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Starting the Zebrafish Pineal Circadian Clock with a Single Photic Transition

Laboratoire Aragó (R.V., L.B., G.B., A.P., J.F.), Observatoire Océanologique de Banyuls, Centre National de la Recherche Scientifique Unité Mixte de Recherche 7628, Université de Paris VI, 66651 Banyuls/Mer-Cédex, France; Biozentrum (R.V., W.G.G.), University of Basel, Cell and Developmental Biology, Basel 4056, Switzerland; Section on Neuroendocrinology (D.C.K.), Office of the Scientific Director, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and Department of Zoology (Y.G.), The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel

Address all correspondence and requests for reprints to: Jack Falcón, Laboratoire Arago, Unité Mixte de Recherche 7628, Centre National de la Recherche Scientifique and University Pierre and Marie Curie, BP 44, F-66651 Banyuls sur Mer, France. E-mail: falcon{at}obs-banyuls.fr.

The issue of what starts the circadian clock ticking was addressed by studying the developmental appearance of the daily rhythm in the expression of two genes in the zebrafish pineal gland that are part of the circadian clock system. One encodes the photopigment exorhodopsin and the other the melatonin synthesizing enzyme arylalkylamine N-acetyltransferase (AANAT2). Significant daily rhythms in AANAT2 mRNA abundance were detectable for several days after fertilization in animals maintained in a normal or reversed lighting cycle providing 12 h of light and 12 h of dark. In contrast, these rhythms do not develop if animals are maintained in constant lighting or constant darkness from fertilization. In contrast to exorhodopsin, rhythmicity of AANAT2 can be initiated by a pulse of light against a background of constant darkness, by a pulse of darkness against a background of constant lighting, or by single light-to-dark or dark-to-light transitions. Accordingly, these studies indicate that circadian clock function in the zebrafish pineal gland can be initiated by minimal photic cues, and that single photic transitions can be used as an experimental tool to dissect the mechanism that starts the circadian clock in the pineal gland.

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