5-HT(2A) receptor activation in the dorsolateral septum facilitates inhibitory avoidance in the elevated T-maze

Serotonin in the lateral septum has been implicated in the modulation of defense and hence in anxiety. However, it deserves investigation how changes in 5-HT-mechanisms in this area modulate defensive responses associated with specific subtypes of anxiety disorders. We evaluated the effects of intra-dorsolateral septum (DLS) injections of the preferential 5-HT_2A receptor agonist DOI (8 and 16 nmol), the 5-HT_2C selective agonist MK-212 (0.1 and 1 nmol) and the preferential 5-HT_2A antagonist ketanserin (10 and 20 nmol) in rats exposed to the elevated T-maze (ETM), a model which allows the measurement of two defensive responses: inhibitory avoidance and escape. These responses have been respectively related to generalized anxiety and panic disorder. All animals were tested in an open-field after the ETM for locomotor activity assessments. Results showed that intra-DLS DOI increased avoidance latencies, an anxiogenic effect. MK and ketanserin were without effect. Also, none of the drugs administered affected the escape performance. Ketanserin blocked the anxiogenic effect caused by DOI. No changes to locomotion were observed. The data suggests that DLS 5-HT_2A receptors are involved in the control of inhibitory avoidance and that a failure in this mechanism may be of importance to the physiopathology of generalized anxiety.     

An autoradiographic study of serotonergic receptors in a murine genetic model of anxiety-related behaviors

Modifications in serotonin (5-HT) neurotransmission have been associated with the physiopathology of anxiety and depression. Among the numerous 5-HT receptor subtypes, several (5-HT_1A, 5-HT_1B, 5-HT₂ and 5-HT₃) could be involved in these etiologies. By using a murine genetic model, we attempted to correlate variations in the density of receptor subtypes with modifications of anxiety-related behaviors. From a classic inbred strain (C57BL/6ByJ) and a linkage-testing inbred strain (ABP/Le), segregated F₂ populations for 3 loci located in the 4th, 7th and 9th chromosomes have been selected for their different responses in anxiety-related behavioral tests. The regional density of 5-HT_1A, 5-HT_1B, 5-HT_2A and 5-HT_2C receptors has been measured in the brains of parental strains, F_1s and F₂ populations by quantitative autoradiography. The results suggest that chromosomal fragments containing the brown, pink-eyed dilution and the short-ear loci, previously shown to be involved in anxiogenic processes, are mainly associated with a variation in the density of the 5-HT_1B receptors.     

Blockade of 5-HT(2) receptors in the periaqueductal grey matter (PAG) abolishes the anxiolytic-like effect of 5-HT(1A) receptor antagonism in the median raphe nucleus in mice

Several lines of evidence support the involvement of serotonergic (5-HT) neurons of the median raphe nucleus (MRN) in anxiety-like behaviour. In this context, it is known that blockade of 5-HT_1A somatodendritic autoreceptors in the midbrain raphe nuclei increases the firing rate of these neurons, disinhibiting 5-HT release in postsynaptic target areas such as amygdala, hippocampus and periaqueductal grey matter (PAG). However, while activation of 5-HT_1A or 5-HT₂ receptors in forebrain targets such as the amygdala or hippocampus enhances anxiety-like behaviours in rodents, stimulation of both receptor subtypes in the midbrain PAG markedly reduces anxiety-like behaviour. In view of these findings, the present study investigated whether the anti-anxiety effects induced by pharmacological disinhibition of 5-HT neurons in the MRN are attenuated by the blockade of 5-HT₂ receptors within the PAG. Mice received combined intra-PAG injection with ketanserin (10 nmol/0.1 μl), a 5-HT₂ receptor antagonist, followed by intra-MRN injection of WAY-100635 (5.6 nmol/0.1 μl), a highly selective 5-HT_1A receptor antagonist. They were then individually exposed to the elevated plus-maze (EPM), with the videotaped behavioural sessions subsequently scored for both conventional and ethological measures. The results confirmed that intra-MRN infusion of WAY100635 reduces behavioural indices of anxiety without significantly altering general activity measures, and further showed that this effect was completely blocked by intra-PAG pretreatment with an intrinsically-inactive dose of ketanserin. Together, these results suggest that 5HT₂ receptor populations located within the midbrain PAG play a significant role in the reduction of anxiety observed following disinhibition of 5-HT neurons in the MRN.     

