Possible involvement of serotonin receptors in the facilitatory effect of a hallucinogenic phenethylamine on single facial motoneurons.

2,5-Dimethoxy-4-methylamphetamine (DOM, "STP") is a potent hallucinogen, proposed to be a serotonin receptor agonist. Its effects have not previously been tested upon central neurons where serotonin is excitatory and serotonin antagonists are effective. Extracellular single unit recordings were obtained from facial motoneurons in anaesthetized rats, and drugs were applied from five-barrelled micropipettes by iontophoresis. Facial motoneurons were commonly silent. During subthreshold application of glutamate, firing could be induced by dopamine and DOM. As reported by others, serotonin and noradrenaline also excited facial motoneurons under these conditions. Methysergide antagonized responses to serotonin and DOM but not those to noradrenaline; methysergide could not usually discriminate between responses to serotonin and dopamine. Ketanserin reversibly antagonized (but could not discriminate between) responses to serotonin, dopamine, and noradrenaline. Chlorpromazine antagonized responses to dopamine at doses that did not alter serotonin-induced excitation, and responses to DOM were not reduced by doses of chlorpromazine, that had no local anaesthetic effect on action potentials elicited by DOM and serotonin. These results suggest that DOM is an agonist on at least one type of central serotonin receptor. This receptor may also be a ketanserin (5-HT2) binding site.