5-HT(1B/D) receptor agonist, SKF99101H, induces locomotor hyperactivity in the guinea pig

Previous studies in guinea pigs have shown that while a serotonin 5-HT(1B/D) receptor agonist, GR46611, does not induce locomotor activation when given alone, it markedly enhances the locomotor response to selective 5-HT(1A) receptor agonists, 8-OH-DPAT and buspirone. In these studies, we found that another 5-HT(1B/D) agonist, 3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate (SKF99101H), significantly elevated locomotor activity in guinea pigs when given alone. We assessed the relative contribution of 5-HT1(1A) and 5-HT(1B/D) receptors in the mediation of this effect.Activity was measured by photobeam interrupts in opaque Perspex cylinders linked to a computer. SKF99101H (20 mg/kg s. c.) significantly increased the locomotor activity in guinea pigs. The locomotor stimulant effect of SKF99101H (20 mg/kg s.c) was reversed by the selective 5-HT(1B/D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl -1,2,4-oxadiazol-3-yl)[1,1biphenyl]4-carboxamide (GR127935; 0.06-0. 25 mg/kg s.c.). The 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY100635; 0.05-0.25 mg/kg s.c.), slightly but significantly attenuated the hyperactivity induced by SKF99101H. These findings suggest that 5-HT(1B/D) receptor agonists may require concomitant activation of 5-HT(1A) receptors to induce locomotor activity in guinea pigs. The 5-HT(2A) receptor antagonist 6[2-[4-[bis(4-fluorophenyl)methylene]-1-piperidinyl]-ethyl]-7-methyl- 5H-thiazol[3,2-a]pyrimidin-5-one (ritanserin) had no effect on SKF99101H-induced hyperactivity, suggesting that these receptors are not involved in the mediation of SKF99101H-induced hyperactivity. SKF99101H-induced hyperactivity was significantly attenuated by the D(1) dopamine receptor antagonist SCH 23390 (0.005-025 mg/kg), but not by the D(2) dopamine receptor antagonist raclopride (0.25-1.0 mg/kg), possibly suggesting the selective involvement of D(1) dopaminergic receptors in the mediation of the stimulant actions of the 5-HT(1B/D) agonist.     

Differential effects of 5-HT2C receptor ligands on place conditioning and locomotor activity in rats

Effects of the 5-hydroxytryptamine (5-HT)(1A/1B/2C) receptor agonist N-[3-(trifluoromethyl)phenyl] piperazine (TFMPP, 0-3.0 mg/kg s.c.) and the 5-HT2C receptor agonist 8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY 161503, 0-3.0 mg/kg s.c.) in place conditioning were measured in male Sprague-Dawley rats. Effects of TFMPP, alone and with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-cyclohexanecarboxamine (WAY 100635), the 5-HT(1B) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1,1'-biphenyl-4-carboxamide (GR 127935) or the 5-HT2C receptor antagonist 6-chloro-5-methyl-1-[[2-(2-methylpyrid-3-yloxy)pyrid-5-yl]carbamoyl]indoline (SB 242084) and of WAY 161503 alone and with SB 242084 on locomotor activity were also assessed. Neither TFMPP nor WAY 161503 induced place conditioning. WAY 161503 (1.0 and 3.0 mg/kg s.c.) decreased locomotor activity; SB 242084 (1.0 mg/kg i.p.) blocked this effect. Reduced locomotor activity following TFMPP was blocked by SB 242084 but not WAY 100635 (0.1 mg/kg s.c.) or GR 127935 (3.0 mg/kg s.c.). Behaviourally relevant levels of 5-HT2C receptor stimulation may not exert reinforcing effects, although other studies indicate that such manipulations alter reinforcing effects of drugs of abuse.     

