title: alpha2-Adrenoceptor antagonists reverse the 5-HT2 receptor antagonist suppression of head-twitch behavior in mice.
abstract: The alpha2-adrenoceptor agonist clonidine, as well as 5-HT2 receptor antagonists, reportedly suppress 5-HT2 receptor-mediated head-twitch behavior. We investigated the effect of alpha2-adrenoceptor antagonists on the suppressive action of 5-HT2 receptor antagonists in mice pretreated with the noradrenaline toxin 6-hydroxydopamine (6-OHDA) or the 5-HT synthesis inhibitor p-chlorophenylalanine (p-CPA). In normal mice, idazoxan (0.08-0.2 mg/kg, IP) or yohimbine (0.2-2.0 mg/kg, IP), both alpha2-adrenoceptor antagonists, had no effect on the head-twitch response caused by 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 16 mg/kg, IP), but idazoxan significantly enhanced the response at 0.5 mg/kg. On the other hand, these alpha2-adrenoceptor antagonists, at doses that had no effect on the basal number of head-twitches (idazoxan 0.2 mg/kg and yohimbine 0.5 mg/kg), significantly attenuated not only the suppressive effect of clonidine (0.01 mg/kg, IP) on head-twitch response but also that of the 5-HT2 receptor antagonist ritanserin (0.03 mg/kg, IP). Moreover, idazoxan (0.2 mg/kg) also significantly reversed the inhibition by 0.01 mg/kg (IP) ketanserin, a selective 5-HT2 receptor antagonist. Pretreatment with 6-OHDA plus nomifensine but not with p-CPA significantly attenuated the effect of idazoxan (0.2-0.5 mg/kg) on the ritanserin inhibition of the head-twitch response. Prazosin, an alpha1-adrenoceptor antagonist, dose-dependently suppressed the response, and the effect of prazosin (1.25 mg/kg) was significantly attenuated by 0.5 mg/kg idazoxan. These results indicate that endogenous noradrenaline is involved in the apparent antagonistic interaction between selective alpha2-adrenoceptor antagonists and 5-HT2 receptor antagonists in the head-twitch response, and suggest that noradrenaline stimulation of alpha1-adrenoceptors may be involved in this apparent antagonism.
authors.affiliation: Department of Pharmacology, Toyama Medical and Pharmaceutical University, Sugitani, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Matsumoto
authors.forename: M
authors.initials: M
authors.lastname: Mizowaki
authors.forename: S
authors.initials: S
authors.lastname: Thongpraditchote
authors.forename: Y
authors.initials: Y
authors.lastname: Murakami
authors.forename: H
authors.initials: H
authors.lastname: Watanabe
mesh_terms: D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D004353:Drug Evaluation, Preclinical; D010134:Fenclonine / Q000494:pharmacology; D019416:Head Movements / Q000187:drug effects*; D007650:Ketanserin / Q000494:pharmacology; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009638:Norepinephrine / Q000502:physiology*; D016627:Oxidopamine; D016713:Ritanserin / Q000494:pharmacology; D012701:Serotonin / Q000502:physiology*; D012702:Serotonin Antagonists / Q000494:pharmacology*
chemical_list: D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D012702:Serotonin Antagonists; D016713:Ritanserin; D012701:Serotonin; D016627:Oxidopamine; D007650:Ketanserin; D003000:Clonidine; D010134:Fenclonine; D009638:Norepinephrine