Deramciclane, a putative anxiolytic drug, is a serotonin 5-HT2C receptor inverse agonist but fails to induce 5-HT2C receptor down-regulation

 Deramciclane (EGIS-3886) is a novel anxiolytic agent that binds with high affinity to 5-HT_2A/2C receptors. The interactions of deramciclane with the serotonin 5-HT_2C receptor were characterized further using receptor phosphoinositide hydrolysis assays and receptor autoradiography. Deramciclane antagonized 5-HT_2C receptor mediated 5-HT-stimulated phosphoinositide hydrolysis with an IC₅₀ value of 168 nM. Deramciclane also decreased basal phosphoinositide hydrolysis by up to 33% (EC₅₀ = 93 nM) in a physiological system in the choroid plexus, suggesting that deramciclane possesses inverse agonist properties at this receptor. Administration of single doses of 0.5 mg/kg and 10 mg/kg resulted in a maximal 5-HT_2C receptor occupancy of up to 45% and 79%, respectively, in the choroid plexus. Chronic (14 days) treatment with 0.5 mg/kg or 10 mg/kg deramciclane did not alter [¹²⁵I]DOI (agonist) or [³H]mesulergine (antagonist) binding to 5-HT_2C receptors in the choroid plexus compared to saline-treated controls, as determined by quantitative receptor autoradiography. In comparison, the effects of deramciclane on 5-HT_2A binding characteristics and receptor occupancy were also studied. Deramciclane treatment resulted in 5-HT_2A receptor occupancy of up to 78%, but no significant effect of chronic treatment on 5-HT_2A receptor agonist binding levels was found. In conclusion, these data indicate that deramciclane is a 5-HT_2C receptor inverse agonist and occupies 5-HT_2C receptors during treatment, and that chronic treatment with deramciclane does not lead to 5-HT_2C receptor down-regulation. Received: 24 October 1996/Final version: 8 August 1997     

5-HT7 receptor deletion enhances REM sleep suppression induced by selective serotonin reuptake inhibitors, but not by direct stimulation of 5-HT1A receptor

5-HT₇ receptors are involved in REM sleep and possibly in mood disorders. REM sleep suppression and antidepressant-like behavior is observed in 5-HT₇^−/− mice and in rats treated with 5-HT₇ receptor antagonists. We recently demonstrated that pharmacological blockade of 5-HT₇ receptors enhances REM sleep suppression and antidepressant-like behavior induced by citalopram in rodents. It has been hypothesized that the effect of citalopram on sleep is essentially mediated by the activation of 5-HT_1A receptors. The present study investigates the impact of 5-HT₇ receptor gene deletion on the effect of various reuptake inhibitors on REM sleep and probes the role of 5-HT_1A receptors in this response. Three SSRIs (citalopram, fluoxetine and paroxetine) but not the tricyclic antidepressant desipramine had a significantly stronger REM sleep suppressive effect in 5-HT₇^−/− mice compared to 5-HT₇^+/+ mice. In contrast, REM sleep was similarly reduced in 5-HT₇^+/+ mice and 5-HT₇^−/− mice after treatment with the 5-HT_1A receptor agonist ipsapirone. Furthermore, both 5-HT₇^+/+ and 5-HT₇^−/− mice displayed the same increase in REM sleep duration produced by the 5-HT_1A receptor antagonist WAY-100635. These findings indicate that 5-HT₇ receptor deletion augments the effect of various SSRIs on REM sleep suppression and that this effect is distinct from those mediated via 5-HT_1A receptors.     

Investigation of the SSRI augmentation properties of 5-HT(2) receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT₂ receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT₂ receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT₂ receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2 mg kg⁻¹) and higher (4 mg kg⁻¹) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT₂ receptor antagonist ketanserin (4 mg kg⁻¹) augmented the effect of 4 mg kg⁻¹ but not 2 mg kg⁻¹ citalopram. The effect of 4 mg kg⁻¹ citalopram was also augmented by pre-treatment with either the 5-HT_2C receptor antagonist SB 242084 (0.5 mg kg⁻¹) or the 5-HT_2A receptor antagonist MDL 100907 (0.5 mg kg⁻¹). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5 mg kg⁻¹) and higher (20 mg kg⁻¹) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg⁻¹). These results confirm recent findings that 5-HT₂ receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT_2C and possibly 5-HT_2A receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT₂ receptor antagonist properties of this drug.     

