The right SL nerve was dissected from surrounding connective tissue and transected for electrical stimulation. In some experiments the right glossopharyngeal (IXth) nerve was dissected and transected for recording of afferent nerve activities (Fig. 1). In all experiments, the hypoglossal nerves (bilateral), the SL nerves (bilateral), and the left IXth nerve were transected to prevent inadvertent tongue and trachea movements. For the insertion of glass microelectrodes, the head of the animal was fixed in a stereotaxic frame, and ~2 mm square hole was made on the rightside of the skull at around bregma and 5-6 mm lateral from the midline. For gustatory stimulation of the posterior tongue, the right masseter muscle and the upper part of the mandible were removed to expose the lining of the mouth. The single vallate papillae (V.P.) and the right foliate papillae (F.P.) were exposed by tension applied to the lower incisors and by stretching the sutures placed in the anterior tongue.

View larger version (19K):
[in this window]
[in a new window]
| FIG. 1.
A: responses of the glossopharyngeal (IXth) nerve to gustatory stimulation (1.0 M NaCl, 30 mM HCl, 30 mM QHCl, and 1.0 M NaCl) of the foliate papillae (F.P.), vallate papillae (V.P.), and both papillae (F.P. & V.P.). B: responses of the IXth nerve to water flow (rinse). C: responses of the IXth nerve to light mechanical stimulation of the F.P. and V.P. using a glass rod. Solid lines under the records indicate the periods of taste or mechanical stimulation. The magnitude of the response is shown as a rate (impulses/s). In A-C, the responses were recorded from the same IXth nerve.
|
|
During the recordings, the animal was paralyzed by tubocurarine (2 mg/kg iv) and artificially ventilated by a Harvard respiratory pump. Whenever paralysis seemed to wear off (usually 30-60 min), the level of anesthesia was assessed by pinching the tail. In addition, the level of anesthesia was continuously checked during the experiment by continuous monitoring of BP, heart rate (HR), and expiratory CO2, which are useful parameters to know the level of the anesthesia. If required, supplemental doses of alpha-chloralose (20 mg/kg iv) were administered (approximately every 2 h). The end-expired CO2 was constantly monitored and maintained at 3.5-4.5%. Rectal temperature was maintained 37-38°C by a thermostatically regulated heating pad.
Stimulation
For electrical stimulation, the central portion of the SL nerve was placed on a pair of platinum wire electrodes. The nerves were stimulated with a train of three rectangular pulses of 0.03 ms duration at 500 Hz. The stimulus strength used was 12 V, which is the maximal intensity for excitation of A
fibers in the SL nerve (Hanamori et al. 1996
). In some experiments, repetitive electrical stimulation of the SL nerve at 50 Hz for 2-20 s was made.
Nociceptive stimulation was made by pinching the tail at the proximal portion using a surgical clamp.
Arterial baroreceptors were stimulated by administration of methoxamine hydrochloride (Mex; 20 µg/0.1 ml iv) or sodium nitroprusside (SNP, 10 µg/0.1 ml), which produces an increase or decrease in BP, respectively. Arterial chemoreceptors were stimulated by administration of sodium cyanide (NaCN, 100 µg/0.1 ml iv). The drugs were injected at doses of 0.05-0.1 ml per rat.
Taste stimulation was delivered using a V-shaped glass pipette. One side of the pipette was connected to a peristaltic pump via polyethylene tubing, which applies negative pressure. The other side of the pipette was connected to a bottle of taste solution via polyethylene tubing. The bottom of the pipette (2-3 mm diam of inner tip) was open so that all F.P. in the right side or a single vallate papillae V.P. could be attached via negative pressure. The flow rate of the taste solution was 0.06 ml/s, and the duration of the stimulation was ~1 min for both the F.P. and the V.P. Before and after each stimulation, the papillae were constantly rinsed with deionized water (water). The taste stimuli used were 1.0 M NaCl, 1.0 M sucrose, 30 mM HCl, and 30 mM quinine HCl (QHCl). All of the stimuli were dissolved in deionized water. The stimuli and rinse were delivered at room temperature (~24°C). In the present study, both the F.P. and the V.P. were stimulated simultaneously by using two V-shaped glass pipettes. The period of the taste stimulation of the papillae was determined by observing the air bubbles contained in the polyethylene tubing between the taste solution and a rinse (water); the beginning of the stimulation was marked by using a foot pedal when the meniscus of the taste solution reached the surface of the papillae after the air bubbles, and the end of the stimulation was marked when the air bubbles reached the papillae after the taste solution.
