5-HT1C Sites Research Articles

Differential effects of novel ligands for 5-HT receptor subtypes on nonopioid defensive analgesia in male mice

The effects of a number of 5-HT receptor ligands were examined on nonopioid defensive analgesia in male DBA/2 mice. MDL 73005EF (0.05-1.0 mg/kg), a selective 5-HT1A receptor agonist, potently and dose-dependently inhibited the analgesic consequences of social defeat. CGS 12066B (0.5-10.0 mg/kg) and MK-212 (0.3-10.0 mg/kg), selective agonists for 5-HT1B and 5-HT1C sites, respectively, failed to influence this particular form of adaptive pain inhibition. Two 5-HT2/1C receptor antagonists, ritanserin (0.05-10.0 mg/kg) and ICI 169.369 (0.3-10.0 mg/kg), were also devoid of specific effects upon defensive analgesia. Both ritanserin and ICI 169,369 were found to have intrinsic analgetic efficacy and to induce behavioural changes indicative of increased defensiveness. These data, together with previous findings, confirm the specific involvement of 5-HT1A receptor mechanisms in the analgesic consequences of social defeat in male mice. Results are discussed in relation to the role of anxiety in adaptive pain inhibition.

Serotonergic modulation of the rat pup ultrasonic isolation call: studies with 5HT1 and 5HT2 subtype-selective agonists and antagonists

A modulatory role for serotonin has been described for the development and expression of the ultrasonic call of infant rat pups during brief maternal separations. In previous studies, serotonin reuptake inhibitors selectively reduced the rate of calling following acute administration to 9-11-day-old pups and a serotonin neurotoxin (MDMA) systematically disrupted the development of ultrasonic vocalizations but not other measures of motor development. In the current studies, we extended our investigations to include drugs with purported receptor subtype selectivities. Consistent with previous reports, acute administration of 5HT1A agonists buspirone and 8-OH-DPAT [+/-)-8-hydroxy-2-(di-N-propylamino)tetralin) reduced the rate of calling at doses which did not affect motor activity or core body temperature. The rate reducing effects of buspirone persisted up to 1 but not 2 h after injection. Administration of purported 5HT1B receptor agonists, CGS12066B (7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a] quinoxaline) and TFMPP (1-[3-fluoromethyl)phenyl]-piperazine) increased the rate of calling depending on the specificity of the drug for the 5HT1B receptor. d,l-Propranolol, a 5HT1 receptor antagonist, blocked the effects of both 8-OH-DPAT and TFMPP. m-CPP (1-(3-chlorophenyl)piperazine) and DOI [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane), drugs with putative actions at 5HT1C and 5HT2 receptor sites both decreased calling but differed according to their effects on motor activity. Ritanserin, a 5HT2 and 5HT1C antagonist, produced a dose-related increase in call rate. A dose of ritanserin with no apparent intrinsic effects effectively antagonized DOI rate reducing effects but potentiated the rate reducing effects of m-CPP.(ABSTRACT TRUNCATED AT 250 WORDS)

CP-93,129, a potent and selective 5-HT1B receptor agonist blocks neurogenic plasma extravasation within rat but not guinea-pig dura mater.

Pretreatment with CP-93,129 blocked plasma extravasation in rat dura mater induced by electrical trigeminal ganglion stimulation when administered at greater than or equal to 140 nmol kg-1, i.v. but did not affect plasma leakage in guinea-pig at 460 or 1400 nmol kg-1. Sumatriptan, a 5-HT1D-like receptor agonist, blocked plasma extravasation in the guinea-pig model when administered at 7 nmol kg-1. In as much as CP-93,129 binds with micromolar affinities to 5-HT1A, 5-HT1C, 5-HT1D, and 5-HT2 recognition sites, and with nanomolar affinity to the 5-HT1B receptor subtype, blockade of plasma extravasation in the rat dura mater may be mediated by 5-HT1B receptors whereas the 5-HT1D receptor may be more relevant to the guinea-pig.

