Primary Somatosensory Cortical Neuronal Activity During Monkey's Detection of Perceived Change in Tooth-Pulp Stimulus Intensity
Koichi Iwata1, 2,
Yoshiyuki Tsuboi1, 2, and
Rhyuji Sumino1, 2
1 Department of Physiology and 2 Division of Pathophysiology in the Dental Research Center, School of Dentistry, Nihon University, Tokyo 101, Japan
 |
ABSTRACT |
Iwata, Koichi, Yoshiyuki Tsuboi, and Rhyuji Sumino. Primary somatosensory cortical neuronal activity during monkey's detection of perceived change in tooth-pulp stimulus intensity. J. Neurophysiol. 79: 1717-1725, 1998. To elucidate the functional properties of primary somatosensory cortical neurons for the perception of tooth-pulp sensation, neuronal activity was recorded from the primary somatosensory cortex (SI) in awake behaving monkeys. Monkeys were trained to detect changes in tooth-pulp stimulus intensity applied to the upper canine or incisor tooth pulp. Stimulus intensities applied to the tooth pulp were multiples of the threshold intensity for the jaw opening reflex (1.0 T) elicited by tooth-pulp stimulation. When monkeys pressed a button, baseline electrical pulses (V1: 0.5 T, 1.0 T, 2.0 T, or 3.0 T) were applied to the tooth pulp. After 4-8 s, a V2 stimulus (0.3 T, 0.5 T, 1.0 T, or 2.0 T) was added to V1. Percent escapes at V1 stimulus intensity of 0.5 T and 1.0 T were ~10%, 22% at 2.0 T, and 40% at 3.0 T (total of 1,997 trials). A total of 862 single units were recorded from the SI. Thirty-seven SI neurons responded to electrical stimulation of the tooth pulp (tooth-pulp-driven neurons; TPNs), 139 SI neurons responded to tactile stimulation of the lateral face area, 90 to upper lip and 99 to lower lip, 44 to tongue and 102 to periodontal membrane, whereas 351 SI neurons were not responsive to tactile stimulation of the orofacial regions. Thirty of 37 TPNs were recorded long enough to test with V1 stimuli ranging from 0.5 T to 3.0 T. Eleven of 30 TPNs linearly increased their firing frequency following increases in stimulus intensity (encoding TPNs), whereas 19 did not (nonencoding TPNs). Mean first spike latency of encoding TPNs was 24.8 ± 1.7 ms (n = 11), that of nonencoding TPNs was 23.6 ± 1.5 ms (n = 19), and that of unclassified TPNs was 24.7 ± 3.7 ms (n = 7). TPNs were distributed in the areas 1-2, 3a, and 3b within the oral projection area and the transition zone between the face and oral projection areas of the SI. All of them received inputs from the intraoral structures, facial skin, or both. The firing frequency of eight encoding and nonencoding TPNs was correlated with detection latency at stimulus intensities of 0.5 and 1.0 T. On the other hand, when the baseline stimulus was increased to 2.0 T and 3.0 T, the discharge of most TPNs did not increase in firing frequency with the reduction in detection latency. These results indicate that the discharge rates of some SI TPNs are correlated with detection latency at near-noxious threshold and noxious stimulus intensities. These findings suggest that some TPNs are involved in the sensory-discriminative aspect of tooth-pulp sensation in the near-pain threshold and pain ranges.
 |
INTRODUCTION |
It is well-known that somatosensory information is represented in the primary somatosensory cortex (SI) with somatotopic organization (Darian-Smith et al. 1966
; Felleman et al. 1983
; Kaas et al. 1979
; Landgren and Olsson 1980
; Lund and Sessle 1974
). Jaw and orofacial sensory inputs project to the lateral part of the SI, and other body parts are represented in the medial portion in several species, such as monkeys (Kaas et al. 1979
; Ogawa et al. 1989
), cats (Darian-Smith et al. 1966
; Felleman et al. 1983
; Landgren and Olsson 1980
; Lund and Sessle 1974
), and rats (Armstrong-James and Fox 1987
; Chapin and Lin 1984
; Welker 1971
). Most of the SI neurons receive nonnoxious cutaneous input, whereas some of them also receive proprioceptive or noxious inputs. It is believed that the major function of SI neurons is to process the spatiotemporal perception of the whole body surface. It is likely that the perception of body parts is processed by low-threshold mechanoreceptive (LTM) neurons that respond to nonnoxious mechanical stimulation of their receptive fields. Those LTM neurons are somatotopically arranged in the SI.
Several lines of evidence reveal that some SI neurons respond to noxious heat and/or mechanical stimulation of the body surface (Chudler et al. 1986
, 1990
; Kenshalo and Isensee 1983
; Kenshalo et al. 1988
; Lamour et al. 1982
). It has been also reported that SI neurons responding to noxious thermal stimulation are located in areas 1, 2, and 3 of the SI (Kenshalo et al. 1983
, 1988). Most of these nociceptive neurons were classified as wide dynamic range (WDR) neurons that responded to both nonnoxious and noxious stimuli and increased their firing frequency following increases in stimulus intensity (Chudler et al. 1990
; Kenshalo et al. 1983
, 1988). A minority of the nociceptive neurons were classified as nociceptive specific (NS) neurons that responded exclusively to noxious thermal and/or mechanical stimulation (Chudler et al. 1990
; Iwata et al. 1990
; Kenshalo et al. 1983
).
