Temporal changes in the pattern of melatonin secretion in sheep held in constant darkness

The pattern of melatonin secretion was investigated in six 2 yr-old Suffolk ewes transferred from a long-day lighting regime (14L:10D) to continuous darkness. During the 9 d in continuous darkness, five of the ewes maintained a characteristic diurnal rhythm in melatonin secretion. However, there was a systematic change in the duration of each episode of melatonin secretion with the result that it extended from 10 h under the entraining lighting regime to about 19 h by the fourth cycle and then contracted to about 14 h by the eighth cycle under continuous darkness. The pattern of individual ewes varied, but overall the time of offset showed a consistent delay each successive day of about 1 h, indicative of control by a pacemaker with a period of greater than 24 h. By contrast, the time of onset tended to occur earlier each day during the first 4 d of continuous darkness, then changed so that after 8 d, it was delayed in all animals compared to the time of onset on d 4. If, as previously postulated, onset is controlled by a separate pacemaker, then this change is accounted for by interaction with the dominant offset control pacemaker; however, control of both onset and offset through a single pacemaker cannot be excluded. The study confirms that the patterns of melatonin secretion developed in sheep held in continuous darkness may be used to gain insight into the activity of the light-sensitive pacemaker centre(s) controlling the offset and onset of pineal function. Further experimentation is required to differentiate whether control is being exerted by one or two or more independent but interacting pacemaker centres.