Serotonin Receptor Subtypes Research Articles

Heterodimers of serotonin receptor subtypes 2 are driven by 5-HT2C protomers

The serotonin receptor subtypes 2 comprise 5-HT2A, 5-HT2B, and 5-HT2C, which are Gαq-coupled receptors and display distinct pharmacological properties. Although co-expressed in some brain regions and involved in various neurological disorders, their functional interactions have not yet been studied. We report that 5-HT2 receptors can form homo- and heterodimers when expressed alone or co-expressed in transfected cells. Co-immunoprecipitation and bioluminescence resonance energy transfer studies confirmed that 5-HT2C receptors interact with either 5-HT2A or 5-HT2B receptors. Although heterodimerization with 5-HT2C receptors does not alter 5-HT2C Gαq-dependent inositol phosphate signaling, 5-HT2A or 5-HT2B receptor-mediated signaling was totally blunted. This feature can be explained by a dominance of 5-HT2C on 5-HT2A and 5-HT2B receptor binding; in 5-HT2C-containing heterodimers, ligands bind and activate the 5-HT2C protomer exclusively. This dominant effect on the associated protomer was also observed in neurons, supporting the physiological relevance of 5-HT2 receptor heterodimerization in vivo Accordingly, exogenous expression of an inactive form of the 5-HT2C receptor in the locus ceruleus is associated with decreased 5-HT2A-dependent noradrenergic transmission. These data demonstrate that 5-HT2 receptors can form functionally asymmetric heterodimers in vitro and in vivo that must be considered when analyzing the physiological or pathophysiological roles of serotonin in tissues where 5-HT2 receptors are co-expressed.

(165) Understanding the role of serotonin receptor subtype 7 (5-HT7) in comorbid pain and depression using novel compounds derived from marine cyanobacteria

(165) Understanding the role of serotonin receptor subtype 7 (5-HT7) in comorbid pain and depression using novel compounds derived from marine cyanobacteria

Risperidone resets the circadian clock in mice

Risperidone is an atypical antipsychotic that is active at multiple dopamine and serotonin receptor subtypes. Based on its high affinity for serotonin receptors, we predicted that it might reset circadian rhythms in a nocturnal rodent. We report temporally differentiated and differential effects of various doses of risperidone on the voluntary locomotor activity rhythm in the Indian field mice, Mus booduga. Risperidone (0.5 mg/kg) elicited phase delays at phases between CT (circadian time) 12 to CT18 and CT0 to CT3, and phase advances at CT6, CT9 and CT21. However, mice injected at CT6 showed maximum advances (1.299 ± 0.286 h), whereas at CT15 showed maximum delays (−1.514 ± 0.312 h). Increasing the dose beyond 0.5 mg/kg at maximally responsive CTs (CT6 and CT15) resulted in progressively smaller but significant shifts. Thus, 0.5 mg/kg is the optimal dose in this species. The fact that risperidone resets the circadian rhythm in a mammal can be extended to clinical studies and used for optimal adjustment of the circadian rhythm in mental disorders. Conversely, risperidone administration for various treatments must be carefully timed to prevent unwanted phase shifts in patients.

The pathophysiological role of serotonin receptor systems in opioid analgesia and tolerance

Continuous treatment with opioid analgesics, such as morphine, leads to the development of ant nociceptive tolerance in patients. Although a lot of information about antinociceptive, the pathophysiological mechanisms of tolerance to opioid analgesia are not yet completely understood. Proposed mechanisms for opioid analgesic tolerance comprise down-regulation of opioid receptors, reduction of sensitivity G-proteins, altered intracellular signalling pathway including nitric oxide, adenyl cyclase, and protein kinase C. Numerous physiological and behavioural studies have shown an interaction of the serotonergic system and opioid antinociception. The serotonin (5-HT) receptor system is a necessary component of the spinal and midbrain pain modulation circuit mediating opioid analgesia. Various types of serotonin receptors demonstrate different effects on morphine analgesia. Systemic administration of opioids rise 5-HT levels in the spinal cord dorsal horn and contribute to opioid analgesia in the normal state but reduce that in neuropathic pain via spinal 5-HT3 receptors. Spinal and supraspinal serotonergic neurons may also play a pathophysiological role in the development of morphine analgesic tolerance. Serotonin receptor subtypes show different effects on opioid tolerance. This review paper focus on the current understanding of the role of serotonin receptor systems in opioid analgesia and tolerance.

