HIPPOCAMPAL MOSSY FIBER SPROUTING AND SYNAPSE FORMATION AFTER STATUS EPILEPTICUS IN RATS - VISUALIZATION AFTER RETROGRADE TRANSPORT OF BIOCYTIN

In complex partial epilepsy and in animal models of epilepsy, hippocampal messy fibers appear to develop recurrent collaterals that invade the dentate molecular layer. Messy fiber collaterals have been proposed to subserve recurrent excitation by forming granule cell-granule cell synapses. This hypothesis was tested by visualizing dentate granule cells and their messy fibers after terminal uptake and retrograde transport of biocytin. Labeling studies were performed with transverse slices of the caudal rat hippocampal formation prepared 2.6-70.0 weeks after pilocarpine-induced or kainic acid-induced status epilepticus. Light microscopy demonstrated the progressive growth of recurrent messy fibers into the molecular layer; the densest innervation was observed in slices from pilocarpine-treated rats that had survived 10 weeks or longer after status epilepticus. Thin messy fiber collaterals originated predominantly from deep within the hilar region, crossed the granule cell body layer, and formed an axonal plexus oriented parallel to the cell body layer within the inner one-third of the molecular layer. When sprouting was most robust, some recurrent messy fibers at the apex of the dentate gyrus reached the outer two-thirds of the molecular layer. The distribution and density of messy fiber-like Timm staining correlated with the biocytin labeling. When viewed with the electron microscope, the inner one-third of the dentate molecular layer contained numerous messy fiber boutons. In some instances, biocytin-labeled messy fiber boutons were engaged in synaptic contact with biocytin-labeled granule cell dendrites. Granule cell dendrites did not develop large complex spines (''thorny excrescences'') at the site of synapse formation, and they did not appear to have been permanently damaged by seizure activity. These results establish the validity of Timm staining as a marker for messy fiber sprouting and support the view that status epilepticus provokes the formation of a novel recurrent excitatory circuit in the dentate gyrus. Retrograde labeling with biocytin showed that the recurrent messy fiber projection often occupies a considerably greater fraction of the dendritic region than previous studies had suggested. (C) 1995 Wiley-Liss, Inc.