Prolonged increases in neurotrophic activity associated with kainate-induced hippocampal synaptic reorganization

Synaptic reorganization occurs in the hippocampus following various forms of seizure activity and injury, and may contribute to epileptogenesis. To address the hypothesis that neurotrophic factors play an inductive role in synaptic reorganization following seizures, we directly measured neurotrophic activity in rat hippocampal extracts after kainate injection or prolonged stimulation of the perforant path. Serial dilutions of hippocampal extracts were added to cultures of chick dorsal root ganglia, which are known to require trophic support from nerve growth factor and other neurotrophins, or ciliary ganglia neurons, which require trophic support from ciliary neurotrophic factor. Neurotrophic activity was significantly increased in hippocampal extracts harvested from 12 h to 2 months after kainate treatment, with the peak effect seen at seven days. This neurotrophic activity was substantially blocked by an anti-nerve growth factor antibody. Extracts at seven days also showed a significant increase in ciliary neurotrophic factor-like activity. Sulfide/silver histochemistry, which stains dentate granule cell axon terminals, revealed that mossy fiber sprouting was evident two weeks following kainate treatment and increased progressively over the next two to six weeks. Perforant path stimulation that produced hyperexcitability in the dentate gyrus, but no sprouting, failed to induce changes in neurotrophic activity. These results suggest there are significant increases in neurotrophic factors following kainate-induced seizures, and the increases may be related to kainate-induced hippocampal injury rather than seizures per se. Furthermore, the timecourse of increased neurotrophic activity parallels that of mossy fiber reorganization, and is consistent with the hypothesis that neurotrophic factors play a role in the injury-induced synaptic reorganization seen in epilepsy.