Effect of nucleus tractus solitarius lesions on cardiovascular responses elicited from the caudal ventrolateral medulla.

Neurons in the caudal ventrolateral medulla (CVLM) which inhibit sympathetic vasomotor tone may have reciprocal connections with the nucleus tractus solitarius (nTS). This study determined whether changes in arterial pressure elicited by chemical excitation or inhibition of neurons in the vasodepressor region of the rabbit CVLM depend on the integrity of the nTS. Unilateral injections of L-glutamate (10 pmol to 100 nmol), or bilateral injections of GABA (1 nmol to 125 nmol), were made into the CVLM, and dose-response effects on arterial pressure determined. The nTS was then bilaterally cauterized, or inhibited by local injections of muscimol, and the dose-response curves were repeated. Neither cauterization nor injection of muscimol significantly altered the slope of the log dose-response curves for L-glutamate, but nTS muscimol increased the fall in arterial pressure for each dose of L-glutamate (P less than 0.01). Cauterization of the nTS significantly (P less than 0.01) increased the slope of the curve relating dose of GABA to rise in arterial pressure observed, after injection of GABA into the CVLM. This increase in slope was similar to the increase observed when GABA is injected into the CVLM in baroreceptor-denervated rabbits. We conclude that neither the depressor nor the pressor response evoked by stimulation or inhibition of the CVLM is dependent on the integrity of the nTS. Inactivation of the nTS tends to increase the magnitude of the CVLM responses, possibly by removal of baroreceptor-mediated buffering of the responses.