Taste in chimpanzees. III: Labeled-line coding in sweet taste.
In peripheral taste the coding mechanism remains an enigma. Among coding theories the "across-fiber pattern" argues that activity across fibers codes for taste, whereas the "labeled line" claims that activity in a particular set of fibers underlies a taste quality. We showed previously that chimpanzee chorda tympani taste fibers grouped according to human taste qualities into an S-cluster, responding predominantly to sweet stimuli, a Q-cluster, sensitive to bitter tastants, and an N-cluster, stimulated by salts. The analysis showed that information in the S-line suffices to distinguish stimuli of one taste quality from the others. However, one condition for the labeled line remained: that blockage of activity in a particular line must cause blockage of one taste quality, but of no other, or its onset give rise to the sensation of a taste quality. Here we studied this requirement with gymnemic acids and miraculin. In humans and chimpanzees, gymnemic acids suppress the sweet taste of all sweeteners whereas miraculin adds a sweet taste quality to sour stimuli. Gymnemic acids also abolish miraculin-induced sweet taste. We found that gymnemic acids practically abolished the response to every sweetener in the chimpanzee S-cluster. Equally important, they had no effect on the responses of the Q- and N-clusters. After miraculin, the S-cluster fibers responded to acids as well as to sweeteners, although they had not responded to acids before miraculin. Gymnemic acids abolished this miraculin-induced response to acids and responses to sweeteners in the S-fibers. These results link the sweet taste quality to activity in fibers of the S-cluster. Thus the S-cluster fibers satisfy the definition of the labeled-line theory: "that activity in a particular fiber type represents a specific taste quality."
Hellekant
G
G
The University of Wisconsin and Wisconsin Regional Primate Center, Madison 53706, USA. GH@ahabs.wisc.edu
Ninomiya
Y
Y
Danilova
V
V
D000818:Animals; D002814:Chorda Tympani Nerve / Q000187:drug effects / Q000502:physiology*; D005260:Female; D006023:Glycoproteins / Q000494:pharmacology; D009412:Nerve Fibers / Q000187:drug effects / Q000502:physiology*; D002679:Pan troglodytes / Q000502:physiology*; D012503:Saponins; D013549:Sweetening Agents / Q000494:pharmacology; D013649:Taste / Q000187:drug effects / Q000502:physiology*; D014315:Triterpenes / Q000494:pharmacology
D006023:Glycoproteins; D012503:Saponins; D013549:Sweetening Agents; D014315:Triterpenes; C008533:miraculin protein, Synsepalum dulcificum; C007898:gymnemic acid