Chem. Senses 27: 81-90,
2002
© Oxford University Press 2002
Gustatory Projections from the Nucleus of the Solitary Tract to the Parabrachial Nuclei in the Hamster
Department of Anatomy and Neurobiology and Program in Neuroscience, University of Maryland School of Medicine, Baltimore, MD 21201, USA
Correspondence to be sent to: David V. Smith, Department of Anatomy and Neurobiology, University of Maryland School of Medicine, 685 West Baltimore Street, Baltimore, MD 21201-1509, USA. e-mail: dvsmith{at}umaryland.edu
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Abstract |
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Taste-responsive cells in the nucleus of the solitary tract (NST) either project to the parabrachial nuclei (PbN) of the pons, through which taste information is transmitted to forebrain gustatory nuclei, or give rise to axons terminating locally within the medulla. Numerous anatomical studies clearly demonstrate a substantial projection from the rostral NST, where most taste-responsive cells are found, to the PbN. In contrast, previous electrophysiological studies in the rat have shown that only a small proportion (21-45%) of taste-responsive NST cells are antidromically activated from the PbN, suggesting that less than half the cells recorded from the NST are actually involved in forebrain processing of gustatory information. In the present experiment we investigated the projections from the NST to the PbN electrophysiologically in urethane anesthetized hamsters. Responses of 101 single neurons in the rostral NST were recorded extracellularly following lingual stimulation with 32 mM NaCl, sucrose and quinine hydrochloride (QHCl) and 3.2 mM citric acid. The taste-responsive region of the PbN was identified electrophysiologically and stimulated with a concentric bipolar electrode to antidromically activate each NST cell. Of the 101 taste-responsive NST cells, 81 (80.2%) were antidromically activated from the ipsilateral PbN. The mean firing rates to taste stimulation and the spontaneous activity of these projection neurons were significantly greater than those of non-projecting cells. Every sucrose-best neuron in the sample projected to the PbN. The mean conduction velocity of the 23 QHCl-best neurons was significantly lower than that of the other 58 PbN projection neurons, suggesting that the most QHCl-responsive cells are a subset of smaller neurons. These data show that a large majority of NST cells responsive to taste stimulation of the anterior tongue project to the gustatory subdivisions of the PbN and that these cells have the most robust responses to gustatory stimulation.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second central relays in the rodent taste pathway, respectively (Norgren and Leonard, 1971
; Norgren,
1978; Travers,
1988; Whitehead,
1990). Taste-responsive cells in the NST either project to the PbN
to transfer taste information rostrally or give rise to axons terminating
within the medulla (Norgren and Leonard,
1971; Norgren,
1978; Travers,
1988; Whitehead,
1990; Beckman and Whitehead,
1991; Halsell et al.,
1996). The NST consists of several subdivisions that differ in
their afferent and efferent connections and their cytoarchitectural features
(Davis and Jang, 1986;
Travers, 1988; Whitehead,
1988,
1990;
Halsell et al.,
1996). The rostral central and rostral lateral subnuclei in the
NST receive input from the oropharyngeal cavity via the chorda tympani and
greater superficial petrosal branches of the VIIth nerve and the
lingual-tonsillar branch of the IXth nerve
(Whitehead and Frank, 1983;
Hamilton and Norgren, 1984;
Brining and Smith, 1996). Most
taste-responsive neurons are found in these two subnuclei
(McPheeters et al.,
1990), from which neurons send axons to the pons to terminate
largely in the caudal and ventral portions of the medial subdivision of the
ipsilateral PbN (Herbert et al.,
1990; Whitehead,
1990). This area corresponds to the region of the PbN where
taste-responsive cells have been recorded
(Norgren and Pfaffmann, 1975;
Van Buskirk and Smith, 1981).
In contrast, neurons projecting to the reticular formation or the caudal NST
have their origins mostly in the ventral and medial subdivisions of the
rostral NST (Travers, 1988;
Halsell et al.,
1996).
Whereas connections within the central taste pathway have been extensively
explored anatomically, there are fewer physiological studies of these
projections. Ogawa and colleagues (Ogawa et al.,
1980,
1984;
Ogawa and Kaisaku, 1982) found
that only 21% of NST neurons that responded to electrical stimulation of
gustatory nerves and 31-34% of cells that responded to oral gustatory
stimulation were antidromically activated by stimulation of the PbN in the
rat. A larger percentage (45%) was reported in an investigation by Monroe and
Di Lorenzo (Monroe and Di Lorenzo,
1995), although it still comprised less than half of the
taste-responsive neurons in their sample. In contrast to these
electrophysiological results, Halsell et al.
