Effects on the 5-hydroxytryptamine-induced head-twitch of drugs with selective actions on alpha1 and alpha2-adrenoceptors.

Effects of agents with selectivity for the alpha1- or alpha2-adrenoceptor have been investigated on the head twitches induced by 5-HT injected into the cerebral ventricles in the mouse. Alpha-adrenoceptor agonists inhibited head-twitching with a rank order of potency: guanabenz, clonidine, phenylephrine, methoxamine. The antagonists yohimbine and piperoxane, potentiated the head-twitch frequency. These results suggest that there may be an involvement of an alpha2-adrenoceptor. Small doses of phenylephrine (s.c.) potentiated the head-twitch. Methoxamine, injected intracerebroventricularly potentiated the head-twitch only when alpha2-adrenoceptors were blocked by a small dose of yohimbine. Prazosin and thymoxamine inhibited the head-twitch, suggesting that alpha1-adrenoceptors also modulate this phenomenon. Yohimbine and phenylephrine increased the incidence of spontaneous head-twitches. A tonic noradrenergic input may be necessary for the occurrence of the head-twitch induced by 5-HT.