Marked sexual dimorphism in 5-HT1 receptors mediating pronociceptive effects of sumatriptan

Amongst the side effects of triptans, a substantial percentage of patients experience injection site pain and tenderness, the underlying mechanism of which is unknown. We found that the dose range from 10fg to 1000ng (intradermal) of sumatriptan induced a complex dose-dependent mechanical hyperalgesia in male rats, with distinct peaks, at 1pg and 10ng, but no hyperalgesia at 1ng. In contrast, in females, there was 1 broad peak. The highest dose (1000ng) did not produce hyperalgesia in either sex. We evaluated the receptors mediating sumatriptan hyperalgesia (1pg, 1 and 10ng). In males, the injection of an antagonist for the serotonin (5-HT) receptor subtype 1B (5-HT1B), but not 5-HT1D, markedly inhibited sumatriptan (1pg)-induced hyperalgesia, at 10ng a 5-HT1D receptor antagonist completely eliminated hyperalgesia. In contrast, in females, the 5-HT1D, but not 5-HT1B, receptor antagonist completely blocked sumatriptan (1pg and 10ng) hyperalgesia and both 5-HT1B and 5-HT1D receptor antagonists attenuated hyperalgesia (1ng) in females, which is GPR30 estrogen receptor dependent. While selective 5-HT1D or 5-HT1B, agonists produce robust hyperalgesia in female and male rats, respectively, when co-injected the hyperalgesia induced in both sexes was attenuated. Mechanical hyperalgesia induced by sumatriptan (1pg and 10ng) is dependent on the G-protein αi subunit and protein kinase A (PKA), in IB4-positive and negative nociceptors. Understanding the mechanisms responsible for the complex dose dependence for triptan hyperalgesia may provide useful information for the design of anti-migraine drugs with improved therapeutic profiles.

5-HT2C Receptor Knockdown in the Amygdala Inhibits Neuropathic-Pain-Related Plasticity and Behaviors.

Neuroplasticity in the amygdala drives pain-related behaviors. The central nucleus (CeA) serves major amygdala output functions and can generate emotional-affective behaviors and modulate nocifensive responses. The CeA receives excitatory and inhibitory inputs from the basolateral nucleus (BLA) and serotonin receptor subtype 5-HT2CR in the BLA, but not CeA, has been implicated anxiogenic behaviors and anxiety disorders. Here, we tested the hypothesis that 5-HT2CR in the BLA plays a critical role in CeA plasticity and neuropathic pain behaviors in the rat spinal nerve ligation (SNL) model. Local 5-HT2CR knockdown in the BLA with stereotaxic injection of 5-HT2CR shRNA AAV vector decreased vocalizations and anxiety- and depression-like behaviors and increased sensory thresholds of SNL rats, but had no effect in sham controls. Extracellular single-unit recordings of CeA neurons in anesthetized rats showed that 5-HT2CR knockdown blocked the increase in neuronal activity (increased responsiveness, irregular spike firing, and increased burst activity) in SNL rats. At the synaptic level, 5-HT2CR knockdown blocked the increase in excitatory transmission from BLA to CeA recorded in brain slices from SNL rats using whole-cell patch-clamp conditions. Inhibitory transmission was decreased by 5-HT2CR knockdown in control and SNL conditions to a similar degree. The findings can be explained by immunohistochemical data showing increased expression of 5-HT2CR in non-GABAergic BLA cells in SNL rats. The results suggest that increased 5-HT2CR in the BLA contributes to neuropathic-pain-related amygdala plasticity by driving synaptic excitation of CeA neurons. As a rescue strategy, 5-HT2CR knockdown in the BLA inhibits neuropathic-pain-related behaviors.SIGNIFICANCE STATEMENT Neuroplasticity in the amygdala has emerged as an important pain mechanism. This study identifies a novel target and rescue strategy to control abnormally enhanced amygdala activity in an animal model of neuropathic pain. Specifically, an integrative approach of gene transfer, systems and brain slice electrophysiology, behavior, and immunohistochemistry was used to advance the novel concept that serotonin receptor subtype 5-HT2C contributes critically to the imbalance between excitatory and inhibitory drive of amygdala output neurons. Local viral vector-mediated 5-HT2CR knockdown in the amygdala normalizes the imbalance, decreases neuronal activity, and inhibits neuropathic-pain-related behaviors. The study provides valuable insight into serotonin receptor (dys)function in a limbic brain area.

Visualization of 5-HT Receptors Using Radioligand-Binding Autoradiography.

