Nursing induces a biphasic release of prolactin in rhesus monkeys.

Abstract

The effect of nursing on serum PRL levels was studied in four conscious unrestrained rhesus monkeys (20--46 days postpartum) fitted previously with exteriorized cardiac catheters. On the day of an experiment, the infant was transferred to a separate room for 9.5 h. Blood samples for PRL assay were collected from the mother at frequent intervals for 1 h before and 3 h after returning the infant. In all instances, vigorous suckling of the nipple was observed within 10 min of reuniting mother and infant and resulted in an elevation of serum PRL from an initial value of 7.4 +/- 0.5 ng/ml (mean +/- SEM) to a peak at 90 min of 339.7 +/- 56.1 ng/ml. Analysis of secretory patterns from individual animals revealed that PRL release was biphasic. During phase I, which began within 10--50 min of returning the infants and lasted for 20--50 min, serum PRL concentrations increased at an average rate of 0.47 ng/ml . min. This was followed by phase II, lasting 20--40 min, during which serum PRL rose at a 20-fold greater rate (10.57 ng/ml . min) to sustained maximum levels. Whether this unusual pattern of PRL secretion occurs as a consequence of neuroendocrine interactions unique to this species or simply reflects the prolongation and partitioning of a secretory process common to other mammals remains to be determined.