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The Journal of Neurophysiology Vol. 84 No. 2 August 2000, pp. 628-638
Copyright ©2000 by the American Physiological Society
Department of Anatomy and Neurobiology and Program in Neuroscience, University of Maryland School of Medicine, Baltimore, Maryland 21201-1509
John, Steven J. St. and
David V. Smith.
Neural Representation of Salts in the Rat Solitary Nucleus:
Brain Stem Correlates of Taste Discrimination. J. Neurophysiol. 84: 628-638, 2000. One mechanism of salt taste
transduction by gustatory receptor cells involves the influx of cations
through epithelial sodium channels that can be blocked by oral
application of amiloride. A second mechanism is less clearly defined
but seems to depend on electroneutral diffusion of the salt through the
tight junctions between receptor cells; this paracellular pathway is
insensitive to amiloride. Because the first mechanism is more sensitive
to sodium salts and the second to nonsodium salts, these peripheral events could underlie the ability of rats to discriminate sodium from
nonsodium salts on the basis of taste. Behavioral experiments indicate
that amiloride, at concentrations that are tasteless to rats, impairs a
rat's ability to discriminate NaCl from KCl and may do so by making
both salts taste like KCl. In the present study, we examined the neural
representation of NaCl and KCl (0.05-0.2 M), and mixtures of these
salts with amiloride (0, 3, and 30 µM), to explore the neural
correlates of this behavioral result. NaCl and KCl were represented by
distinct patterns of activity in the nucleus of the solitary tract.
Amiloride, in a concentration-dependent manner, changed the pattern for
NaCl to one more characteristic of KCl, primarily by reducing activity
in neurons responding best to NaCl and sucrose. The effect of amiloride
concentration on the response to 0.1 M NaCl in NaCl-best neurons was
virtually identical to its effect on behavioral discrimination
performance. Modeling the effects of blocking the amiloride-insensitive
pathway also resulted in highly similar patterns of activity for NaCl and KCl. These results suggest that activity in both the
amiloride-sensitive and -insensitive pathways is required for the
behavioral discrimination between NaCl and KCl. In the context of
published behavioral data, the present results suggest that
amiloride-sensitive activity alone is not sufficient to impart a unique
signal for the taste of sodium salts.
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