Study data published in Nature assessed the biological underpinnings of emotional memories and conditioned threat response. In mouse models, investigators found that protein synthesis in somatostatin (SOM)-expressing inhibitory neurons of the amygdala was critical for the long-term storage of conditioned threat responses. In the same brain region, synthesis in protein kinase Cδ (PKCδ)-expressing inhibitory neurons was associated with conditioned responses to safety cues. Together, these distinct inhibitory neuron populations serve as central loci for emotional memories. These findings may have implications for the treatment of posttraumatic stress disorder (PTSD), which is characterized by dysregulated threat response.
Investigators sought to understand the biological substrates which inform defensive responses to threats. Mice were trained in a cued “threat-conditioning paradigm” in which a shock-predictive tone (conditioned stimulus [CS+]) was followed by an electrical shock to the foot (unconditioned stimulus [US]) and a safety-predictive tone was followed by the absence of a shock (CS-).
Control mice were exposed to US without any conditioning noises, while the “paired” training group was exposed to CS+ and CS- paired with US. When mice were tested for long-term memory, those in the paired group displayed “freezing behavior” after hearing the shock-predictive tone, suggesting memory of the footshock following CS+ tone. This freezing behavior was suppressed in the presence of the CS- tone. Freezing patterns were highly predictive of exposure to CS+ or CS-. Mice who were not exposed to the tones during shock training did not display freezing behaviors when they were exposed to CS+ or CS- tones.
In biochemical analyses of mice amygdala tissue, phosphorylation of protein S6 kinase beta-1 (S6K1) was observed in those exposed to the CS+/CS- stimuli, suggesting activation of mammalian target of rapamycin complex 1 (mTORC1). Mice who were exposed to footshock without CS+/CS- tones did not display increased mTORC1 activation. However, dephosphorylation of eukaryotic initiation factor-2α (eIF2α) was observed in both the shock only and paired tone-shock groups.
Phosphorylation of ribosomal protein S6 was increased at Ser235/236 in the SOM and PKCδ inhibitory neurons of paired mice, though not control mice. Overall, de novo translation of neurons in the centrolateral amygdala (CeL) was increased in paired mice compared to untrained mice and controls.
The role of inhibitory CeL neurons in threat memory consolidation was confirmed using drug-induced manipulation of elF2α. In vivo infusion of Asunaprevir was performed to increase the phosphorylation of eIF2α in inhibitory neurons of the CeL. Mice were then exposed to the differential threat-conditioning training paradigm. While mice responded to training normally, long-term memory deficits were observed in mice exposed to Asunaprevir (ie: mice with increased eIF2α phosphorylation). Mice with increased phosphorylation in SOM inhibitory neurons displayed impaired freezing response to CS+ but had typical response to CS-. By contrast, increased phosphorylation of PKCδ inhibitory neurons resulted in impaired response to CS- tones, but no change in freezing response. As such, investigators hypothesized that the “simultaneous consolidation of long-lasting threat and safety responses requires…protein synthesis in distinct populations of [inhibitory neurons].”
Data from these mouse models elucidate the role of the CeL in emotional memory consolidation. Protein synthesis in distinct populations of inhibitory neurons was associated with long-term memory of threat and safety cues.
“To our knowledge, our study provides the first evidence that the disruption of protein synthesis in discrete [inhibitory neuron] subpopulations in the CeL impairs associative memories related to threat and safety,” the investigators wrote. “[Such disruptions] may contribute to maladaptive behaviour in memory disorders such as PTSD.”
Shrestha P, Shan Z, Mamcarz M, et al. Amygdala inhibitory neurons as loci for translation in emotional memories. Nature. 2020;586(7829):407-411.