Central effect of the enkephalin analogue FK-33824 on vasopressin secretion in conscious sheep.

The role of opioids in the regulation of arginine vasopressin release from the posterior pituitary is a subject of controversy. In the present study, we examined the effects of central administration of met-enkephalin, leu-enkephalin, the enkephalin analogue FK-33824, and the opiate antagonist naloxone, and the effects of systemic administration of met-enkephalin and FK-33824 on AVP secretion in conscious normal sheep. Intracerebroventricular infusion of FK-33824 significantly increased the plasma concentration of immunoreactive AVP in a dose-dependent manner, but met-enkephalin, leu-enkephalin and naloxone failed to change plasma concentration of AVP. Intravenous infusion of met-enkephalin and FK-33824 also failed to change plasma concentration of AVP. The opiate antagonist naloxone given both centrally and systemically attenuated the increase in plasma concentration of AVP induced by FK-33824. We conclude that basal AVP release is stimulated by central administration of FK-33824.