Shift in Acrophase of Circadian Prolactin Plasma Levels in Adult Female Rats Following Neonatal Ovariectomy and Estrogen Treatment

Female rats were ovariectomized 2-4 h postpartum and injected with oil or estradiol benzoate (E2B). Rats injected with 1 mg of E2B, but not with 10 or 100 microgram, had a significantly lower body weight when adult (p < 0.001). In a first series of experiments circadian prolactin plasma concentration between days 60 and 80 postpartum was measured following blood sampling from the tail. Rats injected as neonates with 10 microgram and 1 mg of E2B had elevated mean prolactin plasma levels. It was possible to fit the circadian variations of oil-injected controls and E2B-injected rats to a significant sinusoidal curve with an acrophase at 21.86 h for controls, 2,67 h for rats injected as neonates with 10 microgram of E2B and 10.71 h injected with 1 mg of E2B. The shift in acrophase between oil-injected controls and 1 mg of E2B-injected rats is statistically meaningful. In a second series of experiments rats were injected as neonates with oil or 100 microgram of E2B and circadian prolactin was assayed in plasma and the hypophysis following decapitation at day 65 of age. Again a significant sinusoidal curve could be fitted to the data. Oil-injected controls hat an acrophase at 22.30 +/- 1.17 h and E2B-injected rats at 14.69 +/- 2.52 h. This shift could also be proved to be statistically significant. A similar significant shift in hypophyseal prolactin content was observed. Rats injected neonatally with 10 microgram of E2B had a more pronounced prolactin acrophase might be related to the decrease in body weight. The observed effects can be the result of an altered hypothalamic sensitivity to several stimuli in estrogenized rats.