Gonadotropin inhibitory hormone (GnIH) as a regulator of gonadotropes

• The relevance of GnIH to mammalian reproduction is reviewed.
• GnIH neurons project to GnRH neurons and the median eminence.
• GnIH is secreted into the hypophysial portal blood.
• Levels of GnIH in portal blood are higher in the non-breeding season in sheep.
• GnIH acts on the gonadotropes to reduce synthesis and secretion of GnRH.

Gonadotropin inhibitory hormone (GnIH) has emerged as a negative regulator of gonadotrope function in a range of species. In rodents, such as rats and mice, GnIH exerts influence upon GnRH cells within the brain. In other species, however, the peptide is secreted into hypophysial portal blood to act on pituitary gonadotropes. In particular, a series of studies in sheep have demonstrated potent actions at the level of the pituitary gland to counteract the function of GnRH in terms of the synthesis and secretion of gonadotropins. This review focuses on the action of GnIH at the level of the gonadotrope.