Recording and data analysis
The BP and HR obtained from pulsation of the blood pressure were amplified using a conventional amplifier and recorded on an 8-channel pen recorder.
For whole nerve recording, the peripheral portion of the IXth nerve was placed on a pair of the platinum wire electrodes. Neural activity was amplified, displayed on an oscilloscope, and stored on magnetic tape. Neural discharges were fed into a pulse counter, and the instantaneous changes of the number of the impulses were continuously recorded in the form of cumulative frequency histograms with a reset time of 1 s (see Fig. 1).
Extracellular unit responses of neurons in the insular cortex were recorded with glass microelectrodes filled with pontamine sky blue in 0.5% sodium acetate. Neural activity was amplified, displayed on an oscilloscope, led to an audiomonitor, and stored on a data recorder.
Single sweeps of neural impulses in the insular cortex evoked following electrical stimulation of the SL nerve were stored on an oscilloscope, and recorded on a recorder. To determine the responses of neurons to electrical stimulation of the SL nerve, peristimulus time histograms were made using a histogram analyzer from 50-300 stimulus presentations.
The electrodes were inserted vertically into the insular cortex between 5 and 6 mm lateral to the midline, between 2 mm anterior and 1 mm posterior to bregma, and between 3.5 and 4 mm ventral to the dorsal surface of the cortex. Spontaneously active neurons in the insular cortex encountered during an electrode penetration were tested for their responses to electrical stimulation of the SL nerve, gustatory stimulation of the F.P. and V.P., baroreceptor and chemoreceptor stimulation, and tail pinch. The responses of neurons isolated in the insular cortex were defined as excitatory if the mean number of impulses (per second) over a 30-s period after stimulation increased significantly to that over a 30-s period before stimulation and inhibitory if the mean number of impulses decreased significantly (2-tailed t-test, P < 0.05). The magnitude of the response was expressed as an increase or decrease (%) to the basal activity before stimulation. The values in the results were expressed as means ± SE. Multiple comparisons were made using one-way analysis of variance (ANOVA).
Histology
At the end of the experiments, the recording sites were marked by deposition of the dye when passing 5 µA of DC current through the tip of the electrode for 3-5 min. Then the brains were removed and fixed with 10% Formalin. Fifty-micrometer sections were cut on a freezing microtome and stained with neutral red. Of the 43 neurons isolated in the present study, 2 (4.7%) were located in layers II-III, 5 (11.6%) were in layer IV, 30 (69.8%) neurons in layer V, and the remaining 6 (13.9%) in layer VI. Thus most neurons identified in the present study were located in layer V. The location of neurons isolated in the insular cortex was plotted on two-dimensional maps in which the anterior edge of the joining of the anterior commissure (AC) and the rhinal fissure (RF) were adopted as standard zero point in the anterior-posterior and the dorsoventral axes, respectively (Cechetto and Saper 1987
).
 |
RESULTS |
Responses of the IXth nerve to gustatory stimulation of the posterior tongue
Figure 1 shows sample recordings of the whole-IXth nerve responses (number of impulses/s) to gustatory stimulation of the F.P. and the V.P. For stimulation of the F.P. or the V.P., the IXth nerve responds well to 1.0 M NaCl, 30 mM HCl, and 30 mM QHCl, but shows relatively poor responses to sucrose. The magnitude of the IXth nerve responses to stimulation of the F.P. or the V.P. is small in comparison with stimulation of both F.P. and V.P. The IXth nerve shows also a good mechanical response (Fig. 1C) to light mechanical stimulation of the F.P. or the V.P. with a glass rod. In the present study, water was used as a rinse solution before and after taste stimulation. Figure 1B shows that the IXth nerve responds to water; in this record, the papillae have been exposed to the air before application of water. This water response may be a complex response (water response as a taste, tonic mechanical response induced from a continuous water flow, and thermal response). In Fig. 1A, a decrease in the baseline activity of the IXth nerve was seen at just before taste stimulation. This is due to the disappearance of the water response when the air bubbles contained in the polyethylene tubing between the taste solution and a rinse are passing the papillae after water flow (rinse).