Described in this unit are techniques to visualize the majority of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes in sections of frozen brain tissue using receptor autoradiography. Protocols for brain extraction and sectioning, radioligand exposure, autoradiogram generation, and data quantification are provided, as are the optimal incubation conditions for the autoradiographic visualization of receptors using agonist and antagonist radioligands. © 2016 by John Wiley & Sons, Inc.

Divergent cAMP signaling differentially regulates serotonin-induced spinal motor plasticity

Spinal metabotropic serotonin receptors encode transient experiences into long-lasting changes in motor behavior (i.e. motor plasticity). While interactions between serotonin receptor subtypes are known to regulate plasticity, the significance of molecular divergence in downstream G protein coupled receptor signaling is not well understood. Here we tested the hypothesis that distinct cAMP dependent signaling pathways differentially regulate serotonin-induced phrenic motor facilitation (pMF); a well-studied model of spinal motor plasticity. Specifically, we studied the capacity of cAMP-dependent protein kinase A (PKA) and exchange protein activated by cAMP (EPAC) to regulate 5-HT2A receptor-induced pMF within adult male rats. Although spinal PKA, EPAC and 5-HT2A each elicit pMF when activated alone, concurrent PKA and 5-HT2A activation interact via mutual inhibition thereby blocking pMF expression. Conversely, concurrent EPAC and 5-HT2A activation enhance pMF expression reflecting additive contributions from both mechanisms. Thus, we demonstrate that distinct downstream cAMP signaling pathways enable differential regulation of 5-HT2A-induced pMF. Conditional activation of independent signaling mechanisms may explain experience amendable changes in plasticity expression (i.e. metaplasticity), an emerging concept thought to enable flexible motor control within the adult central nervous system.

Activation of 5-HT2A/2C receptors reduces the excitability of cultured cortical neurons

The abundant forebrain serotonergic projections are believed to modulate the activities of cortical neurons. 5-HT2 receptor among multiple subtypes of serotonin receptors contributes to the modulation of excitability, synaptic transmissions and plasticity. In the present study, whole-cell patch-clamp recording was adopted to examine whether activation of 5-HT2A/2C receptors would have any impact on the excitability of cultured cortical neurons. We found that 2,5-Dimethoxy-4-iodoamphetamine (DOI), a selective 5-HT2A/2C receptor agonist, rapidly and reversibly depressed spontaneous action potentials mimicking the effect of serotonin. The decreased excitability was also observed for current-evoked firing. Additionally DOI increased neuronal input resistance. Hyperpolarization-activated cyclic nucleotide-gated cationic channels (HCN) did not account for the inhibition of spontaneous firing. The synaptic contribution was ruled out in that DOI augmented excitation and attenuated inhibition to actually favor an increase in the excitability. Our findings revealed that activation of 5-HT2A/2C receptors reduces neuronal excitability, which would deepen our understanding of serotonergic modulation of cortical activities.

Redundant Gs-coupled serotonin receptors regulate amyloid-β metabolism in vivo.

BackgroundThe aggregation of amyloid-β (Aβ) into insoluble plaques is a hallmark pathology of Alzheimer’s disease (AD). Previous work has shown increasing serotonin levels with selective serotonin re-uptake inhibitor (SSRI) compounds reduces Aβ in the brain interstitial fluid (ISF) in a mouse model of AD and in the cerebrospinal fluid of humans. We investigated which serotonin receptor (5-HTR) subtypes and downstream effectors were responsible for this reduction.ResultsAgonists of 5-HT4R, 5-HT6R, and 5-HT7R significantly reduced ISF Aβ, but agonists of other receptor subtypes did not. Additionally, inhibition of Protein Kinase A (PKA) blocked the effects of citalopram, an SSRI, on ISF Aβ levels. Serotonin signaling does not appear to change gene expression to reduce Aβ levels in acute timeframes, but likely acts within the cytoplasm to increase α-secretase enzymatic activity. Broad pharmacological inhibition of putative α-secretases increased ISF Aβ and blocked the effects of citalopram.ConclusionsIn total, these studies map the major signaling components linking serotonin receptors to suppression of brain ISF Aβ. These results suggest the reduction in ISF Aβ is mediated by a select group of 5-HTRs and open future avenues for targeted therapy of AD.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-016-0112-5) contains supplementary material, which is available to authorized users.

Preclinical Abuse Potential Assessment of Flibanserin: Effects on Intracranial Self-Stimulation in Female and Male Rats.

