GABA-serotonergic interactions in the regulation of central sympathetic pathways.

Methysergide, a serotonin antagonist, and clonidine, an alpha 2-adrenoceptor agonist, lowered arterial blood pressure, heart rate and inferior cardiac sympathetic nerve activity in the anesthetized cat. Picrotoxin, a GABA antagonist, caused a reduction in the inhibition of sympathetic activity produced by methysergide. In contrast, picrotoxin did not effect the reduction of sympathetic nervous discharge resulting from administration of clonidine. These observations suggest that an interaction exists between GABAergic and serotonergic neurons which regulates central sympathetic pathways, possibly by a tonic GABAergic inhibition of serotonergic sympathoexcitatory pathways.