Naloxone increases maturation rate and ratio of inner cell mass to total cells in blastocysts in pigs

The purposes of the present study were to examine the effect of naloxone, a mu‐opioid receptor ( MOR ) antagonist, on porcine oocyte maturation and embryo development. MOR gene was expressed in germinal vesicle ( GV ) and metaphase II ( M ‐ II ) porcine oocytes, one‐, four‐cell stage embryos and blastocysts. In blastocysts, MOR gene was mainly expressed in inner cell mass ( ICM ) cells. Supplementation of 10 −8  mol/L naloxone in in vitro maturation ( IVM ) medium increased the maturation rate ( P  < 0.05). However, 10 −4  mol/L naloxone reduced the maturation rate ( P  < 0.05) compared with the control. The presence of naloxone during IVM had no effects on fertilization status and subsequent embryonic development after in vitro culture ( IVC ). The addition of 10 −3  mol/L dibutyryl cyclic adenosine monophosphate ( dbcAMP ), and 10 −8  mol/L naloxone together into IVM medium increased nuclear maturation ( P  < 0.05) compared with the addition of either dbcAMP or naloxone alone. Supplementation with naloxone in IVC medium did not improve embryonic development. However, at the concentrations of 10 −6  mol/L and 10 −8  mol/L, naloxone increased the ratio of ICM to total cells in blastocysts ( P  < 0.05). In conclusion, at low concentration, naloxone increases maturation rate and the ratio of ICM to total cells in blastocysts. Naloxone and cAMP have a synergistic effect on oocyte maturation.