Electroretinographic responses, whole retina cGMP and recoverin levels in albino and pigmented mice over different periods of dark adaptation.

Abstract

Abstract: : Purpose: We have previously shown that hypopigmented animals have elevated dark–adapted thresholds. Behn et al (2003) has suggested that some of the threshold deficit might be explained by an altered time course of dark–adaptation in albino animals. We have reported that the sub–retinal calcium level is elevated by roughly 75% in albino mice when compared to black mice after 45 minutes of dark–adaptation. The purpose of this study was to: 1) examine thresholds via the ERG at different dark–adaptation time points in black and albino mice; and 2) to examine the activity of two calcium dependent processes within the photoreceptor: [cGMP] and recoverin in black and albino mice at three different dark–adaptation times. Methods: Scotopic electroretinograms (ERGs) were obtained from both albino (C57BL/6J c^2J/c^2J) and pigmented (C57BL/6J +/+) mice following different periods of dark adaptation. Intensity responses were recorded in response to white light flashes over a 4.3 log unit range. cGMP levels were determined with a cGMP assay kit (Assay Designs Inc.) from cytosol fractions (100mM HCl) from albino (n=5) versus black mice (n=5) at the above dark–adaptation times. Recoverin was assayed in Western blots of membrane vs. cytosol retinal fractions from black versus albino mice. Results: Both albino and black mice showed the maximum b–wave response (V_max) at 0.5 hours of dark adaptation, amplitudes decreased at longer adaptation time. Retinal sensitivity increased in both strains peaking at 3 hours. Pigmented mice demonstrated larger amplitudes compared to albino mice at all time periods. We measured whole retina [cGMP] from albino mice and compared them to black mice at 0, 30, 90, mins and at 12 hours. We found a 0%, 10%, 8%, 0% reduction in albino whole retina [cGMP] at these time points respectively. Conclusions: Our results indicate that albino and pigmented mice differ in their time course of dark adaptation, a finding that supports previous work that albino animals have elevated dark–adapted thresholds. Together with the albino’s elevated sub–retinal calcium space level, these results are consistent with a reduction of internal photoreceptor [cGMP] and altered partitioning of recoverin between the membrane and cytosol fractions, both results that we find. The deficit in [cGMP] is consistent with a more hyperpolarized (light–adapted photoreceptor) in the albino animal.

Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • electroretinography: non–clinical • calcium