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K. Sakurai, J. Chen, V. J. Kefalov; Modulation of Mouse Cone Phototransduction by GCAPs. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1663.
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In addition to the photoresponse, light also triggers decline in intracellular calcium which leads to adaptation through negative feedback on the phototransduction cascade. In mammalian rods, calcium modulates phototransduction via guanylyl cyclase activating proteins (GCAPs), which regulate the synthesis of cGMP by guanylyl cyclase. Cones have wider calcium dynamic range and adapt faster and to wider range of illumination than rods indicating a strong calcium-mediated feedback on cone phototransduction. To determine how GCAPs modulate phototransduction in mammalian cones and affect response kinetics, amplification, and light adaptation, we compared the flash responses from wild type and GCAPs knockout (GCAPs-/-) mouse cones.
We performed single-cell suction recordings from individual cones and transretinal electroretinogram a-wave recordings from isolated whole retina from wild type and GCAPs-/- mice. Suction recordings were done by drawing the cell body of a single cone into the recording electrode. Whole retina recordings were done from dorsal mouse retina, treated with glutamate to suppress inner retina components of flash response and isolate the photoreceptor component of flash response. To facilitate suction recordings and to remove the rod component in the whole retina flash response, all recordings were done from mice in rod transducin alpha knockout (Gnat1-/-) background.
Deletion of GCAPs greatly slowed down the dim flash kinetics of single cones measured by suction recordings. Time-to-peak increased from 128±6 ms (n=11) in wild type cones to 349±65 ms (n=10) in GCAPs-/- cones and the time constant of response shutoff increased from 140±18 ms (n=11) to 894±199 ms (n=11), respectively. Similar slowing down of cone flash response in the absence of GCAPs was observed with whole retina recordings. The half-saturating flash intensity of whole retina responses decreased from 2872±457 photons µm-2 (n=4) in wild type cones to 803±119 photons µm-2 (n=5) in GCAPs-/- cones, a 3.5-fold increase in sensitivity. Surprisingly, in preliminary background adaptation experiments, deletion of GCAPs precipitated the reduction of maximum response amplitude but appeared to have no effect on the decline of sensitivity.
Our results demonstrate that the GCAPs-dependent regulation of guanylyl cyclase strongly modulates phototransduction in mammalian cones. Deletion of GCAPs dramatically slows down kinetics of cone flash response and increases the sensitivity of cones by 3.5-fold.
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