Circuit mechanisms of retrospective memory-linking

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PROJECT SUMMARY/ABSTRACT Post-traumatic stress disorder (PTSD) symptoms are often triggered by environmental stimuli that were not directly present during trauma, but which nevertheless elicit strong fear responses. Thus, understanding how fear spreads to non-trauma associated stimuli is paramount to PTSD treatment. Recently, I found that different contextual memories encoded close in time (i.e., 5 hours) are linked by sharing an overlapping neural ensemble, making recall of one memory more likely to trigger recall of another memory encoded close in time. My lab has preliminary evidence that increasing the negative valence of a memory extends the temporal window of retrospective linking, such that the fear of an aversive context is linked with neutral stimuli experienced days prior. Linking aversive experiences to past events is ecologically valuable, as the past has predictive value for the future. However, overlinking traumatic experiences to everyday memories could be maladaptive and may promote PTSD symptoms. To observe dynamic ensemble activity across long time scales, I have co-developed a wire-free Miniscope which allows in vivo calcium imaging in untethered, freely behaving mice. Using the Miniscope, we found that neutral and aversive memories are linked by an increased overlap in their hippocampal neural ensembles, and that this overlap emerges sometime after learning. We will therefore test the hypothesis that the enhancement of memory-linking by aversive experience emerges through co-reactivation of memory representations during an offline consolidation period. We will test the sufficiency and necessity of ensemble reactivation, as well as synaptic plasticity, in the linking of aversive memories and safe memories. This work could therefore shed fundamental light on the spread of fear in PTSD and how it might be clinically targeted.