In the rat hippocampal formation, degeneration of CA4-derived afferent fibers provokes the growth of mossy fiber collaterals into the fascia dentata. These aberrant fibers subsequently form granule cell-granule cell synapses. The hippocampal slice preparation was employed to determine whether these recurrent connections are electrophysiologically functional. Hippocampal slices were prepared 12 to 21 days after the bilateral destruction of CA4 neurons with either intracerebroventricular or intravenous kainic acid (KA). In slices from control rats, antidromic stimulation of the mossy fibers elicited a single population spike in the granular layer of the fascia dentata. In contrast, when slices from some KA-treated rats were similarly tested, antidromic stimulation elicited multiple population spikes. This effect was not reproduced by blocking inhibitory transmission with bicuculline methiodide. Slices from other KA-treated rats fired a single population spike, but an antidromic conditioning volley increased the amplitude of a subsequent antidromic population spike by 5 to 15%. In slices from control rats, on the other hand, an antidromic conditioning volley always either decreased or failed to alter the amplitude of an antidromic test response. Superfusion with Ca2+-free medium containing 3.8 mM Mg2+ reversibly abolished all effects of KA administration. Abnormal responses to antidromic stimulation correlated with the loss of CA4 neurons and the growth of supragranular mossy fiber collaterals in the same animals. These results suggest that supragranular mossy fiber collateral sprouts form a functional recurrent excitatory circuit. These aberrant connections may further compromise hippocampal function already disrupted by neuronal degeneration, such as by facilitating seizure activity.

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In complex partial epilepsy and in animal models of epilepsy, hippocampal mossy fibers appear to develop recurrent collaterals that invade the dentate molecular layer. Mossy 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 mossy 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 mossy 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 mossy 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 mossy fibers at the apex of the dentate gyrus reached the outer two-thirds of the molecular layer. The distribution and density of mossy 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 mossy fiber boutons. In some instances, biocytin-labeled mossy 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 mossy 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 mossy fiber projection often occupies a considerably greater fraction of the dendritic region than previous studies had suggested.