Nerve Growth Factor Receptors in the Peripubertal Rat Ovary

We previously demonstrated that the immature rat ovary synthesizes nerve growth factor (NGF), and that interference of NGF actions by immunoneutralization during neonatal life prevents development of the ovarian sympathetic innervation and delays follicular maturation. Since the actions of NGF are exerted via binding to specific cell surface receptors, the present study was undertaken to define and characterize the presence of NGF receptors (NGFrec) in the developing rat ovary. NGF interacts with two classes of NGFrec. The most abundant is a low affinity form expressed in the central nervous system and peripheral tissues. This receptor is encoded by a single 3.8-kilobase mRNA species. Cross-linking of [I-125]NGF to ovarian membranes followed by immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and autoradiography showed the presence of a approximately 90-kilodalton molecular species which corresponds in size to the predominant NGF receptor species cross-linked to its ligand. While ovarian NGFrec may be of neuronal origin and reach the gland exclusively by anterograde axonal transport, RNA blot hybridization demonstrated that the ovary expresses the NGFrec mRNA species that encodes the low affinity NGF receptor and, thus, implicated the ovary itself as a site of NGFrec synthesis. NGFrec mRNA levels decreased abruptly after the first ovulation, suggesting that NGFrec may be synthesized in growing follicles and that this capacity is lost after follicular rupture and luteinization. Immunohistochemical examination of the ovary using monoclonal antibody 192 immunoglobulin G, which recognizes the low affinity form of NGFrec, localized the NGFrec protein to ovarian nerve fibers and, unexpectedly, to thecal cells of developing follicles; some immunoreactive material was also detected in follicular granulosa cells but not in luteal cells of newly formed corpora lutea. Detection of NGFrec mRNA by RNase protection assay confirmed the ability of granulosa cells to synthesize NGFrec. Hybridization histochemistry using the same probe used for RNA blots confirmed the thecal cells as a site of NGFrec synthesis and verified the relative absence of NGFrec synthesis in the corpus luteum. The results demonstrate that the gene encoding the predominant low affinity class of NGFrec is expressed in a subset of endocrine steroid-producing cells of the developing ovary and raise the possibility that NGF and other members of the NGF family may act in endocrine glands as neuroendocrinotropic factors that functionally link the nervous and the endocrine systems.