5-Hydroxytryptamine stimulates two distinct adenylate cyclase activities in rat brain: high-affinity activation is related to a 5-HT1 subtype different from 5-HT1A, 5-HT1B, and 5-HT1C

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Identification of 5-hydroxytryptamine receptors positively coupled to adenylyl cyclase in rat cultured astrocytes

1. 5-Hydroxytryptamine (5-HT) elicited a dose-dependent stimulation of intracellular adenosine 3': 5'-cyclic monophosphate (cyclic AMP) accumulation in cultured astrocytes derived from neonatal rat (Sprague Dawley) thalamic/hypothalamic area with a potency (pEC50) of 6.68 +/- 0.08 (mean +/- s.e. mean). 2. In order to characterize the 5-HT receptor responsible for the cyclic AMP accumulation the effects of a variety of compounds were investigated on basal cyclic AMP levels (agonists) and 5-carboxamidotryptamine (5-CT) stimulated cyclic AMP levels (antagonists). The rank order of potency for the agonists investigated was 5-CT (pEC50 = 7.81 +/- 0.09) > 5-methoxytryptamine (5-MeOT) (pEC50 = 6.86 +/- 0.36) > 5-HT (pEC50 = 6.68 +/- 0.08). The following compounds, at concentrations up to 10 microM, did not affect basal cyclic AMP levels 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), cisapride, sumatriptan, DOI and RU 24969. The rank order of potency of antagonists was methiothepin (pKi = 7.98 +/- 0.25) > mesulergine (pKi = 7.58 +/- 0.18) > ritanserin (pKi = 7.20 +/- 0.24) > clozapine (pKi = 7.03 +/- 0.19) > mianserin (pKi = 6.41 +/- 0.19). The following compounds, at concentrations up to 10 microM, were inactive: ketanserin, WAY100635, GR127935. This pharmacological profile is consistent with that of 5-HT7 receptor subtype-mediated effects. 3. The cultured astrocytes exhibited regional heterogeneity in the magnitude of cyclic AMP accumulation (Emax). Cells cultured from the thalamic/hypothalamic area had significantly higher Emax values (588 +/- 75% and 572 +/- 63% of basal levels for 5-CT and 5-HT, respectively) compared to brainstem (274 +/- 51% and 318 +/- 46%, respectively) and colliculus astrocytes (244 +/- 15% and 301 +/- 24%, respectively). No significant differences in pEC50 (for either 5-HT or 5-CT) values were observed. 4. Reverse transcriptase-polymerase chain reaction (RT-PCR) with primers specific for the 5-HT7 receptor confirmed expression of messenger RNA for this receptor subtype by the cultured astrocytes derived from all regions investigated. Primers specific for the 5-HT6 receptor also amplified a cDNA fragment from the same samples. 5. From these findings, we conclude that astrocytes cultured from a number of brain regions express functional 5-HT receptors positively coupled to adenylyl cyclase and that the level of receptor expression or the efficiency of receptor coupling is regionally-dependent. The pharmacological profile of the receptor on thalamic/hypothalamic astrocytes suggests that the 5-HT7 receptor is the dominant receptor that is functionally expressed even though astrocyte cultures have the capacity to express both 5-HT6 and 5-HT7 receptor messenger RNA.

The genes encoding the 5HT1D alpha and 5HT1D beta receptors are unchanged in patients with panic disorder

To determine the serotonergic function in panic disorder, sequencing of the genes encoding the 5HT1D alpha and 5HT1D beta receptors was carried out. The genes for the 5HT1D alpha and 5HT1D beta receptors were amplified by polymerase chain reaction and sequenced by the dideoxy method. Some patients have both nucleotides C and T at position 1080 in 5HT1D alpha receptor gene; however, both of them code the same amino acid, asparagine. The 5HT1D beta receptor gene had a substitution from GCA276 to GCG276, not only panic disorder but also in controls; however, this substitution does not change the corresponding amino acid, alanine92.

