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Negative potentials evoked by nucleus reticularis gigantocellularis in the spinal trigeminal tract of the cat
EXPERIMENTAL
NEUROL.OGY
68. 249-257 (1980)
Negative Potentials Evoked by Nucleus Reticularis Gigantocellularis in the Spinal Trigeminal Tract of the Cat SAMUEL
H. H. CHAN’
In cats that were precollicularly decerebrated. bilateral electrical stimulation of the nucleus reticularis gigantocellularis (NRGC) evoked negative potentials in the spinal trigeminal tract at the level of the subnucleus oralis of the spinal trigeminal nuclear complex. These negative potentials exhibited two types of configurations that differed in the slope of the rising and declining phase, duration, and amplitude of the negative wave. They were also found to develop as a function of the reticular stimulus parameters. Thus, they possessed electrophysiologic characteristics similar to the dorsal root potentials induced by comparable reticular activation in the spinal cord. The time course of the NRGC-evoked negative potential paralleled the inhibition of dental pulp-elicited responses in the subnucleus oralis. promoted by the same reticular stimulation. As the dorsal root potential is generally taken to be a manifestation of primary afferent depolarization, it is suggested, by extrapolation, that the NRGC may, at least in part, suppress the transmission of nociceptive signals from the dental pulp by a depolarization of the pulpal afferent fibers.
INTRODUCTION Chan (3,4) recently demonstrated that microinjection into, and electrical activation of, the nucleus reticularis gigantocellularis (NRGC) in the medulla of the cat resulted in a drastic suppression of the dental pulp-evoked trigeminal neuronal responses. It was suggested that upon morphine or electrical excitation, the reticular neurons may inhibit the Abbreviation: NRGC-nucleus reticularis gigantocellularis. ’ This study was supported in part by the University Research Committee, Indiana State University. Photography by Mr. Anthony J. Brentlingerofthe Audio-Visual Center, Indiana State University, is appreciated. 249 00 14-4886/80/050249-09$02.00/O Copyright 0 1980 by Academic Preu. Inc. All rights of reproduction in any form rewved.
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transmission of nociceptive signals from the dental pulp via a depolarization of the pulpal afferent terminals (4). In the course of those studies, NRGC stimulation was observed to evoke negative potentials in the spinal trigeminal tract which exhibited configurations similar to the negative dorsal root potentials that could be elicited by comparable reticular activation in the spinal cord (5). As the negative dorsal root potential is generally related to primary afferent depolarization, forming the basis of presynaptic inhibition (9), an intriguing question is whether or not the spinal trigeminal tract potential is the physical and functional counterpart of the dorsal root potential. A positive answer to this question will in turn lend support to the aforementioned hypothesis. The present communication provides at least a partial answer to this question. We report the experimental results from an investigation of the electrophysiologic characteristics of the spinal trigeminal tract potentials and attempt to relate these potentials to the NRGC-elicited inhibition of dental pulp-evoked trigeminal neuronal activities. METHODS Experiments were conducted on 15 adult cats, weighing 2 to 3.5 kg. Under ether anesthesia, tracheal intubation, permanent ligation of both carotid arteries, and cannulation of the right femoral vein were routinely carried out. Ether was discontinued immediately after precollicular decerebration. The recording session commenced at least 2 h after discontinuing the anesthetic agent. A pair of Nichrome electrodes implanted into the dental pulp of the left upper canine was used to deliver the intrapulpal stimulation, using single rectangular pulses (100 ps, 1 Hz). The resultant evoked field potential from the ipsilateral subnucleus oralis (oralis potential) was recorded by means of a bipolar concentric electrode (Rhodes Medical Instruments, NE-100, shaft diameter: 500Cl.m; contact diameters: center, 200 pm, outer, 500pm), placed stereotaxically (P 6; L 4.5; H - 5.0 to -5.5) (4). The same type of electrode was also used to record the negative potential from the left spinal trigeminal tract, at the level of the subnucleus oralis. All bioelectric potentials were amplified and filtered by individual differential amplifiers (DC recording, 10x, bandwidth: DC to 100 Hz) and were photographed directly from different storage oscilloscopes using Polaroid films. Electrical activation of the NRGC on either side was induced by means of a train of 100~p.s rectangular pulses. The same type of bipolar concentric electrode as the recording ones was stereotaxically positioned (P 8 to 9; L 2; or R 2; H - 8). The effect of varying the reticular stimulus parameters
