Morphological evidence for the sprouting of inhibitory commissural fibers in response to the lesion of the excitatory entorhinal input to the rat dentate gyrus.

Recently a commissural fiber projection that terminates in the outer molecular layer of the fascia dentata was described in normal rats (Deller et al., 1995). In the present article, Phaseolus vulgaris leucoagglutinin (PHAL) tracing was used to analyze the contribution of this previously unknown projection to the commissural sprouting response after entorhinal cortex lesion. Rats 4-8 weeks after unilateral entorhinal lesion received a single PHAL deposit into the hilus of the fascia dentata contralateral to the lesion side. Unlesioned control animals received a similar PHAL deposit. The degree of axonal arborization and the bouton density per axon length were determined for individual PHAL-labeled commissural axons to the outer molecular layer of the dentate gyrus. A significant increase in both parameters was observed in the lesioned group. The PHAL-labeled commissural fibers established symmetric synapses in the denervated outer molecular layer. Postembedding immunocytochemistry indicated that some of these sprouting commissural fibers are GABAergic. Our findings provide morphological evidence for lamina-specific sprouting of an inhibitory commissural projection that normally terminates in the outer molecular layer. This suggests that inhibitory fibers participate in the replacement of the excitatory perforant pathway after entorhinal lesion.