Novel functions for Period 3 and Exo-rhodopsin in rhythmic transcription and melatonin biosynthesis within the zebrafish pineal organ

Expression of the gene for the opsin protein Exo-rhodopsin (Exorh) in zebrafish pineal photoreceptors is higher at night than during the day.

• Exorh is associated with the regulation of gene transcription in the pineal organ.
• In embryos lacking Exorh, the expression of both the exorh gene and the melatonin synthesis gene aanat2 is significantly reduced.
• Exorh may play a role in a signaling pathway that positively regulates melatonin production in the pineal.
• The transcription factor Otx5 is involved in activating exorh transcription.
• The circadian clock component Per3 is linked to the repression of exorh expression during the day.

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