It has been also reported that some of the WDR, NS,and/or LTM SI neurons receive inputs from the tooth pulp (tooth-pulp-driven neurons: TPNs) (Biedenbach et al. 1979
; Iwata et al. 1986
, 1987
, 1990
, 1994
; Lund and Sessle 1974
; Mason et al. 1985
; Matsumoto et al. 1987
, 1989
). These TPNs are distributed within laminae III-V of areas 3a and 3b of the orofacial area of the SI in cats (Hassler and Muhs-Clement 1964
; Iwata et al. 1986
, 1987
, 1990
, 1994
; Matsumoto et al. 1989
). Most TPNs receive inputs from the intraoral structures, facial cutaneous tissues, or both. Furthermore, many of them exhibit an increase in their firing frequency following increases in stimulus intensity (Iwata et al. 1990
; Matsumoto et al. 1987). These observations suggest that TPNs in SI are possibly involved in the perception of tooth pulp, cutaneous, mucosal, and periodontal inputs.

View larger version (21K):
[in this window]
[in a new window]
| FIG. 1.
Schematic illustration of the experimental setup (A) and behavioral task (B) used in the present study. A press button with light illumination is placed in front of the monkey (A). Recording chamber is placed on the jaw and orofacial areas of the primary somatosensory cortex. Bipolar electrodes are embedded into the upper canine or incisor tooth. An upward movement in each trace indicates the stimulus or switch on response, and a downward movement indicates the off response (B). Tooth-pulp stimulation, electrical stimulation of the tooth pulp (duration: 500 µs, frequency: 1 Hz); V1, baseline stimulus; V2, additional stimulus to the above baseline stimulus.
|
|
However, because previous studies were performed in anesthetized animals, it is not known whether animals perceived the tooth-pulp stimulation. To clarify the functional role of the SI for the perception of tooth-pulp sensation, it is necessary to study awake, behaving animals. Chudler et al. (1986)
recorded large field potentials from the oral projection area of SI and the secondary somatosensory cortex (SII) of awake monkeys after electrical stimulation of the tooth pulp. These potentials were elicited at nonnoxious stimulus intensities (i.e., they did not induce escape behavior). Furthermore, the SI field potentials became larger after increases in stimulus intensity. When a noxious stimulus was applied to the tooth pulp, the large field potentials were recorded from SII as well. Kenshalo et al. (1988)
, also using awake behaving monkeys, reported that the firing frequency of SI nociceptive neurons increased following graded heat stimulation and correlated well with the speed with which the monkeys perceived a change in noxious thermal stimuli. These findings suggest that nociceptive neurons in SI have a role in the perception of sensations produced by noxious stimuli applied to the facial skin.
To clarify the functional role of TPNs for the perception of the sensation produced by electrical stimulation of the tooth pulp, awake behaving monkeys were trained to detect changes in the intensity of a stimulus applied to the tooth pulp. The goal of this study was to elucidate the relationship between the firing frequency of TPNs in the SI and the detection latencies at which the monkeys perceived the changes in stimulus intensity. A part of this study was published in abstract form (Nomura et al. 1996
).

View larger version (13K):
[in this window]
[in a new window]
| FIG. 2.
Histogram of %escape to the V1 stimulus. Note that %escape is ~10% when the stimulus intensity is 0.5 and 1.0 T, whereas it increases when V1 stimulus intensities are increased to 2.0 and 3.0 T the jaw-opening reflex (JOR) threshold.
|
|
 |
METHODS |
This study was approved by the Animal Experimentation Committee at Nihon University School of Dentistry, and the treatment of the animals conformed to the guidelines of the International Association for the Study of Pain (Zimmerman 1983
).
Animal preparation
Two Japanese Macaca monkeys weighing 6.5-9.2 kg (M. fuscata, 4-5 yr old) were used for the present study. Before starting training, the monkey's water was restricted to 150 ml/day for 1 wk. Monkeys were trained to sit in the chair for 2-3 h and also to detect changes in the intensity of a light. They were seated facing an illuminated button and, at head height, a white electric light. In daily sessions they were trained to hold down the button and to release it when the light intensity was increased. The criterion performance was when the monkeys could hold the button down for between 4 and 8 s (the times were selected at random) and to release it within 3 s of the change in light intensity with >80% accuracy. Correct behavior was rewarded with 0.3 ml of orange juice. The training took place daily until criterion performance was reached. At this time, initial surgery was performed.
The monkeys were anesthetized initially with ketamine hydrochloride (10 mg/kg ip). Anesthesia was subsequently maintained with a mixture of halothane (2-3%), nitrous oxide (60%), and oxygen. Monkeys were placed in a stereotaxic frame. A head holder for chronic experiments was implanted onto the surface of the skull and stabilized using stainless steel screws and dental acrylic resin. During the initial surgery, body temperature was maintained at 37-38°C with a heating pad, and heart rate was continually monitored by electrocardiographic recording. Expired CO2 concentration was also monitored and maintained at a level between 3.0 and 4.0%. After the initial surgery, the monkeys were tested daily until the ability to discriminate the change in intensity of the light illumination had been reestablished. After that, they were reanesthetized with ketamine and halothane (2-3%), nitrous oxide (60%), and oxygen for implantation of the tooth-pulp electrodes. Upper canine or incisor teeth were prepared for implantation of wire electrodes. Two small holes were drilled into the dentine on the lingual and labial sides of the tooth, and bipolar platinum wire electrodes were embedded using amalgam, and sealed with acrylic resin. Bipolar Teflon-coated stainless steel wire electrodes were also embedded into the ipsilateral digastric muscles to monitor the jaw-opening reflex (JOR). The monkeys were trained to detect the change in stimulus intensity applied to the tooth pulp. After completion of training on this detection task, the monkeys were reanesthetized using the same anesthetic procedure as used for the initial surgery, and a metal chamber was installed over the orofacial projection area of the SI.

View larger version (26K):
[in this window]
[in a new window]
| FIG. 3.
