, 2006 and Ishizuka et al., 2006). Moreover, while retinoids were already well known to be present find more in large quantities in embryonic tissues and in the retina, it was soon found that mature mammalian brains ( Deisseroth et al., 2006 and Zhang et al., 2006), and indeed all vertebrate tissues thus far examined (e.g., Douglass et al., 2008) contain sufficient all-trans retinal for microbial opsin genes to define a single-component strategy. By 2010 the major classes of ion-conducting microbial opsins (including bacteriorhodopsin, channelrhodopsin, and halorhodopsin) had all proven to function as optogenetic control tools in mammalian neurons, as described

below. Since earlier, multicomponent efforts for photosensitization of cells (for example, involving cascades of multiple genes or combinations of genes and custom organic

chemicals (Zemelman et al., 2002, Zemelman et al., 2003, Banghart et al., 2004, Lima and Miesenböck, 2005, Kramer et al., 2005 and Volgraf et al., 2006) have been recently reviewed (Gorostiza and Isacoff, 2008 and Miesenböck, 2009), here we provide a primer focusing on single-component Erastin optogenetics, delineating guiding principles for scientific investigation and summarizing the enabling technologies for neuroscience application. However, most of the techniques developed for this approach (ranging from genetic targeting methods, to addressing experimental confounds, to intact-system light delivery methods) will be relevant to any biological system or optogenetic strategy. We do not attempt to review in any form the very large number of papers and results that have emerged in this field, nor to address every technique, reagent, and device linked to optogenetics. Rather, here we highlight limitations, challenges, and obstacles in the field and outline general principles for designing, conducting, and reporting optogenetic experiments. Optogenetics is not simply photoexcitation or photoinhibition of targeted cells; rather, optogenetics must deliver gain or loss of function of precise events—just as in genetics, where

single-gene manipulations are the core currency of the field. This means that in neuroscience, millisecond-scale precision is essential to true optogenetics, to keep pace with the known over dynamics of the targeted neural events such as action potentials and synaptic currents. Moreover, this level of precision must be operative within intact systems including freely moving mammals. All strategies to achieve optical control, including those involving microbial opsin genes, initially displayed serious limitations in meeting this goal. The multicomponent character, longer-timescale temporal properties, and/or requirement for high-intensity UV light characteristic of the earlier strategies (Zemelman et al., 2002, Banghart et al., 2004, Lima and Miesenböck, 2005 and Kramer et al.