Extended fear conditioning reveals a role for both N-methyl-D-aspartic acid and non-N-methyl-D-aspartic acid receptors in the amygdala in the acquisition of conditioned fear

Forebrain GluN2A overexpression impairs fear extinction and NMDAR-dependent long-term depression in the lateral amygdala

N-methyl-d-aspartic acid receptor (NMDAR)-dependent synaptic plasticity at the thalamus-lateral amygdala (T-LA) synapses is related to acquisition and extinction of auditory fear memory. However, the roles of the NMDAR GluN2A subunit in acquisition and extinction of auditory fear memory as well as synaptic plasticity at T-LA synapses remain unclear. Here, using electrophysiologic, molecular biological techniques and behavioral methods, we found that the forebrain specific GluN2A overexpression transgenic (TG) mice exhibited normal acquisition but impaired extinction of auditory fear memory. In addition, in vitro electrophysiological data showed normal basal synaptic transmission and NMDAR-dependent long-term potentiation (LTP) at T-LA synapses, but deficit in NMDAR-dependent long-term depression (LTD) at T-LA synapses in GluN2A TG mice. Consistent with the reduced NMDAR-dependent LTD, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) internalization was also weakened during NMDAR-dependent LTD in GluN2A TG mice. Taken together, our findings for the first time indicate that GluN2A overexpression impairs extinction of auditory fear memory and NMDAR-dependent LTD at T-LA synapses, which further confirms the close relationship between NMDAR-dependent LTD and fear extinction.

Infusion of the NMDA receptor antagonist, DL-APV, into the basolateral amygdala disrupts learning to fear a novel and a familiar context as well as relearning to fear an extinguished context

Ample evidence suggests that activation of NMDA receptors (NMDAr) in the basolateral complex of the amygdala (BLA) is necessary for context fear conditioning. The present series of experiments examined whether this activation was still required when the to-be-shocked context had a history. We found that BLA infusion of the selective NMDAr antagonist DL-APV impaired the acquisition of fear responses to a novel context, a moderately familiar context, or a highly familiar context. The same treatment also impaired the reacquisition of fear responses to a dangerous context, a context extinguished to criterion, or a context massively extinguished. Importantly, DL-APV persistently suppressed fear responses, suggesting that the NMDAr antagonist disrupted basal synaptic transmission in the BLA. Therefore, we conclude that neuronal activity in the BLA is critical for learning and relearning context-conditioned fear. This finding is consistent with current neural models that attribute context fear conditioning to interactions among several brain regions, most notably the hippocampus and the BLA.