Central nervous system mechanisms involved in inhibition of renal sympathetic nerve activity induced by arginine vasopressin.

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Abstract
Previous studies have suggested that intravenous infusion of arginine vasopressin (AVP) inhibits sympathetic nerve activity (SNA) in rabbits by its action at the brainstem. This study aimed to determine, first, whether this action of AVP depended on functioning baroreflex mechanisms. The second aim was to determine the role of the nucleus tractus solitarii (NTS) in mediating this action of AVP. Experiments were performed in rabbits anesthetized with alpha-chloralose. Intravenous infusions of AVP (1, 3, and 10 mU/kg/min) resulted in progressive inhibition of renal SNA in rabbits with intact arterial baroreceptors and vagal afferents as well as in rabbits with sinoaortic denervation and bilateral vagotomy. The magnitudes of renal SNA inhibition evoked with AVP were comparable in the two groups. Renal SNA inhibition evoked with AVP at doses of 1 and 3 mU/kg/min in rabbits after sinoaortic denervation and vagotomy was totally blocked by chemical lesions with kainic acid of the bilateral NTS or the area postrema. Lesions of the NTS or the area postrema markedly attenuated (by about 60%) but did not totally block the response evoked with AVP at a dose of 10 mU/kg/min. AVP microinjected directly into the NTS did not evoke changes in renal SNA or arterial pressure, whereas AVP microinjected into the area postrema suppressed renal SNA. These results suggest that intravenous AVP at lower doses inhibited renal SNA by causing excitation of the NTS neurons, and this action of AVP did not depend on functioning baroreflex mechanisms. The second suggestion is that the NTS was unlikely to be the site where AVP directly interacted but instead received a neural connection, presumably from the area postrema, where AVP might directly interact.