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(train duration, train pulse frequency, and train pulse intensity) on the evoked spinal trigeminal tract potential was investigated. For every combination that elicited a negative potential, its effect on the dental pulpinduced oralis potential was also studied using the conditioning-testing technique (4), allowing a comparison of the time courses of both events. All animals were paralyzed with gallamine triethiodide (2 mg/kg/2 h, i.v.) and artificially respired during the recording session. Exposed neural tissues were bathed with warm mineral oil and the body temperature of the animal was maintained by a heating pad. All recording-stimulating sites in the brain stem were verified histologically after each experiment. RESULTS Stimulation of the NRGC on either side consistently evoked potentials in the spinal trigeminal tract at the level of the subnucleus oralis. Systematic mapping in initial experiments revealed that the size and polarity of these potentials varied with the locus of the recording electrode, but large-amplitude negative waves were frequently elicited at the stereotaxic coordinates of P 7; L 5.0 to 5.5; H -4.5 to -5.5. These coordinates were used as the reference in subsequent animals to detect the maximum negative potential evoked in the spinal trigeminal tract. The NRGC-evoked negative spinal trigeminal tract potentials could be grouped into two general categories according to the slope of the rising and descending phase as well as the amplitude and duration of the negative wave. The first category (type I, Fig. 1) had a slowly rising negativity that reached its maximum 30 to 40 ms after the onset of the reticular stimulation. The declining phase then returned to the control value during the next 45 to 90 ms. Most of the negative potentials in this category had a well defined and sometimes lingering peak. The second category (type II, Fig. 2, upper tracings) had a sharply rising phase that was maximized 5 to 10 ms after the beginning of the reticular train. This was followed by a slower, yet still steep declining phase. The total duration of negativity was usually 30 to 50 ms, about one-half to two-thirds of that of the type I. However, the latter type of negative potentials had an amplitude only 60 to 70% of the former. At the present stage of investigation, it is impossible to conclude whether there are differential anatomic loci for these two types of negative potential, although both were evoked by comparable NRGC stimulations in different animals. It is intriguing to note that the development of both types of negative potential was inherently related to the stimulus parameters of the NRGC activation. Generally speaking, increasing the train duration, train pulse frequency, or train pulse intensity resulted in an elevation of either the
252
SAMUEL
H. H. CHAN
A
5 rnsec,
200
HZ. 2 5T
lOmsec,200Hz,2.5T
20msec,200Hz
25T
100mm.200Hz.2.51
.,,, 20
wec,53Hz,2.5T
‘2bmrcJOOHz,25T
20mseC,150Hz,25T
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23msec,200Hr,31
20mscc,ZCOHz,4T
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..,. 2C rnsec
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2O-mec,200Hr,2T
3 i 40mse;
8
FIG. 1. Type I negative spinal trigeminal tract potentials evoked by stimulating the ipsilateral nucleus reticularis gigantocellularis using various combinations of stimulus parameters. Differential effects due to variations of train duration (A), train pulse frequency (B), and train pulse intensity (C). Each dot in the stimulus markers represents one reticular activating pulse. Negativity is denoted by an upward deflation.
amplitude or duration of the negative potential, although changes of both were also observed. Figures 1 and 2 (upper tracings) present some sample illustrations from two typical experiments, in which the ipsilateral NRGC was stimulated. An increase in the reticular stimulus intensity was usually accompanied by an elevation of the amplitude of the spinal trigeminal tract potentials (Figs. 1C and 2B). On the other hand, when the train pulse frequency or train duration was increased, negative potentials were found to increase either in amplitude (Fig. 1B) or duration (Figs. 2A,C). Augmentation in both amplitude and duration was also observed, as exemplified by the responses to an increase in train pulse frequency (Fig. 1A). Of the stimulus parameters hitherto tested, the optimal combination was found to be: train duration, 20 ms: train pulse frequency, 200 Hz; and train pulse intensity, 4 T to 5 T (T is the threshold intensity needed to evoke a spinal trigeminal tract potential). Limited exploration with higher values revealed either no further change or even a decrease in the responses. When the time courses of the NRGC-evoked negative spinal trigeminal tract potential and NRGC-promoted inhibition of the dental pulp-evoked oralis potential were compared, another very interesting phenomenon was unveiled. Figure 2 is a typical example of these observations, using type II negative potentials. Not only were the degree and duration of the oralis potential suppression (lower tracings) correlated nicely with the reticular stimulus parameters, as previously demonstrated (4), they were also found
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....
