Traditionally, circadian clocks have been thought of as a neurobiological phenomenon. This view changed somewhat over\r\nrecent years with the discovery of peripheral tissue circadian oscillators. In mammals, however, the suprachiasmatic nucleus\r\n(SCN) in the hypothalamus still retains the critical role of a central synchronizer of biological timing. Zebrafish, in contrast,\r\nhave always reflected a more highly decentralized level of clock organization, as individual cells and tissues contain directly\r\nlight responsive circadian pacemakers. As a consequence, clock function in the zebrafish brain has remained largely\r\nunexplored, and the precise organization of rhythmic and light-sensitive neurons within the brain is unknown. To address\r\nthis issue, we used the period3 (per3)-luciferase transgenic zebrafish to confirm that multiple brain regions contain\r\nendogenous circadian oscillators that are directly light responsive. In addition, in situ hybridization revealed localised neural\r\nexpression of several rhythmic and light responsive clock genes, including per3, cryptochrome1a (cry1a) and per2. Adult\r\nbrain nuclei showing significant clock gene expression include the teleost equivalent of the SCN, as well as numerous\r\nhypothalamic nuclei, the periventricular grey zone (PGZ) of the optic tectum, and granular cells of the rhombencephalon. To\r\nfurther investigate the light sensitive properties of neurons, expression of c-fos, a marker for neuronal activity, was\r\nexamined. c-fos mRNA was upregulated in response to changing light conditions in different nuclei within the zebrafish\r\nbrain. Furthermore, under constant dark (DD) conditions, c-fos shows a significant circadian oscillation. Taken together,\r\nthese results show that there are numerous areas of the zebrafish central nervous system, which contain deep brain\r\nphotoreceptors and directly light-entrainable circadian pacemakers. However, there are also multiple brain nuclei, which\r\npossess neither, demonstrating a degree of pacemaker complexity that was not previously appreciated.
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