Effects of Opioid Peptides on the Regulation of Vasopressin Secretion.

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1. The role of opioid kappa-, mu- and delta-receptor in the secretion of arginine vasopressin (AVP) was first studied. Opioid kappa- and mu- receptor agonists significantly suppressed plasma AVP increased by hyperosmolar or hypovolemic stimulation. Opioid delta-receptor agonist did not suppressed plasma AVP induced by hypovolemia. These results suggest that suppressive activity of opioid receptor agonist for AVP secretion depends on its biological subclass.2. Secondly, we examined the effect of gamma-aminobutyric acid (GABA) and naloxone, a potent opioid antagonist, on AVP secretion in conscious rats in order to study the relationship of GABA and endogenous opioid peptides in the regulation of AVP secretion. Intracerebroventricular administration of GABA caused a time and dose dependent decrease in the plasma concentration of AVP that was elevated by hypertonic saline injection, whereas it did not affect the basal AVP. Pretreatment with naloxone significantly attenuated the inhibitory effect of GABA on AVP release. These results suggest that GABA produces an inhibition of AVP release stimulated by hypertonic saline, and that this inhibitory effect may be mediated at least in part by the endogenous opioid systems.