Inhibition of reflex vagal bradycardia by a central action of 5-hydroxytryptophan.

1 Vagally mediated reflex bradycardia was elicited in spinal cats with intravenous pressor doses of noradrenaline. Administration of 5-hydroxytryptophan (1.5 and 3 mg total dose) into the fourth cerebral ventricle reduced the reflex bradycardia. 2 Inhibition of central amino acid decarboxylase with R044602 prevented the effects of 5-hydroxytryptophan. After intravenous administration of 5-hydroxytryptophan, vagal reflex bradycardia was not affected. 3 Results suggest that 5-hydroxytryptophan acts in the CNS to inhibit baroreceptor-mediated vagal reflex bradycardia and that action is mediated via conversion to 5-hydroxytryptamine.