A sex difference in endogenous opioid regulation of the posterior pituitary response to stress in the rat.

The influence of endogenous opioids on the posterior pituitary response to stress was investigated by measuring plasma hormone levels in immobilized male and female rats following either acute naloxone treatment or prolonged morphine administration. Naloxone significantly potentiated the oxytocin and arginine vasopressin (AVP) response to immobilization, but in female rats only. The responses of morphine-treated male rats showed differences compared with vehicle-treated controls, although chronic morphine treatment did not reliably alter the oxytocin or AVP responses to immobilization in males or females. In a further experiment to investigate the role of gonadal hormones in determining the sex difference in responsiveness to naloxone, it was found that acute naloxone treatment significantly potentiated the posterior pituitary response to stress in castrated male rats. These results extend previous studies showing a sex difference in stress-induced secretion of posterior pituitary hormones, providing evidence of a sexual dimorphism in the endogenous opioid regulation of this response which is partly determined by circulating gonadal hormones.