Training and characterization of a quinine taste discrimination in rhesus monkeys

There is a limited amount of information available about taste as a discriminative stimulus in nonhuman primates. The objective of this study was to establish a bitter taste (quinine sulfate) as a cue for lever selection and food reward in rhesus monkeys. Training took place in a series of steps that culminated in a schedule in which five lip contacts on a spout produced either quinine solution or water, followed by an opportunity to earn a food pellet by completing 20 presses on one of two levers. Responses on one of the levers resulted in food delivery if the solution contained quinine; responses on the other lever resulted in food delivery if the solution was water. A single session consisted or 100 randomly ordered taste trials with a 60-s interval between each trial. All of the animals acquired the discrimination, and the lowest quinine concentration that maintained consistent behavior was 0.3 mg/ml. To assess the specificity of the discrimination, compounds from other human taste categories were tested. A series of compounds that are detected as 'bitter' by humans (caffeine, 1.5x10^-3 M; strychnine, 9x10^-4 M; PTC, 6x10^-5 M, denatonium benzoate, 2.24x10^-4 M; and urea, 3.0x10^-4 M) produced full generalization to the quinine sulfate discriminative stimulus, while 'sweet' (sucrose, 2.9x10^-2 M) and 'salty' (sodium chloride, 1.4 M) stimuli did not. There was individual variation among animals in response to 'sour' compounds; acetic acid did not generalize to quinine, but HCl acid produced full generalization in one of three animals. These results suggest that a 'bitter' taste cue is controlling the quinine discrimination.