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors,citalopram and fluvoxamine,in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

Rationale Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment. Objectives This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA. Methods The most selective antagonists currently available were used. Results Citalopram-induced LA was dose-dependently attenuated by the 5-HT_1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT_1B antagonist, SB224,289, but unaffected by the selective 5-HT_1A antagonist, WAY100,635. The selective antagonists at 5-HT_2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT_2B antagonist, SB204,741, and the 5-HT_2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT_2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT₃ (ondansetron), 5-HT₄ (GR125,487), 5-HT₆ (SB271,046) and 5-HT₇ (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine. Conclusions The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT_1B and 5-HT_2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.     

Competitive antagonism by recognised 5-HT2 receptor antagonists at 5-HT1C receptors in pig choroid plexus

Summary The effects of several antagonists, known to interact with 5-HT₂ receptors (ritanserin, LY 53857, ICI 169,369, methysergide, mesulergine and ketanserin), were tested against 5-HT-stimulated production of inositol phosphate in pig choroid plexus, a 5-HT_1C receptor model. These antagonists produced dextral shifts of the concentration response curve to 5-HT in a parallel manner, without depressing significantly the maximal response. The following pA₂ values (in parentheses) were obtained: mesulergine (8.88), methysergide (8.85), LY 53857 (8.69), ritanserin (8.69), ICI 169,369 (7.86), and ketanserin (6.57). These pA₂ values were in good agreement with the pK_D values determined in radioligand binding studies performed in pig choroid plexus with [³H]mesulergine. The present data demonstrate that several drugs described as 5-HT₂ receptor selective antagonists (e.g. ritanserin, LY 53857 and ICI 169,369) are also potent, competitive and surmountable antagonists at 5-HT_1C receptors. Thus, the results provide further evidence for the pharmacological similarity of 5-HT_1C and 5-HT₂ receptors. However, in contrast to the situation described with methysergide, ritanserin and LY 53857 in several 5-HT₂ receptor models, none of these antagonists acted in a non-competitive or unsurmountable fashion at 5-HT_1C receptors. These results suggest, but do not firmly rule out, that at least in the presence of the drugs tested in the present study, 5-HT_1C receptors in the choroid plexus do not undergo an allosteric modulation; these findings are apparently in contrast to a model proposed previously for 5-HT₂ receptors (Kaumann and Frenken 1985, Naunyn-Schmiedeberg's Arch Pharmacol 328: 295–300) Send offprint requests to P. Schoeffter at the above address     

Implication of 5-HT(2B) receptors in the serotonin syndrome

The serotonin (5-HT) syndrome occurs in humans after antidepressant overdose or combination of drugs inducing a massive increase in extracellular 5-HT. Several 5-HT receptors are known to participate in this syndrome in humans and animal models. The 5-HT_2B receptor has been proposed as a positive modulator of serotonergic activity, but whether it is involved in 5-HT syndrome has not yet been studied.We analyzed here, a putative role of 5-HT_2B receptors in this disorder by forced swimming test (FST) and behavioral assessment in the open field. In FST, genetic (5-HT_2B^−/− mice) or pharmacological (antagonist RS127445 at 0.5 mg/kg) ablation of 5-HT_2B receptors facilitated selective 5-HT reuptake inhibitors (SSRI)-induced increase of immobility time as well as expression of other symptoms related to 5-HT syndrome like hind limb abduction and Straub tail. Increase in immobility was also developed in FST by both wild type (WT) and 5-HT_2B^−/− mice after the administration of 5-HT_1A, 5-HT_2A or 5-HT_2C receptor agonists, 8-OH-DPAT (5 mg/kg), DOI (1 mg/kg), or WAY161503 (5 mg/kg), respectively. In contrast, the 5-HT_2B receptor agonist BW723C86 (3 mg/kg) or 5-HT_1B receptor agonist CGS12066A (2 mg/kg) decreased immobility time in both genotypes. The 5-HT syndrome induced by fluoxetine at high doses was blocked in WT and 5-HT_2B^−/− mice by administration of 5-HT_1A and 5-HT_2C receptor antagonists (WAY100635 0.5 mg/kg and SB242084 0.5 mg/kg) but not by the 5-HT_2A receptor antagonist MDL100907 (1 mg/kg). By behavioral assessment, we confirmed that 5-HT_2B^−/− mice were more prone to develop 5-HT syndrome symptoms after administration of high dose of SSRIs or the 5-HT precursor 5-Hydroxytryptophan, 5-HTP, even if increases in 5-HT plasma levels were similar in both genotypes.This evidence suggests that the presence of 5-HT_2B receptors hinders acute 5-HT toxicity once high levels of 5-HT are attained. Therefore, differential agonism/antagonism of 5-HT receptors should be considered in the search of therapeutic targets for treating this serious disorder.     

