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Laboratorio de Neurofisiología, Departamento de Fisiología, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina
Submitted 11 May 2004; accepted in final form 6 October 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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mol/0.5 µl) into striatal JOR inhibitory sites significantly decreased the A
and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons (n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Graybiel et al. 1994; Mink 1996; Wichmann and DeLong 2003). However, recent studies have shown that they are implicated in sensory, cognitive, and autonomic functions (Abbruzzese and Berardelli 2003; Graybiel 1998; Green et al. 2002a; Kimura 1995; Packard and Knowlton 2002; Schultz 1998; Soliveri et al. 1997). Furthermore, evidence from experimental and clinical studies has shown that the basal ganglia also have a role in the processing of nociceptive information and pain (for review, see Chudler and Dong 1995). In anesthetized animals, a large proportion of striatal neurons are differentially or exclusively activated by noxious stimuli (Chudler 1998; Chudler et al. 1993; Richards and Taylor 1982; Schneider and Lidsky 1981). Studies in human volunteers have shown that thermal painful stimulation of the hand increases the blood flow measured by PET in the contralateral striatum (Jones et al. 1991). Also, clinical studies have shown that 
40% of Parkinson’s disease patients complain of sensory abnormalities unrelated to motor disturbances and most frequently connected with pain dysfunctions (Factor et al. 2000; Ford 1998; Goetz et al. 1986; Honey et al. 1999; Koller 1984; Snider et al. 1976; Witjas et al. 2002). Similar results were reported in animal models of the disease (Carey 1986; Saade et al. 1997). The basal ganglia and specifically the striatum are among the neural structures with highest concentrations of endogenous opiates and their receptors (Angulo and McEwen 1994; McGeer and McGeer 1993; Parent et al. 1995), which could be relevant in endogenous analgesia (Hebert et al. 1990; Kurumaji et al. 1988; Thorn-Gray and Levitt 1983). Neuropharmacological and electrophysiological studies have evaluated the analgesic behavioral effect of basal ganglia manipulation (Anagnostakis et al. 1992; Baumeister et al. 1988, 1990; Gear et al. 1999; Jurna and Heinz 1979; Lin et al. 1981; Lineberry and Vierck 1975; Schmidek et al. 1971). However, in these studies, it is difficult to separate the effects related to analgesia from those related with motor phenomena because the manipulation of the basal ganglia may have an effect on the motor response of the animal to painful stimuli, without modifying the sensation or perception of pain.
The jaw opening reflex (JOR) elicited by the suprathreshold stimulation of the tooth pulp has been considered a valid measurement of nociceptive response (Boissonade and Matthews 1993; Chiang et al. 1989, 1991; Mahan and Anderson 1970; Schmidt et al. 2002a; Takeda et al. 2002; Tambeli et al. 2002; Zhang et al. 1998, 1999). We have previously shown in anesthetized rats that electrical or chemical stimulation of the striatum, within a specific region, results in the inhibition of the JOR (Belforte et al. 2001). To elucidate the neural substrate of the striatal inhibitory effect, we performed electrophysiological experiments designed to evaluate the effect of the striatal stimulation on the activity of sensory and motor neurons related to the tooth pulp–evoked JOR.
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METHODS |
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All procedures were performed on 42 adult male Sprague-Dawley rats weighing 300–450 g in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals. Before surgery, rats were housed in groups of four to five per cage (cage size: 54 cm length, 36 cm width, 22 cm height) and maintained in a 12-h light/dark schedule with free access to food and water. All efforts were made to minimize animal suffering and to reduce the number of animals used.
