Gonadotropin-Releasing Hormone Receptors: Characterization, Physiological Regulation, and Relationship to Reproductive Function

Abstract

THE TONIC and cyclic secretion of gonadotropins is mediated by the hypothalamic production of gonadotropin releasing hormone (GnRH). The direct assessment of hypothalamic function has been hampered by the absence of reliable methods for the measurement of GnRH production and secretion. Endogenous hypothalamic GnRH probably never reaches the general circulation in sufficiently high concentration to be measured by RIA (1–5), since the pituitary gland has the capacity to metabolize the majority of the peptide secreted into the hypothalamo-hypophyseal portal circulation (6–10). Attempts to determine the rate and amplitude of GnRH secretion have shown that GnRH concentrations in hypophyseal stalk plasma fluctuate between 20 and 800 pM (11–14). Such studies have revealed that GnRH is released in a pulsatile fashion, although rapid changes in secretion rate cannot be detected during the long period required for sample collection. In addition to modulating the concentration of GnRH in hypophyseal portal plasma (14–16), the sex-steroid milieu of the animal may also regulate the frequency of GnRH pulses.