Immunohistochemical study of the pattern of rapid expression of C-Fos protein in the visual cortex of dark-reared kittens following initial exposure to light.

Brief alterations to the nature of the visual input during critical periods in the early life of cats and monkeys can result in rapid anatomical and physiological changes in the central visual pathways. The immediate early genes (IEGs) represent a possible way in which these changes could be mediated since the protein products of a number of these genes have been shown to be induced rapidly in neurons in response to a variety of transsynaptic stimuli. Immunohistochemical methods were employed to examine the tempo and pattern of expression of Fos, the protein product of the c-fos gene, induced in the visual cortex of kittens dark-reared from birth to 30 days of age by brief periods of binocular visual exposure. In visual cortical area 17, the number of Fos immunoreactive cells increased rapidly from virtually zero in control kittens that received no visual exposure, to reach high levels in animals that received between 1 and 2 hours of visual experience. Immunoreactive cells were absent in the dorsal lateral geniculate nucleus, but were numerous in the ventral lateral geniculate nucleus, and in area 17, were most numerous in the extragranular layers (2, 3 and 6) but sparse in lower layer 4 and layer 5, and virtually absent in layer 1. Substantial constitutive Fos immunoreactivity was observed in area 17 of normal 30-day-old kittens but very few immunopositive cells were evident in adult animals. However, Fos immunoreactivity was observed in the visual cortex of a dark-reared (for 30 days) adult animal following a brief period of visual exposure, a finding that suggests that Fos might serve other roles in the visual cortex in addition to those it might play uniquely during development. It is suggested that Fos, in combination with the protein products of a select number of other IEGs, may mediate a variety of rapid changes in the visual cortex including those that underlie visual system plasticity during early postnatal life.