Behavioral displays to gustatory stimuli in newborn rat pups

These experiments examine behavioral responses to taste stimuli in newborn rat pups during the first 4 postnatal days. Motor displays in the face and head regions of 90 neonate rats were recorded during 60-sec observation periods in a double-blind setting. Stimuli, presented as single droplets to the lips, included 2 concentrations each of sucrose, sodium saccharin, citric acid, quinine, and distilled water. Results from 5 different judges making "blind" and independent observations on different animals were compared for concordance. It was agreed that certain features were consistently associated with one stimulus more than another. Licking and rhythmic mouth movements were found to be the most salient features elicited by sweet stimuli, while head movements and gaping most accurately identified quinine. The sour reaction often contained components characterizing both sweet and bitter. Salient features differed not only by their association with certain stimuli, but by their repetitive frequencies as well as by their likelihood to initiate a behavioral sequence. Intensity and hedonic values assigned to taste-induced behaviors were usually different from water and accurately related to stimulus type. Results suggest that the gustatory system becomes functionally mature during the first postnatal days, and that this functionality occurs before the structural development of all taste buds is complete.