Serotonin receptors: Clinical implications

Over the past decade, a variety of serotonin (5-hydroxytryptamine, 5-HT) receptor/binding sites have been identified. These include 5-HT1, 5-HT2, and 5-HT3 sites. The 5-HT1 sites have been further divided into 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D and 5-HT1E sites. It would be of interest to identify those pharmacological effects that are specifically mediated by a particular population of 5-HT sites and, indeed, attempts have been made to do this almost since the initial discovery of multiple populations of sites. Unfortunately, much of the early work made use of serotonergic agents that are now known to be somewhat less selective than originally suspected. Nevertheless, there is ample information in the literature suggesting that site-selective serotonergic agents may ultimately lead (and, in some cases, has already led) to the development of therapeutically-useful agents. The present review examines the pharmacological effects that are thought to be related to the individual types of 5-HT sites and provides some clinical implications for agents that act at these sites.

5-HT1 and 5-HT2 binding profiles of the serotonergic agents .alpha.-methylserotonin and 2-methylserotonin

alpha-Methyl-5-hydroxytryptamine (alpha-Me-5-HT; 2) and 2-methyl-5-hydroxytryptamine (2-Me-5-HT; 3) are considered to be 5-HT2-selective and 5-HT3-selective agents, respectively. These agents were synthesized and examined at serotonin (5-HT) binding sites because there is relatively little documentation as to their selectivity and because they have not been previously examined at the newly discovered 5-HT1D and 5-HT1E sites. As previously reported, 2-Me-5-HT possesses a low affinity (Ki greater than 500 nM) for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT2 sites; this agent also displays a low affinity for 5-HT1D (Ki = 1220 nM) and 5-HT1E (Ki greater than 10,000 nM) sites. However, alpha-Me-5-HT displays little selectivity for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D sites (Ki = 42, 85, 150, and 150 nM, respectively) and a very low affinity for 5-HT1E (Ki greater than 10,000 nM) sites. Depending upon the radioligand used to label the sites, alpha-Me-5-HT displays either a low affinity (Ki = 880 nM with [3H]ketanserin) or a high affinity (Ki = 3 nM with [3H]DOB) for 5-HT2 sites. These results suggest that alpha-Me-5-HT is not as selective as previously considered and that caution should be used when employing this agent in pharmacological studies because it may act as mixed 5-HT1/5-HT2 agonist.

5.HT1 receptors in the vertebrate brain

The regional distribution of high affinity [3H]5-HT recognition sites in the brain of several vertebrates (pigeon, rat, mouse, guinea-pig, cat, dog, monkey and human) was analyzed using in vitro autoradiography. The presence of subtypes of 5-HT1 binding sites was investigated by selective displacements with 8-OH-DPAT, mesulergine and (+/-)SDZ 21-009 at appropriate concentrations to block 5-HT1A, 5-HT1C and 5-HT1B sites respectively. In addition, 5-HT1A and 5-HT1C sites were directly visualized with the more selective radioligands [3H]8-OH-DPAT and [3H]mesulergine, respectively. In the pigeon brain, total [3H]5-HT binding sites were enriched in all telencephalic areas. Densely labelled regions were also present in the optic tectum and the brainstem. No binding was observed in the cerebellum. 8-OH-DPAT and mesulergine only displaced a small proportion of [3H]5-HT binding in most of the areas where high concentrations of 5-HT1 sites were found. (+/-)SDZ 21-009 did not affect [3H]5-HT binding in the regions examined. Taking into account our pharmacological studies, these results suggest that the majority of 5-HT1 sites belong to the 5-HT1D subtype in the pigeon brain. In the mammalian species investigated high levels of [3H]5-HT binding were found in the neo-cortex, hippocampal formation, basal ganglia and related structures (substantia nigra), raphe dorsalis, nucleus superior colliculus and choroid plexus. However, these brain areas were differentially enriched in subtypes of 5-HT1 recognition sites.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT1D receptors in guinea-pig and pigeon brain

