Researchers trained mice to associate a specific environment with cold exposure, then used optogenetics to reactivate the exact neurons tagged during that cold memory, located in the hippocampus's dentate gyrus, without any actual cold or the original environment present. This artificial reactivation alone raised the mice's oxygen consumption, showing that recalling a memory can trigger a real physiological metabolic response. A reverse experiment using a pharmacological inhibitor confirmed the causal link: silencing those same neurons during real cold exposure suppressed the mouse's normal metabolic increase.
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The neurons were tagged using a method called TRAP2 (targeted recombination in active populations), which links gene expression to neural activity so only cells active during the cold experience receive the genetic tag needed for later optogenetic control.
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The positive optogenetic activation result corresponds to figure 4 of the paper, while the inhibitory pharmacology experiment corresponds to figure 5, forming a two-directional causal test.
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The hosts caution that the mouse's physiological response does not tell us anything about its subjective experience or whether it 'feels' cold during reactivation.
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