Phototransduction in Drosophila Is Compromised by Gal4 Expression but not by InsP3 Receptor Knockdown or Mutation.

Drosophila phototransduction is mediated by phospholipase C, leading to activation of transient receptor potential (TRP) and TRP-like (TRPL) channels by mechanisms that are unresolved. A role for InsP3 receptors (IP3Rs) had been excluded because IP3R mutants (itpr) appeared to have normal light responses; however, this was recently challenged by Kohn et al. ("Functional cooperation between the IP3 receptor and phospholipase C secures the high sensitivity to light of Drosophila photoreceptors in vivo," Journal of Neuroscience 35:2530), who reported defects in phototransduction after IP3R-RNAi knockdown. They concluded that InsP3-induced Ca2+ release plays a critical role in facilitating channel activation, and that previous failure to detect IP3R phenotypes resulted from trace Ca2+ in electrodes substituting for InsP3-induced Ca2+ release. In an attempt to confirm this, we performed electroretinograms, whole-cell recordings, and GCaMP6f Ca2+ imaging from both IP3R-RNAi flies and itpr-null mutants. Like Kohn et al., we used GMRGal4 to drive expression of UAS-IP3R-RNAi, but we also used controls expressing GMRGal4 alone. We describe several GMRGal4 phenotypes suggestive of compromised development, including reductions in sensitivity, dark noise, potassium currents, and cell size and capacitance, as well as extreme variations in sensitivity between cells. However, we found no effect of IP3R RNAi or mutation on photoreceptor responses or Ca2+ signals, indicating that the IP3R plays little or no role in Drosophila phototransduction.