The serotonin (5-hydroxytryptamine, 5-HT) 5-HT1 G-protein coupled receptor subtypes (5-HT1A/1B/1D/1E/1F) share a high sequence homology, confounding development of subtype-specific ligands. This study used a 5-HT1 structure-based ligand design approach to develop subtype-selective ligands using a 5-substituted-2-aminotetralin (5-SAT) chemotype, leveraging results from pharmacological, molecular modeling, and mutagenesis studies to delineate molecular determinants for 5-SAT binding and function at 5-HT1 subtypes. 5-SATs demonstrated high affinity (Ki ≤ 25 nM) and at least 50-fold stereoselective preference ([2S] > [2R]) at 5-HT1A, 5-HT1B, and 5-HT1D receptors but essentially nil affinity (Ki > 1 μM) at 5-HT1F receptors. The 5-SATs tested were agonists with varying degrees of potency and efficacy, depending on chemotype substitution and 5-HT1 receptor subtype. Models were built from the 5-HT1A (cryo-EM), 5-HT1B (crystal), and 5-HT1D (cryo-EM) structures, and 5-SATs underwent docking studies with up to 1 μs molecular dynamics simulations. 5-SAT interactions observed at positions 3.33, 5.38, 5.42, 5.43, and 7.39 of 5-HT1 subtypes were confirmed with point mutation experiments. Additional 5-SATs were designed and synthesized to exploit experimental and computational results, yielding a new full efficacy 5-HT1A agonist with 100-fold selectivity over 5-HT1B/1D receptors. The results presented lay the foundation for the development of additional 5-HT1 subtype selective ligands for drug discovery purposes.

Brain serotonin (5-HT) neurotransmission plays an important role in male sexual behavior and it is well established that activating 5-HT1 A receptors in rats facilitate ejaculatory behavior. However, the relative contribution of 5-HT1 A somatodendritic autoreceptors and heteroreceptors in this pro-sexual behavior is unclear. Moreover, it is unclear whether the contribution of somatodendritic 5-HT1 A autoreceptors and postsynaptic 5-HT1 A heteroreceptors alter when extracellular 5-HT levels are chronically increased. Serotonin transporter knockout (SERT-/-) rats exhibit enhanced extracellular 5-HT levels and desensitized 5-HT1 A receptors. These rats model neurochemical changes underlying chronic SSRI-induced sexual dysfunction. We want to determine the role of presynaptic versus postsynaptic 5-HT1 A receptors in the pro-sexual effects of 5-HT1 A receptor agonists in SERT+/+ and in SERT-/- rats. Therefore, acute effects of the biased 5-HT1 A receptor agonists F-13714, a preferential 5-HT1 A autoreceptor agonist, or F-15599, a preferential 5-HT1 A heteroreceptor agonist, and S15535 a mixed 5-HT1 A autoreceptor agonist/heteroreceptor antagonist, on male sexual behavior were assessed. A clear and stable genotype effect was found after training where SERT+/+ performed sexual behavior at a higher level than SERT-/- rats. Both F-15599 and F-13714 induced pro-sexual activity in SERT+/+ and SERT-/- animals. Compared to SERT+/+, the F13714-dose-response curve in SERT-/- rats was shifted to the right. SERT+/+ and SERT-/- rats responded similar to F15599. Within both SERT+/+ and SERT-/- rats the potency of F-13714 was much stronger compared to F-15599. S15535 had no effect on sexual behavior in either genotype. In SERT+/+ and SERT-/- rats that were selected on comparable low sexual activity (SERT+/+ 3 or less ejaculations and SERT-/- 5 or less ejaculations in 10 weeks) S15535 also did not influence sexual behavior. The two biased compounds with differential effects on 5-HT1 A auto- and hetero-receptors, exerted pro-sexual activity in both SERT+/+ and SERT-/- rats. Applying these specific pharmacological tools has not solved whether pre- or post-synaptic 5-HT1 A receptors are involved in pro-sexual activity. Moreover, the inactivity of S15535 in male sexual behavior in either genotype was unexpected. The question is whether the in vivo pharmacological profile of the different 5-HT1 A receptor ligands used, is sufficient to differentiate pre- and/or post-synaptic 5-HT1 A receptor contributions in male rat sexual behavior.