Sample poststimulus time histograms (PSTHs) of tooth-pulp-driven neurons (TPNs), which increased firing frequency following increases in V1 stimulus intensity (A) and did not increase firing frequency following increases in stimulus intensity (B). Arrows indicate the stimulus onset. Peak firing frequencies were observed ~20 ms after tooth pulp stimulation.
|
|
After completing each surgical procedure, the monkeys were routinely sedated with a small amount of ketamine (2-3 mg/kg im) and given penicillin (10,000 U/kg im) and glucose-saline(40-50 ml of a 5% glucose solution in 0.18% NaCl, sc). Mefenamic acid (10 mg/kg, Pontal) dissolved in 200 ml orange juice was given daily after surgery for 3-4 days.
Behavioral task
A schematic illustration of the task used in the study reported here is shown in Fig. 1. Tooth-pulp impedance (mean ± SD) and JOR threshold were measured everyday before training the monkeys [tooth-pulp impedance: 90.7 ± 33.2 (SD) M
ranging from 46.7 to 162.3 M
; JOR threshold: 83.5 ± 21.5 µA ranging from 40 to 200 µA). Stimulus intensity was randomized as multiples of the threshold intensity for the JOR (1.0 T). The monkeys were seated quietly in the monkey chair, and an illuminated button was placed in front of them. When the cue light was illuminated, the monkeys pressed a button that caused electrical pulses (500 µs duration, 1 Hz) to be applied to the tooth pulp at an intensity of 0.5, 1.0, 2.0, or 3.0 T stronger than that that elicited a JOR. After a randomly determined period, ranging from 4 to 8 s, the stimulus intensity increased above the baseline stimulus (V2: 0.3, 0.5, 1.0, or 2.0 T). When the monkeys detected the change in stimulus intensity, they released the button within 3 s and received a reward (0.3 ml orange juice). In the present study, the monkeys could stop the tooth-pulp stimulation at any time by releasing the button. However, this behavior was not rewarded when monkeys released the button before the V2 stimulus came on.
Recording procedure
When the monkeys recovered from surgery and its ability to discriminate change in intensity of tooth-pulp stimulation was restored, the behavioral testing was carried out while simultaneously recording activity of units in the contralateral SI. Recordings were made daily from SI neurons whose activity was evoked by electrical stimulation of the tooth pulp during the detection task, using glass-coated tungsten microelectrodes, but no sessions lasted longer than 2 h. When single or multiunit neuronal activity was encountered, neurons were first classified as TPNs or LTM neurons. Then, mechanical receptive fields of each neuron were analyzed to see whether mechanical stimulation of the intraoral structures and facial skin were effective in activating these neurons. There are many limitations when using awake behaving monkeys in this type of research. It is important that the monkeys are able to terminate any type of noxious stimuli, such as tooth-pulp stimulation in this case. In the present study, the tooth-pulp stimulus was the only noxious stimulus that the monkeys are able to stop at any time by releasing the button. Therefore we did not introduce any natural noxious stimuli, such as pinching or squeezing.
Neuronal activity was fed into the tape recorder (bandwidth: DC-20 kHz) for off-line data analysis. After recording from TPNs, electrodes were advanced into the subcortical white matter, and lesions were made by passing 10 µA DC for 20 s for subsequent analysis of the location of recording sites. Lesions were not made closer than 1.0 mm apart and ranged from 5 to 10 lesions per hemisphere. The monkeys were tested daily until 100 penetrations had been made in each hemisphere (2-3 penetrations per 1-2 h daily recording session).
Data analysis
The waveforms of single or multiple neuronal activity were analyzed using a microcomputer system. The waveforms of each neuron were identified using spike 2 microcomputer software (CED LTD). Peristimulus time histograms (1 bin = 10 ms) were then developed in response to various stimuli. Because each neuron showed fluctuations in firing, we measured spontaneous discharge frequency [background (BG) activity; spikes/100 ms] for 60 s before application of the electrical stimulation of the tooth pulp, and then spontaneous discharge frequency was subtracted from the mean spike frequency within 100 ms after tooth-pulp stimulation (spikes/100 ms). Stimulus-response functions for each TPN were constructed as a function of stimulus intensity.

View larger version (15K):
[in this window]
[in a new window]
| FIG. 4.
Histogram of r2 value of stimulus-response (S-R) functions following the V1 stimulus. , encoding TPNs; , nonencoding TPNs.
|
|
The electrical stimulation of the tooth pulp was considered to have induced an effect if the mean firing frequency within 100 ms after stimulation was higher than the mean spontaneous rate ±2 SD.
Histology
When 200 penetrations had been made in each monkey (100 per each hemisphere), the monkeys were deeply anesthetized with pentobarbital sodium (80 mg/kg ip) and perfused transcardially with 3,000 ml of 10% paraformaldehyde, and the brains were removed. Fifty-micrometer-thick serial sections were cut along the path of the electrode penetrations. Every section was then counterstained with cresyl violet, and the locations of recording sites were subsequently analyzed using a light microscope.
Statistical analysis
Statistical analysis was performed by using analysis of variance (ANOVA) followed by post hoc Fisher's protected least significant difference (PLSD) or Scheffe's tests where appropriate. Differences were considered significant at P < 0.05.
 |
RESULTS |
Monkeys were trained to detect the change in stimulus intensity applied to the tooth pulp. Monkeys were used for recording experiments when >80% correct responses to detect the change in stimulus intensity applied to the tooth pulp were achieved. Monkeys were able to release the button before the stimulus intensity was increased (at the V2 stimulus) to escape from the stimulus. Although >80% correct responses were observed, the percent of escape responses was greater at stimulus intensities of 2.0 and 3.0 T(Fig. 2).

View larger version (19K):
[in this window]
[in a new window]
| FIG. 5.
