Effects of D-Ala2-D-Leu5-enkephalin, microinjected into the supraoptic and paraventricular nuclei, on urine outflow rate

The effects of D-Ala2-D-Leu5-enkephalin (DADL, a delta-opioid agonist), microinjected directly into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, on urine outflow rate, urinary osmotic pressure, blood pressure, heart rate, respiratory rate and rectal temperature were investigated in water-loaded and ethanol-anesthetized rats. The microinjection of DADL into both the nuclei decreased urine outflow rate in a dose-dependent manner with an increase in urinary osmotic pressure, but did not change the other recorded parameters. The DADL-induced antidiuretic effect in the SON was inhibited by naloxone, but not by atropine, phenoxybenzamine, timolol nor a vasopressin antagonist, d(CH2)5-D-Tyr(Et)VAVP. The effect in the PVN was inhibited by naloxone, atropine, timolol and d(CH2)5-D-Tyr(Et)VAVP, but not by phenoxybenzamine. These results suggest that DADL causes antidiuretic effects mediated through opioid receptors in both the SON and PVN, and the underlying mechanisms are different between them. Involvement of delta-opioid receptors in the DADL-induced antidiureses was discussed.