Surgery
The rats were anesthetized with urethane (1.2 g/kg, ip), and additional anesthetic was administered as necessary to maintain a constant level of anesthesia. The depth of anesthesia was systematically checked by the lack of response to paw pinching. The animals were prepared for acute recording of the tooth pulp stimulation–elicited JOR as previously described (Chiang et al. 1989; Myslinski and Mattews 1987; Pazo et al. 2001). Briefly, a silver wire electrode (0.5 mm diam) was inserted into the pulp cavity of each lower incisor and fixed with dental acrylic. The JOR was recorded as EMG activity from the anterior belly of both digastric muscles by means of bipolar twisted stainless steel electrodes (80 µm diam). The head of the animal was fixed to a stereotaxic frame (David Kopf, Tujunga, CA). The skin over the dorsal surface of the skull was reflected, and a small craniectomy was performed over the target structures. All the incisions and pressure points were infiltrated with a long-lasting local anesthetic (bupivacaine). The body temperature was maintained at 37 ± 0.5°C with an electric blanket thermostatically controlled by a rectal probe (FHC, Bowdoinham, ME).
Tooth pulp stimulation
Electrical rectangular pulses of 0.83 Hz and 0.5-ms duration (A300 Pulsemaster WPI and S.I.U. 385) were applied to dental pulp through the electrodes implanted in the lower incisors. The threshold for EMG response of the digastric muscle was determined. The intensity was increased 2–2.5 times the threshold until reproducible and regular responses were obtained.
Electrophysiological recordings
Extracellular single-unit recording was performed by means of glass microelectrodes with tip diameters of 1–5 µm (1–10 M
), filled with 2% Pontamine Sky Blue in 2 M NaCl. Unilateral or bilateral simultaneous single unit recordings were performed with electrodes placed in the Sp5c. The targeted region within the nucleus was 2–3 mm lateral to the midline and between 4.6 and 5.6 mm posterior to the interaural line (IA; Paxinos and Watson 1997). In a second set of experiments, we simultaneously recorded the activity of single units into the Sp5c and the ipsilateral motor trigeminal nucleus (Mo5; AP: –0.3 to –0.8 mm from IA, L: 1.6–2.2 mm from midline, V: 8.5–9 mm from dura; Paxinos and Watson 1997). The electrodes were hydraulically advanced through Sp5c or Mo5 until single units could be isolated. Only neurons with a signal-to-noise ratio >2:1 were analyzed. As the electrodes were lowered, the tooth pulp was periodically stimulated to search for neuronal-evoked activity. The EMG response from digastric muscles and single unit activities was recorded with high-impedance amplifiers and band-pass filters of 30–300 and 300–3,000 Hz, respectively (P511 Grass Instruments, Quincy, MA), displayed on a PC previous A/D conversion, monitored with an audio amplifier, and recorded on videotape with a digital data recorder (VR-100B, Instrutech Corp., Long Island, NY) for subsequent off-line computer-assisted analysis of the data.
Characterization of Sp5c neurons
Each neuron was tested for its responsiveness to innocuous mechanical stimuli (i.e., brushing with a soft brush, light pressure applied to the skin and oral mucosa with a blunt probe, hair movement, and vibrissae displacement). Noxious stimuli consisted of heavy pressure, pinprick, and squeezing, applied with pins and serrated forceps. The latter evoked painful sensations when applied to the experimenter’s skin. For units with no spontaneous activity, any consistent stimulus-evoked discharge was considered to be a response. For units with spontaneous activity, a stimulus-evoked response was defined as an increase in firing rate of 3 SD over the mean spontaneous activity. Neurons were classified according to previously outlined criteria (Hu 1990; Meng et al. 2000; Sessle et al. 1986) as nociceptive specific (NS), wide dynamic range (WDR), or low-threshold mechanoreceptive (LTM) neurons. LTM units responded only to innocuous stimuli (e.g., hair movement and light pressure), WDR units were sensitive to both nonnoxious and noxious stimuli, and NS neurons were activated only by noxious stimuli. Primary afferents were distinguished by their ability to faithfully follow 200-Hz electrical stimulation applied within their receptive field (Hu 1990) and were not considered further. Once the neuron had been identified, the extent of its receptive field was determined and mapped onto scale drawings of the rat’s face. In addition, the responses evoked by tooth pulp stimulation in Sp5c were classified as A or C fiber inputs according to previous criteria (Hu 1990; Hu et al. 1981; McHaffie et al. 1994). To that end, the latency value of the responses was used to determine the conduction velocity of the afferent inputs after making an allowance for the conduction distance (30–40 mm) and 1 ms for the synaptic delay. A response to tooth pulp stimulation with a conduction velocity <2 m/s was considered to be the result of C fiber input to the neuron and termed C input, whereas those with conduction velocity <30 m/s were categorized as A fiber primary afferents, and termed A inputs (Dallel et al. 1998; Hu 1990; Meng et al. 2000; Woda et al. 2001).
