In the epileptic hippocampus, newly sprouted mossy fibers are considered to form recurrent excitatory connections to granule cells in the dentate gyms and thereby increase seizure susceptibility. To study the effects of mossy fiber sprouting on neural activity in individual lamellae of the dentate gyms, we used high-speed optical recording to record signals from voltage- sensitive dye in hippocampal slices prepared from kainate-treated epileptic rats (KA rats). In 14 of 24 slices from KA rats, hilar stimulation evoked a large alepolarization in almost the entire molecular layer in which granule cell apical dendrites are located. The signals were identified as postsynaptic responses because of their dependence on extracellular Ca2+. The depolarization amplitude was largest in the inner molecular layer (the target area of sprouted mossy fibers) and declined With increasing distance from the granule cell layer. In the inner molecular layer, a good correlation was obtained between depolarization size and the density of mossy fiber terminals detected by Timm staining methods. Blockade of GABAergic inhibition by bicuculline enlarged the depolarization in granule cell dendrites. Our data indicate that mossy fiber sprouting results in a large and prolonged synaptic depolarization in an extensive dendritic area and that the enhanced GABAergic inhibition partly masks the synaptic depolarization. However, despite the large dendritic excitation induced by the sprouted mossy fibers, seizure-like activity of granule cells was never observed, even when GABAergic inhibition was blocked. Therefore, mossy fiber sprouting may not play a critical role in epileptogenesis.
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