Common mechanisms regulating dark noise and quantum bump amplification in Drosophila photoreceptors.

Chu B, Liu CH, Sengupta S, Gupta A, Raghu P, Hardie RC. Common mechanisms regulating dark noise and quantum bump amplification in Drosophila photoreceptors. J Neurophysiol 109: 2044-2055, 2013. First published January 30, 2013; doi:10.1152/jn.00001.2013.-Absolute visual thresholds are limited by "dark noise," which in Drosophila photoreceptors is dominated by brief (similar to 10 ms), small (similar to 2 pA) inward current events, occurring at similar to 2/s, believed to reflect spontaneous G protein activations. These dark events were increased in rate and amplitude by a point mutation in myosin III (NINAC), which disrupts its interaction with the scaffolding protein, INAD. This phenotype mimics that previously described in null mutants of ninaC (no inactivation no afterpotential; encoding myosin III) and an associated protein, retinophilin (rtp). Dark noise was similarly increased in heterozygote mutants of diacylglycerol kinase (rdgA/+). Dark noise in ninaC, rtp, and rdgA/+ mutants was greatly suppressed by mutations of the G(q) alpha-subunit (G alpha q) and the major light-sensitive channel (trp) but not rhodopsin. ninaC, rtp, and rdgA/+ mutations also all facilitated residual light responses in G alpha q and PLC hypomorphs. Raising cytosolic Ca2+ in the submicromolar range increased dark noise, facilitated activation of transient receptor potential (TRP) channels by exogenous agonist, and again facilitated light responses in G alpha q hypomorphs. Our results indicate that RTP, NINAC, INAD, and diacylglycerol kinase, together with a Ca2+-dependent threshold, share common roles in suppressing dark noise and regulating quantum bump generation; consequently, most spontaneous G protein activations fail to generate dark events under normal conditions. By contrast, quantum bump generation is reliable but delayed until sufficient G proteins and PLC are activated to overcome threshold, thereby ensuring generation of full-size bumps with high quantum efficiency.