Prolonged glucocorticoid exposure reduces hippocampal neuron number: implications for aging

The hippocampus of the rat loses neurons with age, a loss which may eventuate in some of the functional impairments typical of senescence. Cumulative exposure to corticosterone (CORT) over the lifespan may be a cause of this neuronal loss, as it is prevented by adrenalectomy at mid- age. In this study, we demonstrate that prolonged exposure to CORT accelerates the process of cell loss. Rats were injected daily with sufficient CORT to produce prolonged elevations of circulating titers within the high physiological range. Animals treated for 3 months (chronic subjects) resembled aged rats in a number of ways. First, both groups had extensive and persistent depletions of CORT receptors in the hippocampus; in the case of chronic rats, no recovery of receptor concentrations occurred 4 months after the end of steroid treatment. Second, autoradiographic analysis revealed that the receptor depletion was due, in part, to a loss of CORT-concentrating cells, especially in the CA3 cell field. Remaining cells bound significantly less [3H]corticosterone than did those of control rats. Finally, analysis of size distributions of hippocampal cell bodies indicated that chronic subjects lost neurons of the same size as those lost in the aged hippocampus. Furthermore, chronic subjects also had increased numbers of small, darkly staining cells of CA3; these corresponded in size to the dark glia whose numbers increase in the aged hippocampus, and which are thought to infiltrate in response to neuronal damage or destruction. Thus, this study supports the hypothesis that cumulative exposure to CORT over the lifespan may contribute to age-related loss of neurons in the hippocampus, and that prolonged stress or exposure to CORT accelerates this process.