Cardiovascular functions of central noradrenergic and serotonergic neurons in conscious rabbits. Their contributions to the central actions of clonidine.

Synaptic release of transmitter from central noradrenergic (NA) and serotonergic (5HT) neurons was studied in intact and pontine decerebrate unanesthetized rabbits, following intracisternal injections of the selective neurotoxic drugs 6-hydroxydopamine (6-OHDA) and 5,6-dihydroxytryptamine (5,6-DHT). The NA and 5HT neurons both raise blood pressure through a suprapontine pathway, with 5HT neurons in series with NA neurons. Descending bulbospinal fibers have antagonistic effects on blood pressure, with NA release lowering blood pressure and 5HT release increasing it. The two transmitters also have antagonistic effects on the cardiac vagus, with NA neurons increasing vagal activity and 5HT neurons inhibiting it. Our results suggest that both NA and 5HT neurons contribute to the cardiovascular effects of clonidine. The actions of clonidine on blood pressure and heart rate mimic the effects of NA and are opposite those of 5HT released at synaptic sites in the bulb and spinal cord.