| Abstract |
Melatonin, Ν-acetyl-5-methoxytryptamine, is an indolamide, which is well known to act as an internal zeitgeber, transducing light information into a chemical signal. The daily cycle of illumination is by far the main environmental factor affecting melatonin production. In fish as well as in other vertebrates, melatonin is synthesized by the pineal organ and released into blood in a rhythmic fashion. In all species investigated so far, melatonin production in the pineal is inhibited by light but stimulated by darkness, so that low levels appear during the day and high levels at night. In addition to its role in the circadian system, melatonin is also involved in the control of many other biological functions such as growth, development, reproduction, aging, cellular immunity, skin coloration, thermoregulation, sleep and osmoregulation. The aim of this study was (a) the development and validation of a method for the determination of melatonin concentrations in the plasma of a marine species, the red porgy Pagrus pagrus, (b) the examination of the existence or not of an endogenous rhythm in melatonin production of red porgy, (c) the description of melatonin profile for this species and (d) the investigation of diel rhythms of the hormone under different photoperiodic regimes (12L:12D, LL, DD), light spectrum (full, blue, red) and intensities (high, low) and water temperature levels (15 oC, 19 oC, 23 oC). The radioimmunoassay (RIA), which was developed in the present study, provides firstly the financial and time advantage by skipping the extraction stage and secondly the accuracy and repeatability of the determination. Moreover, this method represents an important tool for measuring melatonin contents in other Mediterranean seawater species. The pattern of changes in plasma melatonin currently reported for the red porgy closely resemble those found in other species of fish. During the photophase plasma melatonin levels fluctuate around 12.5-102 pg/ml, while nighttime melatonin levels are clearly higher around 118.3?469 pg/ml. With this study, it is shown for the first time that in contrast to most teleosts the red porgy doesnt exhibit an endogenous rhythm in melatonin production. The melatonin synthesis occurs primarly in response to the presence or absence of light. Also, the environmental temperature affects melatonin levels, as they are higher in increased temperature. However, neither the light spectrum nor light intensity has an effect on melatonin levels. Finally, the absence of correlation between the chosen stress indicators (lactate, glucose) and the hormone production indicates that probably melatonin doesnt involve with stress in fish.
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Ο ρόλος περιβαλλοντικών παραγόντων στις νυχθημερήσιες διακυμάνσεις της μελατονίνης στο φαγκρί, pagnus pagnus
