NMDA receptors in retrosplenial cortex are necessary for retrieval of recent and remote context fear memory

Abstract

Over time, memory retrieval is thought to transfer from the hippocampus to a distributed network of neocortical sites. Of these sites, the retrosplenial cortex (RSC) is robustly activated during retrieval of remotely acquired, emotionally valenced memories. It is unclear, however, whether RSC is specifically involved in memory storage or retrieval, and which neurotransmitter receptor mechanisms serve its function. We addressed these questions by inhibiting NMDARs in RSC via infusions of APV before tests for context fear in mice. Anterior cingulate cortex (ACC) and dorsal hippocampus (DH), which have been implicated in the retrieval of remote and recent memory, respectively, served as neuroanatomical controls. Surprisingly, infusion of APV only into RSC, but not ACC or DH, abolished retrieval of remote memory, as revealed by lack of freezing to the conditioning context. APV infused into RSC also impaired retrieval of recent memory, but had no effect on conditioning or memory storage. Within-subject experiments confirmed that the role of RSC in memory retrieval is not time limited. RSC-dependent context fear memory retrieval was mediated by NR2A, but not NR2B, subunit-containing NMDARs. Collectively, these data are the first demonstration that NMDARs in RSC are necessary for the retrieval of remote and recent memories of fear-evoking contexts. Dysfunction of RSC may thereby contribute significantly to the reexperiencing of traumatic memories in patients with posttraumatic stress disorder.

Figure 1.

A, Experimental design for experiments in B. B, NMDARs in RSC are necessary for retrieval of remote context fear memory. Infusion of APV into DH (APV: n = 8; Veh: n = 8; left) or ACC (APV: n = 16; Veh: n = 18; center) had no effect, whereas infusion of APV into RSC (APV: n = 13; Veh: n = 12; right) reduced freezing to the conditioning context. C, Experimental design for experiment in D. D, RSC APV blocks remote context fear memory retrieval even when no reminder test is given the day before (APV: n = 6; Veh: n = 6). Arrows indicate infusions. *p < 0.001. E, Cannula placements in DH (left), ACC (center), and RSC (right). The photomicrographs show cresyl violet-stained coronal sections with representative cannula placements in each region. Illustrated below those are the locations of cannula placements in each region. Atlas templates adapted from Paxinos and Franklin (2001).

Figure 2.

A, NMDARs in RSC are not necessary for context fear learning. Mice were infused with Veh (n = 10) or APV (n = 17) before context conditioning. The following day, there was no difference in freezing levels to the conditioning context. B, NMDARs in RSC are necessary for retrieval of recent context fear memory. APV infusions into RSC reduced freezing to the conditioning context on day 3. When the mice were returned to the conditioning context drug-free (day 4), freezing levels returned to normal (APV: n = 6; Veh: n = 6). C, RSC is equally involved in recent and remote memory retrieval. In a within-subjects experiment, infusion of APV into RSC reduced freezing to contexts in which conditioning had occurred 1 d (Recent) and 36 d (Remote) prior (APV: n = 9; Veh: n = 9). Arrows indicate infusions. *p < 0.04.

Figure 3.

NMDAR blockade in RSC does not affect locomotion, tone fear, or freezing behavior. A, Infusion of APV into RSC prevented the conditioning-induced decrease in locomotion, but did not cause hyperlocomotion, as activity during testing in the conditioning context was similar to preconditioning levels (APV: n = 9; Veh: n = 8). B, Infusion of APV into RSC did not prevent freezing to a tone CS in a novel context (APV: n = 8; Veh: n = 9). C, Infusion of APV into RSC does not block postshock freezing (APV: n = 6; Veh: n = 6). Arrows indicate infusions. *p ≤ 0.02.

Figure 4.

NR2A subunit-containing NMDARs mediate context fear memory retrieval by RSC. A, Retrieval of recent context fear memory was blocked by infusion of the preferential NR2A antagonist NVP, but not the specific NR2B antagonist Ro (NVP: n = 8; Ro: n = 8; Veh: n = 7; left), or the AMPAR antagonist CNQX (CNQX: n = 8; Veh: n = 7; right). B, Remote fear memory retrieval was similarly disrupted by NVP, but not Ro (NVP: n = 6; Ro: n = 6; Veh: n = 7; left) or CNQX (CNQX: n = 7; Veh: n = 8; right).