The effects of leumorphin, a K-agonist derived from proenkephalin B (neoendorphin and dynorphin precursor), on vasopressin secretion were studied under basal and stimulated conditions in conscious, unrestrained rats. Intracerebroventricular injection of leumorphin (60 or 600 pmol) significantly inhibited basal vasopressin secretion. The vasopressin response induced by intracerebroventricular injection of angiotensin II (100 pmol) was significantly suppressed, hi a dose-dependent fashion, by the simultaneous intracerebroventricular injection of leumorphin (6, 60, or 600 pmol). Intravenous pretreatment with naloxone (0.5 mg/kg body weight) diminished the inhibitory action of leumorphin (60 pmol) on vasopressin secretion. Moreover, naloxone (0.5 mg/kg body weight) prolonged the vasopressin secretion induced by intracerebroventricular injection of angiotensin II (100 pmol). These results indicate that leumorphin possesses a potent inhibitory effect on vasopressin secretion and that, alone or in combination with other endogenous opioid peptides, it plays an important role hi the control of vasopressin secretion.
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