Control of the reproductive axis: Balancing act between stimulatory and inhibitory input

As for most vertebrates, reproduction in poultry is controlled by an integrated axis [the hypothalamo-pituitary-gonadal (HPG) axis]. External and internal cues are integrated at the level of the hypothalamus to initiate gonadal recruitment and control the subsequent reproductive cycle. Until recently, it was believed that the HPG was exclusively under stimulatory control from hypothalamic gonadotropin releasing hormone (GnRH). However in 2000, the discovery of gonadotropin inhibitory hormone (GnIH), an inhibitory peptide, changed this dogma. Since then, evidence accumulated to confirm that in fact the HPG is under a dual control system with a stimulatory and an inhibitory branch. In this paper, we review the organization of this dual control system, the mechanisms controlling the synthesis and release of GnRH and GnIH, and the possible integration and interactions between the two branches to regulate pituitary gonadotropes' function. Furthermore, as light perception and photoperiod are the primary cues utilized by the poultry industry in controlled environments, special consideration was given to potential practical applications.