Circadian myometrial and endocrine rhythms in the pregnant rhesus macaque: Effects of constant light and timed melatonin infusion

Six chronically catheterized rhesus macaques maintained on a 12-hour-light/dark cycle (lights on from 7 am to 7 pm) showed a nocturnal uterine activity rhythm with peak contractile events between 9 and 11 pm (p < 0.05). In blood samples collected at 3-hour intervals over a 24-hour period, we determined that plasma melatonin and progesterone concentrations were elevated at night whereas estradiol, estrone, and cortisol reached peak concentrations in the early morning (p < 0.05). Lights were then left on for the remainder of the study. After 12 days in constant light, daily rhythms in uterine activity and plasma steriod levels were relatively unchanged, whereas melatonin concentrations were suppressed. Animals then received a timed infusion of melatonin (0.2 mg/kg/hr each day from 7 pm to 6 am daily until delivery). The nocturnal uterine activity rhythm and the rhythms in plasma steroid concentrations were maintained. We conclude that the 24-hour patterns in maternal uterine activity and plasma steroid hormone levels are circadian rhythms generate by an endogenous biologic clock and do not appear to be driven by the pattern of melatonin in circulation.