New players in the 5-HT receptor field: genes and knockouts

In recent years, cloning has revealed the existence of no less than 14 mammalian 5-HT receptors, as well as evidence that isoforms of the 5-HT4 and 5-HT7 receptors are generated by alternative splicing. In addition, molecular biology techniques have provided new tools with which to study the function of 5-HT receptors. For example, new technologies are emerging that will allow the generation of either inducible or tissue-specific knockouts (or both). In this review José Lucas and René Hen focus on the characteristics of the most recently cloned receptors and the contribution of molecular biology to the understanding of 5-HT receptor function.

5-Hydroxytryptamine receptor subtypes in vertebrates and invertebrates

In the last few years, molecular biology has led to the cloning and characterization of several 5-HT receptors (serotonin receptors) in vertebrates and in invertebrates. These studies have allowed identification not only of 5-HT receptors already described but also of novel subtypes. The molecular cloning of 13 different mammalian receptor subtypes revealed an unexpected heterogeneity among 5-HT receptors. Except for the 5-HT3 receptors which are ligand-gated ion channel receptors, all the other 5-HT receptors belong to the large family of receptors interacting with G proteins. Based on their amino acid sequence homology and coupling to second messengers these receptors can be divided into distinct families: the 5-HT1 family contains receptors that are negatively coupled to adenylate cyclase: the 5-HT2 family includes receptors that stimulate phospholipase C; the adenylyl cyclase stimulatory receptors are a heterogeneous group including the 5-HT4 receptor which has not yet been cloned, the Drosophila 5-HTdro1 receptor and two mammalian receptors tentatively named 5-HT6 and 5-HT7 receptors. The 5-HT5A and 5-HT5B receptors might constitute a new family of 5-HT receptors whose effectors are unknown. This review focusses on the molecular characteristics of the cloned 5-HT receptors such as their structure, their effector systems and their distribution within the central nervous system. The existence of a large number of receptors with distinct signalling properties and expression patterns might enable a single substance like 5-HT to generate simultaneously a large panel of effects in many brain structures. The availability of the genes encoding these receptors has already allowed a partial characterization of their structure-function relationship and will probably allow in the future a dissection of the contribution of each of these receptor subtypes to physiology and behaviour.

Characterization of galanin and 5-HT1A receptor coupling to adenylyl cyclase in discrete regions of the rat brain

We have studied the coupling of galanin and 5-HT1A receptors with adenylyl cyclase in the hypothalamus, the entorhinal cortex and the hippocampus of the rat brain. Furthermore, we have evaluated the effects of simultaneous activation of galanin and 5-HT1A receptors on adenylyl cyclase activity. Galanin-(1-29) and galanin-(1-15) showed a dose-dependent inhibitory effect on forskolin-stimulated adenylyl cyclase activity in the hypothalamus and entorhinal cortex. No clear effects were observed in the hippocampus. Neither galanin-(1-29) nor galanin-(1-15) had any effect on the basal activity of adenylyl cyclase in these regions. The selective 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT) induced a dose-dependent inhibition of forskolin stimulated adenylyl cyclase activity in the hippocampus and the entorhinal cortex. 5-HT induced an inhibition in the hypothalamus. In all regions the effects could be fully counteracted by methiothepin. 5-HT was shown to stimulate the basal activity of adenylyl cyclase in the hippocampus and the entorhinal cortex. The effects could be counteracted by methiothepin. When galanin-(1-29) and 5-HT/8-OH-DPAT were incubated simultaneously additive inhibitory effects, but no synergistic interactions, could be observed on the stimulated adenylyl cyclase activity. In conclusion, galanin and 5-HT1A receptors seem to be linked to different independent pools of G proteins, indicating that the previously demonstrated intramembrane interactions between galanin and 5-HT1A receptors involve a mechanism not directly related to adenylyl cyclase.