The anxiolytic-like effect of 5-HT1B receptor ligands in rats: a possible mechanism of action

We have examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg kg(-1)), and the influence of flumazenil (Ro 15-1788, 10 mg kg(-1)), a benzodiazepine receptor antagonist, on the anxiolytic-like activity of CP 94253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine), a 5-HT1B receptor agonist, SB 216641 (N-[3-[3-(dimethylamino)ethoxy]-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide), a 5-HT1B receptor antagonist, and GR 127935 (N-[4-methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-l, l'-biphenyl-4-carboxamide), a 5-HT1B/1D receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. CP 94253 (2.5 mg kg(-1)), SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test in vehicle- and p-CA-pretreated rats. Flumazenil did not change the anxiolytic-like effect of CP 94253 (2.5 mg kg(-1)), but wholly blocked the anxiolytic-like effects of SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)). p-CA and flumazenil alone were inactive in the conflict drinking test. The results suggested that the anxiolytic-like effect of the 5-HT1B receptor ligands CP 94253, SB 216641 and GR 127935 was possibly linked to the postsynaptic 5-HT1B receptors or/and 5-HT1B heteroreceptors. The results suggested also that benzodiazepine receptors were indirectly involved in the effects of SB 216641 and GR 127935 (but not of CP 94253), which might have been due to a possible interaction between the 5-HT and the GABA/benzodiazepine systems.     

Effect of combined administration of 5-HT1A or 5-HT1B/1D receptor antagonists and antidepressants in the forced swimming test

In the present study, we examined effects of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor citalopram, the 5-HT/noradrenaline reuptake inhibitor imipramine, the selective noradrenaline reuptake inhibitor desipramine or the monoamine oxidase-A inhibitor moclobemide, administered in combination with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635) or the 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)1,1'-biphenyl-4-carboxamide (GR 127935) and the 5-HT(1B) receptor antagonist N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641) in the forced swimming test in rats. When given alone, citalopram (20 and 30 mg/kg), imipramine (20 mg/kg), desipramine (20 mg/kg), moclobemide (20 mg/kg), WAY 100635 (0.1 and 1 mg/kg), GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) did not shorten the immobility time of rats. Co-administration of WAY 100635 (0.1 and 1 mg/kg) and citalopram (20 mg/kg), or imipramine (20 mg/kg), or moclobemide (20 mg/kg) did not affect the immobility time of rats, whereas WAY 100635 given jointly with desipramine (20 mg/kg) induced a weak anti-immobility effect. GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) co-administered with imipramine, desipramine or moclobemide, but not citalopram, produced a significant anti-immobility action in the forced swimming test in rats. These results indicate that the blockade of 5-HT(1B) rather than 5-HT(1A) receptors may facilitate the anti-immobility effect of imipramine, desipramine or moclobemide in the forced swimming test. No interaction was observed between 5-HT(1A) or 5-HT(1B/1D) receptor antagonists and citalopram.     

No involvement of 5-HT(7) or 5-HT(1D) receptors in the (R)-8-OH-DPAT-induced depression of the monosynaptic reflex in spinalized rats

(R)-8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) depressed the monosynaptic reflex. This effect was not antagonized by 5-HT(1A) receptor antagonists. We examined whether 5-HT(1D) and 5-HT(7) receptors are involved in (R)-8-OH-DPAT-induced inhibition of the monosynaptic reflex in spinalized rats. Pretreatment with methiothepin and mesulergine, but not clozapine, inhibited (R)-8-OH-DPAT-induced monosynaptic reflex depression. Pretreatment with 2a-(4-phenyl-1,2,3,6-tetrahydropyridal)butyl)-2a,3,4,5-tetrahydrobenzo[c,d]indol-2(1H)-one (DR4004) and (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrolidine (SB-269970), new selective 5-HT(7) receptors antagonists, and N-[methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide (GR127935), a selective 5-HT(1D) receptor antagonist, had no effect on (R)-8-OH-DPAT-induced depression. These results suggested that 5-HT(7) and 5-HT(1D) receptors are not involved in (R)-8-OH-DPAT-induced monosynaptic reflex depression.     