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FIG. 2. Time course comparisons of nucleus reticularis gigantocellularis (NRGC)-evoked type II negative spinal trigeminal tract potentials (upper tracings) and NRGC-conditioned dental pulp-elicited oralis potentials (lower tracings). Differential effects due to variations of train pulse frequency (A). train pulse intensity (B). and train duration(C). The lower tracings were obtained by eliciting the oralis potentials at increasing time intervals (from 0 to 200 ms) after the NRGC stimulation. Each dot in the stimulus markers represents one reticular activating pulse. Negativity is denoted by an upward deflation.
to parallel the development of the evoked negative waves (upper tracings) with the same NRGC stimulation. At the same time, except in two cases (20 ms, 200 Hz, 1.5 T and 3 r) in this series, the maximal possible depression of the oralis response to intradental stimulation invariably occurred at about the same time that the spinal trigeminal tract potential was at its peak of negativity, although the former usually outlasted the negative potential. Similar results were observed when type I spinal trigeminal tract potentials were evoked. They were associated with longer lasting inhibition of the oralis potential, which also persisted beyond the duration of the negative potentials.
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DISCUSSION The present study revealed that electrical activation of the NRGC elicited negative potentials in the spinal trigeminal tract that possessed electrophysiologic characteristics similar to the dorsal root potentials evoked by comparable reticular stimulation in the spinal cord (5). Both potentials (a) exhibited two types of configurations that differ in the slope of the rising and falling phase, as well as the duration and amplitude of the negative wave, (b) developed as a function of the reticular stimulus parameters, and (c) had a time course comparable to the NRGC-elicited suppression of sensory evoked neuronal responses. It is generally accepted that the spinal trigeminal tract and dorsal root are composed primarily of afferent fibers and the dorsal root potential is a manifestation of primary afferent depolarization (9). Thus, it may not be unreasonable to suggest, by extrapolation, that the spinal trigeminal tract potential in the brain stem is the counterpart of the dorsal root potential in the spinal cord and that it reflects a depolarization of the preterminal fibers of the spinal trigeminal nerve. Primary afferent depolarization in the trigeminal sensory complex has been demonstrated using methods varying from measurement of terminal excitability (7, 12, 27), trigeminal dorsal root reflex (28), bulbar dorsum potential (11) to intraaxonal recording from the primary afferent fibers (15, 28). The spinal trigeminal tract potential may then represent another means of assessment for the existence of primary afferent depolarization in the trigeminal system. It may be argued that these negative potentials as recorded from the spinal trigeminal tract do not necessarily take origin from the tract primary afferent fibers as contended. First, they may be simply due to current spread from the stimulating to the recording electrode; in short, they may be stimulus artifacts. This possibility may be dismissed for two reasons. Reversal of the polarity of the reticular activating pulses resulted only in a change in the amplitude of negative potentials instead of their polarity. Furthermore, the polarity of these potentials varied with the position of the recording electrode, signifying a source-sink distribution within the vicinity of the spinal trigeminal tract. Second, the negative potentials may reflect activities evoked by NRGC stimulation in the adjacent subnucleus oralis. A waveform that resembled the spinal trigeminal tract potential was recorded in the subnucleus oralis upon NRGC activation (4). This was judged to be a stimulus artifact because it reversed its polarity with that of the stimulating pulses.
