Neuroplasticity in the damaged dentate gyrus of the epileptic brain

Using Golgi preparations, Cajal described many cell types and connections of the dentate gyrus. He described granule cells as having a round or elliptical cell body with their long axis perpendicular to the granule cell layer, dendrites arising from one pole and an axon arising from the other. Cajal apparently never studied the brains from epileptic animals or humans, and thus did not report on changes in granule cell morphology after epilepsy. Several neuroplastic changes have been described in the dentate gyrus of epileptic mammals in the past decade or so using modern methods. Two changes involving their processes include mossy fiber sprouting of granule cell axons into the inner molecular layer of the dentate gyrus and the formation of hilar basal dendrites. Two changes associated with increased neurogenesis of granule cells in the epileptic brain include hilar ectopic granule cells and the dispersion of the granule cell layer. The significance of the first two changes is that granule cell axon collaterals establish additional synapses with apical and basal dendrites of granule cells, and these connections contribute to new recurrent excitatory circuitry. The significance of increased neurogenesis is that granule cells are migrating into inappropriate areas (deep hilus) or excessive numbers of granule cells accumulate in the layer (dispersion). These data on the epileptic dentate gyrus show that granule cells may change their axonal and dendritic arbors as well as their numbers and position to respond to altered activity possibly caused by decreased inhibition. These findings indicate that the dentate gyrus shows several neuroplastic changes following temporal lobe epilepsy.