A possible mechanism of the tyramine-induced head-twitch response

The effects of monoaminergic drugs on the head-twitch response (HTR) induced by intracerebroventricular (i.c.v.) injection of tyramine (TyA) in mice pretreated with safrazine, a monoamine oxidase inhibitor, were compared with effects on the response induced by the i.c.v. injection of serotonin (5-HT) in safrazine-pretreated mice. The HTR induced by both TyA and 5-HT were suppressed by i.p. injection of p-chlorophenylalanine. Chlorimipramine enhanced the 5-HT response but not the TyA response. Dimethothiazine, a serotonergic blocker, reduced both the TyA and 5-HT responses. The i.c.v. injection of p-chlorophenylalanine methylester resulted in a reduction of the TyA response but not of the 5-HT response. The i.c.v. injection of 5,6-dihydroxytryptamine (5,6 DHT) 1 day before the test suppressed the TyA response but enhanced the 5-HT response. The i.c.v. injection of noradrenaline reduced both the TyA and 5-HT responses. The i.c.v. injection of 6-hydroxydopamine, and i.p. injection of alpha-methyl-p-tyrosine and tolazoline accelerated the TyA response. These results suggest that the TyA response may be based on the release of endogenous 5-HT and can be suppressed by the catecholaminergic system.