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 less than 0.001). Between 3 and 10 weeks of age, plasma concentrations of melatonin in the dark phase were significantly (P less than 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)