(from Latin, ad: to, renes: kidneys) The adrenal gland, situated (as the name indicates) immediately next to the kidneys, is involved in STRESS and OSMOREGULATION. The adrenal gland is composed of two distinct components: the adrenal cortex (the outer portion or shell of the adrenal gland) and the adrenal medulla (the inner portion or core of the adrenal gland). The adrenal gland is related to the tissue of the nervous system, being derived from NEURAL CREST cells which, having moved into position, can develop either into cells capable of releasing HORMONES in the adrenal medulla, or into sympathetic neurons (see SYMPATHETIC NERVOUS SYSTEM). The differentiation of neural crest cells into either of these is chemically controlled: in the presence of GLUCOCORTICOIDS, neural crest cells develop into adrenal medulla cells; in the presence of NERVE GROWTH FACTOR they develop into sympathetic neurons.
Stress triggers activity in the HYPOTHALAMUS, which then generates activity in the adrenal gland by two routes. A neural signal, relayed from the hypothalamus via the sympathetic nervous system, promotes release of ACETYLCHOLINE in the adrenal medulla, which in turn stimulates the release of the catecholamine neurotransmitters, ADRENALINE and NORADRENALINE (also known as, respectively, EPINEPHRINE and NOREPINEPHRINE). Adrenaline and noradrenaline are released from the adrenal medulla into the blood to produce several effects: increased breakdown of GLYCOGEN to GLUCOSE (to fuel ‘FIGHT-OR-FLIGHl’), increases in blood pressure, RESPIRATION and METABOLISM, and alteration of the pattern of blood flow, with decreasing digestive activity and a concomitant increase in alertness. Stressors also generate an ENDOCRINE response from the hypothalamus. Activation causes the hypothalamus to trigger release of ADRENOCORTICOTROPIC HORMONE (ACTH) from the anterior PITUITARY GLAND. ACTH activates the adrenal cortex, causing release of MINERALOCORTICOIDS (such as ALDOSTERONE) which act to retain sodium and water by the kidneys, and increase blood pressure (see OSMOREGULATION). Glucocorticoids such as cortisol are also released by the adrenal cortex, leading to conversion of fats and proteins to glucose (again helping fuel ‘fight or flight’) and suppression of the IMMUNE SYSTEM. The endocrine response to stress by the adrenal cortex, being dependent on diffusion of ACTH from the pituitary gland, is of course much slower than the response made by the adrenal medulla, which is under very fast neural control. In addition to a role in stress and osmoregulation, the adrenal gland also appears to have the capacity to produce STEROID HORMONES related to SEXUAL BEHAVIOUR: the adrenal cortex produces ANDROGENS (such as TESTOSTERONE), ESTROGENS (such as ESTRADIOL) and PROGESTINS (such as PROGESTERONE). These are all made in greater quantities in the GONADS.
Exactly what function adrenal SEX HORMONES have is unclear.
Loss of the adrenal gland—either by disease or by surgical removal (ADRENALECTOMY)—has significant physiological effects, the most pressing of which is an inability to retain sodium. Adrenalectomized animals have to be given additional salt in their water or food in order to maintain sodium levels. Adrenalectomy produces a specific loss of GRANULE CELLS from the HIPPOCAMPUS, a unique event: no other neurons in the brain are damaged by adrenalectomy (see Sloviter et al., 1993). It is thought that glucocorticoids might act as nerve growth factors in the developing hippocampus. Their function in the adult hippocampus is uncertain. In addition, some of the effects psychological of AMPHETAMINE are thought to be mediated, directly or indirectly, via an action on catecholamine release from the adrenal gland (see Martinez et al., 1980)
Campbell N.A., Reece J.B. & Mitchell L.G. (1999) Biology, 5th edn, Addison-Wesley: Menlo Park CA.
Martinez J.L., Jensen R.A., Messing R.B., Vasquez B.J., Soumireu-Mourat B., Geddes D., Liang K.C. & McGaugh J.L. (1980) Central and peripheral actions of amphetamine on memory storage. Brain Research 182:157–166.
Sloviter R.S., Dean E. & Neubort S. (1993) Electron microscopic analysis of adrenalectomy induced hippocampal granule cell degeneration in the rat—apoptosis in the adult central nervous system. Journal of Comparative Neurology 330:337–351.
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