a, The c-fos-tTA mouse was injected with AAV9-TRE-ChR2-EYFP and implanted with an optical fiber targeting DG. b, When off Dox, training induces the expression of c-fos-tTA, which binds to TRE and drives the expression of ChR2-EYFP, labeling a subpopulation of activated cells (yellow) in DG. c, Basic experimental scheme. Mice were habituated in context A with light stimulation while on Dox for five days, then taken off Dox for two days and fear conditioned (FC) in context B. Mice were put back on Dox and tested for five days in context A with light stimulation. d, Representative image showing the expression of ChR2-EYFP in a mouse that was taken off Dox for two days and underwent FC training. An image of each rectangular area in (d) is magnified showing DG (e), CA1 (f), and CA3 (g). The green signal from ChR2-EYFP in the DG spreads throughout entire granule cells, including dendrites (e), while the green signal confined to the nuclei in CA1 and CA3 is due to shEGFP expression from the c-fosshEGFP construct of the transgenic mouse (f, g). Blue is nuclear marker DAPI.