Systems Consolidation of Memory: Engram PDF

Engrams are a population of neurons that: are activated during learning have enduring cellular changes (undergo plasticity) are reactivated by the original stimulus for recall Turning on/o an engram FOS promoter is upregulated > Tta gene is expressed > bing to TRE > ChR2 (activation) /Arc...

Engrams are a population of neurons that: are activated during learning have enduring cellular changes (undergo plasticity) are reactivated by the original stimulus for recall Turning on/o an engram FOS promoter is upregulated > Tta gene is expressed > bing to TRE > ChR2 (activation) /ArchT(inhibition) is activated c-FOS: plasticity product Mice are transgenics for either ChR2 or ArchT DOX is a TRE-inhibitor = TRE is not expressed Can capture an engram Label an engram by removing DOX so that TRE is found and neurons are labelled DOX used in lieu of CRE because of non-permanent gene changes Removal of DOX = label active engrams Induce freezing by turning blue light on to reactivate active engrams during freezing Can inhibit freezing by turning yellow light on to prevent engrams from being active Memory consolidation circuits GENERATION Hippocampus > Medial entorhinal cortex (MEC) > mPFC & BLA RECALL FOR RECENT MEMORY Hippocampus > BLA MATURATION/CONSOLIDATION Consolidation in cortex Hippocampus > (neo)cortex RECALL FOR REMOTE MEMORY Skip the hippocampus & activate mPFC & BLA Hippocampal engrams are sufficient for fear expression Methods context A + DOX Context B (fear conditioning) - DOX Back in context A + light (ChR2) = induce fear Results: Active engram is crucial for fear by activating neurons in the hippocampus Activation of the engram in hippocampus leads to the expression of fear in new contexts ff Inactivation of entorhinal cortex projections to mPFC during learning EXPERIMENT 1: Injection of a virus w/ ChR2 in the mEC Inhibition of mEC terminals in mPFC Implant optic fibres in PFC Results: PFC should not be important for recall of recent memory Day 2-8 = not change from controls Day 15-22 = mEC inhibition impairs freezing EXPERIMENT 2: WHAT HAPPENS IN THE MPFC DURING THE EARLY STAGES OF MEMORY? Inhibition of mEC inputs to the mPFC Look at c-FOS expression Results: Some cells in PFC are still important at early stages But not enough to keep/support the memory Home cage = low c-FOS Context = low c-FOS Context + shock = high c-FOS in PFC Inactivation of BLA projections to mPFC during learning Inhibition of BLA input to mPFC Results: Inhibition from BLA = reduction in number of c-FOS during contextual fear conditioning What happens in the mPFC during the early stages of memory? A subset of FOS+ mPFC neurons undergo plasticity (form an engram) during learning that is dependent on input from the BLA and mEC Is the CFC engram sufficient to recall the fear memory? Injection of ChR2 to activate engram in mEC Results in increased fear expression Consolidation of the fear engram in the mPFC? Looked at spine growth GFP+: cells that are active during CFC on Day 1 (tagged by tTA) c-Fos+: cells that are active during retrieval, either Day 2 or Day 12 Testing is done in either context A (context of training) or B (novel context) Results: Early on, no mapping  Day 13: cells that are active on day 13 are the same that were tagged on day 1 (reactivation) In a different context, the cells are not active Increased spine growth in neurons that were tagged on day 1 Does the formation of the engram in mPFC depend on inputs from the entorhinal cortex? Does inhibition of the mEC projection to mPFC, using ArchT, disrupt the activation of the mPFC fear engram? Inhibit on mEC inputs via green light to mPFC on day 1 Excite marked neurons in context B on day 2 or day 12 Results: Input of mEC to mPFC is necessary for engram formation Is the engram in mPFC necessary for fear expression? Inhibition on day 2: little fear expression Inhibition on day 12: no fear expression CONSOLIDATION CIRCUIT Early on in Hippocampus, there is a subset that is necessary for fear conditioning Send information to mEC & BLA During consolidation, silent engrams in mPFC are being reactivated, strengthening the association LTM: memory is completely stored in mPFC How does the CFC engram develop in the mPFC? 1. Inhibition of the mEC to mPFC pathway during CFC disrupts consolidation of the memory for later recall 2. Tagging c-Fos+ neurons and then stimulating those neurons (using optogenetics) evokes the fear memory, even before the memory is typically reliant on mPFC activation 3. Neurons in mPFC, as determined by c-Fos, are most active during context- induced recall after 12 days of consolidation 4. Inhibition of the mEC to mPFC pathway during CFC disrupts the ability to evoke a fear memory by stimulating the c-Fos+ neurons Monitoring mPFC activity across time monitored activity in mPFC using calcium imaging Calcium-sensitive protein that fluoresce when Ca2+ is in contact with it Comparing neurons that respond to a shock vs those that don’t respond Results: cells in mPFC that respond to the shock on day 1 are ‘reactivated’ by Context- A on day 12 (remote memory) but not on day 2 (recent memory) Silent engrams Silent engrams are not activated by natural cues, but can be activated artificially Engrams in the mPFC soon after contextual fear conditioning are said to be in a silent state They are active during recall but not enough to evoke the memory When a light is flashed on them, they can evoke the memory