Effects of morphine on urine flow in rats. Involvement of vasopressin.

The actions of morphine (1.25 to 20 mg/kg i.p.) were compared in several models of experimental polyuria: alcohol--or water--loaded rats and Brattleboro rats (i.e. animals with congenital lack of vasopressin). In normal rats (without load), morphine induced no change except at low dose. In water-loaded rats, the opiate exhibited its dose-related antidiuretic effect and increased Na+, urea and osmolality excretion. After inhibition of vasopressin secretion by alcohol load as well as in Brattleboro rats, morphine induced antidiuretic response for the higher doses and diuretic effects for the lower doses. The morphine --induced changes in urine flow were suppressed by naloxone in normal, water--and alcohol--loaded rats. These results indicate that morphine is able to induce diuretic or antidiuretic action according to the state of hydratation in conscious rats after peripheral administration. This property is due to an interference with opiate receptors without involvement of vasopressin secretion.