Figure 1.

PDA phenotypes revealed in ERG recordings of nina and ina mutants and wild type. The stimulus protocol is shown at the top: three bright blue stimuli (filled rectangles) each of 4 s duration presented at 20-s intervals followed by three bright orange stimuli (unfilled rectangles) also of 4 s duration presented at 20-s intervals. The first blue stimulus generates a large response that lasts the duration of the stimulus (light-coincident component) in flies of all genotypes (unfilled arrowheads). In wild type, the PDA is generated at the termination of the blue stimulus and maintained throughout the two subsequent blue stimuli (Trace A). No PDA is generated in ninaD^P245, ninaE^P332, or inaE^N125 (Traces B, C, and E), and partial PDAs are generated in ninaC^P238 and inaC^P209 (Traces D and F). During the fully developed PDA in wild type, the R1–6 photoreceptors are inactivated, and only small responses originating from R7/8 photoreceptor are elicited by the second and third blue stimuli (Trace A, filled arrowheads; also see inset). R1–6 photoreceptors of inaE^N125 and inaC^P209 are also inactivated by the first blue stimulus and generate only small responses to the second and third blue stimuli (Traces E and F, filled arrowheads). Thus, in these ina mutants, the afterpotential (PDA) is not present, but the R1–6 photoreceptors are inactivated, hence the name inactivation but no afterpotential. In strong nina mutants, ninaD^P245 and ninaE^P332, the PDA is not present and the R1–6 photoreceptors are not inactivated, generating full-amplitude responses to the second and third blue stimuli (Traces B and C, filled arrowheads). Therefore these mutants were named neither inactivation nor afterpotential. The mutant ninaC^P238 displays a partial PDA and modest inactivation of R1–6 photoreceptors. The inset illustrates the R7/8 origin of the small responses to the second and third blue stimuli in wild type (Trace A, filled arrowheads). It compares the ERG of wild type (bottom) with that of the transgenic fly (top) carrying wild-type norpA cDNA driven by Rh1 promotor on a norpA^P24 mutant background (norpA^P24; Rh1-norpA⁺). The stimulus protocol is shown at the bottom. Since norpA^P24 blocks phototransduction and Rh1 drives the expression of wild-type norpA cDNA only in R1–6 cells, phototransduction is blocked in R7/8 cells but the block is rescued in R1–6 cells in this transgenic fly. Note that the small response to the second blue stimulus superposed on the PDA is not present in the transgenic fly. This figure was originally published in the Journal of Biological Chemistry: Pearn, Randall, Shortridge, Burg, & Pak, 1996, J Biol Chem, 271, 4937–4945.