Double dissociation of the requirement for GluN2B- and GluN2A-containing NMDA receptors in the destabilization and restabilization of a reconsolidating memory

Abstract

Signaling at NMDA receptors (NMDARs) is known to be important for memory reconsolidation, but while most studies show that NMDAR antagonists prevent memory restabilization and produce amnesia, others have shown that GluN2B-selective NMDAR antagonists prevent memory destabilization, protecting the memory. These apparently paradoxical, conflicting data provide an opportunity to define more precisely the requirement for different NMDAR subtypes in the mechanisms underlying memory reconsolidation and to further understand the contribution of glutamatergic signaling to this process. Here, using rats with fully consolidated pavlovian auditory fear memories, we demonstrate a double dissociation in the requirement for GluN2B-containing and GluN2A-containing NMDARs within the basolateral amygdala in the memory destabilization and restabilization processes, respectively. We further show a double dissociation in the mechanisms underlying memory retrieval and memory destabilization, since AMPAR antagonism prevented memory retrieval while still allowing the destabilization process to occur. These data demonstrate that glutamatergic signaling mechanisms within the basolateral amygdala differentially and dissociably mediate the retrieval, destabilization, and restabilization of previously consolidated fear memories.

Figure 1.

Cannulae placements. All cannulae placements were within the BLA. The placements for individual experiments are shown separately. For each placement the white circles represent the vehicle/vehicle group, the white squares the vehicle/anisomycin group, the gray circles the drug/vehicle group, and the gray squares the drug/anisomycin group where the drugs were IFEN (a), NVP-AAM077 (b), LY293558 (c), and LY317206 (d). Coordinates are given from bregma. This figure was modified, with permission, from Paxinos and Watson (2004).

Figure 2.

Effects of the GluN2B-selective NMDAR antagonist on CS–fear memory reconsolidation. Administration of the GluN2B-selective NMDAR antagonist had no effect on the retrieval of the CS–fear memory at reactivation (a), but it did prevent the destabilization of the CS–fear memory (b). Data are presented as means ± SEM. Group sizes (and colors in b) were VEH/VEH, n = 8 (white); IFEN/VEH, n = 8 (mid-gray); VEH/ANI, n = 8 (pale gray); and IFEN/ANI, n = 5 (dark gray). In a the groups are collapsed with the white bar representing the average of the VEH/VEH and VEH/ANI groups, and the gray bar the IFEN/VEH and IFEN/ANI groups. Asterisk (*) denotes p < 0.05.

Figure 3.

Effects of the GluN2A-preferring NMDAR antagonist on CS–fear memory reconsolidation. Administration of the GluN2A-preferring NMDAR antagonist had no effect on the retrieval of the CS–fear memory at reactivation (a), but it prevented the restabilization of the CS–fear memory (b). Data are presented as means ± SEM. Group sizes (and colors in b) were: VEH/VEH, n = 4 (white); NVP/VEH, n = 5 (mid gray); VEH/ANI, n = 5 (pale gray); and NVP/ANI, n = 5 (dark gray). In a the groups are collapsed, with the white bar representing the average of the VEH/VEH and VEH/ANI groups, and the gray bar the NVP/VEH and NVP/ANI groups. Asterisk (*) denotes p < 0.05; ns denotes nonsignificant differences.

Figure 4.

Effects of the AMPAR antagonist on CS–fear memory reconsolidation. The AMPAR antagonist LY293558 acutely impaired the retrieval of the CS–fear memory at reactivation (a), but it affected neither the destabilization nor the restabilization of the CS–fear memory (b). Group sizes (and colors in b) were VEH/VEH, n = 8 (white); LY293558/VEH, n = 6 (mid gray); VEH/ANI, n = 6 (pale gray); and LY293558/ANI, n = 5 (dark gray). In a the groups are collapsed, with the white bar representing the average of the VEH/VEH and VEH/ANI groups and the gray bar the LY293558/VEH and LY293558/ANI groups. Asterisks (*) denote p < 0.05.

Figure 5.

Effects of the mGlu2/3R agonist on CS–fear memory reconsolidation. The mGlu2/3R agonist LY317206 did not prevent retrieval of the CS–fear memory at reactivation (a), and it prevented neither destabilization nor restabilization of the CS–fear memory (b). Group sizes (and colors in b) were VEH/VEH, n = 4 (white); LY317206/VEH, n = 6 (mid gray); VEH/ANI, n = 4 (pale gray); and LY317206/ANI, n = 6 (dark gray). In a the groups are collapsed, with the white bar representing the average of the VEH/VEH and VEH/ANI groups and the gray bar the LY317206/VEH and LY317206/ANI groups. Asterisk (*) denotes p < 0.05; ns denotes nonsignificant differences.

Figure 6.

Hypothesized glutamatergic mechanisms underlying memory stability. GluN2B-containing NMDARs are required for memory destabilization, and IFEN prevents this process. GluN2A-containing NMDARs are required for memory restabilization, which is prevented by NVP-AAM077. The AMPAR antagonist LY293558 reduces memory retrieval but has no effect on destabilization or restabilization. The presynaptic mGlu2/3R agonist LY317206 reduces glutamate release, reducing memory retrieval via AMPARs but leaving the balance of GluN2A and GluN2B activity intact (thus having no effect on destabilization or restabilization).