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

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.