Axon sprouting in a model of temporal lobe epilepsy creates a predominantly excitatory feedback circuit

Abstract

The most common type of epilepsy in adults is temporal lobe epilepsy. After epileptogenic injuries, dentate granule cell axons (mossy fibers) sprout and form new synaptic connections. Whether this synaptic reorganization strengthens recurrent inhibitory circuits or forms a novel recurrent excitatory circuit is unresolved. We labeled individual granule cells in vivo, reconstructed sprouted mossy fibers at the EM level, and identified postsynaptic targets with GABA immunocytochemistry in the pilocarpine model of temporal lobe epilepsy. Granule cells projected an average of 1.0 and 1.1 mm of axon into the granule cell and molecular layers, respectively. Axons formed an average of one synapse every 7 microm in the granule cell layer and every 3 microm in the molecular layer. Most synapses were with spines (76 and 98% in the granule cell and molecular layers, respectively). Almost all of the synapses were with GABA-negative structures (93 and 96% in the granule cell and molecular layers, respectively). By integrating light microscopic and EM data, we estimate that sprouted mossy fibers form an average of over 500 new synapses per granule cell, but <25 of the new synapses are with GABAergic interneurons. These findings suggest that almost all of the synapses formed by mossy fibers in the granule cell and molecular layers are with other granule cells. Therefore, after epileptogenic treatments that kill hilar mossy cells, mossy fiber sprouting does not simply replace one recurrent excitatory circuit with another. Rather, it replaces a distally distributed and disynaptic excitatory feedback circuit with one that is local and monosynaptic.

Fig. 1.

Timm's-stained sections of the hippocampus in a control (a, c) and a pilocarpine-induced epileptic rat (b, d). Boxed regions in a and b are shown at higher magnification in c and d, respectively. Black mossy fiber terminals are evident in the granule cell layer (gcl) and inner one-third of the molecular layer (ml) in the epileptic but not the control rat.

Fig. 2.

EM reconstruction of a sprouted mossy fiber in an epileptic rat. a, A granule cell labeled with biocytinin vivo in an epileptic rat. Photograph of section containing the soma (a1) and a light-microscopic reconstruction (a2) of the cell. Dendrites are thick; axon is thin. All of the axon shown in this reconstruction is within the dentate gyrus, and most is within the hilus. Several sections away from the soma, an axon collateral projected from the hilus through the granule cell layer and into the molecular layer. A segment of that axon collateral (box) was selected for reconstruction at the EM level.h, Hilus; gcl, granule cell layer;ml, molecular layer. b, Photograph of the selected axon segment. c1, EM reconstruction of the selected axon segment (black). Gray contours indicate cell nuclei that were used as landmarks. Thearea in the box is shown at high magnification in d. c2, Side view of three-dimensionally reconstructed axon segment demonstrating how the axon projected through the thickness of the section. d1, Magnified view of boxed region in c1demonstrates that the reconstructed axon segment consists of aligned serial contours (gray) that outlined biocytin-labeled axon profiles in serial electron micrographs. Theblack contour outlined the biocytin-labeled axon (black) in d2, which forms a synapse (arrowhead) with a dendritic spine.

Fig. 3.

Sprouted mossy fibers synapsed with dendritic spines and dendritic shafts. a, A biocytin-labeled axon (black) in the molecular layer formed synapses (arrowheads) with a large and a small spine.b, A biocytin-labeled axon (black) in the molecular layer formed a synapse (arrowhead) with the shaft of a spiny (arrows) dendrite.

Fig. 4.

Sprouted mossy fibers synapsed preferentially with dendritic spines. Reconstructed sprouted mossy fibers with synaptic contacts indicated by markers. Squaresindicate that the postsynaptic target was a dendritic spine;circles indicate a dendritic shaft. The identity of each reconstruction (rat and segment) corresponds to Table 1. Borders between strata (h, hilus; gcl, granule cell layer; ml, molecular layer) are indicated bylines.

Fig. 5.

Sprouted mossy fibers synapsed with GABA-negative and GABA-positive targets. a, A biocytin-labeled axon (black) formed synaptic contacts (arrowheads) with two GABA-negative spines. Nearby GABA-positive structures were labeled with 10-nm-diameter colloidal gold particles, and a GABA-positive axon terminal formed a symmetric synapse (arrow) with a granule cell body.b, A biocytin-labeled axon (black) in the molecular layer formed a synaptic contact (arrowhead) with a GABA-positive dendritic shaft.

Fig. 6.

Sprouted mossy fibers synapsed preferentially with GABA-negative dendritic spines. Reconstructed sprouted mossy fibers with synaptic contacts indicated by markers.Squares indicate that the postsynaptic target was a dendritic spine; circles indicate a dendritic shaft.Open markers indicate that the postsynaptic target was GABA-negative; filled markers indicate GABA-positive. The identity of each reconstruction (rat and segment) corresponds to Table 1. Borders between strata (h, hilus;gcl, granule cell layer; ml, molecular layer) are indicated by lines.