No abstract

Protein kinase C (PKC) is a key enzyme for many cellular processes but its physiological roles are poorly understood. An excellent opportunity to investigate the function of PKC has been provided by the identification of an eye-specific PKC in Drosophila and a null PKC mutant, inaCP209 (refs 5,6). Bright conditioning lights delivered to inaC photoreceptors lead to an abnormal loss of sensitivity in whole cell recordings from dissociated ommatidia; this has been interpreted as 'hyper-adaptation' and PKC's role has been suggested to be distinct from light adaptation. A presumably related finding is that during intense light, the response of inaC declines to baseline. Invertebrate photoreceptors use the phosphoinositide signalling cascade, responding to single photons with so-called quantum bumps which sum to form the macroscopic response to light. Light adaptation allows photoreceptors to adjust their sensitivity over the enormous range of ambient intensities. Although the molecular mechanism of light adaptation remains obscure, it is a negative-feedback process mediated by a rise in cytosolic calcium and a decrease in bump size. We now show that under physiological conditions light adaptation is severely reduced in inaC, suggesting that eye-specific PKC, itself activated by a rise in cytosolic calcium and diacylglycerol, is required for adaptation. Furthermore, we show that in the absence of PKC individual bumps fail to terminate normally, an effect that can account for the pleiotropic manifestations of the inaC phenotype.

Ion-selective calcium microelectrodes were inserted into the compound eyes of the wild-type sheep blowfly Lucilia or into the retina of the no steady state (ms) mutant of Lucilia. These electrodes monitored light-induced changes in the extracellular concentration of calcium (A[Ca2+] minimum within 6 s after light onset, and then rose to a nearly steady-state phase below the dark concentration. When lights were turned off, a rapid increase in [Ca2+]o was observed, reaching a peak above the dark level and then declining again to the dark level within 1 rain. In analogy to similar studies conduced in the honeybee drone, we suggest that the reduction in [Ca2÷]o reflects light-induced Ca 2÷ influx into the photoreceptors, while the subsequent increase in [Ca~÷]o reflects the activation of the Na-Ca exchange which extrudes Ca ~+ from the cells. In the ms mutant in response to intense prolonged light, the receptor potential declines to baseline during light while the Ca 2+ signal is almost abolished, revealing only a short transient reduction in [Ca~+]o. Application of lanthanum (LaSt), but not nickel (Ni2+), into the retinal extracellular space of normal Lucilia mimicked the effect of the ms mutation on the receptor potential, while complete elimination of the Ca ~+ signal in a reversible manner was observed. The results suggest that La 3+ and the ms mutation inhibit light-induced Ca ~+ influx into the photoreceptor in a manner similar to the action of the trp mutation in Drosophila, which has been shown to block specifically a light-activated Ca 2+ channel necessary to maintain light excitation.