Responses of neurons in the insular cortex to electrical stimulation of the SL nerve
Of the 43 neurons, 27 (62.8%) were excited, 8 (18.6%) were inhibited, and the remaining 8 (18.6%) were unresponsive to electrical stimulation of the SL nerve (Fig. 5, A and B). Figure 2B (left) shows an insular cortex neuron showing an excitatory response to a single electrical stimulation of the SL nerve (3 pulses at 500 Hz). In this neuron, two spikes were evoked by the SL nerve stimulation with a latency of 36 ms. Figure 2B (right) shows the peristimulus time histogram of this neuron that was made from 50 stimulations. It can be seen that there is an inhibitory phase after excitation of the neuron. Figure 3B shows a neuron with an inhibitory response. This histogram was made from 200 stimulations. Figure 4B shows a sample of a neuron showing no response to electrical stimulation of the SL nerve. For each neuron, the response type (excitatory, inhibitory, or no response) that was observed in single electrical stimulation of the SL nerve was the same as that seen after repetitive electrical stimulation of the SL nerve (Figs. 2A, 3A, and 4A). Cardiovascular responses were evoked after repetitive electrical stimulation of the SL nerve; increase in BP and decrease or increase in HR (Figs. 2A, 3A, and 4A).

View larger version (47K):
[in this window]
[in a new window]
| FIG. 5.
A: response types (excitatory, inhibitory, or no response) to gustatory stimulation, baroreceptor and chemoreceptor stimulation, electrical stimulation of the SL nerve, and tail pinch are summarized for 43 neurons. Responses of neurons were defined as excitatory if the mean number of impulses (per s) over a 30-s period after stimulation increased significantly to that over a 30-s period before stimulation and inhibitory if the mean number of impulses decreased significantly (2-tailed t-test, P < 0.05). B: number of neurons showing excitatory, inhibitory, or no response are shown for 9 stimuli. C: number of neurons according to the degree of the convergence among the 9 stimuli.
|
|

View larger version (32K):
[in this window]
[in a new window]
| FIG. 2.
A: responses of an insular cortex neuron (neuron) to gustatory (1.0 M NaCl, 30 mM HCl, 30 mM QHCl, and 1.0 M sucrose) stimulation, baroreceptor and chemoreceptor (Mex, SNP, and NaCN) stimulation, electrical stimulation of the superior laryngeal (SL) nerve, and tail pinch with simultaneously recorded blood pressure (BP) and heart rate (HR). Solid lines under the records of the neural activity indicate the periods of taste stimulation. Arrows indicate the time of an intravenous application of the drugs. Mex, methoxamine hydrochloride; SNP, sodium nitroprusside; NaCN, sodium cyanide. B, left: evoked response (2 spikes) of an insular cortex neuron (the same neuron as in A) to electrical stimulation of the SL nerve with a single train pulse (50 Hz, 3 pulses). B, right: peristimulus time histogram that was constructed from 50 such stimulations of the SL nerve. C: position (*) of the same neuron as in A and B is shown by a coronal section of the brain. The location of the neuron was the anterior edge of the joining of the anterior commissure (AC) in the anterior-posterior dimension. RF, the rhinal fissure.
|
|

View larger version (22K):
[in this window]
[in a new window]
| FIG. 3.
A: responses of an insular cortex neuron to various stimuli. B: peristimulus time histogram of the insular cortex neuron (the same neuron as in A) that was constructed from 200 stimulations of the SL nerve. C: position (*) of the same neuron as in A and B is shown by a coronal section of the brain. Other conventions as in Fig. 2.
|
|

View larger version (20K):
[in this window]
[in a new window]
| FIG. 4.
A: responses of an insular cortex neuron to various stimuli. B: peristimulus time histogram of the neuron (the same neuron as in A) that was constructed from 100 stimulations of the SL nerve. C: position (*) of the same neuron as in A and B is shown by a coronal section of the brain. Other conventions as in Fig. 2.
|
|
Responses of neurons in the insular cortex to tail pinch
Of the 43 neurons, 18 (41.9%) were excited, 15 (34.9%) were inhibited, and the remaining 10 (23.2%) were unresponsive to tail pinch (Fig. 5, A and B). Sample recordings of excitatory responses of insular cortex neurons to tail pinch are shown in Figs. 2A and 4A. A typical example of an inhibitory response to tail pinch is shown in Fig. 3A. In all rats, tail pinch induced an increase both in BP and HR (Figs. 2A, 3A, and 4A).
