Parabrachial gustatory neural activity during licking by rats

A total of 51 single neurons was recorded from the pontine parabrachial nuclei of three rats being given sapid stimuli either via intraoral infusions or during spontaneous licking behavior. In 46 neurons, sapid stimuli elicited significant taste responses; of these, 28 responded best to NaCl, 15 to sucrose, 2 to citric acid, and 1 to quinine HCl. The remaining five neurons responded significantly only to water. The mean spontaneous rate of taste neurons during the intraoral infusion and licking sessions was 11.1 ± 1.1 and 10.8 ± 1.2 (SE) spike/s, respectively. Of the 39 neurons tested during both licking and intraoral infusions, four responded significantly only to water via either route. The remaining 35 neurons responded significantly to at least some sapid stimuli. The best-stimulus categories remained the same regardless of the route of fluid delivery (24 NaCl best, 10 sucrose best, 1 citric acid best). When the rats were licking the stimuli, nine taste neurons responded significantly to only one sapid chemical [6 Na specific (Ns) and 3 sucrose specific (Ss)] but were more broadly tuned during intraoral infusions. Conversely, three taste neurons that responded specifically during intraoral infusions (3 Na specific) were not as specific when the animal licked the same fluids. Thirty-five taste neurons were tested via both stimulus routes. These data were compared in three ways. First, for each neuron, the responses elicited during licking and intraoral infusions were compared for each of the four standard sapid stimuli. The Pearson correlation coefficients for the 35 taste neurons ranged from 0.9997 to 0.6785, with a mean at 0.953 ± 0.012 (SE). The second comparison was between stimulus routes across chemicals. With the use of raw responses, the correlation coefficients for NaCl, sucrose, citric acid, and QHCl ranged from 0.925 to 0.778 (t test, P < 0.0001). With the activity elicited by water subtracted (corrected responses), the correlation coefficients for NaCl, sucrose, citric acid, and QHCl were 0.900, 0.795, 0.369, and 0.211, respectively. The coefficient for QHCl was not significant (t test, P > 0.05). Finally, the mean responses to NaCl, sucrose, and citric acid delivered by both routes were compared and found not to differ (paired t test, P > 0.05). In separate hierarchical cluster analyses for the licking and infusion data, the largest cluster in each contained all of the Na-best neurons and the next largest, all of the sucrose-best cells. Similarly, multidimensional scaling of the two data sets resulted in comparable two-dimensional distributions that accounted for similar percentages of variance. A subsample of the taste neurons was tested with a full or a partial concentration series. Two of these cells generally increased their activity with increasing sucrose concentration but did not respond to any concentration of either citric acid or QHCl. In the NaCl concentration series, these neurons responded up to 0.3 M but then failed to respond at 1.0 M. The responsiveness of one of these neurons was tested over a considerable period, and its response to the four sapid chemicals gradually decreased with repetitive stimulation. Of the 51 taste- or water-responsive neurons, the activity of 17 taste cells appeared to be associated with genioglossus electromyographic (EMG) activity. For 9 taste neurons out of 46 for which data was available, activity increased during the second 5-s epoch above the level during the first 5 s. Seven of these nine late responses were to citric acid; none to NaCl. During licking, the latency of taste responses varied depending on the stimulus. The average latencies for NaCl, sucrose, and citric acid were 54.4 ± 6.8, 420.8 ± 98.2, and 718 ± 316.9 (SE) ms, respectively.