Lesion-induced synapse reorganization in the hippocampus of cats: sprouting of entorhinal, commissural/associational, and mossy fiber projections after unilateral entorhinal cortex lesions, with comments on the normal organization of these pathways.

This study evaluates whether three forms of sprouting occur in the hippocampus of the cat following unilateral entorhinal cortex (EC) lesions: (1) sprouting of projections from the EC contralateral to the lesion; (2) sprouting of the commissural/associational system; and (3) sprouting of mossy fibers. Tract tracing techniques were used to define the normal organization of the entorhinal cortical projection system, the commissural/associational (C/A) systems, and the mossy fiber projections in normal cats. The same techniques were then used to evaluate whether there were changes in these projections in animals with long-standing unilateral EC lesions. The projections from the entorhinal cortex were evaluated autoradiographically following injections of 3H proline into the entorhinal area. The projections of the C/A system were traced using the Fink-Heimer technique after lesions of the hippocampal commissures, and by using autoradiographic techniques after injections of 3H proline into the hippocampus. The distribution of mossy fibers was evaluated using the Timm's stain. The results reveal that unilateral lesions of the EC in cats lead to the same sorts of sprouting that have been described in rats. There is: (1) an increase in the density of the crossed projection from the surviving EC to the contralateral dentate gyrus that had been deprived of its normal EC inputs; (2) an expansion of the terminal field of the C/A projection system into portions of the molecular layer of the dentate gyrus normally occupied by EC projections; and (3) an increase in supragranular mossy fibers in some animals. The mossy fiber sprouting was especially prominent when the lesions encroached upon the hippocampus. The studies also reveal additional details about the normal organization of hippocampal pathways in cats. The most important points are: (1) there is a crossed projection from the entorhinal cortex to the contralateral dentate gyrus; and (2) there is a complex laminar organization of the commissural and associational terminal fields in the molecular layer of the dentate gyrus that appears to be related to the point of origin of the projections along the septotemporal axis of the hippocampus. This heretofore unrecognized aspect of the laminar organization of C/A terminations has important implications for the temporal competition hypothesis, which has been advanced to account for the development of these afferent systems.