EFFECTS OF PHOTOPERIOD AND DIFFERENT INTENSITIES OF LIGHT EXPOSURE ON MELATONIN LEVELS IN THE BLOOD, PINEAL ORGAN, AND RETINA OF THE BROOK TROUT (SALVELINUS-FONTINALIS MITCHILL)

Daily variations in the melatonin concentration in the blood, pineal organ, and retina of the brook trout, Salvelinus fontinalis, were investigated in July under constant photoperiod (16 h L : 8 h D) and temperature (12-degrees-C). Diel variations in melatonin levels in the blood and pineal organ showed similar patterns, with higher values during the night, whereas melatonin concentrations in the retina increased slightly in the first half of the light period. Light exposure of 1 h duration at mid-dark decreased melatonin levels in the blood and pineal organ in an intensity-dependent manner, whereas retinal melatonin levels increased with increasing light intensities. The minimum light intensity causing significant effects on the melatonin concentrations was 2 lx for the serum and 20 lx for the pineal organ and the retina. The parallelism between pineal and circulating melatonin patterns suggests that the lateral eyes of the trout have no significant endocrine function as far as the melatonin rhythm in the blood is concerned. Supporting evidence comes from the reversed response of retinal melatonin to light exposure at mid-dark, and from the low retinal melatonin concentrations found in this study. Thus, the serum melatonin rhythm of the brook trout seems to be mainly the result of rhythmic melatonin secretion from the pineal organ.