Intensity coding in pontine taste area: gustatory information is processed similarly throughout rat's brain stem

Abstract

Rats acutely anesthetized with Nembutal were used in recording chemically evoked single neuron responses from the pontine taste area. Stimuli were 5 concentrations each of NaCl, HCl, sucrose and quinine HCl (QHCl). Pontine intensity-response functions were monotonic, and were best fitted by power functions with exponents near 0.2. Interstimulus correlation coefficients indicated that all taste qualities remained quite constant across their concentration range. A rate code is apparently an adequate descriptor for taste intensity in the pons, i.e., stimulus concentration is coded by the total activity evoked from the neural population. The responses of pontine taste cells can be accurately described as a linear function of peripheral and bulbar taste responses. The correlation between pontine and peripheral responses was 0.97 and between pontine and bulbar responses, 0.98. By contrast, pontine responses correlated only 0.55 with those evoked from the thalamic taste area. Thus, a major dichotomy exists between brain stem and forebrain coding for taste. Analysis of the pontine data implicates the toxicity of a chemical as a factor in determining its sensory code. Since toxicity is related to gustatory aversiveness, the pons may be involved in mediating the powerful hedonic qualities of taste stimuli.