The physiological role of β-endorphin in porcine ovarian follicles

β-Endorphin-like immunoreactivity (β-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 β-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 β-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 β-END-LI in FF from medium follicles were relatively equal throughout the cycle (days 6–21). No significant differences in β-END-LI levels were found between small, medium and large follicles from days 17–21. However, β-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 (P4), testosterone (T) and 17 β-oestradiol (E2) on β-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 β-END-LI output in a dose-dependent fashion. This stimulatory effect of the gonadotrophin was inhibited by the highest dose of P4 (10–5 M). The effect of PRL and the steroids added to the cultures on β-END-LI release was negligible. FSHor PRL-induced P4 secretion by GCs was essentially abolished by both FK 33–824 and NAL. However, androstenedione (A4) and testosterone output by the cells was greatly potentiated by FK 33–824. In the presence of NAL, FSH or PRL, A4 release stimulated by FK 33–824 was suppressed to the basal level. Secretion of E2 was completely free from the influence of FK 33–824 or NAL; only oestrone (E1) output was modulated by them in cultures where FSH or PRL was present. In conclusion, FSH appears to be the key regulator of β-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. opioid peptides / β-endorphin / porcine granulosa cells / steroid secretion Reprod. Nutr. Dev. 40 (2000) 63–75 63 © INRA, EDP Sciences * Correspondence and reprints E-mail: [email protected] T. Kaminski et al. 64