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Third, the negative potentials may represent postsynaptic potentials of the nonprimary afferent fibers in the spinal trigeminal tract elicited, e.g., in the subnucleus caudalis, by the NRGC. This is based on the contention that the subnucleus caudalis may modulate the processing of nociceptive information in the subnucleus oralis via an influence on its primary afferent fibers (19.27) and the identification of nonsensory, internuclear fibers in the spinal trigeminal tract connecting various subnuclei of the trigeminal sensory system (10). Although the possibility that the NRGC may influence the subnucleus oralis indirectly via the subnucleus caudalis cannot be ruled out, it is pertinent to relate it to some recent observations in this laboratory (Chart, unpublished data). Partial to complete (80 to 100%) blockade of the NRGC-elicited spinal trigeminal tract potentials in the vicinity of the subnucleus oralis occurred when they were allowed to collide with the potentials evoked at the same recording locus by dental pulp stimulation, suggesting the sharing of common primary afferent terminals at this level by both inputs. Furthermore, this observation implied that the negative potentials may reflect a depolarization of the pulpal terminals. One of the most frequently used pain stimuli is intrapulpal electrical stimulation, based on the assumption that the tooth pulp is a “pure” pain source (1, 2). Dubner et al. (8) discussed clinical reports which overwhelmingly indicate that pain is the only sensation that can be elicited from the dentine and pulp, providing that the stimulus does not spread to gingival and periodontal tissues. Mumford and Bowsher (14), however, revealed that threshold electrical activation of the teeth in man elicited a spectrum of sensations that varied from pain to vibration, tingling, or warmth. In the absence of a “perfect” pain stimulus, the dental pulp stimulation appears to be one of the better experimental pain inputs. Intrapulpal stimulation was reported to evoke neuronal activities in the marginal zone of the subnucleus caudalis, in its magnocellular region, and in the adjacent reticular formation (16, 17,21,25,26). According to Dubner et crl. (8). the rostra1 trigeminal nuclei (subnucleus oralis-main sensory nucleus) appeared to receive a larger representation from the dental pulp, at least electrophysiologically, than the subnucleus caudalis (6, 20-24), suggesting that the subnucleus oralis may be related to the localization of the pulpal stimuli, Keller rf ul. (13) showed that cortical potentials evoked in the somatic cortex by tooth pulp stimulation were abolished by the destruction of the subnucleus oralis. Thus, the subnucleus oralis may be an important relay nucleus for the centripetal transmission of dental pain signals. It follows that the oralis potential evoked by dental pulp stimulation may be a good experimental index of dental pain. The powerful NRGC-elicited suppression of the dental pulp-induced
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oralis potential in this and a previous (4) study may be interpreted to indicate that this reticular nucleus is actively participating in the analgesic processes against dentalgia. Pearl and Anderson (18) found a good number of gigantocellular neurons responded to dental pulp stimulation and suggested that the NRGC is involved in the processing of nociceptive information. Subjected to further verification, the parallel time courses of the negative spinal trigeminal tract potential and the inhibition of the oralis potential, elicited by the same NRGC stimulation, implied that presynaptic inhibitory mechanism may, at least in part, underlie this process. However, because the suppression of oralis potential outlasted the negative potential, it is possible that other synaptic mechanisms may also be involved. REFERENCES 1. ANDERSON, D. J., A. G. HANNAM, AND B. MATTHEWS. 1970. Sensory mechanisms in mammalian teeth and their supporting structures. Physiol. Rev. 50: 171-195. 2. BROOKHART, J. M., W. K. LIVINGSTONE, AND F. P. HAUGEN. 1953. Functional characteristics of afferent fibers from tooth pulp of cat. J. Neurophysiol. 16: 634-642. 3. CHAN, S. H. H. 1979. Participation of the nucleus reticularis gigantocellularis in the morphine suppression of jaw-opening reflex in cats. Bruin Res. 160: 377-380. 4. CHAN. S. H. H. 1979. Suppression ofdental pulp-evoked trigeminalresponses by nucleus reticularis gigantocellularis in the cat. Exp. Neural. 66: 356-364. 5. CHAN, S. H. H., AND C. D. BARNES. 1972. A presynaptic mechanism evoked from brain stem reticular formation in the lumbar cord and its temporal significance. Bruin Res. 45: 101-114. 6. DAVIES. W. I. R., D. SCOTT, JR., K. VESTERSTRBM. AND L. VYKLICK?. 1971. Depolarization of the tooth pulp afferent terminals in the brain stem of the cat. J. Physiol. (London) 218: 515-532. 7. DUBNER, R., AND B. J. SESSLE. 1971. Presynaptic excitability changes ofprimary afferent and cortifugal fibers projecting to trigeminal brain serm nuclei. Exp. Neural. 30: 223-238. 8. DUBNER, R., B. J. SESSLE, AND A. T. STOREY. 1979. The NeuralBasis oj‘Oralund Facial Function. Plenum, New York. 9. ECCLES, J. C. 1964. Presynaptic inhibition in the spinal cord. Pages 65-80in J. C. ECCLES AND J. P. SCHAD~, Eds., Physiology ofspinal Neurons, Progress in Brain Research. Elsevier, Amsterdam. 10. GOBEL, S., AND M. B. PURVIS. 1972. Anatomical studies ofthe organization of the spinal V nucleus: the deep bundles and the spinal V tract. Brain Res. 48: 27-44. 11. HAMMER, B.. R. TARNECKI, L. VYKLICK-?., AND M. WIESENDANGER. 1966. Cortifugal control of presynaptic inhibition in the spinal trigeminal complex of the cat. Brian Res. 2: 216-218. 12. Hu, J. W., J. 0. DOSTROVSKY, AND B. J. SESSLE. 1978. Primary afferent depolarization of tooth pulp afferents is not affected by naloxone. Nurure (London) 276: 283-284. 13. KELLER, O., S. M. BUTKHUZI, L. VYKLICK~, AND G. BROZEK. 1974. Cortical response evoked by stimulation of tooth pulp afferents in the cat. Physiol. Bohemoslov. 23: 45-54.
