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

14 Aug 2017

$\textit{Drosophila}$ 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 InsP$_3$ receptors (IP$_3$Rs) had been excluded because IP$_3$R mutants ($\textit{itpr}$) 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 $\textit{Drosophila}$ photoreceptors in vivo," Journal of Neuroscience 35:2530), who reported defects in phototransduction after IP$_3$R-RNAi knockdown. They concluded that InsP$_3$-induced Ca$^{2+}$ release plays a critical role in facilitating channel activation, and that previous failure to detect IP$_3$R phenotypes resulted from trace Ca$^{2+}$ in electrodes substituting for InsP$_3$-induced Ca$^{2+}$ release. In an attempt to confirm this, we performed electroretinograms, whole-cell recordings, and GCaMP6f Ca$^{2+}$ imaging from both IP$_3$R-RNAi flies and $\textit{itpr}$-null mutants. Like Kohn et al., we used GMRGal4 to drive expression of UAS-IP$_3$R-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 IP$_3$R RNAi or mutation on photoreceptor responses or Ca$^{2+}$ signals, indicating that the IP$_3$R plays little or no role in $\textit{Drosophila}$ phototransduction.