Stimulus-response functions of TPNs to the V1 stimulus. Spike frequencies were plotted against stimulus intensity. Encoding TPNs whose S-R functions were significantly correlated increase in firing frequency following increases in V1 stimulus intensity from 0.5 to 3.0 T (A). Nonencoding TPNs whose S-R functions were not significantly correlated are shown in B. BG, background.
|
|

View larger version (18K):
[in this window]
[in a new window]
| FIG. 6.
Latency histogram of TPNs. First spike latencies following tooth-pulp stimulation were plotted. , encoding TPNs; , nonencoding TPNs; , TPNs that could not be classified as encoding or nonencoding TPNs.
|
|

View larger version (17K):
[in this window]
[in a new window]
| FIG. 7.
Electrode penetration tracks were plotted on the SI of one monkey's hemisphere. Note that tracks where encoding and nonencoding TPNs were encountered were intermingled with each other. CS, central sulcus.
|
|
Responses to the baseline (V1) stimulus
Eight hundred sixty-two single neurons (monkey R: 433 neurons; monkey A: 429 neurons) were isolated from the orofacial projection areas of the SI. Thirty-seven of them responded to electrical stimulation of the tooth pulp (16 TPNs from monkey R and 21 TPNs from monkey A). Tactile stimulation of the lateral face produced responses in 139 neurons, and 90 responded to upper lip stimulation, 9 to lower lip stimulation, 44 to tongue stimulation, and 102 to periodontal membrane stimulation. Three hundred fifty-one neurons did not have any identified receptive fields on the face or intraoral structures to mechanical stimuli. Figure 3 shows sample poststimulus time histograms (PSTHs) of a TPN following application of different intensities of V1 (0.5, 1.0, 2.0, and 3.0 T). When the stimulus intensity of 0.5 T was applied, a small number of spikes were elicited just after the onset of the stimulus (indicated by the arrows in Fig. 3A). When stimulus intensities were increased from 1.0 to 3.0 T, firing frequency increased with increases in stimulus intensity.

View larger version (24K):
[in this window]
[in a new window]
| FIG. 8.
Intracortical distribution of TPNs in one hemisphere. , encoding TPNs; , nonencoding TPNs. Top trace is from the most medial section and the bottom is the lateral. CS, central sulcus; IPS, intraparietal sulcus.
|
|

View larger version (36K):
[in this window]
[in a new window]
| FIG. 9.
Schematic illustrations of typical receptive fields of encoding (top 2 panels and drawings on the right side of the dotted line in the bottom panels) and those of nonencoding TPNs (drawings to the left of dotted lines in the bottom panels).
|
|

View larger version (29K):
[in this window]
[in a new window]
| FIG. 10.
Sample PSTHs of TPNs after V2 stimulation. Arrows indicate the V2 stimulus onset. Each column indicates the histograms at different V1 stimulus intensities. V2 stimulus intensities are 0.3, 0.5, 1.0, and 2.0 T from the top histograms to the bottom histograms, respectively.
|
|
On the other hand, a TPN illustrated in Fig. 3B did not increase firing frequency following increases in stimulus intensity. When the V1 stimulus intensity was 2.0 or 3.0 T, neuronal activity was frequently depressed for 50 ms just after peak firing was reached. Regression analysis of the V1 stimulus-response functions of each TPN revealed two groups of TPNs classified according to their r2 values; encoding TPNs whose r2 values were >0.6 and nonencoding TPNs whose r2 values are <0.6 (Chudler et al. 1990
). Figure 4 illustrates the distribution of r2 values of encoding and nonencoding TPNs. r2 values of encoding TPNs were >0.8 (0.90 ± 0.02, SE), whereas those of nonencoding TPNs were <0.6 (0.20 ± 0.04). Stimulus-response functions of encoding and nonencoding TPNs to the V1 stimulus are illustrated in Fig. 5. Eleven TPNs increased their firing frequencies following increases in stimulus intensity from 0.5 to 3.0 T, and had r2 values of >0.6 (Fig. 5A: encoding TPNs) and 19 did not grade firing frequencies following increases in stimulus intensities (Fig. 5B: nonencoding TPNs, r2 < 0.6). Figure 6 illustrates the histogram of first spike latencies of TPNs after V1 tooth-pulp stimulation(V1 = 1.0). Most of the TPNs responded with first spike latencies of 20-30 ms after tooth-pulp stimulation (SE: encoding: 24.8 ± 1.7 ms; nonencoding: 23.6 ± 1.5 ms). There were no significant differences in latencies between encoding and nonencoding TPNs.
Cortical distribution of TPNs
Figure 7 illustrates the cortical distribution of electrode penetration tracks where TPNs were encountered in one hemisphere. Most TPNs were distributed in the oral projection area, and some were in the intermediate zone between oral and facial projection areas of the SI. Encoding (
) and nonencoding TPNs (
) were intermingled. All TPNs received input from both tooth pulp and orofacial structures. This distribution pattern was observed within the SI cortex as well. Sample intracortical distribution of the TPNs from one hemisphere is illustrated in Fig. 8. TPNs were distributed in the areas 1-2, 3a, and 3b of the SI near the central sulcus. No distribution differences of encoding and nonencoding TPNs were observed in the SI. Receptive fields were mainly located in the intraoral structures, such as periodontal tissues, and some of them were in the face area as shown in Fig. 9. Furthermore, receptive fields of encoding TPNs were restricted to the intraoral structures as illustrated on the left sides of the dotted line of the bottom panels in Fig. 9. On the other hand, receptive fields of nonencoding TPNs were located in both intraoral structures and facial skin.
