The hippocampal formation plays a central role in the brain network essential for memory function, but the contribution of distinct hippocampal regions in specific memory processes is the subject of current debates. The expression of immediate-early genes (IEGs) can be used to map regional brain activity. However, only one IEG is generally used to map regional brain activation, considering that even though different IEGs contribute to different cellular processes they can be used interchangeably to study regional brain activity. First, we evaluated the expression of the three IEGs Arc, zif268 and c-fos in the rat hippocampal formation following the exploration of a novel environment, as compared to control rats that were simply taken out and placed back into their cages. Second, we evaluated the postnatal functional maturation of distinct hippocampal regions using c-fos expression...