EMERGENCE OF THE DIURNAL RHYTHM IN PLASMA MELATONIN CONCENTRATIONS IN NEWBORN LAMBS DELIVERED TO INTACT OR PINEALECTOMIZED EWES

We have monitored the 24-h profiles of plasma melatonin concentrations between birth and 10 weeks of age, in lambs which were delivered to, and suckled, either pineal-intact (control group) or pinealectomized (pinealectomized group) ewes. Between 0 and 2 weeks of age, plasma concentrations of melatonin in lambs suckling either intact or pinealectomized ewes were highest at 01.00 h. At this age, however, there was no significant difference in the mean plasma concentrations of melatonin between the entire dark and light phases in lambs in either the control group (dark, 39.7 ± 6.0 (S.E.M.) pmol/l; light, 39.5 ± 8.1 pmol/l) or the pinealectomized group (dark, 79.8 ± 43.3 pmol/l; light, 60.9 ± 8.7 pmol/l). Between 3 and 4 weeks of age, however, a diurnal rhythm in plasma melatonin concentrations was clearly present in the lambs in both the control and pinealectomized groups (control group: dark, 164.1 ± 5.6 pmol/l; light, 26.2 ± 2.5 pmol/l; pinealectomized group: dark, 52.7 ± 8.0 pmol/l; light, 19.1 ± 5.3 pmol/l; P < 0.001). Between 3 and 10 weeks of age, plasma concentrations of melatonin in the dark phase were significantly (P < 0.05) lower in the lambs suckling pinealectomized ewes than in the control group. In both the control lambs and lambs suckling pinealectomized ewes, the mean plasma concentrations of melatonin in the dark and light phases increased significantly (P < 0.05) between 3 and 6 weeks after birth. In conclusion, we have demonstrated that a clear diurnal plasma rhythm in melatonin concentrations does not emerge until 3-4 weeks of age in lambs suckling either pinealectomized or intact ewes. We have also measured lower melatonin concentrations during the dark phase in lambs suckling pinealectomized ewes when compared with lambs suckling pineal-intact ewes between 3 and 10 weeks of age.