Effects of 5,7-dihydroxytryptamine and 6-hydroxydopamine on head-twitch response induced by serotonin, p-chloroamphetamine, and tryptamine in mice.

Head-twitch response (HTR) in mice was induced by intracerebroventricular injection of tryptamine (TRA) as well as serotonin (5-HT) and p-chloroamphetamine (PCA). Pretreatment with 5,7-dihydroxytryptamine enhanced both the 5-HT-induced and the TRA-induced HTR. The PCA-induced HTR, however, was attenuated by the drug. On the other hand, pretreatment with 6-hydroxydopamine did not alter the 5-HT response but enhanced both the PCA- and the TRA-induced response. These results suggest that 5-HT may directly stimulate the post-synaptic receptors, while the PCA response may be based on the release of endogenous 5-HT. The presynaptic component of the central serotonergic system does not appear to be involved in the TRA response. Both PCA and TRA may affect catecholaminergic systems which can suppress the response.