A Role for Neurotransmitters in Early Follicular Development: Induction of Functional Follicle-Stimulating Hormone Receptors in Newly Formed Follicles of the Rat Ovary*

The initiation of follicular growth in the mammalian ovary is a gonadotropin-independent phenomenon. Although some of the intraovarian signaling molecules that control the later phases of this process have been recently identified, the factors involved in the acquisition of gonadotropin receptors by early growing follicles have not been fully defined. In the rat, development of the ovarian innervation precedes the onset of folliculogenesis and occurs before follicles acquire responsiveness to gonadotropins. Because vasoactive intestinal polypeptide (VIP) and norepinephrine (NE), two of the neurotransmitters contained in ovarian nerves, are present in the ovary before the gland becomes responsive to gonadotropins, we sought to determine if VIP and/or NE are able to act on early follicles to facilitate the process of molecular differentiation that leads to gonadotropin dependency. In vitro exposure of 2-day-old rat ovaries to isoproterenol (ISO), a beta-adrenoreceptor agonist, or VIP, a neurotransmitter contained in both sympathetic and sensory nerves, increased the steady state levels of the messenger RNAs encoding cytochrome P-450 aromatase (P-450arom) and FSH receptors (FSHR) within 8 h of treatment. A similar effect was observed following forskolin-induced activation of cAMP formation. In situ hybridization experiments revealed that both the P-450arom and FSHR hybridization signals were localized to follicles. The increase in FSHR messenger RNA was accompanied by formation of functional receptor molecules, as demonstrated by the ability of FSH to stimulate cAMP formation in ovaries preexposed to either ISO or VIP, but not in untreated ovaries. The stimulatory effect of ISO and VIP on the formation of FSHR coupled to the cAMP generating system was not reproduced by phenylephrine, an alpha-adrenergic agonist, or secretin, a member of the VIP family not recognized by ovarian VIP receptors. Treatment of VIP-primed ovaries with FSH resulted in follicular growth, demonstrating that exposure of the gland to the neurotransmitter led to the formation of a functional complement of FSH receptors. These results suggest that ovarian nerves, acting via neurotransmitters coupled to the cAMP generating system, contribute to the differentiation process by which newly formed primary follicles acquire FSH receptors and responsiveness to FSH. Follicles that begin to grow in more densely innervated ovarian regions, may have a selective advantage over those not exposed to neurotransmitter-activated, cAMP-dependent signals and, thus, may become more rapidly subjected to gonadotropin control.