Lactational anovulation in non-human primates: restriction of nursing inhibits Prl secretion without precipitating the return of ovulatory menstrual cyclicity in cynomolgus monkeys.

To test the hypotheses that nighttime suckling and elevated nocturnal prolactin concentrations are essential for the continuation of lactational anovulation, the effects of restricting nursing to twelve h per day (either day or night) on maternal prolactin (Prl) levels and resumption of ovulatory menstrual cycles were studied in five groups of cynomolgus monkeys: Group 1: baby weaned: Group 2: baby fully nursed: Group 3: baby nursed night only; Group 4: baby nursed day only; and Group 5: baby housed with mother but not allowed to nurse. Restrictions were initiated at approximately 150 days postpartum and were achieved by placing a non-nursing jacket over the nursing jacket, which was worn 24 h/day. Fifteen out of seventeen monkeys remained anovulatory while housed with their infants, irrespective of the type of nursing restriction. First postpartum ovulations occurred approximately two months post-weaning. Plasma prolactin concentrations during both day and nighttime were significantly (p < 0.05) greater in the fully nursing group(s) than in all other groups. Fully nursing mothers had significantly (p < 0.01) greater prolactin concentrations at night than during the day. Among the restricted groups, the night only suckling group had significantly greater prolactin concentrations at night than the other restricted groups. There were no differences between daytime values, and within each restricted group there were no significant differences between day and night prolactin levels. We conclude that 1) nighttime suckling is not an absolute prerequisite for sustained lactational anovulation, and 2) suckling-induced hyperprolactinemia is not the principle mediator of lactational anovulation.