Effect of season on plasma concentrations of prolactin and cortisol in pregnant, non-pregnant and lactating ewes

The objective of this study was to examine the effect of season and the interaction between season and reproductive status on secretion of prolactin and cortisol. Plasma concentrations of these hormones were determined in non-pregnant ewes (n = 3), pregnant ewes mated in January (Group A; n = 5) or June (Group B; n = 5) and in lactating ewes after lambing in June (Group J; n = 5) or November (Group N; n = 5). In non-pregnant ewes, concentrations of prolactin in plasma were significantly higher during spring and summer than during autumn and winter. During the first 2 months of gestation, concentrations of prolactin did not differ between pregnant ewe groups, but from Day 61 until 2 days before parturition, mean values were higher in Group A than in Group B. An abrupt rise in concentration of prolactin the day before parturition, followed by a decrease on the day of lambing, was noted in both groups. The discharge of prolactin before parturition tended to be larger in Group B than in A; mean values did not differ between groups. In lactating animals, suckling increased the secretion of prolactin the magnitude of the response was greater in ewes lambed in November (Group N) than in those lambing in June (Group J), owing to seasonal differences in pre-suckling baseline levels, but no differences between groups were detected in the maximum concentration after suckling. Except for the increased levels observed in non-pregnant animals in September and October, mean concentrations of prolactin did not differ between pregnant and non-pregnant ewes. Higher values were recorded in lactating ewes as compared with non-pregnant and pregnant females. No significant relationship was found between season and circulating levels of cortisol in non-pregnant, pregnant and lactating ewes, but a tendency for greater values during the spring was apparent in non-pregnant ewes. During pregnancy, concentrations of cortisol were similar in Groups A and B; the highest concentrations occurred during the first 60 days and then declined until the day of parturition, when there was a significant but transient increase. The secretion of cortisol rose after suckling in lactating females, but there was no difference in the magnitude of the response between the two groups. Mean values did not differ between ewes lambing in June (Group J) or November (Group N). © 1991.