The characteristics of high affinity [3H]5-HT (5-hydroxytryptamine) binding to non 5-HT1A non 5-HT1C sites were examined in crude membranes prepared from different regions of guinea-pig and pigeon brains. The coupling of these sites to adenylate cyclase was examined, and its pharmacological profile investigated. In the presence of 100 nmol/l 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) and 100 nmol/l mesulergine, [3H]5-HT labelled with nanomolar affinity an apparently homogeneous population of recognition sites in guinea-pig and pigeon brain membranes. The rank order of affinities of agonists and antagonists (5-CT (5-carboxamidotryptamine) greater than 5-HT greater than RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydro-4- pyridinyl)-1H indole succinate) greater than yohimbine greater than or equal to rauwolscine greater than DP-5-CT (N,N dipropyl-5-carboxamidotryptamine) greater than or equal to mianserin greater than 8-OH-DPAT greater than mesulergine greater than SDZ 21-009 ((+/-)-4(3-tert-butyl-amino-2-hydroxypropoxy)-indol-2 carbonic acid isopropyl ester) greater than (-)propranolol), as well as their individual pKD values, were very similar to those at porcine caudate 5-HT1D sites and clearly different from those at rat cortex 5-HT1B sites. In the substantia nigra of the guinea-pig the 5-HT receptor-mediated inhibition of forskolin-stimulated adenylate cyclase had a pharmacological profile fully comparable to that of 5-HT1D binding sites (5-CT greater than 5-HT greater than yohimbine greater than RU 24969 greater than 8-OH-DPAT greater than SDZ 21-009 = isamoltane greater than (-)pindolol greater than (-)propranolol).(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacological properties of the presynaptic serotonin autoreceptor in the pig brain cortex conform to the 5-HT1D receptor subtype.

The effects of serotonin receptor agonists and antagonists on the electrically (3 Hz) evoked 3H overflow were determined on pig brain cortex slices preincubated with 3H-serotonin and superfused with physiological salt solution containing indalpine (an inhibitor of serotonin uptake) plus phentolamine. The potencies of the serotonin receptor agonists and antagonists were compared with their affinities for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D binding sites in pig or rat tissue membranes; in addition, the potencies of the agonists were compared to their potencies in inhibiting adenylate cyclase activity in membranes of calf substantia nigra. In the superfusion experiments on pig brain cortex slices the following rank orders of potencies were obtained: agonists, serotonin greater than 5-methoxytryptamine = 5-carboxamidotryptamine greater than RU 24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole) greater than SDZ 21009 (4(3-terbutylamino-2-hydroxypropoxy)indol-2-carbonic-acid-isopr opylester) greater than or equal to yohimbine greater than or equal to cyanopindolol greater than 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) greater than or equal to CGS 12066 B (7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline); ipsapirone and urapidil were ineffective; antagonists (antagonism determined against 5-methoxytryptamine as an agonist), metitepine greater than metergoline greater than mianserin. Propranolol, spiperone or mesulergine did not produce a shift of the concentration-response curve for 5-methoxytryptamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative Neuropharmacology of Dihydroergotamine and Sumatriptan (GR 43175)

The potency of dihydroergotamine (DHE) was determined at 13 neurotransmitter receptors using radioligand binding techniques. DHE is a potent agent (i.e. affinity value less than 100 nM) at 5-hydroxytryptamine1A (5-HT1A), 5-HT1C, 5-HT1D, 5-HT2, dopamine2, alpha1- and alpha2-adrenergic binding sites. DHE displays moderate affinity (i.e. affinity values = 100 - 1000 nM) for beta-adrenergic and dopamine1 sites and is completely inactive at 5-HT3, muscarinic and benzodiazepine receptors. These results were then compared to similar data on sumatriptan (formerly called GR 43175), a novel acute anti-migraine agent. The only pharmacological property shared by both agents is high affinity for 5-HT1D and 5-HT1A receptors. Therefore, these data suggest that 5-HT1D and/or 5-HT1A receptors may play an important role in the pathophysiology of migraine.

