Characterization of target cells for aberrant mossy fiber collaterals in the dentate gyrus of epileptic rat.

Previous studies have demonstrated formation of recurrent excitatory circuits between sprouted mossy fibers and granule cell dendrites in the inner molecular layer of the dentate gyrus (9, 28, 30). In addition, there is evidence that inhibitory nonprincipal cells also receive an input from sprouted mossy fibers (39). This study was undertaken to further characterize possible target cells for sprouted mossy fibers, using immunofluorescent staining for different calcium-binding proteins in combination with Timm histochemical staining for mossy fibers. Rats were injected intraperitoneally with kainic acid in order to induce epileptic convulsions and mossy fiber sprouting. After 2 months survival, hippocampal sections were immunostained for parvalbumin, calbindin D28k, or calretinin followed by Timm-staining. Under a fluorescent microscope, zinc-positive mossy fibers in epileptic rats were found to surround parvalbumin-containing neurons in the granule cell layer and to follow their dendrites, which extended toward the molecular layer. In addition, dendrites of calbindin D28k-containing cells were covered by multiple mossy fiber terminals in the inner molecular layer. However, the calretinin-containing cell bodies in the granule cell layer did not receive any contacts from the sprouted fibers. Electron microscopic analysis revealed that typical Timm-positive mossy fiber terminals established several asymmetrical synapses with the soma and dendrites of nonpyramidal cells within the granule cell layer. These results provide direct evidence that, in addition to recurrent excitatory connections, inhibitory circuitries, especially those responsible for the perisomatic feedback inhibition, are formed as a result of mossy fiber sprouting in experimental epilepsy.