In vivo and in vitro effects of beta-endorphin and naloxone on corticosterone and cortisol release in male and female water frog, Rana esculenta.

1. beta-Endorphin and naloxone effects on corticosterone and cortisol production in male and female Rana esculenta, were studied in vivo and in vitro. 2. The in vivo and in vitro results were in agreement. 3. beta-Endorphin caused a decrease in corticosterone and cortisol release. 4. Naloxone induced an increase in the two corticosteroids at the same times as the decrease caused by beta-endorphin. 5. beta-Endorphin plus naloxone treatment did not change corticosterone and cortisol levels. 6. These results suggest that in Rana esculenta opioids are involved in the regulation of the hypothalamo-pituitary-interrenal axis; in particular, opioids directly modulated interrenal steroidogenesis.