Relation between firing frequency and detection latency
Figure 10 shows the sample recordings of PSTHs after V2 stimulation at different V1 stimulus intensities. Phasic responses were observed after the V2 stimulus. V2 stimulus onset is indicated by the arrows in each histogram. Firing frequencies increased following increases in V2 stimulus intensity at all of the V1 stimulus intensities. When the stimulus intensity was high (V1 = 3.0 T, V2 = 2.0 T), depression of the spike potentials was observed just after the peak frequency, as previously shown in Fig. 3, for responses to V1 stimulus intensities.

View larger version (19K):
[in this window]
[in a new window]
| FIG. 11.
Histograms of r2 values following V2 stimulus at different V1 stimulus intensities. A: histogram of r2 values when V1 values are 0.5 T and 1.0 T. B: histogram of r2 values when V1 values are 2.0 T and 3.0 T. Sample scatter plots of monkey's detection latency and firing frequency of TPNs are illustrated in Aa and Bb. Spikes/100 ms after stimulus are plotted against as the detection latency. Solid lines in each scatter plot represent the linear regression of the equation fit to the scatter plots. , encoding TPNs; , nonencoding TPNs.
|
|
Regression analysis was applied to the relationship between firing frequency and detection latencies to the V2 stimulus. It has been shown in human psychophysical studies that 0.5 and 1.0 T stimuli do not produce pain sensation, whereas 2.0 and 3.0 T produce pain (McGrath et al. 1981
, 1983
; Mumford and Stanley 1981
). Thus in this study, stimulus intensities of 0.5 to 1.0 T and 2.0 to 3.0 T were categorized as near-noxious threshold and noxious stimuli, respectively. According to the above-mentioned categories, r2 values at 0.5 to 1.0 T stimuli and 2.0 to 3.0 T stimuli are plotted in the histograms in Fig. 11, A and B, respectively. A sample scatter plot between firing frequency and detection latency is shown in Fig. 11Aa. When the V1 values were 0.5 and 1.0 T, for 8 of 30 TPNs there was a significant correlation between firing frequency and detection latency (P < 0.05). On the other hand, when the stimulus intensities were high (2.0 and 3.0 T), firing frequencies of only 3 of 30 TPNs were significantly correlated with detection latency (P < 0.05) as illustrated in Fig. 11Bb.
 |
DISCUSSION |
It is known from human psychophysical experiments that electrical stimulation of the tooth pulp elicits both pain and nonpain (prepain) sensations (Brown et al. 1985
; Byers 1984
; McGrath et al. 1981
, 1983
; Mumford and Bowsher 1988
; Mumford and Stanley 1981
; Shimizu 1964
). Anatomic and electrophysiological studies using anesthetized animals have shown the existence of many large myelinated nerve fibers in the tooth pulp (Beasley and Holland 1978
; Cadden et al. 1983
; Dong et al. 1993
; Holland 1978
; Holland and Robinson 1983
; Lisney 1978
; Sessle 1987
). Recently, Dong et al. (1993)
provided direct evidence of the existence of intra-pulpal A
nerve fibers using a single-unit recording technique. These results suggest that the prepain sensation produced by weak electrical stimulation of the tooth pulp in humans may be elicited by intra-pulpal A
fibers. High-intensity electrical stimulation of the tooth pulp has been known to activate small diameter nerve fibers. It has been also reported that there is a high correlation between stimulus intensity and primary afferent responses in the tooth pulp following graded electrical stimulation (Boissonade and Matthews 1993
; Byers 1984
; Mason et al. 1985
). Although electrical stimulation is not the most accurate procedure for activating only C or A
nerve fibers in the tooth pulp, it is convenient to use this technique for analyzing the relationship between stimulus intensity and neuronal activity in a quantitative manner. Furthermore, natural tooth-pulp stimuli such as thermal or chemical stimulation is difficult to apply in awake behaving animals. Thus electrical stimulation of the tooth pulp is a suitable method for quantitatively activating pulpal small and large diameter nerve fibers. Therefore awake behaving monkeys were studied to analyze the relationship between cortical neuronal activity and the perceived intensity of electrical tooth-pulp stimulation.
The tooth-pulp stimulus intensity was defined relative to the threshold intensity for the JOR to normalize the stimulus intensities in different animals. The relationship between the detection latencies of changes in tooth-pulp stimulus intensities and neuronal responses from SI neurons were analyzed precisely. In human pain reports, it has been shown that when the stimulus intensity was increased to >2.0 T, a pain sensation occurred (McGrath et al. 1981
, 1983
; Mumford and Bowsher 1988
; Mumford and Stanley 1981
). In this study, when the stimulus intensity was lower than the JOR threshold, escape occurred on ~10% of the trials, whereas when the stimulus intensity was at 2.0 and 3.0 T, escape frequencies increased to 20-40%. These findings suggest that stimulus intensities of 0.5 and 1.0 T for the JOR are near-noxious, whereas 2.0 and 3.0 T are noxious for monkeys.
It is well-known that SI neurons are somatotopically arranged such that each SI neuron receives somatosensory information from a restricted region of the body (Darian-Smith et al. 1966
; Felleman et al. 1983
; Kaas et al. 1979
; Landgren and Olsson 1980
; Taira 1987
). It has been reported that hindlimb regions in monkeys are represented in the medial portion of the posterior part of the central sulcus and intraoral structures are in the most lateral part (Kaas et al. 1979
; Ogawa et al. 1989
). Most of the neurons in SI responded to mechanical stimulation of the receptive fields, and some of them also received nociceptive inputs (Chudler et al. 1990
; Kenshalo et al. 1983
; Lamour et al. 1982
). Furthermore, it has been also reported that a large number of SI neurons responded to electrical, chemical, or thermal stimulation of the tooth pulp in the orofacial projection area in the SI (Iwata et al. 1986
, 1987
, 1990
, 1994
; Lund and Sessle 1974
; Matsumoto 1984
; Matsumoto et al. 1987
, 1989
). Most of the TPNs were located in the oral projection area of SI, and they were located in areas 1-2 and 3 cytoarchitectonically. In previous studies, it has been reported that TPNs were located in the areas 3a and 3b of the orofacial projection area in cats (Iwata et al. 1986
, 1987
, 1990
, 1994
; Lund and Sessle 1974
; Matsumoto 1984
; Matsumoto et al. 1987
, 1989
). These cytoarchitectonic differences of TPN distribution between previous results are likely related to species differences.