Interaction of arylpiperazines with 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptors: do discriminatory 5-HT1B receptor ligands exist?

Summary The effects of several putative 5-HT₁ receptorsubtype selective ligands were investigated in biochemical models for 5-HT_1A, 5-HT_1B, and 5-HT_1D receptors (inhibition of forskolin-stimulated adenylate cyclase activity in calf hippocampus, rat and calf substantia nigra, respectively) and 5-HT_1C receptors (stimulation of inositol phosphates production in pig choroid plexus). Following compounds were studied: 5-HT (5-hydroxytryptamine), TFMPP (1-(mtrifluoromethylphenyl)piperazine), mCPP (1-m-chlorophe-nyl)piperazine, 1 CGS 12066 (7-trifluoromethyl-4-(4-methyl1-piperazinyl)-pyrrolo[1,2-a]quinoxaline 1), isamoltane (CGP 361A, 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propranol), quipazine, 1-NP (1-(1-naphthyl)piperazine), and PAPP (LY165163, 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)-piperazine). Among reported 5-HT_1B receptor selective drugs, TFMPP had similar potency at 5HT_1A, 5-HT_1B and 5-HT_1C receptors, mCPP did not separate between 5-HT_1B and 5-HT_1C receptors, CGS 12066 was equipotent at 5-HT_1B and 5-HT_1D receptors, and isamoltane was only slightly 5-HTIB versus 5-HT_1A selective. Quipazine showed equal potency at 5-HTIB and 5-HT_1C receptors and 1-NP did not discriminate between the four receptor subtypes. PAPP described as 5-HT_1A receptor selective, was equally potent at 5-HT_1A and 5-HT_1D receptors. The potencies determined in second messenger studies were in good agreement with the affinity values determined in radioligand binding studies. Thus 5-HT_1A, 5-HT_1B, 5-HT_1C and 5-HT_1D receptors have different pharmacological profiles as predicted from radioligand binding studies. Despite claims to the contrary, none of the tested compounds had actual selectivity for a given 5-HT₁ receptor subtype. Of interest were the properties of several of these drugs, which behaved as agonists at some receptors and as antagonists at others (e. g. quipazine, 1-NP, PAPP and isamoltane). Send offprint requests to D. Hoyer at the above address     

Functional crosstalk and heteromerization of serotonin 5-HT2A and dopamine D2 receptors

The serotonin 5-HT_2A receptor (5-HT_2AR) and dopamine D₂ receptor (D₂R) are high-affinity G protein-coupled receptor targets for two different classes of antipsychotic drugs used to treat schizophrenia. Interestingly, the antipsychotic effects are not based on the regulation of same signaling mediators since activation of the 5-HT_2AR and of the D₂R regulate G_q/11 protein and G_i/o protein, respectively. Here we use radioligand binding and second messenger production assays to provide evidence for a functional crosstalk between 5-HT_2AR and D₂R in brain and in HEK293 cells. D₂R activation increases the hallucinogenic agonist affinity for 5-HT_2AR and decreases the 5-HT_2AR induced inositol phosphate production. In vivo, 5-HT_2AR expression is necessary for the full effects of D₂R antagonist on MK-801-induced locomotor activity. Co-immunoprecipitation studies show that the two receptors can physically interact in HEK293 cells and raise the possibility that a receptor heterocomplex mediates the crosstalk observed. The existence of this 5-HT_2AR-D₂R heteromer and crosstalk may have implications for diseases involving alterations of serotonin and dopamine systems and for the development of new classes of therapeutic drugs.     