Responses of neurons in the insular cortex to baroreceptor and chemoreceptor stimulation
Of the 43 neurons, 11 (25.6%) were excited, 9 (20.9%) were inhibited, and the remaining 23 (53.5%) were unresponsive to baroreceptor stimulation with an intravenous injection of Mex, which induces an increase of the basal level of afferent discharge from baroreceptors (Fig. 5, A and B). Samples of insular cortex neurons showing an excitatory response to stimulation with Mex are shown in Figs. 3A and 4A. A neuron showing an inhibitory response to stimulation with Mex is shown in Fig. 2A.
The baroreceptors were stimulated with an intravenous injection of SNP, which induces a decrease of the basal level of afferent discharges from baroreceptors. Of the 43 neurons, 10 (23.3%) were excited, 17 (39.5%) were inhibited, and 16 (37.2%) were unresponsive to SNP (Fig. 5, A and B). Neurons showing an inhibitory response to SNP are seen in Figs. 3A and 4A. The neuron shown in Fig. 2A was not sensitive to SNP stimulation.
The chemoreceptors were stimulated by an intravenous injection of NaCN. Of the 43 neurons, 10 (23.3%) were excited, 16 (37.2%) were inhibited, and the remaining 17 (39.5%) were unresponsive to NaCN (Fig. 5, A and B). The neuron in Fig. 2A shows an excitatory response to NaCN stimulation, whereras the neuron in Fig. 3A shows an inhibitory response. A sample of a neuron showing no response is shown in Fig. 4A.
Responses of neurons in the insular cortex to gustatory stimulation of the posterior tongue
Relatively few gustatory-responsive neurons were found in the insular cortex in comparison with other stimuli used in the present study (Fig. 5, A and B). For NaCl stimulation, of the 43 neurons, 5 (11.6%) were excited, 3 (7.0%) were inhibited, and the remaining 35 (81.4%) were unresponsive. For HCl stimulation, of the 43 neurons, 6 (14.0%) were excited, 9 (20.9%) were inhibited, and the remaining 28 (65.1%) did not respond. For QHCl stimulation, of the 43 neurons, 4 (9.3%) were excited, 4 (9.3%) were inhibited, and the remaining 35 (81.4%) were unresponsive. For sucrose stimulation, of the 43 neurons, 5 (11.6%) were excited, 8 (18.6%) were inhibited, and the remaining 30 (69.8%) gave no response. Figure 4A shows a sample of a neuron showing an excitatory response to sucrose stimulation.
Convergence of neurons in the insular cortex
The types of responses shown by the 43 insular cortex neurons are summarized in Fig. 5A. All of the neurons responded to at least one of the nine stimuli used in the present study. In Fig. 5, A and B, it can be seen that the neurons in the insular cortex show both excitatory and/or inhibitory responses to various kinds of stimuli. A large number of the neurons (42/43; 97.7%) responded to more than one stimulus (Fig. 5C). The neurons receiving inputs from three or five stimuli are observed most frequently (Fig. 5C). When the eight natural stimuli are grouped into three, visceral (Mex, SNP, and NaCN), gustatory (NaCl, HCl, QHCl, and sucrose), and nociceptive stimuli (tail pinch), 42 neurons that were responsive to these stimuli can be classified into 3: 37 visceral-sensitive neurons responding to the visceral stimuli, 26 gustatory-sensitive neurons responding to the gustatory stimuli, and 33 nociceptive-sensitive neurons responding to tail pinch. Concerning the convergence of these neurons, 4 neurons were responsive to one specific stimulus group, 22 neurons received convergent inputs from two stimulus groups, and 16 neurons received convergent inputs from all three stimulus groups (visceral, gustatory, and nociceptive).
Distribution of neurons in the insular cortex
The locations of the 43 neurons responding to each stimulus are shown in Fig. 6 in the dorsoventral and anterior-posterior dimensions in the insular cortex. The neurons were distributed between 0.3 and 3.1 mm dorsal to the RF (1.8 ± 0.1 mm, n = 43), and between 2.8 mm anterior to the AC and 1.1 mm posterior to the AC (
1.0 ± 0.1 mm, n = 43). In the anterior-posterior distribution, the mean location of neurons responding to Mex, SNP, NaCN, and tail pinch (between
0.6 and
0.9 mm from the AC) were posterior to those of the neurons responding to gustatory stimulation (between
0.9 and
1.4 mm from the AC; see Table 1), although these differences were not statistically significant [F(8,176) = 0.835, P = 0.573]. The mean locations of the excitatory and inhibitory neurons responding to the nine stimuli are shown in Table 1. There were also no significant differences in the distribution between excitatory and inhibitory neurons for all stimuli (2-tailed t-test, P > 0.05). There were no differences in the dorsoventral distributions for the neurons responding to each of the nine stimuli [F(8,176) = 0.337, P = 0.951, Table 1].

