Effects of chemical stimulation and lesion of the rostral ventrolateral medulla in spontaneously hypertensive rats.

We investigated whether or not the rostral ventrolateral medulla (RVL) participates in maintaining a higher arterial pressure of spontaneously hypertensive rats (SHR). SHR and Wistar-Kyoto rats (WKY) were anesthetized with chloralose, paralyzed with tubocurarine, and artificially ventilated. Chemical excitation of the RVL neurons by microinjection of excitatory amino acids (L-glutamate) elicited a dose-dependent pressor response in both the SHR and WKY groups. The magnitude of increase in the arterial pressure (AP) was similar in both groups, but the present increase of AP was higher in WKY than in SHR. Chemical lesions of the bilateral RVL produced by the microinjection of tetrodotoxin decreased the AP to the spinal level, which was similar in both groups. The results indicate that the RVL functions as the sole sympathetic output of the brain in SHR, and suggest that the RVL neurons may be tonically overactive to maintain hypertension in SHR.