LSD, mescaline and serotonin injected into medial raphe nucleus potentiate apomorphine hypermotility

Microinjections of LSD, mescaline, and serotonin into the medial raphe nucleus of rats strongly potentiated the increase in locomotor activity caused by apomorphine. This potentiating effect of LSD or serotonin was blocked by simultaneous injections of methysergide or cyproheptadine into the same brain region. Injections of the same doses of LSD into the dorsal raphe nucleus or of LSD and mescaline into the nucleus accumbens did not affect locomotor activity, while higher doses into the nucleus accumbens inhibited both spontaneous and apomorphine-stimulated activity. The findings suggest that low systemic doses of hallucinogens potentiate behavior by preferentially acting on the serotonergic system in the medial raphe nucleus.

Study at a glance

Abstract

Microinjections of LSD (0.05 microgram), mescaline (0.5 microgram) and serotonin (10 microgram) into the medial raphe nucleus of rats resulted in a strong potentiation of apomorphine (1 mg/kg i.p.)-induced hypermotility. The potentiating effect of LSD or serotonin was suppressed by simultaneous injections of methysergide (0.05 microgram) or cyproheptadine (0.05 microgram) into the medial raphe nucleus. The same doses of LSD injected into the dorsal raphe nucleus and of LSD and mescaline injected into the nucleus accumbens failed to influence locomotor activity, whereas injections of higher doses of LSD and mescaline into the nucleus accumbens inhibited spontaneous and apomorphine-stimulated locomotor activity. It is concluded that the potentiating effect of systemically administered low doses of hallucinogens was triggered by preferential actions on the serotonergic system in the medial raphe nucleus.