Cardiovascular responses to 5-HT in the ventrolateral medulla of the rat.

Bilateral microinjection of 5-HT into the rostral ventrolateral medulla (RVLM) produced a fall in blood pressure in anaesthetised rats. Injection of 10-20 nmol 5-HT produced a fall in mean arterial pressure of 5-55 (mean 15.7) mm Hg which lasted 1-69 (mean 25.2) min. Larger doses (up to 120 nmol) produced a similar, but longer lasting fall in pressure. The depressor response was accompanied by bradycardia but tachycardia was sometimes seen in response to injections made at the rostral tip of the RVLM. An analysis of haemodynamic changes in response to 40-50 nmol 5-HT showed the depressor response was accompanied by an increase in vascular conductance of hindlimb muscle (20-200, mean 87.2%) and a rise of 2-9 (mean 5.9)degrees C, in the surface temperature of the tail. In contrast, an increase in renal vascular conductance (8-66 (mean 19.6)%) was only evoked after injections of 5-HT into the caudal half of the RVLM whereas a decrease in conductance (14-48 (mean 24.5)%) was measured after injections of 5-HT into the rostral tip of the nucleus. The hindlimb muscle and tail vascular beds therefore appear to make a larger contribution to the fall in peripheral resistance than the renal bed.