Abstract
Purpose :
Determine the role of the circadian rhythm regulatory gene Rev-Erbα (Rev) and melanopsin (OPN4) in the circadian modulation of light responses in mouse retina
Methods :
2-4 months old mice with 3 different genotypes (Rev-/-, OPN4-/- and Rev-/-/OPN4-/-) were compared with the wildtype (WT) littermates. The animals were reared under 12hr light/12hr dark cycles and dark adapted for at least 2 hour before recording. Scotopic single-flash electroretinograms (ERG) were recorded at increasing intensities from 3x10-4 to 10 mcds/m2 at the following times: Zeitgeber Time: 3, 4, 6, 8, 9, 11, 14, 18 and 22 (n=3). Comparisons of groups were performed by a Student’s t-test, and p<0.05 was considered statistically significant.
Results :
There were no significant differences in a- and b-wave amplitudes or latencies between WT and Rev-/- at any time point. However, the b-wave amplitude at night for the OPN4-/- and the Rev-/-/OPN4-/- was significantly larger than for the WT. Further analysis indicated that for all genotypes the ERG waveform differed strongly, suggesting that the animals were “stuck” in a particular circadian phase. While WT mice showed distinctly lower responses during the day compared to the night, Rev-/- ERG maintained a “night-time” response throughout the 24hr period; while OPN4-/- ERG maintained a “daytime phase” across 24hrs. The Rev-/-/OPN4-/- mice showed major differences in the wave form for both day and night. We measure these differences by analyzing the area under the b-wave curve (late b-wave). This parameter showed a clear circadian rhythm in WT, which was significantly greater in OPN4-/- mice and absent in Rev-/- and Rev-/-/OPN4-/-mice.
Conclusions :
We found that Rev-Erbα, which is in charge of the inhibitory feedback loop with Bmal1 and Clock genes, modulates photoreceptor/bipolar cell responses especially during the day. The effect of Rev deletion could not be via activation of melanopsin since the light stimuli used in this study were too dim to stimulate intrinsically photosensitive retinal ganglion cells (ipRGC). The pronounced phenotype seen in OPN4-/- mice indicates that the lack of unstimulated melanopsin affects the system strongly by pushing it to a “day” state. We speculate that non-activated melanopsin could actively hyperpolarize ipRGC, maintaining the threshold response of the cell throughout the whole circadian cycle and that these effects are controlled via the ipRGC input to the dopaminergic amacrine cells.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.