A High Throughput In Vivo Assay for Taste Quality and Palatability

Abstract

Taste quality and palatability are two of the most important properties measured in the evaluation of taste stimuli. Human panels can report both aspects, but are of limited experimental flexibility and throughput capacity. Relatively efficient animal models for taste evaluation have been developed, but each of them is designed to measure either taste quality or palatability as independent experimental endpoints. We present here a new apparatus and method for high throughput quantification of both taste quality and palatability using rats in an operant taste discrimination paradigm. Cohorts of four rats were trained in a modified operant chamber to sample taste stimuli by licking solutions from a 96-well plate that moved in a randomized pattern beneath the chamber floor. As a rat’s tongue entered the well it disrupted a laser beam projecting across the top of the 96-well plate, consequently producing two retractable levers that operated a pellet dispenser. The taste of sucrose was associated with food reinforcement by presses on a sucrose-designated lever, whereas the taste of water and other basic tastes were associated with the alternative lever. Each disruption of the laser was counted as a lick. Using this procedure, rats were trained to discriminate 100 mM sucrose from water, quinine, citric acid, and NaCl with 90-100% accuracy. Palatability was determined by the number of licks per trial and, due to intermediate rates of licking for water, was quantifiable along the entire spectrum of appetitiveness to aversiveness. All 96 samples were evaluated within 90 minute test sessions with no evidence of desensitization or fatigue. The technology is capable of generating multiple concentration–response functions within a single session, is suitable for in vivo primary screening of tastant libraries, and potentially can be used to evaluate stimuli for any taste system.

A Pharmacological Perspective on the Study of Taste

“…The brief access assay also has been used to establish concentration-response functions for appetitive tastants such as sweeteners. Sucrose potencies determined from concentration-response functions for sucrose using Sprague-Dawley rats have been shown to range narrowly between 40 and approximately 150 mM (Grobe and Spector, 2008;Palmer et al, 2013;Hashimoto and Spector, 2014). This range is consistent with the only published concentration-response function for rTAS1R2/3 (Nelson et al, 2001) found for this review.…”

“…The heterodimeric functional receptor was responsive to a broad variety of compounds, across multiple chemical classes, reflective of the great variety of compounds that are detected as sweet tasting. Those cell lines expressing rat versions of the receptor were not responsive to the human sweeteners aspartame, cyclamate, and the protein thaumatin, consistent with the lack of sensitivity to these compounds by rodents in behavioral assays (Bachmanov et al, 2001;Palmer et al, 2013). This species-dependence of responsiveness to sweeteners has been exploited to identify likely agonist binding sites within the structure of the receptor (Jiang et al, 2004(Jiang et al, , 2005aZhang et al, 2010).…”

“…In operant taste discrimination, the animal reports the taste quality it detects through operation of manipulanda. The procedure usually requires first training the animal to perform an operant task such as foodreinforced lever presses (Morrison and Norrison, 1966;Morrison, 1969;Palmer et al, 2013) or water-reinforced spout licking (Swartzwelder et al, 1980;St John and Spector, 1998;Grobe and Spector, 2008). Then through additional training, an antecedent, specific taste cue becomes associated with reinforcement on one of two or more operant choices.…”

“…Once animals are fully trained, the distribution of responses from one operant choice to the other following a taste stimulus is taken as a direct indication of its similarity or disparity with the sensory qualities of the training cues. The paradigm can be used to test the organism's ability to discriminate one taste quality from another (Stapleton et al, 2002;Grobe and Spector, 2008;Palmer et al, 2013) or a taste stimulus from water (Mariotti and Fiore, 1980;Slotnick, 1982;Delay et al, 2004). In the latter case, the lowest concentration of a tastant that reliably can be discriminated from water is defined as the taste detection threshold.…”

“…With accommodation to the different statistical methods for their criterion, these threshold values appear consistent with the low ends of concentration-response function data obtained for sucrose from rat brief access assays and the cell-based rTAS1R2/R3 assay described above. In another taste discrimination method (Palmer et al, 2013), rats were trained to discriminate a single concentration of sucrose (100 mM) from water, NaCl, quinine, and citric acid randomly presented in a 96-well plate with at least 90% accuracy. Rather than licking from spouts, rats in this study sampled stimulus solutions by licking from individual wells of the 96-well plate.…”