The actions of gonadotropic hormones in the ovary and testis are influenced by several hypothalamic peptides, including GnRH, vasopressin, and CRF, which act as local regulators of gonadal function. The 43 amino acid hypothalamic polypeptide, growth hormone releasing hormone (GHRH) is found in several sites in addition to the brain, and acts on a common VIP/GRF receptor in peripheral tissues as well as on the specific GHRH receptor of somatotrophs in the anterior pituitary gland. Treatment with GHRH has been shown to stimulate follicular development when administered with FSH to infertile women who are resistant to conventional gonadotropin therapy. The presence of GHRH in the gonads, and in rat and human ovarian follicular fluid, suggests that these effects reflect direct ovarian actions of the peptide. In cultured granulosa cells, GHRH and its potent analogs and derivatives were previously found to bind to high-affinity receptors that also recognized VIP. Both GHRH and VIP stimulated cAMP production, and markedly potentiated FSH-induced cAMP production, as well as progesterone production, aromatase activity, and the expression of LH receptors. These findings indicate that GHRH acts on a common VIP/GHRH receptor to promote gonadotropin-induced maturation of ovarian granulosa cells, and suggest that the peptide's action in gonadotropin-resistant infertile women is at least partly mediated by this mechanism. The expression of GHRH receptors in the ovary was increased by in vivo treatment with FSH. Rat granulosa cells cultured without hormone supplements contained low levels of GHRH binding sites, and these were increased by incubation with FSH. GHRH receptors were also increased by agents that elevate cAMP production (choleragen, forskolin) and also by dibutyryl cAMP. Low forskolin concentrations potentiated the action of FSH on GHRH receptors. The actions of FSH on expression of GHRH receptors are mediated by cAMP-- dependent mechanisms, and since GHRH enhances the actions of FSH on cAMP production and granulosa cell differentiation, local production of GHRH within the ovary could initiate a position feedback loop to accelerate follicular maturation by amplifying the granulosa-cell response to FSH. Analysis of the expression of the GHRH gene and its peptide product in the ovary revealed that the major ovarian GHRH mRNA (1750 nt) was much larger than that expressed in the hypothalamus and placenta (750 nt), but similar to that present in the rat testis. Granulosa cells were found to release GHRH into the incubation medium, and the immunoreactive GHRH in ovarian extracts co-eluted on gel filtration with authentic rat GHRH (5.2 kDa). The larger mRNA species derived from ovarian expression of the GHRH gene could arise by tissue-specific initiation or processing of the primary transcript.
