Structural alterations of the hippocampal formation of adrenalectomized rats: an unbiased stereological study

Previous studies have demonstrated that adrenalectomy rapidly induces cell death in hippocampal formation. However, these previous studies have involved only qualitative observations or biased estimates. Therefore, the selectivity of the effects of adrenalectomy and the magnitude of changes occurring, remain controversial. The present work employed unbiased stereological tools to examine the effects of adrenalectomy on the number of neurons in, and the volume of, the hippocampal formation. Male rats were adrenalectomized 15,30 or 120 days before sacrifice at 180 days of age. The total number of neurons in the somal layers and hilus of the hippocampal formation was estimated using the optical fractionator. The volume of the different layers of each subdivision in the hippocampal formation was determined according to the Cavalieri principle. A progressive reduction, reaching 43%, was found in the total number of granule cells. Adrenalectomized animals exhibited a reduction in the volume of all layers of the dentate gyrus. No other region of the hippocampal formation displayed significant cell loss or a reduction in volume. In addition, the main neuronal subpopulations of the dentate gyrus were also evaluated, and a reduction in the total number of GABA- and neuropeptide Y-immunoreactive neurons in the molecular and granule cell layers of adrenalectomized rats was found. No quantitative changes were observed in the hilus. To characterize the glial response to the neuronal degeneration, we estimated the total number of cells immunoreactive for glial fibrillary acidic protein in the dentate gyrus. Although no variation in the total number of glial cells was found, signs of astroglial activation were observed in the adrenalectomized group. The present data strengthen the evidence pointing to the critical role of corticosteroids in maintaining the structural integrity of the dentate gyrus.