A high throughput in vivo assay for taste quality and palatability.

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

Effect of Alloxan on Sucrose Concentration–response Function.

<p>Each concentration of sucrose plus alloxan was dispensed in 3 wells, and of sucrose alone in 2 wells, per plate; controls were dispensed in 4-6 wells each per plate (See Figure S7 for plate configurations corresponding to the experiment illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072391#pone-0072391-g008" target="_blank">Figure 8</a>). Upper panel shows sweet taste quality plotted as percent of responses made on the sucrose lever. Lower panel shows concentration–response function for palatability plotted as mean licks per trial. Data are plotted as mean of responses averaged across 4 rats. Error bars are SEM. SUC = sucrose, ALOX = alloxan, CIT = citrate, QUI = quinine. Data are representative of 3 equivalent experiments.</p

Concentration–response Functions are Stable Across Tests Following Completion of Discrimination Training.

<p>Upper graphs in each panel show sweet taste quality plotted as percent of responses made on the sucrose lever. Lower panels show concentration–response functions for palatability plotted as mean licks per trial. Data are plotted as mean of responses to contents from 4 wells for sucrose concentration range and 12-14 wells for controls per rat, averaged across 4 rats. Error bars are SEM. Open symbols represent values obtained for control tastants: ○ = 100 mM sucrose; ▽ = water; □ = 100 mM NaCl; △ = 10 mM citric acid; ◇ = 1 mM quinine. Closed symbols = test article: ● = sucrose. Results in each panel are of single experiments performed in sequential weekly intervals. See Figure S4 for plate configuration corresponding to the experiment illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072391#pone-0072391-g003" target="_blank">Figure 3</a>.</p

Effect of ZnSO₄ on Sucrose Concentration–response Function.

<p>Each concentration of sucrose plus ZnSO₄ was dispensed in 3 wells, and of sucrose alone in 2 wells, per plate; controls were dispensed in 4-6 wells each per plate (See Figure S7 for plate configuration corresponding to the experiment illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072391#pone-0072391-g009" target="_blank">Figure 9</a>). Upper panel shows sweet taste quality plotted as percent of responses made on the sucrose lever. Lower panel shows concentration–response function for palatability plotted as mean licks per trial. Data are plotted as mean of responses averaged across 4 rats. Error bars are SEM. SUC = sucrose, CIT = citrate, QUI = quinine. Data are representative of 3 equivalent experiments.</p

Concentration–response Functions for Salt and Umami Tastes.

<p>Upper graphs in each panel show taste quality plotted as percent of responses made on the standard-appropriate lever. Lower panels show concentration–response functions for palatability plotted as mean licks per trial. Data are plotted as mean of responses to contents from 4 wells per concentration of test article and 10-14 wells for controls per rat, averaged across 4 rats. Error bars are SEM. <b>A</b>: Rats were trained to discriminate the taste of 100 mM NaCl (○) from water (▽), 100 mM sucrose (□), 10 mM citric acid (△), and 1 mM quinine (◇). Test article: ● = NaCl. <b>B</b>: Rats were trained to discriminate the taste of 100 mM MSG+100 µM amiloride (○) from water (▽), 100 mM sucrose (*), 100 mM NaCl (□), 10 mM citric acid (△) and 1 mM quinine (◇). Test article: ● = MSG + amiloride. See Figure S8 for plate configurations corresponding to the experiment illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072391#pone-0072391-g010" target="_blank">Figure 10A</a>; Figure S9 corresponds to the experiment of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072391#pone-0072391-g010" target="_blank">Figure 10B</a>.</p

Function Relating Licks to Consumption.

<p>Licks recorded during individual trials of water and 100 mM sucrose over 3 sessions (12 each of water and sucrose per session, total of 36 water and sucrose trials) are plotted as a function of volume consumed on each trial. Shown are the data obtained from 2 rats tested on 3 consecutive days.</p