Release of Antidiuretic Hormone by Morphine in Rats: An in Vivo and in Vitro Study 1

Abstract

Morphine has been reported to cause either diuresis or antidiuresis in rats. It is not clear whether antidiuretic hormone (ADH) release contributes to antidiuresis. In this paper we report that iv injection of morphine in doses of 0.5, 1, and 2 mg/kg caused antidiuresis and not diuresis in conscious rats. There was a dose-related increase in plasma ADH levels. The peak level of hormone occurred in the sample taken within 30 sec of morphine administration. The levels of hormone gradually decreased in samples taken at 5, 10, and 15 min after morphine administration. Prior injection of naloxone, 1 mg/kg, did not prevent the release of ADH in response to morphine. In in vitro experiments morphine did not release ADH into the medium when incubated with isolated neural lobes although a gradual release was observed when incubated with intact hypothalamoneurohypophysial preparations. The in vitro ADH release by morphine was also not prevented by naloxone. These results thus demonstrate that morphine can release ADH in vivo and in vitro but this action is not blocked by naloxone.