Neuropharmacological reassessment of the discriminative stimulus properties ofd-lysergic acid diethylamide (LSD)

The neuropharmacological mechanisms underlying the behavioral effects ofd-lysergic acid diethylamide (LSD) were assessed by comparing the discriminative stimulus properties of LSD with those of agonists and antagonists that act selectively at putative serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes (5-HT₁ and 5-HT₂). Male Sprague-Dawley rats (N=23) were trained to discriminate LSD (0.08 mg/kg) from saline and given substitution tests with the following agents: 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT; 0.02–0.64 mg/kg), Ru 24969 (0.2–3.2 mg/kg),m-chlorophenylpiperazine (MCPP; 0.1–1.6 mg/kg), 1-(m-trifluoromethylphenyl)piperazine (TFMPP; 0.1–1.6 mg/kg), and quipazine (0.2–3.2 mg/kg). Only quipazine mimicked LSD. In combination tests, BC 105 (0.2–3.2 mg/kg), 2-bromolysergic acid diethylamide (BOL; 0.1–1.6 mg/kg), Ly 53857 (0.4–3.2 mg/kg), metergoline (0.05–0.8 mg/kg), ketanserin (0.2–3.2 mg/kg), and pipenperone (0.0025–0.08 mg/kg), all of which act as 5-HT₂ antagonists, blocked the LSD cue; only spiperone (0.02–0.32 mg/kg) was without effect. Although commonalities may exist among 5-HT agonists, the present results demonstrate that such agonists are not identical. Since putative 5-HT₁ agonists do not mimic LSD and the LSD cue is potently blocked by 5-HT₂ antagonists, it appears that 5-HT₂ neuronal systems are of greater importance than 5-HT₁ systems in mediating the discriminative stimulus and, perhaps, other effects of LSD.     

Characterizing the effects of 5-HT2C receptor ligands on motor activity and feeding behaviour in 5-HT2C receptor knockout mice

5-HT_2C receptor agonists have considerable therapeutic potential, however there is little in vivo data to compare the potency and selectivity of 5-HT_2C receptor agonists. Since 5-HT_2C receptor agonists reduce locomotor activity and food intake, changes in these drug-induced behaviours in 5-HT_2C receptor knockout mice could provide a means to examine receptor selectivity in-vivo. Initially this study compared older 5-HT_2C agonists mCPP and MK212, to newer, apparently more selective compounds: Ro 60-0175, WAY161503, CP809,101 and lorcaserin (APD356) on motor activity in wild-type, and 5-HT_2C receptor knockout mice. Two 5-HT_2C receptor antagonists SB242084 and SDZ SER 082 were also examined. mCPP did not significantly alter activity in wild-type mice, but enhanced activity in knockout animals. MK212 (3 and 10 mg/kg) and Ro 60-0175 (1 and 3 mg/kg) reduced activity in wild-type but not knockout animals. At 10 mg/kg, Ro 60-0175 reduced activity in knockout animals, suggesting loss of 5-HT_2C receptor selectivity. CP809,101 and lorcaserin reduced activity in wild-type but not knockout mice. In subsequent feeding studies, Ro 60-0175 and lorcaserin reduced food intake in wild-type animals only. Selectivity of effect for mCPP was marginal. The antagonist SB242084 increased activity in wild-type animals but not in knockout mice; SB242084 did not alter feeding in either genotype. SDZ SER 082 reduced activity in both genotypes implying poor selectivity for 5-HT_2C receptors. The data demonstrate that studying food intake, and particularly motor behaviour, in the 5-HT_2C receptor knockout mouse is a useful and relatively simple approach for screening 5-HT_2C receptor ligands in vivo.     

