Biology of Reproduction, Vol 23, 276-280, Copyright © 1980 by Society for the Study of Reproduction

Reinitiation of the Diurnal Rhythm of Prolactin Secretion in Postpartum Rhesus Monkeys

¹ Section on Endocrinology, Pregnancy Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20205

To determine if rhesus monkeys have a diurnal rhythm of prolactin secretion during pregnancy and in the postpartum interval, as occurs throughout the normal menstrual cycle, femoral blood samples were collected at 0400, 0800, 1200, 1600, 2000, and 2400 h from four pregnant (152 days of pregnancy) and six postpartum (weeks 1, 3, 5, 7, 9, 11, 13, 15, 17, and 19) non-nursing monekys. All births were spontaneous vaginal deliveries at term. Femoral blood samples were collected under light ketamine HCl anesthesia. Serum concentrations of prolactin, estradiol-17, and progesterone were determined by radioimmunoassay. In the near-term pregnant monkeys, prolactin values during the 24 h sampling intervals varied only slightly from 52 ± 10 ng/ml to 65 ± 1 ng/ml ( x ± SEM; not significant, P>0.05). In contrast, during the first postpartum week, prolactin levels underwent daily fluctuations from 9 ± 2 ng/ml at 0800 h to 71 ± 30 ng/ml at 2000 h (P<0.05). This immediate return of the diurnal rhythm of prolactin levels in circulation continued throughout the postpartum interval. As expected, estradiol and progesterone levels were significantly lower in postgestational monkeys than in those pregnant. Thus, the nyctohemeral rhythmicity, which is characteristic of prolactin secretion during ovulatory menstrual cycles, was not evident during late pregnancy, but was rapidly reinstated within 1 week into the puerperium. The rapid renewal of the diurnal pattern of prolactin secretion postpartum may reflect escape from the milieu of advanced gestation, particularly from estrogens and progesterone that arise within the fetoplacental unit.

Note:
ACKNOWLEDGMENTS The authors express their appreciation to Dr. G. D. Niswender and Dr. D. C. Collins for providing the progesterone and estradiol antisera, respectively, to Mr. D. Barber, Mr. A. Coleman, Mr. A. Tanner, Mr. J. Kim for their expert assistance, and to Ms. A. Montague for typing the manuscript.