Chronic cerebroventricular infusion of hypertonic sodium chloride in rats reduces hypothalamic sympatho-inhibition and elevates blood pressure.

To determine whether or not salt loading restricted to the brain would elevate blood pressure, chronically implanted osmotic minipumps were used to infuse hypertonic sodium chloride solutions made in artificial cerebrospinal fluid into a jugular vein intravenously or the third cerebral ventricle, intracerebroventricularly, for 11 days in awake rats. During intracerebroventricular infusion of hypertonic sodium chloride, tail-cuff systolic pressures began to rise on day 4 and were significantly elevated by day 9. In contrast, infusions of either artificial cerebrospinal fluid alone, intracerebroventricularly, or hypertonic sodium chloride, intravenously, were ineffective. Confirming the blood pressure elevation thereby detected, systolic and mean pressures recorded directly from indwelling aortic catheters after the same rats had been anesthetized with urethane on day 11, were also significantly higher following intracerebroventricular infusion of hypertonic sodium chloride than after infusion of artificial cerebrospinal fluid alone. Magnitude of depressor and sympatho-inhibitory responses elicited by graded electrical stimulation of the anterior hypothalamus invariably increased with the current strength used for stimulation. At all current strengths used for hypothalamic stimulation, depressor responses as well as attendant decreases in sympathetic neural firing, were smaller in rats that had been infused intracerebroventricularly with hypertonic sodium chloride than in any of the controls. Inhibition seemed specific for the anterior hypothalamus because pressor responses to stimulation of the ventromedial hypothalamus were the same whether or not the intracerebroventricular infusion contained hypertonic sodium chloride. An explanation based on diminished cardiovascular sensitivity also appeared unlikely, since depressor responses to intravenously injected histamine were almost equal in both groups. Our results are compatible with the interpretation that hypertonic sodium chloride infused chronically into the 3rd ventricle acts by reducing anterior hypothalamic inhibition of sympathetic vasomotor tone, and that this, in turn, then elevates blood pressure.