Effects of naloxone on vasopressin secretion in conscious rats: Evidence for inhibitory role of endogenous opioid peptides in vasopressin secretion

The effects of naloxone, an opioid antagonist, on arginine vasopressin (AVP) secretion were examined in conscious unrestrained rats under both basal and stimulated conditions. Intravenous injection of naloxone in a dose of 0.1 mg/kg did not significantly affect the basal plasma AVP level. However, 0.5 or 2.5 mg/kg naloxone significantly raised the basal AVP level in euhydrated rats. Naloxone (0.5 mg/kg) significantly enhanced AVP secretion after 72-h water deprivation. However, the enhancement was more prominent in euhydrated rats than in dehydrated rats. Pretreatment with naloxone (0.5 mg/kg) also significantly prolonged AVP secretion induced by intracerebroventricular injection of angiotensin-II (100 ng). Moreover, naloxone (0.5 mg/kg) significantly increased AVP secretion induced by intracerebroventricular injection of carbachol (10 ng). Naloxone (0.5 mg/kg) altered neither basal blood pressure nor the angiotensin-II-induced pressor response, but augmented the carbachol-induced pressor response. This suggests that facilitation of AVP secretion by naloxone is not due to a reflex mechanism resulting from decreased blood pressure. These results indicate that endogenous opioid peptides exert a tonic inhibitory control on AVP secretion in rats.