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It is now possible to cause a rat to walk in circles at the flick of a switch; turn a light on and the rat will start to move, switch it off and it will stop . Welcome to the strange world of optogenetics, a technique which combines optical methods and genetic modifications with remarkable results.
This ingenious method was first discovered by a team led by Karl Deisseroth and Edward Boyden in 2005 . They took a light-sensitive ion channel, called Channelrhodopsin 2 (ChR2), from the green algae Chlamydomonas reinhardtii and introduced it into neurones. In nature these channels play a role in phototaxis, enabling the algae to sense light and move to a position with the best growth conditions. In neurones, ChR2 inserts itself into the cell membrane, so that exposure to blue light causes a gate to open and Na+ ions to flood in. This depolarised the membrane to trigger and action potential, therefore stimulating the neurones at will.
However, the real potential of optogenetics became clear when it began to be used in live organisms. The genetic construct for the channelrhodopsin can be introduced to brain cells using viruses. More importantly, expression can be restricted to a specific group of neurones (e.g. those for the neurotransmitter dopamine) using a cell-type specific promoter . This had never previously been possible, so was a huge breakthrough. A light can be shone on the brain (via an optic fibre), so the method is completely non-invasive, reducing damage caused and allowing optogenetics to be used in freely moving organisms. Therefore you can test specific neuronal circuits, simply by turning on a light and watching what the animal does. Plus, if activation in surrounding neurones is tracked, a map of neuronal connections can be created. For example, the fear circuit in the amygdala of the brain has been mapped .
Since then, other possible channels have been identified. As each channel responds to a different wavelength of light and has different properties, a ‘toolbox’ is being developed to enable greater control. Take the example of the light-driven Cl- pump halorhodopsin (NpHR). NpHR has an inhibitory effect on neurones and responds to red light, so can be combined with ChR2 to allow the neuron to be switched on or off .
However, looking for new variants is very time consuming and has its limits, so scientists have gone one step further and created their own ones. Either they merge bits of two different channels to produce ‘chimeras’, or they mutate certain areas to refine the function of the channel (or a combination of both). For example, ChR1 has been combined with VChR1 to create CIVI. This is known as channelrhodopsin engineering .
This very month, the field of optogenetics has received a great boost; the crystal structure of a chimeric channelrhodopsin has been determined . This provides new insights into how these channels work and will allow them to be further engineered to tailor their properties.
Optogenetics has given neuroscience the tools it really needs to make the link from electrical activity at a neuronal level to control of behaviour in an entire organism. And with the newly discovered crystal structure of channelrhodopsin, the future of optogenetics is looking bright.
 Gradinaru, V. et al (2007). Targeting and Readout Strategies for Fast Optical Neural Control In Vitro and In Vivo. The Journal of Neuroscience. 27 (52), 14231-14238.
 Boyden, E.S. et al (2005). Millisecond-timescale, genetically targeted optical control of neural activity. Nature neuroscience. 8 (9), 1263-1268.
 Buchen, L. (2010). Neuroscience: Illuminating the brain. Nature. 465 (7294), 26-28.
 Tye, K.M. et al(2011). Amygdala circuitry mediating reversible and bidirectional control of anxiety. Nature. 417 (7338), 358-362.
 Han, X. & Boyden, E.S. (2007). Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution. PloS one. 2 (3), e299.
 Hegemann, P. & Möglich, A. (2011). Channelrhodopsin engineering and exploration of new optogenetic tools. Nature methods.8 (1), 39-42.
 Kato, H.E. et al. (2012). Crystal structure of the channelrhodopsin light-gated cation channel. Nature. 482 (7385), 369-374.
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