Fos labels a distinct excitatory neuron ensemble for memory retrieval

Experience-triggered expression of Fos has been extensively used as a marker representing memory engram. It is generally assumed that Fos-positive neurons are preferentially excited by the experience, yet this has not been directly examined. In this study, we tracked neuron activity and Fos expression in somatosensory cortex and dentate gyrus of hippocampus in mice, during electric shock-induced fear memory formation and retrieval. Contrary to the prevailing view, we found that calcium features fail to predict Fos for individual cells. Fos-positive neurons show no specific activation to stimuli during memory encoding, but their activities become stimuli-selective during retrieval. Surprisingly, inhibiting neurons that will express Fos during encoding does not affect the induction of memory-guided behavior upon their activation. Network simulation and voltage imaging further showed that connectivity to interneurons, not stimulus responsiveness, is a major factor determining Fos expression. Our results indicate that Fos labels excitatory neurons specific for memory retrieval.