Differential contribution of some cortical sites to the formation of memory traces supporting fear conditioning

Abstract

The aim of the present work was to assess the role of some cortical sites of the rat (the prefrontal, PFC, frontal, FC, parietal, PAC, and perirhinal, PC, cortices) in the acquisition of classical fear conditioning (CS and context conditioned freezing). Using the reversible ablation technique the sites were inactivated with lidocaine (LIDO), administered before the one-trial training session. The freezing response, taken as memorization index, was measured in conditions of full functional recovery after the short reversible LIDO inactivation either 3 h (“short-term” memory) and/or 72 h (“long-term” memory) later, so as to follow the temporal dimension of mnemonic elaboration. The results of the inactivations performed during the training session show that PFC, FC, PAC and PC play contemporaneous but distinct roles in the memorization of aversive responses to CS and context. PC inactivation weakened the retention of both mnemonic traces at the 3-h delay. At the same delay FC and PAC inactivation weakened only freezing to acoustic CS while PFC inactivation improved the retention of both traces. Inactivation of all four sites was followed by significant amnesia for both traces at the 72-h after-acquisition delay. The present findings show that PFC, FC, PAC and PC reversible inactivation during the acquisition training session diversely interferes with the memorization of conditioned freezing to acoustic CS and context. Moreover, the different results obtained at the two different retention intervals support the hypothesis that “short-term” and “long-term” memories are not necessarily linked, the earlier one not always influencing the subsequent one.