Lesion-induced synaptogenesis in the dentate gyrus of aged rats: II. Demonstration of an impaired degeneration clearing response.

Previously we reported that a delayed onset in the reinnervation of the outer two-thirds of the dentate molecular layer occurred in aged rats after an entorhinal lesion. Several factors associated with formation of new synaptic contacts and removal of degenerative debris may affect the reinnervation process. In this study the appearance and removal of degeneration was analyzed and evaluated with respect to the delayed reinnervation process in aged rats. After a complete lesion of the entorhinal cortex, 85-90% of the input to the outer two-thirds of the ipsilateral molecular layer is lost. Electron-dense and electron-lucent degeneration are present throughout the outer two-thirds of the denervated molecular layer. In both aged and young adult rats, the electron-lucent degeneration disappears by 10 days postlesion. The predominant electron dense degeneration, however, is removed at a different rate by young adult and aged rats. Young adults demonstrate a biphasic degeneration removal process, with almost half of this degeneration rapidly lost by 10 days postlesion, and nearly all by 60 days postlesion. Aged animals in contrast, have lost only 16% of the dense degeneration at 10 days postlesion, with about 30% of the degeneration remaining at 60 days postlesion. The impaired removal of the degeneration from the denervated zone appears to be reciprocally related to the reinnervation response in both age groups and may be related to the normal astrocyte hypertrophy and elevated corticosteroid levels in aged rats.