This project seeks to identify the cellular mechanisms that regulate neurite outgrowth and synapse replacement in the hippocampus in response to injury and the effect of aging on these processes. Studies conducted during the previous funding period argue against the notion that reactive synaptogenesis involves a unitary set of cellular responses common to all neurons in the brain but more likely involves specific sets of partially overlapping developmental signals that are cell type, brain region and lesion specific. In addition, while it was once believed that degenerative disease of the brain can be represented by simple cell loss that leads to functional decline, it is now known that the injured brain represents a composite of various adaptive responses which work to preserve and repair the brain's synaptic circuits against a background of ongoing cell death. However, while previous anatomical studies have readily demonstrated the ability of neurons to adapt and form new synaptic circuits in response to brain injury the cellular and molecular mechanisms that regulate reactive synaptogenesis and the effect of aging on these processes remain unclear. The studies proposed are a direct extension of the work completed previously and will test three working hypotheses: 1) that the observed delayed but enhanced sprouting response of commissural/associational axons following a combined lesion of the entorhinal cortex/fimbria fornix (EC/FF) is selective for the loss of cholinergic input from the septum/diagonal band and input from the entorhinal cortex; 2) that in response to injury different sets of growth associated proteins and trophic factors regulate neurite outgrowth in projection neurons that terminate in different laminae of the dentate gyrus (DG); and 3) that the new pattern of synaptic innervation that is formed in the DG following the EC/FF lesion results in the formation of a pattern of aberrant neural circuits which increases the vulnerability of target neurons to injury, inhibits information processing and thus, limits the chance of functional recovery. The investigators will use experimental deafferentation lesions in rats to model different combinations of neurotransmitter deficits that may affect neurite outgrowth and synapse replacement in the DG following brain injury. In addition, they will screen aged rats prior to surgery, using the Morris water maze, to identify subsets of old rats with hippocampal deficits that may alter their ability to respond to the lesion. They will also assess the rate of functional recovery at various times postlesion to correlate with their morphological and molecular data. Morphological remodeling of afferent input to the DG will be evaluated using light microscopic histochemical and ultrastructural methods; while the synaptic physiology of the anatomically reorganized hippocampus will be examined by electrophysiological analysis. In addition, they will use in situ hybridization and western blot methods to define the time course of changes in the levels of mRNAs and proteins that they hypothesize are associated with the promotion of neurite outgrowth and synapse replacement in the hippocampus (i.e. GAP-43, SCG-10, BDNF, trkB and SNAP-25).