View larger version (27K):
[in this window]
[in a new window]
| FIG. 6.
Distribution of the 43 neurons in the insular cortex and their responsiveness to each stimulus. The ordinate and abscissa show the distance from the rhinal fissure (RF) and the anterior edge of the joining of the anterior commissure (AC), respectively.
|
|
View this table:
[in this window]
[in a new window]
|
TABLE 1.
Mean locations of insular cortex neurons responding to each nine stimuli in the anterior-posterior and dorsoventral dimensions
|
|
Response profiles of neurons in the insular cortex
Figure 7 shows the response profiles of the 43 neurons to the 8 stimuli. The neurons are arranged in order of the magnitude of the responses to NaCl. The similarity of the response profiles between the stimuli is shown in Table 2 with a correlation coefficient. The similarities are low in most of the pairs, although a significant correlation between QHCl and HCl (r = 0.57) and between sucrose and tail pinch(r = 0.47) are indicated (P < 0.01).

View larger version (36K):
[in this window]
[in a new window]
| FIG. 7.
Response profiles of the 43 neurons for each stimulus. Response types (excitatory, inhibitory, or no response) are shown for each neuron. Neurons are arranged along the abscissa according to the order of the magnitude of the response to NaCl. Response magnitude is expressed as an increase or decrease (%) to the basal activity before stimulation.
|
|
View this table:
[in this window]
[in a new window]
|
TABLE 2.
Correlation coefficients between pairs of stimuli and spontaneous rate across the 43 insular cortex neurons
|
|
Spontaneous discharge rate
Rate of the spontaneous discharges in the insular cortex neurons collected in the present study ranged from 0.1 to 11.3 Hz (mean, 2.2 Hz, n = 43). The rate of the spontaneous discharge had no relationship to the responses to the eight stimuli (r, between
0.22 and 0.31; see Table 2), or with the location of neurons (in the anterior-posterior axis, r = 0.25; in the dorsoventral axis, r =
0.03). Also, there was no significant difference in the spontaneous rate between the excitatory and the inhibitory neurons for all stimuli (2-tailed t-test, P > 0.05).
 |
DISCUSSION |
We have previously shown that insular cortex neuronswere responsive to electrical stimulation of the CT,LT-IXth, PH-IXth, and SL nerves (Hanamori et al. 1997b
). The present study shows that insular cortex neurons are responsive to visceral, gustatory, and nociceptive stimuli as well as to electrical stimulation of the SL nerve.
Responses of neurons in the insular cortex to baroreceptor and chemoreceptor stimulation
Cechetto and Saper (1987)
for the first time recorded visceral responses from insular cortex neurons to baroreceptor and chemoreceptor stimulation. They found seven neurons that were responsive to baroreceptor stimulation by an intravenous injection of phenylephrine hydrochloride (PE); three were inhibitory neurons and four were excitatory. They also found six neurons that were sensitive to chemoreceptor stimulation by an intravenous injection of NaCN; one neuron was inhibitory and five were excitatory. They showed that most of the neurons responded to either chemoreceptor or baroreceptor stimulation. Among the 13 neurons, only 1 responded to both PE and NaCN stimulation.
Our data are similar to those of Cechetto and Saper (1987)
. In the present study, 37 neurons showed an excitatory, inhibitory, or both types of responses to the baroreceptor and chemoreceptor stimulation. Of the 37 neurons, 21 (56.8%) were responsive to both baroreceptor (Mex and/or SNP) and chemoreceptor stimulation (NaCN). Thus in the present study many neurons receiving convergent inputs from the baroreceptors and chemoreceptors were observed in comparison with the report by Cechetto and Saper (1/13: 7.7%). This difference in the amount of the convergence may be attributable to the methods for sampling of neurons and the dose of the drugs for baroreceptor and chemoreceptor stimulation. In addition, because chemoreceptor stimulation by an intravenous injection of NaCN induced more or less changes in BP, there is a possibility that the baroreceptors might be also stimulated as well as the chemoreceptors. Thus our data might not provide a stringent test of the convergence of inputs from the baroreceptors and chemoreceptors. Although the present study does not show the pathways from the baroreceptors and the chemoreceptors to the insular cortex, some previous reports suggest that the parabrachial nucleus may be included in these neuronal pathways (cf. Cechetto and Saper 1990
). Hayward and Felder (1995)
have recorded 32 parabrachial neurons that responded to selective stimulation of the baroreceptors and chemoreceptors. They showed that, of the 32 neurons, 11 (34%) were sensitive to stimulation of both types of receptors. This result suggests the possibility of the large amount of convergence in insular cortex neurons from the baroreceptors and chemoreceptors. However, more data are needed to know precisely the amount of convergence in insular cortex neurons from the baroreceptors and chemoreceptors.
