5-Methoxytryptamine-induced head twitches in rats.

In rats pretreated with pargyline ip injection of 5-methoxytryptamine (5-MT), but not of 5-HT, produced characteristic head twitches. 5-HT receptor blocking agents (cyproheptadine, methergoline, mianserine, WA-335-BS, and methsergide) reduced the effect of 5-MT. These substances in doses 20-160 times higher than their ED50 in head twitch test did not antagonize the pinna reflex. 5-MT-induced head twitches were also inhibited by imipramine, morphine, phenoxybenzamine and aceperone. However, their ED50 were only 3-8 times below the doses necessary to inhibit the pinna reflex. Reserpine, phentolamine, spiroperidol, pimozide and LiCl did not modify head twitches produced by 5-MT, while PCPA even potentiated them. Our results suggest the 5-MT-induced head twitches are due to the activation of the central 5-HT mechanisms probably as a result of direct stimulation of 5-HT receptors by 5-MT. Antagonism of 5-MT-induced head twitches and the dissociation of doses effective in this test from those inhibiting the pinna reflex may be of value in the prediction of central 5-HT receptor blocking properties.