5-HT1C receptor-mediated stimulation of inositol phosphate production in pig choroid plexus. A pharmacological characterization.

1) 5-HT (5-hydroxytryptamine, serotonin) induces inositol phosphate production in a pig choroid plexus preparation. This effect has been pharmacologically characterized and the data compared to those obtained from radioligand binding studies performed with [3H]mesulergine to 5-HT1C sites in pig choroid plexus membranes. 2) The rank order of potency of agonists stimulating inositol phosphate production was: alpha-methyl-5-HT greater than 1-methyl-5-HT greater than DOI greater than bufotenine = SKF 83566 = 5-HT greater than 5-MeO-DMT greater than 5-MeOT = RU 24969 greater than SCH 23390 greater than 5-CT. 8-OH-DPAT was virtually devoid of activity at 100 mumol/l. 3) The increase in inositol phosphate production induced by 5-HT and other agonists was surmountably antagonised by mesulergine, ketanserin and spiperone with pKB values of 8.7, 6.7 and 5.3, respectively. 4) The rank order of potency of antagonists was: metergoline greater than mesulergine greater than LY 53857 greater than ritanserin greater than methiothepin greater than mianserin greater than cyproheptadine greater than pirenperone greater than cinanserin greater than ketanserin greater than spiperone. The following antagonists were virtually devoid of activity at 100 mumol/l; pindolol, 21-009 and yohimbine. 5) The results obtained both with agonists and antagonists strongly support the view that 5-HT1C receptors mediate agonist induced production of inositol phosphates in pig choroid plexus.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic Effects of Imipramine and Lithium on 5-HT Receptor Subtypes in Rat Frontal Cortex, Hippocampus and Choroid Plexus: Quantitative Receptor Autoradiographic Analysis

The effects of chronic treatment with imipramine or lithium on serotonin (5-HT) receptor subtypes were analyzed in the frontal cortex, hippocampus and choroid plexus of rat brain by quantitative receptor autoradiographic procedures, using radioligands [3H]-5-HT, [3H]-8-hydroxy-2-(di-n-propylamine) tetralin ([3H]-8-OH-DPAT), [125l]-iodocyanopindolol ([125I]-CYP), [3H]-mesulergine and [1251]-7-amino-8-iodo-ketanserin ([1251]-ketanserin) or [3H]-spiperone. Chronic i.p. administration of imipramine (20 mg/kg/day for 21 days) decreased the densities of 5-HT1, 5-HT1A, 5-HT1c and 5-HT2 sites in the frontal cortex, hippocampus and choroid plexus. Lithium (2 mEq/kg/day for 21 days) also decreased the densities of 5-HT1, 5-HT1C and 5-HT2 sites in the frontal cortex, and the densities of those including 5-HT1A sites in the hippocampus and choroid plexus. Imipramine and lithium very markedly decreased the density of 5-HT1c sites in the choroid plexus. We propose that methods employing quantitative receptor autoradiographic analysis can be used to characterize and understand the local effects of these drugs on 5-HT receptor subtypes.

Chronic effects of imipramine and lithium on 5-HT receptor subtypes in rat frontal cortex, hippocampus and choroid plexus: quantitative receptor autoradiographic analysis.

The effects of chronic treatment with imipramine or lithium on serotonin (5-HT) receptor subtypes were analyzed in the frontal cortex, hippocampus and choroid plexus of rat brain by quantitative receptor autoradiographic procedures, using radioligands [3H]-5-HT, [3H]-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]-8-OH-DPAT), [125I]-iodocyanopindolol ([125I]-CYP), [3H]-mesulergine and [125I]-7-amino-8-iodo-ketanserin ([125I]-ketanserin) or [3H]-spiperone. Chronic i.p. administration of imipramine (20 mg/kg/day for 21 days) decreased the densities of 5-HT1, 5-HT1A, 5-HT1C and 5-HT2 sites in the frontal cortex, hippocampus and choroid plexus. Lithium (2 mEq/kg/day for 21 days) also decreased the densities of 5-HT1, 5-HT1C and 5-HT2 sites in the frontal cortex, and the densities of those including 5-HT1A sites in the hippocampus and choroid plexus. Imipramine and lithium very markedly decreased the density of 5-HT1C sites in the choroid plexus. We propose that methods employing quantitative receptor autoradiographic analysis can be used to characterize and understand the local effects of these drugs on 5-HT receptor subtypes.