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14. MUMFORD, .I., AND D. BOWSHER. 1976. Pain and protopathic sensibility. A review with particular reference to the teeth. Pain 2: 223-243. 15. NAKAMURA, Y..T. MURAKAMI. M. KIKUCHI,~. KUBO. AND% ISHIMINE. 1977. Primary afferent depolarization in the trigeminal spinal nucleus of cats. E?rp. Brain Res. 29: 45-56. 16. NORD, S. G. 1976. Responses of neurons in rostra1 and caudal trigeminal nuclei to tooth pulp stimulation. Brain Res. Bull. 1: 489-492. 17. NORD, S. G. 1976. Bilateral projection of the canine tooth pulp to bulbar trigeminal neurons. Brain Res. 113: 517-523. 18. PEARL, G. S.. AND K. V. ANDERSON. 1978. Response patterns ofcells in the feline caudal nucleus reticularis gigantocellularis after noxious trigeminal and spinal stimulation. Exp. Neural. 58: 231-241. 19. SESSLE, B. J., AND L. F. GREENWOOD. 1974. Influence oftrigeminal nucleus caudalis on the responses of cat trigeminal brain stem neurons with orofacial mechanoreceptive fields. Bruin Res. 67: 330-333. 20. SESSLE. B. J., AND L. F. GREENWOOD. 1976. Inputs to trigeminal brain stem neurons from facial, oral, tooth pulp and pharyngolaryngeal tissues: I. Responses to innocuous and noxious stimuli. Brain Res. 117: 221-226. 21. SESSLE, B. J., R. DUBNER, L. F. GREENWOOD, AND G. E. LUCIER. 1976. Descending influences of periaqueductal gray matter and somatosensory cerebral cortex on neurons in trigeminal brain stem nuclei. Can. J. Physiol. Phnrmacol. 54: 66-69. 22. TAMAROVA. Z. A., A. I. SHAPOVALOV. AND L. VYKLICK~. 1973. Projection oftooth pulp afferents in the brain stem of rhesus monkey. Brain Res. 64: 442-445. 23. VYKLICKP, L.. AND 0. KELLER. 1973. Central projection of tooth pulp primary afferents in the cat. Acta Neurohid. Exp. 33: 803-809. 24. WODA, A., J. AZERAD. AND D. ALB~-FESSARD. 1977. Mapping of the trigeminal sensory complex of the cat. Characterization of its neurons by stimulations of peripheral field, dental pulp afferents and thalamic projections. J Phpsiol. (Paris) 73: 367-378. 25. YOKOTA, T. 1975. Excitation of units in marginal rim of trigeminal subnucleus caudalis elicited by tooth pulp stimulation. Brain Res. 95: 154- 158. 26. YOKOTA. T., AND S. HASHIMOTO. 1976. Periaqueductal gray and tooth pulp afferent interaction on units in caudal medulla oblongata. Bruin Res. 117: 508-512. 27. YOUNG, R. F., AND R. B. KING. 1972. Excitability changes in trigeminal primary afferent fibers in response to noxious and nonnoxious stimuli, J. Neurophysio[. 35: 87-95. 28. Yu, H. H., AND J. K. AVERY. 1974. Primary afferent depolarization: direct evidence in the trigeminal system. Brain Res. 75: 328-333.
