Since its initial publication 10 years ago (Nagel et al., 2003), Channelrhodopsin-2 (ChR2) has achieved immense popularity as an optogenetic tool for the control of cellular activity. Genetically expressed wild-type Channelopsin-2 (wt-Chop2) requires addition of its chromophore all-trans-retinal (RAL) to yield functional wt-ChR2. In Drosophila melanogaster, RAL must be added as a food supplement before blue light can be employed to stimulate selected cell types in vivo (Schroll et al., 2006). Correspondingly, wt-ChR2 function strongly depends on RAL concentration and light intensity. This poses an obstacle for various experimental approaches. For example, the dark cuticle of adult flies necessitates high light intensities to stimulate cells for behavioural manipulation. In addition, light-induced muscle contractions in larvae expressing wt-ChR2 in motorneurons subside after several seconds of continuous stimulation. To date, it is therefore difficult to apply long-term stimulation in vivo and many light-controlled behavioural assays are not feasible.

Here, we employ two new ChR2 variants for the control of neuronal activity in Drosophila. Continuous light-application led to a long-lasting (> 1 hour) immobilisation of larvae expressing either mutant in motorneurons. Mutant-1 required roughly 100 times lower light intensity than wt-ChR2 to trigger comparable muscle contractions. Strikingly, mutant-2 required no RAL food supplement for the induction of neuronal activity in adult flies using daylight.

Nagel G., Szellas T., Huhn W., Kateriya S., Adeishvili N., Berthold P., Ollig D., Hegemann P., Bamberg E. (2003) Channelrhodopsin-2, a directly light-gated cation-selective membrane channel. PNAS 100(24): 13940-5

Schroll C., Riemensperger T., Bucher D., Ehmer J., Völler T., Erbguth K., Gerber B., Hendel T., Nagel G., Buchner E., Fiala A. (2006) Light-induced activation of distinct modulatory neurons triggers appetitive or aversive learning in Drosophila larvae. CB 16(17): 1741-7