Data analysis
The stored signal was digitized off-line by means of an A/D converter (sampling frequency, 10 kHz; DigiData 1200, Axon Instruments) and analyzed (PSW V5.1, DataWave Technologies) to study spontaneous and evoked activity. Spontaneous activity was expressed as mean spike count per second. To quantify the tooth pulp–evoked activity, the responses (number of spikes) to 50 consecutive stimuli were counted and plotted as poststimulus time histograms (PSTHs; bin width: 0.5–1 ms, 50 stimuli) to measure A and C responses. Responses with values below 3 SD from the control mean were designated inhibitory. The latency of the tooth pulp–evoked responses was determined from PSTHs constructed for this purpose (bin width: 0.1 ms, 150 stimuli). The peak-to-peak amplitude of each JOR was measured, and the mean and SE of 50 consecutive JORs were calculated for the same periods of time used for the PSTH and expressed as percentage of control baseline. A response was considered inhibitory only when the mean amplitude of the JOR differed significantly from control values as determined by one-way ANOVA for repeated measures.
Intrastriatal microinjection
To stimulate the striatum, monosodium-L-glutamate (80 moles dissolved in 0.5 µl of isotonic saline) was slowly microinjected over a period of 60 s through a stainless steel cannula (0.3 mm OD) connected via polyethylene tubing to a 5-µl Hamilton microsyringe driven by a microdrive unit (Baltimore Instruments). This dose was selected based on previous experiments (Belforte et al. 2001) and bibliographical data (Behbehani and Fields 1979; Chiang et al. 1989; Janss and Gebhart 1988; Meng et al. 2000). The cannula was stereotaxically directed to the central-medial region of the striatum (AP: 0.2–0.48 mm anterior to bregma, L: 2.7 mm, V: 5–6 mm below dura), because this area has been shown to induce inhibition of the JOR in previous experiments (Belforte et al. 2001). Some animals were microinjected with solvent as control. In addition, 14 animals were also injected into striatal JOR-ineffective sites as control (A: 0.2–0.48 mm from bregma, L: 2.7 mm, V: 3.5 mm below dura). All coordinates were taken from the atlas of Paxinos and Watson (1997). Microinjections were performed ipsilateral to the single unit recording except when two neurons were studied simultaneously in different sides of the brain stem (bilateral single unit recordings). In these cases, the glutamate was microinjected first in one side, and 20–40 min after the effect had subsided, in the opposite side, to explore the effect of ipsilateral and contralateral injection of glutamate in both neurons.
Experimental design
The isolated units were monitored for 
10 min prior to recording to assure their stability, and 3–5 min of spontaneous activity was recorded. Next, the properties of the units were evaluated to classify them as LTM, WDR, or NS, and the receptive field was delimited. Afterward, the WDR and NS Sp5c units were recorded in response to the noxious stimulation of the tooth pulp simultaneously with the JOR, for 3–6 min as control, followed by an intrastriatal microinjection of glutamate and recorded for a postinjection period of 7–15 min (n = 30). Some animals in this group (n = 8) received a second microinjection with solvent into the same position 20–40 min later. Another subgroup of those animals (n = 14) received two microinjections of glutamate at different vertical positions: one dorsal to the postulated inhibitory striatal zone (JOR-ineffective sites) and the other in the inhibitory area. In a second group of eight animals, nociceptive neurons of the Sp5c were simultaneously analyzed with Mo5 neurons and the JOR. In these rats, we first studied the effect of striatal chemical stimulation on the spontaneous activity of Sp5c and Mo5 units. In addition, after 20–40 min, a second microinjection of glutamate was delivered in the same striatal position while the dental pulp was stimulated. With this experimental approach, we attempted to establish a temporal relationship between the striatal effect exerted on the spontaneous and evoked activity of the Sp5c and Mo5 neurons and the JOR. We evaluated the striatal modulation of the response in LTM Sp5c neurons evoked by mechanical innocuous stimulation in an additional group of eight animals. In all cases, only one neuron was analyzed per side, and the JOR was measured simultaneously whenever it would correspond.