The response of neurons in the bed nucleus of the stria terminalis to serotonin: Implications for anxiety

Substantial evidence has suggested that the activity of the bed nucleus of the stria terminalis (BNST) mediates many forms of anxiety-like behavior in human and non-human animals. These data have led many investigators to suggest that abnormal processing within this nucleus may underlie anxiety disorders in humans, and effective anxiety treatments may restore normal BNST functioning. Currently some of the most effective treatments for anxiety disorders are drugs that modulate serotonin (5-HT) systems, and several decades of research have suggested that the activation of 5-HT can modulate anxiety-like behavior. Despite these facts, relatively few studies have examined how activity within the BNST is modulated by 5-HT. Here we review our own investigations using in vitro whole-cell patch-clamp electrophysiological methods on brain sections containing the BNST to determine the response of BNST neurons to exogenous 5-HT application. Our data suggest that the response of BNST neurons to 5-HT is complex, displaying both inhibitory and excitatory components, which are mediated by 5-HT_1A, 5-HT_2A, 5-HT_2C and 5-HT₇ receptors. Moreover, we have shown that the selective activation of the inhibitory response to 5-HT reduces anxiety-like behavior, and we describe data suggesting that the activation of the excitatory response to 5-HT may be anxiogenic. We propose that in the normal state, the function of 5-HT is to dampen activity within the BNST (and consequent anxiety-like behavior) during exposure to threatening stimuli; however, we suggest that changes in the balance of the function of BNST 5-HT receptor subtypes could alter the response of BNST neurons to favor excitation and produce a pathological state of increased anxiety.     

Synthesis and Structure–Activity Relationships of N-Benzyl Phenethylamines as 5-HT2A/2C Agonists

相似文献   

Activation and blockade of dorsal hippocampal Serotonin6 receptors regulate anxiety-like behaviors in a unilateral 6-hydroxydopamine rat model of Parkinson’s disease

ABSTRACT

Objectives: This study aimed to investigate the effect of serotonin₆ (5-HT₆) receptors in the dorsal hippocampus (dHip) on the regulation of Parkinson’s disease (PD)-associated anxiety.

Methods: We examined whether intra-dHip injection of both 5-HT₆ receptor agonist and antagonist was involved in the regulation of anxiety-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle by the open-field and elevated plus maze (EPM) tests. Four weeks after injection of 6-OHDA, the concentrations of dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) in relative related brain regions were measured by reverse-phase high-performance liquid chromatography.

Results: In sham-operated rats, intra-dHip injection of both 5-HT₆ receptor agonist WAY208466 (3 and 6 µg/rat) and antagonist SB258585 (4 µg/rat) increased the percentage of time spent in the center area in the open-field test and percentages of open arm entries and open arm time in EPM test, indicating that induced anxiolytic effects. In the lesioned rats, WAY208466 (1.5, 3 and 6 µg/rat) produced anxiolytic responses, whereas SB258585 (2 and 4 µg/rat) produced anxiogenic effects. Neurochemical results showed that intra-dHip injection of WAY208466 (6 µg/rat) decreased NA level in the amygdala, and SB258585 (4 µg/rat) increased DA levels in the dHip and vHip in sham-operated rats, whereas WAY208466 increased DA levels in the dHip, vHip, and amygdala in the lesioned rats.

Discussion: dHip 5-HT₆ receptors are involved in the regulation of anxiety-like behaviors, which may be mediated through different neurochemical mechanisms, and the dHip is an important site involved in these effects.

Abbreviation: PD: Parkinson’s disease;6-OHDA: 6-hydroxydopamine; dHip: dorsal hippocampus; vHip: ventral hippocampus; 5-HT: serotonin; MFB: medial forebrain bundle; DA: dopamine; NA: noradrenaline; EPM: elevated plus-maze; GABA: gamma-aminobutyric acid; BLA: basolateral amygdala     

Colocalisation of serotonin2A receptors with the glutamate receptor subunits NR1 and GluR2 in the dentate gyrus: an ultrastructural study of a modulatory role

