EN
Extinction of conditioned fear leads to formation of a new memory trace. There are, however, factors altering behaviors associated with such a memory trace, such as CS presentation outside the extinction context (promoting fear renewal) and re-emerging of fear with the passage of time after extinction (spontaneous recovery). The neuronal basis of these phenomena is poorly understood. The involvement of hippocampal-prefrontal cortical circuits was investigated only during initial processing of fear extinction memory. As has been shown before for fear conditioning, the mechanisms underlying matured memory may differ from those of recent memory. In our study we used c-Fos immunohistochemistry to generate a functional map of the neural circuits involved in contextual retrieval of recent and remote memories of extinguished fear. Presentation of the CS in the extinction context 24 h after extinction yielded low freezing and induced strong activation of infralimbic cortex (IL) and ventral hippocampus (vHIPP). Similar presentation after 28 days resulted in high freezing and much lower activity of IL and vHIPP. In contrast, presentation of the CS outside the extinction context after either 24 h or 28 days yielded high freezing and induced strong activation of prelimbic cortex. These results suggest remodelling of the fear extinction memory trace over time, as well as dissociable neuronal mechanisms underlying fear renewal and spontaneous recovery.