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Johan Pahlberg, Haruhisa Okawa, Alapakkam P. Sampath; The Effect Of Reduced Rod Noise On Light Sensitivity In The Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1180.
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Noise in receptor neurons is believed to limit the detection of stimuli near threshold in many sensory systems. In rods, two forms of dark noise in the outer segment photocurrent can occlude signals generated by single absorbed photons; discrete noise events believed to arise from the thermal activation of rhodopsin, and continuous noise originating in phototransduction downstream of rhodopsin. We characterized the reduction in discrete events when rhodopsin expression is reduced in rhodopsin (Rh+/-) heterozygous mice to determine how this might influence visual threshold.
We estimated the rate of thermal noise in the absence of light for wild-type (WT) and Rh+/- rods, where rhodopsin expression is reduced ~50% compared to WT (Calvert 2001, Nature 411, 90-94). To count discrete events we studied these genotypes in a GCAPs-/- background, which allows single photon events to be easily counted due to the elimination of Ca2+ dependent feedback in phototransduction (Burns 2002, Neuron 36:81-91). Dark noise and responses to brief flashes of light were recorded with suction electrode from the outer segment of WT, Rh+/-, GCAPs-/- and Rh+/-GCAPs-/- mouse rods.
: In Rh+/+GCAPs-/- mice we counted 27 discrete noise events in 2013 sec of recording across 4 cells, corresponding to a thermal rate of 0.013 sec-1rod-1, in agreement with previous estimations of their rate (Burns 2002). In Rh+/-GCAPs-/-mice we observed 18 events in 2858 sec across 6 cells corresponding to a thermal rate of 0.006 sec-1rod-1. Previous studies and our early results also indicated that the dim flash response of Rh+/- rods was accelerated compared to WT, proposed to be due to reduced diffusional hindrance in the outer segment disc membranes (Calvert 2001). Surprisingly we observed that the dim flash response in Rh+/- mice slowed to match the time course of the Rh+/+ as these lines were bred into a common C57Bl/6 background.
Discrete noise events were reduced in Rh+/- rods by 2-fold compared to WT, consistent with their origin in thermal activation of rhodopsin. We surprisingly found that reduced rhodopsin expression in Rh+/- rods did not accelerate the dim flash response in mice bred into a C56Bl/6 background, suggesting that the previously observed acceleration of the dim flash response may be attributable to strain differences. Future experiments will determine how reduced discrete rod noise and its synaptic processing influences light sensitivity and signal transmission in the primary rod pathway.
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