Regulation of hypothalamic GnRH system and pituitary gonadotropin synthesis and release. Kisspeptin, the ligand of the GPR54 receptor located on GnRH neurons, is a key regulator of GnRH production. Steroids, leptin, and Ghrelin are believed to modulate GnRH secretion via the kisspeptin/GPR54 system. GnRH I and II molecules are present in primates, and both molecules act via GnRH I receptors in gonadotropes. Synthesis and secretion of LH and FSH are locally modulated by a variety of paracrine factors.