Mechanisms of control of milk intake in suckling rats.

To reconcile reported differences in the development of internal controls over milk intake via suckling, infant rats 5-20 days of age were allowed to withdraw milk from anesthetized dams periodically induced to let down milk by intravenous infusion of oxytocin. In experiment 1, milk supply was made unlimited by providing pups with a series of milk-replete dams. Five- and ten-day-old pups withdrew inappropriately large volumes of milk, whereas 15- and 20-day-old pups limited intake to moderate volumes. In experiment 2, intake of deprived and nondeprived pups was measured after only 10 oxytocin-induced milk letdowns, comparable to the normal nursing bout. When milk availability was thus limited, pups of all ages consumed volumes of milk dependent on their level of deprivation. Moreover intake of pups 10-20 days of age was highly correlated with incidence of nipple shifting. Thus rats as young as 5 days of age can vary the rate at which milk is taken from a limited source; however, not until 15 days are they able to control the volume withdrawn from an unlimited source.