Circadian pacemaker in the suprachiasmatic nuclei of teleost fish revealed by rhythmic period2 expression

In mammals, the role of the suprachiasmatic nucleus (SCN) as the primary circadian clock that coordinates the biological rhythms of peripheral oscillators is well known. However, in teleosts, it remains unclear whether the SCN also functions as a circadian pacemaker. We used in situ hybridization (ISH) techniques to demonstrate that the molecular clock gene, per2, is expressed in the SCN of flounder (Paralichthys olivaceus) larvae during the day and down-regulated at night, demonstrating that a circadian pacemaker exists in the SCN of this teleost. The finding that per2 expression in the SCN was also observed in the amberjack (Seriola dumerili), but not in medaka (Oryzias latipes), implies that interspecific variation exists in the extent to which the SCN controls the circadian rhythms of fish species, presumably reflecting their lifestyle. Rhythmic per2 expression was also detected in the pineal gland and pituitary, and aperiodic per2 expression was observed in the habenula, which is known to exhibit circadian rhythms in rodents. Since the ontogeny of per2 expression in the brain of early flounder larvae can be monitored by whole mount ISH, it is possible to investigate the effects of drugs and environmental conditions on the functional development of circadian clocks in the brain of fish larvae. In addition, flounder would be a good model for understanding the rhythmicity of marine fish. Our findings open a new frontier for investigating the role of the SCN in teleost circadian rhythms.