Histology
At the end of the experiments, the position of the recording electrode(s) was marked by iontophoretic application of Pontamine Sky Blue. The rats were deeply anesthetized with urethane and perfused transcardially with saline followed by 10% buffered formalin. The brains were removed, postfixed in 10% buffered formalin, sectioned coronally (60 µm) with a freezing microtome, and stained with cresyl violet. The localization of the striatal injection and recording sites were reconstructed from the microdrive readings, cannulae tracts, and deposits of dye spots. Composite diagrams of these positions were drawn using a microprojector and the atlas of Paxinos and Watson (1997). Only those animals with the microinjection and the recording electrode positioned within the intended target were analyzed.
Statistical analysis
Statistical comparison was performed with one-way ANOVA for repeated and unrepeated measurements, followed by Newman-Keuls (NK) posthoc test for comparison of treatment means. Student’s t-test was also used for comparing between means. Frequency distributions were compared with 
2. All values are expressed as means ± SE, and a probability of 5% (P < 0.05) was considered significant.
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RESULTS |
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Extracellular single unit activity was recorded from a total of 42 neurons in the Sp5c responding to dental pulp stimulation; 8 of these neurons were recorded simultaneously with Mo5 units. These neurons were located throughout the dorso-ventral extent of the Sp5c nucleus and were functionally identified as WDR neurons (n = 17) and NS neurons (n = 25) and are shown in Fig. 1A. The recording sites of 39 of 42 neurons were histologically recovered (16 WDR and 23 NS neurons). The WDR units were concentrated in deeper laminae in all cases (14 of 16 WDR units, 88%), whereas NS units predominated in laminae I/II (20 of 23 NS units, 87%) as previously described (Hu 1990; Meng et al. 2000; Raboisson et al. 1995; Sessle et al. 1986). These NS units did not respond to innocuous mechanical stimulation of the skin or tongue (Fig. 2A). On the other hand, WDR units responded to low-intensity tactile stimulation of the skin and exhibited graded responses from innocuous stimuli to noxious compression or pinching of the skin (Fig. 3A). All neurons had an ipsilateral receptive field that included the perioral region in 93% and the intraoral in 36% of the neurons. The recorded neurons had very low spontaneous activity (mean firing rate = 0.7-Hz; range, 0–4.5 Hz; n = 42). Indeed, 31% had no spontaneous activity (13 of 42 neurons).
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Typical evoked responses of the Sp5c neurons and digastric EMG to electrical stimulation of the dental pulp are shown in Figs. 2B and 3B. The mean threshold for eliciting the JOR was 720 ± 100 µA (n = 42), and the mean latency of the reflex was 6.9 ± 0.3 ms (range, 5.3–9.6 ms). The neurons of the Sp5c responded to dental stimulation with time-locked, probably monosynaptic mediated (latency jitter, 2–3 ms), short latency response that was highly reliable (mean: 1.2 ± 0.2 spikes/stimuli, n = 42). Primary afferent recordings were previously discarded based on their ability to respond to high-frequency stimulation applied in the receptive field. On the basis of their latency, all units showed an A fiber input response (mean latency: 4.4 ± 0.23 ms, range: 3.1–8.7 ms, n = 42, Figs. 2C and 3C). In addition, 33% of the units (n = 14) presented a second activation peak clearly differentiated from the short-latency response, corresponding to C fiber inputs (mean latency: 41.9 ± 3.5 ms, range: 32–84 ms, Fig. 3C). These values are similar to those reported in previous studies (Belforte et al. 2001; Chiang et al. 1989; Hu 1990; Meng et al. 2000; Raboisson et al. 1995).
