In vertebrates, the pineal gland hormone melatonin plays a crucial role in the inner clock system. Produced at night, it regulates daily rhythms, and also annual rhythms such as seasonal changes in reproduction and energy balance.
The activity of melatonin is modulated by external photic signals, and in mammals it is driven by the suprachiasmatic nucleus (SCN) in the hypothalamus. In fish, scientists have found no SCN or any structure homologous to it. However, the fish pineal gland seems to have all the elements for the generation of circadian rhythmicity.
The Yoav Gothilf Lab (Tel Aviv University, Israel) has done some very in-depth research on the mechanism behind the light entrainment of the circadian system in zebrafish.
For example, Sima Smadja Storz and her colleagues at this lab investigated the role of casein kinase 1 (CK1), which are enzymes that function as regulators of circadian rhythmicity (research published in PLoS ONE, 2013). They studied both cell cultures and zebrafish at different developmental stages.
To determine rhythmicity, the researchers looked at the activity patterns of zebrafish larvae in 48-well plates in DanioVision. Before the experiment, larvae were kept under 12h light/12h dim light conditions. The effects of specific CK1 inhibitors were measured by comparing the affected larvae’s total distance moved during a 10 minute dim light experiment to the activity of a control group.
Zebrafish larvae normally develop circadian rhythmicity when exposed to light/dark cycles, after which the animals are able to maintain this rhythmicity even under continuous light conditions. In larvae that were exposed to the CK1δ inhibition, rhythmicity was completely diminished. Specific CK1ε inhibition did not have as significant an effect.
The complete publication is accessible online: Smadja Storz, S.; Tovin, A.; Mracek, P.; Alon, S.; Foulkes, N.S.; Gothilf, Y. (2013). Casein Kinase 1δ Activity: A Key Element in the Zebrafish Circadian Timing System. PLoS ONE, 8 (1), e54189.