Some comparisons among the calming and pain-relieving effects of sucrose, glucose, fructose and lactose in infant rats.

Ultrasonic vocalizations were recorded from 10-day-old albino rats while they were isolated from their dam and siblings. Each rat received a 1.0% BW intra-oral infusion of sucrose, fructose, glucose or lactose at a concentration range of 0.22-0.66 M for 3 min and their vocalizations were determined during the infusion and for an additional 7 min. Sucrose, fructose and glucose all significantly reduced vocalizations to about 50% of baseline levels, whereas lactose, the milk sugar, was ineffective. Moreover, the dose-response function was flat for the three effective sugars. In a second experiment, the effects of these sugars on heat escape latency were measured. Sucrose, fructose and glucose each elevated the latency with which infant rats removed a paw from a 48 degrees C surface; lactose did not. These findings of lactose ineffectiveness and the flat dose-response function for the other three sugars exactly parallel those obtained for human newborns. Their implications are discussed.