Striatal inhibition of tooth pulp–evoked responses
As expected from previous work of this laboratory (Belforte et al. 2001), the microinjection of glutamate into the central-medial region of the striatum (Fig. 1B) resulted in a significant inhibition of the JOR amplitude, as shown in Figs. 2E, 3E, and 4A. The magnitude and time course characteristics of the effect were similar to those previously reported (Belforte et al. 2001). Following intrastriatal microinjection of glutamate, a marked reversible depression was observed for the A fiber response evoked by dental stimulation in Sp5c NS (17/19, 90%) and WDR (14/15, 93%) neurons (Figs. 2 and 3, respectively). Maximal inhibition of the evoked neural response appeared as soon as 1 min after the injection. This effect occurred before the maximal inhibition of the JOR in 
53% of cases and simultaneously in 47% of cases. The time course of the effect is shown in representative examples in Figs. 2E and 3E. Moreover, the striatum was also capable to inhibit the C fiber noxious-evoked response in Sp5c neurons in 14 of 14 neurons with C fiber input (shown by a representative example in Fig. 3). The cumulative results obtained from these neurons are presented in Fig. 4. The A and C fiber–evoked responses were significantly reduced by glutamate microinjection to 53 ± 4.2 and 43.6 ± 6.4% of control values, respectively (P < 0.001, NK post-ANOVA for repeated measures, factor:time; F2,92 = 30.9; P < 0.001). There were no significant differences in the magnitude of the inhibitory effect between both types of inputs (ANOVA for repeated measures, interaction fiber x time F2,92 = 2.24; P > 0.05). The microinjection of vehicle in the same striatal regions did not modify the JOR amplitude or the neuronal evoked response (Fig. 4B). We did not find any significant difference in the inhibitory effect obtained by either ipsilateral or contralateral stimulation of the striatum (49.6 ± 8.8 vs. 52.1 ± 9.6%, respectively, Student’s t-test for repeated measures, t5 = 0.61; P = 0.57, n = 6 from bilateral recordings). In addition, the microinjection of glutamate into the JOR ineffective sites (Fig. 1B) did not produce any significant changes in the evoked response in Sp5c neurons (Fig. 4C).
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The activity of Mo5 neurons was studied simultaneously with Sp5C units in eight animals. Neurons recorded within the Mo5 nucleus discharged tonically with a regular-irregular pattern (mean spontaneous discharge frequency, 5.9 Hz; range, 0.9–13 Hz). The tooth pulp stimulation evoked an excitatory response, with a significant temporal relationship to digastric EMG activity (JOR), followed by a slight short-lasting inhibition (7/8 neurons; Fig. 5, B and C). The mean latency for the excitatory response was 6.35 ms (range, 5.1–7.8 ms). These results were in accordance with previous reports (Sotgiu and Bellinzona 1991). The recording sites were located in the ventromedial part of the Mo5 nucleus (Fig. 1C), a region reported to contain the motoneurons that innervate the anterior belly of the digastric muscle (Fay and Norgren 1997; Mizuno et al. 1975). In addition, three of eight of these neurons were activated antidromically at short and constant latency (0.7–1 ms) by digastric muscle stimulation.
