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Alex McKeown, Timothy Kraft; A New Form of Photoreceptor Light Adaptation: Enhanced Current Following Sustained Illumination. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2461.
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© ARVO (1962-2015); The Authors (2016-present)
Photoreceptors have been extensively studied for the past half century, but important details remain undiscovered in the realm of biochemical adaptation. Certain stimulus conditions can elicit protein translocation, but these occur well outside the working range of rods. More relevant to fast adaptation are the biochemical changes that occur in response to changing calcium levels, which result in altered phototransduction response kinetics. Here we present a unique form of photoreceptor adaptation that results in increased total circulating dark current following sustained illumination. We also examine the idea that the adaptation is caused by the dynamic interaction of calmodulin with the cyclic nucleotide gated (CNG) channels of the rod outer segment.
Using single cell suction electrode recordings of mouse photoreceptors, we have discovered a form of adaptation that occurs in response to a steady illumination capable of bleaching only 1-4% of the photopigment in mouse rod cells. Following illumination of 1-3 minutes (5000 photons µm-2 s-1) there is a transient 35% increase in response amplitude that returns to baseline with a time constant of 10 seconds. We also investigated this unique form of adaptation in a mutant mouse lacking the calmodulin binding site on the beta subunit of the rod outer segment CNG channel (CaM-del mouse).
Following exposures of sustained illumination, rods exhibit a 35% increase in circulating current upon return to darkness (n=11 WT). Following identical duration and intensity of sustained illumination, calmodulin mutant rods exhibit a 65% (vs. 35% in WT) increase in circulating current upon returning to darkness (p = 0.006) (n=14 CaM-del).
These findings demonstrate both a unique form of light adaptation in photoreceptors that we call dark potentiation, and suggest a role for calmodulin in attenuating this form of adaptation. The excess channel opening following the light step is likely due to fluctuations in the concentration of cGMP, as determined by guanylate cyclase and phosphodiesterase. The role of calcium and GCAPs in this form of adaptation is being investigated. Additional psychophysical studies may reveal how such photoreceptor changes can impact human perception. The adaptation may enhance sensitivity in dim environments like those found in a deep forest, or during specific times of day, such as dawn or dusk.
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