The amphibian clock system

Abstract

The variety of experimental approaches possible in amphibians, in particular, in the anuran species Xenopus laevis, have been crucial in discovering key regulators of circadian rhythms, such as melatonin and melanopsin. Differently from mammals, amphibians are characterized by the peculiar presence of multiple anatomical structures and cell types that feature photosensitive and self-sustained circadian activities. In particular, in amphibians, both the retina and the pineal complex are photosensitive and display circadian melatonin secretion. Furthermore, skin melanophores are light responsive and represent an exclusive model to study a peripheral circadian clock. In this chapter, we will review (1) the cellular and molecular mechanisms regulating circadian rhythms in amphibian retina, (2) the molecular bases of pineal circadian rhythms and its link to cell differentiation and cell proliferation, and (3) the Xenopus melanophore system as an example of a well-described peripheral, light-sensitive, clock.