Modulation of the carotid baroreceptor reflex by substance P in the nucleus tractus solitarius

Previous studies have shown that administration of substance P (SP) into th e nucleus tractus solitarius (NTS) can evoke a depressor response similar t o that produced by activation of the arterial baroreceptors. In addition, s ome studies have suggested that SP increases the reflex responses to activa tion of baroreceptor input. The present study was performed to determine th e effects of SP on the carotid sinus baroreceptor reflex at the level of th e NTS by examining the effects of both exogenous SP microinjected into diff erent rostrocaudal locations in the NTS and blockade of the effects of endo genous SP, through the microinjection of a substance P antagonist (SPa; [D- Pro, D-Trp]-substance P). Changes in pressure in an isolated carotid sinus in anesthetized dogs were used to evoke baroreflex changes in arterial bloo d pressure (BP) before and after microinjection of SP (0.5 mu M) or SPa (10 mu M) into barosensitive regions of the NTS. Microinjection of SP or its a ntagonist did not alter baseline, resting BP but did produce significant ch anges in baroreflex sensitivity. Microinjection of SP into different rostro caudal regions of the NTS produced different responses, with rostral and ca udal NTS microinjections producing significant increases in sensitivity. No effects on baroreflex sensitivity were obtained in response to SP microinj ections into the intermediate NTS. Unlike SP, microinjection of the SPa sig nificantly decreased baroreflex sensitivity at all rostrocaudal levels of t he NTS. These data demonstrated that SP has the capability to modulate the carotid baroreflex at the level of the NTS and support a physiological role for endogenously released SP. (C) 2000 Published by Elsevier Science B.V. All rights reserved.