The reptilian clock system: circadian clock, extraretinal photoreception and clock-dependent celestial compass orientation mechanisms in reptiles

Reptiles because of their phylogenetic position and ecology are one of the most interesting model to understand the circadian organization, its evolution and its adaptation to the different environments. The present review summarizes the current knowledge about the circadian organization of this taxon. In the detail, Paragraph 3 analyzes studies concerning the existence of peripheral and central circadian oscillators in reptiles, with complementary data gathered using physiological, behavioral and molecular approaches, particularly the role of SCN and the reactions of both central and peripheral oscillators to drastic changes in ambient temperatures. Paragraph 4: as it is peculiar to other non-mammalian vertebrates, also lizards behavioral and hormonal rhythms (particularly melatonin) can be entrained by extra-retinal and deep brain photoreceptors, whose position in the brain seems to vary somewhat in different lizard species. In paragraph 5, the seasonal changes in circadian organization are analyzed in fine detail in a model species, the ruin lizard Podarcis sicula, in which most research on seasonality has be done. Paragraph 6 reports some data on the role of ambient light irradiance in the circadian organization. Paragraph 7 deals with problems of orientation in space, with particular interest in celestial compass mechanisms which need a functional circadian clock to work properly. In this context recent results are reported on the functioning of a sun compass and a sky polarization compass, and the central role played by the parietal eye in those mechanisms of orientation. Paragraph 8 reports conclusion and perspectives.