References SUPPORTING ONLINE MATERIAL MATERIALS AND METHODS

Viral vectors preparation A replication-deficient retroviral vector based on the Moloney murine leukemia virus was used to express RFP under a CAG promoter (1). Retroviral particles were assembled using three separate plasmids containing the capside (CMV-vsvg), viral proteins (CMV-gag/pol) and the transgene (CAG-RFP). For channelrhodopsin-2 expression, retroviral particles were prepared by using a Ubi-ChR2-GFP plasmid (kindly provided by S. Ge, SUNY Stony Brook). Plasmids were transfected onto HEK 293T cells using deacylated polyethylenimine. Virus-containing supernatant was harvested 48 h after transfection and concentrated by two rounds of ultracentrifugation. Virus titer was typically ~105 particles/μl. Animals and surgery for retroviral delivery Female C57Bl/6J mice 6–7 weeks of age were housed at 4 mice per cage, with two running wheels. Running wheel housing started 2-4 days before surgery and continued until the day of slice preparation. For surgery, mice were anesthetized (150 µg ketamine/15 µg xylazine in 10 µl saline/g), and virus (1 µl at 0.15 µl/min) was infused into the dorsal area of the right dentate gyrus using sterile microcapillary calibrated pipettes and stereotaxic references (coordinates from bregma:-2 mm anteroposterior,-1.5 mm lateral,-1.9 mm ventral). Animals were killed for acute slice preparation at the indicated times. Experimental protocols were approved by the Institutional Animal Care and Use Committee of the Fundación Instituto Leloir according to th