Rats are unable to discriminate quinine from diverse bitter stimuli

Compounds described by humans as "bitter" are sensed by a family of type 2 taste receptors (T2Rs). Previous work suggested that diverse bitter stimuli activate distinct receptors, which might allow for perceptually distinct tastes. Alternatively, it has been shown that multiple T2Rs are expressed on the same taste cell, leading to the contrary suggestion that these stimuli produce a unitary perception. Behavioral work done to address this in rodent models is limited to Spector and Kopka (Spector AC, Kopka SL. J Neurosci 22: 1937-1941, 2002), who demonstrated that rats cannot discriminate quinine from denatonium. Supporting this finding, it has been shown that quinine and denatonium activate overlapping T2Rs and neurons in both the mouse and rat nucleus of the solitary tract (NTS). However, cycloheximide and 6-n-propylthiouracil (PROP) do not appear to overlap with quinine in the NTS, suggesting that these stimuli may be discriminable from quinine and the denatonium/quinine comparison is not generalizable. Using the same procedure as Spector and Kopka, we tasked animals with discriminating a range of stimuli (denatonium, cycloheximide, PROP, and sucrose octaacetate) from quinine. We replicated and expanded the findings of Spector and Kopka; rats could not discriminate quinine from denatonium, cycloheximide, or PROP. Rats showed a very weak ability to discriminate between quinine and sucrose octaacetate. All animals succeeded in discriminating quinine from KCl, demonstrating they were capable of the task. These data suggest that rats cannot discriminate this suite of stimuli, although they appear distinct by physiological measures.