Abstracts

RATIONALE: Epilepsy is a common consequence of traumatic brain injuries; its cause is unknown. Axonal sprouting is observed after many forms of CNS injury and it has been hypothesized that this synaptic reorganization is a critical cause of posttraumatic epilepsy. A full test of this hypothesis requires the means to stop injury-induced axonal sprouting, but the factors that trigger sprouting after injury are unknown. Secretion of neurotrophins and expression of NT receptors is increased after many forms of CNS injury and can trigger axonal sprouting, even in mature tissue. We hypothesized that injury-induced NT secretion triggers axonal sprouting.
METHODS: Schaffer collateral transection in hippocampal slice cultures has been shown previously to result in a delayed sprouting of CA3 cell axons and in hyperexcitability. Furthermore, application of NTs triggers axonalsprouting by pyramidal cells in cultures maintained in vitro for [gt]14 days. The role of NTs in injury-induced sprouting was therefore tested in hippocampal slice cultures prepared from postnatal day 5-7 mice in which the predominant CA3 cell NT receptor, trkB, has been replaced with a [ssquote]floxed[ssquote] trkB transgene, resulting in a 75% reduction in trkB expression (Xu et al., Neuron., 26:233, 2000). After 14 days in vitro, lesions were placed in the Schaffer collateral pathway at the border between areas CA3 and CA1 in cultures from wild type and homozygous [ssquote]knock-down[ssquote] litter mates. Seven days later, cultures were fixed and processed for immunocytochemistry using an antibody against the growth associated protein GAP-43. Previous work has shown that GAP-43 expression is substantially up-regulated in newly sprouted axons after injury.
RESULTS: Decreased trkB expression had no adverse effects on the cytoarchitecture or health of the cultures. Large numbers of immunoreactive fibers, terminating in growth cones, were observed in lesioned wild type cultures. Cultures from mice with decreased trkB expression, in contrast, displayed no or very few immunoreactive fibers.
CONCLUSIONS: We conclude that Schaffer collateral transection fails to trigger axonal sprouting when the level of trkB expression by CA3 cells is below some critical level. Signaling via the trkB receptor, presumably in response to NTs secreted as a result of the tissue injury, is required for the induction of axonal sprouting by pyramidal cells after traumatic injury. This model system thus provides the experimental means to test the hypothesized role of axonal sprouting as a cause of posttraumatic epilepsy.
[Supported by: NINDS]