Flibanserin is a serotonin receptor subtype 1A agonist and 2A antagonist that has been approved by the Food and Drug Administration for treating female sexual interest and arousal disorder. Little is known about the abuse potential of flibanserin. To examine abuse-related effects of flibanserin in rats using an intracranial self-stimulation (ICSS) procedure previously used to evaluate the abuse potential of other drugs. Adult female and male Sprague-Dawley rats with electrodes implanted in the medial forebrain bundle were trained to press a lever for electrical brain stimulation under a "frequency-rate" ICSS procedure. In this procedure, increasing frequencies of brain stimulation maintain increasing rates of responding. Drugs of abuse typically increase (or "facilitate") ICSS rates and produce leftward and upward shifts in ICSS frequency-rate curves, whereas drugs that lack abuse potential typically do not alter or only decrease ICSS rates. Initial studies determined the potency and time course of effects on ICSS produced by acute flibanserin administration (1.0, 3.2 and 10.0 mg/kg). Subsequent studies determined the effects of flibanserin (3.2-18 mg/kg) before and after a regimen of repeated flibanserin administration (5.6 mg/kg/d for 5 days). Effects of the abused stimulant amphetamine (1.0 mg/kg) were examined as a positive control. Flibanserin effects on ICSS frequency-rate curves in female and male rats were examined and compared with the effects of amphetamine. Baseline ICSS frequency-rate curves were similar in female and male rats. Acute and repeated administrations of flibanserin produced only decreases in ICSS rates, and rate-decreasing effects of the highest flibanserin dose (10 mg/kg) were greater in female than in male rats. In contrast to flibanserin, amphetamine produced an abuse-related increase in ICSS rates that did not differ between female and male rats. These results suggest that flibanserin has low abuse potential. In addition, this study suggests that female rats might be more sensitive than male rats to the rate-decreasing effects of high flibanserin doses.

Editorial: Serotonin and Memory.

Several neurotransmission systems have been involved in function and dysfunctional memory (e.g., Myhrer, 2003; Decker and McGaugh, 2004; Reis et al., 2009; Cassel, 2010; Rodriguez et al., 2012; Komal and Nashmi, 2015), including serotonin (5-hydroxytryptamine, 5-HT), which accounts with multiple neural markers (receptors, transporter; e.g., Hannon and Hoyer, 2008; Saulin et al., 2012; Seyedabadi et al., 2014; McCorvy and Roth, 2015). Indeed, the 5-HT system can be manipulated in multiple ways with pharmacological tools and possesses well characterized downstream signaling in mammals' species (e.g., Marin et al., 2012; McCorvy and Roth, 2015). Emergent evidence indicates that this monoamine system might be a therapeutic target and neural marker regarding function and dysfunctional memory. This issue presents recent advances including the role of 5-HT2A and 5-HT1A receptors in the medial prefrontal cortex during recognition memory (Morici et al.). Hippocampal 5-HT1A receptors and spatial and memory is revised by Glikmann-Johnston et al. Ochoa et al. report that post-training serotonergic depletions of the basolateral amygdala did not disrupt discrimination, retention or reversal learning; suggesting that this serotonergic activity is not required for formation and flexible adjustment of new stimulus-reward associations when the strategy to efficiently solve the task has already been learned. Hernandez-Perez et al. report that serotonin reduction in the supramammillary nucleus alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in spatial memory. Zhang and Stackman review progress in the 5-HT2A receptor distribution, signaling, polymerization, and allosteric modulation; as well as functions in learning and memory, hallucination and spatial cognition, and mental disorders. Pereira et al. show us that 5-HT6 receptor agonism facilitates emotional learning and involves prefrontal cortex and hippocampal signaling. Serotonergic transporter function is reported by Sivamaruthi et al. demonstrating that cronobacter sakazakii infection alters serotonin transporter and improved fear memory retention. Stiedl et al. discuss the role of the serotonin receptor subtypes 5-HT1A and 5-HT7 and their interaction in emotional learning and memory; including the role of these receptors and their interplay at the molecular, neurochemical, and behavioral level. The potential involvement of serotonergic neural markers with respect to memory is reviewed by Meneses. Special mention and thanks to the expert work of the referees, who made professional and careful reviews that improved the papers in this topic.