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A representative experiment in Fig. 5 shows the effect of glutamate microinjection on the spontaneous and evoked activity in Mo5 and Sp5c neurons. Spontaneous activity of the motoneuron and the Sp5c units was unaffected by glutamatergic stimulation of the striatum in any of the eight pairs recorded (Fig. 5A, see population results in Fig. 6A). However, a subtle inhibition of the Sp5c neurons firing rate cannot be ruled out, because the very low spontaneous activity of these neurons (0.35–1.7 Hz, n = 8) makes the assessment of an inhibitory effect difficult. To confirm the inhibitory nature of the microinjection positions within the striatum in these experiments, a second application of glutamate was delivered to evaluate the effect on the tooth pulp–evoked activity. As expected, the intrastriatal microinjection induced a marked decrease in the evoked response in the Sp5c units and the JOR, with an amplitude and time course similar to those previously observed (Fig. 5, C and E). The evoked response in Mo5 neurons decreased with activation of the striatum (27.7 ± 4.3 vs. 59.1 ± 6.1 spikes/50 stimuli, n = 8, P < 0.001, NK). Moreover, the time course of this suppression paralleled that of the Sp5c neuron simultaneously recorded, presumably reflecting the decrease of the inputs from the Sp5c nucleus. The magnitude of the inhibitory effect exerted by the striatum on the noxious evoked response in the Sp5c, Mo5 neurons, and the JOR was comparable (63.9, 47.7, and 47.0%, respectively, from control values, one-way ANOVA, F2,21 = 1.32; P = 0.29; not significant; Fig. 6B).
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In an additional series of eight experiments, 12 LTH neurons were recorded within the spinal trigeminal nucleus pars caudalis (Sp5c) and adjacent pars interpolaris, mainly in laminae III/IV (Fig. 1A). LTH units responded to hair movement and to light tactile stimulation but did not respond to nociceptive stimulation, including tooth pulp activation. In only 50% (6/12) of the LTH neurons evaluated di the chemical activation of the striatum result in a decrease of the innocuous-evoked response (4.4 ± 0.8 spikes/stimulus) compared with preinjection values (7.6 ± 1.4 spikes/stimulus, Student’s t-test for repeated measures; t5 = 3.36; P < 0.02, n = 6). However, the incidence of striatal-induced inhibition was significantly greater for nociceptive neurons than for LTH ones (WDR, 93%; NS, 90%; vs. LTH, 50%; P < 0.05; 2 = 6.3).
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DISCUSSION |
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; Baumeister et al. 1988, 1990; Belforte et al. 2001; Gear et al. 1999; Jurna and Heinz 1979; Lin et al. 1981; Magnusson and Fisher 2000). However, the studies based on reflexes alone have a drawback because they cannot distinguish between a direct effect on the motor output and a true antinociceptive action on the sensory input. This study, to our knowledge, is the first report in which a noxious stimulus–evoked response in sensory neurons can be significantly inhibited by the stimulation of the striatum. This suggests a role of the basal ganglia in pain modulation.
The sensory innervation of the tooth pulp is considered to be nociceptive because it arises almost exclusively from small-diameter myelinated and unmyelinated axons as has been shown in humans (Edwall and Olgart 1977) and animals (Bishop 1981; Holland 1978; Johnsen and Karlsson 1974). Most of the tooth pulp afferents have a high threshold and a conduction velocity in the range of A and C fibers (Bishop 1981; Dostrovsky 1984; Greenwood et al. 1972; Johnsen and Karlsson 1974; Narhi et al. 1983; Takeda et al. 1998; Wakabayashi et al. 1993). However, Dong et al. (1990) have reported that innocuous stimulation of the tooth pulp activated intrapulpal A
fibers, which are known to innervate low-threshold mechanoreceptors in cutaneous tissue (Birder and Perl 1994). These observations raise the possibility that electrical stimulation of the tooth pulp activates intrapulpal A fibers, conveying nonpain sensation described by human volunteers and named "prepain" (Ahlquist et al. 1984). For this reason, the JOR evoked by tooth pulp stimulation is considered a valid model for pain studies only when elicited by suprathreshold stimuli, as in our case (Mason et al. 1985). Also, it has been extensively employed in pain research (Chiang et al. 1990, 1991; Gear and Levine 1995; Gear et al. 1999; Grassi and Passatore 1987; Hu et al. 1986; Schmidt et al. 2002b; Sessle and Hu 1981; Takeda et al. 1998; Tambeli et al. 2002; Vassel et al. 1986; Zhang et al. 1998, 1999). Moreover, the magnitude of experienced pain in human volunteers is related to the amplitude of A fiber discharge of the dental pulp afferents (Ahlquist et al. 1984; Olgart et al. 1988). Another important issue regarding the JOR induced by tooth pulp stimulation is the control of current spreading to the periodontum (Hayashi 1980; Myslinski and Matthews 1987). In our experiments, the stimulus intensity was set at a level that ensures the activation of dental pulp without stimulating the periodontal tissues (Pazo et al. 2001).
