The effects of opiate receptor agonists and antagonists on the stress-induced secretion of corticosterone in mice.

1 Intraperitoneal administration of normorphine, morphine or naloxone or exposure to ether vapour for 1 min, elevated plasma corticosteroid concentrations in mice. 2 Injection of saline or exposure to ether vapour rendered mice less sensitive to a subsequent exposure to ether vapour 15 min later. 3 Treatment with normorphine (50 mg/kg) potentiated the corticosteroid response to ether stress whilst pentazocine (20 mg/kg), naltrexone (10 mg/kg), morphine (24 mg/kg), levorphanol (20 mg/kg) and naloxone (50 mg/kg) prevented the stress-induced elevation of plasma corticosteroids. 4 Both naloxone and morphine inhibited the potentiation by normorphine of the response to ether, the dose of naloxone required being higher than that for inhibition of normorphine analgesia. 5 It is concluded that endogenous opioid peptides may be involved in the control of the response to ether stress in mice.