Novel N-acyl-carbazole derivatives as 5-HT7R antagonists

To discover a novel 5-HT7R antagonist for treatment of depression, we designed N-acyl-carbazole derivatives which were synthesized and biologically evaluated against 5-HT7R. Among total 30 compounds synthesized, four compounds 27–30 showed good binding affinities with Ki values of <100 nM. The compound 28, 1-(9H-carbazol-9-yl)-6-(4-(2-methoxyphenyl)piperazin-1-yl)hexan-1-one, showed good selectivity over other serotonin receptor subtypes and turned out to be a novel selective 5-HT7R antagonist following functional assays. The compound 28 showed moderate activity on hERG channel and good stability in microsomal stability test. The compound 28 exhibited a good pharmacokinetic profile with 67.8% oral bioavailability and good penetration to the brain. The compound 28 was also tested in in vivo depression animal model and showed antidepressant effect in the forced swimming test. Therefore, the selective 5-HT7R antagonist 28 can be considered as a good lead for discovery of novel 5-HT7R antagonists as antidepressants.

Variable Effects of Different Serotonin Receptor Subtype Antagonists on the Development of Transplant Vasculopathy in Murine Aortic Allografts

Variable Effects of Different Serotonin Receptor Subtype Antagonists on the Development of Transplant Vasculopathy in Murine Aortic Allografts

The serotonergic system in the neurobiology of depression: Relevance for novel antidepressants.

The monoamine hypothesis of depression posits that an imbalance in monoaminergic neurotransmission is causally related to the clinical features of depression. Antidepressants influencing serotonin mainly aim at raising serotonin concentrations, thereby increasing serotonergic transmission at the level of the synapse, for example by inhibiting the serotonin transporter. However, the serotonin system is multifaceted. Different serotonin receptor subtypes turn the serotonergic system into a complex neurochemical arrangement that influences diverse neurotransmitters in various brain regions. Classical antidepressants as well as other psychopharmacological agents have various crucial effects on serotonin receptors. We aim at providing a clinically useful characterization of serotonin receptor subtypes in the treatment of depression. Clarifying the mode of action and the interplay of serotonin receptors with pharmacological agents should help antidepressant mechanisms and typical side effects to be better understood. Against this background, we feature the novel antidepressants vortioxetine, vilazodone and milnacipran/levomilnacipran with regard to their serotonin receptor targets such as the 5-HT1A, 5-HT3 and 5-HT7 which may account for their specific effects on certain symptoms of depression (e.g. cognition and anxiety) as well as a characteristic side-effect profile.

Development of Tinnitus at a Low Dose of Sertraline: Clinical Course and Proposed Mechanisms

Introduction. Serotonin is involved in filtering of auditory stimuli. Cochlear input is processed through complex interactions between serotonergic, glutamatergic, and GABAergic neurotransmitter systems. Options for treatment of tinnitus include selective serotonin reuptake inhibitors (SSRIs); however in rare instances this symptom may occur as a side effect of this class of medications. Case Presentation. A 50-year-old woman developed bilateral tinnitus after several weeks of being treated with sertraline 50 mg. She had been on a long-standing daily dose of aspirin 325 mg which had been discontinued shortly before starting sertraline. Medical work-up was negative for her symptom. Shortly after discontinuation of the medication, her tinnitus subsided completely. Discussion. Tinnitus is a rare side effect of sertraline and may be related to particular distribution of serotonin receptor subtypes within the auditory system, and serotonergic agents may reinforce or desensitize the activity of different receptors. Also, there may be a priming effect of salicylate agents on the auditory system, predisposing particular patients to be more sensitive to how auditory stimuli are processed.

Investigation of 5-HT2A gene expression in PBMCs of patients with allergic asthma.

Asthma is an inflammatory airway disorder in which different immune cells in the blood and lungs play a fundamental role. In asthma condition, the airway inflammation accompanied by bronchial smooth muscle spasm cause airway obstruction. A study showed that high concentration of blood serotonin is associated with the intensity and exacerbation of asthma disease. Other studies showed that a subtype of serotonin receptor called 5-Hydroxytriptamine 2A receptor (5- HT2A) can enhance T-cell blastogenesis and production of pro-inflammatory cytokines such as IFNγ. The objective of this study was to assess the level of 5-HT2A in peripheral blood mononuclear cells (PBMCs) of asthmatic patients. PBMCs were extracted from blood of 30 patients with asthma and 30 normal people. After synthesizing cDNAs from total mRNAs, real-time PCR was performed to amplify 5-HT2A and β-actin (as an internal control). The expression ratios were analyzed in patients with asthma in comparison with normal group. The results indicated that gene expression is significantly increased in peripheral blood mononuclear cells (PBMCs) of asthma patients in comparison with normal group (P = 0.003). The results of this study can suggest designing a protocol by using of the 5-HT2A receptor expression in PBMCs as a biomarker of asthma, but this requires further studies on a larger number of patients. In addition, the potential role of this receptor in bronchoconstriction can lead us to use its antagonists as a new treatment in asthma.

