mental events, e.g. decision making. Optical control of behavior has been demonstrated in nematodes, fruit flies, zebrafish, and mice. Recently, chloride-conducting channelrhodopsins have been engineered and were also found in nature. These tools can be used to silence neurons in cell culture and in live animals by shunting inhibition. Using multiple colors of light expands the possibilities of optogenetic experiments. The blue-light sensitive ChR2 and the yellow light-activated chloride pump halorhodopsin together enable multiple-color optical activation and silencing of neural activity. Another interesting pair is the blue-light sensitive chloride channel GtACR2 and the red-light sensitive cation channel Chrimson which have been combined in a single protein (BiPOLES) for bidirectional control of membrane potential. Using fluorescently labeled ChR2, light-stimulated axons and synapses can be identified. This is useful to study the molecular events during the induction of synaptic plasticity. Transfected cultured neuronal networks can be stimulated to perform some desired behaviors for applications in robotics and control. ChR2 has also been used to map long-range connections from one side of the brain to the other, and to map the spatial location of inputs on the dendritic tree of individual neurons. In 2006, it was reported that transfection with Channelrhodopsin could restore eyesight to blind mice. Neurons can tolerate ChR expression for a long time, and several laboratories are testing optogenetic stimulation to solve medical needs. In blind mice, visual function can be partially restored by expressing ChR2 in inner retinal cells. In 2021, the red-light sensitive ChR ChrimsonR was virally delivered to the eyes of a human patient suffering from retinal degeneration (Retinitis pigmentosa), leading to partial recovery of his vision. Optical cochlear implants have been shown to work well in animal experiments and are currently undergoing clinical trials. In the future, ChRs may find even more medical applications, e.g. for deep-brain stimulation of Parkinson patients or to control certain forms of Epilepsy. References Further reading (Naturel function of channelrhodopsins and other photoreceptors in green) (Using channelrhodopsin in transgenic mice to study brain circuitry) (Using channelrhodopsin potentially to treat blindness) External links Optogenetics Resource Center / Deisseroth lab Boyden lab Hegemann lab Spudich lab Kato lab Ion channels Integral membrane proteins Neurotechnology