Serotonin 5-HT1A receptor activation increases cyclic AMP formation in the rat hippocampus in vivo

In vivo microdialysis was used to examine the efflux of cyclic AMP (cAMP) into the extracellular fluid of the ventral hippocampus in the freely moving rat. The changes in extracellular cAMP concentration were monitored in response to forskolin and the serotonin 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The basal level of hippocampal extracellular cAMP was 2.3 +/- 0.2 pmol/ml (n = 6), after a 3-h postsurgery stabilisation period. Perfusion of forskolin (100 microM) through the probe for 30 min significantly increased the efflux of cAMP, which returned to baseline levels within 90 min. 8-OH-DPAT (0.3 mg/kg s.c.) also significantly increased cAMP efflux, whereas a similar volume of saline had no effect. Desensitisation of the 8-OH-DPAT-induced increase in cAMP efflux was observed following a second administration of 8-OH-DPAT after a 4-h interval. Administration of 8-OH-DPAT did not alter the efflux of cAMP when forskolin was perfused through the probe. Pretreatment with WAY 100135 [N-tert-butyl 3-4-(2-methoxyphenyl)piperazine-1-yl-2-phenylpropanamide dihydrochloride] (5 mg/kg s.c.), a specific 5-HT1A receptor antagonist, prevented the 8-OH-DPAT-induced increase in cAMP efflux. The data indicate that the 8-OH-DPAT-induced increase in cAMP efflux in vivo is mediated by a 5-HT1A receptor.

Quantitative autoradiography of 5-HT1E binding sites in rodent brains: effect of lesion of serotonergic neurones

Binding sites corresponding to 5-HT1E receptors were labelled in mouse, rat, and guinea-pig brains by using [3H]5-hydroxytryptamine ([3H]5-HT) in the presence of 5-carboxamidotryptamine (5-CT) (0.1 microM), and their distribution within the brain was studied by quantitative autoradiography. The results obtained with mouse brain show that 5-HT1E binding sites are particularly present in the cortex, caudate-putamen and claustrum, where they showed the highest density. Lower densities were measured in other regions. Saturation experiments showed that the affinity of [3H]5-HT for 5-HT1E binding sites (nanomolar range) was very similar in the different structures. The distribution of 5-HT1E binding sites was similar in rat and guinea-pig brains. In rat brain, selective lesioning of serotonergic fibres by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), a specific 5-HT neurotoxin, did not affect the density of 5-HT1E binding, indicating that these receptors are mainly localized on non-serotonergic neurones.

Molecular cloning, characterization, and localization of a high-affinity serotonin receptor (5-HT7) activating cAMP formation

By using a strategy based on nucleotide sequence homology, we have cloned a cDNA encoding a functional serotonin (5-HT) receptor. The deduced amino acid sequence of the 5-HT7 receptor displays limited homology with that of other 5-HT receptors. In addition to the seven stretches of hydrophobic amino acids that characterize the superfamily of receptors interacting with guanine nucleotide-binding proteins, the 448-aa sequence of the 5-HT7 receptor contains a hydrophobic domain located at its N-terminal end. Genomic analysis indicated the presence of introns interrupting the coding sequence. The 5-HT7 receptor, stably expressed in transfected CHO cells, bound [3H]5-HT with high affinity (Kd = 1 nM), like receptors of the 5-HT1 subfamily from which, however, it was clearly distinguished by its pharmacology. 5-HT in nanomolar concentrations stimulated cAMP accumulation in these CHO cells by approximately 10-fold, whereas lysergic acid diethylamide displayed low intrinsic agonist activity. These various properties differentiate the 5-HT7 receptor from the four other subfamilies of mammalian 5-HT receptors (i.e., the 5-HT1-, 5-HT2-, 5-HT3-, and 5-HT4-like subfamilies) and, therefore, appear to define another receptor subfamily. Northern blot and in situ hybridization analyses showed the 5-HT7 transcripts to be expressed in discrete areas of the limbic brain (e.g., pyramidal hippocampus cells, tenia tecta, amygdaloid, or mammillary nuclei), suggesting that the receptor mediates serotoninergic controls in functions like mood, learning, or neuroendocrine and vegetative behaviors.

