Sucrose vs. maltose taste discrimination by rats depends on the input of the seventh cranial nerve.

Abstract

Although rats treat the taste of sucrose and maltose as perceptually similar, they nonetheless appear to be able to distinguish between the two sugars, as suggested from prior work examining the cross-generalization of conditioned taste aversions. This study explictly tested whether rats could behaviorally discriminate sucrose from maltose and examined the relative importance of the gustatory input of the seventh and ninth cranial nerves in maintaining such performance. Water-restricted rats were presurgically trained in a conditioned avoidance task to suppress licking to sucrose or maltose and to maintain licking to the other sugar. Concentration (0.05, 0.1, 0.2, and 0.4 M) was varied to make intensity an irrelevant cue. Stimuli were randomly presented in 5-s trials during 50-min sessions. Bilateral transection of the chorda tympani nerve (CT) or the glossopharyngeal nerve or sham surgery did not disrupt discrimination performance. In contrast, combined transection of the CT and greater superficial petrosal nerve, which collectively removes the taste input of the seventh cranial nerve, caused severe impairments in sugar discriminability. In these rats, performance was more disturbed at the lower concentrations. These findings confirm that rats can discriminate sucrose from maltose and that this capability relies heavily on the taste input of the seventh cranial nerve. Although the input of the ninth cranial nerve is unnecessary, it may help sustain partial competence in this task, especially at high concentrations, in the combined absence of the CT and greater superficial petrosal nerve.