(Halsell et al.,
1996) reported that more cells in the rostral NST of the rat send
axons to the PbN (67%) than to the reticular formation (33%). Thus the
percentage of PbN projection cells in previous electrophysiological studies
was likely underestimated. In these earlier experiments the PbN stimulating
sites were determined solely on the basis of stereotaxic coordinates. In the
present study we placed the stimulating electrode under electrophysiological
guidance to ensure that it was positioned within the taste-responsive region
of the PbN.
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Materials and methods |
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Animals and surgery
Seventy-three male Syrian golden hamsters (Mesocricetus auratus) weighing 150-250 g were used in this experiment. Animals were deeply anesthetized with urethane (1.7 g/kg i.p.) and a cannula was inserted into the trachea to aid breathing. The animals were placed in a non-traumatic head holder with the head angled nose downward at 27° from the horizontal to minimize movement of the brainstem. The animal's body temperature was maintained at 37°C with a heating pad. The muscle over the occipital plate was cut along the midline and separated and a portion of the skull and dura was removed. The posterior portion of the cerebellum was aspirated to expose the floor of the IVth ventricle for 4-5 mm anterior to the obex, allowing direct access to the NST and PbN.
The concentric PbN stimulating/recording electrode was constructed by
inserting an Epoxylite-insulated 33 gauge stainless steel tube into a 27 gauge
stainless steel tube, the two being cemented together with Epoxylite 6001
(Epoxylite Corp., Irvine, CA). The inner tubing protruded 
500 µm from
the outer barrel and was exposed at its tip for 200 µm. The outer
tubing was exposed concentrically for 150 µm. A 75 µm diameter
tungsten microelectrode (Frederick Haer & Co., Bowdoinham, ME) was
inserted through the inner barrel of the stimulating electrode, its tip
protruding 1 mm from the tip of the inner barrel. This combination
stimulating/recording electrode was initially positioned 4.0 mm rostral
and 1.4 mm lateral to the obex to search for the taste-responsive region of
the PbN (Van Buskirk and Smith,
1981). The microelectrode was lowered slowly into the pons to the
depth at which the strongest neuronal activity was recorded in response to
anodal current (50 µA, 0.5 s, 0.33 Hz) applied to the anterior tongue,
which drives taste fibers of the chorda tympani (CT) nerve
(Smith and Bealer, 1975). At
that point the PbN electrode was lowered an additional 1 mm to position the
stimulating electrode in the most taste-responsive area. It was then fixed to
the adjacent skull with dental cement.
NST recording and taste stimulation
The taste responsiveness of a cell was initially determined by a change in
neural activity associated with the application of anodal current pulses (50
µA, 0.5 s, 0.33 Hz) applied to the anterior tongue
(Smith and Bealer, 1975).
Action potentials were recorded from 101 taste-responsive NST cells with glass
micropipettes (tip diameter 2 µm, resistance 7-10 M
) filled with a
2% (w/v) solution of Chicago Blue dye in 0.5 M sodium acetate. Cells
responsive to lingual stimulation were encountered from 0.5 to 1.1 mm below
the surface of the brain stem, with mean coordinates relative to the obex of
2.06 ± 0.09 mm anterior and 1.31 ± 0.09 mm lateral. Action
potentials of single cells were isolated, displayed on oscilloscopes and
monitored with an audio monitor. Extracellular potentials were differentially
amplified (Bak MDA-41) and discriminated with a dual time—amplitude
window discriminator (Bak DDIS-1). The amplified action potentials were
counted online using a Pentium computer, configured with a CED 1401
plus interface board and Spike2 software (Cambridge Electronic
Design, Cambridge, UK).
