A Circadian Clock in the Retina Regulates Rod-Cone Coupling

Abstract

Purpose:: A circadian (24-hr) clock in the fish retina regulates rod and cone pathways. Specifically, by increasing dopamine release and activating dopamine D₂-like receptors in the day, the clock regulates rod and cone input to cone horizontal cells (cHCs) so that cone input dominates during the day and rod input dominates at night (Wang & Mangel, 1996; Ribelayga et al., 2002, 2004). Because cHCs receive synaptic contact from cones, but not from rods (Stell & Lightfoot, 1975) and because rods and cones are connected by gap junctions (Witkovsky et al., 1974), we have previously suggested that rod input reaches cones and then cHCs at night via rod-cone gap junctions (Wang & Mangel, 1996; Ribelayga et al., 2002). We therefore directly tested this hypothesis.

Methods:: Whole-cell patch clamp recordings from the inner segments of cones in intact neural goldfish retinas were obtained under continuous dark-adapted conditions in the subjective day (CT 5-11) and subjective night (CT 13-20). Cone responses to dim full-field white and spectral light stimuli were measured to assess the light responsivity of the cones as well as to determine whether they received rod input. The recorded cones were injected either with neurobiotin or biocytin tracer (0.3%) to assess the extent and type of photoreceptor cell coupling.

Results:: Following injection into single cones, tracer was observed on average in 3 +/- 1 (SEM) rods and 5 +/- 1 cones during the subjective day (n = 21), but was observed on average in 208 +/- 65 rods and 83 +/- 12 cones during the subjective night (n = 10). The light responses of cones in the subjective day (n = 23) had a threshold that was 2.0 log units higher and a shorter latency, time-to-peak, and duration than those of cones in the subjective night (n = 15). Dark-adapted cones responded to light in the scotopic range at night, but not in the day. Light responses to dim spectral stimuli distinguished three cone types in the subjective day (B: n = 1; G: n = 6; R: n = 8). In contrast, all cones tested (n = 9) were most sensitive to middle (G) wavelength light in the subjective night. Finally, application of the dopamine D₂-like receptor antagonist spiperone (10 µM) during the subjective day (n = 4) decreased response threshold by 2.0 log units and modified other response characteristics so that cone light responses resembled those typically observed at night.

Conclusions:: These findings indicate that the circadian clock in the fish retina increases rod-cone gap-junctional coupling at night so that rod input reaches cones and then reaches cHCs. The clock-induced increase in dopamine D₂-like receptor activation during the day decreases rod-cone coupling.

Keywords: gap junctions/coupling • circadian rhythms • dopamine