Radioligand binding evidence implicates the brain 5-HT2 receptor as a site of action for LSD and phenylisopropylamine hallucinogens

Alterations in brain serotonergic function have been implicated in the mechanism of action of LSD, mescaline, and other similarly acting hallucinogenic drugs of abuse such as STP (2,5-dimethoxyphenylisopropylamine; DOM). In order to test the hypothesis that the mechanism of action of LSD and phenylisopropylamine hallucinogens is through stimulation of a specific brain serotonin receptor sub-type, the affinities of these compounds for radiolabelled 5-HT₂, 5-HT_1A, 5-HT_1B, and 5-HT_1C receptors have been determined using recently developed in vitro radioligand binding methodologies. The 5-HT₂ receptor was labelled with the agonist/hallucinogen radioligand ³H-DOB (4-bromo-2,5-dimethoxyphenylisopropylamine). The 5-HT_1A, 5-HT_1B, and 5-HT_1C receptors were labelled with ³H-OH-DPAT, ³H-5-HT, and ³H-mesulergine, respectively. In general, the phenylisopropylamines displayed 10–100 fold higher affinities for the 5-HT₂ receptor than for the 5-HT_1C receptor and 100–1000 fold higher affinities for the 5-HT₂ receptor than for the 5-HT_1A or 5-HT_1B receptor. There was a strong correlation between hallucinogenic potencies and 5-HT₂ receptor affinities of the phenylisopropylamines (r=0.90); the correlation coefficients for the 5-HT_1A, 5-HT_1B, and 5-HT_1C were 0.73, 0.85, and 0.78, respectively. Because there is no evidence that 5-HT_1A-selective or 5-HT_1B-selective agonists are hallucinogenic and because the phenylisopropylamines are potent hallucinogens, a 5-HT₂ receptor interaction is implicated and supports our previous suggestions to this effect. A secondary role for 5-HT_1C receptors cannot be discounted at this time. These results, when integrated with other receptor pharmacological information, indicate that an important component of the mechanism of action of LSD and the phenylisopropylamine hallucinogens is through stimulation of brain 5-HT₂ receptors. Offprint requests to: M. Titeler     

Frontal 5-HT2A receptors studied in depressive patients during chronic treatment by selective serotonin reuptake inhibitors

To investigate adaptative changes of 5-HT_2A receptors induced by SSRIs, six patients chronically treated for a depressive episode (four with fluoxetine, two with fluvoxamine) were studied with PET and [¹⁸F]setoperone. They were compared to eight untreated depressive patients. The mean frontal to cerebellum radioactivity concentration ratio, an index of the [¹⁸F]setoperone specific binding to 5-HT_2A receptors, was higher in treated than in untreated patients, when age was taken into account. This suggests that chronic treatment by SSRIs could induce an up-regulation of the 5-HT_2A receptors, and that 5-HT_2A receptor down-regulation is not a common mechanism for the therapeutic effects of all serotoninergic antidepressive drugs. Received: 9 January 1997 /Final version: 22 May 1997     

Repeated administration of the 5-HT<Subscript>1B</Subscript> receptor antagonist SB-224289 blocks the desensitisation of 5-HT<Subscript>1B</Subscript> autoreceptors induced by fluoxetine in rat frontal cortex

Desensitisation of 5-HT_1A and 5-HT_1B autoreceptors is thought to be the mechanism underlying the therapeutic effects of fluoxetine and other selective serotonin re-uptake inhibitors (SSRIs) when these are administered chronically, while blockade of these autoreceptors occurring on administration of an SSRI together with an autoreceptor antagonist is responsible for the acute increase in 5-HT levels in vivo observed under these circumstances. The effects of repeated administration of SSRIs together with 5-HT_1B receptor antagonists on 5-HT levels and autoreceptor activity have not been studied previously with an in vivo method. In this work we found, using in vivo microdialysis that the effect of fluoxetine (5 mg/kg i.p. daily for 7 days) to desensitise 5-HT_1B autoreceptors in frontal cortex, as measured by the action of CP 93129 (10 M) to reduce 5-HT levels, was prevented by concomitant administration of the 5-HT_1B receptor antagonist SB 224289 (2.5 mg/kg s.c.). 5-HT_1B receptor activity in hypothalamus and 5-HT_1A autoreceptor activity, as determined by the effects of s.c. 8-OH-DPAT to reduce 5-HT levels, were not altered either by fluoxetine alone at this dose or by fluoxetine in the presence of SB 224289. We conclude that the effects obtained when 5-HT_1B autoreceptor antagonists are administered acutely together with SSRIs may not be maintained after repeated administration.     