Rabies virus selectively alters 5-HT1 receptor subtypes in rat brain

Rabies virus infection in man induces a series of clinical symptoms, some suggesting involvement of the central serotonergic system. The results of the present study show that, 5 days after rabies virus infection in rat, the total reversible high-affinity binding of [3H]5-HT in the hippocampus is not affected, suggesting that 5-HT1A binding is not altered. 5-HT1B sites identified by [125I]cyanopindolol binding are not affected in the cortex 3 and 5 days after the infection. Accordingly, the cellular inhibitory effect of trifluoromethylphenylpiperazine (TFMPP) on the [3H]acetylcholine-evoked release, presumably related to 5-HT1B receptor activity, is not modified 3 days after infection. In contrast, [3H]5-HT binding determined in the presence of drugs masking 5-HT1A, 5-HT1B and 5-HT1C receptors, is markedly (50%) reduced 3 days after the viral infection. These results suggest that 5-HT1D-like receptor subtypes may be affected specifically and at an early stage after rabies viral infection.

Effects of inhibitory and excitatory drugs on the metabolic rhythm of the hamster suprachiasmatic nucleus in vitro

In order to elucidate the role of excitatory and inhibitory transmitters within the suprachiasmatic nucleus (SCN) in the circadian change of 2-deoxyglucose (2-DG) uptake in this nucleus, the effects of 8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), muscimol, flurazepam, pentobarbital and glutamate on uptake of 2-DG by hamster SCN were examined in hypothalamic slice preparations. 2-DG uptake in the SCN was high during the subjective day and low during the subjective night. The high uptake of 2-DG in the SCN during the daytime was inhibited by the superfusion of 8-OH-DPAT, muscimol, flurazepam and pentobarbital in a dose-dependent manner, but the low uptake of 2-DG during the night was unaffected. The low uptake during the night was significantly increased by treatment with glutamate, whereas 2-DG uptake during the day was unaffected. In contrast to the above results, 20 mM KCl and 1 microM tetrodotoxin increased and decreased 2-DG uptake during both the day and night, respectively. The present results strongly suggest that agonists of 5-HT1A receptors and GABAA-benzodiazepine-barbiturate complex receptors regulate the function of the SCN through their inhibitory action on 2-DG uptake during the day, and that glutamate also regulates SCN function through it stimulatory action on 2-DG uptake during the night.

8-hydroxy-2-(di-N-propylamino)tetralin increases the activity of adenylate cyclase in the hippocampus of freely-moving rats

The present study was designed to examine the effects of intraperitoneal (i.p.) administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on the efflux of cyclic adenosine monophosphate (cAMP) in the extracellular fluid of the dorsal hippocampus, using in vivo microdialysis. One week after implantation of the guide, probes were inserted in conscious rats and perfused with Ringer solution. Steady basal levels of cAMP (2.9 +/- 0.1 pmol/ml, n = 74 rats) were obtained after at least three hours of stabilisation. The 8-OH-DPAT dose-dependently increased the basal efflux of cAMP, which was most apparent between 20-40 min after the injection. The largest dose of 8-OH-DPAT (1 mg/kg) tested, induced a maximum response of approximately 50%, whereas injections of saline did not alter the efflux of cAMP. Treatment with (+/-)pindolol (10 mg/kg) did not significantly affect the basal efflux of cAMP, whereas it markedly inhibited the increase in levels of cAMP, induced by 0.5 mg/kg 8-OH-DPAT (injected 40 min later). Simultaneous behavioural observations demonstrated that (+/-)pindolol also attenuated various components of the 8-OH-DPAT-induced behavioural syndrome. Addition of 3-isobutyl-1-methylxanthine (IBMX), forskolin or noradrenaline, to the perfusion fluid, strongly enhanced the levels of cAMP in the extracellular fluid from the hippocampus. Injection of 8-OH-DPAT (1 mg/kg) during perfusion with IBMX induced a similar increase in levels of cAMP, as under normal perfusion conditions. However, 8-OH-DPAT did not significantly alter the efflux of cAMP, when probes were perfused with either forskolin or forskolin and IBMX.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzamide derivatives provide evidence for the involvement of a 5-HT4 receptor type in the mechanism of action of serotonin in frog adrenocortical cells