It has been reported that many TPNs receive inputs from the intraoral structures, facial skin, or both (Iwata et al. 1986
, 1987
, 1990
, 1994
; Lund and Sessle 1974
; Matsumoto 1984
; Matsumoto et al. 1987
, 1989
). Based on receptive-field properties, TPNs were classified as 1) LTM neurons that responded exclusively to innocuous mechanical stimulation of the receptive field, 2) WDR neurons that responded to both innocuous and noxious stimulation of the receptive fields and increased their firing frequency following increases in stimulus intensity, or 3) NS neurons that exclusively responded to noxious mechanical or thermal stimulation of the receptive fields (Iwata et al. 1990
). The previous findings revealed that most of the TPNs were activated by both tooth pulp and cutaneous noxious and/or nonnoxious stimulation, suggesting that TPNs in SI are involved in processing innocuous and noxious sensory information from cutaneous, mucosal, and/or periodontal inputs as well as from the tooth pulp. In the present study, we did not use any noxious natural stimuli, such as pinching or squeezing of the skin. Thus WDR and NS TPNs could not be distinguished. The data from the present study describe similar convergence properties of TPNs, in which all TPNs receive inputs from the intraoral mucous membrane, facial skin, or periodontal structures. The convergence of peripheral sensory inputs on TPNs suggests that these neurons may be involved in signaling spatial localization and referred pain. We also found that encoding TPNs had receptive fields restricted to the intraoral structures, whereas nonencoding TPNs had receptive fields on the facial skin and/or intraoral structures, and some of the nonencoding TPNs had larger receptive fields than those of the encoding neurons. This may suggest that TPNs with restricted receptive fields in the intraoral structures are involved in the sensory encoding process related to tooth-pulp sensation.
Many TPNs increase their firing frequency following increases in stimulus intensity (Iwata et al. 1990
; Matsumoto et al. 1987
). In the present study, many TPNs increased their firing frequency dependent on increases in the baseline (V1) stimulus intensity. Furthermore, in 11 of 30 TPNs firing frequency and stimulus intensity were significantly correlated with each other. These response properties of TPNs recorded from the SI in awake behaving monkeys are similar to those reported previously in anesthetized animals (Iwata et al. 1986
, 1987
, 1990
, 1994
; Lund and Sessle 1974
; Matsumoto 1984
; Matsumoto et al. 1987
, 1989
). Although TPNs in the anesthetized and awake behaving animals show similar electrophysiological characteristics, it is still useful to study the behavioral and neuronal correlates in awake monkeys to clarify the central mechanism of perception of tooth-pulp stimulation. It has been reported that the perception of nociceptive information can be separated into two components (Melzack and Casey 1968
; Price and Dubner 1977
): a sensory discriminative component and a motivational-affective component. The sensory discriminative component refers to the ability to judge the quality, localization, and intensity of painful stimulation. The motivational-affective component refers to the aversive qualities of painful stimulation that lead to avoidance and escape behavior. A large number of nociceptive neurons in the medullary dorsal horn, thalamus, or SI show a high correlation between firing frequency and the monkey's detection latency. Kenshalo et al. (1988)
have reported that the firing frequency of SI nociceptive neurons and detection speed are highly correlated and concluded that SI nociceptive neurons have a role in the sensory discriminative aspect of pain. In the present study, 11 of 30 TPNs showed a significant correlation between firing frequency and detection latency. The proportion of TPNs with significant correlation between firing frequency and detection latency is relatively low as compared with those reported for cutaneous nociceptive neurons in the SI (Kenshalo et al. 1988
). This difference between our results and previous data might be the result of a difference in stimulus modality used in the two studies (thermal vs. electrical). Also WDR and NS TPNs were not classified in the present study, because we did not test any natural noxious mechanical or thermal stimuli. It is possible that for WDR or NS TPNs, there are significant correlations between behavior and neuronal discharge. Furthermore, 8 of 11 TPNs exhibited a significant correlation between firing frequency and detection latency at V1 stimulus intensities of 0.5 and 1.0 T, whereas only 3 of them exhibited significant correlation at stimulus intensities of 2.0 and 3.0 T. Our findings suggest that TPNs may play more of a role in sensory discrimination in the near-noxious threshold range than at higher intensities of noxious stimulation.
 |
ACKNOWLEDGEMENTS |
We thank Profs. R. Dubner and J. D. Stephenson, and Dr. D. A. Thomas for helpful comments on this manuscript.
This study was supported by the Sato Fund, Nihon University School of Dentistry and in part by Grants-in-Aid for Scientific Research (7457442, 9671907, 9771551) from the Japanese Ministry of Education, Science and Culture.
 |
FOOTNOTES |
Address reprint requests to K. Iwata.
Received 12 September 1997; accepted in final form 9 December 1997.
 |
REFERENCES |
-
ARMSTRONG-JAMES, M.,
FOX, K.
Spatiotemporal convergence and divergence in the rat SI "barrel" cortex.
J. Comp. Neurol.
263: 265-281, 1987.[Medline]
-
BEASLEY, W. L.,
HOLLAND, R. A
quantitative analysis of the innervation of the pulp of the cat's canine tooth.
