The astrocytic ensemble is a multiday eligibility trace stabilizing memory

Recalled memories become transiently labile and require stabilization. The mechanism for specifically stabilizing memories of critical experiences essential for survival, which are often emotionally salient and repeated, remains unclear. Here, we identified an astrocytic ensemble that is transcriptionally primed by emotional experience and functionally triggered by repeated experience to stabilize labile memory. Using our novel brain-wide Fos tagging and imaging method, we found that astrocytic Fos ensembles were preferentially recruited in regions with neuronal engrams and were more widespread during fear recall than conditioning. We established the induction mechanism of the astrocytic ensemble, which involves two steps: (i) an initial fear experience inducing day-long, slow astrocytic state changes with noradrenaline (NA) receptor upregulation, and (ii) enhanced NA responses during recall, a repeated experience, enabling astrocytes to integrate coincident signals from local engrams and long-range NA projections, inducing secondary astrocytic state changes, including the upregulation of Fos and the neuromodulatory molecule Igfbp2. Pharmacological and genetic perturbation of the astrocytic ensemble signaling modulate engrams, and memory stability and precision. The astrocytic ensemble therefore is as a multiday trace left in a subset of astrocytes after experience-dependent neural activity, making them eligible to capture future repeated experiences for stabilizing memories.