A variety of procedures were used to modify the light response of Drosophilaphotoreceptors in order to find out if these manipulations produce effects that mimic some aspects of light adaptation. The ultimate goal of our approach is to use these experimental manipulations to dissect the different stages of the phototransduction process.
The means which were used to modify the light response were as follows: (a) light adaptation of normal photoreceptors, (b) exposure of normal photoreceptors to various levels of CO2 (c) light adaptation of the trp Drosophila mutant in which the receptor potential decays to baseline during illumination, (d) exposure of the temperature sensitive norpAH52 mutant of Drosophila to elevated temperatures. Intensity‐response functions were obtained using intracellular and extracellular recordings. The maximal response amplitude (Vmax) was then plotted as a function of the light intensity (s̀) which evoked a response of half maximal amplitude. This plot was used to compare the effects of the procedures described above. Qualitatively all the manipulations had similar effects, they reduced Vmax and shifted s̀ to higher levels of light intensity, however, quantitatively the various effects were different.
The finding that each experimental manipulation gave a different Vmax versus s̀ plot suggests that they affect different stages in phototransduction. We suggest that the slope of the Vmax versus a function can be interpreted in terms of an ordered cascade of events in phototransduction. This interpretation might give us a tool to distinguish between early and late stages in phototransduction.