Circadian regulation of iodopsin and clock is altered in the retinal degeneration chicken retina.
We are interested in determining if the visual phototransduction cascade plays a role in light entrainment of photoreceptor circadian oscillators. In this study, we compared mRNA levels of iodopsin and the chicken homolog of Clock (cClock) in the retinas of normal and rd (retinal degeneration) chickens that lack functional rod and cone phototransduction cascades. Iodopsin is a circadian-regulated, photoreceptor-specific gene expressed in chicken retina, and Clock is a transcription factor that has been shown to play a role in the circadian clock mechanism in mouse and Drosophila. The results of our analyses show that cClock and iodopsin transcript levels undergo daily oscillations in retinas of normal animals housed under 12 h light:12 h dark (12L:12D) conditions, and that these oscillations are maintained in the absence of light. Levels of these transcripts in the retinas of rd/rd chickens housed under cyclic light conditions did not change significantly over the course of a 12L:12D cycle; however, there was evidence that the photoreceptor oscillators were entrained in these animals. Comparisons of our normal and rd/rd data suggest that there are at least two light entrainment pathways that impinge on the oscillators found in photoreceptor cells, one of which is effectively disabled by the GC1 null mutation carried by the rd chicken.