Melatonin and rhythmic photoreceptor metabolism: melatonin-induced cone elongation is blocked at high light intensity.

We have proposed a model for circadian regulation of cone position in Xenopus laevis that involves interaction of melatonin and dopamine as signals for darkness and light respectively. One problem, however, is that the effects of melatonin have not been detected in eye cups prepared from animals maintained on a cyclic light schedule. Since melatonin's effect would be expected to occur in low light intensity at night, we have investigated the relationship among melatonin, light intensity, and cone length. We report that melatonin mimics the effects of darkness and stimulates cone elongation in eye cups from cyclic light animals incubated at low but not at high light intensities.