NEUROL.OGY
68. 249-257 (1980)
Negative Potentials Evoked by Nucleus Reticularis Gigantocellularis in the Spinal Trigeminal Tract of the Cat SAMUEL
H. H. CHAN’
In cats that were precollicularly decerebrated. bilateral electrical stimulation of the nucleus reticularis gigantocellularis (NRGC) evoked negative potentials in the spinal trigeminal tract at the level of the subnucleus oralis of the spinal trigeminal nuclear complex. These negative potentials exhibited two types of configurations that differed in the slope of the rising and declining phase, duration, and amplitude of the negative wave. They were also found to develop as a function of the reticular stimulus parameters. Thus, they possessed electrophysiologic characteristics similar to the dorsal root potentials induced by comparable reticular activation in the spinal cord. The time course of the NRGC-evoked negative potential paralleled the inhibition of dental pulp-elicited responses in the subnucleus oralis. promoted by the same reticular stimulation. As the dorsal root potential is generally taken to be a manifestation of primary afferent depolarization, it is suggested, by extrapolation, that the NRGC may, at least in part, suppress the transmission of nociceptive signals from the dental pulp by a depolarization of the pulpal afferent fibers.
INTRODUCTION Chan (3,4) recently demonstrated that microinjection into, and electrical activation of, the nucleus reticularis gigantocellularis (NRGC) in the medulla of the cat resulted in a drastic suppression of the dental pulp-evoked trigeminal neuronal responses. It was suggested that upon morphine or electrical excitation, the reticular neurons may inhibit the Abbreviation: NRGC-nucleus reticularis gigantocellularis. ’ This study was supported in part by the University Research Committee, Indiana State University. Photography by Mr. Anthony J. Brentlingerofthe Audio-Visual Center, Indiana State University, is appreciated. 249 00 14-4886/80/050249-09$02.00/O Copyright 0 1980 by Academic Preu. Inc. All rights of reproduction in any form rewved.
250
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transmission of nociceptive signals from the dental pulp via a depolarization of the pulpal afferent terminals (4). In the course of those studies, NRGC stimulation was observed to evoke negative potentials in the spinal trigeminal tract which exhibited configurations similar to the negative dorsal root potentials that could be elicited by comparable reticular activation in the spinal cord (5). As the negative dorsal root potential is generally related to primary afferent depolarization, forming the basis of presynaptic inhibition (9), an intriguing question is whether or not the spinal trigeminal tract potential is the physical and functional counterpart of the dorsal root potential. A positive answer to this question will in turn lend support to the aforementioned hypothesis. The present communication provides at least a partial answer to this question. We report the experimental results from an investigation of the electrophysiologic characteristics of the spinal trigeminal tract potentials and attempt to relate these potentials to the NRGC-elicited inhibition of dental pulp-evoked trigeminal neuronal activities. METHODS Experiments were conducted on 15 adult cats, weighing 2 to 3.5 kg. Under ether anesthesia, tracheal intubation, permanent ligation of both carotid arteries, and cannulation of the right femoral vein were routinely carried out. Ether was discontinued immediately after precollicular decerebration. The recording session commenced at least 2 h after discontinuing the anesthetic agent. A pair of Nichrome electrodes implanted into the dental pulp of the left upper canine was used to deliver the intrapulpal stimulation, using single rectangular pulses (100 ps, 1 Hz). The resultant evoked field potential from the ipsilateral subnucleus oralis (oralis potential) was recorded by means of a bipolar concentric electrode (Rhodes Medical Instruments, NE-100, shaft diameter: 500Cl.m; contact diameters: center, 200 pm, outer, 500pm), placed stereotaxically (P 6; L 4.5; H - 5.0 to -5.5) (4). The same type of electrode was also used to record the negative potential from the left spinal trigeminal tract, at the level of the subnucleus oralis. All bioelectric potentials were amplified and filtered by individual differential amplifiers (DC recording, 10x, bandwidth: DC to 100 Hz) and were photographed directly from different storage oscilloscopes using Polaroid films. Electrical activation of the NRGC on either side was induced by means of a train of 100~p.s rectangular pulses. The same type of bipolar concentric electrode as the recording ones was stereotaxically positioned (P 8 to 9; L 2; or R 2; H - 8). The effect of varying the reticular stimulus parameters
RETICULAR
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251
