Regulation of C-Fos MRNA and Fos Protein Expression in Olfactory Bulbs from Unilaterally Odor-deprived Adult Mice

Odorant deprivation, produced by unilateral naris closure, profoundly reduces tyrosine hydroxylase (TH) expression within intrinsic olfactory bulb dopamine neurons. The TH gene contains an AP-1 site, which interacts with the product of the immediate early gene, c-fos. c-Fos exhibits activity dependent regulation in the CNS. The hypothesis that odorant stimulation and deprivation might modify c-fos expression in TH neurons was tested in adult CD-1 mice, subjected to unilateral naris closure. After 2 months, naris closed and control mice were exposed to either clean air for 60 min or clean air for 60 min followed by 30 min of alternating exposure to 10% isoamyl acetate (1 min) and air (4 min). A parallel reduction occurred in TH and fos expression (both c-fos mRNA and fos-like immunoreactivity) in the glomerular layer of the odorant-deprived olfactory bulb. Odor stimulation induced a short-lived increase in c-fos mRNA and fos-like immunoreactivity in olfactory bulbs contralateral to naris closure. The increase in fos expression was region-specific in the glomerular layer but more diffuse in mitral and granule cell layers. In olfactory bulbs ipsilateral to naris closure, odor stimulation also induced c-fos mRNA expression in the mitral and granule cell layers and sparsely within limited periglomerular regions. Odor induced expression in mitral and granule cell layers may represent increased centrifugal activity acting on as yet unknown genes. These results suggest a correlation between c-fos mRNA expression and increased neuronal activity in the olfactory bulb which, in turn, acts to regulate TH expression in periglomerular neurons.