Serotonin receptors represent highly favorable molecular targets for cognitive enhancement in schizophrenia and other disorders

Rationale Current treatments for schizophrenia adequately treat the positive symptoms of schizophrenia but only modestly improve cognitive deficits. This review provides evidence for and against the use of selective 5-HT receptor drugs as cognition enhancing agents for schizophrenia and other disorders.Methods Pre-clinical and clinical literature concerned with the role of the serotonergic system in cognition and memory as it relates to schizophrenia is reviewed. Individual 5-HT receptor subtypes for which selective drugs are available that are likely to improve cognition are reviewed. Recommendations for clinical testing are proposed.Results and conclusions Four 5-HT receptor systems (5-HT_1A, 5-HT_2A, 5-HT₄, 5-HT₆) are highlighted as suitable targets for enhancing cognition and memory. Because many clinically available antipsychotic drugs already target 5-HT_1A, 5-HT_2A and 5-HT₆ receptors, design of clinical trials will need to take into account the serotonergic pharmacology of concurrently administered antipsychotic medications. 5-HT_1A partial agonists and 5-HT_2A antagonists have shown modest effectiveness in improving cognition in schizophrenia. 5-HT₆-selective compounds for cognition enhancement are in late-stage clinical trials, while 5-HT₄ compounds have not yet been tested in humans for cognition enhancement.Recommendations For stand-alone therapy for enhancing cognition, 5-HT_1A partial agonists, 5-HT_2A antagonists, 5-HT₄ partial agonists and 5-HT₆ antagonists are all likely to induce at least modest improvement in cognition in schizophrenia. If add-on therapy is contemplated, antipsychotic drugs with weak affinities for serotonin receptors should be used to avoid confounds. It is likely that serotonergic drugs will soon be available as cognition enhancing medications for disorders other than schizophrenia (e.g. dementia).     

Effects of 5-HT3 agonists on reproductive behaviors in rats

Activity at 5-HT₁ and 5-HT₂ receptor sites influences sexual behavior in male and female rats. 5-HT₃ antagonists reportedly have no effect on copulatory activity in rats of either sex although they influence a variety of other behaviors. The effects of 5-HT₃ agonists on sexual behavior are unknown. The following experiments were undertaken to assess the influence of the 5-HT₃ agonists 1-phenylbiguanide (PBG) and 2-methyl-serotonin (2-Me-5-HT) on sexual behavior, when administered intracerebroventricularly. Consistent with earlier reports indicating that 5-HT₁ and 5-HT₂ receptor activity influences reproductive activity in a sex-dependent manner, PBG was found to facilitate male, but not female, rat sexual behavior. 2-Me-5-HT, however, failed to modify either female or male rat sexual activity. Evidence that PBG, but not 2-Me-5-HT, induces carrier-mediated dopamine release suggests that the effect of PBG in male rats is due to dopaminergic mediation. Overall, the present data indicate that 5-HT₃ receptor activation has only slight effects on rat sexual behavior.     

Involvement of 5-HT1C-receptors in drug-induced penile erections in rats

Drug-induced penile erections (PE) were initially suggested to be 5-HT_1B receptor mediated. However, since the discovery of the 5-HT_1C receptor a number of compounds, considered to be 5-HT_1B selective, appear to bind more strongly to the 5-HT_1C receptor and this prompted a re-evaluation of the receptor subtype involved in PE induction. PE could be induced by the 5-HT agonists mCPP (0.22–2.2 mg/kg), TFMPP (0.46–1.0 mg/kg) and MK 212 (0.1–1.0 mg/kg). The 5-HT agonist DOI (0.022–0.22 mg/kg) did not induce PE in placebo-pretreated rats but in rats pretreated with various 5-HT₂ antagonists it did. These compounds have in common a strong affinity for the 5-HT_1C receptor. mCPP (0.46 mg/kg)-induced PE could be antagonized by the 5-HT antagonists metergoline, cyproheptadine, mesulergine, mianserin, ritanserin and ketanserin. Their ED₅₀s were 0.04, 0.4, 0.03, 0.06, 0.4 and 2 mg/kg, respectively. The potency of both the agonists to induce, and the antagonists to inhibit, PE was found to be dependent on their selectivity for the 5-HT_1C receptor versus the 5-HT₂ receptor. Spiperone (0.1–1.0 mg/kg) and GR 38032F (1–10 mg/kg) did not antagonise mCPP-induced PE. 8-OH-DPAT and 5MeODMT counteracted mCPP (0.46 mg/kg)-induced PE. Their ED₅₀s were 0.03 and 0.4 mg/kg, respectively. DOI counteracted mCPP induced PE only at doses above 1 mg/kg, whereas CGS 12066B (1.0–10 mg/kg) was inactive. The results suggest that PE are induced by activation of the 5-HT_1C receptor and are functionally inhibited by activation of 5-HT_1A or 5-HT₂ receptors.     