The serotonin_2A receptor (5-HT_2AR) is implicated in many neurological disorders and has a role in cognitive processes, reliant upon hippocampal glutamate receptors. Recent studies show that 5-HT_2AR agonists and/or antagonists can influence cognitive function, suggesting a critical hippocampal role for these receptors, yet their cellular and subcellular distribution within this region has not been comprehensively analysed. Here, we have conducted an electron microscopic examination of 5-HT_2AR distribution with the glutamate N-methyl-d-aspartate (NMDA) and amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor subunits NR1 and GluR2 in the hippocampal dentate gyrus (DG) in order to investigate whether 5-HT_2AR location is compatible with a modulatory role over NMDA and/or AMPA receptor mediated neurotransmission. Of 5-HT_2AR positive profiles, 56% were dendrites and 16% were dendritic spines. Labelling was both cytoplasmic and membranous. Spinous labelling was more frequently membranous at peri- and extra-synaptic sites, though was also associated with synaptic specialisations. Profiles displaying colocalisation of immunoreactivity for 5-HT_2ARs with NR1 or GluR2 were predominantly dendrites, representing 11% and 8% of 5-HT_2AR positive profiles, respectively. Additionally, 12% of 5-HT_2AR labelled profiles also displayed immunoreactivity for γ-aminobutyric acid (GABA). These data indicate most 5-HT_2ARs are expressed on granule cell projections, with a smaller subpopulation expressed on GABAergic interneurons.     

In vivo effects of activation and blockade of 5-HT2A/2C receptors in the firing activity of pyramidal neurons of medial prefrontal cortex in a rodent model of Parkinson's disease

In the present study, we examined changes in the firing rate and firing pattern of pyramidal neurons in medial prefrontal cortex (mPFC), and the effects of 5-HT_2A/2C receptor agonist DOI and antagonist ritanserin 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 pyramidal neurons compared to sham-operated rats, and the firing pattern of these neurons also changed significantly towards a more bursty one. Systemic administration of DOI (20–320 μg/kg, i.v.) increased the mean firing rate of pyramidal neurons in sham-operated and the lesioned rats. The excitation was significant only at doses higher than 160 μg/kg and 320 μg/kg in sham-operated and the lesioned rats, respectively. In addition, the local application of DOI, 5 μg, in mPFC inhibited the firing rate of pyramidal neurons in sham-operated rats, while having no effect on firing rate in the lesioned rats. After treatment with GABA_A receptor antagonist picrotoxinin, the local application of DOI, at the same dose, increased the mean firing rate of the neurons in sham-operated rats; however, DOI did not alter the firing activity of the neurons in the lesioned rats. These results indicate that the lesion of the nigrostriatal pathway leads to hyperactivity of pyramidal neurons in mPFC, and the decreased response of pyramidal neurons to DOI, suggesting dysfunction of 5-HT_2A and 5-HT_2C receptors on pyramidal neurons and GABAergic interneurons in the 6-OHDA-lesioned rats.     

Expression of 5-HT1A receptor mRNA in rat nucleus raphe magnus neurons after peripheral inflammation

The present study demonstrated that 5-HT_1A receptor mRNA was expressed with moderate level in the NRM neurons. Most of 5-HT_1A receptor mRNA positive cells were 5-HT neurons, suggesting the majority of 5-HT_1A receptor in the NRM might be autoreceptors. Eight hours after carrageenan inflammation, the expression of 5-HT_1A receptor mRNA in the NRM neurons, especially in the 5-HT neurons, was significantly increased. These results suggest that synthesis of 5-HT_1A receptors, including 5-HT_1A autoreceptors, is increased in the NRM during peripheral inflammation.     

The 5-HT1A receptor antagonist (S)-UH-301 blocks the (R)-8-OH-DPAT-induced inhibition of serotonergic dorsal raphe cell firing in the rat

Summary (S)-UH-301 [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin, 0.5–4.0 mg/kg i.V.] did not significantly alter the firing rate of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN) as a group, although some individual cells were activated whereas others were depressed. However, (S)-UH-301 (2.0mg/kg i.v.) consistently reversed the inhibition of DRN-5-HT cells produced by the selective 5-HT_1A receptor agonist (R)-8-OH-DPAT (0.5 g/kg i.v.) and the dose-response curve for this effect of (R)-8-OH-DPAT was markedly shifted to the right by pretreatment with (S)-UH-301 (1.0mg/kg i.v.). These results support the notion that (S)-UH-301 acts as an antagonist at central 5-HT_1A receptors.     

