Effects of Housing Conditions and Circadian Time on Baseline c-Fos Immunoreactivity in C57BL/6J Mice.

• Housing conditions and circadian time of analysis affect basal c-Fos expression in untreated "control" mice. • Basal c-Fos level in suprachiasmatic nucleus and dentate gyrus is regulated by the circadian time of analysis. • In many brain regions, social housing affects basal c-Fos levels only during the dark phase of circadian cycle. • In regions of amygdala, individual housing leads to increased c-Fos levels only during the dark phase of circadian cycle. Analysis of expression of the immediate early gene c-Fos in neuronal populations is a commonly used method to assess changes in neuronal activity due to various factors of interest. However, different levels of c-Fos have been observed between control animals across studies. The present investigation assessed whether such differences could reflect different behavioral or physiological states in housing conditions that are typically considered naïve controls. Specifically, we assessed c-Fos expression in 19 brain regions in male C57BL6/J mice that were housed either socially (in groups of four/cage) or individually. c-Fos expression was compared with socially-housed mice under either normal or reverse light conditions to assess the effect of light cycle on neuronal activity. We identified three main patterns of differences between groups. Light, but not social housing conditions, influenced c-Fos expression in the suprachiasmatic nucleus of hypothalamus and the dentate gyrus (DG). A large number of brain regions across cortex, hypothalamus, ventral striatum and midbrain showed increased activity during the dark phase of circadian cycle only in the social, but not individual, housing. Finally, activity in the amygdala appeared to be induced by social housing conditions only during the dark phase of circadian cycle. Taken together, our experiment identified differential regulation of c-Fos expression by basal housing conditions and circadian phase. It also indicates that despite the well-known habituation of c-Fos expression to repeated stimulation, this expression is sensitive to basal housing conditions. This sensitivity needs to be taken into account when analyzing c-Fos data in various studies. [ABSTRACT FROM AUTHOR]