Seasonal changes in the hypothalamo-pituitary-gonadal axis in birds.

Seasonal reproduction in birds is the adaptation to breed at the time of the greatest survival of young. This exact moment is mainly imposed by the photoperiod which stimulates the hypothalamo-pituitary-gonadal axis and starts breeding season. This article summarizes present knowledge concerning: 1/ perception and transduction of light into the biological signal; 2/ model of the avian photoperiodic response; 3/ seasonal changes in hypothalamic secretion of gonadotropin-releasing hormone (GnRH), vasoactive intestinal polipeptide (VIP), as well as the gonadotropin inhibitory hormone (GnIH); 4/ seasonal interactions between pituitary hormones; 5/ seasonal morphological and functional changes within the avian gonads.