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.[ref]
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
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.[ref]
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
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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