Calcium-dependent inactivation of light-sensitive channels in Drosophila photoreceptors.

Abstract

Whole-cell voltage clamp recordings were made from photoreceptors of dissociated Drosophila ommatidia under conditions when the light-sensitive channels activate spontaneously, generating a "rundown current" (RDC). The Ca 2+ and voltage dependence of the RDC was investigated by applying voltage steps (+80 to -100 mV) at a variety of extracellular Ca 2+ concentrations (0-10 mM). In Ca2+-free Ringer large currents are maintained tonically throughout 50-ms-long voltage steps. In the presence of external Ca 2+, hyperpolarizing steps elicit transient currents which inactivate increasingly rapidly as Ca 2+ is raised. On depolarization inactivation is removed with a time constant of ~10 ms at +80 mV. The Ca2+-dependent inactivation is suppressed by 10 mM internal BAPTA, suggesting it requires Ca 2+ influx. The inactivation is absent in the trp mutant, which lacks one class of Ca~+-selective, light-sensitive channel, but appears unaffected by the inaC mutant which lacks an eye-specific protein kinase C. Hyperpolarizing voltage steps applied during light responses in wild-type (WE) flies before rundown induce a rapid transient facilitation followed by slower inhibition. Both processes accelerate as Ca 2+ is raised, but the time constant of inhibition (12 ms with 1.5 mM external Ca 2+ at -60 mV) is ~ 10 times slower than that of the RDC inactivation. The Ca2+-mediated inhibition of the light response recovers in ~50-100 ms on depolarization, recovery being accelerated with higher external Ca 2+. The Ca z+ and voltage dependence of the light-induced current is virtually eliminated in the trp mutant. In inaC, hyperpolarizing voltage steps induced transient currents which appeared similar to those in WT during early phases of the light response. However, 200 ms after the onset of light, the currents induced by voltage steps inactivated more rapidly with time constants similar to those of the RDC. It is suggested that the Ca2+-dependent inactivation of the light-sensitive channels first occurs at some concentration of Ca 2+ not normally reached during the moderate illumination regimes used, but that the defect in inaC allows this level to be reached.

Phototransduction and retinal degeneration in Drosophila

“…Ca 2+ is the critical second messenger that regulates termination of the photoresponse in Drosophila [4,160,161]. The central role for Ca 2+ in response termination is illustrated by the observations that the termination time is increased either by removing or lowering extracellular Ca 2+ from the bath solution during whole-cell recordings from photoreceptor cells [2,4,25] or by overexpressing the Na + /Ca 2+ exchanger, CalX [157].…”

“…Ca 2+ influx through the TRP channels is much higher in the inaC mutant than in the wild-type, indicating that INAC is required for inactivation of the TRP channels [161]. TRP is phosphorylated by INAC [164,165] and a mutation that abolishes the primary INAC-dependent phosphorylation site in TRP causes slow termination of the light response [166].…”

Phototransduction in Drosophila

“…Additional, Ca 2+ also plays a critical role in the deactivation of light response [68,69]. In the whole-cell recordings of photoreceptor cells, termination speed depends on the Ca 2+ [69][70][71].…”

Protein Gq Modulates Termination of Phototransduction and Prevents Retinal Degeneration

“…In contrast, the dephosphorylation of rhodopsin by retinal degeneration C (RDGC) is essential for receptor deactivation (14). Ca 2ϩ also plays critical roles in regulating the termination of the photoresponse in Drosophila (8,15,16). Several proteins that mediate this Ca 2ϩ -regulated termination have been identified, such as eye-specific protein kinase C (INAC), calmodulin, INAD, and myosin III (NINAC) (8,(17)(18)(19)(20).…”