Stimulation of central 5-HT1D receptors causes hypothermia in the guinea-pig

The 5-HT(1) receptor agonist GR46611 (3-30 mg/kg s.c.) caused a dose-related decrease in rectal temperature in the adult guinea-pig. A lower dose (20 μg) administered directly into the lateral cerebral ventricle also caused a hypothermic response, suggesting that this effect is centrally mediated. GR46611-induced (10 mg/kg s.c.) hypothermia was not attenuated by WAY100135 (3-10 mg/kg s.c.), ritanserin (0.3-1 mg/kg s.c.), spiperone (0.1-0.3 mg/kg s.c.) and ondansetron (0.1-1 mg/kg s.c.), suggesting that 5-HT(1A), 5-HT(2A), 5-HT( 2C) and 5-HT(3) receptors are unlikely to be involved in this response. In contrast, the poorly selective 5-HT receptor antagonist, metergoline (1-10 mg/kg s.c.), and the potent 5-HT(1D) receptor antagonist, GR127935 (0.1-1 mg/kg p.o.), antagonized the effects of GR46611. The present data suggest that antagonism of GR46611-induced hypothermia may be useful for assessing the potency and duration of action of centrally-acting 5-HT( 1D) receptor antagonists in the guinea-pig.     

Regulation of sensitivity to 5-hydroxytryptamine in pulmonary supernumerary but not conventional arteries by a 5-HT(1D)-like receptor

Bovine pulmonary supernumerary arteries are more sensitive to 5-hydroxtryptamine (5-HT) (pD(2) 6.43+/-0.25) than conventional arteries (pD(2) 5.32+/-0.16). This study investigated receptors for 5-HT in ring segments of these arteries. The 5-HT(2) receptor agonist, 2,5 dimethoxy-4-iodoamphetamine hydrobromide (DOI) constricts both arteries. The selective 5-HT(2) receptor antagonist ritanserin produced insurmountable antagonism of 5-HT concentration-response curves in both arteries, whereas the 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'(5-methyl- 1,2,4-oxadiazol-3-yl[1,1,-biphenyl]-4-carboxamide hydrochloride (GR127935) produced much greater antagonism in supernumerary arteries. In rings preconstricted with 9,11-dideoxy-9, 11-methanoepoxy prostalagdin F(2alpha) (U46619) and relaxed with the adenylyl cyclase activator forskolin, the selective 5-HT(ID) receptor agonist 2-[5-[3-(4-methylsulphonylamino) benzyl-1,2, 4-oxadiazol-5-yl]-1H-indole-3-yl] ethylamine (L694247) reversed the relaxation. Concentration-response curves for L694247-induced reversal of forskolin-relaxation were antagonised by GR127935 in supernumerary (pK(B) 8.6) and conventional (pK(B) 8.4) arteries, whereas concentration-response curves to 5-HT-were less sensitive to antagonism by GR127935T and this was more obvious in conventional (pK(B) 7.6) than supernumerary (pK(B) 8.1) arteries. Neither the selective 5-HT(1D) receptor antagonist (1-(3-chlorophenyl)-4-[3, 3-diphenyl (2-(S,R) hydroxypropanyl)piperazine] hydrochloride (BRL15572) nor the 5-HT(1B) receptor antagonist (2,3,6, 7-tetrahydro-1'-methyl-5-[2'methyl-4'5-(methyl-1,2,4-oxadiazol-3-y l) biphenyl-4-carbonyl]furo[2,3-f]indole-3-spiro-4'-piperidine hydrochloride (SB224289) antagonised concentration-response curves induced by 5-HT or 5-HT(1)-receptor-selective agonists. In addition to the 5-HT(2A) receptor, 5-HT activates a GR127935-sensitive and a GR127935-insensitive receptor in these arteries. Supernumerary arteries have a greater proportion of GR127935-sensitive receptors, which display only some of the pharmacological characteristics of the cloned 5-HT(ID) receptor. It is possible that the GR127935-sensitive receptor could be a species homologue of the human 5-HT(1B) receptor that is insensitive to SB224289.     

