The effect of photoperiod on diel rhythms in serum melatonin, cortisol, glucose, and electrolytes in the common dentex, Dentex dentex.

Diel rhythms in serum concentrations of melatonin, cortisol, glucose, sodium, chloride, and potassium were studied in the common dentex, Dentex dentex, under different photoperiods (DD, 8L:16D, 12L:12D, 16L:8D). Photoperiod affected both the diel rhythms and the absolute values of the estimated blood components. Regardless of the photoperiod, melatonin titers were elevated during the scotophase (384.3 +/- 13.9 pg/ml) compared with a mean baseline level of 54.4 +/- 2.7 pg/ml during the photophase. Serum melatonin concentrations reflected the prevailing photoperiod and constantly elevated melatonin levels with no diel rhythmicity were evident in fish held in the DD protocol. A circadian-like pattern in serum cortisol was observed in fish that were kept at the DD and 8L:16D protocols with cortisol peak at 18:00 h in the night. Fish exposed to the 16L:8D regime showed highest cortisol levels at 10:00 h, while no rhythmicity was evident under the 12L:12D protocol. A phase shift of 4 h between the peaks of cortisol and glucose was evident in fish exposed to the DD, 8L:16D, and 12L:12D regimes. Diel patterns of changes in serum Na+ and Cl- were observed only in the fish held in the DD protocol. Serum K+ values were lowest during the first part of the scotophase under all regimes, except the 16L:8D where no diel rhythmicity was detected. During the photophase, cortisol was positively correlated with glucose, Na+, and Cl- and negatively with K+. During the scotophase, melatonin was positively correlated with glucose and electrolytes. Results indicated that cortisol may be responsible for the observed rhythmicity of glucose and that melatonin may play a role in glucose and ion regulation in common dentex. Copyright 1999 Academic Press.