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150 μL of XenoLight RediJect D-luciferin (30 mg mL^− 1; PerkinElmer, Waltham, MA, USA) was injected intraperitoneally into HCN4^Luc/tTA_TRE mice. The mouse was deeply anesthetized 30 min after the injection, and the lumbar spinal cord was dissected out. Transverse sections (400 μm thickness) of the spinal cord were obtained using a microslicer (Linear Slicer PRO 1; Dosaka EM, Kyoto, Japan). Luminescent imaging was performed using an EM-CCD camera (ImagEM2; Hamamatsu Photonics, Hamamatsu, Japan).

One hundred and fifty microlitres of XenoLight RediJect d‐luciferin (30 mg ml⁻¹, ParkinElmer, Waltham, MA, USA) was injected into the peritoneal cavity of HCN4^+/luc mice. In vivo imaging was performed 15 min after the injection using IVIS‐100 (ParkinElmer) under general anaesthesia with 1.5% isoflurane. Thereafter, the hearts were removed under general anaesthesia with 3.0% isoflurane, and immersed in nominally Ca²⁺‐free Tyrode solution containing 30 μg ml⁻¹d‐luciferin. The right atrium and both ventricles were opened and fixed on silicone rubber. In vitro luminescence images were taken by an EM‐CCD camera (ImagEM2, Hamamatsu Photonics, Hamamatsu, Japan) on a macro zoom microscope (MV‐10; Olympus, Tokyo, Japan) or on a Slice scope (Scientifica, Uckfield, UK) with a water immersion objective lens (×40, NA = 0.8, Olympus).