Subchronic fluoxetine administration to rats: effects on 5-HT autoreceptor activity as measured by in vivo microdialysis.

Subchronic administration of fluoxetine to rats has been shown to induce subsensitivity of presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors, and also postsynaptic 5-HT(1A) receptors in the hypothalamus. We investigated the effects of administration of fluoxetine (10 mg/kg i.p.) to rats for 6 days on presynaptic 5-HT(1A) receptor activity in the hypothalamus, postsynaptic 5-HT(1A) receptor activity in the hippocampus, and presynaptic 5-HT(1B) autoreceptor activity in both areas, using in vivo microdialysis. The effect of the 5-HT(1B/1D) antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5- methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to elevate 5-hydroxytryptamine (5-HT) levels was reduced in hippocampus but not hypothalamus of fluoxetine-treated rats. Fluoxetine did not alter either presynaptic 5-HT(1A) autoreceptor activity, as measured by the effect of injection of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) (0.2 mg/kg or 50 microg/kg s.c.) on 5-HT levels in the hypothalamus, or postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT (0.2 mg/kg s.c.) on cyclic AMP accumulation, in the hippocampus.     

Serotonin 2C receptor agonists and the behavioural satiety sequence in mice

The studies reported here examined the role of the 5-hydroxytryptamine (5-HT)(2C) receptor subtype in the control of ingestive behaviour in mice. Behavioural satiety sequence (BSS) and food intake measurements were taken, comparing the selective 5-HT(2C) receptor agonist (S)-2-(6-chloro-5-fluoro-indol-l-yl)-l-methylethylamine hydrochloride (Ro 60-0175; 1.0, 3.0 and 10.0 mg/kg) and D-fenfluramine (3.0 mg/kg). Ro 60-0175 produced a dose-dependent decrease in food intake. The effects of Ro 60-0175 (3.0 mg/kg) on the BSS were similar to the hypophagic effects of D-fenfluramine (3.0 mg/kg). In a second experiment, the specific effects on feeding produced by Ro 60-0175 (5.6 mg/kg) were attenuated by pretreatment with the selective 5-HT(2C) receptor antagonist 6-chloro-5-methyl-1-[2(2-methylpyridyl-3-oxy)-pyrid-5-yl carbamoyl] indoline (SB 242084; 0.5 mg/kg). The 5-HT(1B/2C) receptor agonist 1-(m-chlorophenyl)piperazine (mCPP; 3 mg/kg) also produced a substantial decrease in food intake, which was attenuated by SB 242084 (0.5 mg/kg). A dose of the selective 5-HT(1B/1D) antagonist 2'-methyl-4'(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-(5-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]amide (GR 127935; 3.0 mg/kg) that successfully attenuated the action of the 5-HT(1B) agonist 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969; 5.0 mg/kg) failed to attenuate mCPP-induced hypophagia. These data suggest that Ro 60-0175- and mCPP-induced hypophagia in mice are mediated via activation of 5-HT(2C) receptors and that stimulation of 5-HT(1B) receptors plays only a minor role in mCPP-induced hypophagia.     

Functional characterization of the 5-HT terminal autoreceptor in the guinea-pig brain cortex.

