= Channelrhodopsin-2
<Channelrhodopsin-2> is a microbial light-gated cation channel originally identified in the green alga <Chlamydomonas reinhardtii>. Its retinal-containing membrane protein couples light absorption to channel opening. Unlike a light-driven pump that transports charge through a directed cycle, an open <Channelrhodopsin-2> provides a conductive path whose net current depends on ion concentrations and <membrane potential>.<Channelrhodopsin-2, a directly light-gated cation-selective membrane channel by Georg Nagel and others>[Abstract and functional expression experiments, PNAS 100, 13940–13945, 2003]{ref}
The original experiments expressed the protein in frog oocytes and mammalian cells and recorded light-induced membrane currents. Testing the isolated membrane response helped show that light directly gates the conductive protein, rather than acting only through a separate intracellular signaling cascade. Responses depended on voltage and ionic conditions, and prolonged illumination did not simply maintain an unchanged initial peak current.<Channelrhodopsin-2, a directly light-gated cation-selective membrane channel by Georg Nagel and others>[Electrophysiological experiments, light-switched conductance and transient/steady current behavior]{ref}
A useful conceptual example is a cell held below the channel's reversal potential: opening the channel can give net inward positive charge and <depolarization>. Near reversal there can be little net current despite channel opening. Consequently a blue-light pulse does not guarantee an <action potential>; channel abundance, ionic driving force and the cell's excitability all matter.
This direct connection between light and conductance made <Channelrhodopsin-2> useful for <optogenetics>. The measurement logic remains important: a light-controlled current is evidence about channel function, whereas precise control of firing in a particular cell type also requires testing the resulting electrical response.
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