The influence of tactile stimulation of nipples on the milk ejection reflex in the rat.

Changes of positive pressure exerted by pups on nipples during sucking were investigated using anesthetized, lactating dams. It was found that, every 50-60 s, individual pups performed bouts of pressure oscillations (3/s) of high amplitude which lasted about 10-12 s and coincided with periods of increased motor activity. During the intervals, when pups were quiet, series of low-amplitude oscillations (3/s) were also observed. Using a strain measuring method to record the activity of sucking pups, synchronization of activity of two or more pups was found to occur periodically every 25-30 s and, most frequently, 10-30 s before the reflex increase of milk pressure. In further experiments, artificial tactile stimulation was applied to the dam's nipples using the joint action of suction and positive pressure. Following a short-term (10-20 s) increase in frequency and amplitude of artificial nipple stimulation, 60%-80% of all reflexive peaks of milk pressure were elicited with a latency of 19 +/- 5 s. This suggests that there are specific conditions under which the stimulation of nipples by pups may trigger the formation of the milk ejection reflex in the rat.