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A. P. Sampath, H. Okawa; The Impact of the Prolonged Rod Photoresponse Rising Phase in GCAP-/- Mice on Downstream Retinal Processing. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2847.
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Dendrites of bipolar cells sense a common pool of glutamate released from a rod terminal. Interestingly, rod bipolar cells and Off-bipolar cells have similar dim flash response kinetics, which are much faster than the slow rod photoresponse. This suggests that the light response is transformed presynaptically at the rod synapse, and appears to influence the retinal output as light-evoked synaptic currents in On-ganglion cells are similar in time course to the bipolar responses. To probe the mechanism underlying the transformation of slow rod photoresponses to fast bipolar responses, we have recorded bipolar cell light responses from transgenic mice where the rod single-photon response is altered. Here, we report results from GCAP1/2 knockout (GCAP-/-) mice, whose rod photoresponses have a common rising phase slope but whose time-to-peak is prolonged significantly compared to wild-type (WT) mice.
Whole-cell patch clamp recordings (Vm = -60 mV) were made from rod bipolar cells in retinal slices (200µm) from WT or GCAP-/- mice. The slices were perfused with oxygenated Ames solutions at physiological temperatures (~35-37 oC). Responses to a series of flashes (10ms) delivered from a blue LED (max ~ 470nm) were recorded and analyzed.
The half-saturating flash strength of GCAP-/- rod bipolar cells determined from Hill equation fits to response-flash intensity curves was ~3 times smaller than that of WT rod bipolar cells, reflecting the higher flash sensitivity of GCAP-/- rods. Surprisingly the rising phase slope and time-to-peak of rod bipolar responses to dim flashes was nearly identical in WT and GCAP-/-, despite the 3-fold difference in the time-to-peak of their respective rod photoresponses. The prolongation of GCAP-/- rod photoresponses was rather reflected in rod bipolar response by an ~3-fold prolongation in the recovery phase.
The prolongation of the rising phase in GCAP-/- rod photoresponses did not alter the rising phase slope or the time-to-peak of rod bipolar responses compared to WT. For both GCAP-/- and WT mice, the rod bipolar responses reach their peak while rod responses were still rising. This suggests that the time-to-peak of rod bipolar responses was determined by a fixed time or voltage in the rising phase of rod photoresponses. Recordings from other mouse mutants with altered rod photoresponse rising phases will provide more insights into mechanisms controlling the properties of the bipolar response.
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