A novel psychophysical procedure for bitter taste assessment in rats

A persistent problem with attempts to examine bitter taste mechanisms has been the lack of adequate behavioral methodology providing data which parallels that obtained from physiological investigations. We developed a brief contact procedure to assess the ability of rats to detect the presence of a weak bitter compound dissolved in a strong sucrose solution. Male Fischer 344 rats were trained to drink immediately to multiple 10-s presentations of acetaminophen (2, 8, 32, 128 mM), chlorpheniramine maleate (1, 3, 9, 27 mM) L-tryptophan (13.5, 27, 54, 108 mM), pseudoephedrine hydrochloride (1, 4, 16, 64 mM) and quinine hydrochloride (0.008, 0.04, 0.2, 1.0 mM) dissolved in 0.8 M sucrose. The number of licks to sucrose and water were also measured. A microcomputer controlled stimulus presentations and measured the animal's licks of each solution during each 10-s presentation. The responses to the bitter+sucrose mixture were significantly decreased at most concentrations with increasing levels of the bitter component. This was true for all five bitter-tasting compounds, but over different concentration ranges relatively unique to each compound. The present study is the first to characterize the sensory effects of acetaminophen, pseudoephedrine, and chlorpheniramine maleate, all purported to taste bitter to humans. These results demonstrate rats' acute ability to discriminate by taste not only the presence but the concentration of a dilute bitter compound dissolved in a strong sucrose solution.