The retinal photoreceptors of Xenopus laevis contain a circadian clock that controls the synthesis and release of melatonin, resulting in high levels during the night and low levels during the day. Light is also an important regulator of melatonin synthesis and acts directly to acutely suppress melatonin synthesis during the day and indirectly to entrain the circadian clock. We examined the development of circadian and light regulation of melatonin release in Xenopus retinas and pineal glands. Pineal glands are capable of making measurable melatonin in culture soon after they evaginate from the diencephalon at stage 26. In cyclic light, the melatonin rhythms are robust, with higher overall levels and greater amplitudes than in constant darkness. However, the rhythm of melatonin release damps strongly and quickly toward baseline in constant darkness. Similar results are observed in older (stage 47) embryos, indicating that cyclic light has a positive effect on melatonin synthesis in this tissue. Optic vesicles dissected at stage 26 do not release melatonin in culture until the second or third day. It is weakly rhythmic in cyclic light, but in constant dark it is released at constitutively high levels throughout the day. By stage 41, the eyes release melatonin rhythmically in both cyclic light and constant darkness with similar amplitude. Our results show that Xenopus embryos develop a functional, photoresponsive circadian clock in the eye within the first few days of life and that rhythmic melatonin release from the pineal gland at comparable stages is highly dependent on a light-dark cycle.
