EXPRESSION OF THE NEURAL ADHESION MOLECULE L1 IN THE DEAFFERENTED DENTATE GYRUS

Expression of the neural adhesion molecule L1 and its potential involv ement in axonal sprouting were examined in the deafferented rat dentat e gyrus. We focused on the dentate gyrus because of its well-defined c ytoarchitecture and well-characterized neuronal degeneration and sprou ting response following entorhinal cortex lesions. In the molecular la yer of the dentate gyrus, a trilaminar staining pattern was observed, with the middle molecular layer exhibiting slightly denser immunolabel ing compared to both inner and outer molecular layers. Two to 12 days after a unilateral entorhinal cortex lesion, a progressive loss of L1 immunolabeling was noted in the ipsilateral middle and outer molecular layers, followed by a substantial reappearance of immunostaining 65 d ays after lesion incidence. The width of the immunostained ipsilateral inner molecular layer revealed a progressive widening and by postlesi on day 65 occupied about 50% of the total width of the molecular layer . Immunoelectron microscopy localized L1 to the surface of unmyelinate d axons in both normal and deafferented dentate gyrus. In situ hybridi zation revealed LI messenger RNA confined to neurons throughout the hi ppocampal formation, but did not indicate changes in L1 messenger RNA levels in the hippocampus, dentate gyrus, entorhinal cortex or basal f orebrain in response to unilateral entorhinal cortex lesions. Changes in L1 immunolabeling in the deafferented dentate gyrus corresponded in a spatial and temporal manner to changes of the synaptic marker synap tophysin and axonal marker phosphorylated tau. Results of the present study are most consistent with the view that L1 is expressed on reinne rvating fibers after they make synaptic contacts with other structures . Thus, L1 appears to be involved in the maturation and stabilization of reinnervating fibers and consequently may play an important role in the repair process of the lesioned adult CNS. Copyright (C) 1996 IBRO .