Corticofugal influence on taste responses in the nucleus of the solitary tract in the rat

1. Previous work has revealed a pervasive influence of the gustatory neocortex (GN) on the electrophysiological responses to taste in the parabrachial nucleus of the pons (PbN), the second synapse in the central pathway for gustation. Subsequent experiments have further suggested that direct projections from the GN to the PbN are not sufficiently dense to account for the widespread effects of cortical input. Because the main source of input to the PbN, i.e., the nucleus of the solitary tract (NTS), also receives input from the GN, the present experiment was conducted to test the hypothesis that changes in taste responses in the PbN after temporary elimination of GN input may be a normal reaction to altered input originating in the NTS. 2. Fourty-three taste-responsive neurons in the NTS were isolated initially in urethan-anesthetized rats. Single units were then classified as "relay" (n = 12) or "nonrelay" (n = 13) on the basis of their electrophysiological response to electrical shocks delivered to the taste-responsive portion of the PbN. After histological analyses, 18 units were classified as "unknown" because the PbN stimulating electrode was found to be outside the anatomically defined taste area in the pons. 3. Electrophysiological responses to sapid solutions of the NaCl (0.1 M), HCl (0.01 M), quinineHCl (0.01 M), sucrose (0.5 M), and Na-saccharin (0.004 M) were then recorded before and after recovery from infusions of procaineHCl into the GN. Both the ipsilateral and contralateral sides of the GN, in that order, received procaine infusions separated by a recovery period of at least 45 min. 4. Analysis of across-unit patterns of response was accomplished with the use of a vector space analysis. With this approach, the response of a given neuron to a given tastant is considered as a coordinate in n-dimensional space, where n is the number of neurons tested. The responses to each stimulus generate vectors whose length relates to the overall magnitude of response across the sample and whose relative directionality indicates similarity to other across-unit patterns. Measures derived from this type of analysis were used as input in a multidimensional scaling (MDS) analysis designed to summarize the organization of the across-unit patterns of response generated by the taste stimuli. This type of analysis creates a "taste space" in which similar across-unit patterns of response are placed close together and dissimilar patterns are placed far apart.(ABSTRACT TRUNCATED AT 400 WORDS)