Changes in the distribution of the dentate gyrus associational system following unilateral or bilateral entorhinal lesions in the adult rat.

The distribution of the dentate gyrus associational system was analyzed in naive adult rats and in those with either unilateral or bilateral lesions of the entorhinal cortex. Horseradish peroxidase histochemistry was used to trace the origin and course of this intrinsic fiber system. The fibers originated in the CA3-4 pyramidal cell field, apparently medial to the origin of the Schaffer collateral system, and followed a trajectory which was essentially identical to that described for this system by Zimmer36. The associational terminal field occupied the inner 26% of the dentate gyrus molecular layer in normal rats and 35-38% of the normal width of that layer following either ipsilateral or bilateral entorhinal lesion. These measurements are quite similar to those previously obtained on the commissural system terminal field in the normal and partially deafferented dentate gyrus. These results are interpreted to reflect axon sprouting by the associational fibers into the adjacent deafferented dendritic field.