Serotonin (5-hydroxytryptamine, 5-HT) is known for its key role in modulating diverse physiological processes and behaviors by binding various 5-HT receptors. However, a lack of pharmacological knowledge impedes studies on invertebrate 5-HT receptors. Moreover, pharmacological information is urgently needed in order to establish a reliable classification system for invertebrate 5-HT receptors. In this study we report on the molecular cloning and pharmacological characterization of a 5-HT1 receptor from the red flour beetle, Tribolium castaneum (Trica5-HT1). The Trica5-HT1 receptor encoding cDNA shows considerable sequence similarity with members of the 5-HT1 receptor class. Real time PCR showed high expression in the brain (without optic lobes) and the optic lobes, consistent with the role of 5-HT as neurotransmitter. Activation of Trica5-HT1 in mammalian cells decreased NKH-477-stimulated cyclic AMP levels in a dose-dependent manner, but did not influence intracellular Ca(2+) signaling. We studied the pharmacological profile of the 5-HT1 receptor and demonstrated that α-methylserotonin, 5-methoxytryptamine and 5-carboxamidotryptamine acted as agonists. Prazosin, methiothepin and methysergide were the most potent antagonists and showed competitive inhibition in presence of 5-HT. This study offers important information on a 5-HT1 receptor from T. castaneum facilitating functional research of 5-HT receptors in insects and other invertebrates. The pharmacological profiles may contribute to establish a reliable classification scheme for invertebrate 5-HT receptors.

Serotonin (5-HT) is implicated in several aspects of brain development, yet the ontogenetic expression patterns of 5-HT receptors responsible for transducing specific effects have largely not been characterized. Fifteen different 5-HT receptor genes have been cloned; therefore any spatial and/or temporal combination of their developmental expression could mediate a wide array of 5-HT effects. We undertook a detailed analysis of expression mapping of the Gi/o-coupled 5-HT1 (5-HT1A, 1B, 1D and 1F) receptor subtypes in the fetal and early postnatal mouse forebrain. Using receptor subtype-specific riboprobes and in situ hybridization, we observed that all 5-HT1 receptor subtypes are expressed as early as embryonic day (E) 14.5 in the forebrain, typically in gradients within specific structures. Among 5-HT1 receptors, the 5-HT1A receptor transcript is expressed densely in E14.5-16.5 thalamus, in hippocampus, and in a medial to lateral gradient in cortex, whereas the 5-HT1B receptor mRNA is expressed in more lateral parts of the dorsal thalamus and in the striatum at these ages. The 5-HT1D receptor transcript, which also is expressed heavily in E14.5-E16.5 thalamus, appears to be down-regulated at birth. The 5-HT1F receptor transcript is present in proliferative regions such as the cortical ventricular zone, ganglionic eminences, and medial aspects of the thalamus at E14.5-16.5, and otherwise presents similarities to the expression patterns of 5-HT1B and 1D receptor transcripts. Overall, the 5-HT1 subfamily of Gi/o-coupled 5-HT receptors displays specific and dynamic expression patterns during embryonic forebrain development. Moreover, all members of the 5-HT1 receptor class are strongly and transiently expressed in the embryonic dorsal thalamus, which suggests a potential role for serotonin in early thalamic development.

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-Hydroxytryptamine (5-HT) receptors 1B, 1D and 1F have key roles in migraine pharmacotherapy. Selective agonists targeting these receptors, such as triptans and ditans, are effective in aborting acute migraine attacks and inhibit the in vivo release of calcitonin gene-related peptide (CGRP) in human and animal models. The study aimed to examine the localization, genetic expression and functional aspects of 5- HT1B/1D/1F receptors in the trigeminal system in order to further understand the molecular sites of action of triptans (5-HT1B/1D) and ditans (5-HT1F).

Dihydroergotamine (DHE) is an antimigraine drug that produces cranial vasoconstriction and inhibits trigeminal CGRP release; furthermore, it inhibits the vasodepressor sensory CGRPergic outflow, but the receptors involved remain unknown. Prejunctional activation of α2A/2C-adrenergic, serotonin 5-HT1B/1F, or dopamine D2-like receptors results in inhibition of this CGRPergic outflow. Since DHE displays affinity for these receptors, this study investigated the pharmacological profile of DHE-induced inhibition of the vasodepressor sensory CGRPergic outflow.