In the present study, neuronal activity in the insular cortex did not always correlate with changes in BP, as shown in Fig. 5A. Of the 32 baroreceptor-sensitive neurons, 5 (15.6%) showed responsiveness in the same direction as the changes in BP (an excitatory response when the BP is increased by Mex stimulation and an inhibitory response when the BP is decreased by SNP stimulation), and 2 (6.3%) showed a reversal in the direction of the neural activity and the changes in BP. Most of the neurons (25, 78.1%) responded to either the baroreceptor and the chemoreceptor stimulation (17) or responded to these stimuli with the same direction of the responses (8). These results indicate that most of the insular cortex neurons do not directly reflect the changes in the activity of the baroreceptors. Similar results have been shown for the neural activity in the bed nucleus of the stria terminals (Wilkinson and Pittman 1995
). They have shown that the neuronal responses to changes in BP in one direction did not necessarily infer the opposite neuronal response to BP change in the other direction.
Responses of neurons in the insular cortex to gustatory stimulation of the posterior tongue
It has been shown that the neurons in the anterior portion of the insular cortex respond to gustatory stimulation of the anterior tongue or entire oral cavity (Kosar et al. 1986
; Ogawa et al. 1990
, 1992
; Yamamoto et al. 1980
, 1984
). The present study is the first to record the activity of neurons in the insular cortex to selective stimulation of the posterior tongue.
In the present study, 26 neurons (60.5%) were responsive to at least one of the taste stimuli (NaCl, HCl, QHCl, and sucrose) to stimulation of the V.P. and F.P. In 26 taste-responsive neurons, 16 (61.5%) responded to more than one taste stimulus and 10 (38.5%) were responsive to only one stimulus (3 to NaCl; 5 to HCl; 2 to sucrose). The order of the stimuli in the number of the taste-responsive neurons was HCl (15), sucrose (12), QHCl (8), and NaCl (8). The responsiveness of the LT-IXth nerve to the four basic taste stimuli can be arranged in the order of HCl, QHCl, sucrose, and NaCl, considering the number of the best fibers for each stimuli (Frank 1991
; Hanamori and Smith 1988). Thus the responsiveness of the neurons in the insular cortex roughly reflects the activity of the fibers in the LT-IXth nerve.
The present study shows low values in the correlation coefficients for pairs of stimuli obtained from the taste response profiles across the 43 neurons with the exception of HCl and QHCl (r = 0.57, Table 2). In entire oral cavity stimulation (Ogawa et al. 1992
) or anterior tongue stimulation (Yamamoto et al. 1985
), moderate or low correlation coefficients between pairs of the four basic taste stimuli have been reported (r < 0.6).
Many neurons in the insular cortex show an inhibitory response to taste stimulation of the posterior tongue. In the present study, about one-half of the taste-sensitive neurons show an inhibitory response for each taste stimuli (Fig. 5B). Similar results have been reported by many authors in gustatory stimulation of the anterior tongue or entire oral cavity (London 1994; Ogawa et al. 1992
; Yamamoto et al. 1989
).
Localization of insular cortex neurons
Cechetto and Saper (1987)
have first shown that the insular cortex can be divided into two areas in the anterior-posterior axis, the taste area and the visceral area. That is, the neurons in the anterior portion of the insular cortex respond to taste stimulation, and those in the posterior portion of the insular cortex respond to general visceral stimulation. Although Cechetto and Saper recorded neural activity in the insular cortex to stimulation of both taste and visceral stimulation, the borderline between the taste area and the visceral area was not clear. According to their report, the neurons responding to general visceral stimulation are distributed between 2.0 mm anterior and 0.5 mm posterior to the AC with a peak around AC (0.0 mm), whereas the taste-sensitive neurons are seen between 0.5 and 2.0 mm anterior to the AC with a peak around 1.5 mm (
1.5 mm).
