Lesions of entorhinal cortex produce a calpain-mediated degradation of brain spectrin in dentate gyrus. II. Anatomical studies

Abstract

Lesions of the various afferents to the hippocampus have been widely used to investigate the mechanisms underlying growth and degeneration in adult mammalian CNS. It has been proposed that disturbances in intracellular calcium and activation of calcium-dependent proteases represent key steps in producing some of the consequences of the lesions. In this study, we show that lesions of the entorhinal cortex or of the commissural pathway result in profound changes in the distribution of brain spectrin. At 2 days after lesions of the entorhinal cortex, immunoreactivity to spectrin is markedly increased in the outer molecular layer (OML) of the dentate gyrus; conversely at 2 days after commissural lesions, immunoreactivity to the same antigen is increased in the inner molecular layer. The increase in immunoreactivity to spectrin varies with survival time after lesions of the entorhinal cortex. By 24 h post lesion, the increase is homogeneous across the OML, and becomes more intense by 48 h. Between 1 and 3 weeks the increase is much less than at 48 h and is concentrated at the inner border of the OML. Pretreatment of the animal with the calpain inhibitor leupeptin reduces the increase in spectrin immunoreactivity normally seen 48 h after the lesion of the entorhinal cortex. Changes in the pattern of immunoreactivity to GFAP are very different to that seen with spectrin antibodies and are consistent with the known modifications in astrocytes that follow lesions of hippocampal afferents. The results are in good agreement with the biochemical data (Seubert et al., 1988) and indicate that denervation of hippocampus results in a spatially restricted increase in the breakdown and synthesis of cytoskeletal proteins into deafferented dendrites soon after the lesion. The results also indicate that the activation of calpain appears to be necessary for some but not all the anatomical modifications that accompany lesions of the entorhinal cortex. In particular, while calpain activation might be required for the increased synthesis of cytoskeletal proteins in denervated dendrites it does not participate in astrocyte hypertrophy.