The aim of this review was to identify the mechanisms by which serotonin receptors involved at the central level are able to modulate the nociceptive response. Pain is a defense mechanism of the body that entails physiological, anatomical, neurochemical, and psychological changes, and is defined as an unpleasant sensory and emotional experience with potential risk of tissue damage, comprising the leading cause of appointments with Physicians worldwide. Treatment for this symptom has generated several neuropharmacological lines of research, due to the different types of pain and the various drugs employed to treat this condition. Serotonin [5- HydroxyTryptamine (5-HT)] is a neurotransmitter with seven families (5-HT1-5-HT7) and approximately 15 receptor subtypes. Serotonin modulates neuronal activity; however, this neurotransmitter is related with a number of physiological processes, such as cardiovascular function, gastric motility, renal function, etc. On the other hand, several researches reported that serotonin modulates nociceptive response through 5-HT1, 5-HT2, 5-HT3, and 5-HT7 receptors in the Central Nervous System (CNS).

In the vertebrate central nervous system as well as in the periphery, serotonin, also known as 5-hydroxytriptamine (5-HT), function as a neurotransmitter, a hormone or a mitogen. 5-HT receptors are composed of 7 family 5-HT1-7 receptors, comprising of 14 structurally and pharmacologically distinct 5-HT receptor subtypes. Previous experimental studies showed that mouse 5-HT1A , 5-HT4 and 5-HT7 receptors are regulated by post-translational protein palmitoylation, the reversible attachment of the lipid palmitate to intracellular cysteine residues. Here, we further focused on conservation of these putative palmitoylation sites found in vertebrate 5-HT receptor orthologs.

G protein-coupled receptors (GPCRs), which regulate numerous intracellular signaling cascades that mediate many essential physiological processes, are attractive yet underexploited insecticide targets. RNA interference (RNAi) technology could facilitate the custom design of environmentally safe pesticides that target GPCRs in select target pests yet are not toxic to non-target species. This study investigates the hypothesis that an RNAi yeast insecticide designed to silence mosquito serotonin receptor 1 (5-HTR1) genes can kill mosquitoes without harming non-target arthropods. 5-HTR.426, a Saccharomyces cerevisiae strain that expresses an shRNA targeting a site specifically conserved in mosquito 5-HTR1 genes, was generated. The yeast can be heat-inactivated and delivered to mosquito larvae as ready-to-use tablets or to adult mosquitoes using attractive targeted sugar baits (ATSBs). The results of laboratory and outdoor semi-field trials demonstrated that consumption of 5-HTR.426 yeast results in highly significant mortality rates in Aedes, Anopheles, and Culex mosquito larvae and adults. Yeast consumption resulted in significant 5-HTR1 silencing and severe neural defects in the mosquito brain but was not found to be toxic to non-target arthropods. These results indicate that RNAi insecticide technology can facilitate selective targeting of GPCRs in intended pests without impacting GPCR activity in non-targeted organisms. In future studies, scaled production of yeast expressing the 5-HTR.426 RNAi insecticide could facilitate field trials to further evaluate this promising new mosquito control intervention.

We studied the dural plasma protein extravasation response after unilateral electrical stimulation of the trigeminal ganglion in mice lacking serotonin 5-HT1B (5-HT1D beta) receptors by modifying a technique previously described in rats or guinea pigs. We investigated the inhibitory effects of six 5-HT1 receptor agonists in this model: 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP-93,129), sumatriptan, serotonin-5-O-carboxymethyl-glycyl -tyrosinamide (GTI), 5-methylaminosulfonylmethyl-3-(N-methylpyrrolidin-2R -ylmethyl)-1H-indole (CP-122,288), 5-carboxamido-tryptamine (5-CT), and dihydroergotamine. The plasma extravasation response did not differ between wild-type and mutant after vehicle injection. The potency of sumatriptan, CP-122,288, CP-93,129, and 5-CT in wild-type mice was similar to that previously reported for rats. CP-122,288 (1 nmol kg), 5-CT (1 nmol/kg), and dihydroergotamine (72 nmol/kg) inhibited plasma protein extravasation within dura mater after electrical trigeminal ganglion stimulation in both wild-type and knockout mice, which suggests that these agonists act predominantly via receptors other than 5-HT1B. Unlike the wild-type mice, CP-93,129 (1.4 mumol/kg), a specific 5-HT1B receptor agonist, had no effect in knockout mice. The same held true for sumatriptan (0.7 mumol/kg) and GTI (0.6 mumol/kg). These results suggest that CP-93,129, sumatriptan, and GTI exert their effects via 5-HT1B (5-HT1D beta) receptors in mice.

