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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 2970-2988
Copyright ©1999 by the American Physiological Society
Department of Psychology, Florida State University, Tallahassee, Florida 32306-1270
Lundy Jr., Robert F. and
Robert J. Contreras.
Gustatory Neuron Types in Rat Geniculate Ganglion. J. Neurophysiol. 82: 2970-2988, 1999. We
used extracellular single-cell recording procedures to characterize the
chemical and thermal sensitivity of the rat geniculate ganglion to
lingual stimulation, and to examine the effects of specific ion
transport antagonists on salt transduction mechanisms. Hierarchical
cluster analysis of the responses from 73 single neurons to 3 salts
(0.075 and 0.3 M NaCl, KCl, and NH4 Cl), 0.5 M sucrose,
0.01 M HCl, and 0.02 M quinine HCl (QHCl) indicated 3 main groups that
responded best to either sucrose, HCl, or NaCl. Eight narrowly tuned
neurons were deemed sucrose-specialists and 33 broadly tuned neurons as
HCl-generalists. The NaCl group contained three identifiable
subclusters: 18 NaCl-specialists, 11 NaCl-generalists, and 3 QHCl-generalists. Sucrose- and NaCl-specialists responded specifically
to sucrose and NaCl, respectively. All generalist neurons responded to
salt, acid, and alkaloid stimuli to varying degree and order depending
on neuron type. Response order was NaCl > HCl = QHCl > sucrose in NaCl-generalists, HCl > NaCl > QHCl > sucrose in HCl-generalists, and QHCl = NaCl = HCl > sucrose in QHCl-generalists. NaCl-specialists responded robustly to low and high NaCl concentrations, but weakly, if at all, to high KCl and
NH4 Cl concentrations after prolonged stimulation.
HCl-generalist neurons responded to all three salts, but at twice the
rate to NH4 Cl than to NaCl and KCl. NaCl- and
QHCl-generalists responded equally to the three salts. Amiloride and
5-(N,N-dimethyl)-amiloride (DMA), antagonists of
Na+ channels and Na+/H+ exchangers,
respectively, inhibited the responses to 0.075 M NaCl only in
NaCl-specialist neurons. The K+ channel antagonist,
4-aminopyridine (4-AP), was without a suppressive effect on salt
responses, but, when applied alone in solution, it evoked a response in
many HCl-generalists and one QHCl-generalist neuron so tested. Of the
39 neurons tested for their sensitivity to temperature,
23 responded to cooling and chemical stimulation, and
20 of these neurons were HCl-generalists. Moreover, the responses to
the four standard stimuli were reduced progressively at lower temperatures in HCl- and QHCl-generalist neurons, but not in
NaCl-specialists. Thus sodium channels and
Na+/H+ exchangers appear to be expressed
exclusively on the membranes of receptor cells that synapse with
NaCl-specialist neurons. In addition, cooling sensitivity and
taste-temperature interactions appear to be prominent features of
broadly tuned neuron groups, particularly HCl-generalists. Taken all
together, it appears that lingual taste cells make specific connections
with afferent fibers that allow gustatory stimuli to be parceled into
different input pathways. In general, these neurons are organized
physiologically into specialist and generalist types. The sucrose- and
NaCl-specialists alone can provide sufficient information to
distinguish sucrose and NaCl from other stimuli, respectively.
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