title: The 24-hour rhythm of oxytocin and beta-endorphin secretion in human pregnancy.
abstract: OBJECTIVE
  Animal work suggests that maternal oxytocin secretion is influenced by the secretion of endogenous opioids in pregnancy. Spontaneous labour and pre-labour uterine activity follow a 24-hour rhythm the origin of which has not been explained but may be related to diurnal changes in oxytocin secretion. This study was performed to document the changes over a 24-hour period in maternal oxytocin and beta-endorphin secretion.
  
  
  METHODS
  A 4-hourly blood profile was undertaken for a 24-hour period.
  
  
  METHODS
  Sixteen women with singleton pregnancies of more than 36 weeks gestation and 10 women with pregnancies in the mid trimester were studied.
  
  
  METHODS
  Blood was sampled 4-hourly for 24 hours beginning at 1200 h. Oxytocin was measured in all patients and beta-endorphin-like immunoreactivity was measured in 15 patients.
  
  
  RESULTS
  A simple index was defined for comparing night-time levels to daytime levels for both oxytocin and beta-endorphin. In all cases more than 36 weeks gestation the index was positive for oxytocin (night-time levels were higher) and in all cases the index was negative for beta-endorphin (night-time levels were lower). In the mid trimester women all values of the index for oxytocin were positive but in the beta-endorphin group equal numbers demonstrated a positive or a negative index.
  
  
  CONCLUSIONS
  Reciprocal 24-hour rhythms were demonstrated between oxytocin and beta-endorphin; however, it is not clear whether this relationship is causal.
authors.affiliation: Department of Obstetrics and Gynaecology, Medical School, University of Cape Town, South Africa.
authors.forename: S W
authors.initials: SW
authors.lastname: Lindow
authors.forename: A
authors.initials: A
authors.lastname: Newham
authors.forename: M S
authors.initials: MS
authors.lastname: Hendricks
authors.forename: J W
authors.initials: JW
authors.lastname: Thompson
authors.forename: Z M
authors.initials: ZM
authors.lastname: van der Spuy
mesh_terms: D002940:Circadian Rhythm; D005260:Female; D006801:Humans; D010121:Oxytocin / Q000097:blood / Q000378:metabolism*; D011247:Pregnancy / Q000097:blood / Q000502:physiology*; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D001615:beta-Endorphin / Q000097:blood / Q000378:metabolism*
chemical_list: D010121:Oxytocin; D001615:beta-Endorphin