J. Comp. Neurol.
178: 487-494, 1978.[Medline]
-
BIEDENBACH, M. A.,
VAN HASSEL, H. J.,
BROWN, A. C.
Tooth pulp-driven neurons in somatosensory cortex of primates: role in pain mechanism including a review of the literature.
Pain
7: 31-50, 1979.[Medline]
-
BOISSONADE, F. M.,
MATTHEWS, B.
Responses of trigeminal brain stem neurons and the digastric muscle to tooth-pulp stimulation in awake cats.
J. Neurophysiol.
69: 174-186, 1993.[Abstract/Free Full Text]
-
BROWN, A. C.,
BEELER, W. J.,
KLOKA, A. C.,
FIELDS, R. W.
Spatial summation of pre-pain and pain in human teeth.
Pain
21: 1-16, 1985.[Medline]
-
BUSHNELL, M. C.,
DUNCAN, G. H.
Sensory and affective aspects of pain perception is medial thalamus restricted to emotional issue?
Exp. Brain Res.
78: 415-418, 1989.[Medline]
-
BUSHNELL, M. C.,
DUNCAN, G. H.,
DUBNER, R.,
HE, L. F.
Activity of trigeminothalamic neurons in medullary dorsal horn of awake monkeys trained in a thermal discrimination task.
J. Neurophysiol.
52: 170-187, 1984.[Abstract/Free Full Text]
-
BUSHNELL, M. C.,
DUNCAN, G. H.,
TREMBLAY, N.
Thalamic VPM nucleus in the behaving monkey. I. Multimodal and discriminative properties of thermosensitive neurons.
J. Neurophysiol.
69: 739-752, 1993.[Abstract/Free Full Text]
-
BYERS, M. R.
Dental sensory receptors.
Int. Rev. Neurobiol.
25: 39-94, 1984.[Medline]
-
CADDEN, S. W.,
LISNEY, S.J.W.,
MATTHEWS, B.
Thresholds to electrical stimulation of nerves in cat canine tooth-pulp with A
, A
and C-fiber conduction velocities.
Brain Res.
261: 31-41, 1983.[Medline] -
CHAPIN, J. K.,
LIN, C.
Mapping the body representation in the SI cortex of anesthetized and awake rats.
J. Comp. Neurol.
229: 199-213, 1984.[Medline]
-
CHUDLER, E. H.,
ANTON, F.,
DUBNER, R.,
KENSHALO, D. R. JR
Responses of nociceptive neurons in monkeys and pain sensation in humans elicited by noxious thermal stimulation: effect of interstimulus interval.
J. Neurophysiol.
63: 559-569, 1990.[Abstract/Free Full Text]
-
CHUDLER, E. H.,
DONG, W. K.,
KAWAKAMI, Y.
Cortical nociceptive responses and behavioral correlates in the monkey.
Brain Res.
397: 47-60, 1986.[Medline]
-
DARIAN-SMITH, I.,
ISBISTER, J.M.H.,
YOKOTA, T.
Somatic sensory cortical projection areas excited by tactile stimulation of the cat, a triple representation.
J. Physiol. (Lond.)
182: 671-689, 1966.[Medline]
-
DONG, W. K.,
SHIWAKU, T.,
KAWAKAMI, Y.,
CHUDLER, E. H.
Static and dynamic responses of periodontal ligament mechanoreceptors and intradental mechanoreceptors.
J. Neurophysiol.
69: 1567-1582, 1993.[Abstract/Free Full Text]
-
DUBNER, R.
The effect of behavioral state on the sensory processing of nociceptive and non-nociceptive information.
In: Progress in Brain Research. Pain Modulation, edited by
H. L. Field,
and J. M. Besson
Amsterdam: Elsevier, 1988, vol. 77, p. 213-223.
-
DUBNER, R.,
KENSHALO, D. R. JR,
MAXINER, W.,
BUSHNELL, M. C.,
OLIVERAS, J.-L.
The correlation of monkey medullary dorsal horn neuronal activity and the perceived intensity of noxious heat stimuli.
J. Neurophysiol.
62: 450-457, 1989.[Abstract/Free Full Text]
-
FELLEMAN, D. J.,
WALL, J. T.,
CUSICK, C. G.,
KAAS, J. H.
The representation of the body surface in SI of cats.
J. Neurosci.
3: 1684-1669, 1983.
-
HASSLER, R.,
MUHS-CLEMENT, K.
Architectonisher aufbar des sensomotorishen cortex der katze.
J. Hirnforsch.
6: 377-420, 1964.
-
HOFFMAN, D. S.,
DUBNER, R.,
HAYES, R. L.,
MEDLIN, T. P.
Neuronal activity in medullary dorsal horn of awake monkeys trained in a thermal discrimination task. I. Responses to innocuous and noxious thermal stimuli.
J. Neurophysiol.
46: 409-427, 1981.[Free Full Text]
-
HOLLAND, G. R.
Fiber numbers and sizes in the inferior alveolar nerve of the cat.
J. Anat.
127: 343-352, 1978.[Medline]
-
HOLLAND, G. R.,
ROBINSON, P. P.
The number and size of axons at the apex of the cat's canine tooth.
Anat. Rec.
205: 215-222, 1983.[Medline]
-
IWATA, K.,
ITOGA, H.,
IKUKAWA, A.,
TAMURA, K.,
SUMINO, R.
Cortical cells driven by the low threshold tooth-pulp afferent in cats.
Brain Res.
368: 399-403, 1986.[Medline]
-
IWATA, K.,
ITOGA, H.,
MURAMATSU, H.,
TODA, K.,
SUMINO, R.
Responses of bradykinin sensitive tooth pulp-driven neurons in cat cerebral cortex.
