Corticofugal influence on taste responses in the nucleus of the solitary
tract in the rat.
Di Lorenzo, Patricia M., and Scott Monroe.
Department of Psychology, P.O. Box 6000, SUNY at Binghamton, Binghamton,
N.Y. 13902-6000.
APStracts 2:0048N, 1995.
SUMMARY AND CONCLUSIONS
(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 following temporary
elimination of GN input may be a normal reaction to altered input originating
in the NTS. (2) 43 taste-responsive neurons in the NTS were isolated initially
in urethane-anesthetized rats. Single units were then classified as
"relay" (n=12) or "non-relay" (n=13) based on their
electrophysiological response to electrical shocks delivered to the taste-
responsive portion of the PbN. Following 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 (.1 M), HCl
(.01 M), quinineHCl (.01 M), sucrose (.5 M) and Na-saccharin (.004 M) were
then recorded before, after, and following 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 using 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. (5) The influence of
procaine infusions into the GN on NTS units was evidenced by changes in the
responses to a subset of effective taste stimuli within each unit. In some
units, the ipsilateral and contralateral GN procaine infusions influenced
responses to different stimuli. Moreover, there were occasions when the
response to one stimulus was enhanced and the response to another stimulus was
attenuated within the same unit. (6) Of 30 NTS units that were held through
both the ipsilateral and contralateral GN procaine infusions, 22 (73%) were
affected by infusions on at least on side of the brain. Fourteen (47% of
total) of these were affected by procaine infusions into both sides of the GN.
(7) Although the effects of ipsilateral and contralateral GN procaine
infusions were similar in some ways, e.g., both infusions had the effect of
decreasing the number of taste stimuli to which a unit responded and both
infusions most frequently resulted in attenuated responses, the elimination of
each side of the GN resulted in unique effects on the taste response in the
NTS. Results of several analyses point to the conclusion that, although the
ipsilateral GN may affect the responses to each tastant independently, the
most profound effect is on the responses to the sweet stimuli, sucrose and
saccharin. In contrast, the contralateral GN may be more concerned with
general distinctions between palatable and unpalatable stimuli. (8) Two points
can be made concerning differences between relay and non-relay NTS units: i.)
The most profound effects of the elimination of GN input are seen in the
responses to sweet stimuli in non-relay units. Since it is predominantly the
non-relay units in the NTS that showed an altered sucrose response pattern
following procaine infusions into the GN, it is unlikely that the change in
this pattern in the PbN is mediated through the NTS. Rather these data suggest
that the GN may have a common effect on both the NTS and the PbN. ii.) It is
likely that at least some differences that have been reported to exist between
NTS-PbN relay and non-relay units may be accounted for by corticofugal
influence. (9) The capability of the cortex to single out and modify the NTS
and PbN response patterns for particular tastants suggests that the GN may act
like a cognitive filter. That is, the cortex may select incoming signals,
based, for example on their behavioral or physiological significance, and then
feed back onto lower structures. In the intact animal, this type of descending
influence may be used to enhance behavioral responses to taste stimuli.
Received 8 December 1993; accepted in final form 8 February 1995.
APS Manuscript Number J627-3.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.