Evidence that suckling pups, through an exteroceptive mechanism, inhibit the milk stimulatory effects of prolactin in the rat during late lactation

Lactating rats were removed from the animal room on postpartum Day 20 and placed in a room where there were no other rats. On postpartum Day 21, these rats released prolactin either in response to exteroceptive signals from their own pups which were placed underneath or in response to those from 25 to 30 other lactating rats which along with their litters were placed in a rack 3 ft in front of the mothers' cage. Milk secretion was stimulated in the isolated rats on Day 21 by the prolactin released in response to exposure to the rack of lactators, but paradoxically was not stimulated by that released in response to exposure of the mother to her own pups. In fact, the stimulatory effects upon milk secretion, resulting from exposure to the rack of lactators was totally prevented if the mother was exposed to her pups 3–4 min before exposure to the rack of lactators. The blocking effect of the pups, however, did not occur when the pups were placed in cages alongside the mothers' cage. From subsequent experiments, it was concluded that sensory cues, from the pups, appeared to activate the sympathetico-adrenal system of the mother to release catecholamines which then blocked the milk stimulatory effects of prolactin which already had been discharged into the circulation. The pups did not inhibit milk secretion on Day 14 which indicates that the inhibiting mechanism becomes established sometime between Days 14 and 21 of lactation. These data suggest that a peripheral mechanism may operate to reduce milk secretion in the rat during late lactation, and thus, may be involved in the normal weaning process.