Differential activation of c-Fos and Egr1 during development of the mouse visual cortex

Background: Critical periods (CP) in brain development are characterized by heightened neural plasticity in the relevant brain regions.  They are associated with changes in gene expression cascades, in particular with altered expression of genes involved in plasticity regulation, such as immediate early genes.  Here we examine the expression of the immediate early genes c-Fos and Egr1 at different stages of mouse visual cortex (VC) development. Methods: Mice 11, 25, and 50 days of age were maintained under standard light-dark conditions, deprived of light for 5 days, or deprived of light for 5 days and then exposed to light for 90 min. Their brains were analyzed at PND16 (before the onset of the CP), PND30 (during the CP) and PND55 (after the CP) to determine the changes in the number of cells expressing c-Fos and Egr1 in the binocular primary visual and primary somatosensory cortices. Results: We found highly specific induction of c-Fos expression in the primary VC in response to light. We also observed transient cross-modal activation of c-Fos in the barrel field of the primary somatosensory cortex in response to light before and during the CP; such activation disappeared after the CP. Expression of Egr1 was not induced by light in the VC before the CP, but was evident during and after the CP, although the induction was much less pronounced than that of c-Fos . Conclusions: Dynamic changes in c-Fos and Egr1 expression may reflect their contribution to the VC plasticity during the CPs of postnatal brain development.