Differences in lesion-induced hippocampal plasticity between mice and rats

We studied the differences between mice and rats in lesion-induced sprouting in the hippocampus. The entorhinal cortex was unilaterally lesioned with ibotenic acid in adult, female mice and rats. Four weeks later the subsequent axonal sprouting in the dentate gyrus was analysed, by measuring the density of the synaptophysin immunohistochemical and acetylcholinesterase histochemical staining in the termination area of the entorhinal cortex axons. The data demonstrate that both mice and rats display a significantly increased density of staining for synaptophysin and acetylcholinesterase in the molecular layer of the dentate gyrus, indicative of axonal sprouting. Both species also show an upregulation in the density of staining for acetylcholinesterase in the molecular layer of the dentate gyrus. Further, rats, but not mice, show a significant upregulation of synaptophysin staining in stratum lacunosum moleculare of CA1 following the lesions. However, whereas rats show significant shrinkage of the molecular layer of the dentate gyrus, mice do not show any shrinkage of that layer following entorhinal cortex lesions. Taken together, these data indicate that whereas the process of reinnervation in the hippocampus is similar between the mouse and the rat, the hippocampal response to denervation shows clear differences between these two species.