Trajectory of contralateral entorhinal axons which reinnervate the fascia dentata of the rat following ipsilateral entorhinal lesions.

Unilateral destruction of the entorhinal area in the rat results in the proliferation of a pathway from the surviving contralateral entorhinal area to the fascia dentata denervated by the lesions (the crossed temporodentate pathway). The present study analyzes the point of entry of these reinnervating fibers into the fascia dentata, and their trajectory within the reinnervated zones utilizing orthograde transport of tritiated proline or horseradish peroxidase (HRP). Following injections of tritiated proline or HRP into the surviving entorhinal area in animals with long standing unilateral entorhinal lesions labeled axons could be visualized entering the contralateral fascia dentata via two routes. Labeled fibers could be traced from the dorsal hippocampal commissure (the dorsal psalterium) into the rostral tip of the fascia dentata (the fasciola cinerea) and from the terminal field of the crossed temporo-ammonic tract in regio superior into adjacent portions of the stratum moleculare of the fascia dentata. Within the stratum moleculare, most of the labeled axons had predominantly a caudal and lateral orientation. Exceptions to this predominant trajectory were found in the case of some of the axons which entered the ventral blade of the rostral fascia dentata, and coursed laterally from their point of entry. Comparisons of the trajectory of the crossed temporodentate projections with that of the normal ipsilateral pathway indicated that while the predominant trajectory of the fibers was roughly comparable, the polarity of the projections was in part opposite. Specifically, the normal ipsilateral pathway travels in a caudorostral direction, while the majority of fibers of the crossed temporodentate pathway apparently project rostrocaudally. The significance of this difference in the pattern of innervation is discussed with respect to the normal functioning of the temporodentate circuitry.