5-HYDROXYTRYPTAMINE INHIBITS PRESSOR-RESPONSES TO PREGANGLIONIC SYMPATHETIC-NERVE STIMULATION IN PITHED RATS

5-Hydroxytryptamine (5-HT) inhibits contractile responses to adrenergi c nerve stimulation in several blood vessels and organs. We have now i nvestigated the potential ability of 5-HT to inhibit the presser respo nses caused by preganglionic sympathetic stimulation (T-7-T-9) in pith ed rats. Sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) resulted in frequency-dependent increases in diastolic blood pressure; these e ffects were augmented after i.v. treatment with desipramine (50 mu g/k g). During continuous infusions of 5-HT (1.8, 3.1, 5.6 and 10 mu g/kg. min, i.v.), but not of saline, the presser responses were dose-depende ntly inhibited in both control and desipramine-pretreated rats; this i nhibitory effect of 5-HT was significantly more pronounced at lower fr equencies of stimulation. In contrast, the above infusions of 5-HT did not inhibit the presser responses induced by i.v. bolus injections of exogenous norepinephrine (up to 3 mu g/kg). Taken together, the above findings suggest an operative 5-HT-induced prejunctional inhibition o f sympathetic nerve activity on the systemic vasculature in vivo.