We propose the possibility of 5-hydroxytryptamine (5-HT)1A receptor involvement in mild serotonin toxicity. A 64-year-old woman who experienced hallucinations was treated with perospirone (8 mg/day). She also complained of depressed mood and was prescribed paroxetine (10 mg/day). She exhibited finger tremors, sweating, coarse shivering, hyperactive knee jerks, vomiting, diarrhea, tachycardia, and psychomotor agitation. After the discontinuation of paroxetine and perospirone, the symptoms disappeared. Another 81-year-old woman, who experienced delusions, was treated with perospirone (8 mg/day). Depressive symptoms appeared and paroxetine (10 mg/day) was added. She exhibited tachycardia, finger tremors, anxiety, agitation, and hyperactive knee jerks. The symptoms disappeared after the cessation of paroxetine and perospirone. Recently, the effectiveness of coadministrating 5-HT1A agonistic psychotropics with selective serotonin reuptake inhibitors (SSRIs) has been reported, and SSRIs with 5-HT1A agonistic activity have been newly approved in the treatment of depression. Perospirone is a serotonin-dopamine antagonist and agonistic on the 5-HT1A receptors. Animal studies have indicated that mild serotonin excess induces low body temperature through 5-HT1A, whereas severe serotonin excess induces high body temperature through 5-HT2A activation. Therefore, it could be hypothesized that mild serotonin excess induces side effects through 5-HT1A, and severe serotonin excess induces lethal side effects with hyperthermia through 5-HT2A. Serotonin toxicity via a low dose of paroxetine that is coadministered with perospirone, which acts agonistically on the 5-HT1A receptor and antagonistically on the 5-HT2A receptor, clearly indicated 5-HT1A receptor involvement in mild serotonin toxicity. Careful measures should be adopted to avoid serotonin toxicity following the combined use of SSRIs and 5-HT1A agonists.

We previously reported that site-selective claudin-5 (CLDN5) breakdown and protein kinase A (PKA) activation are observed in brain microvessels of schizophrenia, but the underlying molecular basis remains unknown. The 5-HT1 receptors decline the intracellular cAMP levels and inactivate the major downstream PKA, and the 5-HT1A receptor is a promising target for schizophrenia. Therefore, we elucidated the involvement of serotonin/5-HT1A signaling in the endothelial CLDN5 expression. We demonstrate, by immunohistochemistry using post-mortem human brain tissue, that the 5-HT1A receptor is expressed in brain microvascular endothelial cells (BMVECs) and mural cells of the normal prefrontal cortex (PFC) gray matter. We also show that PKA is aberrantly activated not only in BMVECs but also in mural cells of the schizophrenic PFC. We subsequently revealed that the endothelial cell-pericyte tube-like structure was formed in a novel two-dimensional co-culture of human primary BMVECs and a human brain-derived pericyte cell line, in both of which the 5-HT1A receptor was expressed. Furthermore, we disclose that the serotonin/5-HT1A signaling enhances endothelial CLDN5 expression in BMVECs under two-dimensional co-culture conditions. Our findings provide novel insights into the physiological and pathological significance of serotonin/5-HT1A signaling in the region-specific regulation of the blood-brain barrier.

5-HT1 receptor agonists have been shown to reduce abnormal involuntary movements (AIMs) in the rat and monkey models of L-DOPA-induced dyskinesia. Different mechanisms have been proposed to underlie this effect. Activation of pre-synaptic 5-HT1 receptors has been suggested to inhibit dysregulated release of dopamine from the serotonin terminals, and thus, abnormal activation of striatal dopamine receptors. Activation of post-synaptic 5-HT1 receptors expressed in non-serotonergic neurons in different brain areas, by contrast, has been shown to result in decreased glutamate and GABA release, which may also contribute to the antidyskinetic effect. To unveil the relative contribution of these mechanisms, we have investigated the effect of increasing doses of 5-HT1A and 5-HT1B receptor agonists on AIMs induced by either L-DOPA or apomorphine. In contrast to L-DOPA-induced AIMs, which were dampened already at low doses of 5-HT1 agonists, reduction of apomorphine-induced AIMs required higher doses. Removal of the serotonin innervation suppressed L-DOPA-induced AIMs, but neither affected apomorphine-induced AIMs nor the inhibiting effect of 5-HT1 agonists on AIMs induced by the direct dopamine agonist, suggesting that such effect is independent on activation of pre-synaptic 5-HT1 receptors.