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

Summary 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 -adrenoceptor blocking agents) on the electrically evoked ³H overflow were determined. 1. 5-HT, 5-carboxamidotryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969), 5-methoxytryptamine, N,Ndimethyl-5-HT, 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-HT_1B binding sites, but not with their affinities for 5-HT_1A, 5-HT_1C or 5-HT₂ binding sites. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5HT_1A 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-HT_1B (but not 5-HT_1A, 5-HT_1c or 5-HT₂) binding sites. Ketanserin, a 5-HT₂ receptor antagonist, and spiperone, which blocks 5-HT₂ and 5-HT_1A but not 5-HT_1B or 5-HT_1C receptors, failed to antagonize the effect of 5-HT. These results suggest that the inhibitory presynaptic 5-HT receptors on the sympathetic nerve terminals of the rat vena cava appear to belong to the 5-HT_1B subtype. Cyanopindolol may act as a partial agonist at these receptors, as it does at the facilitatory prosynaptic -adrenoceptors.This study was supported by a grant of the Deutsche Forschungsgemeinschaft Send offprint requests to M. Göthert     

Pharmacological profile of 5-hydroxytryptamine-induced inhibition on the pressor effect elicited by sympathetic stimulation in long-term diabetic pithed rats

We analysed the type and/or subtype of 5-hydroxytryptamine (5-HT) receptors involved in the inhibitory mechanisms of 5-HT on the pressor responses induced by stimulation of sympathetic vasopressor outflow in long-term diabetic pithed rats. Diabetes was induced in male Wistar rats by a single subcutaneous injection of alloxan. Eight weeks later, rats were anaesthetized, pre-treated with atropine, and pithed. The effect of 5-HT on the pressor responses elicited by stimulation of the sympathetic outflow was analysed in eight-week alloxan-induced diabetic pithed rats. 5-HT (20 μg/kg/min) reduced the pressor action obtained by electrical stimulation of the sympathetic outflow. However, there was no effect on exogenous noradrenaline-induced pressor responses. 5-CT (5 μg/kg/min), 8-OH-DPAT (5 μg/kg/min), and α-methyl-5-HT (5 μg/kg/min), selective 5-HT₁, 5-HT_1A and 5-HT₂ receptor agonists, respectively, reproduced the 5-HT inhibitory action. Nevertheless, infusion of 5 μg/kg/min of 1-phenylbiguanide, CGS-12066B, L-694,247, BW273C86 or MK212 (5-HT₃, 5-HT_1B, 5-HT_1D, 5-HT_2B and 5-HT_2C receptor agonists, respectively) had no effect on the pressor responses elicited by stimulation of the sympathetic outflow. Methiothepin (100 μg/kg) and a cocktail of WAY-100,635 (100 μg/kg) and spiperone (125 μg/kg) blocked the 5-HT inhibitory effect on the pressor action obtained by sympathetic stimulation. Moreover, WAY-100, 635 abolished the 8-OH-DPAT inhibitory effect and spiperone blocked α-methyl-5-HT action. In conclusion, this study revealed that long-term experimental diabetes induces changes in the receptor type/subtype involved in the 5-HT inhibitory action on the sympathetic pressor responses produced by electrical stimulation. This is mainly mediated by pre-junctional 5-HT_1A and 5-HT_2A receptors.     

5-HT1C receptor-mediated stimulation of inositol phosphate production in pig choroid plexus

