Myoclonus in guniea pigs is induced by indole-containing but not piperazine-containing 5HT agonists

L-5-Hydroxytryptophan (5HTP) induces in guinea pigs a myoclonic jerking which is dependent upon stimulation of brainstem 5-hydroxytryptamine (5HT) receptors. We have investigated the ability of 5HT precursors and a range of synthetic 5HT agonists to produce myoclonus. The 5HT precursors and 5HT agonists containing an indole nucleus induced dose-dependent jerking in guinea pigs. In contrast, 5HT agonists possessing a piperazine moiety induced occasional jerking only at toxic doses, but not a those doses normally associated with 5HT agonist activity. The difference in activity between the indole-containing compounds and piperazine-containing 5HT agonists suggests that myoclonus is due to activation of an indole-selective brainstem 5HT receptor and provides further evidence for multiple cerebral 5HT receptors.