Many taste researchers have recorded from neurons of the insular cortex responding to gustatory stimulation of the anterior tongue and entire oral cavity (Kosar et al. 1986
; Ogawa et al. 1990
, 1992
; Yamamoto et al. 1980
, 1984
, 1985
, 1989
). However, the several different zero points for anterior-posterior axis were used by these authors; the intersection of the middle cerebral artery and the rhinal fissure (MCA), the bed nucleus of the anterior commissure (BNAC), and the most rostral extent of the corpus callosum. We could presume the location of the AC as 1.2 mm posterior to the MCA; in 32 rats the location that was marked by a dye was measured as the distance from the MCA (0.4 ± 0.2 mm posterior to the MCA, n = 32) in the anterior-posterior axis, and also the same location was determined histologically as the distance from the AC (0.8 ± 0.2 mm anterior to the AC, n = 32) by examining the serial coronal sections. In addition, using an atlas of the rat brain (Paxinos and Watson 1982
), we could determine the location of the taste-sensitive neurons obtained by these authors as the distance from the AC. By these considerations, the taste-sensitive neurons obtained by these authors are distributed in the area between 0.5 and 2.5 mm anterior to the AC with a peak around 1.3-1.7 mm (
1.3 to
1.7 mm). In the present study the mean location of the taste-sensitive neurons is shown to be 1.0 mm anterior to the AC (
1.0 ± 0.1 mm, n = 26). Thus the neurons responsive to stimulation of the posterior tongue are distributed in the area posterior to the taste area or in the region anterior to the visceral area in the insular cortex; they might also be an area of overlap between these two areas.
It has been shown that electrical stimulation of the posterior insular cortex elicits cardiovascular responses (Ruggiero et al. 1987
; Sun 1992
; Yasui et al. 1991
). These studies have shown that the posterior portion of the insular cortex can be further divided into two areas, caudal and rostral areas, according to the cardiovascular responses following electrical stimulation of the insular cortex; a decrease in BP is induced by electrical stimulation of the caudal area of the posterior insular cortex, and an increase in BP is induced by electrical stimulation of the rostral area of the posterior insular cortex (Butcher and Cechetto 1995
; Yasui et al. 1991
). Interestingly, the mean location of the neurons showing an excitatory response to stimulation of the SNP or the Mex was more anterior region than that of the neurons showing an inhibitory response, although the difference was not significant (P > 0.05, Table 1). In our previous study this difference for SNP is shown to be statistically significant (Hanamori et al. 1997a
). Thus there is a possibility that more refined research can show a function-dependent localization in the posterior insular cortex.
Speculation of functional roles for the insular cortex
Most of the neurons recorded in the present study from the insular cortex received convergent inputs from different kinds of sensory organs and were localized around 1.0 mm anterior to the AC. All taste-sensitive neurons were also responsive to other kinds of stimuli. It is clear that these neurons are distributed in a region posterior to the taste area, which receive projections from the CT. It is likely that this area is compatible with the IXth nerve projection area as has been reported by Yamamoto et al. (1980)
using field potential recording. In addition, the position of this area is anterior to the visceral area. It has been suggested that the function of the IXth nerve may be different from that of the CT; namely, information from the CT is important for taste quality determination, but that from the IXth nerve is important for aversive responses (Frank 1991
; Hanamori et al. 1988
). Most of the taste-sensitive neurons (76.9%, 20/26) were also responsive to tail pinch. This is not a surprising result because it has been demonstrated by anterograde tracing methods using herpes simplex virus type 1 that nociceptive information projects to the insular cortex (Barnett et al. 1995
). In addition, of 26 taste-sensitive neurons, 21 neurons (80.8%) were also sensitive to baroreceptor and/or chemoreceptor stimulation. It has been shown anatomically that the neurons in the insular cortex project to the amygdala, hypothalamus, and caudal portion of the nucleus of solitary tract (Allen et al. 1991
; Cechetto and Saper 1987
; Ruggiero et al. 1987
) and receive convergent inputs from visceral, limbic, and association cortex (Krushel and Kooy 1988
). Taken together, these results suggest that the neurons located between the taste area and the visceral area have important roles for taste-aversion behavior or regulation of visceral response to stress.