Effects of beta-endorphin and Naloxone on in vitro maturation of bovine oocytes

Bovine cumulus-oocyte complexes (COCs) and mural granulosa cells express the mRNA coding for the micro-opioid receptor. The addition of beta-endorphin (beta-end) to oocytes cultured in hormonally-supplemented in vitro maturation (IVM) medium had no effect on the rates of oocytes reaching the metaphase II (MII) stage, but significantly decreased the maturation rate (P < 0.05) and arrested oocytes at metaphase I (MI) after culture in hormone-free medium (P < 0.001). Naloxone (Nx) reverted this inhibitory effect of beta-end. Moreover, Nx "per se" showed a dose-dependent dual effect. When added at high concentration (10 x (-3) M), it significantly reduced the rate of oocytes in MII (P < 0.001), thus increasing the rate of oocytes arrested in MI. However, Nx added at low concentration (10 x (-8) M) significantly increased oocyte maturation (P < 0.001). High concentration of Nx induced an increase in both intracellular calcium concentration (Ca(2+)](i)) and in the activity of the mitogen-activated protein kinase (MAPK) also called extracellular-regulated kinase (ERK) in cumulus cells of bovine COCs. Blocking the rise in Ca(2+)](i) with the calcium chelator acetoxymethylester-derived form of bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) reversed the Nx-dependent inhibition of meiotic maturation observed at high Nx concentrations. Whereas blocking ERK with the MAPK/ERK kinase (MEK) inhibitor, PD98059, had no effect on this process. Therefore, we concluded that the mocro-opioid receptor, by inducing Ca(2+)](i) increase, participates in the cumulus-oocyte coupled signaling associated with oocyte maturation.