Neuroendocrine Responses

The neuroendocrine responses to stressors are important adaptation and coping mechanisms that occur in response to a threatening stimulus. The adaptive changes initiated by stressors involve activation of the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus and the

STRESS

STRESS

Adrenal

Adrenal

Cortex

Medulla

--Glucocorticoids J J Catecholamines

--Glucocorticoids J J Catecholamines

Homeostasis

Fig. 1 This diagram depicts the activation of the HPA axis in response to stress. The response is perceived and organized in the CNS, which in turn activates either the endocrine pathway or fight or flight response so that the animal can return to homeostasis. The type of response(s) the animal initiates is dependent upon various modifiers.

brainstem are pivotal regions of the brain that control the animal's response to stress. Once the threat to homeostasis is perceived, the HPA axis is activated and the hormones corticotropin releasing hormone (CRH) and vasopressin (VP) are released from the neurons of the paraventricular nuclei (Fig. 1). CRH stimulates the pituitary gland to secrete adrenocorticotropin hormone (ACTH) and other peptides (i.e., p-endorphin). VP plays a role in sustaining HPA responsiveness and, along with CRH, has a syn-ergistic impact on ACTH secretion. Elevated ACTH stimulates the adrenal cortex to increase synthesis and production of glucocorticoid hormones and regulates the secretion of glucocorticoids.

The glucocorticoids influence homeostasis and the biological response to stress. The glucocorticoids are essential for regulating basal activity of the HPA axis and terminating the stress response. Glucocorticoids terminate the stress response through an inhibitory feedback loop at the pituitary and hypothalamus (Fig. 1). Further responsiveness within the HPA is dependent upon this negative feedback, which is influenced by HPA facilitation. In addition, stress activates the secretion of the catecholamines, which influence the HPA axis, and mediates many changes associated with the stress response.

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