The Neuroendocrinology of Stress and Aging: The Glucocorticoid Cascade Hypothesis

Over the past 5 yr, we have examined some of the sharpest edges of the pathology of aging. We have studied the capacity of aged organisms to respond appropriately to stress and the capacity of stress to cumulatively damage aging tissue. The idea of a relationship between stress and aging has permeated the gerontology literature in two forms. First, senescence has been thought of as a time of decreased adaptiveness to stress. This idea has been supported frequently, as many aged physiological systems function normally under basal conditions, yet do not adequately respond to a challenge. For example, aged and young humans have similar basal body temperatures, but the former are relatively impaired in thermoregulatory capacities when heat- or cold-challenged. A second theme in gerontology concerning stress is that chronic stress can accelerate the aging process. Selye and Tuchweber for example, postulated a finite "adaptational energy" in an organism, with prolonged stress prematurely depleting such reserves, thus accelerating the onset of senescence. This idea was derivative of earlier idea that the "rate of living" could be a pacemaker of aging. Experimentally, varied approaches have supported the notion that at least some biomarkers of age can be accelerated by stress. The above hypotheses led us to examine the adrenocortical axis, the endocrine axis which is among the most central to the stress response. Our findings support both of these concepts. We find that the aged male rat is impaired in terminating the secretion of adrenocortical stress hormones, glucocorticoids, at the end of stress. This hormonal excess may be due to degenerative changes in a region of the brain which normally inhibits glucocorticoid release; the degeneration, in turn, is caused by cumulative exposure to glucocorticoids. Together, these effects form a feed-forward cascade with potentially serious pathophysiological consequences in the aged subject. Reproduced by permission. Robert M. Sapolsky, Lewis C. Krey, Bruce S. McEwen, The Neuroendocrinology of Stress and Aging: The Glucocorticoid Cascade Hypothesis. Endocr. Rev. 7, 284-301 (1986).