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 albinorats while they were isolated from their dam and siblings. Eachrat received a 1.0% BW intra-oral infusion of sucrose, fructose,glucose or lactose at a concentration range of 0.22–0.66M for 3 min and their vocalizations were determined during theinfusion and for an additional 7 min. Sucrose, fructose andglucose 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 thethree effective sugars. In a second experiment, the effectsof these sugars on heat escape latency were measured. Sucrose,fructose and glucose each elevated the latency with which infantrats removed a paw from a 48°C surface; lactose did not.These findings of lactose ineffectiveness and the flat dose-responsefunction for the other three sugars exactly parallel those obtainedfor human newborns. Their implications are discussed.