Systems reconsolidation reveals a selective role for the anterior cingulate cortex in generalized contextual fear memory expression

Abstract

After acquisition, hippocampus-dependent memories undergo a systems consolidation process, during which they become independent of the hippocampus and dependent on the anterior cingulate cortex (ACC) for memory expression. However, consolidated remote memories can become transiently hippocampus-dependent again following memory reactivation. How this systems reconsolidation affects the role of the ACC in remote memory expression is not known. Using contextual fear conditioning, we show that the expression of 30-day-old remote memory can transiently be supported by either the ACC or the dorsal hippocampus following memory reactivation, and that the ACC specifically mediates expression of remote generalized contextual fear memory. We found that suppression of neural activity in the ACC with the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) impaired the expression of remote, but not recent, contextual fear memory. Fear expression was not affected by this treatment if preceded by memory reactivation 6 h earlier, nor was it affected by suppression of neural activity in the dorsal hippocampus with the GABA-receptor agonist muscimol. However, simultaneous targeting of both the ACC and the dorsal hippocampus 6 h after memory reactivation disrupted contextual fear memory expression. Second, we observed that expression of a 30-day-old generalized contextual fear memory in a novel context was not affected by memory reactivation 6 h earlier. However, intra-ACC CNQX infusion before testing impaired contextual fear expression in the novel context, but not the original training context. Together, these data suggest that although the dorsal hippocampus may be recruited during systems reconsolidation, the ACC remains necessary for the expression of generalized contextual fear memory.

Figure 1

Effects of CNQX infusion into the ACC on recent and remote contextual fear memory expression, and expression following remote memory reactivation. (a) Experimental designs used in testing 30-day-old recent memory (left) and 3-day-old remote memory (right). (b) At 3 days after training, pre-test CNQX infusions into the ACC did not impair contextual fear memory expression. (c) At 30 days after training, pre-test CNQX infusions into the ACC impaired contextual fear memory expression (test 1), but if preceded by memory reactivation 6 h earlier (vehicle group on test 1; 6 h inter-test-interval, ITI), the same treatment did not impair contextual fear memory expression (test 2). (d) At 30 days after training, pre-test CNQX infusions into the ACC impaired contextual fear memory expression (test 1), and when preceded by memory reactivation 24 h earlier (vehicle group on test 1), the same treatment was similarly effective in impairing contextual fear memory expression. (e) Testing 30-day-old contextual fear memory twice with 24 h inter-test-interval did not impair contextual fear memory expression on the second test. *P<0.05. Data are means±SEM.

Figure 2

Effects of CNQX infusion into the ACC and muscimol infusion into the dorsal hippocampus on remote contextual fear memory preceded by memory reactivation. (a) Experimental design used with data presented below. (b) At 30 days after training and preceded by memory reactivation 6 h earlier, only simultaneous pre-test infusions of CNQX into the ACC and muscimol into the dorsal hippocampus impaired contextual fear memory expression, whereas simultaneous infusions of either drug alone with the vehicle of the other drug did not impair contextual fear memory expression. A control for anatomical specificity of dorsal hippocampus infusions, where muscimol was infused dorsal to the dorsal hippocampus with CNQX infused into the ACC, did not impair contextual fear memory expression. *P<0.05. Data are means±SEM.

Figure 3

Effects of CNQX infusion into the ACC on generalized contextual fear memory preceded by memory reactivation. (a, upper) Experimental design used with data presented below. (a, lower) When tested 3 days after training, rats showed high levels of freezing in the original training context, but not in a novel context. At 30 days, however, rats showed contextual fear generalization with similar levels of freezing to both the original training context and a novel context. (b, upper) Experimental design used with data presented below. (b, lower) Remote memory reactivation in the original training context (test 1) did not affect contextual generalization 6 h later (test 2) where similar high levels of freezing were expressed in both the training context and the novel context. (c, upper) Experimental design used with data presented below. (c, lower) At 6 h following the reactivation of a 30-day-old memory, CNQX infusions into the ACC impaired contextual fear expression in the novel context, whereas vehicle infusions into the ACC did not impair contextual fear expression in either the training or the novel context (d). **P<0.01. Data are means±SEM.