Naltrexone normalizes the suppression but not the surge of delta 5-3 beta-hydroxysteroid dehydrogenase activity in Leydig cells of stressed rat fetuses.

Rat fetuses from mothers stressed chronically by immobilization and high intensity illumination beginning on day 14 of gestation have higher than normal levels of delta 5-3 beta-hydroxysteroid dehydrogenase (3 beta HSD) activity in Leydig cells on day 17 of gestation and lower than normal levels on days 18 and 19. Plasma testosterone titers in normal and stressed male fetuses closely parallel the activity of 3 beta HSD in fetal Leydig cells. In the present study quantitative cytochemistry was used to determine whether the stress-induced alterations in 3 beta HSD activity could be prevented by treating the mother with naltrexone, an opioid receptor blocker, before each stress session. Naltrexone normalized 3 beta HSD activity on days 18 and 19 of gestation, suggesting that the stress-induced suppression involves the endogenous opioid system. In contrast, naltrexone did not prevent the elevation in enzyme activity seen on day 17 in stressed fetuses. The persistence of a stress-induced surge on day 17, in spite of naltrexone therapy, suggests that some nonopioid mechanism is operational at that time.