Tonic Sympathoinhibition in the Baroreceptor Denervated Cat

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Abstract
Lesions of those portions of the paramedian reticular, raphe obscurus and raphe pallidus nuclei which extend 0-2 mm rostral to the obex increased renal sympathetic nerve discharge (SND) without affecting the baroreceptor reflexes in cats with intact carotid sinus, aortic depressor and vagus nerves. Lesions of these medial medullary structures in baroreceptor denervated cats produced an equivalent increase in SND. The effect of medial medullary lesions on SND, however, was prevented by prior decerebration. Tonic sympathoinhibition involving the medial medulla is of nonbaroreceptor origin and is dependent on the integrity of forebrain-medullary connections. The increase in SND produced by medial medullary lesions was not significantly different from that produced by section of the baroreceptor nerves or by ablation of the medullary nucleus of baroreceptor fiber termination, NTS [nucleus of the tractus solitarius]. The nonbaroreceptor sympathioinhibitory system of the medial medulla is potentially as important as the baroreceptor reflexes in the control of SND in the anesthetized cat. This study demonstrated that sympathoinhibitory elements in NTS are not tonically active in the absence of baroreceptor nerve input.

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