Time course and distribution of neuronal degeneration in the dentate gyrus of rat after adrenalectomy: A silver impregnation study

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Abstract
Recently, Sloviter et al. reported that adrenalectomy (ADX) of young adult rats after 3 months led to a selective loss of granule neurons in the dentate gyrus (DG) and that this loss could be prevented by low doses of corticosterone. In the present study, the ADX‐induced neuronal degeneration was investigated in Wistar rats, using a silver impregnation method for degenerating neurons. To examine the time course and distribution of the ADX‐induced degeneration, young adult male rats were allowed to survive 2, 3, and 5 days and 1, 2, and 3 weeks after ADX. Argyrophilic neurons were present in the dentate granule cell layer on the second day following ADX. Three days after ADX, the number of argyrophilic granule neurons was much more abundant, and it increased gradually with longer post‐ADX survival times. Argyrophilia was specifically confined to dentate granule cells and was accompanied by the occurrence of pyknotic nuclei as observed in adjacent cresyl violet‐stained section. There were significant differences between individual rats in quantity of argyrophilia. About one fifth of the ADX rats showed sporadic or no argyrophilia, in spite of plasma corticosterone levels below the detection limit (10 ng/mL). Sham‐operated rats and ADX rats receiving corticosterone (10 mg/L) or dexamethasone (15 mg/L) in their drinking water did not display any argyrophilic neurons in the dentate gyrus. The distribution of the argyrophilia within the DG was highly characteristic with the highest number of degenerating cells in the hidden blade of the middle and the temporal thirds of the DG. There was a strong crest‐to‐tip and deep (hilar)‐to‐superficial increase in density of degenerating neurons. Along the entire septotemporal axis of the free blade and at the septal pole of the hidden blade, the number of argyrophilic cells was low. Wistar rats of different sex and aged 1–6 months all showed dentate granule cell degeneration 1 week following ADX. The distribution of the degeneration was identical to that of the young adult male group.In order to examine whether the absence of corticosteroids affects dentate granule cells directly or through other mechanisms induced by ADX, low doses of dexamethasone were infused with a microdialysis probe directly into the hippocampus on one side of ADX rats. As a result, the ADX‐induced neuronal degeneration was selectively prevented at the treated side.