Developmental changes in c-fos expression to an age-specific social stressor in infant rats

Young rats become immobile when exposed to a potentially infanticidal adult male rat. Male-induced immobility declines during the preweaning period, p aralleling the decrease in infanticidal threat. To investigate the neural s ubstrates underlying the developmental change in immobility, male-induced e xpression of the immediate-early gene c-fos was assessed on postnatal days 7, 14 and 21. A huddle of three young rats was exposed to an adult male beh ind a screen. As control, three littermates were put in the testing chamber but not exposed to the male. On day 7, male exposed and control pups were immobile most of the time and c-fos expression did not differ between condi tions. On day 14, rats in the presence of the male stopped ongoing behavior s and became immobile. They had significantly higher c-fos expression in th e paraventricular nucleus of the hypothalamus, the amygdala, the periaquedu ctal gray, and the locus ceruleus. On day 21, the male-exposed rats that we re immobile had elevated c-fos expression in a similar pattern as on day 14 , however, different nuclei of the amygdala were activated. In contrast, ma le-exposed 21-day-old rats that showed control levels of immobility did not have elevated c-fos expression in these areas. These results demonstrate t hat male exposure induced c-fos expression in brain areas of young rats in an age-specific pattern. Some of the activated brain areas seem to have con tributed to immobility. Differential activation of neuronal populations may underlie developmental changes in defensive immobility during early ontoge ny. (C) 2001 Elsevier Science B.V. All rights reserved.