Further studies on the role of indoleamines in the responses of cortical neurones to stimulation of nucleus raphe medianus: effects of indoleamine precursor loading.

Responses of cortical neurones to stimulation of nucleus raphe medianus (RM) were determined before and after parenteral administration of l-tryptophan or l-5-hydroxytryptophan (5HTP). The short latency inhibitory effects of stimulation and the longer latency excitation were enhanced by l-tryptophan. 5-Hydroxytryptophan, on the other hand, enhanced the excitatory effects but had no effects on the initial inhibition. Both precursors were able to induce a third type of response, a long-latency inhibition which succeeded the excitation. Inhibition of tryptophan hydroxylase abolished the facilitatory effects of tryptophan on the excitatory and long-latency inhibitory effects of stimulation of the raphé medianus but the enhancement of the short-latency inhibition remained intact. Inhibition of tryptophan hydroxylase failed to alter any of the effects of 5HTP on evoked responses to stimulation of the raphé medianus. Finally, inhibition of aromatic amino acid decarboxylase abolished the effects of both indoleamine precursors on all response phases. The results are consistent with a previous suggestion that the short-latency inhibition may be tryptamine-mediated while the other response phases are mediated by 5-hydroxytryptamine.