We have previously shown that serotonin (5-HT) is a potent stimulator of corticosterone and aldosterone secretion by frog adrenocortical cells and we have demonstrated that the action of 5-HT is not mediated by the classical 5-HT receptor subtypes i.e. 5-HT1, 5-HT2 and 5-HT3. Recently, a non-classical 5-HT receptor (termed 5-HT4) has been characterized using 4-amino-5-chloro-2-methoxy-benzamide derivatives as serotonergic agonists. In the present report, we have investigated the possible involvement of the 5-HT4 receptor subtype in the mechanism of action of 5-HT on steroid secretion. Increasing concentrations of benzamide derivatives (zacopride, cisapride and BRL 24924) gave rise to a dose-related stimulation of corticosteroid production, zacopride being the most potent compound of this series to enhance steroidogenesis. Prolonged administration (230 min) of zacopride induced a rapid increase in corticosterone and aldosterone output followed by a gradual decline of corticosteroid secretion. During prolonged exposure of adrenal tissue to zacopride (10(-5) M), the corticotropic activity of 5-HT (10(-6) M) was totally abolished. The stimulatory effects of 5-HT and zacopride were abolished by the non-selective 5-HT3 antagonist ICS 205 930. In contrast methysergide, a 5-HT1 receptor antagonist, and MDL 72222, a selective 5-HT3 antagonist did not block zacopride-induced corticosteroid secretion. Both 5-HT and zacopride induced a dose-related increase in cAMP production by frog adrenal slices. Taken together, these results indicate that the stimulatory effect of 5-HT on frog adrenocortical tissue is mediated by activation of a 5-HT4 receptor subtype positively coupled to adenylate cyclase.

The inhibitory effect of trifluoromethylphenylpiperazine on [3H]acetylcholine release in guinea pig hippocampal synaptosomes is mediated by a 5-hydroxytryptamine1 receptor distinct from 1A, 1B, and 1C subtypes

The effect of the serotonergic receptor agonist 1-(m-trifluoromethylphenyl)piperazine (TFMPP) was studied on the K(+)-evoked [3H]acetylcholine [( 3H]ACh) release from guinea pig hippocampal synaptosomes loaded with [3H]choline. TFMPP (5-1,000 microM) inhibited the evoked ACh release in a dose-dependent manner (IC50 = 81.8 microM). The inhibitory effect of TFMPP was mimicked by CGS-12066B (10, 30, and 100 microM), a 5-hydroxytryptamine1B (5-HT1B)/5-HT1D receptor agonist; 1-(m-chlorophenyl)piperazine (100 microM), a 5-HT1C/5-HT1B receptor agonist; and 5-carboxamidotryptamine (10 microM), a nonselective 5-HT1 receptor agonist. 8-Hydroxy-2-(di-n-propylamino)tetralin (10 and 100 microM), a 5-HT1A receptor agonist, and quipazine (10 and 100 microM), a 5-HT2 receptor agonist, did not have any significant effect. Serotonergic antagonists, such as dihydroergotamine (0.1 and 1 microM), metergoline (0.1 microM), methysergide (0.5 and 1 microM), or yohimbine (1 and 10 microM), blocked the TFMPP effect dose-dependently. In contrast, methiotepine (0.3 and 1 microM), propranolol (1 microM), ketanserin (0.1 microM), mesulergine (0.1 microM), ICS 205930 (0.1 and 1 microM), and spiroperidol (1 and 7 microM) did not affect the TFMPP-induced inhibition of the evoked ACh release. These data suggest that, in guinea pig hippocampus, the K(+)-evoked ACh release is modulated by a 5-HT1 receptor distinct from the 5-HT1A, 5-HT1B, and 5-HT1C subtypes.

