Convergence of sweet and umami in the mammalian taste bud

The sense of taste enables animals to identify nutrients and avoid harmful substances. Although the five basic taste qualities—sweet, umami, bitter, sour, and salty—are mediated by distinct molecular receptors, the extent and functional significance of receptor co-expression remain intensely debated. Here, we use in vivo two-photon calcium imaging in mouse taste buds to directly visualize how taste qualities are encoded by taste receptor cells (TRCs) and by innervating gustatory fiber terminals. We find that most TRCs are selectively activated by a single taste quality. However, a significant population of TRCs responds to both sweet and umami stimuli, consistent with extensive sweet-umami double tuning among gustatory ganglion neurons. Moreover, in situ hybridization reveals co-expression of sweet and umami receptors in TRCs from both mouse tongue and human tongue biopsies. Finally, experiments with conditioned taste aversion, a basic form of learned food avoidance, show that sweet and umami taste cross-generalize behaviorally. Together, our findings reveal significant convergence between sweet and umami taste coding both in the tongue and in the peripheral gustatory neurons.