Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: September 3, 2004
Publication Date: N/A
Technical Abstract: ACTH is the abbreviation for adrenocorticotropic hormone. ACTH is a 39 amino acid, peptide hormone produced in the anterior pituitary gland. Production of this hormone is stimulated by the corticotropic-releasing factor (CRF) which travels from the hypothalamus through the blood to bind to receptors on corticotropic cells of the anterior pituitary. Activation of these receptors then cause the production of pro-opiomelanocortin, referred to as POMC. Enzymatic cleavage of POMC in the anterior pituitary gland forms several hormones, including: ß-endorphin, melanocyte stimulating hormone, and ACTH. ACTH is released into the peripheral vasculature, which allows it to travel to the cortex of the adrenal gland to bind to receptors, causing the production of glucocorticoids. The predominant glucocorticosteroid in cattle, swine, and sheep is cortisol, whereas in poultry and rodents the predominant glucocorticosteroid is corticosterone. The release of glucocorticosteroids cause the liver to produce glucose (gluconeogenesis). ACTH production and excretion is controlled by the negative feedback effect of glucocorticosteroids on the anterior pituitary gland to directly decrease ACTH synthesis and excretion. Glucocorticosteroids also have a negative feedback effect on the hypothalamus to decrease CRF synthesis and excretion. Because ACTH plays a dominant role in increasing energy availability in the form of glucose, it has been considered of major interest in terms of how animals cope with and respond to environmental challenges and stress. Stress research has long focused on the concept of the Flight or Fight response which requires the animal have rapid availability to energy (glucose) to mount a response to a threat. Therefore, researchers have used the measurement of ACTH and its associated activation path to measure a response to stress. One such test is the ACTH Challenge test which involves injecting ACTH to stimulate the adrenal gland to release glucocorticosteroid. Researchers can then measure the circulating concentrations. The reason that this test may show when an animal is stressed is because under chronic stress, the adrenal gland of animals enlarges due to hyperplasia and hypertrophy. This enlargement enables the animal to produce more glucocorticosteroid. When given ACTH the circulating concentrations of a chronically stressed animal should be greater than an animal that is not stressed. Thus circulating glucocorticosteroids can give us a somewhat quantitative measure of the level of stress an animal is experiencing, although it has its limitations.