Molecular biology of the 5-HT1 receptor subfamily

Receptor-Based 3D QSAR Analysis of Serotonin 5-HT1D Receptor Agonists

A three-dimensional quantitative structure-activity relationship study (3D QSAR) has been successfully applied to explain the binding affinities for the serotonin 5-HT1D receptor of a triptan series. The paper describes the development of a receptor-based 3D QSAR model of some known agonists and recently developed triptans on the 5-HT1D serotonergic receptor, showing a significant correlation between predicted and experimentally measured binding affinity (pIC50). The pIC50 values of these agonists are in the range from 5.40 to 9.50. The ligand alignment obtained from dynamic simulations was taken as basis for a 3D QSAR analysis applying the GRID/GOLPE program. 3D QSAR analysis of the ligands resulted in a model of high quality (r2 = 0.9895, q2LOO = 0.7854). This is an excellent result and proves both the validity of the proposed pharmacophore and the predictive quality of the 3D QSAR model for the triptan series of serotonin 5-HT1D receptor agonists.

Effects of serotonin-1 and serotonin-2 receptor agonists on neuronal activity in the nucleus tractus solitarius

Spontaneous neuronal activity in the solitary tract nucleus was recorded extracellularly in a brain slice preparation during bath-application of 5-HT1 and 5-HT2 receptor-selective agonists and antagonists. The 5-HT1A/5-HT1B agonist 5-carboxamidotryptamine depressed activity in 20 of 25 neurons studied. The remaining five neurons were unaffected. The 5-HT1A/5-HT1B antagonist pindolol prevented the 5-carboxamidotryptamine-induced changes, whereas the 5-HT1A antagonist spiroxatrine and the 5-HT2 antagonists ketanserin and mianserin were ineffective. Application of the 5-HT1/5-HT2 agonist alpha-methylserotonin depressed activity in 16 of 19 neurons, whereas the remaining three neurons were unresponsive. Pindolol blocked alpha-methylserotonin-induced changes of activity, but spiroxatrine, ketanserin and mianserin were ineffective. Finally, the 5-HT2 agonist DOI was applied to seven neurons. Six were unresponsive to DOI, and one responded with a depression of activity. These data provide electrophysiological evidence for the presence of 5-HT1 receptors in the nTS, presumably of the 5-HT1B subclass, but cast further doubt on the contribution of 5-HT2 and 5-HT1A receptors to the actions of serotonin in the nucleus.

Quantitative autoradiographic characterisation of the binding of [3H]WAY-100635, a selective 5-HT1A receptor antagonist

The binding characteristics of [3H]WAY-100635 ([O-methyl 3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride), a potent and selective 5-HT1A antagonist radioligand, were examined in the rat brain using in vitro quantitative receptor autoradiography. The regional distribution of specific [3H]WAY-100635 binding sites was heterogeneous and demonstrated a strong correlation with that of [3H]8-OH-DPAT binding. The highest concentrations of [3H]WAY-100635-labelled sites were found in the lateral septal areas, dorsal raphe n., entorhinal cortex and the hippocampal formation (CA1, CA3 and dentate gyrus). Scatchard transformation of saturation isotherms revealed saturable [3H]WAY-100635 binding sites of high-affinity: in the hippocampal formation, Kd was approximately 1 nM and Bmax ranged between 187 and 243 fmol/mg tissue wet weight, in the entorhinal cortex, Kd = 0.44 nM and Bmax = 194 fmol/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 0.52 nM and Bmax = 157 fmol/mg tissue wet weight. The affinity of [3H]WAY-100635 for the 5-HT1A binding site tended to be higher in the dorsal raphe n. and entorhinal cortex compared with that of the hippocampal formation. In contrast, the binding affinity of [3H]8-OH-DPAT in the hippocampal formation was between 1.1 and 2.3 nM and the Bmax was 137 to 183 fmoles/mg tissue wet weight; in the entorhinal cortex, Kd = 3.2 nM and Bmax = 141 fmoles/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 3.4 nM and Bmax = 163 fmol/mg tissue wet weight.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine2 and 5-hydroxytryptamine3 receptors mediate serotonin-induced short-circuit current in pig jejunum

The purpose of this study was to determine the effect of methysergide, ketanserin, granisetron, cisapride, and renzapride on serotonin (5-hydroxytryptamine-evoked short-circuit current in muscle and myenteric plexus-stripped pig jejunum using the Ussing chamber technique. Ketanserin, granisetron, cisapride, and renzapride all reduced the 5-hydroxytryptamine-induced increase in short-circuit current by about 50%. Combination of ketanserin and granisetron only reduced the 5-hydroxytryptamine-induced peak increase in short-circuit current by 25%. Cisapride caused a small concentration-dependent increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Ketanserin, granisetron, methysergide, and renzapride did not alter the basal short-circuit current. These results suggest that 5-hydroxytryptamine elicits an increase in short-circuit current by activating epithelial and submucosal 5-hydroxytryptamine2 and 5-hydroxytryptamine3 receptor subtypes. Furthermore, the short-circuit current-increasing effect of cisapride, is due to activation of at least muscarinic and nicotinic receptors.

