Relationship between endogenous melatonin concentrations and uterine contractions in late third trimester of human pregnancy

Abstract

In humans, circulating levels of the hormone melatonin and the initiation of spontaneous labor are both higher at night than during the day. Since activation of uterine melatonin receptors can stimulate human in vitro uterine contractions and these receptors are only expressed on the uterine tissue of women in labor, we hypothesized that circulating melatonin concentrations would affect uterine contractions in vivo. We evaluated the impact of light-induced modulation of melatonin secretion on uterine contractions in women during late third trimester (~36-39 weeks) of pregnancy in two inpatient protocols. We found a significant (P < 0.05) positive linear association between circulating melatonin concentrations and the number of uterine contractions under both protocols. On average, uterine contractions increased between 1.4 and 2.1 contractions per 30 minutes for every 10 pg/mL*h increase in melatonin concentration. These findings have both basic science and clinical implications for pregnant women, since endogenous melatonin levels and melatonin receptor activity can be altered by light and/or pharmaceutical agents.

Keywords: circadian; contractions; human; light; melatonin; pregnancy; uterus.

Figure 1:

Full term pregnant women were studied in a 1-night (n=8, panel A) inpatient protocol. White bars represent wake episodes under typical indoor light intensity (~90 lux), gray bars represent wake episodes under dim lighting (<15 lux), gray hashed bars represent 4-h experimental light exposure with either short- or long-wavelength visible light, and black bars represent sleep in dim lighting (<3 lux). Time-course of salivary melatonin levels under long- () and short-wavelength light () and uterine contractions under long- () and short-wavelength light () are shown for one individual in each light exposure condition (panels B-E). Unadjusted linear regression results testing the association between uterine contractions and salivary melatonin AUC are shown (panel F). Corresponding uterine contractions and melatonin AUC values from each individual are shown under long- () and short-wavelength light () conditions.

Figure 2:

Full term pregnant women were studied in a 2-night (n=17, panel A) inpatient protocol. White bars represent wake episodes under typical indoor light intensity (~90 lux), gray bars represent wake episodes under dim lighting (<15 lux), gray hashed bars represent 4-h experimental light exposure with either short- or long-wavelength visible light, and black bars represent sleep in dim lighting (<3 lux). Time-course of salivary melatonin levels under long- () and short-wavelength light () and uterine contractions under long- () and short-wavelength light () are shown for one individual (panels B-E). Unadjusted linear regression results testing the association between uterine contractions and salivary melatonin AUC are shown (panel F). Corresponding uterine contractions and melatonin AUC values from each individual are shown under long- () and short-wavelength light () conditions. Values from the first night are shown as filled symbols and values from the second night are shown as unfilled symbols.

Figure 3:

The box and whisker plots show the median (solid horizontal line), 25th and 75th percentile (box limits), the 10th and 90th percentiles (whiskers), and the mean (dashed horizontal line) for uterine contractions (A) and melatonin AUC (B) for the each of the two nights in the 2-night protocol under the long- (left two box plots) and short-wavelength (right two box plots) light exposure conditions. Data were compared between light exposure conditions and study days using repeated measures mixed-model regression. * Signifies p<0.0001 between the first and second night within a light exposure condition.