The vasotocinergic system and its role in the regulation of stress in birds.

Abstract

The regulation of stress in birds includes a complex interaction of neural systems affecting the hypothalamic-pituitary-adrenal (HPA) axis. In addition to the hypothalamic paraventricular nucleus, a structure called the nucleus of the hippocampal commissure likewise affects the output of pituitary stress hormones and appears to be unique to avian species. Within the anterior pituitary, the avian V1a and V1b receptors were found in corticotropes. Based on our studies with central administration of hormones in the chicken, corticotropic releasing hormone (CRH) is a more potent ACTH secretagogue than arginine vasotocin (AVT). In contrast, when applied peripherally, AVT is more efficacious. Co-administration of AVT and CRH peripherally, resulted in a synergistic stimulation of corticosterone release. Data suggest receptor oligomerization as one possible mechanism. In birds, vasotocin receptors associated with stress responses include the V1a and V1b receptors. Three-dimensional, homology-based structural models of the avian V1aR were built to test agonists and antagonists for each receptor that were screened by molecular docking to map their binding sites on each receptor. Additionally, binding affinity values for each available peptide antagonist to the V1aR and V1bR were determined. An anterior pituitary primary culture system was developed to determine how effective each antagonist blocked the function of each receptor in culture when stimulated by a combination of AVT/CRH administration. Use of an antagonist in subsequent in vivo studies identified the V1aR in regulating food intake in birds. The V1aR was likewise found in circumventricular organs of the brain, suggesting a possible function in stress.