Met-enkephalin enhances follicle-stimulating hormone-dependent progesterone production from cultured granulosa cells.

beta-Endorphin (beta-EP) and methionine-enkephalin (Met-Enk) have been detected in human follicular fluid in concentrations several times higher than those in plasma. These data stimulated us to study the possible physiological role of ovarian opioids. We, therefore, determined the effects of both beta-EP and Met-Enk, alone or in combination with naloxone, on FSH-induced progesterone (P) secretion by cultured granulosa cells. Granulosa cells were collected from follicular fluid recovered at laparoscopy in seven superovulated women. The cells were preincubated with RPMI-1640 medium containing 20\% fetal calf serum in 5\% CO2 for 48 h, followed by the addition of 100 mU purified FSH and the various test substances for 48 more h. beta-EP (10 nM to 1 pM) had no effect on P secretion either alone or in combination with FSH and/or naloxone. Micro- to picomolar amounts of Met-Enk increased FSH-induced P secretion up to 186.9 +/- 35.1\% (+/- SEM). Met-Enk had no affect in the absence of FSH, and its action was significantly blunted by the concomitant addition of 10(-5) M naloxone. These data provide evidence for a dose-dependent naloxone-reversible synergistic action of Met-Enk and FSH on P secretion by cultured granulosa cells. This finding supports the hypothesis of the existence of an ovarian opioid system.