Neuroendocrine Regulation of Adaptive Mechanisms in Livestock

Abstract

Neuroendocrine responses to stress play an integral role in the maintenance of homeostasis in livestock. In general, activation of the stress circuitry inhibits functions such as growth and reproduction. Substantial evidence suggests that neuroendocrine responses varies with the type of stressor and are specific and graded, rather than ‘all or none’. While acute responses have important adaptive functions and are vital to coping and survival, chronic stressors elicit endocrine responses that may actually contribute to morbidity and mortality. Integration of these responses is possible through the network of mutual interactions that exist between the immune system, the central nervous system, and the endocrine system. A crucial component of this network is the stress axis. Activation of the stress axis is accomplished through the release of several neurotransmitters and hormones. The stress axis or the hypothalamo-pituitary-adrenal (HPA) axis consists of 3 components: corticotrophin releasing hormone (CRH) neurons in the hypothalamus, corticotrophs in the anterior pituitary, and the adrenal cortex. A variety of molecular mediators have been implicated in the stimulation of CRH neurons ranging from neurotransmitters such as catecholamines to proinflammatory cytokines. CRH is an obligatory and primary stimulus for adrenocorticotropin hormone (ACTH) secretion by the pituitary gland. Subsequently, ACTH stimulates glucocorticoid synthesis from the cells of adrenal cortex. The secretion of ACTH, which is very crucial in this neuroendocrine response, seems to be regulated by a variety of peptides, but principally by CRH and vasopressin (VP; arginine vasopressin in most mammalian farm animal species; lysine vasopressin in pigs). CRH seems to be active mainly in the acute phase of stress while VP is proposed to maintain HPA axis activity after repeated stimulation. In addition to the more traditional regulation of pituitary corticotrope function by hypothalamic peptides, proinflammatory cytokines are now recognized to play an important regulatory role in the HPA axis. Currently, interleukin-1, interleukin-6, and tumor necrosis factor are implicated strongly as stimulators of the stress axis. Cytokines are also capable of stimulating the secretion of the hormone leptin from adipocytes. Leptin is now recognized as an inhibitor of stress axis activity. Therefore, both leptin and glucocorticoids complete the negative feedback circuit to suppress stress axis activity and maintain homeostasis. We will explore the relationship between these hormonal mediators, immune cytokines, brain neurochemicals, and stress axis activation and the implications for livestock production.