Decreases in arterial pressure activate oxytocin neurons in conscious rats

Hemorrhage and nonhypotensive hypovolemia are known to increase plasma levels of oxytocin (OT) and vasopressin (VP) in rats. The present experiments demonstrated that secretion of OT and VP also are stimulated by acute drug-induced hypotension. Injection of hydralazine abruptly decreased arterial blood pressure in conscious rats and induced Fos expression, a marker of neuronal activation, within OT and VP neurons in the hypothalamus. Hydralazine also elicited substantial increases in plasma levels of both OT and VP. Injection of chlorisondamine similarly elicited acute hypotension and increased plasma levels of OT and VP. Furthermore, when the hypotensive effect of chlorisondamine was blunted by coinfusion of phenylephrine, the induced increases in OT and VP were markedly attenuated. Across all treatments, arterial blood pressure was inversely related to plasma levels of OT and VP. Plasma osmolality was not increased by hydralazine, nor was there evidence of gastric malaise, two known stimuli for OT secretion in rats. These results suggest that arterial hypotension increases neurohypophysial release of OT and VP in conscious rats.