Enhanced but delayed axonal sprouting of the commissural/associational pathway following a combined entorhinal cortex/fimbria fornix lesion.

From previous lesion studies of the hippocampus it has been reported that axons of the commissural/associational pathway expand their termination zone in the molecular layer of the dentate gyrus by 20-25% in response to loss of input from the entorhinal cortex. However, although much is known about the response of the commissural/associational pathway with regard to extent, latency, and speed of the reinnervation response following an entorhinal cortex lesion, little is known about how the loss of additional afferent systems might modulate this response. To address this issue, we examined at 14, 30, and 45 days postlesion, the sprouting of commissural/associational afferents following either a unilateral fimbria fornix transection, a unilateral entorhinal cortex lesion, or combined lesions of both the entorhinal cortex and the fimbria fornix. Loss of septal innervation to the hippocampus was assessed using the cholinesterase stain, whereas sprouting from the commissural/associational pathway was determined from Holmes fiber-stained sections. In addition, the Timms stain was used to examine the time course of the loss of terminal fields of the various zinc-containing afferent systems within the hippocampus. Following the removal of input to the hippocampus via the fimbria fornix transection, there was no evidence of sprouting of the commissural/associational fibers into the deafferented portion of the dentate gyrus. In contrast, rats receiving an entorhinal cortex lesion showed a significant increase (28%) in the width of the commissural/associational fiber plexus that was present by 14 days postlesion. By comparison, the magnitude of the expansion of the commissural/associational fiber plexus was significantly larger after lesioning both the entorhinal cortex and the fimbria than after the entorhinal cortex lesion alone (45% vs. 28%). In addition, the expansion of the commissural/associational fiber plexus was not increased at 14 days postlesion but was significantly increased at 30 days postlesion. The delay in the sprouting of the commissural/associational pathway coincided with the time course of loss of zinc-containing fibers in the outer molecular layer of the dentate gyrus as assessed with the Timms stain. These results suggest that the magnitude and time course for the sprouting of axons from the commissural/associational pathway into the partially deafferented hippocampus of the adult rat is lesion dependent and that the effect of the loss of input from the entorhinal cortex can be modulated and enhanced by the concomitant depletion of input from the fimbria fornix.