A taste profile for each cell was established in response to four taste
solutions, 32 mM sucrose, NaCl and quinine hydrochloride (QHCl) and 3.2 mM
citric acid, applied to the anterior portion of the tongue. These
concentrations evoke roughly equal multi-unit responses in the hamster NST
(Duncan and Smith, 1992). The
taste solutions were delivered by a gravity flow system composed of a two-way
solenoid-operated valve connected via tubing to a distilled water rinse
reservoir and a stimulus reservoir. The stimulation sequence, during which
data were accumulated, was a continuous flow initiated by the delivery of 5 s
of distilled water, followed by 10 s of stimulus and then by 5 s of distilled
water. The flow rate was 2 ml/s. Following each taste stimulus, the tongue was
rinsed with distilled water (50 ml) and individual stimulations were separated
by at least 2 min to avoid adaptation
(Smith and Bealer, 1976). Each
cell was categorized as responding best to sucrose, NaCl, citric acid or QHCl
on the basis of its response profile (see
Frank, 1973).
Classification of PbN projection and non-projection neurons
To test each NST cell for antidromic invasion, rectangular pulses (0.5 ms,
<120 µA) were delivered to the taste-responsive area of the PbN. Three
criteria defined antridomic activation
(Iggo, 1958). First, the
action potentials of a projection neuron should be evoked at a constant
latency. Second, the projection neuron should be able to follow paired pulse
stimulation at >200 Hz. Finally, a collision test was conducted between
spontaneous and stimulus-evoked action potentials. If a taste-responsive NST
cell failed to meet any one of these criteria, it was classified as a
non-projection neuron. For each PbN projection neuron the threshold of PbN
stimulation was defined as the lowest stimulus intensity that would produce an
antidromic action potential on five consecutive trials. Both antidromic
threshold and latency were measured for each cell.
At the end of each experiment the recording site in the NST was marked with Chicago Blue dye by passing a 10 µA cathodal current through the recording electrode for 10 min. The stimulating site in the PbN was evident from tissue damage produced by the stimulating electrode. The linear distance between the stimulating site in the PbN and recording site in the NST was measured to estimate the conduction velocity of each PbN projection neuron. The hamster was then given an overdose of urethane and perfused through the heart with 4% formaldehyde containing 3% potassium ferrocyanide and ferricyanide. Following removal of the brain, 40 µm frozen sections were cut in the coronal plane and stained with Neutral Red.
Data analysis
Responses to taste stimuli were quantified by subtracting the 5 s pre-stimulus baseline from the first 5 s of the evoked response to yield a net response (impulses/5 s). Responses are reported as means ± SEM. Differences in mean firing rates between projection and non-projection neurons, among taste stimuli and in conduction velocities were compared using ANOVA.
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Results |
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Histology
The recording and stimulating sites were examined histologically. A
recording site in the NST is shown in
Figure 1A. This cell was a
QHCl-best PbN projection neuron, located medial to the solitary tract, most
likely in the rostral central subdivision. Most recording sites were found
near the level of the NST where the dorsal cochlear nucleus (DC) is first
apparent on the dorsolateral margins of the medulla. This region of the NST
receives its predominant gustatory input from the VIIth nerve
(Whitehead and Frank, 1983;
Whitehead, 1988). We were
unable to unambiguously assign each recorded cell to a nuclear subdivision,
although all of the recorded cells appeared to be in the region of the NST
corresponding to the rostral central or rostral lateral subdivisions, where
the majority of PbN projecting neurons have been localized anatomically
(Whitehead, 1990).
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An example of the tissue damage induced by the stimulating/recording
electrode in the PbN is shown in Figure
1B. The electrode was positioned ventral to the superior
cerebellar peduncle (scp) within the medial PbN, fairly caudal within the
parabrachial nuclear complex. This area was highly responsive to anodal
stimulation of the tongue (see Materials and methods) and has previously been
shown to contain taste-responsive neurons
(Van Buskirk and Smith, 1981;
Halsell and Frank, 1991) and
to receive the majority of axons from the taste-responsive portions of the NST
(Whitehead, 1990). The mean
coordinates of the stimulating sites were 4.08 ± 0.12 mm anterior to
the obex and 1.48 ± 0.11 mm lateral to the midline. The positions of 28
of the stimulating electrodes are depicted in
Figure 1C on a standard drawing
of the hamster brain (Morin and Wood,
2001), showing them to be distributed within the medial
parabrachial nucleus (MPB). Although not all of the sites were histologically
reconstructed, every stimulating electrode was placed in a taste-responsive
site in the PbN (see Materials and methods).
