Effect of brain 5-hydroxytryptamine alterations on reflex bradycardia in rats.

The vasopressor and bradycardia responses to an intravenous dose of epinephrine were assessed in saline-controlled, 5-hydroxytryptamine-(5-HT) depleted, and 5-HT-potentiated rats. Regardless of the previous treatment epinephrine produced an insignificant change in the basal levels of mean arterial pressure and heart rate. However, brain serotonin alteration did produce some influences on the reflex bradycardia in response to an elevation in arterial pressure. Elevating 5-HT contents in brain with 5-hydroxytryptophan (5-HTP) after peripheral decarboxylase inhibition with Ro 4-4602 produced a significant reduction in reflex bradycardia compared to the controls. In contrast, depleting 5-HT contents in brain with either p-chlorophenylalanine (PCPA) or 5,7-dihydroxytryptamine (5,7-DHT) led to an enhancement of epinephrine-induced bradycardia. Moreover, the enhanced reflex bradycardia induced by PCPA treatment was readily blocked by the replacement of the depleted brain 5-HT with 5-HTP and Ro 4-4602. The results suggest that serotoninergic systems play a role in the elaboration or modulation of reflex bradycardia. Specifically, 5-HT appears to inhibit reflex bradycardia since its depletion facilitated and its elevation inhibited reflex bradycardia.