Detection and characterization of a 5HT1D serotonin receptor‐gtp binding protein interaction in porcine and bovine brain

Radioligand binding studies were performed to characterize serotonin 5HT1D binding sites in porcine and bovine brain. 3H-5HT binding, in the presence of 1 microM (+/-)pindolol (to block 5HT1A and 5HT1B receptors) and 100 nM mesulergine (to block 5HT1C receptors), was specific, saturable, and of high affinity. In porcine and bovine cortex and striatum the majority of 5HT1 sites (80%-90%) were of the 5HT1D subtype. In competition experiments 8-OH-DPAT, TFMPP, mesulergine, DOB, and ICS 205-930 had low affinity for 3H-5HT-labeled 5HT1D sites, indicating that the pharmacology of the 5HT1D site is distinct from previously identified 5HT1A, 5HT1B, 5HT1C, 5HT2, and 5HT3 sites. Guanyl nucleotides, GTPgammaS, and Gpp(NH)p, and divalent cations potently modulated the binding of 3H-5HT to 5HT1D sites in porcine and bovine striatum. Mg++ ions increased the number and affinity of 3H-5HT-labeled 5HT1D sites, while guanyl nucleotides decreased the number of 3H-5HT-labeled 5HT1D sites. These results demonstrate the presence of the 5HT1D receptors in porcine striatum and bovine cortex and provide direct demonstration that the radioligand binding assay for the 5HT1D receptor can monitor the interaction of this receptor with a GTP-binding protein.

Evidence that hypophagia induced by mCPP and TFMPP requires 5-HT1C and 5-HT1B receptors; hypophagia induced by RU 24969 only requires 5-HT1B receptors.

Male Sprague-Dawley rats deprived of food for 18 h were injected with the 5-HT agonists RU 24969, 1-(3-chlorophenyl)piperazine (mCPP) or 1-[3-(trifluoromethyl)phenyl)]piperazine (TFMPP) and 20 min later presented with their normal diet. Food intake was determined 1, 2 and 4 h later. All three drugs reduced intake over 1 and 2 h. Three out of four drugs with high affinity for 5-HT1C receptors (metergoline, mianserin, and mesulergine but not cyproheptadine) opposed hypophagia caused by mCPP. Another drug reported to have high affinity for the 5-HT1C site, 1-naphthyl-piperazine (1-NP), also blocked the hypophagic response to mCPP at doses which attenuated mCPP-induced hypolocomotion. Only one of the above drugs (metergoline) which also has high affinity for other 5-HT sites opposed hypophagia caused by RU 24969. Two out of three 5-HT1B receptor antagonists [(+/-) cyanopindolol, (-) propranolol, but not (-) pindolol)] which oppose hypophagia caused by RU 24969 (Kennett et al. 1987) also opposed hypophagia caused by mCPP. The 5-HT2 antagonists ketanserin and ritanserin, the 5-HT3 antagonist ICS 205-930 and the alpha 2 adrenoceptor antagonist idazoxan did not oppose the hypophagic effect of mCPP. In agreement with results for mCPP, hypophagia caused by TFMPP was opposed by both, mianserin and (+/-) cyanopindolol. Given alone, mianserin 1-NP and cyproheptadine but not ICS 205-930 increased food consumption of normally fed rats. The results suggest that RU 24969-induced hypophagia depends on 5-HT1B receptors but not on 5-HT1C receptors, while mCPP (and TFMPP)-induced hypophagia may depend on both receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular pharmacology of 5-HT1D recognition sites: radioligand binding studies in human, pig and calf brain membranes.

