Differences in the induction of Fos protein in cat visual cortex during and after the critical period

The aim of this study was to compare the inducibility of Fos protein, in terms of magnitude and laminar distribution in visual cortex, between young (5-week-old) and adult cats. Immunohistochemical methods were used to detect Fos protein in visual and frontal cortex of young and adult cats who experienced brief (1 or 4 h) visual experience after a 1 week period of total darkness. In the 5-week-old kittens, densely stained Fos-immunoreactive neurons were found throughout all visual cortical layers as a result of 1 h of visual experience. In the adults, immunoreactive cells were concentrated in supra- and infragranular layers and only very faint labeling was found in layer IV. Immunoreactivity in young kittens persisted at much greater dilutions of primary antibody than in adults, suggesting a difference in the magnitude of induction between ages. The inductions were markedly greater in visual than in frontal cortex, where only scattered immunopositive cells were seen at the highest antibody concentration. The induction of Fos protein in visual cortex was transient and largely disappeared by 4 h at both ages. The differences between ages, both in the magnitude and laminar distribution of immunoreactivity, are inconsistent with a simple explanation of Fos inductions in terms of neural activity level. Rather, these results suggest a relation between Fos inducibility and the level of plasticity in visual cortex.