High NaCl diet reduces hypothalamic norepinephrine turnover in hypertensive rats

The current study tested the hypothesis that high NaCl diets elevate blood pressure in NaCl-sensitive spontaneously hypertensive rats (SHR-S) by reducing noradrenergic input to depressor neurons in the anterior hypothalamus. SHR-S were studied at 7 weeks of age, and age-matched salt resistant SHR (SHR-R) and normotensive Wistar-Kyoto rats (WKY) were controls. Rats were fed either high (8%) NaCl or control (1% NaCl) diets for 2 weeks, following which norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions), brainstem (pons and medulla), and thoracic spinal cord was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole (CHMI). Regional brain catecholamines were measured by high performance liquid chromatography with electrochemical detection following intraperitoneal injection of CHMI or vehicle. Disappearance of norepinephrine following CHMI was used as an index of noradrenergic neuronal activity. The 8% NaCl diet caused a significant elevation in blood pressure in SHR-S but not in SHR-R or WKY. Endogenous norepinephrine levels and turnover were lower in the anterior hypothalamus of SHR-S fed 8% NaCl than in those fed 1% NaCl but were not significantly different in other groups. Endogenous norepinephrine levels and turnover were greater in pons of 8% NaCl--fed SHR-S than in those fed 1% NaCl but were not significantly different in other groups. These observations support the hypothesis that reduced noradrenergic input to depressor neurons in the anterior hypothalamus and increased noradrenergic input to neurons in the pons are related to NaCl sensitivity in the SHR-S.