The role and significance of endomorphin-1 and μ-opioid receptor in rats with endometriosis.

Endomorphin-1 (EM-1) was reported to have very high affinity and selectivity for μ-opioid receptor (MOR). However, it remained unclear whether EM-1 and MOR were involved in the pathologies of endometriosis resulting in reduced fertility. In this study, RT-PCR, radioimmunoassay, immunohistochemistry, and Western blot were used, respectively. The results showed that the immune positive cells of EM-1 in hypothalamus, pituitary, and ovaries were significantly increased in endometriosis model rats, accompanied by the increase of plasma level of EM-1 and the decrease of follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol (E2), and progesterone (P). Interestingly, EM-1 was negatively correlated with FSH and LH (p < 0.05). More importantly, Naloxone (MOR antagonist) can significantly reduce the levels of EM-1 in serum, hypothalamus, pituitary, and ovaries, while increased the levels of FSH and LH. In conclusion, our results suggested that EM-1 may be involved in the pathogenesis of the endometriosis-associated infertility by regulating hypothalamus-pituitary-ovarian axis, and Naloxone may be a new alternative drug for the treatment of endometriosis.