Single unit recordings were performed in the nucleus caudalis of the trigeminal nerve because clinical (Fox 1971; Green et al. 2002b; Morita and Hosobuchi 1992; Rosenkopf 1989), behavioral (Bohotin et al. 2003; Duale et al. 1996; Luccarini et al. 1998; Rosenfeld et al. 1983), anatomical (Clements et al. 1991; Coimbra and Coimbra 1994; Strassman and Vos 1993; Voisin et al. 2002), immunohistochemical (Bereiter et al. 1994; Lu et al. 1993; Meng and Bereiter 1996; Oakden and Boissonade 1998; Strassman et al. 1993), and electrophysiological (Amano et al. 1986; Carstens et al. 1998; Chiang et al. 1998; Dallel et al. 1998; Hu et al. 1981; Tsai et al. 1999) evidence suggests that this is the most important site for relay of orofacial nociceptive information. The latencies of the evoked noxious responses in the Sp5c neurons were consistent with previous reports (Hu 1990; Meng et al. 2000; Raboisson et al. 1995), and the calculated conduction velocities were compatible with A and C fiber inputs (10.3 ± 0.6 and 0.86 ± 0.09 m/s, respectively), which would confirm the nociceptive nature of the studied neural response.
In a previous paper, we showed that stimulation of the central-medial region of the striatum results in an inhibition of the JOR evoked by tooth pulp stimulation (Belforte et al. 2001). In this study, we show that the tooth pulp–evoked neuronal activity in the Sp5c, temporally related to the JOR, was inhibited by the stimulation of the same striatal region. However, other striatal areas could also be involved in the antinoceptive effect. The inhibition was evident on both A an C fiber–mediated responses in NS and WDR neurons. In addition, the time course of the inhibitory effect paralleled that of the JOR suppression. The activation of the striatum also inhibited the excitatory response evoked by the tooth pulp stimulation in the digastric motoneurons. However, it did not change the spontaneous activity of the motor units. On the basis of these results, we can assume that striatal inhibition of the tooth pulp–evoked response in digastric motoneurons may be the consequence of a decrease in the inputs arriving from the sensory nucleus rather than a direct inhibitory action of the striatum on motoneurons. Furthermore, the striatal inhibitory region capable of decreasing the sensory-evoked responses was not involved in the generation of rhythmical jaw movements (vacuous chewing) observed by stimulation of the striatum (Hashimoto and Amano 1998; Kelley et al. 1989; Kolomiets et al. 2001; Koshikawa et al. 1989; Nakamura et al. 1990), which may support the assumption that this area subserves sensory function.
Nociceptive somatosensory information arrives to the basal ganglia from high-threshold receptors through several sources such as cerebral cortex, intralaminar nuclei of thalamus, superior colliculus, raphe nuclei, amygdala, and probably from direct spinal projections (Bernard et al. 1992; Corvaja et al. 1993; Grunwerg et al. 1992; Lapper and Bolam 1992; McGeorge and Faull 1989; Newman et al. 1996; Shinonaga et al. 1992; Yasui et al. 1987). Indeed, experiments performed in cats have shown that the striatal neurons were activated by stimulation of the inferior dental nerve only with a stimulus intensity that elicited the jaw opening reflex (Lidsky et al. 1978). The inhibitory striatal region studied in this work receives afferents from the lateral and ventrolateral orbital cortex (Deniau et al. 1996; McGeorge and Faull 1989; Sesack et al. 1989). These cortical regions have been implicated in the inhibition of the JOR induced by noxious tooth pulp stimulation, an effect that was mediated by the periaqueductal gray matter (Zhang et al. 1998, 1999). This suggests that at least part of the cortical inputs arriving to the striatal inhibitory region are involved in modulation of nociceptive inputs, which may indicate the participation of the striatum in the mechanisms of the endogenous analgesic system.
