Expression of the μ Opioid Receptor and Effects of the Opioid Antagonist Naloxone on In Vitro Maturation of Oocytes Recovered from Anoestrous Bitches

Abstract

The mu-opioid receptor (MOR) is expressed in bovine, human, equine and canine oocytes, and in seasonal breeders, it is expressed with higher intensity during the anoestrous phase. Supplementation of in vitro maturation (IVM) medium with opioid agents, agonists or antagonists, was shown to affect oocyte maturation in several species such as rat, bovine and equine. This study reports the effects of supplementing IVM medium with naloxone (Nx), an opioid antagonist, on nuclear and cytoplasmic maturation rate of oocytes recovered from anoestrous bitches. Cytoplasmic maturation was examined in terms of mitochondrial (mt) distribution. In order to confirm the receptor-mediated action of Nx, in oocytes of anoestrous bitches, MOR expression was analyzed by Western blot. Cumulus-oocyte complexes, recovered from the ovaries of bitches in anoestrous, were cultured in vitro and Nx was added at the concentrations of 1 x 10(-6), 1 x 10(-8) and 1 x 10(-10) M. The rate of oocytes resuming meiosis after culture in presence of 1 x 10(-6) M Nx (29%) was significantly higher than that of oocytes of control group (12%; p < 0.05). However, treatment with Nx did not affect mt distribution pattern. In denuded oocytes and in corresponding cumulus cells, a doublet of 65 and 50 kDa was observed. We conclude that, in oocytes of anoestrous bitches, MOR is expressed and Nx significantly improves nuclear maturation rate. Further studies should be performed to elucidate the expression of other opioid receptors, such as delta and kappa, and possible interactive effects of their antagonists on canine oocyte maturation.

Exposure to opiates in female adolescents alters mu opiate receptor expression and increases the rewarding effects of morphine in future offspring

“…The mechanism of transmission is currently unknown but may include direct oocyte exposure to opioids. It is known that opioid receptors are expressed on oocytes and that stimulation delays oocyte maturation (Agirregoitia et al, 2012; Iorga et al, 2009), which can be reversed using the opioid antagonist naloxone (Dell’Aquila et al, 2002; Iorga et al, 2009). In fact, multiple studies have shown that endogenous opioids and their receptors play a substantial role in the development and function of the reproductive axis (Vuong et al, 2010).…”

Regulation of Male Fertility by the Opioid System

“…Recent studies have demonstrated that MOR is expressed in the bovine (91), human (92), canine (93) and equine (94) cumulus-oocyte complex (COC) (95). MOR, by inducing an increase in intracellular calcium, has been shown to participate in the cumulus-oocyte coupled signaling associated with oocyte maturation (94,95).…”

Implication of mu opioid receptor in the in vitro maturation of oocytes and its effects on subsequent fertilization and embryo development in mice

“…Opioid receptors have been described in oocytes and granulosa cells from various species at the different maturation stages (Agirregoitia et al, ; Dang‐Nguyen et al, ; Dell'Aquila et al, ; Iorga et al, ; Lunger et al, ). In mammals, oocytes are arrested at germinal vesicle stage (GV) and during meiotic resumption they evolve to mature oocytes at metaphase II (MII).…”

“…Several studies have been carried on to determine the function of opioid peptides during this event. The opioid antagonist naloxone appeared to modify the canine and porcine oocyte in vitro maturation, obtaining higher maturation rates when oocytes were matured in culture medium supplemented with low concentrations of naloxone (Dang‐Nguyen et al, ; Iorga et al, ). Besides, the addition of ß‐endorphin to the IVM culture resulted in an inhibition of the maturation rates in rats, bovine and equine oocytes (Dell'Aquila et al, ; Wai‐sum, ).…”