Axonal Sprouting and Epilepsy after Traumatic CNS Injury

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The Problem: Epilepsy is a common consequence of traumatic head injury. Its cause is unknown. The Hypotheses: 1) The release of neurotrophins after traumatic CNS injury triggers axonal sprouting by pyramidal cells. 2) The glutamate sensitivity and excitability of postsynaptic target cells increase as a consequence of partial denervation. 3) Injury-induced presynaptic axonal sprouting and increased postsynaptic excitability combine synergistically to cause posttraumatic epilepsy. The Model: After Schaffer collateral transection, CA3 cells in hippocampal slice cultures sprout new axon collaterals and CA1 cells become supersensitive to glutamate. These phenomena may account for the lesion-induced hyperexcitability. This model provides an experimentally tractable and informative approach for studying pre- and postsynaptic mechanisms of posttraumatic epilepsy. The genesis of hyperexcitability after axonal injury in this model will therefore be investigated using neuroanatomical, cell biological, and electrophysiological techniques. AIM 1: Determine the presynaptic mechanisms underlying injury- induced hyperexcitability. TrkB immunoadhesins, biolistic transfection with full length and dominant negative neurotrophin receptor constructs, and cultures derived from trk receptor knockout mice will be used to test the hypothesis that activation of trk receptors is required for injury-induced axonal sprouting. The hypothesis predicts that lesion-induced sprouting will not occur in the presence of trkB immunoadhesin, in cells transfected with dominant negative irk receptors or in cultures made from trk receptor knock-out mice. Lack of axonal sprouting is predicted to eliminate injury-induced hyperexcitability. AIM 2: Determine the postsynaptic mechanisms underlying injury- induced hyperexcitability. Using whole-cell voltage-clamp and laser microphotolysis of caged neurotransmitters targeted to individual distal dendrites, we will test the hypotheses that glutamate supersensitivity and intrinsic hyperexcitability occur in CA1 cells after denervation. The hypothesis predicts that changes in the levels of expression of neurotransmitter receptors and/or changes in intrinsic voltage-dependent ionic conductances underlie the potentiation of dendritic glutamate responses observed previously after Schaffer collateral transection. The Goal: to better understand the causes of posttraumatic epilepsy and, ultimately, to offer new and improved prophylactic therapeutic strategies to cure this disease.