1 In guinea-pig cerebral cortical slices in vitro we have shown that the rank order of potency of 5-hydroxytrptamine (5-HT), 5-carboxamidotryptamine and sumatriptan for inhibition of electrically stimulated [3H]-5-HT release correlates well with published data on their 5-HT1D receptor binding affinities. 2 Both the non-selective 5-HT1D receptor antagonist, methiothepin and the selective 5-HT1D receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl]phenyl]-2'-methyl-4'- (5-methyl-1,2,4-oxadiazole-3-yl) [1,1-biphenyl]4-carboxamide (GR127935) increased stimulated [3H]-5-HT release per se and also attenuated agonist-induced inhibition of [3H]-5-HT release. GR127935 (10 nM-100 nM) produced a pA2 of 9.0 against 5-HT, which is consistent with its 5-HT1D receptor binding affinity. 3 From these findings we conclude that, in guinea-pig cerebral cortex, the 5-HT terminal autoreceptor is of the 5-HT1D receptor subtype. However, three observations suggest the presence of multiple terminal autoreceptors: shallow inhibition curves to the agonists; a shallow Schild slope of GR127935 antagonism and differences in the maximal responses to 5-HT between whole cortex and frontal cortex.     

Involvement of dopamine autoreceptors in the hypoactivity induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in mice.

The effect of 8-OH-DPAT, a 5-HT1A receptor agonist, on the locomotor activity was analyzed in Albino Swiss mice. The studied drug (0.5-5 mg/kg) inhibited the spontaneous locomotor activity in mice. The hypoactivity induced by 8-OH-DPAT (1.5 mg/kg) was abolished by the dopamine (D1 and D2) receptor antagonist-haloperidol (0.00125 and 0.0025 mg/kg, but not in higher doses) and by the D2 antagonist with affinity for 5-HT1A and 5-HT2 receptors-spiperone (0.0025 and 0.005 mg/kg, but not in higher doses). The effect of 8-OH-DPAT was slightly reduced by the alpha 2-adrenoceptor antagonists: idazoxan (4 mg/kg), yohimbine (2 and 4 mg/kg) and rauwolscine (4 mg/kg). On the other hand, the non-selective 5-HT antagonist metergoline (0.5-4 mg/kg), the 5-HT1A antagonist NAN-190 (0.5-2 mg/kg), the beta-adrenoceptor blockers with high affinity for 5-HT1A and 5-HT1B receptors: pindolol and SDZ 21009 (2-8 mg/kg) and the agonist/antagonist of 5-HT1A receptors ipsapirone (2.5 and 5 mg/kg) did not affect the 8-OH-DPAT-induced hypoactivity. The obtained results suggest that the reduction of the spontaneous locomotor activity induced by 8-OH-DPAT results from a stimulation of dopamine autoreceptors, but not 5-HT receptors. Involvement of an alpha 2-adrenergic mechanism cannot be excluded.     

SB-272183, a selective 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptor antagonist in native tissue.

A novel compound, SB-272183 (5-Chloro-2, 3-dihydro-6-[4-methylpiperazin-1-yl]-1[4-pyridin-4-yl]napth-1-ylaminocarbonyl]-1H-indole), has been shown to have high affinity for human 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with pK(i) values of 8.0, 8.1 and 8.7 respectively and is at least 30 fold selective over a range of other receptors. [(35)S]-GTPgammaS binding studies showed that SB-272183 acts as a partial agonist at human recombinant 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with intrinsic activities of 0.4, 0.4 and 0.8 respectively, compared to 5-HT. SB-272183 inhibited 5-HT-induced stimulation of [(35)S]-GTPgammaS binding at human 5-HT(1A) and 5-HT(1B) receptors to give pA(2) values of 8.2 and 8.5 respectively. However, from [(35)S]-GTPgammaS autoradiographic studies in rat and human dorsal raphe nucleus, SB-272183 did not display intrinsic activity up to 10 microM but did block 5-HT-induced stimulation of [(35)S]-GTPgammaS binding. From electrophysiological studies in rat raphe slices in vitro, SB-272183 did not effect cell firing rate up to 1 microM but was able to attenuate (+)8-OH-DPAT-induced inhibition of cell firing to give an apparent pK(b) of 7.1. SB-272183 potentiated electrically-stimulated [(3)H]-5-HT release from rat and guinea-pig cortical slices at 100 and 1000 nM, similar to results previously obtained with the 5-HT(1B) and 5-HT(1D) receptor antagonist, GR127935. Fast cyclic voltammetry studies in rat dorsal raphe nucleus showed that SB-272183 could block sumatriptan-induced inhibition of 5-HT efflux, with an apparent pK(b) of 7.2, but did not effect basal efflux up to 1 microM. These studies show that, in vitro, SB-272183 acts as an antagonist at native tissue 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors.     

