Effect of photoreceptor degeneration on circadian photoreception and free-running period in the Royal College of Surgeons rat.

The study of how the retina processes the photic information required for the entrainment of the circadian system is an exciting new topic in retinal neurobiology. We have recently shown that in RCS/N-rdy rats melanopsin mRNA levels are dramatically reduced (about 90%) and melanopsin immunoreactivity cannot be detected in the retina of these rats at 60 days of age. Although RCS/N-rdy rats are a widely used model to investigate mechanisms of photoreceptor degeneration, no study has investigated circadian photoreception in these animals. The aim of this study was to examine circadian photoreception in RCS/N-rdy(+) (rdy(+)) rats homozygous for the normal rdy allele and age-matched RCS/N-rdy (rdy) homozygotes with retinal dystrophy. No differences between RCS/N-rdy and rdy(+) were observed in light-induced phase shift of locomotor activity at the three light intensities used (1 x 10(-3), 1 x 10(-1), and 1 x 10(1) microW cm(-2)). Surprisingly, we observed that in RCS/N-rdy the free-running period of the circadian rhythm of locomotor activity was shorter (P<0.01) than in rdy(+), thus suggesting that photoreceptor degeneration may affect the free-running period of the locomotor activity rhythm.