Use of Fos as an Index of Altered Neuronal Activation in Aging Animals

Abstract The immediate early gene c- fos is rapidly and transiently induced in response to a variety of extracellular stimuli and codes for a protein, Fos, that regulates transcription of target genes in neurons. Detection of Fos can serve as a marker of neuronal activation at the individual cell level. Thus, localization of Fos provides investigators with a widely applicable tool for assessment of neuronal activation during age-related changes in brain function. This article describes a method for the immunocytochemical localization of Fos in neurons of the aging brain using free-floating sections and the avidin-biotin detection system. This technique is particularly useful when Fos expression is cobcalized with other neuropeptides, permitting the assessment of age-related changes in discrete neuronal populations. Application of this method to studies of age-associated changes in hypothalamic neurons involved in reproductive cyclicity is described.