Conditioned suppression as a method of detecting taste thresholds in the rat

Taste thresholds of seven male Sprague—Dawley rats (meanage 10 weeks, mean weight 250 g) were determined for four basictaste qualities: sweet, sour, salty and bitter. The method ofconditioned suppression was employed. An apparatus capable ofpresenting any one of eight separate drinking tubes during atesting session was designed. Animals were reduced to 80–85%ad lib. body weight. They were then trained to lick a sippertube through a slot in the back of an experimental chamber forpellet reinforcements. Animals progressed through a series ofreinforcement schedules starting with a fixed ratio (FR) scheduleof five licks for each reinforcement. They advanced to a variableratio (VR) schedule of reinforcement and finally a variableinterval (VI) schedule with a mean of 17.5 s was used. Whileon the VI schedule animals were trained to suppress lickingwhen any tastant other than water was presented. The first lickon any tastant was followed 10 s later by a mild electric shockif a rat made more than 20 licks on the tube in the 10-s period.Less than 20 licks on a tastant tube resulted in no shock anda 5-s time out before proceeding to the next tube. Thresholdwas determined using a suppression ratio formula. Thresholdwas defined as the 0.33 suppression ratio. Results from thisexperiment reveal mean thresholds for the seven animals as:sucrose = 2.3 mM, NaCl = 0.63 mM, quinine HCl = 0.005 mM andcitric acid = 0.085 mM.