1) The binding characteristics of [3H]5-HT (5-hydroxytryptamine, serotonin) were investigated in membrane preparations of several regions from calf, pig and human brain in the presence of 100 nmol/l 8-OH-DPAT (8-hydroxy-2[di-n-dipropylamino]tetralin) and 100 nmol/l mesulergine in order to mask 5-HT1A and 5-HT1C sites. 2) [3H]5-HT bound rapidly, reversibly and stereo-selectively to a population of high affinity recognition sites in membranes from pig caudate, calf caudate and human cortex, caudate and substantia nigra. 3) Saturation experiments carried out with [3H]5-HT in the presence of 100 nmol/l 8-OH-DPAT and 100 nmol/l mesulergine revealed that non-5-HT1A non-5-HT1C sites represented from 50 to more than 90% of the total 5-HT1 sites (determined with [3H]5-HT in the absence of 8-OH-DPAT and mesulergine), depending on the tissue source. 4) The pharmacological profile of these sites was characterized in competition experiments performed with a variety of ligands in membranes of calf, pig and human caudate membranes. Under these conditions, [3H]5-HT labelled a population of "5-HT1-like" sites which display nanomolar affinity for tryptamines (5-carboxamidotryptamine greater than 5-HT greater than or equal to 5-methoxytryptamine greater than tryptamine) and some ergolines (metergoline greater than methysergide). In contrast, these sites showed low affinity for drugs with high affinity and/or selectivity for 5-HT1A (8-OH-DPAT, buspirone), 5-HT1B (21-009, RU 24969), 5-HT1C (mesulergine, mianserin) and 5-HT2 sites (ketanserin, cinanserin).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of isamoltane (CGP 361A), an anxiolytic phenoxypropanolamine derivative, with 5-HT1 receptor subtypes in the rat brain.

Isamoltane (CGP 361A; (1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol hydrochloride), a beta-adrenoceptor ligand (IC50 = 8.4 nmol/l) which has reported activity as an anxiolytic in man was found to be a reasonably active inhibitor of the binding of [125I]ICYP to 5-HT1B recognition sites in rat brain membranes with 27-fold selectivity (IC50 = 39 nmol/l) as compared to the inhibition of binding of [3H]8-OH-DPAT to 5-HT1A receptors (IC50 = 1070 nmol/l). This selectivity was considerably greater than that observed for other beta-adrenoceptor ligands including propranolol (5-HT1A/5-HT1B ratio = 2), oxpenolol (3.5) and cyanopindolol (8.7). The 5-HT1B activity of the compound resided in the (-)-enantiomer. (-)-Isamoltane had weak activity (IC50 3-10 mumol/l) at 5-HT2 and alpha 1-adrenoceptors. The compound was devoid of activity at a number of other central neurotransmitter recognition sites including the 5-HT1C site. Isamoltane increased the electrically evoked release of [3H]5-HT from prelabeled rat cortical slices in a manner similar to that of cyanopindolol. While both compounds were similar in potency to methiothepin, they had lower efficacy. Oxprenolol was less potent that both isamoltane and cyanopindolol while propranolol was essentially inactive. The effects of the compounds on 5-HT release appeared to be correlated with their 5-HT1B rather than 5-HT1A activity. In vivo, isamoltane increased 5-HTP accumulation in rat cortex following central decarboxylase inhibition at doses of 1 and 3 mg/kg i.p. At higher doses this effect was gradually diminished. Similar, but less clearcut results were obtained with cyanopindolol and oxprenolol, but propranolol was ineffective. No changes in brain tryptophan levels were associated with the isamoltane-evoked changes in brain 5-HTP levels. In reserpinized animals, isamoltane reduced 5-HTP accumulation even at doses which enhanced accumulation of this metabolite when given alone. The effects of the putative 5-HT1B agonist, m-trifluoromethylphenylpiperazine (TFMPP), the mixed 5-HT autoreceptor agonist/antagonist/beta-adrenoceptor antagonist, pindolol, the 5-HT uptake inhibitor, CGP 6085A and the MAO-A inhibitor, brofaromine, were not antagonized by pretreatment with isamoltane. The possibility that isamoltane and the other beta-adrenoceptor antagonists are antagonists at 5-HT1B receptors and that their effect on 5-HT synthesis in vivo is the net result of their agonist/antagonist effects at 5-HT1A and 5-HT1B receptors is discussed in relation to the potential mechanism of the anxiolytic activity of isamoltane.

