Melatonin patterns in ewes maintained under skeleton or resonance photoperiodic regimens

Two experiments were designed to study the mechanism for measurement of daylength by ewes. 1) Two groups of 8 adult ewes were exposed for more than one year to either 16L:8D (long days) or 7L:9D:1L:7D alternated every 3 months with 8L:16D (short days). Ovulatory activity was followed during the total duration of the experiment and compared to that of a third group of 8 females under simulated natural conditions of lighting. Plasma prolactin and melatonin were measured in blood samples collected hourly over a 24-h period, more than 1.5 months after a light shift. Ovulatory activity and patterns of prolactin showed that one hour of light given 16 to 17 hours after the onset of the main light phase led to the measurement of a long day by the animals. Plasma melatonin was high during darkness in ewes under 8L:16D or 16L:8D. So, the duration of melatonin secretion was about twice as long in short days (8L:16D) as in long days (16L:8D). Interruption of the dark phase by the light pulse induced a dramatic decrease in the melatonin levels which increased again in only 6 of the 8 ewes studied. 2) Four groups of 7-8 ewes were subjected for at least 6 months to one of the following treatments: 4L:8D, 4L:20D, 4L:32D or 4L:44D. Blood samples were collected twice each week in order to monitor the ovulatory activity and plasma prolactin levels. In addition, to follow the plasma prolactin and melatonin patterns, blood samples were collected hourly over a 48-h period, 4 months after the onset of the experiment. The ovulatory activity was not indicative of the daylength measured by the animals. However, plasma prolactin levels suggested that 4L:8D and 4L:32D were considered as long days and 4L:20D as short days. Four months after the onset of the experiment, a 24-h rhythm of melatonin secretion was found regardless of the photoperiodic treatment: in all groups, low melatonin levels were coincident in time once every 24 h, and high melatonin levels displayed similar coincidence. Low levels of melatonin were observed during each period of light. Melatonin secretion was interrupted by light in groups 4L:8D and 4L:32D, both treatments to which the ewes responded as long days. Results of both experiments are compatible with the hypothesis that periods of melatonin secretion throughout the day are more important than the duration of secretion itself.