5-Hydroxytryptamine inhibits pressor responses to preganglionic sympathetic nerve stimulation in pithed rats

5-Hydroxytryptamine (5-HT) inhibits contractile responses to adrenergic nerve stimulation in several blood vessels and organs. We have now investigated the potential ability of 5-HT to inhibit the presser responses caused by preganglionic sympathetic stimulation (T₇-T₉) in pithed rats. Sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) resulted in frequency-dependent increases in diastolic blood pressure; these effects were augmented after i.v. treatment with desipramine (50 μg/kg). During continuous infusions of 5-HT (1.8, 3.1, 5.6 and 10 μg/kg.Min, I.v.), but not of saline, the pressor responses were dose-dependently inhibited in both control and desipramine-pretreated rats; this inhibitory effect of 5-HT was significantly more pronounced at lower frequencies of stimulation. In contrast, the above infusions of 5-HT did not inhibit the pressor responses induced by i.v. bolus injections of exogenous norepinephrine (up to 3 μg/kg). Taken together, the above findings suggest an operative 5-HT-induced prejunctional inhibition of sympathetic nerve activity on the systemic vasculature in vivo.