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

5-HT binding sites of the 5-HT1 type are heterogeneous and appear to comprise several subtypes (5-HT1A, 5-HT1B and 5-HT1C); their physiological role is as yet unclear. The stimulation of adenylate cyclase induced by 5-HT has been investigated in membrane fractions prepared from rat brain cortex. Enzymatic activity was determined by measuring cAMP production with an HPLC technique. It was shown that 5-HT stimulates adenylate cyclase activity with 2 activation constants (Kact): one shows a high apparent affinity (Kact = 0.8 nM) and the other a lower apparent affinity (Kact = 0.30 microM). The latter activity, induced by micromolar concentrations of 5-HT, was inhibited by spiperone at concentrations that block 5-HT1A binding. 5-Methoxytryptamine, bufotenin, and LSD also had a stimulatory biphasic effect on adenylate cyclase activity, whereas trifluoromethylphenylpiperazine, 5-carboxyamidotryptamine, 8-hydroxy-(2-di-n-propylamino)tetralin, RU 24969 had a monophasic effect. Enzyme activation by drugs acting in the micromolar range was inhibited by spiperone (1 microM), suggesting a link between this activation and 5-HT1A sites. On the other hand, the high-affinity activation of the enzyme induced by 5-HT, 5-methoxytryptamine, bufotenin, LSD, and the activation induced by TFMPP were not inhibited by spiperone (1 microM), by propranolol (3 microM), or by mesulergine (0.1 microM), which selectively block 5-HT1A, 5-HT1B, and 5-HT1C sites. Inhibition was produced by dihydroergotamine, methysergide, cinanserin, and mianserin, but not by naloxone, phenoxybenzamine, and phentolamine. Therefore, these activations seem related to 5-HT1 receptors but not to 5-HT1A, 5-HT1B, or 5-HT1C sites. Accordingly, binding of [3H]5-HT to 5-HT1-like sites was examined in the presence of spiperone (1 microM) and propranolol (3 microM); in these conditions, a high-affinity site (KD = 3.4 nM) was indeed revealed. The relative potencies of a series of drugs that stimulate or inhibit the activation of the adenylate cyclase with a high affinity and their ability to inhibit this binding of [3H]5-HT showed a positive correlation, strongly suggesting a direct relation between this recognition site for 5-HT and the production of a second messenger (cAMP). Moreover, this potential receptor is shown to be heterogeneously distributed within the brain, and was localized postsynaptically at serotonergic synapses.

Evidence that mCPP may have behavioural effects mediated by central 5-HT1C receptors.

