Effect of different photoperiods on concentrations of 5-methoxytryptophol and melatonin in the pineal gland of the Syrian hamster

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Abstract
Specific, sensitive and direct radioimmunoassays have been used to determine the daily patterns of 5-methoxytryptophol (ML) and melatonin in the pineal glands of Syrian hamsters kept in different photo-periods: 8 h light: 16 h darkness (8L:16D), 14L: 10D and 16L: 8D. A rhythm in pineal ML was evident in animals in all the photoperiods, with high daytime levels (641±35 (s.e.m.) fmol/gland; n=162) which dropped to 119±16 fmol/gland (n = 44) 7·1– 7·5 h after lights out. The duration of low night-time ML levels was proportional to the length of the dark phase (1·2 h in 16L:8D, 5·4 h in 14L: 10D and 8·4 h in 8L: 16D). A marked daily rhythm in melatonin was also present in hamsters in the different photoperiods, with daytime levels of 323 ± 34 fmol/gland (n = 129) and night-time peak concentrations of 3676 ± 336 fmol/gland (n = 22). The duration of high nocturnal melatonin levels was dependent upon the length of the dark phase (4·1 h in 16L: 8D, 4·5 h in 14L: 10D and 12·5 h in 8L: 16D). Linear regression analysis revealed a statistically significant inverse relationship between pineal ML and melatonin levels in 8L: 16D (P< 0·001), 14L: 10D normal (PPJ. Endocr. (1987) 114, 301–309