Summary 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-HT_1C sites in pig choroid plexus membranes. 2) The rank order of potency of agonists stimulating inositol phosphate production was: -methyl-5-HT > 1-methyl-5-HT > DOI > bufotenine = SKF 83566 = 5-HT > 5-MeO-DMT > 5-MeOT = RU 24969> SCH 23390> 5-CT. 8-OH-DPAT was virtually devoid of activity at 100 mol/l. 3) The increase in inositol phosphate production induced by 5-HT and other agonists was surmountably antagonised by mesulergine, ketanserin and spiperone with pK_B values of 8.7, 6.7 and 5.3, respectively. 4) The rank order of potency of antagonists was: metergoline > mesulergine > LY 53857 > ritanserin > methiothepin > mianserin > cyproheptadine > pirenperone > cinanserin > ketanserin > spiperone. The following antagonists were virtually devoid of activity at 100 mol/l; pindolol, 21-009 and yohimbine. 5) The results obtained both with agonists and antagonists strongly support the view that 5-HT_1C receptors mediate agonist induced production of inositol phosphates in pig choroid plexus. This is illustrated by the close similarity between 5-HT_1C binding and stimulation of inositol phospholipid turnover in this preparation. 6) The present data also show that compounds believed to be selective for dopamine D1 receptors (SKF 83566, SCH 23390) or 5-HT₂ receptors (DOI, -methyl-5-HT, LY 53857, ritanserin, cyproheptadine) also interact with 5-HT_1C receptors. 7) A case can be made for the 5-HT_1C receptor, with its similarities to the 5-HT₂ receptor in terms of pharmacology and second messenger coupling, being a 5-HT₂ receptor subtype.These data have been presented in part at the Spring Meeting of the German Pharmacological Society, March 1987 (Hoyer et al. 1987) Send offprint requests to D. Hoyer at the above address     

Evidence that 5-HT agonist-induced rotational behaviour in the rat is mediated via 5-HT1 receptors

The rotational behaviour induced by 5-HT agonists has been investigated in rats with lesions of the dorsal raphe' nucleus (DRN). We have previously reported that 5-methyoxy-N,N-dimethyl-tryptamine (5-MeODMT) caused dose-related contralateral rotation in rats with 5,7-dihydroxytryptamine (5,7-DHT) lesions of the DRN. Similar findings are now presented for the 5-HT₁ agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl) (1H indole) (RU24969). In this model, in agreement with the behavioural studies, both agonists were shown to have a greater affinity for the 5-HT₁ binding site when compared with the 5-HT₂ binding site. Antagonist studies using selective 5-HT₂ antagonists (ketanserin and pirenperone) at non-sedative doses failed to inhibit this behaviour. In contrast, the classical 5-HT antagonist methysergide caused significant inhibition of the rotational behaviour. These results suggest that 5-HT agonist-induced rotation in the rat is mediated via 5-HT₁ receptors, probably located in the substantia nigra.     

5-Hydroxytryptamine potentiates neurogenic contractions of rat isolated urinary bladder through both 5-HT(7) and 5-HT(2C) receptors

Serotonin (5-HT) enhances the neurogenic contractile response induced by electrical field stimulation (EFS) in the rat isolated urinary bladder. The aim of this study was to functionally characterize the receptors involved in this effect by using a range of 5-HT receptor subtype selective agonists and antagonists. 5-HT produced a concentration-dependent potentiation of contractile responses to EFS with a pEC₅₀ value of 6.86 ± 0.24. SB-269970 (0.01, 0.1 and 1 μM), a selective 5-HT₇ receptor antagonist, caused a concentration-dependent rightward shift of the 5-HT-induced response. The pA₂ value was 8.16 with a slope of 0.46 ± 0.08. Neither ketanserine nor SB-204741, 5-HT_2A and 5-HT_2B receptors antagonists, respectively, affected the concentration–response curve to 5-HT. However, 5-HT response was antagonized by the selective 5-HT_2C receptor antagonist SB-242084 (0.1 and 1 μM). In the presence of 1 μM of both antagonists SB-269970 and SB-242084, 5-HT response was almost fully inhibited. 5-CT, a 5-HT₇ receptor agonist, induced a biphasic concentration-dependent potentiation of neurogenic contractions. SB-269970 concentration-dependently antagonized the first phase of 5-CT response with a pA₂ value of 8.77 and a slope not significantly different from unity (0.91 ± 0.11) that suggests a competitive antagonism. WAY-161503, a 5-HT_2C receptor agonist (0.01–10 μM), induced a concentration-dependent potentiation of contractile response to EFS while DOI (a selective 5-HT_2A agonist) had no effect. SB-242084 (0.1 and 1 μM) antagonized the effect of WAY-161503 in a concentration-dependent manner. The current results demonstrate that 5-HT potentiates neurogenic contractions of rat isolated detrusor muscle through both 5-HT₇ and 5-HT_2c receptors.     

