Differential involvement of the medial prefrontal cortex in the expression of learned fear across development

Studies have shown that in adult animals the medial prefrontal cortex (mPFC) is a critical brain region involved in fear regulation, with the prelimbic (PL) subregion being important for fear expression. However, few studies have examined whether the PL cortex is also involved in fear expression in infant animals. Five experiments, using immunohistochemical and temporary inactivation procedures, assessed the role of the PL in the expression of learned fear in postnatal day (PND) 18 (infant) and PND25 (juvenile) rats. We found that in juvenile rats expressing fear (measured through freezing) there was an increase in the number of phosphorylated mitogen-activated protein kinase (pMAPK)-labeled neurons in the PL; this increase was not observed in the infralimbic cortex. Furthermore, inactivation of the PL at test, using muscimol, decreased freezing in the juvenile rat. In contrast, expression of learned fear in infant rats did not require the PL, as there was neither an increase in the number of pMAPK-labeled cells in the PL nor was there any effect of PL inactivation on freezing levels. Taken together, these experiments suggest that a different neural circuitry underlies fear regulation early in life and that the lack of mPFC involvement may reflect a less flexible emotional regulation system in infant animals.