Kainic acid-induced mossy fiber sprouting and synapse formation in the dentate gyrus of rats

In the kainic acid (KA) model of temporal lobe epilepsy, mossy fibers (MFs) are thought to establish recurrent excitatory synaptic contacts onto granu le cells. This hypothesis was tested by intracellular labeling of granule c ells with biocytin and identifying their synaptic contacts in the dentate m olecular layer with electron microscopic (EM) techniques. Twenty-three gran ule cells from KA-treated animals and 14 granule cells from control rats we re examined 2 to 4 months following KA at the light microscopic (LM) level; four cells showing MF sprouting were further characterized at the EM level . Timm staining revealed a time-dependent growth of aberrant MFs into the d entate inner molecular layer. The degree of sprouting was generally (but no t invariably) correlated with the severity and frequency of seizures. LM ex amination of individual biocytin-labeled MF axon collaterals revealed enhan ced collateralization and significantly increased numbers of synaptic MF bo utons in the hilus compared to controls, as well as aberrant MF growth into the granule cell and molecular layers. EM examination of serially reconstr ucted, biocytin-labeled MF collaterals in the molecular layer revealed MF b outons that form asymmetrical synapses with dendritic shafts and spines of granule cells, including likely autaptic contacts on parent dendrites of th e biocytin-labeled granule cell. These results constitute ultrastructural e vidence for newly formed excitatory recurrent circuits, which might provide a structural basis for enhanced excitation and epileptogenesis in the hipp ocampus of KA-treated rats. (C) 2000 Wiley-Liss, Inc.