Nerve growth factor produced by forebrain neurons is thought to provide essential trophic support for the cholinergic neurons of basal forebrain which degenerate in behaviorally impaired aged rats and in diseases of cognitive impairment. This has stimulated interest in understanding the regulation of NGF expression in brain and attempts to augment NGF availability in aged brain. The recent collaborative studies of GAll and Isackson have demonstrated that, in addition to localization in hippocampus and cortex, neurons containing NGF mRNA are distributed within basal forebrain fields containing cholinergic neurons. In addition, seizure activity has been found to stimulate a dramatic increase in NGF mRNA content of dentate gyrus granule cells and neurons of olfactory- and neocortex. The goals of the 5 studies proposed here are to further evaluate the mechanisms by which physiological activity regulates NGF expression in young adult brain and to determine if there are differences in the cellular localization and """"""""reactivity"""""""" of NGF expression in aged brain which correlate with the degree of behavioral impairment. Throughout the proposed work in situ hybridization and S1 nuclease protection techniques will be used to localize and quantify mRNA levels. Exp. 1 will utilize acute electrical stimulation to determine if the activity dependent stimulation of NGF expression is dependent upon synaptic mechanisms; specifically, if orthodromic, as opposed to antidromic, activation and NMDA receptor mechanisms; specifically, if orthodromic, as opposed to antidromic, activation and NMDA receptor mechanisms are required. Exp. 2 will determine if NGF mRNA expression becomes refractory to further stimulation in the wake of seizure-induced increases in this mRNA species. Exp. 3 will examine whether the seizure-stimulation of increased hippocampal NGF synthesis leads to changes in the biosynthetic activities of basal forebrain neurons; specifically, amyloid precursor protein mRNA, NGF receptor mRNA< and choline acetyltransferase (ChAT) immunoreactivity will be evaluated. In Exp. 4 in situ hybridization and immunohistochemical techniques will be used to determine if there are changes in the neuronal localization and abundance of mRNAs for NGF and NGF receptor that correspond with the degree of behavioral impairment and the apparent viability of basal forebrain cholinergic neurons in aged rat brain. Finally, in Exp. 5 acute electrical stimulation techniques will be used to determine if the response of NGF mRNA expression to physiological activation differs in old and aged (impaired and non-impaired) rats as compared to young adults. The proposed studies should further resolve the relationship of NGF trophic support to degenerative processes in the aged brain and begin to explore the feasibility of using physiological stimulation of NGF as a therapeutic strategy for maintaining viability of cholinergic forebrain neurons.
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