Parabrachial neural coding of taste stimuli in awake rats

In awake, behaving rats, the activity of 74 single neurons in the pontine parabrachial nucleus (PEN) was recorded in response to sapid stimulation by 15 chemicals. Of these, 44 taste cells were tested with all 15 stimuli. Based on their responsiveness to 4 standard stimuli, these neurons were categorized as follows: 23 NaCl-best, 15 sucrose-best, 5 citric acid-best, and 1 quinine HCl-best. Several forms of multivariate analyses indicated that the taste responses matched both the behavioral responses to and, less well, the chemical structure of, the sapid stimuli. A hierarchical cluster analysis of the neurons substantially confirmed the best-stimulus categorization, but separated the NaCl-best cells into those that responded more to Na+-containing salts and those that responded more to Cl-containing salts. The cells that responded best to the Na+ moiety actually were somewhat more correlated with the sucrose-best cells than with those that responded to the Cl-containing stimuli. Citric acid-best neurons and the lone quinine-best unit formed a single cluster of neurons that responded well to acids, as well as to NH4Cl and, to a lesser extent, NaNO3. A factor analysis of the neuronal response profiles revealed that three factors accounted for 78.8% of the variance in the sample. Similar analyses of the stimuli suggested that PEN neurons respond to four or five sets of stimuli related by their chemical makeup or by human psychophysical reports. The capacity of rats to make these discriminations has been documented by other behavioral studies in which rodents generalize across sapid chemicals within each of 5 stimulus categories. Furthermore, a simulation analysis of the neural data replicated behavioral results that used amiloride, a Na+ channel blocker, in which rats generalized NaCl to non-Na+, Cl- salts. Thus, using a variety of analyses, in awake rats, the activity of PEN taste neurons tracks their behavioral responses to a variety of chemical stimuli.