Pharmacological characterisation of the agonist radioligand binding site of 5-HT<Subscript>2A</Subscript>, 5-HT<Subscript>2B</Subscript> and 5-HT<Subscript>2C</Subscript> receptors

In the present study we compared the affinity of various drugs for the high affinity agonist-preferring binding site of human recombinant 5-HT_2A, 5-HT_2B and 5-HT_2C receptors stably expressed in monoclonal mammalian cell lines. To ensure that the agonist-preferring conformation of the receptor was preferentially labelled in competition binding experiments, saturation analysis was conducted using antagonist and agonist radiolabels at each receptor. Antagonist radiolabels ([³H]-ketanserin for 5-HT_2A receptor and [³H]-mesulergine for 5-HT_2B and 5-HT_2C receptor) bound to a larger population of receptors in each preparation than the corresponding agonist radiolabel ([¹²⁵I]-DOI for 5-HT_2A receptor binding and [³H]-5-HT for 5-HT_2B and 5-HT_2C receptor binding). Competition experiments were subsequently conducted against appropriate concentrations of the agonist radiolabels bound to the agonist-preferring subset of receptors in each preparation. These studies confirmed that there are a number of highly selective antagonists available to investigate 5-HT₂ receptor subtype function (for example, MDL 100907, RS-127445 and RS-102221 for 5-HT_2A, 5-HT_2B and 5-HT_2C receptors respectively). There remains, however, a lack of highly selective agonists. (–)DOI is potent and moderately selective for 5-HT_2A receptors, BW723C86 has poor selectivity for human 5-HT_2B receptors, while Org 37684 and VER-3323 display some selectivity for the 5-HT_2C receptor. We report for the first time in a single study, the selectivity of numerous serotonergic drugs for 5-HT₂ receptors from the same species, in mammalian cell lines and using, exclusively, agonist radiolabels. The results indicate the importance of defining the selectivity of pharmacological tools, which may have been over-estimated in the past, and highlights the need to find more selective agonists to investigate 5-HT₂ receptor pharmacology.     

Acute and chronic tryptophan depletion differentially regulate central 5-HT1A and 5-HT2A receptor binding in the rat

Rationale Tryptophan depletion is used to reduce central serotonergic function and to investigate its role in psychiatric illness. Despite widespread clinical use, its effects on serotonin (5-HT) receptors have not been well characterized. Objective The aim of this study was to examine the effect of acute (ATD) and chronic tryptophan depletion (CTD) on free-plasma tryptophan (TRP), central TRP and 5-HT and brain 5-HT_1A and 5-HT_2A receptor binding in the rat. Methods TRP and 5-HT were measured by high-performance liquid chromatography and receptor levels determined by homogenate radioligand binding and in-vitro receptor autoradiography. Results Free-plasma TRP, central TRP and central 5-HT levels were significantly and similarly reduced by ATD and 1- and 3-week CTD compared to controls. ATD significantly reduced 5-HT_1A binding in the dorsal raphe (14%) but did not significantly alter postsynaptic 5-HT_1A binding (frontal cortex, remaining cortex and hippocampus) or 5-HT_2A binding (cortex and striatum). One-week CTD did not significantly alter cortical 5-HT_2A binding or postsynaptic 5-HT_1A binding. Furthermore, 3-week CTD did not significantly alter 5-HT_1A binding but significantly increased cortical 5-HT_2A binding without affecting striatal or hippocampal levels. In the CTD 1 and 3-week groups, rat body weight was significantly decreased as compared to controls. However, weight loss was not a confounding factor for decreased cortical 5-HT_2A-receptor binding. Conclusion ATD-induced reduction in somatodendritic 5-HT_1A autoreceptor binding may represent an intrinsic ‘homeostatic response’ reducing serotonergic feedback in dorsal raphe projection areas. In contrast, the increase in 5-HT_2A receptor after CTD may be a compensatory response to a long-term reduction in 5-HT.     

