Nutritive and nonnutritive determinants of milk intake of suckling rats.

In order to determine the factors that enhance milk intake during deprivation, albino rats 15, 20, and 25 days of age were subjected, for 8 hr, to one of the following regimens: (1) privation, that is, separation from the dam and food; (2) privation with a maternal, thelectomized female; (3) privation with a maternal female whose nipples had been surgically ligated; (4) separation from the dam and food but receiving three 2% body weight intragastric preloads of milk; (5) with a dam whose nipples had been ligated and receiving the same intragastric preloads as Group 4; and (6) nondeprived rats. Rats were then allowed 45 min to suckle an anesthetized dam that was induced to let down milk every 4 min by intravenous oxytocin infusion. Intake at Day 15 was reduced by the opportunity to suckle, independent of receiving a milk load. This same trend was apparent, although not as strong, among Day 20 rats. By Day 25, nonnutritive suckling during the privation period no longer attenuated milk intake, although preloads did, whether or not they were paired with nonnutritive suckling. Thus, suckling in albino rats becomes increasingly freed from oral demands and more responsive to the nutritive consequences.