Changes of cyclic alternating pattern (CAP) parameters in situational insomnia under brotizolam and triazolam

The present study was designed to investigate the hypothesis that agonist interactions at 5-HT_2C receptors mediate the discriminative stimulus properties ofm-chlorophenylpiperazine (mCPP). Three structural classes of compounds have been described to stimulate increases in phosphoinositide (PI) hydrolysis at the 5-HT_2C receptor site: phenylpiperazines, phenylalkylamines, and indolamines. Four representative phenylpiperazines, mCPP, TFMPP, MK-212 and quipazine, one phenylalkylamine, (-)DOM, and one indolamine, LSD, were employed in the present study. The efficacies of these compounds were defined (1) in vitro, with respect to their abilities to stimulate increases in PI hydrolysis in the choroid plexus, and (2) in vivo with respect to their abilities to substitute for the mCPP discriminative stimulus. In vitro intrinsic activity at the 5-HT_2C site was expressed as a fraction of the maximal PI hydrolysis response elicited by serotonin (5-HT). MK-212 (fractional efficacy=1.1) and (-)DOM (0.77) were full agonists, while mCPP (0.72), LSD (0.27), quipazine (0.24), and TFMPP (0.22) were partial agonists with respect to the stimulation of PI hydrolysis at the 5-HT_2C receptor. In vivo, each of the phenylpiperazines fully substituted for the mCPP stimulus, while (-)DOM (75%), and LSD (67%) elicited only partial substitution. While compounds with agonist activity at the 5-HT_2C receptor in vitro substitute for the mCPP stimulus in vivo, no clear relationship exists between in vitro intrinsic activity at the 5-HT_2C receptor with respect to the stimulation of PI turnover and maximal substitution for the mCPP stimulus in vivo. The present data suggest that mCPP elicits a compound stimulus which is mediated by agonist interactions at the 5-HT_2C receptor and possibly additional interactions with 5-HT_2A, 5-HT₃, and/or 5-HT_1B receptors.This study was supported in part by US Public Health Service grant DA 03385 (J.C.W., R.A.R.), by National Research Service Award MH 10567 (D.F.), and by a fellowship from Schering-Plough Research Institute (D.F.). Animals used in these studies were maintained in accordance with the Guide for Care and Use of Laboratory Animals of the Institute of Laboratory Animal Resources, National Research Council.     

Evidence for 5-HT2 receptor mediation in quipazine anorexia

Doses ofd-amphetamine (3.2 mg/kg), fenfluramine (10 mg/kg) and quipazine (8 mg/kg) cause a significant reduction in food intake during a 30-min daily feeding session in food-deprived rats. Pirenperone and ritanserin, 5-HT₂ receptor antagonists, significantly blocked the anorectic effect of quipazine, whiled-amphetamine and fenfluramine effects were not modified. Metergoline, a non-specific blocker of 5-HT receptors, significantly blocked the anorectic effects of fenfluramine and quipazine, but not thed-amphetamine effect. Pretreatment with alpha- and beta-adrenergic receptor antagonists (prazosin, propranolol and pindolol), dopamine receptor antagonists (haloperidol and pimozide), the catecholamine synthesis inhibitor alphamethyl-para-tyrosine, and the opioid receptor antagonist naloxone failed to modify the anorectic effects of all three agents, with the exception that quipazine-induced anorexia was significantly reduced by pimozide. These results suggest that the quipazine anorexia is largely mediating through 5-HT₂ receptors, although the effect of pimozide remains to be explained. Consistent with previous studies, the fenfluramine effect appears to be mediated through 5-HT_1B receptors. Receptors involved in the anorectic effect of higher doses ofd-amphetamine are still unidentified by this analysis. Further investigation is required to define the mechanisms by which quipazine and larger doses ofd-amphetamine bring about a reduced appetite for food.     

