Light’s role in timing the proteins for light sensing and melatonin production in the pineal gland of carp Catla catla

Daily rhythmicity of pineal proteins and melatonin levels peaked at 12:00 h in fish under natural light conditions.

• Pineal rod-like opsin and alpha-transducin showed a peak band intensity at 12:00 h and a low intensity at 24:00 h in fish exposed to natural photoperiod.
• In continuous illumination, band intensities of these proteins remained high, while under darkness, they were low at all times measured.
• Pineal AANAT, serum melatonin, and calcium levels were highest at 24:00 h and lowest at 12:00 h in fish under natural light.
• Both serum and intra-pineal calcium levels were significantly lower in continuous illumination and higher in darkness at each measurement point.
• The absence of daily rhythm in these variables under continuous light and darkness indicates dependence on external light conditions for circadian organization.

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