The effect of beta-endorphin on rat oocyte maturation in vitro

Cumulus enclosed or denuded oocytes obtained from ovaries of 25- to 27-day Sprague-Dawley rats underwent spontaneous germinal vesicle breakdown (GVBD) when cultured for 6 h in Krebs-Ringer's buffered solution (KRBS). This spontaneous division was found to be inhibited by adding beta-endorphin to the culture system and the inhibition was dose dependent, ranging from 200 to 800 pg/ml KRBS. Naloxone, a potent opioid antagonist without any agonistic action, did not stimulate spontaneous GVBD when added to the KRBS at doses ranging from 80 to 120 pg/ml. However, by adding 80 pg/ml naloxone to the culture system containing 600 pg/ml beta-endorphin, the inhibitory effect of beta-endorphin on spontaneous GVBD could be reversed completely.