Effects of circadian phase on cone retinomotor movements in the Midas cichlid

Abstract

In retinas of the Midas cichlid ( Cichlasoma citrinellum), cone myoid lengths change in response to both light and circadian signals. Cone myoids are short in day (or in the light), and long at night (or in darkness). During the normal light/dark cycle, cone myoids elongate to 58 μm after light offset at dusk, and remain long until just before dawn, at which time they begin to contract before the time of expected light onset and eventually (after light onset) assume daytime lengths of 5 μm. In continuous light, cone myoids remain fully contracted regardless of the time in the cycle. In continuous darkness, cone myoids undergo circadian movements; they elongate fully at night and contract partly during expected day to 23 μm (65% of the contraction seen at dawn in a normal light/dark cycle). To clarify further the regulation of these retinomotor movements, we have investigated the effects of circadian phase on cone movements induced by light or dark onset. Circadian phase had no significant effect on either the initial rate or the final extent of light-induced cone myoid contraction: at mid-dark cones began to contract immediately after light onset and contracted fully at 3·0±0·5 μm min −1; at mid-night cones contracted immediately and fully at 4·0±0·2 μm min −1. At all of the relatively high intensities of light tested (30–5000 lx) cones contracted at similar rates and to similar final extents. In contrast, circadian phase did affect dark-induced cone myoid elongation: both the delay before initiation and the final extent of cone elongation were strongly influenced by circadian phase. At dusk, elongation began immediately after dark onset and plateaued at 58 μm, while at dawn, elongation was delayed by 50 min and plateaued at 30 μm. At mid-day, elongation began 30 min after dark onset and plateaued at 25 μm. Our observations indicate that circadian phase has significant effects on the time of initiation and the final extent of dark-induced cone elongation, but has little or no effect on light-induced cone contraction at light intensities between 30 and 5000 lx.