Morphine antidiuresis in conscious rats: contribution of vasopressin and blood pressure

Intracerebroventricular injections of morphine (5-20 micrograms) produced a dose dependent antidiuresis in the conscious hydrated rat. Naloxone pretreatment completely abolished this antidiuretic effect, but pretreatment with a specific antidiuretic vasopressin antagonist did not change the morphine antidiuresis. The vasopressin concentration in the first voided urine after antidiuresis from morphine treated rats was found to be in the same range as the vasopressin concentration in urine from saline treated rats. Injections of 20 micrograms morphine intracerebroventricularly in conscious hydrated rats gave a short decrease in heart rate but not in mean arterial blood pressure. This indicates, that either liberation of vasopressin nor a fall in systemic blood pressure contribute to the morphine antidiuresis.