In vivo dendritic calcium imaging with a fiberoptic periscope system

Dendritic recordings in freely moving animals present great challenges using the current approaches. Here we present in detail a microendoscopic technique (the 'periscope' method) for measuring intracellular calcium activity directly from the apical dendrites of L5 pyramidal neurons from the pia down to depths of similar to 700 mu m in anesthetized and freely moving rats. This method gives high signal-to-noise dendritic fluorescence responses to sensory stimuli, and has been proven to be inexpensive, straightforward and reliable, allowing essentially unrestricted behavior. We describe refinements and practical optimizations of procedures aimed at achieving dendritic Ca2+ imaging in freely moving animals. The periscope imaging technique presented here is also ideal for combining with other in vivo recording techniques. The protocol, from the beginning of anesthesia to starting dendritic imaging, can be completed in 5 h.