Abstract
Abstract: :
Purpose:Several minutes of saturating light exposure produces a long–lasting acceleration of transducin/PDE deactivation in mouse retinal rods (Krispel et al 2003). The purpose of this study was to elucidate the mechanism underlying this long–lasting form of adaptation. Methods:Mice were dark adapted overnight prior to an experiment. On the day of the experiment, the animals were sacrificed and the retinas removed. Using suction electrodes, we recorded responses of individual photoreceptors to brief flashes (500 nm, 10 ms) before and after exposure to a bright adapting light (520 nm) for three minutes. Results: To test whether this long–lasting adaptive acceleration is independent of other adaptation mechanisms, we used dim background lights to reduce the inward current and thus the intracellular calcium levels before and after subjecting the cell to the saturating light protocol which gives rise to the long–lasting adaptation. We found that the extent of the response shortening due to the saturating light protocol was not reduced in cells in which calcium levels had been lowered and known calcium feedback mechanisms (e.g., cyclase activation and shortening of rhodopsin lifetime) had already been invoked. We also examined several transgenic mice to test the involvement of proteins known to regulate cascade deactivation and other forms of light adaptation, but found that adaptation was not abolished in any of the lines tested. We additionally employed pharmacological manipulations to test the involvement of post translational modifications in this adaptation. We found that adaptation persisted despite the presence of okadaic acid, an inhibitor of PP2A and PP1, suggesting that neither PP2A nor PP1 play a role in the induction, expression, or recovery of this adaptation. Conclusions:Our results suggest that the mechanism that speeds deactivation is independent of feedback mechanisms involved in classical adaptation. PP2A and PP1 do not seem to be involved in the induction or expression of the speeding of deactivation, but the involvement of other phosphatases and kinases remains to be determined. Because transgenic manipulations of several known regulators of cascade deactivation had no effect on adaptation, it is still unclear how transducin/PDE deactivation is speeded during adaptation.
Keywords: photoreceptors • signal transduction • calcium