Daily and light-at-night induced variations of circulating 5-methoxytryptophol (5-ML) in ewes with respectively high and low nocturnal melatonin secretion.

The aim of the present study was to determine whether the genetic differences previously reported in ewe plasma melatonin concentrations were correlated with differences in the synthesis and release of other 5-methoxyindoles. To determine if 5-methoxytryptophol (5-ML), which is known to be present in large amounts in the sheep pineal gland, is released, as is melatonin, into the general circulation, and if some temporal relationships between 5-ML and melatonin release could be observed, two groups of ewes were selected with respect to their endogenous melatonin secretion: in the first experiment, ten ewes from the low melatonin group (low group) and ten ewes from the high melatonin group (high group). 5-ML was measured every hour during a 24-hr period by radioimmunoassay. In all ewes, 5-ML was released during day-time, the rhythm of 5-ML concentrations being inversely related with the melatonin rhythm. Both day-time and night-time 5-ML concentrations were higher in the ewes from the high group than in the ewes from the low group (14.7 +/- 1.0 pg/mL plasma versus 6.4 +/- 0.3 pg/mL plasma during the day, 3.1 +/- 0.2 pg/mL plasma versus 1.9 +/- 0.2 pg/mL plasma during the night). The 5-ML/melatonin ratio appeared much higher during the day than during the night but was very similar in both groups (day-time: 1.03 in the high group versus 1.16 in the low group, night-time: 0.01 in both groups). In a second experiment, six low group and seven high group ewes were submitted to 1 hr of extra light at night. 5-ML increased and melatonin decreased during extra light. Our results clearly show for the first time a daily variation in circulating 5-ML, and that the strong genetic contribution in the variability in melatonin concentrations in sheep are clearly correlated with a similar variability in 5-ML concentrations. Whether 5-ML, like melatonin, plays a physiological role in the different adaptation processes to the environment remains to be determined.