In Vitro Melatonin Rhythm Reveals A Clocked Pineal In The European Sea Bass, Dicentrarchus Labrax

The melatonin-generating system of the European sea bass, Dicentrarchus labrax, was studied in vitro using a perfusion tissue culture methodology. The examined properties included photic entrainment in the pineal gland, and circadian rhythmicity in alternating dark and light, in contin- uous darkness, and in continuous light. Pineal glands were removed from adult fish and placed in a perfusion tissue culture at a constant temperature of 24°C. Melatonin discharge into the cul- ture medium was measured using radioimmunoassay. Melatonin concentrations during a pho- toperiod of 12h light:12h dark were low during the light periods and high during the dark. The melatonin discharge was inhibited by continuous light but strong and rhythmic in continuous dark with a circadian period (Tau) of about 24 h (n = 10). The results demonstrate that the European sea bass pineal gland is photosensitive, with ability to be entrained, and contains an internal cir- cadian oscillator that regulates melatonin production.