Abstract
The effects of vasopressin and oxytocin on acute morphine antinociception and on tolerance development were examined in mice and rats. The studies failed to demonstrate any alteration of chronic morphine effects using two separate models of tolerance development in the mouse. Adrenalectomy enhanced the antinociceptive, hyperthermic, and cataleptic effects of acute morphine treatment, but vasopressin was without additional effect in either adrenalectomized or sham-control rats. Furthermore, neither vasopressin nor oxytocin pretreatment altered brain concentrations of acutely injected morphine. It is concluded that the role of vasopressin and oxytocin as endogenous mediators of opiate analgesia or tolerance/dependence is minimal at best.
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