(train duration, train pulse frequency, and train pulse intensity) on the evoked spinal trigeminal tract potential was investigated. For every combination that elicited a negative potential, its effect on the dental pulpinduced oralis potential was also studied using the conditioning-testing technique (4), allowing a comparison of the time courses of both events. All animals were paralyzed with gallamine triethiodide (2 mg/kg/2 h, i.v.) and artificially respired during the recording session. Exposed neural tissues were bathed with warm mineral oil and the body temperature of the animal was maintained by a heating pad. All recording-stimulating sites in the brain stem were verified histologically after each experiment. RESULTS Stimulation of the NRGC on either side consistently evoked potentials in the spinal trigeminal tract at the level of the subnucleus oralis. Systematic mapping in initial experiments revealed that the size and polarity of these potentials varied with the locus of the recording electrode, but large-amplitude negative waves were frequently elicited at the stereotaxic coordinates of P 7; L 5.0 to 5.5; H -4.5 to -5.5. These coordinates were used as the reference in subsequent animals to detect the maximum negative potential evoked in the spinal trigeminal tract. The NRGC-evoked negative spinal trigeminal tract potentials could be grouped into two general categories according to the slope of the rising and descending phase as well as the amplitude and duration of the negative wave. The first category (type I, Fig. 1) had a slowly rising negativity that reached its maximum 30 to 40 ms after the onset of the reticular stimulation. The declining phase then returned to the control value during the next 45 to 90 ms. Most of the negative potentials in this category had a well defined and sometimes lingering peak. The second category (type II, Fig. 2, upper tracings) had a sharply rising phase that was maximized 5 to 10 ms after the beginning of the reticular train. This was followed by a slower, yet still steep declining phase. The total duration of negativity was usually 30 to 50 ms, about one-half to two-thirds of that of the type I. However, the latter type of negative potentials had an amplitude only 60 to 70% of the former. At the present stage of investigation, it is impossible to conclude whether there are differential anatomic loci for these two types of negative potential, although both were evoked by comparable NRGC stimulations in different animals. It is intriguing to note that the development of both types of negative potential was inherently related to the stimulus parameters of the NRGC activation. Generally speaking, increasing the train duration, train pulse frequency, or train pulse intensity resulted in an elevation of either the
252
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H. H. CHAN
A
5 rnsec,
200
HZ. 2 5T
lOmsec,200Hz,2.5T
20msec,200Hz
25T
100mm.200Hz.2.51
.,,, 20
wec,53Hz,2.5T
‘2bmrcJOOHz,25T
20mseC,150Hz,25T
20msec,2CQrz,2
23msec,200Hr,31
20mscc,ZCOHz,4T
51
..,. 2C rnsec
200Hz.T
2O-mec,200Hr,2T
3 i 40mse;
8
FIG. 1. Type I negative spinal trigeminal tract potentials evoked by stimulating the ipsilateral nucleus reticularis gigantocellularis using various combinations of stimulus parameters. Differential effects due to variations of train duration (A), train pulse frequency (B), and train pulse intensity (C). Each dot in the stimulus markers represents one reticular activating pulse. Negativity is denoted by an upward deflation.
amplitude or duration of the negative potential, although changes of both were also observed. Figures 1 and 2 (upper tracings) present some sample illustrations from two typical experiments, in which the ipsilateral NRGC was stimulated. An increase in the reticular stimulus intensity was usually accompanied by an elevation of the amplitude of the spinal trigeminal tract potentials (Figs. 1C and 2B). On the other hand, when the train pulse frequency or train duration was increased, negative potentials were found to increase either in amplitude (Fig. 1B) or duration (Figs. 2A,C). Augmentation in both amplitude and duration was also observed, as exemplified by the responses to an increase in train pulse frequency (Fig. 1A). Of the stimulus parameters hitherto tested, the optimal combination was found to be: train duration, 20 ms: train pulse frequency, 200 Hz; and train pulse intensity, 4 T to 5 T (T is the threshold intensity needed to evoke a spinal trigeminal tract potential). Limited exploration with higher values revealed either no further change or even a decrease in the responses. When the time courses of the NRGC-evoked negative spinal trigeminal tract potential and NRGC-promoted inhibition of the dental pulp-evoked oralis potential were compared, another very interesting phenomenon was unveiled. Figure 2 is a typical example of these observations, using type II negative potentials. Not only were the degree and duration of the oralis potential suppression (lower tracings) correlated nicely with the reticular stimulus parameters, as previously demonstrated (4), they were also found
RETICULAR
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20msec,50Hz,5T
20mscc,lOOHz.
20 msec. 200Hql.5T
20msec,2OOHz,3T
POTENTIALS
253
5T
....
