SYNAPTIC AND AXONAL REMODELING OF MOSSY FIBERS IN THE HILUS AND SUPRAGRANULAR REGION OF THE DENTATE GYRUS IN KAINATE-TREATED RATS

Seizures evoked by kainic acid and a variety of experimental methods i nduce sprouting of the mossy fiber pathway in the dentate gyrus. In th is study, the morphological features and spatial distribution of sprou ted mossy fiber axons in the dorsal dentate gyrus of kainate-treated r ats were directly shown in granule cells filled in vitro with biocytin and in vivo with the anterograde lectin tracer Phaseolus vulgaris leu coagglutinin (PHAL). Sprouted axon collaterals of biocytin-filled gran ule cells projected from the hilus of the dentate gyrus into the supra granular layer in both transverse and longitudinal directions in kaina te-treated rats but were not observed in normal rats. The sprouted axo n collaterals projected into the supragranular region for 600-700 mu m along the septotemporal axis. Collaterals from granule cells in the i nfrapyramidal blade crossed the hilus and sprouted into the supragranu lar laver of the suprapyramidal blade. Sprouted axon segments in the s upragranular layer had more terminal boutons per unit length than the axon segments in the hilus of both normal and kainate-treated rats hut did not form giant boutons, which are characteristic of mossy fiber a xons in the hilus and CA3. Mossy fiber axons in the hilus of kainate-t reated rats had more small terminal boutons, fewer giant boutons, and there was a trend toward greater axon length compared with mossy fiber s in the hilus of normal rats. With the additional length of supragran ular sprouted collaterals there was an overall increase in the length of mossy fiber axons in kainate-treated rats. The synaptic and axonal remodeling of the mossy fiber pathway could alter the functional prope rties of hippocampal circuitry by altering synaptic connectivity in lo cal circuits within the hilus of the dentate gyrus and by increasing t he divergence of the mossy fiber terminal field along the septotempora l axis. (C) 1998 Wiley-Liss, Inc.