Neuroendocrine regulation of reproduction in birds and clinical applications of GnRH analogues in birds and mammals

Avian species are extremely diverse in their lifetime reproductive patterns and strategies. Because of this diversity, it is often difficult to ascertain the environmental triggers that regulate reproduction. This becomes a critical issue for captive birds that become reproductive at sporadic intervals or develop pathologic conditions associated with inappropriate hypothalamic pituitary gonadal axis (HPG) activation. The HPG axis affects all stages in the life cycle: organization of the axis during embryonic and perinatal periods, activation during sexual maturation, peak reproductive function in the adult, and declining reproductive function during aging. At each of these stages, the HPG axis responds in an age- and species-specific manner. This article reviews some of these differences in the HPG axis associated with age and stage of the life cycle. The focus is the neuroendocrine regulation of reproduction at the level of the hypothalamus, since this region of the brain modulates both endocrine and behavioral components of reproduction in many birds. Further, neuroendocrine modulation of the gonadotropin-releasing hormone (GnRH) system is similar in birds and mammals. As such, clinical applications in mammals that modulate the GnRH system to resolve reproductive dysfunctions also provide useful information for bird reproductive disorders. Some of this literature on the use of GnRH agonists and antagonists in mammals is discussed in light of clinical applications in birds.