Modulation by 5-HT3 and 5-HT4 receptors of the release of 5-hydroxytryptamine from the guinea-pig small intestine

Summary The effects of agonists and antagonists of 5-hydroxytryptamine (5-HT) receptors on the release of endogenous 5-HT from enterochromaffin cells were studied in the vascularly perfused isolated guinea-pig small intestine. The experiments were done in the presence of tetrodotoxin in order to exclude a neuronally mediated influence on 5-HT release.The 5-HT₃ receptor agonist 2-methyl-5-HT increased 5-HT release, and this effect was antagonized by 1 nmol/l tropisetron. Nanomolar concentrations of tropisetron, MDL 72 222 and granisetron decreased 5-HT release. Ondansetron (0.1 and 1 mol/1) did not modify 5-HT release.5-Methoxytryptamine, BIMU8 and cisapride concentration-dependently inhibited 5-HT release. BIMU8 was more potent than 5-methoxytryptamine. Micromolar concentrations of tropisetron (1 and 10 mol/1) enhanced the release, whilst methiothepine (0.1 mol/l) did not affect the release of 5-HT.The results suggest that enterochromaffin cells of the guinea-pig ileum do not contain 5-HT₁ and 5-HT₂ receptors, but are endowed with 5-HT₃ and 5-HT₄ autoreceptors. Activation of the 5-HT₃ receptors triggers a positive feedback mechanism leading to an increase of 5-HT release. The 5-HT₃ receptors on the enterochromaffin cell differ from neuronal 5-HT3 receptors on guinea-pig myenteric plexus by their high affinity for tropisetron and MDL 72 222, and their very low affinity for ondansetron. Stimulation of 5-HT₄ receptors causes inhibition of release; the inhibitory 5-HT₄ receptor mechanism appears to predominate.Correspondence to H. Kilbinger at the above address     

A neuropharmacological analysis of the discriminative stimulus properties of fenfluramine

Rats were trained to discriminate fenfluramine (1.0 mg/kg) from saline in a two-lever drug discrimination task. The dose-response curve for this discrimination was orderly with an ED₅₀ of about one-half of the training dose (0.52 mg/kg). In substitution tests, indirect (p-chloroamphetamine) and direct (quipazine, MK-212, lisuride) serotonin (5-HT) agonists substituted for fenfluramine. Since none of these compounds have been reported to be hallucinogenic and the potent hallucinogen LSD did not substitute completely, it was suggested that the discriminative stimulus properties of fenfluramine are not related to its ability to produce hallucinations in humans. The fenfluramine cue, like the quipazine cue, was antagonized by the 5-HT antagonists cyproheptadine and methiothepin. Unlike quipazine, fenfluramine was also partially antagonized by the 5-HT uptake inhibitor, fluoxetine, and the 5-HT synthesis inhibitor, p-chlorophenylalanine. Thus, the fenfluramine cue differs from that of quipazine in that it is mediated via indirect actions on 5-HT receptors. Since the indirect dopamine (DA) agonist d-amphetamine failed to substitute and the DA antagonist haloperidol failed to block the fenfluramine cue, a mediating role for DA was not indicated. Another indirect DA agonist, cocaine, substituted partially for fenfluramine, a result which paralleled that seen with fluoxetine. Both of these partial substututions were reduced by cyproheptadine; therefore, it was concluded that these effects may be due to the common ability of cocaine, fluoxetine, and fenfluramine to inhibit 5-HT uptake.Portions of this research were presented at the Ninth Annual Meeting of the Society for Neuroscience Atlanta, Georgia, November 2–6, 1979     

Effect of pindolol on thel-5-htp-induced increase in plasma prolactin and cortisol concentrations in man

