In Search of the Molecular Mechanisms of Adaptation Memory in Rods: Basal Phosphodiesterase Activity

Retinal rods, the photoreceptors of twilight vision, are able to adapt to a wide range of illumination levels. The molecular mechanisms of light adaptation have received extensive study, though no less interesting is the question of what changes occur in the phototransduction cascade after the adaptive light stimuli terminate. Our previous studies demonstrated the phenomenon of adaptation memory in the rods of amphibians: after background illumination, photoreceptor light sensitivity was reduced for several minutes, while in the dark the current was restored in 20–30 sec. This indicates the existence of additional, as yet unknown, mechanisms regulating the phototransduction cascade and operating after the adaptive effect of light. In search of specific mechanisms capable of explaining the effect of adaptive memory, we conducted experiments recording currents from single rods in frogs to assess the basal activity of the effector enzyme of the phototransduction cascade, phosphodiesterase type 6, in the dark state and after saturating background illumination. This study showed that the post-adaptation state of rods is characterized by elevated basal phosphodiesterase activity, with gradual reduction to the dark value within tens of seconds after terminating the adapting background light. These results also suggest that the components of the phototransduction cascade in rods may undergo as yet unstudied post-adaptation changes.