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A. S. Pawar, N. M. Qtaishat, D. R. Pepperberg; Initial Recovery Kinetics of Post-Bleach Photoreceptor Responses in abcr-/- Mice. Invest. Ophthalmol. Vis. Sci. 2007;48(13):608.
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© ARVO (1962-2015); The Authors (2016-present)
ABCR protein in rod outer segment disks is thought to function in the retinoid visual cycle by facilitating all-trans retinal movement from the disk lumen to the cytosol. Recent ERG experiments on ABCR-deficient (abcr-/-) vs. wildtype (C57BL/6J) mice (ref. 1) examined rod recovery after ~1% rhodopsin bleaching. Here, we have investigated rod recovery following bleaching of a substantial fraction of the rhodopsin.
abcr-/- and C57BL/6J mice (ages: 6-12 weeks) were dark-adapted overnight and anesthetized with ketamine/xylazine. Under dark-adapted conditions, the rod response to a weak (0.3 sc cd s m-2) test flash, determined by paired-flash ERGs (80-ms test-probe interval), was taken as a measure of weak-flash sensitivity. Following a 2-min bleach, recovery of the derived rod response was measured using bright probe flashes presented at intervals of 2-4 min. The derived response’s initial recovery rate (fractional response recovered per min) was quantified by fitting a line to data (n≥3 points) obtained over a 12-min post-bleach interval.
Under dark-adapted conditions, abcr-/- and wildtype mice exhibited similar weak-flash sensitivity (respective normalized derived responses: 0.67+0.06 and 0.63+0.03); respective a-wave peak amplitudes were -321+19 and -387+86 µV. Initial bleaching extents produced by the 2-min illumination were, on average, 40% (n=3) and 34% (n=2) for abcr-/- and wildtype, respectively. Initial rates of post-bleach recovery were 0.028+0.008 min-1 for abcr-/- (n=4) and 0.020+0.013 min-1 (n=5) for wildtype; these values did not differ significantly (P=0.32). The initial rate for abcr -/-’s was comparable with data reported by Weng et al. (2) following ~45 % bleaching (0.03 min-1).
In previous experiments with ~1% bleaches (1), exponential functions were fitted to the (more complete) recovery measured over ~30-min post-bleach. Initial recovery rates obtained from these exponential functions were about 0.37 and 0.10 min-1 for abcr-/- and wildtype, respectively. Together, the present and previous data indicate that in abcr-/-'s, increasing the bleach from ~1% to ~40% reduces initial recovery rate by about 13-fold. The similarity of abcr-/- and wildtype rates observed at ~34-40% bleaching raises the possibility that in wildtype rods, instantaneous levels of all-trans retinal exceeding ~30% of the molar amount of opsin essentially saturate the ABCR-facilitated movement of all-trans retinal. (1) Pawar et al. 2006 ARVO. (2) Weng et al. (1999) Cell 98:13-23.
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