Activation of central 5HT1B receptors increases locomotor activity in mice

The effects of 5-HT1B/D receptor agonists anpirtoline, S-CM-GTNH2 and CP93129 were examined in locomotor activity tests in mice. Anpirtoline and S-CM-GTNH2 both significantly increased locomotor activity (ambulation) (minimum affective doses; 3 and 30 mg/kg respectively). CP 93129 injected subcutaneously was without effect, but increased locomotor activity when given i.c.v. Thus activation of central 5-HT1B receptors results in increased locomotor activity. The effects of anpirtoline and S-CM-GTNH2 were blocked by the antagonist GR 127935 suggesting that GR 127935 is an effective antagonist of 5-HT1B receptors in vivo. © 1997 John Wiley & Sons, Ltd.     

Pharmacological evidence that 5-HT(1B/1D) receptors mediate hypotension in anesthetized rats

5-Carboxamidotryptamine (5-CT; 0.003-310 microg/kg, i.v.) produced dose-dependent hypotensive responses which were blocked in a complex manner by the 5-HT(7) receptor antagonist, (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl) ethyl] pyrrolidine (SB-269970; 1 mg/kg, i.v.), in anesthetized vagosympathectomized rats. Interestingly, the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide hydrochloride monohydrate GR-127935 (1 mg/kg, i.v.), also inhibited 5-CT-induced hypotension but the effect was clearly noncompetitive. Finally, the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) produced a further decreased of 5-CT-induced responses as compared to the effect of individual treatments. These data suggest that, in addition to 5-HT(7) receptors, 5-HT(1B/1D) receptors may also mediate hypotension in rats.     

Effects of PNU-109,291, a selective 5-HT1D receptor agonist, on electrically induced dural plasma extravasation and capsaicin-evoked c-fos immunoreactivity within trigeminal nucleus caudalis.

We studied the effects of PNU-109291 [(S)-(-)-1-[2-[4-(4-methoxyphenyl)-1-piperazinyl]ethyl]-N-methyl-isoc hroman-6-carboxamide], a receptor agonist showing 5000-fold selectivity for primate 5-HT1D versus 5-HT1B receptors (Ennis et al., J. Med. Chem. 41, 2180-2183), on dural neurogenic inflammation and on c-fos like immunoreactivity within trigeminal nucleus caudalis evoked by electrical and chemical activation of trigeminal afferents, respectively. Subcutaneous injection of PNU-109291 in male guinea pigs dose-dependently reduced dural extravasation of [125I]-labeled bovine serum albumin evoked by trigeminal ganglion stimulation with an IC50 of 4.2 nmol kg(-1). A dose of 73.3 nmol kg(-1) blocked the response completely. The selective 5-HT1B/1D receptor antagonist GR-127935 (> or = 2 micromol kg(-1) i.v.) prevented this effect. In addition, the number of c-fos immunoreactive cells within guinea pig trigeminal nucleus caudalis induced by chemical meningeal stimulation (intracisternally administered capsaicin) was reduced by more than 50% with PNU-109291 (> or = 122.2 nmol kg(-1) administered s.c. 45 min before and 15 min after capsaicin). These data indicate that the 5-HT1D receptor subtype plays a significant role in suppressing meningeal neurogenic inflammation and attenuating trigeminal nociception in these guinea pig models. Since 5-HT1D receptor mRNA and protein are expressed in trigeminal ganglia but not vascular smooth muscle, the 5-HT1D receptor subtype may become a useful therapeutic target for migraine and related headaches.     