Previous studies with direct-acting serotonin (5-HT) agonists and antagonists have demonstrated that stimulation of 5-HT_1A, 5-HT_1C and 5-HT₂ receptors may promote cortisol and prolactin (PRL) secretion in man. There is also evidence that 5-HT_1C/2 receptor stimulation contributes to the cortisol and PRL responses following administration of the 5-HT precursor,l-5-hydroxytryptophan (l-5-HTP), in man. To clarify the possible contribution of 5-HT_1A receptor stimulation to the ability ofl-5-HTP to stimulate cortisol and PRL secretion in man, the effect of pindolol, a beta adrenoceptor antagonist that is also a 5-HT_1A partial agonist, on thel-5-HTP-induced increases in cortisol and PRL secretion, was examined in 12 normal male volunteers. Pretreatment with pindolol, 30 mg orally, significantly inhibited the PRL but not the cortisol response tol-5-HTP, 200 mg PO. Pindolol alone decreased basal plasma PRL levels and increased basal plasma cortisol levels, possibly due to 5-HT_1A antagonist and agonists effects, respectively. These data, coupled with observations from other studies, suggest that thel-5-HTP-induced increase in PRL but not cortisol secretion requires 5-HT_1A receptor activation. PRL secretion due to 5-HT formed from exogenousl-5-HTP may require the availability of both intact 5-HT_1A and 5-HT₂/5-HT_1C receptors, since blockade of either receptor type inhibited the PRL response tol-5-HTP. The implication of this synergistic effect for interpretation of neuroendocrine studies involving the serotonergic system in man is discussed.     

Evidence that blockade of post-synaptic 5-HT1 receptors elicits feeding in satiated rats

The effects of nine central 5-HT antagonists on food intake in free feeding male rats were examined. The 5-HT₂ antagonists ritanserin and ketanserin and the selective 5-HT₃ antagonists ICS 205-930 and MDL 72222 had no effect on food intake. In contrast, the non-selective 5-HT antagonists metergoline, methiothepin, mesulergine, mianserin and methysergide (all of which have high affinity for various 5-HT₁ receptor subtypes), dose-dependently increased food intake during a 4-h daytime test. Furthermore, metergoline dose dependently increased food intake over a 24-h period. Suprisingly, mesulergine decreased food intake over a 24-h period at the same doses that increased daytime food intake. This may indicate that the increase in daytime feeding produced by mesulergine is a non-specific response. Although the antagonists used have varying degrees of selectivity for 5-HT receptor subtypes, the pattern of results suggests that postsynaptic 5-HT₁ receptors (possibly of the 5-HT_1C type) play an important role in the control of feeding in rats.     

Blockade of the antidepressant-like effects of 8-OH-DPAT,buspirone and desipramine in the rat forced swim test by 5HT1A receptor antagonists

This study examined whether the antidepressant-like effect of serotonin (5-HT)_1A receptor agonists in the forced swim test (FST) is mediated by 5-HT_1A receptors. The 5-HT_1A receptor agonists 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and buspirone decreased immobility in the FST. The effect of 8-OH-DPAT was blocked by the 5-HT_1A receptor antagonists NAN 190, BMY 7378 and pindolol. The effect of buspirone was blocked by NAN 190 and pindolol. The antagonists produced no effects on their own. The norepinephrine (NE) uptake inhibitor desipramine (DMI) also reduced immobility, and this was also blocked by NAN 190, BMY 7378 and pindolol. The ₁, ₁ and ₂ adrenergic antagonists prazosin, betaxolol and ICI 118,551 did not block either 8-OH-DPAT or DMI, and produced no effects on their own. These results provide evidence that the antidepressant-like effects of 5-HT_1A receptor agonists in the FST are mediated through 5-HT_1A receptors, probably located postsynaptically. The finding that the 5-HT_1A receptor antagonists blocked the effect of DMI suggests that the NE and 5-HT systems interact in the FST.     

5-Hydroxytryptamine 5-HT1B and 5-HT1D receptors mediating inhibition of adenylate cyclase activity

