ONTOGENY OF THE CIRCADIAN VARIATION OF PLASMA PROLACTIN IN SHEEP
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Date
1989
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Abstract
The ontogeny of circadian rhythms is unknown. The newborn sheep has a circadian rhythm of temperature; to study the ontogeny of other rhythms, we examined the 24-h variation of plasma prolactin concentration in fetal and newborn sheep. To this effect, we measured plasma prolactin concentration in chronically catheterized fetuses (n = 7) and in newborn lambs raised under short day nycthemeral (12 light:12 dark n = 13) or constant light conditions (n = 5). Indwelling catheters were implanted into the jugular vein and carotid artery of late gestation fetuses (0.9 gestation) and newborns (5-29 days old). Experiments were performed 4 or more days after surgery. Lambs were kept in a canvas sling and were fed cow''s milk either by mouth or through a nasogastric catheter at established time intervals. Haematocrit, pH, and blood gases were measured before and after the experiments in all cases and remained within normal values. Lights were on and room temperature was maintained constant during the whole experiment. Samples were obtained every 1-2 h for 24 h in fetuses and newborn lambs under nycthemeral conditions and every hour for 48 h in newborn lambs kept under constant light. Plasma prolactin was measured by radioimmunoassay. The presence of a 24 h rhythm was determined by Cosinor analysis. Fetuses, aged 129 .+-. 6 days (SD) n = 7, showed a variation in plasma prolactin concentration with a period of 24 h that fits the equation: plasma prolactin (ng ml-1) = 97.0 + 15.4 cos 15 (t-23.0), P = 0.035. Newborn lambs, aged 7.9 .+-. 2.3 days old (n = 11), kept under nycthemeral conditions showed a variation of plasma prolactin with a period of 24 h that fits the equation: plasma prolactin (ng ml-1) = 75.3 + 12.9 cos 15 (t-12.0), P = 0.036. This rhythm disappeared in newborns of 23.4 .+-. 3.8 days of age (n = 7). Newborn lambs, aged 22.0 .+-. 2.7 days (n = 5), raised under constant light, showed a 24 h variation of plasma prolactin in the individual data. These newborns showed a tendency to higher prolactin values and a wider dispersion of plasma prolactin concentration than the two other groups. To normalize the variance the group mean was calculated after log transformation. When log values were synchronized considering the acrophase of the theoretical function as 24 h a rhythm that fits the equation: log plasma prolactin (ng ml-1) = 1.8 + 0.29 cos 15 (t-0.5), P = 0.031, was observed. This indicates that the rhythm of prolactin in the newborn sheep free-runs under constant light. The presence of a rhythm in plasma prolactin with a period of 24 h in fetuses and in the newborn, and the existence of this free running rhythm in newborns kept under constant light, suggest that the prolactin rhythm observed is a circadian rhythm endogenously generated. In addition, the disappearance of the rhythm in older newborns raised under short nycthemeral conditions, and its maintenance in newborns of similar age kept under constant light, suggests that the newborn sheep is sensitive to photoperiod.