Expression and regulation of the fatty acid amide hydrolase gene in the rat uterus during the estrous cycle and peri-implantation period

Fatty acid amide hydrolase (FAAH) is involved in embryo development and implantation. Sex hormones down-modulate FAAH activity in the mouse uterus. However, the regulation of the FAAH gene in the uterus is unknown. Our results showed that FAAH mRNA is localized to uterine epithelial cells and circular myometrium during the estrous cycle. In ovariectomized rats, estradiol (E2) plus progesterone (P4) increased FAAH levels in both epithelial cells and circular myometrium. Interestingly, during the implantation period, FAAH mRNA was detected not only in epithelial cells and circular myometrium, but also in the primary decidual zone surrounding the implanting embryo on day 6 and in whole decidualized stromal cells on day 7. Its levels in the stromal cells were markedly higher at the implantation sites than at the inter-implantation sites on days 6 and 7. When implantation was delayed and then induced by E2 or E2 plus P4, FAAH mRNA levels were significantly increased in subepithelial stromal cells and circular myometrium, indicating that blastocyst activation and initiation of implantation in rats requires higher expression of the FAAH gene in subepithelial stromal cells and circular myometrium. In conclusion, the expression of FAAH mRNA is different in the non-pregnant and pregnant rat uterus and sex hormones up-regulate FAAH gene expression.