Ascending projection from the NST to the gustatory region of the PbN
A total of 101 taste-responsive cells were recorded extracellularly from the NST of 73 hamsters. To determine the projection status of an NST cell, we tested whether it was antidromically invaded from the gustatory portion of the ipsilateral PbN. Electrical stimulation of the PbN gustatory area produced one of three effects in NST taste-responsive neurons: each cell discharged a spike either antidromically or orthodromically or showed no response to PbN stimulation.
Among these 101 NST neurons, 81 (80.2%) were driven antidromically by
ipsilateral PbN stimulation and classified as PbN projection neurons. These
projection neurons showed constant latencies to PbN stimulation. We measured
the threshold of the PbN stimulation required to evoke an antidromic response
and the antidromic latency for every PbN projection cell. The mean (±
SEM) antidromic threshold was 48.33 ± 5.54 µA and the mean latency
was 4.14 ± 0.41 ms. An electrical stimulus of 50 µA would be
expected to affect an area 0.5 mm in diameter
(Ranck, 1975). Each NST cell
produced action potentials with a constant latency following paired pulse
stimulation of the PbN at high frequency (>200 Hz at 1.2x threshold).
Collision tests were conducted at the same intensity as the paired pulse
stimulation for all 81 PbN projection cells, as all of these neurons were
spontaneously active. The lowest spontaneous rate was 0.2 impulses/s, which
occurred in two cells. In the collision test the first evoked action potential
of each pair was cancelled as it met the spontaneously generated action
potential, which was used to trigger the paired pulse stimulation.
Figure 2 demonstrates the
fulfillment of criteria for antidromic invasion from the ipsilateral gustatory
PbN in two NST neurons.
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Taste-responsive NST cells that did not show antidromically evoked action potentials following PbN stimulation were categorized as non-projection neurons. Of the 20 non-projection neurons, action potentials were evoked orthodromically in six cells, as evidenced by variable response latencies indicative of synaptic transmission. The variances in the latency of action potentials of three non-projection cells are shown in Figure 3. The other 14 neurons did not respond to PbN stimulation.
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Conduction velocity of PbN projection neurons
The conduction velocity of each PbN projection neuron was estimated by dividing the distance between the PbN stimulating and NST recording sites by the latency of the antidromic response. The frequency distribution of conduction velocities was essentially bimodal (Figure 4). Although all of these cells were relatively slowly conducting, most displayed conduction velocities >0.6 m/s. However, there was a substantial subset of neurons (20/81, 24.7%) with much slower conduction velocities (<0.3 m/s), all of which responded best to QHCl. The 23 QHCl-best neurons each had a conduction velocity <0.5 m/s. The mean conduction velocity of all 23 QHCl-best neurons was 0.25 ± 0.02 m/s, whereas that of the other 58 PbN projection cells was 0.95 ± 0.04 m/s. The difference between the QHCl-best neurons and all of the other groups was significant [F(3,77) = 41.66, P < 0.001; Bonferroni post hoc test, P < 0.05]. This result suggests that QHCl-best neurons are smaller and/or have axons of smaller diameter than other cell types.
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Taste response characteristics of PbN projection versus non-projection neurons
Each of the 101 NST neurons was tested for its responsiveness to the four taste stimuli and categorized as sucrose-, NaCl-, citric acid- or QHCl-best on the basis of its response profile. Among 81 PbN projection neurons, NaCl-best neurons (n = 35) comprised the largest category. There were 23 QHCl-best neurons, 14 sucrose-best neurons and nine citric acid-best cells in the PbN projection group. Figure 5 shows gustatory responses of four PbN projection neurons, one from each best stimulus category. In comparison, among the non-projection neurons there were no sucrosebest, eight NaCl-best, seven citric acid-best and five QHCl-best cells.