5-HT7 receptor-mediated dilatation in the middle meningeal artery of anesthetized rats

Topical administration of 5-carboxamidotryptamine (5-CT; 0.01-1000 microM) to the exposed dura mater encephali of anesthetized rats produced decreases in blood pressure and dilatation in the middle meningeal artery. Pretreatment with the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide hydrochloride monohydrate (GR-127935; 1 mg/kg, i.v.), unmasked meningeal dilator responses to lower concentrations of 5-CT, and attenuated those to higher concentrations; GR-127935 also inhibited 5-CT-induced hypotension. The 5-HT7 receptor antagonist, (R)-1-{(3-hydroxyphenyl)sulfonyl}-2-{2-(2-(4-methyl-1-piperidinyl) ethyl} pyrrolidine (SB-269970; 1 mg/kg, i.v.), strongly inhibited dilator and hypotensive responses to 5-CT; the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) further inhibited meningeal and hypotensive responses. Thus, 5-CT may produce dilatation in the middle meningeal artery via 5-HT7 receptors; complex effects appear to involve 5-HT(1B/1D) receptors.     

Functional effects of corticosterone on 5-HT(1A) and 5-HT(1B) receptor activity in rat brain: in vivo microdialysis studies

Glucocorticoid hormones are known to be elevated in depression, and to interact with serotonin 5-HT(1A) receptors at both the presynaptic and postsynaptic levels. Since one of the presumed mechanisms of action of antidepressant drugs is induction of changes in sensitivity of 5-HT(1A) and also 5-HT(1B) receptors, the effects of repeated administration of corticosterone (50 mg/kg s.c. b.i.d. for 10 days) on activities of these receptors were determined using in vivo microdialysis in freely moving rats. Presynaptic 5-HT(1A) receptor activity, as measured by the effect of a challenge dose (0.2 mg/kg s.c.) of the 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) to reduce 5-HT levels in the hypothalamus, was not affected by corticosterone administration. Presynaptic 5-HT(1B) receptor activity, as measured by the effect of the 5-HT(1B) receptor antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to increase 5-HT levels, was increased in hypothalamus but not hippocampus of corticosterone-treated rats. Postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT to increase cyclic AMP levels in the hippocampus, was not affected by corticosterone administration. The decrease in presynaptic 5-HT(1B) receptor activity after chronic administration of antidepressant drugs complements the increases in 5-HT(1B) receptor number observed in animal models of depression.     

Effects of a selective 5-HT1B receptor agonist and antagonists in animal models of anxiety and depression

The purpose of the present study was to investigate the effects of the selective 5-HT1B receptor agonist CP 94253, the selective 5-HT1B receptor antagonist SB 216641, and the 5-HT1B/1D receptor antagonist GR 127935 in behavioral tests commonly used to predict anxiolytic- and antidepressant-like activity. Diazepam and imipramine were used as reference drugs. In the Vogel conflict drinking test, CP 94253 (1.25-5 mg/kg), SB 216641 (2.5-5 mg/kg) and GR 127935 (5-10 mg/kg) showed anxiolytic-like effects comparable to that of diazepam (2.5-5 mg/kg). In the elevated plus-maze test, antianxiety-like activity of all the compounds tested was also observed: the effects of CP 94253 (2.5 mg/kg) and SB 216641 (5 mg/kg) were similar to that of diazepam (5 mg/kg), while GR 127935 (up to 40 mg/kg) was less active. In the four-plate test, the compounds tested (5-10 mg/kg) produced anxiolytic-like effects which were weaker than that of diazepam (2.5-5 mg/kg). In the forced swimming test, CP 94253 (5-10 mg/kg), like imipramine (30 mg/kg), showed anti-immobility action, whereas SB 216641 (2.5-10 mg/kg) and GR 127935 (20-40 mg/kg) did not affect the immobility time in mice. The results indicate that the selective agonist (CP 94253) and antagonists (SB 216641 and GR 127935) of 5-HT1B receptors produce effects that are characteristic of anxiolytics, in the preclinical models used; however, CP 94253 also behaves like an antidepressant drug.     

