ABSTRACT
Francis Crick was the first to speculate that it might be possible to use light to control one type of cell in the brain without affecting other cells (Crick 1999). However, this idea languished owing to lack of a viable strategy to render specific cells responsive to light. Meanwhile, in another corner of the scientific world, microbiologists discovered proteins that function as lightactivated ion pumps or channels, including bacteriorhodopsin, halorhodopsin,
17.1 Overview and History of Optogenetics 435 17.2 Imaging in Optogenetics 440
17.2.1 Imaging Neural Activity 440 17.2.1.1 Neural Signals Recorded by Fluorescent Biosensors 440 17.2.1.2 Imaging Animal Behaviors 442 17.2.1.3 Tracking of Brain Activity by fMRI 443
17.2.2 Imaging in Study of Optogenetics Control of Biochemical Processes 444 17.2.2.1 Designs of Optogenetic Biochemical Tools 444 17.2.2.2 Imaging Systems Used in Optogenetic
Biochemical Studies 448 17.3 Future Directions 450 References 451
and channelrhodopsin (Oesterhelt and Stoeckenius 1971; Matsuno-Yagi and Mukohata 1977; Nagel et al. 2002). It took several years for these two fields to come together, in part because it did not seem particularly likely that microbial opsins would function properly in eukaryotic organisms. Instead, neurobiologists experimented with manipulating signaling cascades that respond to chemical stimulation. In 2002, Boris Zemelman and Gero Misenbock established a multiple-component optogenetic system (named “chARGe”) in which coexpression of rhodopsin, a G protein-coupled light receptor, with its binding partners arrestin-2 and the α subunit of the heterotrimeric G protein, rendered non-photoreceptor Drosophila neurons responsive to visible light (Zemelman et al. 2002). Miesenbock and other groups later expressed TRPV1, a cation channel that is activated by capsaicin, or P2X2, an ATP-activated cation channel, heterologously in cultures of primary hippocampal neurons. They then used photo-uncaging of chemically caged agonists to activate the transfected neurons with light (Zemelman et al. 2003; Banghart et al. 2004; Lima and Miesenbock 2005).
