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ME Pennesi, KA Howes, W Baehr, SM Wu; ERG Analysis of Cone Recovery in GCAP1/GCAP2 and GCAP2 Null Mice Using Paired Flashes . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3759.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Guanylate cyclase activating proteins (GCAPs) have been shown to be necessary for the normal recovery of rod photoreceptor responses. We now examine if these proteins also play a role in the recovery of cones. Methods: Two lines of GCAP1 transgenic mice were bred to GCAP1/GCAP2 double knockouts (generously provided by Jeannie Chen) and, through subsequent breedings; the transgene was selected for expression on a double GCAP1/GCAP2 knockout background. Immunohistochemistry was used to verify GCAP1 expression in cone cells of transgenic mice and for comparison to levels seen in wildtype mouse cones. G1T4 mice show GCAP1 levels similar to that of wild type cones, while G1T3 levels in cones are higher than that seen in wild type cones. Paired Flash ERG's were performed on anesthetized mice using a white conditioning flash that produced an estimated 2.3x106 phi/M-cone and 130,000 phi/UV-cone, followed by a probe flash producing 2.0x105 phi/M-cone and 12,000 phi/UV-cone. The interstimulus interval (ISI) was varied from 300ms to 2500 ms. Results: The conditioning flash completely suppressed any rod response up to 3000ms. The cone a-wave and b-wave were measured and normalized to the response with an ISI of 2500ms. The normalized amplitude of the a-wave and b-wave versus the ISI were plotted and fit to the following equation: e(-(t-T)/tau). T is the critical period before which there is no recovery, and tau is the time constant of recovery. In wild type mice the cone b-wave is suppressed at ISI<300ms and recovers completely by 2500 ms (T=234ms, Tau=275ms). Double knockout mice showed a significant delay in recovery of the cone b-wave (T=750ms, Tau=477ms). Both G1T4 and G1T3 mice showed recovery kinetics similar to wild type mice. We are now analyzing the recovery of the cone a-wave to confirm that the delay in recovery results from a change in photoreceptor kinetics and not from a change in the second order bipolar cell. Conclusion: Previous work has demonstrated that GCAP1 is necessary for normal recovery in rod photoreceptors. We now show that GCAP1/GCAP2 null mice also have deficits in cone recovery. Additionally, preliminary evidence suggests, similar to the results seen with rods, that GCAP1 can rescue the cone recovery response.
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