Functional role of the perirhinal cortex in encoding and retrieval of Pavlovian trace fear conditioning.

The perirhinal cortex (PRC) and the thalamic reuniens (Re)/rhomboid (Rh) nuclei (ReRh) participate in the regulation of trace fear learning, likely through their respective reciprocal interconnections with the medial prefrontal cortex (mPFC) and the hippocampus (HPC). We previously demonstrated that ReRh inactivation impaired the acquisition, but not the retrieval, of trace fear memory. Moreover, the retrieval of trace fear memory acquired under ReRh inactivation only reprised when the ReRh was brought off-line. To explore the role of the PRC in trace fear regulation, we pharmacologically inactivated the PRC before trace fear acquisition and/or retrieval in this study. Our results showed that PRC inactivation during either phase resulted in a decrease in fear response during the early test session compared to controls. We also noticed that without the proper function of the PRC during both the acquisition and retrieval, the rats displayed a high level of fear during the early test session but declined rapidly toward the latter trials compared to controls. The results suggested that although the retrieval of trace fear memory acquired under PRC inactivation recurred during PRC inactivation, the fear memory was relatively fragile. Finally, for rats with or without functional PRC, disruption of NMDA-dependent plasticity in the dorsal HPC (DH) during conditioning resulted in defects of trace fear learning in both scenarios. Collectively, our results indicated that functional PRC is involved in the regulation of trace fear acquisition and retrieval, and that trace fear acquisition is hippocampus-dependent regardless of the PRC being on-line or off-line.