Endocrine Response of Birds to Stress
The preoptic anterior hypothalamus induced by thermoreceptors stimulates the hypothalamic paraventricular nucleus which resulted in higher secretion of TRH. The TRH activates the thyrotrophs in the anterior pituitary to secrete thyroid stimulating hormone (TSH) which consequently activates adenylate cyclase and cAMP production enhanced thyroid hormones.
The pituitary secretion of TSH is controlled through a negative feedback mechanism of T3. Further, transformation of T4 to T3 is increased during cold conditions by the enzyme deiodinase in tissues and liver and results in higher concentration of circulating T3. The binding ability of T3 with nuclear and mitochondrial receptors in tissues enhances the expression pattern of genes in relation to metabolic rate and respiration. Thyroid hormones are essential for the regulation of thermogenesis below TNZ. The potential thermoregulatory mechanisms to enhance the capacity for thermogenesis in birds include increasing shivering and non-shivering thermogenesis. The enhancement of mass-specific aerobic enzyme capacity of muscle tissue and increasing mitochondrial density helps in aerobic metabolism. Further, enhancing the capacity to uncouple mitochondria from energy production, causing energy that would have been used to phosphorylate ADP to ATP to be released as heat. Thyroid hormones involve in the regulation of non-shivering thermogenesis whereas shivering thermogenesis is principally controlled directly by neuronal mechanisms in birds.Glucocorticoids are essential in the maintenance of physiological and energy homeostasis in the birds. The primary stressor hormone of bird is corticosterone which is secreted from the adrenal glands in response to stress on activation by ACTH from the hypothalamus. The corticosterone concentration increases across captive and wild species of birds in response to quick changes in Ta due to cold and heat.
Thus, corticosterone is prerequisite for the production of glucose from liver glycogen or fat reserves which catalyse internal energy reserves that may stimulate food seeking behaviour to provide the higher metabolic rate in cold conditions. The mitochondrial glucocorticoid receptors are present in avian muscle cells that enhance the regulation of mitochondrial function for the energy production. During heat stress, both glycogenolysis and gluconeogenesis pathways are activated in the liver thereby providing endogenous energy sources under stress. Further, the HPA axis augments the avian thermoregulation in correlation with the HPT axis especially secreting CRH. The increased level of CRH induces thermogenesis and increases the Tc in chicks along with higher concentration of TSH and circulating T4 and T3 which indicates an interaction between CRH and thyroid hormones on thermoregulation. The hypothalamic orexigenic neuropeptide Y responds to cold exposure in poultry. Further, the higher level of melatonin secreted form pineal gland increases the Tc which induce enhanced cold resistance, thermal insulation and maximal heat production in chicken and quails.28.20