Antidepressant-like activity of aroxyalkyl derivatives of 2-methoxyphenylpiperazine and evidence for the involvement of serotonin receptor subtypes in their mechanism of action

Since serotonin (5-HT) is strongly involved in the etiology and pathophysiology of depression, the development of new antidepressants is still based on the serotonergic system. The complexity of serotonergic system provides an opportunity for the development of compounds with multiple and complementary mechanism of action. This study describes serotonin receptor profile, functional characterization, and pharmacological in vivo evaluation of new aroxyalkyl derivatives of 2-methoxyphenylpiperazine. The obtained results allowed for the identification of compound 3, (1-[3-(2,6-dimethylphenoxy)propyl]-4-(2-methoxyphenyl)piperazine hydrochloride), a partial 5-HT1A receptor agonist, and 5-HT2A receptor antagonist, with high affinity toward 5-HT7 receptors, showing antidepressant- and anxiolytic-like properties. Moreover, 5-HT1A receptor activation is crucial for the antidepressant-like activity of compound 3. The rest of the compounds (except compounds 1 and 9) showed antidepressant but not anxiolytic-like properties, which did not result from 5-HT1A receptors activation. Furthermore, the compounds are 5-HT1A and weak 5-HT3 receptors antagonists, and some of them 5-HT2A antagonists. Moreover, none of the studied compounds impaired motor coordination at antidepressant-like doses. Since the studied compounds exhibited activity in behavioral assays and interacted with various receptors, the results of our experiments are very promising and require further studies.

Discovery of Natural Product-Derived 5-HT1A Receptor Binders by Cheminfomatics Modeling of Known Binders, High Throughput Screening and Experimental Validation.

The human 5-hydroxytryptamine receptor subtype 1A (5-HT1A) is highly expressed in the raphe nuclei region and limbic structures; for that reason 5-HT1A has served as a promising target for treating human mood disorders and neurodegenerative diseases. We have developed binary quantitative structure-activity relationship (QSAR) models for 5- HT1A binding using data retrieved from the WOMBAT database and the k-Nearest Neighbor (kNN) machine learning method. A rigorous QSAR modeling and screening workflow had been followed, with extensive internal and external validation processes. The models' classification accuracies to discriminate 5-HT1A binders from the non-binders are as high as 96% for the external validation. These models were employed further to mine two major natural products screening libraries, i.e. TimTec Natural Product Library (NPL) and Natural Derivatives Library (NDL). In the end five screening hits were tested by radioligand binding assays with a success rate of 40%, and two Library compounds were confirmed to be binders at the μM concentration against the human 5-HT1A receptor. The combined application of rigorous QSAR modeling and model-based virtual screening presents a powerful means for profiling natural products compounds with important biomedical activities.

5-HT7 receptor modulates GABAergic transmission in the rat dorsal raphe nucleus and controls cortical release of serotonin.

The 5-HT7 receptor is one of the several serotonin (5-HT) receptor subtypes that are expressed in the dorsal raphe nucleus (DRN). Some earlier findings suggested that 5-HT7 receptors in the DRN were localized on GABAergic interneurons modulating the activity of 5-HT projection neurons. The aim of the present study was to find out how the 5-HT7 receptor modulates the GABAergic synaptic input to putative 5-HT DRN neurons, and whether blockade of the 5-HT7 receptor would affect the release of 5-HT in the target structure. Male Wistar rats with microdialysis probes implanted in the prefrontal cortex (PFC) received injections of the 5-HT7 receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine hydrochloride (SB 269970), which induced an increase in the levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA) in the PFC. In another set of experiments whole-cell recordings from presumed projection neurons were carried out using DRN slices. SB 269970 application resulted in depolarization and in an increase in the firing frequency of the cells. In order to activate 5-HT7 receptors, 5-carboxamidotryptamine (5-CT) was applied in the presence of N-[2-[4-(2-methoxyphenyl)-1piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Hyperpolarization of cells and a decrease in the firing frequency were observed after activation of the 5-HT7 receptor. Blockade of 5-HT7 receptors caused a decrease in the mean frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), while its activation induced an increase. The mechanism of these effects appears to involve tonically-active 5-HT7 receptors modulating firing and/or GABA release from inhibitory interneurons which regulate the activity of DRN serotonergic projection neurons.