Facial taste responses of the channel catfish to binary mixtures of amino acids

We investigated the neural processing of binary gustatory mixtures of amino acids by the facial taste system of the channel catfish, Ictalurus punctat us. In vivo electrophysiological recordings indicated that the magnitude of both integrated and single-unit facial taste responses to binary mixtures of amino acids was greatest if the components bound to independent receptor sites. Facial taste responses were obtained from 32 multiunit and 55 singl e taste fiber preparations to binary mixtures of amino acids whose componen ts bind to independent taste receptor sites (group I) or to the same or hig hly cross-reactive taste receptor sites (group II). All component stimuli w ere adjusted in concentration to provide approximately equal response magni tude as determined by either the height of the integrated multiunit taste r esponse or by the number of action potentials generated/3 s of response tim e/single taste fiber. The mixture discrimination index (MDI), defined as th e response to the mixture divided by the average of the responses to the co mponent stimuli, was calculated for each test of a binary mixture. MDIs of group I binary mixtures for both the integrated multiunit and single fiber data were significantly greater than those for either the control or group II binary mixtures. In a subset of multiunit recordings, the MDIs of a grou p I binary mixture across three log units of stimulus concentration were si milar and significantly greater than those of a group II binary mixture. An alysis of the single fiber data also indicated that the MDIs of group I bin ary mixtures were significantly larger than those of group II binary mixtur es for both alanine-best and arginine-best taste fibers; however, the MDIs of group I binary mixtures calculated from recordings from arginine-best ta ste fibers were significantly greater than those recorded from alanine-best taste fibers.