Although it is clear that the heterodimer formed by the T1R2 and T1R3 proteins serves as the primary taste receptor for sweeteners, there is growing evidence that responses to glucose polymers may be mediated by a different taste receptor. Here we report that although T1R2 knockout (KO) and T1R3 KO mice displayed severely impaired responding to glucose, maltose, and maltotriose in an initial session of a brief-access taste test (5 s trials, 25 min sessions) relative to wild-type (WT) mice, they subsequently increased their licking as a function of concentration for maltose and maltotriose with continued testing, presumably due to associating weak oral cues with positive post-ingestive consequences. Interestingly, these KO mice displayed relatively normal concentration-dependent licking to Polycose,a mixture of glucose polymers,even in the first session. Importantly, the experience-dependent increase in responsiveness to the sugars observed with the T1R2 and T1R3 single KO mice was not statistically significant in theT1R2/3 double KO mice.ThedoubleKOmice, however, still displayed significant concentration-dependent responding to Polycose in the first test session, albeit lick rates were slightly lower than those seen for WT mice, perhaps because small amounts of glucose, maltose,and maltotriose found in Polycose were enhancing the signal in WT mice or because T1R2 or T1R3 can possibly heteromerize with another protein to form afully functional glucose polymer receptor.These findings provide behavioral evidence that glucose polymers,with an optimal chain length greater than three glucose moieties, stimulate a taste receptor independent of the T1R2+3 heterodimer.
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