Neurobiology of molt in avian species

Abstract

Postnuptial molt is the major molt that occurs in most wild and domestic avian species each year. The process is much more than the replacement of feathers. Studies have shown that a significant increase in metabolic rate, increase in whole body protein synthesis, osteoporosis, loss of body fat, and a suppression of the immune system occur during this event of a bird's annual cycle. Several procedures have been developed to initiate feather replacement, and this review addresses hormonal and neuropeptide treatments that effected molt. The administration of thyroxine, progesterone, gonadotropin-releasing hormone agonist, and prolactin were examined. Of the four, the two most effective were thyroxine and prolactin administration. The neural modulators known to release thyroxine and prolactin, respectively, are thyroid hormone releasing hormone (TRH) and vasoactive intestinal polypeptide (VIP). To gain insight into the possible functions of VIP and TRH, the distribution of these neuromodulators and their terminal fields were reviewed in the avian brain. It was found that VIP-containing neurons and fibers identified a neural system in birds comparable to the visceral forebrain system (VFS) described in mammals. The VFS functions to regulate the balance of the autonomic nervous system. The autonomic nervous system and, therefore, the VFS have been proposed to regulate the many behavioral and physiological events in the annual cycle of a bird, including postnuptial molt. In contrast to VIP that has an extensive brain distribution throughout the forebrain and brainstem, TRH is relatively restrictive and has a main concentration in nerve cells in and about the paraventricular nucleus, a key neural component of the VFS. Due to the roll of TRH in regulating the thyroid axis and operating within the framework of the VFS, it is proposed that the peptide functions to shift the balance of the autonomic nervous system in the direction of the sympathetic nervous system. A shift toward the dominance of the sympathetic nervous system appears to be required during this phase of a bird's yearly cycle.