Visual defects in mice lacking circadian rhythm function in cone photoreceptors

Abstract

Purpose : To test the hypothesis that disruption of circadian clocks in cone photoreceptors would result in visual defects, the clock gene Bmal1 was selectivity ablated in cones of HRGP^creArntl^fl/fl mice.

Methods : Mice approximately 3-6 months of age, kept in constant darkness, were subjected to ERG testing at subjective midday (circadian time [CT] 4-8) and subjective midnight (CT 16-20). Spatial frequency thresholds and contrast sensitivity were assessed using the optomotor response (OMR) test. Scotopic and photopic electroretinograms were performed. Dark-adapted a- and b-wave amplitudes were recorded over a range of intensities from 0.0003 to 20 cd^.sec/m². Mice were then exposed to a steady rod saturating background light (40 cd/m²) and stimulated with cone activating flashes (0.9 cd^.sec/m²) for 18 min. SD-OCT was used to measure photoreceptor layer thickness.

Results : Spatial frequency thresholds were slightly lower in HRGP^creArntl^fl/fl mice compared to Arntl^fl/fl controls lacking the Cre (p<0.01). Contrast sensitivity was higher during the subjective day compared to the subjective night in both genotypes, but was dramatically lower in HRGP^creArntl^fl/fl mice compared to Arntl^fl/fl controls (p<0.002). No differences were observed between genotypes in dark-adapted ERG a- or b-waves. During light-adaptation, there was a circadian rhythm in photopic b-wave amplitude, with higher amplitudes during the day than at night. Photopic b-wave amplitudes were significantly lower in HRGP^creArntl^fl/fl mice compared to controls Arntl^fl/fl (p<0.001), and the magnitude of the day-night difference was reduced. There was no difference between genotypes in photoreceptor layer thickness when measured at 6 months of age.

Conclusions : Disruption of the circadian clock in cones results in visual deficits, particularly in contrast sensitivity and photopic ERG. BMAL1 cone knockout mice adapt to light, however they have suppressed b-wave amplitudes suggesting that cone signaling to bipolar cells may be regulated by circadian rhythms in cone photoreceptors. Circadian rhythms persisted in mice with cone Bmal1 disruption, but were reduced in amplitudes. Thus, although cone circadian clocks contribute to rhythmic visual function, circadian clocks in other cell types also play a major role.

This is a 2020 ARVO Annual Meeting abstract.