In vivo intracellular analysis of granule cell axon reorganization in epileptic rats

In vivo intracellular recording and labeling in kainate-induce depileptic r ats was used to address questions about granule cell axon reorganization in temporal lobe epilepsy. Individually labeled granule cells were reconstruc ted three dimensionally and in their entirety. Compared with controls, gran ule cells in epileptic rats had longer average axon length per cell; the di fference was significant in all strata of the dentate gyrus including the h ilus. In epileptic rats, at least one-third of the granule cells extended a n aberrant axon collateral into the molecular layer. Axon projections into the molecular layer had an average summed length of 1 mm per cell and spann ed 600 mu m of the septotemporal axis of the hippocampus-a distance within the normal span of granule cell axon collaterals. These findings in vivo co nfirm results from previous in vitro studies. Surprisingly, 12% of the gran ule cells in epileptic rats, and none in controls, extended a basal dendrit e into the hilus, providing another route for recurrent excitation. Consist ent with recurrent excitation, many granule cells (56%) in epileptic rats d isplayed a long-latency depolarization superimposed on a normal inhibitory postsynaptic potential. These findings demonstrate changes, occurring at th e single-cell level after an epileptogenic hippocampal injury, that could r esult in novel, local, recurrent circuits.