatus putamen (NCP), cerebral cortex and the cerebellum. Biogenic amines were assayed employing HPLC electrochemistry in these regions 30 min following different doses of atropine (5, 10, 25mg/kg; i.p.), and at various time points (15, 30, 60, 120 min) after 25mg/kg of the drug. The cholinergic receptor antagonist caused a dose-dependent alteration in the level of serotonin in NRD, but the increase was not dose-dependent for other regions studied. The metabolite of serotonin, 5-hydroxyindoleacetic acid was unaffected. Atropine did not affect the levels of dopamine or its metabolites dihydroxyphenyl acetic acid and homovanillic acid. The present study suggests significant effect of this antimuscarinic agent on the synthesis of serotonin in the central serotoninergic pathways, which may have clinical relevance.

Serotonin transporter binding in bipolar disorder assessed using [11C]DASB and positron emission tomography

BACKGROUND

Evidence from neuroimaging post-mortem, and genetic studies suggests that bipolar disorder (BD) is associated with abnormalities of the serotonin-transporter (5-HTT) system. Because of various limitations of these studies, however, it has remained unclear whether 5-HTT binding is abnormal in unmedicated BD-subjects. This study used PET and [(11)C]DASB, a radioligand that afforded higher sensitivity and specificity for the 5-HTT than previously available radioligands, to compare 5-HTT binding between BD and control subjects.

METHODS

The 5-HTT binding-potential (BP) was assessed in 18 currently-depressed, unmedicated BD-subjects and 37 healthy controls using PET and [(11)C]DASB.

RESULTS

In BD, the mean 5-HTT BP was increased in thalamus, dorsal cingulate cortex (DCC), medial prefrontal cortex and insula and decreased in the brainstem at the level of the pontine raphe-nuclei. Anxiety ratings correlated positively with 5-HTT BP in insula and DCC, and BP in these regions was higher in subjects manifesting pathological obsessions and compulsions relative to BD-subjects lacking such symptoms. Subjects with a history of suicide attempts showed reduced 5-HTT binding in the midbrain and increased binding in anterior cingulate cortex versus controls and to BD-subjects without attempts.

CONCLUSIONS

This is the first study to report abnormalities in 5-HTT binding in unmedicated BD-subjects. The direction of abnormality in the brainstem was opposite to that found in the cortex, thalamus, and striatum. Elevated 5-HTT binding in the cortex may be related to anxiety symptoms and syndromes associated with BD.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

Modulation of serotonergic neurotransmission by short- and long-term treatments with sigma ligands

1. Sigma receptors were first described in 1976 as opiate receptors but were later determined to be a distinct class of receptors with two subtypes, sigma(1) and sigma(2). Although the endogenous ligand is yet to be elucidated, the sigma(1) receptor has recently been cloned. 2. Behavioural models used to test potential antidepressants have shown sigma ligands to produce antidepressant effects but their mechanism of action is unknown. 3. The goal of the present study was to assess the effects of various sigma(1) ligands on the firing activity of serotonin (5-HT) neurons of the dorsal raphe nucleus (DRN) using extracellular in vivo recordings in anaesthetized rats. 4. The sigma(1) ligands (+)-pentazocine and 4-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4-IBP) (2 mg kg(-1) day(-1)) increased markedly 5-HT firing activity after 2 days of treatment and maintained the same increased firing rate after long-term (21 days) treatments. Furthermore, the increased firing rate produced by 2 and 21 day treatments with (+)-pentazocine was prevented by the co-administration of N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)-thylamine (NE-100) (10 mg kg(-1) day(-1)) a selective sigma(1) antagonist, confirming the sigma(1) receptor's modulation of these effects. In contrast, the sigma(1) ligands (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-1-ethyl-but-3-en-1-ylamine hydrochloride (JO-1784) and 2-(4-morpholinoethyl 1-phenyl-cyclohexane-1-carboxylate hydrochloride (PRE-084) had no effect. 5. Following a 21-day treatment with (+)-pentazocine there was a marked reduction in the number of neurons found per track. This decrease was not seen after chronic treatment with 4-IBP and may represent a depolarization block. 6. These results suggest a modulation of serotonergic neurotransmission by some sigma receptors and provide a potential mechanism for the 'antidepressant effects' reported and provide evidence toward sigma(1) ligands as potential antidepressants with a rapid onset of action.

Different tonic regulation of neuronal activity in the rat dorsal raphe and medial prefrontal cortex via 5-HT(1A) receptors

It has been established that 5-HT(1A) receptors are expressed both presynaptically as autoreceptors by 5-HT containing neurones, and postsynaptically by a variety of other neurones. Activation of either somatodendritic 5-HT(1A) autoreceptors or postsynaptic 5-HT(1A) receptors induces hyperpolarisation and inhibition of action potential discharge of the neurones, but it is unclear whether 5-HT(1A) receptors are under a general tonic influence by 5-HT. In the present study, using single unit recordings from both anesthetized and non-anesthetized rats, we show that the activity of neurones in the medial prefrontal cortex is not altered by systemic administration of the selective 5-HT(1A) receptor antagonist, WAY 100635. In contrast, WAY 100635 increased the firing rate of 5-HT neurones in the dorsal raphe nucleus. Our findings indicate a tonic activation of presynaptic somatodendritic but not postsynaptic cortical 5-HT(1A) receptors.