Local production of the gonadotropic hormones in the rat ovary

Abstract

The gonadotropic hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are synthesized by and released from the anterior pituitary in response to the hypothalamic gonadotropin-releasing hormone (GnRH) signaling. In the female, LH and FSH affect folliculogenesis, ovarian steroid production, oocyte maturation, ovulation and corpus luteum formation. We have recently studied the expression of GnRH and its receptor in the rat ovary and found organ-specific, estrous cycle-dependant, fluctuations. Subsequently, we wished to determine whether rat ovaries also express gonadotropic hormones. Using RT-PCR, we detected LHβ, FSHβ and the common α-subunit mRNA's in intact follicles, theca cells, corpora lutea and in meiotically competent and incompetent oocytes. Granulosa cells, however, express mRNA's for LHβ and the common α-subunit, but not for FSHβ. We cloned and sequenced the ovarian LHβ transcript and found it to be longer (2.3 kb) than the one produced by pituitary gonadotropes (0.8 kb), due to a longer 5′-UTR. We studied the regulation of ovarian LHβ mRNA in sexually immature female rats administered with pregnant mare serum gonadotropin (PMSG) and in adult cyclic rats. PMSG administration caused a significant decrease in LHβ mRNA expression, detected by real-time PCR. Similarly, LHβ mRNA levels were lower on estrous morning versus proestrous evening. Interestingly, ovarian content of LH remained unchanged following hypophysectomy, although ovarian weight was immensely reduced. Taken together, it seems probable that ovarian LH is heterologously/homologously regulated by pituitary, and possibly also by local gonadotropins. Thus, these findings may imply the existence of a local GnRH–gonadotropin axis in the mammalian ovary that may be involved in the management of processes that lead to ovulation.