Noradrenergic and serotonergic modulation of a hyperpolarization-activated cation current in thalamic relay neurones

1. Modulation of the hyperpolarization-activated cation current, Ih, by noradrenaline (NA) and serotonin (5-HT) was examined in guinea-pig and cat medial and lateral geniculate relay neurones using the in vitro slice technique. 2. In the absence of pharmacological antagonists, local application of NA resulted in a slow depolarization and decrease in apparent input conductance, a response which was blocked by local or bath application of the alpha 1-adrenoceptor antagonist prazosin. Application of NA after pharmacological block of alpha 1- and alpha 2-adrenoceptors, or application of 5-HT in all conditions, induced a 1-3 mV slow depolarization which was associated with a pronounced increase in apparent input conductance. This response to NA and 5-HT persisted during blocked synaptic transmission and was present in both the guinea-pig and cat medial and lateral geniculate nuclei. 3. The increase in membrane conductance elicited by NA was mimicked by the beta-specific agonist isoprenaline and blocked by the beta-antagonists propranolol and atenolol, indicating that it is mediated by beta-adrenoceptors. The response to 5-HT was blocked by the 5-HT1 and 5-HT2 antagonist methysergide, but not by the 5-HT2 antagonist ritanserin. Applications of either the 5-HT1A agonist ipsapirone or the partial agonist 8-hydroxy-dipropylaminotetralin (8-OHDPAT) were without effect. 4. Current versus voltage relationships obtained under voltage clamp revealed NA and 5-HT to cause a voltage-dependent inward shift at membrane potentials negative to approximately -60 mV. This response appeared to be shared by NA and 5-HT since maximal application of 5-HT greatly reduced or abolished the response to NA. 5. Application of NA and/or 5-HT during hyperpolarizing voltage steps in voltage clamp resulted in a marked increase in amplitude of the hyperpolarization-activated cation current, Ih. In addition, the rate of activation of Ih was strongly increased during activation of beta-adrenoceptors. 6. The activation curve of the conductance underlying Ih (Gh) was shifted by 4-6 mV on the voltage axis with NA and/or 5-HT. The positive shift of Gh activation in the voltage domain resulted in an increase in the amplitude of Gh which is active at resting, and more hyperpolarized, membrane potentials. The subsequent increase in resting membrane conductance decreased the responsiveness of thalamic neurones to hyperpolarizations of all durations. 7. Local or bath application of caesium blocked both Ih and the increase in membrane conductance in response to NA and 5-HT. By contrast, barium blocked neither Ih nor the responses to NA and 5-HT.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Hydroxytryptamine receptor agonists influence calcium-stimulated adenylate cyclase activity in the cerebral cortex and hippocampus of the rat

The effects of 5-hydroxytryptamine (5-HT) receptor agonists on calcium (Ca2+)-stimulated adenylate cyclase activity in the hippocampus and cerebral cortex of the rat were studied. In the presence of Ca2+ (1.5 microM), 5-HT dose dependently inhibited adenylate cyclase activity (EC50 = 10 +/- 2 nM). The inhibitory effect of 5-HT on Ca2(+)-stimulated adenylate cyclase was antagonized by spiperone (KB = 2 +/- 0.8 nM). The rank order of potency of 5-HT agonists to inhibit Ca2(+)-stimulated adenylate cyclase in the hippocampus was: 5-carboxamidotryptamine (5-CT) greater than 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) greater than 5-hydroxytryptamine (5-HT) = 5-methoxytryptamine (5-OCH3-T) greater than trifluoromethylphenylpiperazine (TFMPP) greater than m-chlorophenylpiperazine (mCPP). 2-Methyl-5-hydroxytryptamine (2-CH3-5-HT) did not exert an effect on Ca2(+)-stimulated enzyme activity. In the cerebral cortex 5-HT exerted a biphasic stimulatory effect on adenylate cyclase activity in the absence of Ca2+ (EC50 = 0.2 +/- 0.04 nM and 10 +/- 3 microM), whereas 8-OH-DPAT, 5-CT and 2-CH3-5-HT exerted a monophasic effect. In the presence of Ca2+ (1.5 microM), low concentrations of 5-HT, 8-OH-DPAT, 5-CT and 2-CH3-5-HT potentiated adenylate cyclase activity, whereas higher concentrations, except 2-CH3-5-HT, inhibited the enzyme activity. We propose that the 5-HT receptor mediating inhibition of Ca2(+)-stimulated adenylate cyclase in the rat hippocampus corresponds to the 5-HT1A subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical profile of eltoprazine