Electrophysiological effects of serotonin in the solitary tract nucleus of the rat

The nucleus tractus solitarius (nTS) is an important site for the integration of visceral information and its modification by afferent neural systems. One such afferent system arises from the raphe nuclei. This study investigated the electrophysiological effects of the primary transmitter of the raphe nuclei, serotonin (5-HT), on neurones in the nTS of the rat. Extracellular single unit recordings were made of the spontaneous activity of nTS neurones in isolated, superfused brainstem slices during bath-application of 5-HT (50, 100, 250, 500, 1000 nM). Twenty-seven of 46 neurones studied (approximately 59%) showed concentration-dependent decreases of firing rate, with a calculated EC50 of 261 nM. An additional 3 neurones displayed excitatory responses, while the remaining 16 were unaffected. The broad-spectrum 5-HT1 antagonist methysergide (200 nM) was highly effective in producing blockade of 5-HT-evoked depressions of neuronal activity (4 of 4), whereas the 5-HT1A receptor-selective antagonist spiroxatrine (5 nM) and the 5-HT2-selective antagonist mianserin (200 nM) were considerably less effective (1 of 5 and 2 of 5, respectively). Seven additional neurones were examined during exposure to the 5-HT1-selective agonist 5-carboxamidotryptamine (5-CT) or the 5-HT2-selective agonist DOI. 5-CT depressed the activity of three of the four neurones tested, the remaining neurone being unresponsive at concentrations up to 50 nM. DOI at concentrations up to 100 nM failed to affect the activity of two of the three neurones tested, and depressed the activity of the third at a concentration of 50 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin-induced short-circuit current in pig jejunum

This paper presents the effects of serotonin (5-HT) on short-circuit current (SCC), sodium and chloride fluxes, and prostaglandin E2 release in pig jejunum, using the Ussing-chamber technique. 5-HT elicited a dose-dependent increase in SCC, yielding an EC50 of 6 +/- 4 microM and EMAX of 77 +/- 8 microA.cm-2 using about 100 microM. Inhibited sodium absorption and stimulated chloride secretion carried part of the 5-HT-induced SCC. 5-HT caused a dose-independent PGE2 release, and indomethacin reduced the SCC-inducing effect of 5-HT by 40%. Octreotide, a long-lasting somatostatin analogue, also reduced 5-HT-induced SCC by about 40%, while tetrodotoxin (TTX) did not alter the effect of 5-HT. In conclusion, 5-HT causes a dose-dependent indomethacin and octreotide-sensitive, and TTX-insensitive increase in SCC, and a chloride secretion and inhibited sodium absorption and an increased release of PGE2 in pig jejunum in vitro.

5-HT receptors as targets for the development of novel anxiolytic drugs: models, mechanisms and future directions

The introduction of buspirone for the treatment of anxiety, together with the eventual suggestion of a mode of action involving the serotonin (5-HT)1A receptor subtype, has generated considerable research activity and renewed interest in the potential role of 5-HT in anxiety. The further identification of multiple 5-HT1 receptors, coupled with the possibility that these subtypes potentially are involved in discrete biobehavioral regulation and pathophysiological conditions, has greatly expanded the search for tools capable of probing these receptors and has raised hopes for a new generation of more specific compounds to treat other disorders associated with the 5-HT system such as depression, aggression, and sleep and eating disturbances. The involvement of 5-HT in anxiety has prompted a careful reevaluation of several traditional areas of research. This has included those methods used in the in vivo evaluation of drugs in preclinical animal test procedures used to assess potential anxiolytic activity, as well as the mechanisms associated with adaptive changes occurring during long-term drug administration. The proliferation of various procedures for studying the anxiolytic effects of 5-HT drugs has not always been accompanied by systematic behavioral and pharmacological validation. At the present time, this area of research is characterized by numerous inconsistent findings. Procedures that are objective and impartial to the behavioral effects of drugs provide distinct advantages for addressing some of these issues, as will the results from carefully controlled clinical studies. The main objective of this article is to provide an overview of the recent developments in research involving the 5-HT system and anxiety. The emphasis will be on the 5-HT1 receptor system and a review of the results in the predominant animal models used to evaluate these drugs, as well as an overview of the mechanisms currently believed to be responsible for the therapeutic activity of this class of compounds. Studies with the pigeon are reviewed, since this species appears distinctly sensitive to the anxiolytic-like effects of 5-HT1A drugs in conflict procedures. Although chronic administration of 5-HT1A drugs appears necessary for clinical anxiolytic and antidepressant activity, the most noteworthy neuropharmacological effects in animals seem to occur in 5-HT2 and, perhaps, 5-HT3 receptors which are downregulated. Studies summarizing the activity of drugs interacting with 5-HT1C/2 and 5-HT3 receptor sites are also discussed as they too may be involved in anxiety or the actions of anxiolytic drugs.(ABSTRACT TRUNCATED AT 400 WORDS)