Imaging calcium signals in vivo: a powerful tool in physiology and pharmacology

The design and engineering of organic fluorescent Ca2+ indicators approximately 30 years ago opened the door for imaging cellular Ca2+ signals with a high degree of temporal and spatial resolution. Over this time, Ca2+ imaging has revolutionized our approaches for tissue-level spatiotemporal analysis of functional organization and has matured into a powerful tool for in situ imaging of cellular activity in the living animal. In vivo Ca2+ imaging with temporal resolution at the millisecond range and spatial resolution at micrometer range has been achieved through novel designs of Ca2+ sensors, development of modern microscopes and powerful imaging techniques such as two-photon microscopy. Imaging Ca2+ signals in ensembles of cells within tissue in 3D allows for analysis of integrated cellular function, which, in the case of the brain, enables recording activity patterns in local circuits. The recent development of miniaturized compact, fibre-optic-based, mechanically flexible microendoscopes capable of two-photon microscopy opens the door for imaging activity in awake, behaving animals. This development is poised to open a new chapter in physiological experiments and for pharmacological approaches in the development of novel therapies.