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Comparison of the response profiles revealed differences between PbN projection and non-projection neurons. Mean (± SEM) firing rates to taste stimulation and the spontaneous activity of 81 PbN projection neurons were 32.9 ± 2.6 and 19.0 ± 2.2 impulses/5 s, respectively. In contrast, those values in the 20 non-projection neurons were 16.1 ± 2.2 and 7.7 ± 1.7 impulses/5 s. All responses, including spontaneous rate, were significantly greater in PbN projection than non-projection cells [F(1,495) = 13.59, P < 0.001]. Not only were all sucrose-best cells found in the PbN projection group, but sucrose showed a larger percentage difference between the PbN projection and non-projection cells (72.1%) than the other three stimuli (NaCl, 57.9%; citric acid, 23.0%; QHCl, 48.5%). Figure 6 shows the mean response rates to each taste stimulus and spontaneous rates in the PbN projection and non-projection neurons. Although concentrations of taste stimuli were chosen that evoke roughly equal multi-unit responses in the NST, the mean responses to the four stimuli differed slightly from one another [F(3,396) = 2.69, P < 0.05) due to the greater numbers of NaCl-best neurons (Bonferroni post hoc test, P < 0.05). The taste responses of all 101 NST cells are shown in Figure 7, where the differences in firing rate between projection and non-projection cells are readily apparent.
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Discussion |
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Projection status of NST taste-responsive neurons
This study provides evidence that a large majority (80.2%) of
taste-responsive neurons in the hamster NST project to the gustatory area of
the ipsilateral PbN. Numerous anatomical studies show that the next synapse
beyond the NST in the ascending taste pathway of rodents occurs in the PbN
(Contreras et al.,
1982; Hamilton and Norgren,
1984). Although Halsell et al.
(Halsell et al.,
1996) demonstrated that more cells in the rostral NST project to
the PbN than to the reticular formation or the caudal NST in rats, it is
impossible in such anatomical studies to know whether these cells are
gustatory in function. Indeed, previous electrophysiological studies have
reported that considerably fewer than half of the gustatory neurons in the NST
are antidromically activated by stimulation of the PbN (Ogawa et al.,
1980,
1984;
Ogawa and Kaisaku, 1982;
Monroe and Di Lorenzo,
1995).
Either the percentage of NST cells projecting to the PbN in these previous
electrophysiological experiments was underestimated or many of the neurons
shown to project to the PbN in anatomical studies are non-gustatory in
function. In these earlier electrophysiological investigations the PbN
stimulating electrode was positioned stereotaxically. Monroe and Di Lorenzo
(Monroe and Di Lorenzo, 1995)
used only those placements determined to be within the PbN taste area on
histological examination for their analysis. These investigators reported that
45% of the NST neurons were PbN projection cells, compared with the 21-34%
reported in earlier studies (Ogawa et al.,
1980,
1984;
Ogawa and Kaisaku, 1982).
However, even such histological confirmation cannot eliminate the possibility
that the stimulating electrode was positioned in a non-gustatory region of the
PbN. In the present study our combination stimulating/recording electrode made
it possible to place the stimulating electrode precisely at a taste-responsive
location within the PbN. This approach resulted in 80.2% of the neurons being
antidromically activated by PbN stimulation. Therefore, the larger differences
reported here between these two categories of cells than in previous studies
probably reflects the inclusion of many PbN projection neurons in the
non-projection group in the earlier investigations. In the present study PbN
projection neurons were significantly more numerous and more responsive than
non-projection cells. Although it is possible that species difference could
explain the discrepancies between the present hamster experiment and the
previous rat work, anatomical studies have shown no remarkable differences
between rats and hamsters in the size or distribution of the projection from
the NST to the PbN (Contreras et
al., 1982; Whitehead and
Frank, 1983; Hamilton and
Norgren, 1984; Whitehead,
1990).
The proportion of projection neurons could also be related to the
distribution of cells recorded from the NST. In earlier studies the recorded
neurons, regardless of their projection status, were distributed across the
entire mediolateral extent of the NST
(Ogawa et al., 1984)
or confined to the lateral half of the rostral NST
(Monroe and Di Lorenzo, 1995).
In both series of studies in the rat the PbN projection neurons were
intermingled with those that were not antidromically activated by PbN
stimulation. In addition, the entire oral cavity was stimulated in these
earlier studies, whereas stimuli were applied only to the anterior tongue in
the present experiment. Thus the proportion of PbN projection neurons in the
present study reflects NST cells activated by VIIth nerve input. Because the
subdivisions of the hamster NST
(Whitehead, 1988) are
difficult to visualize in this counterstained material, we were not confident
in determining the cytoarchitectural boundaries of the NST subdivisions with
sufficient precision to relate the positions of the cells to these landmarks.
However, the recorded cells were located centrally within the NST, medial to
the solitary tract, in a region that likely corresponds to either the rostral
central or rostral lateral subdivision (see
Figure 1A), where most PbN
projecting neurons have been located anatomically
(Whitehead, 1990).
