Increasing the GluN2A/GluN2B Ratio in Neurons of the Mouse Basal and Lateral Amygdala Inhibits the Modification of an Existing Fear Memory Trace

Abstract

Reconsolidation updating is a form of memory modification in which an existing memory can become destabilized upon retrieval and subsequently be modified via protein-synthesis-dependent reconsolidation. However, not all memories appear to destabilize upon retrieval and thus are not modifiable via reconsolidation updating approaches and the neurobiological basis for this remains poorly understood. Here, we report that auditory fear memories created with 10 tone-shock pairings are resistant to retrieval-dependent memory destabilization and are associated with an increase in the synaptic GluN2A/GluN2B ratio in neurons of the basal and lateral amygdala (BLA) compared with weaker fear memories created via one or three tone-shock pairings. To increase the GluN2A/GluN2B ratio after learning, we generated a line of mice that expresses an inducible and doxycycline-dependent GFP-GluN2A transgene specifically in ␣-CaMKII-positive neurons. Our findings indicate that increasing the GluN2A/GluN2B ratio in BLA ␣-CaMKII-positive neurons after a weak fear memory has consolidated inhibits retrieval-dependent memory destabilization and modification of the fear memory trace. This was associated with a reduction in retrieval-dependent AMPA receptor trafficking, as evidenced by a reduction in retrieval-dependent phosphorylation of GluR1 at serine-845. In addition, we determined that increasing the GluN2A/GluN2B ratio before fear learning significantly impaired long term memory consolidation, whereas short-term memory remained unaltered. An increase in the GluN2A/GluN2B ratio after fear learning had no influence on fear extinction or expression. Our results underscore the importance of NMDAR subunit composition for memory destabilization and suggest a mechanism for why some memories are resistant to modification.

Noradrenergic projections from the locus coeruleus to the amygdala constrain auditory fear memory reconsolidation

“…Also, memories created with the 10 pairings training are much more robust and display impaired extinction learning, a hallmark of posttraumatic stress disorder 33 . When looking at GluN2B expression, we confirmed previous reports showing that strongly-trained animals display lower GluN2B levels than mildly trained ones 11,16 . However, here we included a home cage control group in our analysis which revealed that GluN2B is not actually downregulated by strong training.…”

“…Previous studies revealed that unlike mild fear memories, very strong ones do not destabilize upon recall 11,16 . Some of the mechanisms required to induce destabilization have been identified 1,31 , but not much is known regarding how severe fear experiences end up forming reconsolidationresistant memories.…”

“…In rodent experiments, this is particularly the case in memories generated by high intensity fear conditioning or protocols that induce asymptotic levels of learning [12][13][14][15] . For instance, we 11 and others 16 have found that while a memory acquired by a 1-shock fear conditioning session can be attenuated by reconsolidation blockade, a 10-shock protocol creates an intense fear memory that is resistant to this treatment.…”

An Update on Memory Reconsolidation Updating

“…At the cell surface, there is a functional involvement of cholinergic [108] and dopaminergic receptors [109], at least for non-fear memory destabilization. Of particular relevance to fear memories are the necessity for cannabinoid CB1 receptor and calcium channel activity in the dorsal hippocampus [110] and NMDA receptor activity (specifically NR2B-containing NMDA receptors) in the basolateral amygdala [111–114]. We and others have exploited some of these mechanisms to stimulate fear memory destabilization, in order to render effective post-reactivation treatment even under conditions that do not normally trigger reconsolidation [115–117].…”

Increasing Synaptic GluN2B levels within the Basal and Lateral Amygdala Enables the Modification of Strong Reconsolidation Resistant Fear Memories

“…Typically, strong or remote memories have been shown to be difficult to disrupt via reconsolidation blockade. This appears to be due to the resistance of these memories to undergo retrieval-dependent memory destabilization (13)(14)(15)(16). Therefore, targeting the reconsolidation process will likely never be an effective treatment to attenuate pathological memories unless methods are devised to enhance the induction of their reconsolidation.…”