Structures belonging to the endogenous analgesic system, periaqueductal gray matter, nucleus raphe magnus, and nuclei of the parabraquial region are projecting to the sensory trigeminal nucleus and are able to inhibit the JOR (Chiang et al. 1991, 1994; Chung et al. 1987; Dostrovsky et al.1982). These analgesic nuclei are directly and indirectly related to the globus pallidus and substantia nigra pars reticulata (Chudler 1995; Kirouac et al. 2004), which receive inhibitory projections from the striatum (Boraud et al. 2002; Hornykiewicz 2001; Whichmann and DeLong 2003). The globus pallidus and substantia nigra pars reticulata have been related to the endogenous mechanisms of analgesia (Anagnostakis et al. 1992; Baumeister et al. 1988) and may be involved in the mediation in the analgesic effect of the striatum.
The basal ganglia have also been implicated in focusing and selection of competing motor programs as a result of the temporal related inhibitory-disinhibitory action of the output nuclei over their target structures (Mink 1996). We can speculate about a similar role of the basal ganglia in the selection and funneling of sensory information by activation-disinhibition mechanisms of structures related to the analgesic system; however, further studies are required.
The striatum was able to inhibit not only noxious responses but also innocuous responses in low-threshold neurons. However, the inhibitory effect was more conspicuous for nociceptive units than for low threshold neurons, as was evidenced by the high magnitude and incidence of the effect on NS and WDR neurons. This is in agreement with observations from other neural structures that belong to the endogenous analgesic system. For example, Sessle et al. (1981) reported that electrical stimulation of the periaqueductal gray matter resulted in the inhibition of the sensory evoked activity in 64% of the low-threshold units in opposition to the 97% observed for the tooth pulp–evoked response in nociceptive neurons. However, it is possible that, given the somatosensory topography of the striatum, the stimulated sites optimized for suppressing the tooth pulp–evoked JOR were suboptimal for perioral stimuli.
The modulatory action of the striatum on lamina I trigeminal neurons may implicate the nucleus in the tuning of sensory information that is necessary for maintenance of homeostatic balance. In line with this hypothesis, the striatum is a suitable structure for coordination of sensory (Chudler and Dong 1995; Lidsky et al. 1985; Schwarting et al. 1991), motor (Boraud et al. 2002; Chesselet and Delfs 1996; Graybiel et al. 1994; Marsden and Obeso 1994; Mink 1996; Wichmann and DeLong 2003), autonomic (Chaudhuri 2001; Pazo and Belforte 2002; Quadri et al. 2000), and emotional (Brown et al. 1997; Graybiel 1997; Graybiel and Rauch 2000) responses necessary for correct, context-dependent, behavior response.
In conclusion, the activation of the striatum inhibits the JOR produced by the noxious stimulation of the dental pulp. This effect is elicited by suppression of the sensory response of second-order trigeminal neurons, which in turn do not exert an excitatory action on digastric motoneurons. Based on these results, it seems that the striatum could be considered part of the endogenous analgesic system.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Present address of J. E. Belforte: Unit on the Genetics of Cognition and Behavior, Mood and Anxiety Disorders Research Program, National Institute of Mental Health, NIH, 49 Convent Drive, B1C20, Bethesda, MD 20892-4405.
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. H. Pazo, Lab. de Neurofisiología, Dept. de Fisiología, Facultad de Medicina, Univ. de Buenos Aires, Paraguay 2155, Buenos Aires 1121, Argentina (E-mail: jpazo{at}fmed.uba.ar)
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REFERENCES |
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