The location of the sites of PbN stimulation corresponded quite closely to
the area from which taste responses to anterior tongue stimulation have been
recorded from the hamster PbN (Van Buskirk
and Smith, 1981; Halsell and
Frank, 1991). The mean coordinate of the PbN stimulation sites was
4.08 ± 0.12 mm anterior to the obex, which is relatively caudal in the
PbN. In hamsters many taste-responsive neurons are found ventral to the
brachium conjunctivum (see Figure
1C) at this level [cf. figure
1A and B in (Van Buskirk and
Smith, 1981)] and axons from the taste-responsive regions of the
NST terminate most heavily in this part of the pons
(Whitehead, 1990). We expect
that even NST neurons that project to more rostral levels of the PbN would be
stimulated in these experiments, as their axons must pass the stimulating
electrode en route to the rostral PbN.
Taste responsiveness of PbN projection versus non-projection neurons
The present results show that PbN projection neurons respond more
vigorously to taste stimuli than NST neurons that do not send axons to the PbN
(see Figures 6 and
7). This difference was
greatest for the response to sucrose and least for the response to citric
acid. There were no sucrose-best neurons that did not project to the PbN.
Ogawa et al. (Ogawa et
al., 1984) found fewer sucrose-best cells in the
non-projection group than in the PbN projection group and the average response
to sucrose in the non-projection cells was less than half that of the PbN
projection neurons. In the investigation by Monroe and Di Lorenzo
(Monroe and Di Lorenzo, 1995)
the response to sucrose in the PbN projection neurons was also greater than in
non-projection neurons. All of these studies suggest that information about
the taste of sucrose is preferentially directed toward fore-brain gustatory
areas.
In contrast to the responsiveness to sucrose, previous studies have
reported that the response to QHCl was not significantly different between PbN
projection and non-projection cells and that only a few QHCl-best cells were
found in the PbN projection category
(Ogawa et al., 1984;
Monroe and Di Lorenzo, 1995).
These differences in the responsiveness of NST neurons to sucrose and QHCl led
these previous investigators to suggest that the responses of PbN neurons
serve to enhance the differences between palatable and unpalatable taste
stimuli. However, our findings do not support this hypothesis. First, the mean
response of PbN projection neurons to QHCl was almost twice that of
non-projection cells (35.51 versus 18.25 impulses/5 s). Second, we found many
QHCl-best cells among the PbN projection neurons: more than the number of
sucrose-best neurons and more than the number of QHCl-best cells that do not
project to the PbN. This result indicates that information about unpalatable
tastes is readily transferred to the gustatory PbN from the NST.
QHCl has been used as an exemplary bitter taste stimulus in behavioral and
electrophysiological studies and evokes avoidance behavior in rodents even at
very low concentrations (Grill and
Norgren, 1978). The PbN plays an important role in conditioned
taste aversion, which is a critically important learning mechanism that
prevents the repeated ingestion of toxic food
(Kiefer, 1985). The PbN must
be intact for the retention of a conditioned taste aversion
(Grigson et al.,
1997). In addition, Yamamoto and Sawa
(Yamamoto and Sawa, 2000) have
demonstrated a population of cells within the rat PbN that show Fos activation
following lingual application of QHCl. Electrophysiological evidence from
awake rats also suggests that PbN neurons can differentiate QHCl from the
other stimuli (Nishijo and Norgren,
1997). Thus the transfer of information about unpalatable stimuli
to the pons clearly occurs and may be necessary for ingestive decisions based
on prior gustatory experience.
Conduction velocity of QHCl-best neurons
We found many more QHCl-best neurons in the present investigation than
previous electrophysiological studies have reported; there are several factors
that might account for this discrepancy. We used glass micropipettes (7-10
M) to record from cells in the NST, which would facilitate the
isolation of relatively small cells. In previous electrophysiological studies
in our laboratory using these electrodes we have found QHCl-best neurons to be
a considerable portion of taste-responsive NST cells in hamsters when this
relatively high concentration (32 mM) of QHCl was used
(Li and Smith, 1997; Smith and
Li, 1998,
2000). The concentration of
QHCl in the present study was relatively higher than in many previous
electrophysiological investigations (cf.
