Optogenetic tools are a broad class of genetically engineered proteins activated or inactivated by light of an appropriate wavelength. These include activators, such as the Channelrhodopsins, and suppressors, such as the Halorhodopsins and Archaerhodopsins, along with other variants.
Many of the optogenetic tools currently in use in mammalian systems are derived from microbial opsins. These generally incorporate light-sensing and ion flux components in the same protein, as in Bacteriorhodopsin (Oesterhelt and Stoeckenius 1971), Channelrhodopsin-1 (ChR1) and Channelrhodopsin-2 (ChR2) (Nagel et al. 2002, 2003), and Halorhodopsin (eNpHR) (Gradinaru et al. 2008; Han and Boyden 2007; Zhang et al. 2007). In response to light stimulation, the Channelrhodopsins undergo conformational changes and allow nonspecific cation flow, resulting in depolarization and increased action potential output (Bamann et al. 2010; Hegemann et al. 2005; Kato et al. 2012...