Aging attenuates glucocorticoid negative feedback in rat brain

Aging is thought to be a risk factor to develop vulnerability of the neuroendocrine system, including the hypothalamic-pituitary–adrenal (HPA) axis, and dysregulation of this axis characterized by dexamethasone (DEX)-mediated negative feedback resistance is sometimes observed in elderly humans and animals. However, the influence of aging on the feedback system including an involvement of the brain is not fully understood. In the present study, we examined the suppressive effects of DEX by the systemic injection or the intracranial infusion into the prefrontal cortex (PFC), hippocampus, and hypothalamus on circulating corticosterone levels, and compared between young (3-month-old) and aged (24-month-old) rats. Moreover, we examined expression levels of glucocorticoid receptors (GRs) and their translocation from the cytoplasm to the nucleus using immunohistochemical and Western immunoblot techniques in the pituitary in addition to three brain regions. When DEX was injected systemically, the suppressive response was significantly enhanced in aged rats, compared with young rats. When DEX was infused into three brain regions, the suppressive response to DEX was abolished in aged rats. The immunohistochemical analysis revealed that the number of GR positive cells in the PFC, hippocampus, and hypothalamus was decreased, but that in the pituitary was increased, in aged rats, compared with young rats. The Western immunoblot analysis confirmed these results. Thus, basal expression levels of GRs in three brain regions were decreased, but those in the pituitary were increased, in aged rats. After the injection or infusion of DEX, the translocation of GRs in three brain regions was reduced, but that in the pituitary was enhanced, in aged rats. These results suggest that aging in rats enhances the feedback ability at the systemic level, which mainly involves the pituitary, but it attenuates the ability in the brain. These mechanisms may underlie the vulnerable neuroendocrine systems associated with aging.