Intracranial Viral Injections in Mice

For intracranial injections, post-natal day 14–40 mice were anesthetized with isofluorane and placed into a stereotaxic apparatus. For co-injections, viruses were mixed at ratios that were adjusted empirically to achieve approximately equal expression levels of GFP and mCherry/TdTomato fluorophores. Virus was delivered at 100nl/min using a Microinject system (WPI). Striatum was targeted by directing the needle approximately 0.9mm anterior and 2.4mm lateral from Bregma and 2.1mm from the pial surface. Cortical injections were targeted using the same anterior–posterior and medial-lateral coordinates, but the needle was directed 0.4mm from the pial surface. Coordinates were adjusted slightly by age and 300–1000nl were injected bilaterally. Following injections and wound closure, mice received ketoprofen (10 mg/kg) for analgesia and were returned to home cages for 3–4 weeks. All surgical procedures, animal handling, and euthanasia were carried out in accordance with federal guidelines and were approved by the Harvard Medical School Institutional Animal Care and Use Committee.