(an aldehyde derivative of vitamin A). The retinal chromophore is covalently linked to the rest of the protein through a protonated Schiff base. Whereas most 7-transmembrane proteins are G protein-coupled receptors that open other ion channels indirectly via second messengers (i.e., they are metabotropic), channelrhodopsins directly form ion channels (i.e., they are ionotropic). This makes cellular depolarization extremely fast, robust, and useful for bioengineering and neuroscience applications, including photostimulation. Function The natural ("wild-type") ChR2 absorbs blue light with an absorption and action spectrum maximum at 480 nm. When the all-trans-retinal complex absorbs a photon, it induces a conformational change from all-trans to 13-cis-retinal. This change introduces a further one in the transmembrane protein, opening the pore to at least 6 Å. Within milliseconds, the retinal relaxes back to the all-trans form, closing the pore and stopping the flow of ions. Most natural channelrhodopsins are nonspecific cation channels (CCRs), conducting H+, Na+, K+, and Ca2+ ions. Recently discovered anion-conducting channelrhodopsins (ACRs) and potassium-selective channelrhodpsins (HcKCR1, HcKCR2) were structurally analyzed to understand their ion selectivity. ACRs and KCRs have been used to inhibit neuronal activity. Development as a molecular tool In 2005, three groups sequentially established ChR2 as a tool for genetically targeted optical remote control (optogenetics) of neurons, neural circuits and behavior. At first, Karl Deisseroth's lab demonstrated that ChR2 could be deployed to control mammalian neurons in vitro, achieving temporal precision on the order of milliseconds (both in terms of delay to spiking and in terms of temporal jitter). Because all opsins require retinal as the light-sensing co-factor and it was unclear whether central mammalian nerve cells would contain sufficient retinal levels, but they do. It also showed, despite the small single-channel conductance, sufficient potency to drive mammalian neurons above action potential threshold. From this, channelrhodopsin became the first optogenetic tool, with which neural activity could be controlled with the temporal precision at which neurons operate (milliseconds). A second study was published later confirming the ability of ChR2 to control the activity of vertebrate neurons, at this time in the chick spinal cord. This study was the first wherein ChR2 was expressed alongside an optical silencer, vertebrate rhodopsin-4 in this case, demonstrating for the first time that excitable cells could be activated and silenced using these two tools simultaneously, illuminating the tissue at different wavelengths. It was demonstrated that ChR2, if expressed in specific neurons or muscle cells, can evoke