Methyl methacrylate liquid

To prepare for the optical imaging experiment, mice underwent transparent skull surgery as described earlier (for detailed protocol, see Steinzeig et al., 2017). Briefly, animals were anaesthetized with a mixture of fentanyl (Hameln, Germany) 0.05 mg/kg, midazolam (Hameln, Germany) 5 mg/kg and medetomidine (Orion Pharma) 0.5 mg/kg. Additionally, carprofen 5 mg/kg (ScanVet) was administrated subcutaneously for postsurgery analgesia. During the surgery, animals were fixed to a stereotaxic frame and kept on a heating pad at 37°C to prevent hypothermia. The eyes were protected with eye gel (Viscotears, Alcon). The scalp around the visual cortex was removed, and the skull surface was cleaned of periosteum, blood and debris. A layer of cyanoacrylate glue Loctite 401 (Henkel) was applied, followed by two layers of acryl (EUBECOS) mixed with methyl methacrylate liquid (Dentsply). Finally, a metal bar holder was attached to the surface of the skull and then covered with a mixture of cyanoacrylate glue and dental cement (Dentsply) to guarantee a secure positioning during the optical experiments.