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K. Sakurai, J. Chen, V. J. Kefalov; Modulation of Mouse Cone Phototransduction by Recoverin. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3005.
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Cones have wider calcium dynamic range and adapt faster and to wider range of light than rods indicating a strong calcium modulation on cone phototransduction. Calcium-bound recoverin interacts with rhodopsin kinase to inhibit rhodopsin inactivation. To determine how recoverin modulates cone function, we compared flash responses from wild type and recoverin knockout (Rec-/-) mouse cones.
We performed single-cell suction recordings from individual cones and transretinal electroretinogram (ERG) a-wave recordings from isolated whole retina using wild type and Rec-/- mice. Suction recordings were done from a single cone cell body drawn into the recording electrode. Whole retina ERG recordings were done from dorsal mouse retina, with synaptic transmission inhibited pharmacologically to isolate the photoreceptor flash response. To facilitate suction recordings and to remove the rod component of the whole retina flash responses, all recordings were done from mice in rod transducin alpha knockout (Gnat1-/-) background.
Deletion of recoverin greatly accelerated the dim flash kinetics of single cones measured by suction recordings. Time-to-peak decreased from 128 ± 6 ms (n = 11) in wild type cones to 107 ± 4 ms (n = 10) in Rec-/- cones and the time constant of response shutoff decreased from 140 ± 18 ms (n = 11) to 71 ± 8 ms (n = 11), respectively. The half-saturating flash intensity of cone ERG responses increased from 1,400 ± 220 photons µm-2 (n = 7) in wild type cones to 3,800 ± 410 photons µm-2 (n = 8) in Rec-/- cones, a 2.7-fold decrease in sensitivity. Interestingly, background adaptation experiments showed that while deletion of recoverin decreased cone sensitivity in darkness and in dim light, its effect gradually declined in brighter light.
Our results demonstrate that, like in rods, recoverin modulates flash response kinetics. However, unlike in rods, recoverin also modulates sensitivity and background light adaptation in mammalian cones. In darkness or in dim light, with high cone calcium level, recoverin extends the dynamic range of cones most likely by inhibiting cone visual pigment inactivation by rhodopsin kinase. The effect of recoverin lessens in brighter light, as cone calcium declines.
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