In this paper we present the neurochemical profile of eltoprazine, a drug that specifically inhibits offensive aggression. Eltoprazine interacts selectively with serotonin (5-HT) receptor subtypes (Ki-values for 5-HT1A, 5-HT1B and 5-HT1C receptors are 40, 52 and 81 nM respectively). Affinity for other neurotransmitter receptors is much lower (Ki-values greater than 400 nM) than for 5-HT1 receptors. The selective interaction with 5-HT1 receptor subtypes is confirmed by in vitro autoradiographic studies using radiolabelled eltoprazine. The overall distribution of [3H]eltoprazine bears a strong resemblance to the localization of 5-HT1 binding sites labelled by [3H]5-HT, although some differences are observed. Eltoprazine (1 microM) inhibits the forskolin stimulated c-AMP production in hippocampus slices of the rat, indicating an agonistic action on the 5-HT1A receptor. The K+ stimulated release of 5-HT from rat cortex slices is inhibited by eltoprazine (pD2 = 7.8). The maximal response, however, was clearly less than that of the full agonist 5-HT, indicating partial agonistic activity on the 5-HT1B receptor (alpha = 0.5). Eltoprazine has a weak antagonistic action (IC50 = 7 microM) on the 5-HT1C receptor as revealed by inhibition of the 5-HT-induced accumulation of inositol phosphates in the choroid plexus of the pig. In vivo, eltoprazine reduces 5-HIAA levels in the striatum, without affecting the 5-HT levels. Eltoprazine also reduces the 5-HT synthesis rate as shown by 5-HTP accumulation after decarboxylase inhibition. These data indicate that eltoprazine acts as a 5-HT agonist in vivo in a dose range that affects aggressive behaviour (0.3-3 mg/kg p.o.). Taken together from a variety of neurochemical studies there is strong evidence both in vitro and in vivo that the pharmacological actions of eltoprazine can be attributed to an interaction with the 5-HT system, most probably via a (partial) agonistic action on 5-HT1A and 5-HT1B receptors.

Postnatal development of 5-HT1 receptors: [3H]5-HT binding sites and 5-HT induced adenylate cyclase activations in rat brain cortex

The postnatal development of the 5-HT1 receptor system was studied in young rat brain cortex from birth to adulthood (14 successive ages). The high-affinity binding of [3H]5-HT was low at birth but developed markedly between the 8th and the 15th day postnatally. The basal adenylate cyclase activity produced 50 pmoles cAMP/mg protein/min at birth and increased from the 8th to the 15th day. 5-HT could stimulate the adenylate cyclase activity in adult rat brain cortex with two different affinity constants: Km = 1 nM and Km = 0.5 microM; these low- and high-affinity constants presumably correspond to 5-HT1A and 5-HT1non-A.non-B.non-C (5-HT1D) respectively. These two activities developed parallelly from the 14-15th to the 28th day. The 8-hydroxy-2-(di-n-propylamino-tetralin) (8-OH-DPAT)-induced activity described a curve similar to the one that corresponded to 10 microM 5-HT. These results establish that 5-HT1A and 5-HT1non-A.non-B.non-C receptors mainly develop during the synaptogenesis.