Modulatory effects of nitric oxide on baroreflex activation in the brainstem nuclei of rats

It has been shown that nitric-oxide (NO) was synthesized in the central nervous system as well as in vascular endothelial cells. We reported recently that nitric oxide was involved in central cardiovascular regulation of rat brainstem nuclei. In the present study, we evaluated further the baroreflex response of NO in the nucleus tractus solitarii (NTS) of rats. Male Sprague-Dawley rats were anesthetized with urethane, blood pressure and heart rate was monitored intra-arterially. Intramedullary microinjection (60 nl) of NO synthase inhibitor NG-methyl-L-arginine (L-NMMA), NG-nitro-L-arginine methyl ester(L-NAME) or normal saline was made into the NTS. Baroreflex response were elicited by increasing doses of phenylephrine (10-30 micrograms/Kg i.v.) before and after intra-NTS administration of L-NMMA, L-NAME or normal saline. The reflex bradycardia elicited by phenylephrine was significantly inhibited by the pretreatment of L-NMMA or L-NAME. Furthermore, the inhibitory effects on baroreflex activation by L-NMMA were significantly reversed by pretreatment with L-arginine. These results suggested that central endogenous NO was involved in the medullary regulation of blood pressure and that NO synthase inhibitor attenuated baroreflex activation.