Multimodal responses of taste neurons in the frog nucleus tractus solitarius.

The responses of 216 neurons in the nucleus tractus solitarius (NTS) of the American bullfrog were recorded following taste, temperature, and tactile stimulation. Cells were classified on the basis of their responses to 5 taste stimuli: 0.5 M NaCl, 0.0005 M quinine-HCl (QHCl), 0.01 M acetic acid, 0.5 M sucrose, and deionized water (water). Neurons showing excitatory responses to 1, 2, 3, or 4 of the 5 kinds of taste stimuli were named Type I, II, III, or IV, respectively. Cells whose spontaneous rate was inhibited by taste and/or tactile stimulation of the tongue were termed Type V. Type VI neurons were excited by tactile stimulation alone. Of the 216 cells, 115 were excited or inhibited by taste stimuli (Types I-V), with 35 being Type I, 34 Type II, 40 Type III, 2 Type IV and 4 Type V. The remaining 101 cells were responsive only to tactile stimulation (Type VI). Of those 111 cells excited by taste stimulation (Types I-IV), 106 (95%) responded to NaCl, 66 (59%) to acetic acid, 44 (40%) to QHCl, 10 (9%) to water, and 9 (8%) to warming. No cells responded to sucrose. Of the 111 cells of Types I-IV, 76 (68%) were also sensitive to mechanical stimulation of the tongue. There was some differential distribution of these neuron types within the NTS, with more narrowly tuned cells (Type I) being located more dorsally in the nucleus than the more broadly tuned (Type III) neurons. Cells responding exclusively to touch (Type VI) were also more dorsally situated than those responding to two or more taste stimuli (Types II and III).