Light-induced Egr-1 expression in the striate cortex of the opossum

In the present study, immunocytochemistry was used to assess the expression of Egr-1 nuclear protein across selected regions of the opossum visual system. In light-deprived (LD) animals, only a few scattered cell nuclei were found throughout the striate cortex (V1). Exposure to light promoted a significant increase in the density of Egr-1 labeled nuclei in V1. Laminar distribution of immunoreactive nuclei in light-stimulated animals (LS) tended to vary with topography: the lateral region, which corresponds to the central representation of the visual field, appeared to have higher density of cells expressing protein in the supragranular layers, as compared to the medial region, which corresponds to the representation of the peripheral field of vision. Finally, LS animals displayed a narrow band of labeled cell nuclei in the intergeniculate leaflet (IGL) and throughout the anteroposterior extent of the superior colliculus (SC). In contrast, almost no Egr-1 immunolabeling was found in the IGL and SC of LD animals. Our report is the first demonstration of light-regulated expression of the Egr-1 gene in the opossum visual system and provides evidence that the expression of an activity-dependent gene related to neural plasticity is evolutionarily conserved in the visual cortex of the mammalian lineage.