The physiological role of $\beta$-endorphin in porcine ovarian follicles

Overview & Synopsis

$\beta$-Endorphin-like immunoreactivity ($\beta$-END-LI) was measured by radioimmunoassay in porcine ovarian follicular fluid (FF) from small, medium and large follicles throughout the oestrous cycle. The concentration of $\beta$-END-LI in FF from small follicles collected on days 1-5 of the cycle was at least tenfold higher than in the fluid from any other follicles independently from their size and the period of the cycle. The level of $\beta$-END-LI in small follicles on days 6-10 was drastically decreased. Subsequently, on days 11-16 its concentration was enhanced and reduced again in preovulatory period of the cycle. Concentrations of $\beta$-END-LI in FF from medium follicles were relatively equal throughout the cycle (days 6-21). No significant differences in $\beta$-END-LI levels were found between small, medium and large follicles from days 17-21. However, $\beta$-END-LI concentrations in medium follicles on days 11-13 and 14-16 were statistically lower than those in small follicles. Moreover, the effects of FSH, prolactin (PRL), progesterone (P$_4$), testosterone (T) and 17 $\beta$-oestradiol (E$_2$) on $\beta$-END-LI release by granulosa cells (GCs) from large follicles and, on the other hand, the effects of the opioid agonist FK 33-824 alone or in combination with FSH, PRL or naloxone (NAL) on follicular steroidogenesis were studied. FSH drastically increased $\beta$-END-LI output in a dose-dependent fashion. This stimulatory effect of the gonadotrophin was inhibited by the highest dose of P$_4$ (10$^{-5}$ M). The effect of PRL and the steroids added to the cultures on $\beta$-END-LI release was negligible. FSH- or PRL-induced P$_4$ secretion by GCs was essentially abolished by both FK 33-824 and NAL. However, androstenedione (A$_4$) and testosterone output by the cells was greatly potentiated by FK 33-824. In the presence of NAL, FSH or PRL, A$_4$ release stimulated by FK 33-824 was suppressed to the basal level. Secretion of E$_2$ was completely free from the influence of FK 33-824 or NAL; only oestrone (E$_1$) output was modulated by them in cultures where FSH or PRL was present. In conclusion, FSH appears to be the key regulator of $\beta$-END-LI secretion by porcine granulosa cells. Moreover, steroidogenesis in pig granulosa cells is modulated by opioid peptides acting both alone and by way of interaction with FSH or PRL.