Richard N. Clayton

Regulation of peptide hormone receptors and gonadal steroidogenesis.

Publisher Summary This chapter discusses regulation of peptide hormone receptors and gonadal steroidogenesis. The control of gonadal function by gonadotropins and prolactin is expressed through activation of plasma-membrane effector systems following hormone binding to specific, high-affinity receptors on the target-cell surface. Hormonal stimulation of characteristic gonadal responses by gonadotropins is predominately mediated by the adenylate cyclase protein kinase pathway, whereas the nature and mechanism of prolactin actions are less defined. Peptide hormones comprise one of the major groups of chemical signals by which intercellular communications operate within the organism. The low concentrations at which hormones circulate are commensurate with the high affinities of their cell-surface receptor sites, and the marked specificities of such receptors permit recognition of the circulating hormone amid a vast excess of other protein molecules. These well-defined binding properties of peptide receptors are similar to those of antigen-antibody reactions. In B lymphocytes, the immunoglobulin molecule serves as a surface receptor comparable to the peptide hormone receptor. The ability of peptide hormones to regulate the concentrations of their specific receptor sites in endocrine target cells is demonstrated in numerous tissues.

Gonadotropin-releasing hormone binding and activation of enriched population of pituitary gonadotrophs.

The properties of GnRH receptors were analyzed in isolated rat pituitary cells by binding studies with the labeled GnRH agonist, [125I]iodo-[D-Ser(tBu)6]des-Gly10-GnRH-N-ethylamide. The concentration of GnRH-binding sites in pituitary cells from immature female rats was twice as high as in cells from adult females. Electron-microscopic immunocytochemistry revealed twice as many gonadotrophs in the immature rat pituitary, indicating that gonadotrophs from immature and mature female rats contain the same number of binding sites. An enriched population of gonadotrophs prepared from immature female rat pituitaries by velocity sedimentation at unit gravity contained 77% of the total radioimmunoassayable LH, and 71% of the total GnRH receptors. A second population of small, poorly granulated gonadotrophs was distributed among the cells of other fractions of the gradient, and could be detected only by immunocytochemistry. The smaller, possibly immature, gonadotrophs did not contain a measurable number of GnRH receptors. When incubated with the cells recovered from the gradient, GnRH stimulated LH release only in the mature gonadotroph-enriched fraction, and not in other regions of the gradient. These results indicate that GnRH interacts preferentially with gonadotrophs in the pituitary. The findings also suggest that functional heterogeneity exists among pituitary gonadotrophs, and that GnRH binds to and activates only the mature gonadotrophs which exhibit the highest LH-storage capacity.

Effect of hypophysectomy and growth hormone replacement on hypothalamic growth hormone-releasing factor messenger ribonucleic Acid levels.

The mechanisms by which the pituitary gland, and growth hormone (GH) in particular, affect growth hormone‐releasing factor (GRF) gene expression have been addressed using the technique of in situ hybridization. Anatomically matched sections through the mediobasal hypothalamus of control and hypophysectomized male rats, with or without GH hormone replacement, were analysed to obtain information on GRF mRNA levels within the arcuate nucleus and around the ventromedial hypothalamus. Hypophysectomy resulted in a 70% increase in the amount of GRF mRNA per cell (P<0.001), within neurons in the arcuate nucleus. GH replacement and T4 replacement separately partially attenuated this increase (GH replacement P< 0.001 versus hypophysectomy, T4 replacement P<0.05 versus hypophysectomy). Additionally, after hypophysectomy there was an 80% increase in the number of cells expressing the GRF gene in neurons around the ventromedial hypothalamus, when compared to shamoperated controls (P<0.01). Both GH and T4 replacement separately partially attenuated this phenomenon (P<0.01 versus hypophysectomized animals). Hypothyroidism alone did not affect GRF mRNA levels in either the arcuate nucleus or in the area surrounding the ventromedial hypothalamus. These results show that hypophysectomy increases GRF mRNA levels in two separate ways: by increasing the amount of mRNA produced per cell within the arcuate nucleus, and by increasing the number of cells expressing the gene in the area surrounding the ventromedial hypothalamus. This increase in the number of GRF mRNA‐containing cells after hypophysectomy could result from the recruitment of neurons which previously did not express the GRF gene, and may reflect the plasticity of the adult central nervous system in response to a changing endocrine environment. This could represent part of a sensor mechanism to drive the production of GRF in the arcuate nucleus in response to extreme disruption of the GRF/ GH feedback loop.