ON THE MECHANISM OF LACTATIONAL ANOVULATION IN THE RHESUS-MONKEY

The relative roles of infant suckling and of maternal prolactin (PRL) secretion in lactational anovulation were studied in ovary-intact and ovariectomized rhesus monkeys nursing young that had been removed from their natural mothers. Hypothalamic gonadotropin-releasing hormone (G nRH) pulse generator activity was monitored electrophysiologically in freely behaving animals by radiotelemetry. Serum luteinizing hormone, PRL, estradiol, and progesterone were also measured. Suckling inhibite d GnRH pulse generator activity and ovarian cyclicity in all ovary-int act females but had no such effect on the pulse generator in long-term ovariectomized animals. When PRL secretion was suppressed by daily br omocriptine administration, GnRH pulse generator activity remained sig nificantly inhibited and ovulation was prevented in four monkeys (6 tr ials), whereas in two females (6 trials) a rapid increase in pulse gen erator frequency and the resumption of ovarian cyclicity were observed although suckling activity was maintained. One monkey displayed both response types. Although these results indicate that suckling per se i s able to restrain GnRH pulse generator activity in the absence of PRL , they also suggest that the relative importance of these determinants is variable depending on factors that remain to be determined. The pr esent study also confirms the permissive role of the ovary in the lact ational suppression of GnRH pulse generator activity.