Purpose:
To determine whether visual receptors in Drosophila have normal phototransduction when labeled with Green Fluorescent Protein (GFP). Previously, we used GFP as a reporter to demonstrate transcriptional activation of the opsin genes by retinoids and other substances (Stark, Pineda and Thomas, ARVO, 2003); at that time, we also showed that we could observe the expected photoconversions of rhodopsin-480 with metarhodopsin-570 in R1-6 receptors even if they expressed GFP.
Methods:
The UAS-Gal4 system was used to produce Drosophila expressing GFP in R1-6, the predominant compound eye receptor, in a cross of stocks kindly provided by Claude Desplan and Franck Pichaud, then of Rockefeller University (New York). Electroretinograms (ERGs) were recorded as in many previous studies (e.g. van Huizen et al, EMBO J 17, 2285-2297, 1998).
Results:
The ERG waveform was normal at and slightly above threshold in Drosophila expressing GFP. At these low light levels, positive on- and negative off-transients demonstrated normal receptor interaction at the first synaptic neuropil, the lamina ganglionaris. Also, a normal negative wave sustained for the duration of the stimulus indicated normal light-elicited depolarization of receptors. The threshold was normal (Drosophila are extremely sensitive). Stimulation with 480 nm light sufficiently intense to convert rhodopsin-480 to metarhodopsin-570 within a few seconds induces a prolonged depolarization (PDA) in normal white-eyed Drosophila; intense 570 nm stimulation repolarizes the ERG. The PDA properties were unchanged for Drosophila expressing GFP. All of the above results were obtained in newly-emerged Drosophila as well as in Drosophila aged up to 3 weeks.
Conclusions:
Even if the photoreceptor cells are loaded with GFP, photoreceptor transduction and adaptation are unimpaired as assessed electrophysiologically.