1. Taste buds in catfish are found not only within the oropharyngeal cavity, as in mammals, but are also located along the external body surface of the animal from the barbels and lips to the caudal fin. Because these taste buds are innervated by the facial (cranial VII) nerve, the extraoral taste system of catfish is analogous to the mammalian taste system of the anterior two-thirds of the tongue, which contains taste buds innervated by the chorda tympani nerve, and of the soft palate and nasoincisor ducts, which contain taste buds innervated by the greater superficial petrosal nerve. 2. The majority of information concerning the specificity of individual taste fibers in vertebrates has been obtained primarily in mammals to stimuli representing the four basic human taste qualities (i.e., salty, sweet, sour, and bitter). In the present report, we examine the evidence for gustatory fiber types within the stimulus class of amino acids, compounds known to be especially relevant gustatory stimuli for catfish and other teleosts. 3. Action potentials were recorded from 60 individual facial taste neurons obtained from 28 sea catfish (Arius felis). Stimuli were 10(-4) M concentrations Of L-alanine, D-alanine, glycine, L-proline, L-histidine, and L-arginine, compounds selected from an original stimulus list of 28 amino acids. Responses were quantified as the number of action potentials evoked at various time intervals from the first 0.5 s up to 10 s of response time. 4. The spontaneous activity of 42 fully characterized neurons was 0.8 +/- 2.1 SD spikes/3 s. The average rate of spike discharge increased 50-fold during stimulation with the most effective amino acid (42 +/- 31 spikes/3 s, mean +/- SD). The majority of the sampled neurons were not narrowly tuned to the amino acid stimulants tested (mean breadth of responsiveness, H = 0.60; range 0-0.95). 5. Hierarchical cluster analysis of the fully characterized neurons identified two large and two small groups of cells. The largest group (n = 22) of neurons was stimulated most by L-alanine and glycine; the other large group (n = 17) was stimulated most by D-alanine. For this latter group, the response to glycine was relatively low, whereas the responses to L-alanine varied from 0 to nearly 100% of the D-alanine response. Although spontaneous activity of neurons within the two major groups was similar (average < 1 spike/3 s), the mean response of neurons within the L-alanine/ glycine group (63.8 +/- 33.9 action potentials/3 s, mean +/- SD) to its most effective stimulus was significantly greater than that for the neurons within the D-alanine group (39.6 +/- 16.9 action potentials/ 3 s, mean +/- SD). A small group comprised of two of the remaining three neurons was most stimulated by L-alanine and L-proline. The remaining neuron, which was united with the other neurons after a large cluster distance, was most stimulated by L-histidine. Neuron classification into specific groups was little influenced by three of the response times (0-0.5, 0-3, and 0-10 s) analyzed. 6. Principal components analysis identified four relatively distinct patterns of stimulus activation of the 42 fully characterized taste fibers by the six amino acid stimulants. D-alanine and L-proline activated patterns distinct from the other amino acids and each other. L-arginine and L-histidine elicited common patterns across the facial taste fibers tested but were distinct from those elicited by L-alanine and glycine. The stimulus activation patterns based on the responses of the 22 neurons in the L-alanine/glycine cluster were similar to those formed by all 42 neurons; however, the stimulus activation patterns based on the 17 neurons in the D-alanine cluster were different from the previous patterns.