Anxiogenic-like effect of serotonin(1B) receptor stimulation in the rat elevated plus-maze

Perturbations in serotonin [5-hydroxytryptamine (5-HT)] neurotransmission have been implicated in several psychiatric illnesses including depression and anxiety disorders. It is not yet clear, however, which of the 14 currently identified 5-HT receptor subtypes in the brain participate in the regulation of emotional states. This study investigates a role for the 5-HT(1B) receptor subtype in anxiety-related behaviors using the elevated plus-maze paradigm in rats. The selective 5-HT(1B) receptor agonist 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94,253; 1--5.6 mg/kg) dose-dependently decreased the amount of exploration on the open arms of the plus-maze without altering overall locomotor activity. This 5-HT(1B) agonist-induced increase in anxiety-like behavior was dose-dependently reversed by coadministration of the selective 5-HT(1B/1D) receptor antagonist 2'-methyl-4'-(5-methyl[1,2,4]oxadiazol-3-yl)-biphenyl]-amide (GR 127,935). There was no significant effect of GR 127,935 administration alone on plus-maze behavior. These results indicate that 5-HT(1B) receptor activation increases anxiety-like behavioral responses as measured by the elevated plus-maze. Since 5-HT(1B) receptors modulate the activity of multiple neurotransmitter systems that have been implicated in anxiety disorders, these findings suggest that this receptor subtype may represent an important therapeutic target for the treatment of anxiety.     

The role of the 5-HT1D receptor as a presynaptic autoreceptor in the guinea pig

The present study investigated the role of the 5-hydroxytryptamine (5-HT, serotonin)1D receptor as a presynaptic autoreceptor in the guinea pig. In keeping with the literature, the 5-HT1B selective antagonist, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydrospiro [furo[2,3-f]indole-3,4'-piperidine]oxalate (SB224289) potentiated [3H]5-HT outflow from pre-labelled slices of guinea pig cerebral cortex confirming its role as a presynaptic autoreceptor in this species. In addition, the 5-HT1D receptor-preferring antagonists, 1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-2H-pyridin-1-yl]-ethyl]-3-pyridin-4-yl-methyl-tetrahydro-pyrimidin-2-one (LY367642), (R)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456219), (S)-1-[2-(4-(6-fluoro-1H-indol-3-yl-)-3,6-dihydro-1(2H)-pyridinyl)ethyl]-3,4-dihydro-1H-2-benzopyran-6-carboxamide (LY456220) and 1-[2-[4-(4-fluoro-benzoyl)-piperidin-1-yl]-ethyl]-3,3-dimethyl-1,2-dihydro-indol-2-one (LY310762), potentiated [3H]5-HT outflow from this preparation with potencies (EC50 values=31-140 nM) in the same range as their affinities for the guinea pig 5-HT1D receptor (Ki values=100-333 nM). The selective 5-HT1D receptor agonist, R-2-(4-fluoro-phenyl)-2-[1-[3-(5-[1,2,4]triazol-4-yl-1H-indol-3-yl)-propyl]-piperidin-4-ylamino]-ethanol dioxylate (L-772,405), inhibited [3H]5-HT outflow. In microdialysis studies, administration of either SB224289 or LY310762 at 10 mg/kg by the intraperitoneal (i.p.) route, potentiated the increase in extracellular 5-HT concentration produced by a maximally effective dose of the selective serotonin re-uptake inhibitor, fluoxetine (at 20 mg/kg i.p.). In addition, the 5-HT1D receptor-preferring antagonist and 5-HT transporter inhibitor, LY367642 (at 10 mg/kg i.p.), elevated extracellular 5-HT concentrations to a greater extent than a maximally effective dose of fluoxetine. It is concluded that the 5-HT1D receptor, like the 5-HT1B receptor, may be a presynaptic autoreceptor in the guinea pig.