1. The effects of 1-(3-chlorophenyl)piperazine (mCPP) and 1-[3-(trifluoromethyl)phenyl] piperazine (TFMPP) on activity of rats in a novel cage, and on the rotorod and elevated bar co-ordination tests was examined. 2. Peripherally administered mCPP and TFMPP dose-dependently reduced locomotion, rearing, and feeding scores but not grooming of freely fed rats placed in a novel observation cage. Yawning behaviour was increased. Similar effects were also observed after injection of mCPP into the 3rd ventricle. 3. Co-ordination on a rotating drum of both untrained and trained rats was impaired following mCPP but co-ordination on an elevated bar was not. 4. The hypoactivity induced by mCPP was opposed by three antagonists with high affinity for the 5-hydroxytryptamine (5-HT1C) site; metergoline, mianserin, cyproheptadine and possibly also by a fourth antagonist mesulergine. Metergoline, mianserin and cyproheptadine also opposed the reduction in feeding scores. However, neither effect of mCPP was antagonized by the 5-HT2-receptor antagonists ketanserin or ritanserin, the 5-HT3-receptor antagonist ICS 205-930, the 5-HT1A and 5-HT1B-receptor antagonists (-)-pindolol, (-)-propranolol and (+/-)-cyanopindolol or the 5-HT1A-, 5-HT2- and dopamine receptor antagonist spiperone. The specific alpha 2-adrenoceptor antagonist idazoxan was also without effect. 5. Hypoactivity induced by TFMPP was similarly antagonized by mianserin but unaffected by (+/-)-cyanopindolol. 6. These results suggest that the hypoactivity is mediated by central 5-HT1C-receptors and that mCPP and possibly TFMPP may be 5-HT1C-receptor agonists. 7. As mianserin, cyproheptadine and mesulergine in the absence of mCPP did not increase locomotion but increased the number of feeding scores, the activation of 5-HT1C-receptors may be of physiological importance in the control of appetite. The possible relevance of these results to the therapeutic and side-effects of clinically used antidepressants (particularly trazodone and mianserin) and anorexigenic drugs is discussed.

Inhibition of noradrenaline release via presynaptic 5-HT1B receptors of the rat vena cava.

In the rat inferior vena cava preincubated with 3H-noradrenaline, the effects of nine serotonin (5-HT) receptor agonists and of eight antagonists (including two beta-adrenoceptor blocking agents) on the electrically evoked 3H overflow were determined. 1. 5-HT, 5-carboxamido-tryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969), 5-methoxytryptamine, N,N-dimethyl-5HT, tryptamine and 5-aminotryptamine inhibited the evoked 3H overflow. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B binding sites, but not with their affinities for 5-HT1A, 5-HT1C or 5-HT2 binding sites. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and ipsapirone, a partial agonist at these receptors, did not inhibit overflow. 2. Cyanopindolol facilitated the evoked 3H overflow, an effect which was abolished by propranolol. The maximum inhibition of overflow obtainable with 5-HT was diminished by cyanopindolol. 3. The concentration-response curve for 5-HT was shifted to the right by metitepine, metergoline, quipazine, 6-chloro-2-(1-piperazinyl)pyrazine (MK 212) and propranolol which, given alone, did not affect 3H overflow. The apparent pA2 values of these antagonists tended to be correlated with their affinities for 5-HT1B (but not 5-HT1A, 5-HT1C or 5-HT2) binding sites. Ketanserin, a 5-HT2 receptor antagonist, and spiperone, which blocks 5-HT2 and 5-HT1A but not 5-HT1B or 5-HT1C receptors, failed to antagonize the effect of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Central serotonin receptors: effector systems, physiological roles and regulation.

Radioligand binding studies have revealed four distinct serotonin (5HT) binding sites in rat brain that are thought to function as 5HT receptors. These include the 5HT-1a, 5HT-1b, 5HT-1c, and 5HT-2 binding sites. Studies have shown that the 5HT-2 binding site mediates a number of effects of 5HT agonists and serves as a 5HT receptor in neuronal and non-neuronal tissues. The 5HT-2 site employs phosphoinositide hydrolysis for signal transduction. The 5HT-1c binding site is also a functional receptor that is linked to phosphoinositide hydrolysis. However, the physiological role of the 5HT-1c receptor is not yet known. Lack of appropriate pharmacological tools for probing the 5HT-1a and 5HT-1b binding sites has made it difficult to definitively determine whether these binding sites are coupled to biochemical effector systems or mediate any of the physiological responses to 5HT agonists. However, there is some evidence that the 5HT-1a site is coupled to adenylate cyclase, and a number of functional roles for the 5HT-1a and 5HT-1b sites have been proposed.