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GD Field, F Rieke; Reproducibility of the Single Photon Response in Mammalian Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3754.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Single photon responses in rod photoreceptors are remarkably similar. These responses are produced by a single rhodopsin molecule. Simple stochastic models for how this molecule shuts off predict much more variability than observed. The mechanisms responsible for this reproducibility are unclear. Furthermore reproducibility has not been characterized in mammalian rods. Methods: We used suction electrodes to measure the dim flash responses in guinea pig and primate rods under control and BAPTA loaded conditions. Single photon responses were isolated from failures and multiple photon responses. The Na-K-Ca exchanger time constant was also measured to estimate the amount of BAPTA loaded into each cell and thus determine how much calcium changes were slowed. Results: Consistent with previous studies, the variability in the single photon response was much less than expected if rhodopsin's activity was terminated by a single stochastic shut-off step. Several models could in principle explain the low variability in the single photon response: feedback regulating rhodopsin shut-off, saturation within the transduction cascade, or a rhodopsin shut-off process involving multiple steps, such as phosphorylation or conformational changes. Fortunately, each model makes a different prediction about the time course and total variability in the rod responses. Comparing these predictions to the measured values, we show that multi-step rhodopsin shut-off best accounts for the characteristics of the response variability. Because the above models are not mutually exclusive, we also tested whether combinations of the models could provide a better description of our measured results. A model combining multi-step rhodopsin shut-off with saturation of transducin described the observed variability equally well. Conclusion: The reproducibility of the single photon response in mammalian rods is similar to previous observations in toad. This reproducibility is most consistent with a multi-step rhodopsin shut-off model, or a model combining mult-step shut-off with saturation of transducin.
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