Behavioral profiles of SSRIs in animal models of depression, anxiety and aggression

The behavioral profiles of five clinically used selective serotonin reuptake inhibitors (SSRIs) citalopram, paroxetine, sertraline, fluvoxamine and fluoxetine, have been compared in animal models of antidepressant (mouse forced swim test), anxiolytic (exploration of black and white test box and foot-shock-induced ultrasonic vocalization in the rat) and antiaggressive (isolation-induced aggressive behavior in the mouse) activity. The results are discussed in relation to receptor binding data from the literature. Furthermore, affinities for the σ₁ and σ₂ binding sites are presented. Citalopram reversed the immobility induced by forced swimming with a potency similar to that of imipramine. Paroxetine, fluvoxamine and fluoxetine reversed swim-induced immobility less potently and with a maximum of 40–50% reversal. Citalopram produced a mixed anxiogenic-/anxiolytic-like response in rats tested in the two-compartment black and white box. Paroxetine induced an anxiogenic-like response at low doses and the other SSRIs were without major effects. Citalopram and paroxetine inhibited footshock-induced ultrasonic vocalization with high potencies. The dose-response curve was biphasic for citalopram with a maximum of 64% inhibition. Sertraline and fluvoxamine inhibited the vocalization less potently, and fluoxetine induced a weak inhibitory effect corresponding to a maximum of 32%. Sertraline, fluvoxamine and fluoxetine inhibited isolation-induced aggressive behavior, whereas citalopram and paroxetine were inactive. Both 5-HT₁ and 5-HT₂ receptors are involved, and there was a functional interaction between 5-HT_1A and 5-HT_2A or 5-HT_2C receptors, as ritanserin potentiated the antiaggressive effect of l,5-HTP as well as that of 8-OH-DPAT. Received: 5 March 1996/Final version: 5 September 1996     

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     

Stimulus properties of fluvoxamine in a conditioned taste aversion procedure

A conditioned taste aversion (CTA) procedure in mice was used to investigate the stimulus effects of the serotonin reuptake inhibitors (SSRIs) fluvoxamine and fluoxetine. Fluvoxamine elicited a reliable CTA (ED₅₀ = 24 mg/kg, SC) and a number of drugs were tested as pre-exposure drugs. Pre-exposure to the serotonin (5-HT)_1A receptor agonists flesinoxan and ±-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) prevented the CTA induced by fluvoxamine (50 mg/kg, SC). Pre-exposure with the 5-HT_2C receptor agonist MK 212 [6-chloro-2(1-piperazinyl)pyrazine] partially prevented the fluvoxamine-induced CTA, pre-exposure with the 5-HT_2A/2C receptor agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl] did not prevent the CTA induced by fluvoxamine. Flesinoxan pre-exposure also prevented the taste aversion induced by fluoxetine (10 mg/kg, SC) completely. This contrasts previous results obtained with fluoxetine, where was found that its stimulus is primarily mediated by 5-HT_2C, and to a lesser degree by 5-HT_1A receptors. Therefore, we compared the two SSRIs directly. Pre-exposure to fluvoxamine prevented the fluoxetine-induced CTA, whereas pre-exposure to fluoxetine only partially prevented the fluvoxamine-induced CTA. We conclude that 5-HT_1A receptors are involved in the stimulus properties of both fluvoxamine and fluoxetine, that 5-HT_2C receptors are involved in fluvoxamine and especially fluoxetine, and , based primarily on the cross-comparison tests, that the two SSRIs have somewhat different stimulus properties. Received: 3 March 1998 / Final version: 26 May 1998     

Molecular structural basis of ligand selectivity for 5-HT2 versus 5-HT1C cortical receptors

Summary A molecular structural criterion of ligand selectivity for the 5-HT₂ versus 5-HT_1C receptor was hypothesized on the basis of radioligand binding data. Despite the large number of compounds which have been tested at both receptors, analysis of published data led to the identification of only five agents which are greater than 10-fold selective for the 5-HT₂ versus the 5-HT_1C receptor. Comparison of the two-dimensional structures revealed that, although these five compounds represent three distinct structural classes, they share a common structural feature located in the region hypothesized to be involved in receptor binding: a carbonyl or carboxyl oxygen interposed spatially between an aromatic ring and nitrogen atom. This structural feature was used to predict the relative selectivity of compounds that had not previously been analyzed at both the 5-HT₂ and 5-HT_1C receptors.All six drugs tested which contain the identified reactive carbonyl or carboxyl group were found to be selective for the 5-HT₂ versus the 5-HT_1C receptor with selectivity ratios ranging from 26 to 380. By contrast, three agents which are structurally similar but do not contain the reactive carbonyl or carboxyl group displayed equally high affinity for both receptor binding sites. Since the physiological roles of the 5-HT₂ and 5-HT_1C receptor are markedly different, it would be of potential clinical and scientific value to utilize this molecular structural feature to further identify chemical compounds which would selectively interact with only one of the two receptors. Send offprint requests to S. J. Peroutka at the current address     