Summary 5-Hydroxytryptamine_1B (5-HT_1B) receptor mediated-inhibition of forskolin-stimulated adenylate cyclase activity in rat substantia nigra was characterized pharmacologically and compared to 5-HT_1D receptor mediated-inhibition of forskolin-stimulated adenylate cyclase activity in calf substantia nigra. Special attention was paid to the effects of drugs known to bind with high affinity to 5-HT_1B (pindolol, propranolol, cyanopindolol, SDZ 21-009, isamoltane) or 5-HT_1D recognition sites (yohimbine, rauwolscine).PEC₅₀ or pK _B values of a variety of 5-HT-receptor ligands (6 agonists including 5-HT, and 12 antagonists) for the inhibition of adenylate cyclase activity in rat substantia nigra, correlated significantly to the corresponding pK _D values at 5-HT_1B binding sites (r = 0.90, P = 0.0001). Amongst the ₂- and -adrenoceptor antagonists tested, none of the drugs expressed more than 35% of the intrinsic activity of 5-HT at 5-HT_1B receptors. When tested as antagonists, their pK _B values were in good agreement with their pK _D values for 5-HT_1B sites. By contrast, these drugs displayed marked intrinsic activity at 5-HT_1D receptors: their pEC₅₀ values were close to their pK _D values for 5-HT_1D sites and their effects could be potently antagonized by methiothepin. The rank orders of potency of the tested compounds at 5-HT_1B and 5-HT_1D were markedly different.The results strengthen the identity between 5-HT receptors mediating inhibition of adenylate cyclase activity in rat and calf substantia nigra and 5-HT_1B and 5-HT_1D binding sites, respectively. They underline the differences between these receptors in terms of intrinsic activities and potencies of drugs. Send offprint requests to: D. Hoyer at the above address     

Evidence that hypophagia induced bymCPP 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-HT_1C receptors (metergoline, mianserin, and mesulergine but not cyproheptadine) opposed hypophagia caused bymCPP. Another drug reported to have high affinity for the 5-HT_1C site, 1-naphthyl-piperazine (1-NP), also blocked the hypophagic response tomCPP at doses which attenuatedmCPP-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-HT_1B receptor antagonists [(±) cyanopindolol, (−) propranolol, but not (−) pindolol)] which oppose hypophagia caused by RU 24969 (Kennett et al. 1987) also opposed hypophagia caused bymCPP. The 5-HT₂ antagonists ketanserin and ritanserin, the 5-HT₃ antagonist ICS 205-930 and the α₂ adrenoceptor antagonist idazoxan did not oppose the hypophagic effect ofmCPP. In agreement with results formCPP, 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-HT_1B receptors but not on 5-HT_1C receptors, whilemCPP (and TFMPP)-induced hypophagia may depend on both receptors. Thus, 5-HT_1C and 5-HT_1B receptors may evoke hypophagia via a common pathway but the effect of antagonists implies that at the doses usedmCPP and TFMPP act predominantly at 5-HT_1C receptors. Since only the hypophagic response tomCPP is blocked by cyanopindolol and (−) propranolol (Kennett and Curzon 1988) it is unlikely to be secondary to hypoactivity induced by the drug.     

Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT<Subscript>2A</Subscript> receptors

Rationale Temporal differentiation in the free-operant psychophysical procedure is sensitive to the 5-hydroxytryptamine (5-HT)_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT₂ receptor agonist 2,5-dimethoxy-4-iodo-amphetamine (DOI); both drugs shift the psychophysical curve leftwards, reducing the indifference point, T ₅₀. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation.Objective We examined whether fenfluramines effect on temporal differentiation can be antagonised by the 5-HT_1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT_2A receptor antagonist ketanserin, and compared the effects of fenfluramine, DOI and 8-OH-DPAT in intact rats and rats whose 5-HTergic pathways had been destroyed by 5,7-dihydroxytryptamine.Methods Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data for derivation of timing indices (T ₅₀, time corresponding to %B=50%, and Weber fraction). Experiment 1 examined the effects of acute treatment with fenfluramine, and the interaction between fenfluramine and the 5-HT_1A and 5-HT_2A receptor antagonists WAY-100635 and ketanserin; experiment 2 compared the effects of fenfluramine, 8-OH-DPAT and DOI in intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Concentrations of 5-HT and catecholamines in the brain were measured by high-performance liquid chromatography.Results Experiment 1: fenfluramine (2 mg/kg) reduced T ₅₀; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 g/kg). Experiment 2: 8-OH-DPAT (100 g/kg) and DOI (250 g/kg) reduced T ₅₀ in both groups; fenfluramine reduced T ₅₀ only in the sham-lesioned group. Levels of 5-HT were reduced by 80% in the lesioned group; catecholamine levels were not affected.Conclusions The results suggest that fenfluramine affects temporal differentiation via the release of endogenous 5-HT which acts mainly on postsynaptic 5-HT_2A receptors.