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.^[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.^[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.