Possible involvement of K(ATP) channels in the control of 5-HT neurons

The possible involvement of ATP-sensitive potassium channels in the control of the electrical activity of central serotoninergic neurons was investigated by recording their firing rate in the dorsal raphe nucleus of rat brain stem slices exposed to various blockers and openers of these channels. Whereas the channel openers lemakalim and aprikalim produced no change in the firing rate of these neurons, the channel blockers glibenclamide and gliquidone were strongly inhibitory. As expected from an effect through ATP-sensitive potassium channels, the inhibition by glibenclamide could be prevented in a competitive manner by lemakalim and aprikalim. In contrast, the inactive isomer of the latter drug, RP 61499, did not alter the glibenclamide effect. In addition to the channel openers, the GABA receptor antagonists, bicuculline and phaclofen, but not the antagonist of somato-dendritic 5-HT_1A autoreceptors, (-)tertatolol, prevented the negative influence of glibenclamide on the firing rate of serotoninergic neurons. This suggests that GABA acting at both GABA_A and GABA_B receptors (but not serotonin through the possible stimulation of autoreceptors) was responsible for the effect of glibenclamide. Accordingly, the blockade by the latter drug of ATP-sensitive potassium channels on GABAergic interneurons probably triggered the release of GABA, which in turn, inhibited serotoninergic neurons. In agreement with this hypothetical mechanism, autoradiographic studies demonstrated that ATP-sensitive potassium channels are not located on serotoninergic neurons (but probably on GABAergic interneurons) as the extensive lesion of these neurons by 5,7-dihydroxytryptamine did not reduce the specific labelling of the dorsal raphe nucleus by [³H]glibenclamide.     

Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)1A receptors: WAY 100,635-reversible actions of the highly selective ligands,flesinoxan and S 15535

This study examined the influence of the highly selective 5-HT_1A receptor ligands, flesinoxan, S 15535, and WAY 100,635, upon the electrical activity of dopaminergic neurons in the ventral tegmental area (VTA), as compared to serotonergic neurons in the dorsal raphe nucleus (DRN) of anesthetized rats. Flesinoxan, a high-efficacy agonist at both pre- and postsynaptic 5-HT_1A receptors, dose-dependently (inhibitory dose (ID)₅₀ = 19.5 μg/kg, i.v.) inhibited the firing of DRN serotonergic neurons. This action was abolished by WAY 100,635 (31 μg/kg i.v.) which is an antagonist at pre- and postsynaptic 5-HT_1A receptors. S 15535, which behaves as an agonist and partial agonist at pre- and postsynaptic 5-HT_1A receptors, respectively, similarly abolished DRN firing in a WAY 100,635-reversible fashion with an ID₅₀ of 6.1 μg/kg, i.v. In contrast to these actions, both flesinoxan (≥500 μg/kg, i.v.) and S 15535 (≥125 μg/kg, i.v.) dose-dependently and monophasically increased the firing rate of dopaminergic neurons in the VTA, with maximal effects of 70.1 ± 17.2% and 33.7 ± 5.3%, respectively. Further, VTA dopaminergic neurons displaying a regular firing pattern were transformed into a bursting mode. This influence of flesinoxan and S 15535 on VTA cells was abolished by WAY 100,635. Administered alone, WAY 100,635 did not significantly modify the activity of either serotonergic or dopaminergic neurons. In conclusion, the present findings show that selective activation of 5-HT_1A receptors not only inhibits serotonergic neurones but also elicits a (possibly related) increase in VTA dopaminergic output. A facilitatory influence of flesinoxan, S 15535, and other selective 5-HT_1A receptor ligands upon mesocortical dopaminergic pathways may contribute to their putative antidepressant properties. Synapse 30:172–180, 1998. © 1998 Wiley-Liss, Inc.     

Favouring inhibitory synaptic drive mediated by GABAA receptors in the basolateral nucleus of the amygdala efficiently reduces pain symptoms in neuropathic mice

Pain is an emotion and neuropathic pain symptoms are modulated by supraspinal structures such as the amygdala. The central nucleus of the amygdala is often called the ‘nociceptive amygdala’, but little is known about the role of the basolateral amygdala. Here, we monitored the mechanical nociceptive thresholds in a mouse model of neuropathic pain and infused modulators of the glutamate/GABAergic transmission in the basolateral nucleus of the amygdala (BLA) via chronically‐implanted cannulas. We found that an N‐methyl‐D‐aspartate‐type glutamate receptor antagonist (MK‐801) exerted a potent antiallodynic effect, whereas a transient allodynia was induced after perfusion of bicuculline, a GABA_A receptor antagonist. Potentiating GABA_A receptor function using diazepam or etifoxine (a non‐benzodiazepine anxiolytic) fully but transiently alleviated mechanical allodynia. Interestingly, the antiallodynic effect of etifoxine disappeared in animals that were incapable of producing 3α‐steroids. Diazepam had a similar effect but of shorter duration. As indicated by patch‐clamp recordings of BLA neurons, these effects were mediated by a potentiation of GABA_A receptor‐mediated synaptic transmission. Together with a presynaptic elevation of miniature inhibitory postsynaptic current frequency, the duration and amplitude of GABA_A miniature inhibitory postsynaptic currents were also increased (postsynaptic effect). The analgesic contribution of endogenous neurosteroid seemed to be exclusively postsynaptic. This study highlights the importance of the BLA and the local inhibitory/excitatory neuronal network activity while setting the mechanical nociceptive threshold. Furthermore, it appears that promoting inhibition in this specific nucleus could fully alleviate pain symptoms. Therefore, the BLA could be a novel interesting target for the development of pharmacological or non‐pharmacological therapies.     

Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT_1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT_1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20–60 nA caused an inhibition of tilt-induced firing of α-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of β-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20–60 nA), a 5-HT_1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT_1A receptor to inhibit transmission of otolith information to α- and β-type MVN neurons.     

Conditioned place aversion organized in the dorsal periaqueductal gray recruits the laterodorsal nucleus of the thalamus and the basolateral amygdala

The amygdala-ventral periaqueductal gray circuit is crucial for the expression of contextual conditioned fear. However, little is known about the neural circuits activated when the stimulation of the dorsal periaqueductal gray (dPAG) is used as unconditioned stimulus (US) in conditioned fear paradigms. The present paper examines the Fos-protein distribution in the brain of rats submitted to a conditioned place aversion (CPA) paradigm using the dPAG chemical stimulation with semicarbazide (SMC), an inhibitor of the GABA synthesizing enzyme, as US and the quadrant of an arena where the drug was injected as the paired neutral stimulus. Our results show that CPA associated with SMC injections caused a significant Fos labeling in the laterodorsal nucleus of the thalamus (LD), basolateral nucleus of amygdala (BLA) and in the dorsomedial PAG (dmPAG). This pattern of brain activation is clearly different from the neural substrates of the classical fear conditioning reported in the literature. Moreover, this paper shows that CPA with the use of chemical stimulation of the dPAG could be used as an experimental model of panic disorder with agoraphobia in the extent that panic attacks repeatedly associated with specific contexts may turn in this condition in the clinics. This condition activates the BLA probably through the LD. Besides, it indicates that the dPAG can be the link between amygdala and the brainstem motor regions that controls CPA when dPAG stimulation is used as US instead of footshocks. From this evidence we suggest that a loop dPAG-LD-BLA-dPAG is activated during the panic disorder with agoraphobia.     

Regulation of GABA release via NMDA and 5-HT1A receptors in guinea pig dentate gyrus

The regulation by N-methyl-d-aspartate (NMDA) and 5-HT_1A receptors of the endogenous γ-aminobutyric acid (GABA) release was investigated in slices of the guinea pig dentate gyrus. The release of GABA was increased in a concentration-dependent fashion by NMDA. The release of GABA evoked by NMDA was Ca²⁺-dependent, tetrodotoxin-resistant, Mg²⁺-sensitive and inhibited by MK-801, a selective non-competitive NMDA receptor antagonist. These results suggest that the NMDA receptor present on GABAergic neurons is involved in the stimulatory regulation of GABA release. The release of GABA was increased concentration-dependently by NAN-190, a 5-HT_1A receptor antagonist, but was not affected by 8-OH-DPAT, a 5-HT_1A receptor agonist. The release of GABA evoked by NAN-190 was Ca²⁺-dependent, tetrodotoxin-resistant and inhibited by 8-OH-DPAT. These results suggest that the 5-HT_1A receptor present on GABAergic neurons is involved in the inhibitory regulation of GABA release. The release of GABA evoked by NMDA from the dentate gyrus was inhibited by pretreatment with 8-OH-DPAT. The release of GABA evoked by NAN-190 was inhibited by pretreatment with MK-801. The release of GABA evoked by NMDA from the dentate gyrus was augmented by the concurrent application of NAN-190. Taken together, the results indicate that the NMDA receptor and the 5-HT_1A receptor, which are both located on GABAergic neurons in the guinea pig dentate gyrus, exert stimulatory and inhibitory regulation of neuronal GABA release, respectively.     