Van Buskirk and Smith, 1981);
a lower QHCl concentration would have resulted in many of these QHCl-best
cells being classified as citric acid-best (see
Figure 7) or as NaCl-best (see
PbN 33 in Figure 5). However,
cells classified as QHCl-best by this stronger QHCl concentration displayed a
distinctive feature: they all had very slow conduction velocities, i.e. even
though the classification of these cells by their best stimulus depends
strongly on the stimulus concentrations used
(Smith and Travers, 1979),
this strong QHCl concentration identified a physiologically unique group of
neurons.
Since conduction velocity is directly related to cell or fiber size
(Webber and Pleschka, 1976;
Harper and Lawson, 1985), we
conclude that QHCl-best neurons are likely to be considerably smaller than
other cell types. In addition to classifying gustatory cells according to
their physiological properties, there have been attempts to categorize them
based on morphological features (Davis and Jang,
1986,
1988;
Lasiter and Kachele, 1988;
Whitehead, 1990; Renehan
et al., 1994,
1996;
Leonard et al.,
1999). However, it has been difficult to relate these two
classifications to one another. For example, there is no clear relationship
between morphological and functional features of rat NST neurons, except that
those cells responding very narrowly to QHCl have been shown to be
significantly smaller than other neurons
(Renehan et al.,
1996). Even though these data could not account for all QHCl-best
neurons in the NST, they implied that QHCl-sensitive neurons might be
morphologically unique. The present finding of slow conduction velocities of
QHCl-best neurons supports this hypothesis.
Orthodromic activation
The 20 neurons that were not antidromically activated by PbN stimulation
could be interneurons or neurons that project to the reticular formation or
motor nuclei in the medulla (Travers and
Norgren, 1983). It is also possible that some of these neurons
project to the contralateral PbN, although that possibility was not tested in
the present investigation. A bilateral projection from the rostral NST to the
medial PbN has been demonstrated anatomically in rats
(Williams et al.,
1996). Whereas the primary role of NST neurons that project to the
PbN is most likely to convey taste information to higher gustatory nuclei,
non-projection neurons probably play a role in brainstem circuits or as
interneurons within the NST. About 18% of neurons in the hamster NST are small
ovoid cells expressing GABA-like immunoreactivity
(Davis, 1993). These stand in
contrast to the numerous elongate and stellate neurons that project axons to
the PbN (Whitehead, 1990).
Thus the 20% of taste-responsive cells that were not antidromically activated
by PbN stimulation in the present study likely correspond to these small
GABAergic interneurons.
We found six taste-responsive cells in the NST that were orthodromically
driven from the ipsilateral PbN. Karimnamazi and Travers
(Karimnamazi and Travers,
1998) showed that neurons in the taste-responsive region in the
waist area of the PbN project caudally to the lateral parvo-cellular reticular
formation in the medulla; some of the descending fibers appeared to terminate
within the rostral NST. These combined results imply that taste information in
the NST could be modulated by descending input from the PbN. Taste-responsive
cells in the NST are under the influence of corticofugal projections in the
rat (Di Lorenzo and Monroe,
1995) and hamster (Smith and
Li, 2000). Descending modulation of NST cells by the lateral
hypothalamus and the central nucleus of the amygdala has also been shown in
the rat (Bereiter et al.,
1980; Matsuo et al.,
1984; Murzi et al.,
1986) and hamster (Cho et
al., 2000; Li et
al., 2000). Whether descending fibers from the PbN alter
taste function in NST cells has not been investigated.
In conclusion, we have demonstrated that the vast majority of taste-responsive NST cells transfer taste information, including information about both palatable and unpalatable tastes, to the gustatory PbN. We also found that QHCl-best neurons appear to be relatively smaller in size than other cell types, based on their slow antidromic conduction velocities. In contrast to previous electrophysiological investigations in the rat, the present results confirm that most taste-responsive cells of the NST participate in the projection of taste information to the forebrain.
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Acknowledgments |
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Thanks are due to Drs John D. Boughter Jr and Christian H. Lemon for valuable comments on the manuscript. A portion of these results was presented at the 1999 meeting of the Society for Neuroscience (Miami Beach, FL) and at the 2000 meeting of the Association for Chemoreception Sciences (Sarasota, FL). This work was supported in part by NIDCD grant DC00066 to D.V.S.
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Accepted October 12, 2001
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