Exp. Brain Res.
66: 435-439, 1987.[Medline]
-
IWATA, K.,
TSUBOI, Y.,
MURAMATSU, H.,
SUMINO, R.
Distribution and response properties of cat SI neurons responsive to changes in tooth temperature.
J. Neurophysiol.
64: 822-834, 1990.[Abstract/Free Full Text]
-
IWATA, K.,
TSUBOI, Y.,
YAGI, J.,
KITAJIMA, K.,
SUMINO, R.
Morphology of primary somatosensory cortical neurons receiving input from the tooth pulp.
J. Neurophysiol.
72: 831-845, 1994.[Abstract/Free Full Text]
-
KAAS, J. H.,
NELSON, M.,
LIN, C. S.,
MERZENNICH, M. M.
Multiple representations of the body within the primary somatosensory cortex of primates.
Science
63: 521-523, 1979.
-
KENSHALO, D. R. JR,
CHUDLER, E. H.,
ANTON, F.,
DUBNER, R.
SI nociceptive neurons participate in the encoding process by which monkey perceive of noxious thermal stimulation.
Brain Res.
454: 378-382, 1988.[Medline]
-
KENSHALO, D. R. JR,
ISENSEE, O.
Responses of primate SI cortical neurons to noxious stimuli.
J. Neurophysiol.
50: 1479-1496, 1983.[Abstract/Free Full Text]
-
LAMOUR, Y.,
WILLER, J. C.,
GUILBAUD, G.
Neuronal responses to noxious stimulation in rat somatosensory cortex.
Neurosci. Lett.
29: 35-40, 1982.[Medline]
-
LANDGREN, S.,
OLSSON, K. A.
Low threshold afferent projections from the oral cavity and the face to the cerebral cortex of the cat.
Exp. Brain Res.
39: 133-147, 1980.[Medline]
-
LISNEY, S.J.W.
Some anatomical and electrophysiological properties of tooth-pulp afferents in the cat.
J. Physiol. (Lond.)
284: 19-36, 1978.[Medline]
-
LUND, J. P.,
SESSLE, B. J.
Oral-facial and jaw muscle afferent projections to neurons in cat frontal cortex.
Exp. Neurol.
45: 324-331, 1974.
-
MASON, P.,
STRASSMAN, A.,
MACIEWICZ, R.
Is the jaw-opening reflex a valid model of pain?
Brain Res. Rev.
10: 137-146, 1985.
-
MATSUMOTO, N.
Functional difference of tooth pulp-driven neurons in oral and facial areas of the somatosensory cortex (SI) of the cat.
Exp. Neurol.
85: 437-451, 1984.[Medline]
-
MATSUMOTO, N.,
SATO, T.,
SUZUKI, T. A.
Characteristics of the tooth pulp-driven neurons in a functional column of the cat's somatosensory cortex (SI).
Exp. Brain Res.
74: 263-271, 1989.[Medline]
-
MATSUMOTO, N.,
SATO, T.,
YAHATA, F.,
SUZUKI, T. A.
Physiological properties of tooth pulp-driven neurons in the first somatosensory cortex (SI) of the cat.
Pain
31: 249-262, 1987.[Medline]
-
MCGRATH, P. A.,
GRACELY, R. H.,
DUBNER, R.,
HEFT, M. W.
Non-pain and pain sensations evoked by tooth pulp stimulation.
Pain
15: 377-388, 1983.[Medline]
-
MCGRATH, P. A.,
SHARAV, Y.,
DUBNER, R.,
GRACELY, R.
Masseter inhibitory periods and sensations evoked by electrical tooth pulp stimulation.
Pain
15: 1-17, 1981.
-
MELZACK, R.,
CASEY, K. L.
Sensory, motivational and central control determinants of pain, a new conceptual model.
In: The Skin Sense, edited by
and D. R. Kenshalo
Springfield, IL: Thomas, 1968, p. 423-439.
-
MUMFORD, J. M.,
BOWSHER, D.
Pain and protopathic sensitivity. A review with particular reference to teeth.
Pain
2: 223-243, 1988.
-
MUMFORD, J. M.,
STANLEY, S. J.
Sensations on stimulating the pulps of human teeth, threshold and tolerance ratio.
Pain
10: 391-398, 1981.[Medline]
-
NOMURA, H.,
IWATA, K.,
SHIMIZU, N.,
TSUBOI, Y.,
SUMINO, R.
Distribution and response properties of tooth pulp-driven neurons in the primary somatosensory cortex in awake behaving monkeys.
Pain
8, Suppl.: 146, 1996.
-
OGAWA, H.,
ITO, S.,
NOMURA, T.
Oral cavity representation at the frontal operculum of macaque monkeys.
Neurosci. Res.
6: 283-298, 1989.[Medline]
-
PRICE, D. D.,
DUBNER, R.
Neurons that subserve the sensory discriminative aspects of pain.
Pain
3: 307-338, 1977.[Medline]
-
SESSLE, B. J.
The neurobiology facial and dental pain: present knowledge, future directions.
J. Dent. Res.
66: 962-981, 1987.[Abstract]
-
SHIMIZU, T.
Tooth pre-pain sensation elicited by electrical stimulation.
J. Dent. Res.
43: L467-475, 1964.
-
TAIRA, K.
The representation of the oral structures in the first somatosensory cortex of the cat.
Brain Res.
409: 41-51, 1987.[Medline]
-
WELKER, C.
Microelectrode delineation of fine grain somatotopic organization of SM1 cerebral neocortex in albino rat.
Brain Res.
26: 259-275, 1971.[Medline]
-
ZIMMERMAN, M.
Ethical guide-lines for investigation of experimental pain in conscious animals.
Pain
16: 109-110, 1983.[Medline]