Cerebral 5-HT2A receptor and serotonin transporter binding in humans are not affected by the val66met BDNF polymorphism status or blood BDNF levels

Recent studies have proposed an interrelation between the brain-derived neurotrophic factor (BDNF) val66met polymorphism and the serotonin system. In this study, we investigated whether the BDNF val66met polymorphism or blood BDNF levels are associated with cerebral 5-hydroxytryptamine 2A (5-HT_2A) receptor or serotonin transporter (SERT) binding in healthy subjects. No statistically significant differences in 5-HT_2A receptor or SERT binding were found between the val/val and met carriers, nor were blood BDNF values associated with SERT binding or 5-HT_2A receptor binding. In conclusion, val66met BDNF polymorphism status is not associated with changes in the serotonergic system. Moreover, BDNF levels in blood do not correlate with either 5-HT_2A or SERT binding.     

Characterization of the K current mediated by 5-HT1A receptor in the acutely dissociated rat dorsal raphe neurons

The action of 5-hydroxytryptamine (5-HT) via the 5-HT_1A receptor on dissociated rat dorsal raphe neurons was characterized under the whole-cell mode by using the nystatin-perforated patch-clamp technique. Under voltage-clamp conditions, 5-HT induced an inwardly rectifying K⁺ current (I_5-HT) in a concentration-dependent manner. I_5-HT was mimicked by 8-OH-DPAT and buspirone, which are both 5-HT_1A receptor agonists. I_5-HT was reversibly blocked by such 5-HT_1A receptor antagonists as (S)-UH-301 and spiperone but not by ketanserin, a 5-HT₂ receptor antagonist, granisetron, a 5-HT₃ receptor antagonist, and GR-113808, a 5-HT₄ receptor antagonist. I_5-HT was antagonized concentration-dependently by such K⁺ channel blockers as quinine, Ba²⁺ and 4-aminopyridine but was relatively insensitive to both Cs⁺ and tetraethylammonium. When the neurons were loaded with guanosine 5′-O-3-thiotriphosphate through a patch pipette, the K⁺ current induced by 5-HT became irreversible. N-ethylmaleimide (NEM), a sulfhydryl alkylating agent, irreversibly blocked I_5-HT. The intracellular perfusion with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), a Ca²⁺ chelator, or neomycine, a phospholipase C inhibitor, never significantly affected the 5-HT-induced response. 12-Myristate 13-acetate diester (PMA), a protein kinase C (PKC) activator, had only a weak inhibitory effect on I_5-HT, and staurosporine, a PKC inhibitor, failed to significantly occlude I_5-HT. Therefore, the K⁺ conductance activated via the 5-HT_1A receptor of dorsal raphe neurons was thus characterized by the sensitivity to such K⁺ channel blockers as quinine, Ba²⁺ and 4-aminopyridine. Moreover, G protein, which is NEM-sensitive and can couple to the 5-HT_1A receptor, is thus considered to activate the inwardly rectifying K⁺ conductance without being mediated by such second messengers as Ca²⁺ and PKC.     

5-HT7 receptors mediate serotonergic effects on light-sensitive suprachiasmatic nucleus neurons

Serotonin (5-HT) has been shown to phase shift circadian rhythms in mammals and to affect responses of the circadian system to light, but it is not clear which receptors are involved in these actions. We found that drugs which act as 5-HT_1A receptor agonists suppressed photic responses of hamster SCN cells, but these drugs also exhibit high affinity for the recently cloned 5-HT₇ receptor. We therefore studied the effects of 5-HT agonists and antagonists with differential affinities for 5-HT₇ and 5-HT_1A receptors on responses of hamster SCN cells to retinal illumination. We confirmed that the 5-HT receptor agonists 5-HT, 8-OH-DPAT and 5-CT, dose-dependently reduced photic activation of SCN cells. These effects could be blocked by co-application of antagonists with high affinities for 5-HT₇ receptors: ritanserin or clozapine. The 5-HT_1A/B/D antagonist, cyanopindolol, which is inactive at 5-HT₇ receptors, did not antagonize the actions of 8-OH-DPAT. Selective 5-HT_1A antagonists, WAY100635 and p-MPPI, had weak or no antagonist effects on the responses to 8-OH-DPAT in the SCN, but they effectively antagonized the actions of 8-OH-DPAT in the hippocampus. In the cerebellar cortex where few 5-HT₇ receptors are present, ritanserin failed to antagonize the effects of 8-OH-DPAT. Our results indicate that the 5-HT₇ receptor subtype plays a major role in mediating the effects of 5-HT on photic responses of SCN cells in the hamster.