Taste responses in neurons in the nucleus of the solitary tract that do and do not project to the parabrachial pons. Monroe, Scott, and Patricia M.Di Lorenzo. Department of Psychology, P.O. Box 6000, SUNY at Binghamton, Binghamton, N.Y. 13902-6000.
APStracts 2:0061N, 1995.
SUMMARY AND CONCLUSIONS
(1) Mechanisms of neural coding of gustatory stimuli were studied in the nucleus of the solitary tract (NTS), the first relay in the neural pathway for gustation, in anesthetized rats. Taste-responsive NTS units were identified as "relay" or "non-relay" based on the electrophysiological response to electrical pulses delivered to the parabrachial nucleus of the pons (PbN), the second relay in the neural pathway for gustation. Coding mechanisms in each group were analyzed separately. (2) Taste responses to sapid solutions of NaCl (.1 M), HCl (.01 M), quinineHCl (.01 M), sucrose (.5 M) and Na-saccharin (.004 M) were recorded in single units in the NTS. Following gustatory stimulation, electrophysiological responses to electrical stimulation of the taste-responsive part of the ipsilateral PbN were recorded. A .2 msec pulse was delivered at 75-250 [mu]A at a rates of 1, 25, 50 and 100 pps through a bipolar stainless steel electrode. An antidromic response was defined as a time-locked spike that occurred at a fixed latency following PbN stimulation that followed high stimulation frequencies. A collision test was also performed. (3) Of 42 taste-responsive NTS units, 19 (45%) were relay units, 22 (52%) were non-relay and one unit was orthodromically activated by PbN stimulation. Latencies of evoked spikes ranged from 1.75 to 4.0 msec (mean, 2.1 msec +/- .2 s.e.m., median, 1.75 msec). (4) Examination of general response characteristics revealed few differences among relay and non-relay units. The mean response rates produced by taste stimuli were similar in relay and non-relay units, with the exception of saccharin which evoked a significantly larger response in relay units compared with non-relay units. Classification of units based on their best, i.e. most effective, stimulus showed that most units in both groups were either NaCl- or HCl-best. Relay units were most frequently HCl-best; non-relay units were most frequently NaCl-best. (5) Both relay and non-relay units were broadly responsive across the taste stimuli that were tested. Almost all units in both groups responded to more than one taste stimulus; however significantly more relay units responded to three or more of the representatives of the four basic taste qualities (sweet, sour, salty and bitter). The Uncertainty measure, an index of the breadth of tuning, was not different in relay and non-relay units. (6) The Labeled Line Index was also calculated to assess the extent to which pairwise comparisons of taste stimuli by the associated across unit patterns of response were accomplished by a labeled line code, where one separate subsets of units respond to each stimulus of a pair, or by a frequency code, where both stimuli evoke responses in the same subset of units, albeit with different magnitudes. Results suggested that pairwise comparisons of taste stimuli among relay units were characterized predominantly by labeled line types of codes but showed more evidence of frequency codes among non-relay units. (7) Across unit patterns of response were analyzed using a vector space analysis. In this approach, the response of each unit to a given stimulus is treated as a coordinate in n-dimensional space, where n is the number of units tested. The vector defined by the responses of the sample of units literally represents the across unit pattern evoked by a taste stimulus. When the relationships among these vectors were analyzed for NTS units, results suggested that, for relay units, the across unit patterns associated with palatable stimuli, i.e. NaCl and sucrose, were more similar to each other than they were to the across unit patterns produced by unpalatable stimuli, i.e. HCl and quinine. The converse was also true. In contrast to previous reports that proposed that the NTS preferentially conveys information about hedonically positive stimuli to the PbN, the present results suggest that the signal conveyed to the PbN from the NTS may enhance the differences between palatable and unpalatable stimuli to more rostral structures. 

Received 8 December 1993; accepted in final form 8 February 1995.
APS Manuscript Number J626-3.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.