Stochastic properties of gustatory impulse discharges in rat chorda tympani fibers

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Abstract
The role of temporal patterns of impulse discharges of the gustatory primary afferents in the coding of taste qualities was studied using simultaneous analyses of the across-neuron patterning (5) and the temporal patterning of impulses. Impulse discharges of 43 rat chorda tympani fibers elicited by gustatory stimulation (0.1 M NaCl, 0.5 M sucrose, 0.01 N HCl, 0.02 M quinine hydrochloride, 0.3 M KCl and 0.02 M saccharin sodium) were recorded. The temporal patterns of impulse discharges elicited by these stimuli were statistically analyzed both in an initial dynamic phase and a succeeding stationary phase of responses. In the dynamic phase, the time courses of firing frequency were studied; these were not clearly associated with the qualities of taste stimuli. In the stationary phase, the stochastic properties of impulse trains were studied regarding the impulse trains as a stochastic point process (30). Interspike interval histograms (ISI histograms), joint interval histograms, serial correlograms and autocorrelograms were computed. In general, the distribution patterns of ISI histogram were not absolutely specific to the taste qualities. The .gamma. or the exponential distribution occurred most often with 0.1 M NaCl, and the bimodal or the skewed distribution was dominated with 0.5 M sucrose stimulation among the stimuli tested. The rhythmic discharges did not occur exclusively with the sweet taste stimulation, though they occurred more often with the sweet taste stimulation than with other taste stimulations. The joint interval histogram and the serial correlogram were used to examine the stochastic properties of impulse trains with particular reference to the time dependence in the impulse trains. Generally, time dependence was not associated with taste qualities. The chorda tympani fibers were classified into 3 types according to the magnitude of responses and the temporal patterns of impulse discharges. The neurons predominantly responsive to NaCl and as responsive to sucrose were classified as type I neurons. The remaining neurons, which moderately responded to both NaCl and sucrose, were further categorized into type II neurons and type III neurons, depending on their temporal patterns of impulse discharge. The type II neurons tended to show the rhythmic or burstlike firing of impulses in response to more than 1 kind of taste stimuli. The taste quality was represented in the characteristic temporal patterning of impulses as well as in the across-neuron patterning of responses. The possible origin of the temporal discharge pattern and its significance in gustatory information processing were discussed on the basis of the neuron types described.