POWERFUL DEPRESSOR AND SYMPATHOINHIBITORY EFFECTS EVOKED FROM NEURONSIN THE CAUDAL RAPHE PALLIDUS AND OBSCURUS

Microinjection of glutamate into sites within the medullary raphe nucl ei (pallidus and obscurus) at levels caudal to the obex resulted in a dose-dependent decrease in mean arterial pressure (MAP), renal sympath etic nerve activity (RSNA), and heart rate in anesthetized rabbits. Th e depressor and sympathoinhibitory responses were similar in magnitude to those elicited from the previously described depressor region in t he caudal ventrolateral medulla (CVLM) but had a shorter duration, in both intact and barodenervated animals. The bradycardia was not altere d by barodenervation but was reduced after administration of propranol ol or atropine and abolished after administration of both drugs. The n euroinhibitory compounds gamma-aminobutyric acid or muscimol had no ef fect on MAP or RSNA when injected into the caudal medullary raphe nucl ei but evoked a presser and sympathoexcitatory response when injected into the CVLM. The results indicate that neurons within the caudal rap he pallidus and obscurus can powerfully inhibit sympathetic activity, but unlike sympathoinhibitory neurons in the CVLM, they are not tonica lly active and are not capable of producing sustained changes in arter ial pressure and sympathetic activity.