5-HT2A and 5-HT2C receptor antagonists have opposing effects on a measure of impulsivity: interactions with global 5-HT depletion

Rationale Global serotonin (5-HT) depletion increases the number of premature responses made on the five-choice serial reaction time task (5CSRT) in rats. In contrast, the 5-HT_2A receptor antagonist M100907 decreases this measure of impulsivity. Mounting evidence suggests that 5-HT_2A and 5-HT_2C receptors have opposing effects on behaviour, and that the 5-HT_2C receptor antagonist SB 242084 produces a pattern of behaviour similar to 5-HT depletion.Objectives To assess the effects of 5-HT_2A and 5-HT_2C receptor antagonists on performance of the 5CSRT, to directly compare the effects of these drugs with those of ICV 5,7-dihydroxytryptamine (5,7-DHT) lesions and to investigate whether 5-HT depletion affects the action of these agents.Methods The effects of M100907 (0, 0.01, 0.03, 0.1 mg/kg IP) and SB 242084 (0, 0.1, 0.25, 0.5 mg/kg IP) were investigated on performance of the 5CSRT in both ICV 5,7-DHT-lesioned and sham-operated rats.Results ICV 5,7-DHT lesions, which significantly decreased forebrain levels of 5-HT by around 90%, increased levels of premature responding, decreased omissions and the latency to respond correctly, yet did not affect performance accuracy. M100907 decreased premature responding in sham-operated controls but not in 5-HT-depleted rats. In contrast, SB 242084 increased premature responding in all animals, and also decreased the latency to make a correct response in sham-operated controls.Conclusions These data support the view that serotonergic regulation of impulsive behaviour through different members of the 5-HT₂ receptor family is functionally heterogeneous. Although both 5-HT_2A and 5-HT_2C receptors participate in controlling this form of impulsive action, their relative contribution may depend on the endogenous state of the 5-HT system.     

Sedative-Hypnotic and Receptor Binding Studies of Fermented Marine Organisms

This study was performed to investigate the sedative-hypnotic activity of γ-aminobutyric acid (GABA)-enriched fermented marine organisms (FMO), including sea tangle (FST) and oyster (FO) by Lactobacillus brevis BJ20 (L. brevis BJ20). FST and FO were tested for their binding activity of the GABA_A-benzodiazepine and 5-HT_2C receptors, which are well-known molecular targets for sleep aids. We also measured the sleep latency and sleep duration during pentobarbital-induced sleep in mice after oral administration of FST and FO. In GABA_A and 5-HT_2C receptor binding assays, FST displayed an effective concentration-dependent binding affinity to GABA_A receptor, similar to the binding affinity to 5-HT_2C receptor. FO exhibited higher affinity to 5-HT_2C receptor, compared with the GABA_A receptor. The oral administration of FST and FO produced a dose-dependent decrease in sleep latency and increase in sleep duration in pentobarbital-induced hypnosis. The data demonstrate that FST and FO possess sedative-hypnotic activity possibly by modulating GABA_A and 5-HT_2C receptors. We propose that FST and FO might be effective agents for treatment of insomnia.     

5.HT1 receptors in the vertebrate brain

Summary The regional distribution of high affinity [³3H]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-HT_1A, 5-HT_1c and 5-HT_1B sites respectively. In addition, 5-HT_1A, and 5-HT_1c 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-HT_1D 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. 5-HT_1A sites were observed in the neo-cortex, the hippocampal formation and the raphe nucleus, whereas 5-HT_1C sites accounted for all 5-HT₁ binding in the choroid plexus. In the mouse and rat brain, 5-HT_1B binding sites were enriched in the basal ganglia and associated regions (substantia nigra). These areas were enriched in 5-HT_1D sites in the brain of the other mammals studied. In these animals, no site with a 5-HT_1B pharmacological profile were detected.These results indicate that 5-HT_1A 5-HT_1c and 5-HT_1D sites are present already in the lower vertebrate species investigated and that 5-HT_1B appear to be exclusive of the myomorph rodents (mouse, rat). Furthermore, the different subtypes of the 5-HT₁, receptors present a conserved regional distribution with the 5-HT_1D sites being enriched in the basal ganglia and the 5-HT_1A sites predominating in the hippocampal formation.