I read this after my last entry, honest.

The problem with not reading printed journals is you miss stuff. After I wrote my entry on ChR2 + NpHR, I read the News & Views by Häusser & Smith. It is a very nice introduction to this important paper, with a useful figure (above) showing the same type of spectral separation I illustrated in my figure. Here we can see the absorption spectrum of fura-2 (in peurple) added to that of the light-activated channels. In their article, Häusser & Smith point out one current weakness of ChR2: it is a non-selective cation channel, so allows calcium, as well as sodium to go into cells. They say this can be overcome by mutagenesis. There is a second problem that seems impossible to deal with currently, the protein expression levels vary from cell to cell, so one cannot predict how much current a photon will generate.

Neuroscience: Controlling neural circuits with light. Nature (2007) 446: 617-619.