To clarify the role of endogenous opioid peptides in the control of vasopressin (AVP) secretion, the effects of an endogenous k-agonist, leumorphin, derived from proenkephalin B and an opioid antagonist, naloxone, on AVP secretion were examined in conscious and freely moving rats. Intraperitoneal injection of nicotine markedly increased AVP secretion in rats. The nicotineinduced AVP secretion was significantly suppressed by intracerebroventricular (i.c.v.) pretreatment with leumorphin. Intravenous injection of naloxone significantly increased the basal AVP level and carbachol-induced AVP secretion. These results indicate that endogenous opioid peptides have an inhibitory effect on AVP secretion in rats.
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