Drinking and subsequent suppression of vasopressin is unaltered by naloxone in dogs

The effect of the opioid antagonist naloxone on drinking and the subsequent suppression of plasma vasopressin were evaluated in seven dogs following 24 hr of water deprivation. Each animal underwent an intravenous injection of vehicle as a control and a low (0.05 mg/kg) and high (1 mg/kg) dose of naloxone. Plasma vasopressin was significantly (p less than 0.05) increased from a control value of 4.6 +/- 1.9 microU/ml to 9.9 +/- 3.1 microU/ml after the high dose of naloxone. Fluid intake was not altered by naloxone; 42 +/- 6 ml/kg for the control, 45 +/- 8 ml/kg at the low dose, and 49 +/- 7 ml/kg for the high dose. Six minutes after the onset of drinking vasopressin was reduced by 48% for the control, 41% for the low dose and 45% for the high dose, with no significant difference among treatments. Thus, in dehydrated dogs naloxone presumably blocks endogenous opioids, elevates vasopressin following dehydration, but does not affect drinking behavior or the subsequent suppression of vasopressin after drinking.