The circadian clock functions independently of the presence of a functional eye. The locomotor activity rhythm of zebrafish larvae is measured by calculating the distance traveled, both in the presence and absence of a functional eye, under constant darkness, after five days of exposure to a light/dark cycle. This allows for the assessment of spontaneous locomotor rhythm and, consequently, the activity of the circadian clock. The black areas on the x-axis represent night periods, and the gray areas represent day periods.
The eye is not essential to the functioning of the circadian clock in zebrafish. This is what Elise Cau and her colleagues from MCD-CBI, Clair Chaigne, Dora Sapède and Patrick Blader, from CBI Laurent Sanchou, and from Montpellier Xavier Cousin have just demonstrated.
Although it is accepted that the eyes play a key role in the adaptation of mammals to the alternation of day and night, the circadian clock is currently most often studied in nocturnal vertebrates such as mice. By observing the zebrafish, a species of diurnal vertebrate, this new study shows that the circadian rhythm can also be established in the absence of an eye. By studying a variety of larvae without functional eyes, Elise Cau and her colleagues have found that their circadian rhythm remains synchronised with the alternations of day and night produced in the laboratory. This finding suggests the existence of other neuronal circuits regulating the circadian clock in certain animal species.
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