Purchase this article with an account.
M.L. Woodruff, G.L. Fain, P.A. Sieving, A.M. Dizhoor; Equivalent Light Adapts the Response of G90D Rods . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4319.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To determine the nature and mechanism of constitutive activation of the cascade produced in an intact photoreceptor by the rhodopsin G90D mutation, which is known to produce congenital night blindness in humans.
Suction electrode recordings were made from rods of G90D mice that were G90D+/– and Rh+/–, identical to the mice used in Sieving et al (J Neurosci 21: 5449–60, 2001), whose rods were previously shown not to degenerate but to behave as if light–adapted even in darkness. Measurements were made from between 40 and 50 WT rods and 16 to 22 G90D rods.
Rods from G90D mice had smaller circulating currents than WT mice (7.0 + 0.7 vs. 11.9 + 0.5 pA) and were desensitized even in darkness: flash sensitivity (in pA photon–1 µm2) was 0.030 + 0.003 for G90D but 0.26 + 0.02 for WT, and the intensity at half saturation was 161 + 20 for G90D but 31 + 2 photons µm–2 for WT. The decrease in current and sensitivity was accompanied by an acceleration in the decay of the wave form of the light response. The best–fitting single exponential fit to the declining phase of the just–saturating response changed from 205 + 19 ms for WT to 98 + 10 ms for G90D. The integration time decreased from 199 + 20 ms in WT to 88 + 8 ms in G90D. There was in addition a decrease in Td, the dominant time constant of response decay, as determined from Pepperberg plots: Td was 176 + 9 ms in WT but 118 + 8 ms in G90D.
Our suction–electrode recordings confirm previous measurements of Sieving et al of the a–wave of the ERG from G90D retinas, that the rods in these animals, even though dark–adapted, behave as if in the presence of a continuous (equivalent) background light. By comparing the decrease in the flash sensitivities of WT rods in actual background light and G90D rods in darkness, we estimate the intensity of the equivalent background to be about 380 photons µm–2 s–1, or about 190 Rh* rod–1 s–1. The stimulation of the cascade and reduction in circulating current and sensitivity is quantitatively much less in G90D rods than in Rpe65–/– rods, perhaps explaining why G90D produces congenital night blindness but Rpe65–/–, the degeneration of the photoreceptors.
This PDF is available to Subscribers Only