Corticosterone replacement restores normal morphological features to the hippocampal dendrites, axons and synapses of adrenalectomized rats

A thorough evaluation of hippocampal dendrites, axons and synaptic contacts has not been undertaken following prolonged periods of absence of corticos teroids despite the marked granule cell loss which occurs in the dentate gy rus of adrenalectomized rats. Thus, we have applied morphometric techniques to analyse the dendrites of granule and pyramidal cells, the mossy fiber s ystem, and the number and morphology of synapses between the mossy fibers a nd the excrescences of CA3 pyramidal cells in rats submitted to different p eriods of adrenalectomy. In addition, to search for the presence of neuriti c reorganisation in the hippocampal formation once normal corticosteroid le vels were re-established, we incorporated in this study a group of rats rep laced with corticosterone one month after adrenalectomy. The results obtain ed in adrenalectomized rats showed a striking impoverishment of the dendrit es of surviving granule cells, subtle alterations in the apical dendritic a rborization of CA3 pyramidal cells and no changes in the apical dendrites o f CA1 pyramidal cells. In addition, in adrenalectomized rats there was a pr ogressive reduction in the total number of synapses established between mos sy fibers and CA3 pyramids, as a consequence of a reduction in the volume o f the suprapyramidal part of the mossy fiber system, and profound changes i n the morphology of mossy fiber terminals and CA3 dendritic excrescences. A remarkable reorganisation of neurites was found to occur following the adm inistration of low doses of corticosterone, completely reversing the adrena lectomy-induced synaptic loss and partially restoring the morphology of hip pocampal axons and dendrites. These plastic mechanisms provide a sound stru ctural basis for the reversibility of cognitive deficits observed after cor ticosterone administration to adrenalectomized rats.