The microalgae Chlamydomonas has ion channels that can be activated by brief light flashes within a few milliseconds to depolarize membranes. After absorbing a photon, the ion channel channelrhodpsin undergoes a chemical change from the trans to the cis conformation, allowing the inflow of mainly H+, but also Li+, Na+, and K+. Via transfection, electroporation or gene gun blasting scientists can express the ion channel in the neurons of other animals, including C. elegans, Drosophila, and mice. In order to increase the knowledge base for its protein engineering, Ernst et al. set out to characterize the properties of the ion channel.
Among many other things, the researchers found via two-electrode voltage clamping that light pulses with a 1 second time difference cause a diminished transient peak current on the second attempt (by about half), although the stationary current is the same. Waiting about 60 seconds between pulses causes the membrane depolarization abilities of the two currents to be equal. They also found that the peak amount of depolarization elicited from various wavelengths of light varied based on the pH level of the surroundings, peaking at 7.5 pH at 470 nm, and peaking at 4.0 pH at 484 nm. So under acidic conditions different techniques should perhaps be used. The potential applications of this type of research are impressive, which will hopefully stimulate further action.
Ernst OP, et al. 2008 Photoactivation of channelrhodospin. J. Biol. Chem. doi:10.1074/jbc.M708039200.