CARDIOVASCULAR EFFECTS OF NITRIC-OXIDE IN THE BRAIN-STEM NUCLEI OF RATS

Nitric oxide, synthesized from the semiessential amino acid L-arginine by nitric oxide synthase, is a remarkable regulatory molecule and pla ys an important role in physiological functions. However, the physiolo gical role of nitric oxide in cardiovascular regulation by the central nervous system is net well understood. In this study we investigated the cardiovascular effects of nitric oxide in the lateral ventricle, n ucleus tractus solitarii. area postrema, and rostral ventrolateral med ulla in urethane-anesthetized male Sprague-Dawley rats. Microinjection of N-G-monomethyl-L-arginine, a nitric oxide synthase inhibitor, into the cerebral ventricle of rats elicited a dose-dependent increase in blood pressure and heart rate. This suggests that nitric oxide may be involved in central cardiovascular regulation. Unilateral microinjecti on (60 nL) of L-arginine (1 to 100 nmol) into the nucleus tractus soli tarii and rostral ventrolateral medulla produced prominent dose-relate d depressor and bradycardic effects and reduced renal sympathetic nerv e activity. However, L-arginine had no significant cardiovascular effe cts in the area postrema. In addition, 4 to 6 hours after intravenous injection of bacterial endotoxin-lipopolysaccharide (10 mg/kg), there was a time-related potentiation of the L-arginine-induced depressor an d bradycardic effects in the nucleus tractus solitarii. These results indicate that nitric oxide is involved in central cardiovascular regul ation. The depressor effect of nitric oxide in the nucleus tractus sol itarii and rostral ventrolateral medulla may be through inhibition of renal sympathetic nerve activity.