Effect of Various Lesions in the Nucleus Tractus Solitarii of the Rat on Blood Pressure, Heart Rate and Cardiovascular Reflex Responses

Abstract

The inhibitory role of the nucleus tractus solitarii (NTS) in cardiovascular control was studied in the rat. Bilateral electrolytic lesions of the dorsal medulla oblongata, located in the NTS, acutely caused hypertension and tachycardia. Effective sites were at the level of the obex and just rostral and caudal of this level. More rostrally or more caudally located lesions were ineffective in changing blood pressure and heart rate. The ascending or descending pathways from the NTS structures in the dorsal midline appear not to be essential. Frontal transections rostral of the obex or lesions and transections in the commissural part of the NTS did not block the hypertension caused by lesions at the level of the obex. The latter lesions completely blocked the reflex-induced changes of heart rate caused by administration of bradykinin, angiotensin II and the diving reflex bradycardia. The hypertension was associated with a marked increase in plasma renin activity and plasma adrenaline concentration. Adrenalectomy or nephrectomy, however, failed to prevent the hypertension.

Chronically, up to six weeks after the operation, moderate hypertension was observed in rats bearing a lesion in the NTS at the level of the obex. Tachycardia was present during the first four weeks. Daily water intake was increased. A t six weeks after the operation the cardiovascular reflex responses were blocked, with the exception of the diving reflex bradycardia. These data suggest that the NTS, apart from rnediating the baroreceptor reflex responses, also influences the release of renin by the kidneys and the release of adrenaline by the adrenals and that these mechanisms contribute t o the inhibitory control of blood pressure exerted via the NTS.