Mitochondria in neurons contribute to energy supply, the regulation of synaptic transmission, Ca2+ homeostasis, neuronal excitability, and stress adaptation. In recent years, several studies have highlighted that the neurotransmitter serotonin (5-HT) plays an important role in mitochondrial biogenesis in cortical neurons, and regulates mitochondrial activity and cellular function in cardiomyocytes. 5-HT exerts its diverse actions by binding to cell surface receptors that are classified into seven distinct families (5-HT1 to 5-HT7). Recently, it was shown that 5-HT3 and 5-HT4 receptors are located on the mitochondrial membrane and participate in the regulation of mitochondrial function. Furthermore, it was observed that activation of brain 5-HT7 receptors rescued mitochondrial dysfunction in female mice from two models of Rett syndrome, a rare neurodevelopmental disorder characterized by severe behavioral and physiological symptoms. Our Western blot analyses performed on cell-lysate and purified mitochondria isolated from neuronal cell line SH-SY5Y showed that 5-HT7 receptors are also expressed into mitochondria. Maximal binding capacity (Bmax) obtained by Scatchard analysis on purified mitochondrial membranes was 0.081 pmol/mg of 5-HT7 receptor protein. Lastly, we evaluated the effect of selective 5-HT7 receptor agonist LP-211 and antagonist (inverse agonist) SB-269970 on mitochondrial respiratory chain (MRC) cytochrome c oxidase activity on mitochondria from SH-SY5Y cells. Our findings provide the first evidence that 5-HT7 receptor is also expressed in mitochondria.

Hepatic autonomic nerves regulate postprandial hepatic glucose uptake, but the signaling pathways remain unknown. We tested the hypothesis that serotonin (5-hydroxytryptamine [5-HT]) exerts stimulatory and inhibitory effects on hepatic glucose disposal. Ligands of diverse 5-HT receptors were used to identify signaling pathway(s) regulating glucose metabolism in hepatocytes. 5-HT had stimulatory and inhibitory effects on glycogen synthesis in hepatocytes mediated by 5-HT1/2A and 5-HT2B receptors, respectively. Agonists of 5-HT1/2A receptors lowered blood glucose and increased hepatic glycogen after oral glucose loading and also stimulated glycogen synthesis in freshly isolated hepatocytes with greater efficacy than 5-HT. This effect was blocked by olanzapine, an antagonist of 5-HT1/2A receptors. It was mediated by activation of phosphorylase phosphatase, inactivation of glycogen phosphorylase, and activation of glycogen synthase. Unlike insulin action, it was not associated with stimulation of glycolysis and was counteracted by cyclin-dependent kinase (cdk) inhibitors. A role for cdk5 was supported by adaptive changes in the coactivator protein p35 and by elevated glycogen synthesis during overexpression of p35/cdk5. These results support a novel mechanism for serotonin stimulation of hepatic glycogenesis involving cdk5. The opposing effects of serotonin, mediated by distinct 5-HT receptors, could explain why drugs targeting serotonin function can cause either diabetes or hypoglycemia in humans.

Serotonin is a monoamine neurotransmitter that signals through a wide array of receptors (5-HT1-7) many of which are also involved in immune processes. Dendritic cells (DCs) are crucial players in immune defense by bridging innate and adaptive immune responses via their vast repertoire of pattern recognition receptors and antigen-presenting capability. Although serotonin is known to influence immunity at many levels, cell type-specific expression and function of its receptors remains poorly understood. Here we aimed to study 5-HT1-7 expression and function in CD1a- and CD1a+ human monocyte-derived DCs (moDCs). We found that the 5-HT2B receptor-subtype is solely expressed by the inflammatory CD1a+ moDC subset. Specific 5-HT2B activation potently inhibited TLR2, TLR3, and TLR7/8-induced proinflammatory cytokine and chemokine (TNF-α, IL-6, IL-8, IP-10, IL-12) but not type I interferon-β responses. 5-HT2B agonism also interfered with the polarization of CD1a+ moDC-primed CD4+ T cells towards inflammatory Th1 and Th17 effector lymphocytes. Here we report the subset-specific expression and immunomodulatory function of 5-HT2B in human moDCs. Our results expand the biological role of 5-HT2B which may act not only as a neurotransmitter receptor, but also as an important modulator of both innate and adaptive immune responses.

