Circadian rhythmicity and light sensitivity of the zebrafish brain

Light is important for entraining circadian rhythms, which regulate a wide range of biological processes. Zebrafish have directly light responsive tissues (Whitmore et al 2000) and are thus a useful vertebrate model for circadian rhythmicity and light sensitivity. Recent studies show the pineal regulates locomotor rhythms (Li et al 2012). However, there are many unresolved questions concerning the neurobiological basis of the zebrafish clock, such as whether neuronal pacemakers, which drive rhythms in other tissues, are present throughout the brain. In this study, per3-luc zebrafish confirm that both central and peripheral tissues are directly light sensitive and have endogenous circadian rhythmicity. Chromogenic in situ hybridization reveals localised expression of several core zebrafish clock genes, a rhythmic gene, per3, and two light responsive genes, cry1a and per2. Adult brain nuclei with expression include the suprachiasmatic nucleus, periventricular grey zone of the optic tectum, and granular cells of the rhombencephalon. Pilot experiments using high-resolution spatial recording of per3-luc brain slices show some of these regions can display robust rhythmicity in DD. Some of the cells expressing clock genes are neurons, and therefore neurons were further investigated. C-fos, a marker for neuronal activity in mammalian photoreceptors, is upregulated in at least four different responses to light in zebrafish, in different brain nuclei. This suggests the brain contains several types of photosensitive cells, which respond to different lighting conditions. Zebrafish larvae exhibit developmental changes in spatial circadian gene expression of per3 and light induction of c-fos. Finally, the photopigment group of opsins were investigated for their potential role in light entrainment. Exorh was prominent solely in the pineal. Rgr1 was found in numerous nuclei, many of which had shown expression of cry1a, per2 and per3. Overall, this thesis shows that the zebrafish brain is not uniformly light sensitive. Localised regions in the zebrafish brain with strong rhythmicity and light sensitivity are neuronal pacemaker candidates.