Channelrhodopsin & the engineered opsin toolkit

Channelrhodopsins are light-gated ion channels, originally from algae, that when expressed in neurons open on illumination and depolarize the cell on a millisecond timescale — the core actuator of optogenetics. The research frontier is no longer ChR2 itself but a large engineered toolkit selected for specific photophysical properties: fast off-kinetics for high-frequency, temporally precise spiking (Chronos, ChroME), high single-channel conductance for two-photon single-cell drive, step-function / bistable opsins (SFO, SSFO) that latch on and off with two different wavelengths, and inhibitory effectors (halorhodopsin, archaerhodopsin, anion channelrhodopsins) for optical silencing.

The design parameters that matter at this level are the channel's on/off time constants, its photocurrent amplitude (which sets how many channels — and thus how much light — are needed to reach spike threshold), its desensitization under sustained light, and its ion selectivity. A slow opsin blurs spike timing; a weak one demands damaging light doses; a leaky one is hard to keep off. Matching opsin to experiment is the first design choice in any optical write-in system.