Clozapine is an effective atypical antipsychotic agent, with serious side effects. JL13 [5-(4-methylpiperazin-1-yl)-8-chloropyrido[2,3-b][1,5]benzoxazepine] is a potential new atypical antipsychotic, structurally modified from clozapine to resist oxidation so as to reduce haematological and cardiological side effects. To assess the potential clinical potency of JL13 we tested its action in a newly described animal model based on the ability of clozapine-like agents to affect brain mechanisms controlling sympathetic outflow to thermoregulatory cutaneous vascular beds. We determined whether JL13 has clozapine-like inhibitory actions on alerting-induced falls in tail artery blood flow (sympathetic cutaneous vasomotor alerting responses, SCVARs) in rats, and whether actions on dopamine D(2), and/or 5-HT(1A) receptors are involved in these effects of JL13. The tail artery Doppler flow signal was recorded in conscious freely moving Sprague-Dawley rats before and after alerting stimuli (e.g. cage tap). The percentage fall in flow in response to an alerting stimulus was quantified as a SCVAR index (fall to zero flow implies SCVAR index of 100%, no fall implies 0%). We used pre-treatment with spiperone and WAY100635, before JL13, to assess the role of D(2) and 5-HT(1A) receptors. In addition, the role of 5-HT(2A) receptors in the action of JL13 was assessed by determining whether JL13 prevented and reversed the CNS-mediated tail artery vasoconstricting actions of DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane), an agonist at 5-HT(2A) receptors. JL13 (0.0625-5.0mg/kg s.c.) dose-dependently inhibited SCVARs, less potently than clozapine. WAY100635 but not spiperone reduced the inhibition. JL13 prevented and reversed DOI-induced vasoconstriction. Thus JL13 has clozapine-like actions on thermoregulatory cutaneous blood flow, but the drug is 5 times less potent than clozapine. Stimulation of 5-HT(1A) and blockade of 5-HT(2A) receptors may contribute to the effects, but dopamine D(2) receptors are apparently not involved in the action of JL13.

Serotonin plays an important role of controlling the physiology of the cerebellum. However, serotonin receptor expression has not been fully studied in the developing cerebellum. We have recently shown that cerebellar granule cells transiently express 5-HT3 receptors. In the present study, we investigate expression of 5-HT1 and 5-HT2 receptors in the mouse cerebellum both during postnatal development and in juvenile mice. Here, we show for the first time that 5-HT1A and 5-HT2A receptors are present on cerebellar granule cells with a distinct temporal expression pattern: 5-HT1A receptors are expressed only during the first 2 weeks, while 5-HT2A receptor expression persists until at least 8 weeks after birth. Because of its prolonged expression pattern, we investigated the electrophysiological properties of the 5-HT2A receptor. 5-HT2A receptors expressed by cerebellar granule cells promote stability by reducing variability of the synaptic response, and they modulate the paired-pulse ratio of the parallel fibre-Purkinje cell synapse. Furthermore, pharmacological block of 5-HT2A receptors enhances short-term synaptic plasticity at the parallel fibre-Purkinje cell synapse. We thus show a novel role for serotonin in controlling function of the cerebellum via 5-HT2A receptors expressed by cerebellar granule cells.

Serotonin (1-40 microM) reduced input resistance by 20.6 +/- 6% and hyperpolarized stellate and pyramidal neurons of layers two and three of the lateral entorhinal cortex. 5-Carboxamidotryptamine, a 5-HT1 agonist, and the selective 5-HT1A agonist 8-hydroxy-dipropylaminotetralin mimicked the action of serotonin. The reversal potential of 5-HT-mediated hyperpolarizations was sensitive to the extracellular K+ concentration, indicating a potassium conductance change. Serotonin treatment suppressed excitatory amino acid-mediated synaptic potentials (by 48%, Kd = 6.9 microM) and responses to exogenously applied glutamate (70.1 +/- 17% of control, n = 7), but did not alter paired-pulse facilitation, indicating a postsynaptic site of action. Intracellular application of QX-314, a blocker of potassium conductance, significantly reduced depression of synaptic potentials by 5-HT agonists. In cells filled with QX-314, responses to exogenously applied glutamate were not reduced by serotonin or 5-carboxamidotryptamine application. These results indicate that the observed conductance increase associated with 5-HT application accounts for most if not all of the observed depressant effects of 5-HT1A agonists on excitatory amino acid-mediated neurotransmission.