Abstract
BackgroundPsychophysically, sweet and bitter have long been considered separate taste qualities, evident already to the newborn human. The identification of different receptors for sweet and bitter located on separate cells of the taste buds substantiated this separation. However, this finding leads to the next question: is bitter and sweet also kept separated in the next link from the taste buds, the fibers of the taste nerves? Previous studies in non-human primates, P. troglodytes, C. aethiops, M. mulatta, M. fascicularis and C. jacchus, suggest that the sweet and bitter taste qualities are linked to specific groups of fibers called S and Q fibers. In this study we apply a new sweet taste modifier, lactisole, commercially available as a suppressor of the sweetness of sugars on the human tongue, to test our hypothesis that sweet taste is conveyed in S fibers.ResultsWe first ascertained that lactisole exerted similar suppression of sweetness in M. fascicularis, as reported in humans, by recording their preference of sweeteners and non- sweeteners with and without lactisole in two-bottle tests. The addition of lactisole significantly diminished the preference for all sweeteners but had no effect on the intake of non-sweet compounds or the intake of water. We then recorded the response to the same taste stimuli in 40 single chorda tympani nerve fibers. Comparison between single fiber nerve responses to stimuli with and without lactisole showed that lactisole only suppressed the responses to sweeteners in S fibers. It had no effect on the responses to any other stimuli in all other taste fibers.ConclusionIn M. fascicularis, lactisole diminishes the attractiveness of compounds, which taste sweet to humans. This behavior is linked to activity of fibers in the S-cluster. Assuming that lactisole blocks the T1R3 monomer of the sweet taste receptor T1R2/R3, these results present further support for the hypothesis that S fibers convey taste from T1R2/R3 receptors, while the impulse activity in non-S fibers originates from other kinds of receptors. The absence of the effect of lactisole on the faint responses in some S fibers to other stimuli as well as the responses to sweet and non-sweet stimuli in non-S fibers suggest that these responses originate from other taste receptors.
Highlights
Sweet and bitter have long been considered separate taste qualities, evident already to the newborn human
A series of elegant studies in genetically modified mice show that sweet and umami tastes are dependent on T1Rreceptors, that bitter taste is caused by stimulation of T2R receptors, that these two receptors never are found in the same taste receptor cell (TRC) and that the TRC determines the behavioral response [1,2,3,4,5,6,7]
It identified that different taste fibers respond to different taste qualities and noted that the NG nerve contains a larger proportion of mechanosensitive fibers than the chorda tympani (CT)
Summary
Sweet and bitter have long been considered separate taste qualities, evident already to the newborn human. The above-mentioned discovery of a unique set of taste receptors for the sweet and bitter taste qualities has provided one answer to the long lasting question on how sweet or bitter taste is created on the tongue. The first suggestion that each of the human taste qualities is related to a particular type of taste fiber was based on recordings of the chorda tympani (CT) and glossopharyngeal (NG) nerves of cat [8] It was in many ways a seminal study and presented several observations that later studies confirmed. It correctly connected a lack of response to sucrose with the inability of cats to appreciate sucrose. The sweet sensitive taste fibers were later discovered in dog [10]
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