Orosensory detection of sucrose, maltose, and glucose is severely impaired in mice lacking T1R2 or T1R3, but Polycose sensitivity remains relatively normal

Abstract

Evidence in the literature supports the hypothesis that the T1R2+3 heterodimer binds to compounds that humans describe as sweet. Here, we assessed the necessity of the T1R2 and T1R3 subunits in the maintenance of normal taste sensitivity to carbohydrate stimuli. We trained and tested water-restricted T1R2 knockout (KO), T1R3 KO and their wild-type (WT) same-sex littermate controls in a two-response operant procedure to sample a fluid and differentially respond on the basis of whether the stimulus was water or a tastant. Correct responses were reinforced with water and incorrect responses were punished with a time-out. Testing was conducted with a modified descending method of limits procedure across daily 25-min sessions. Both KO groups displayed severely impaired performance and markedly decreased sensitivity when required to discriminate water from sucrose, glucose, or maltose. In contrast, when Polycose was tested, KO mice had normal EC(50) values for their psychometric functions, with some slight, but significant, impairment in performance. Sensitivity to NaCl did not differ between these mice and their WT controls. Our findings support the view that the T1R2+3 heterodimer is the principal receptor that mediates taste detection of natural sweeteners, but not of all carbohydrate stimuli. The combined presence of T1R2 and T1R3 appears unnecessary for the maintenance of relatively normal sensitivity to Polycose, at least in this task. Some detectability of sugars at high concentrations might be mediated by the putative polysaccharide taste receptor, the remaining T1R subunit forming either a homodimer or heteromer with another protein(s), or nontaste orosensory cues.

Ingestion of bacterial lipopolysaccharide inhibits peripheral taste responses to sucrose in mice

“…The maintenance of normal CT responses to polycose after LPS ingestion is anticipated. Aversion to sucrose in rats is not generalized to polycose, which is transduced independently of Tas1r2/3 (Zukerman et al, 2009, Treesukosol and Spector, 2012). The maintenance of normal glucose, saccharin and aceK responses were more surprising given the significant decrease—though not elimination—in Tas1r2/3 expression.…”

Taste buds: cells, signals and synapses

“…However, rodents show a strong appetitive preference for solutions containing smaller polymers such as Polycose 35 . Intriguingly, mice in which Tas1r3 and Tas1r2 have been knocked out can still detect Polycose 36, 37 . Given the nutritional importance of carbohydrates in most animals, the detection of sugars by glucose transporters and KATP channels may reflect a parallel mechanism for ensuring caloric sufficiency.…”

Perceptual and Neural Responses to Sweet Taste in Humans and Rodents

“…Slopes for least-squares fits (solid lines) to activity to 0.1, 0.17, 0.31, and 0.56 M sucrose measured at 18°, 22°, 30°, and 37°C are as follows: 1.44, 0.94, 0.39, and 0.50. Dashed lines extend fits for comparison against activity to 0.05 M, a near-threshold concentration of sucrose (Treesukosol and Spector 2012). Reprinted from Wilson and Lemon (2014) with permission from the publisher.…”