Sensory stimuli from pups involved in inhibition of milk secretion in rats during late lactation

Lactating rats of two strains, Holtzman and Sprague-Dawley-Rolfsmeyer, were removed from the animal room on postpartum Day 20 and were placed in a testing room where there were no other rats. Milk secretion was stimulated in mothers of each strain 24 hr later in response to 30 min of exposure to a rack of lactators and their pups, placed 3 ft in front of the test mothers; this stimulation to milk secretion was prevented, provided their own pups were placed under, but not when they were placed alongside, the mothers 3–4 min beforehand. The milk secretion responses on the whole were quantitatively less in the Holtzman rats. The sensory signals emanating from the pups which were responsible for the inhibitory effect upon milk secretion were analyzed in Holtzman rats. Sound and odor each proved to be inhibitory; sight, however, was not. The function of the exteroceptive inhibition of milk secretion in the control of milk secretion in late lactation is discussed.