I
2ooJa
40msec
I
4oo~V
FIG. 2. Time course comparisons of nucleus reticularis gigantocellularis (NRGC)-evoked type II negative spinal trigeminal tract potentials (upper tracings) and NRGC-conditioned dental pulp-elicited oralis potentials (lower tracings). Differential effects due to variations of train pulse frequency (A). train pulse intensity (B). and train duration(C). The lower tracings were obtained by eliciting the oralis potentials at increasing time intervals (from 0 to 200 ms) after the NRGC stimulation. Each dot in the stimulus markers represents one reticular activating pulse. Negativity is denoted by an upward deflation.
to parallel the development of the evoked negative waves (upper tracings) with the same NRGC stimulation. At the same time, except in two cases (20 ms, 200 Hz, 1.5 T and 3 r) in this series, the maximal possible depression of the oralis response to intradental stimulation invariably occurred at about the same time that the spinal trigeminal tract potential was at its peak of negativity, although the former usually outlasted the negative potential. Similar results were observed when type I spinal trigeminal tract potentials were evoked. They were associated with longer lasting inhibition of the oralis potential, which also persisted beyond the duration of the negative potentials.
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DISCUSSION The present study revealed that electrical activation of the NRGC elicited negative potentials in the spinal trigeminal tract that possessed electrophysiologic characteristics similar to the dorsal root potentials evoked by comparable reticular stimulation in the spinal cord (5). Both potentials (a) exhibited two types of configurations that differ in the slope of the rising and falling phase, as well as the duration and amplitude of the negative wave, (b) developed as a function of the reticular stimulus parameters, and (c) had a time course comparable to the NRGC-elicited suppression of sensory evoked neuronal responses. It is generally accepted that the spinal trigeminal tract and dorsal root are composed primarily of afferent fibers and the dorsal root potential is a manifestation of primary afferent depolarization (9). Thus, it may not be unreasonable to suggest, by extrapolation, that the spinal trigeminal tract potential in the brain stem is the counterpart of the dorsal root potential in the spinal cord and that it reflects a depolarization of the preterminal fibers of the spinal trigeminal nerve. Primary afferent depolarization in the trigeminal sensory complex has been demonstrated using methods varying from measurement of terminal excitability (7, 12, 27), trigeminal dorsal root reflex (28), bulbar dorsum potential (11) to intraaxonal recording from the primary afferent fibers (15, 28). The spinal trigeminal tract potential may then represent another means of assessment for the existence of primary afferent depolarization in the trigeminal system. It may be argued that these negative potentials as recorded from the spinal trigeminal tract do not necessarily take origin from the tract primary afferent fibers as contended. First, they may be simply due to current spread from the stimulating to the recording electrode; in short, they may be stimulus artifacts. This possibility may be dismissed for two reasons. Reversal of the polarity of the reticular activating pulses resulted only in a change in the amplitude of negative potentials instead of their polarity. Furthermore, the polarity of these potentials varied with the position of the recording electrode, signifying a source-sink distribution within the vicinity of the spinal trigeminal tract. Second, the negative potentials may reflect activities evoked by NRGC stimulation in the adjacent subnucleus oralis. A waveform that resembled the spinal trigeminal tract potential was recorded in the subnucleus oralis upon NRGC activation (4). This was judged to be a stimulus artifact because it reversed its polarity with that of the stimulating pulses.
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Third, the negative potentials may represent postsynaptic potentials of the nonprimary afferent fibers in the spinal trigeminal tract elicited, e.g., in the subnucleus caudalis, by the NRGC. This is based on the contention that the subnucleus caudalis may modulate the processing of nociceptive information in the subnucleus oralis via an influence on its primary afferent fibers (19.27) and the identification of nonsensory, internuclear fibers in the spinal trigeminal tract connecting various subnuclei of the trigeminal sensory system (10). Although the possibility that the NRGC may influence the subnucleus oralis indirectly via the subnucleus caudalis cannot be ruled out, it is pertinent to relate it to some recent observations in this laboratory (Chart, unpublished data). Partial to complete (80 to 100%) blockade of the NRGC-elicited spinal trigeminal tract potentials in the vicinity of the subnucleus oralis occurred when they were allowed to collide with the potentials evoked at the same recording locus by dental pulp stimulation, suggesting the sharing of common primary afferent terminals at this level by both inputs. Furthermore, this observation implied that the negative potentials may reflect a depolarization of the pulpal terminals. One of the most frequently used pain stimuli is intrapulpal electrical stimulation, based on the assumption that the tooth pulp is a “pure” pain source (1, 2). Dubner et al. (8) discussed clinical reports which overwhelmingly indicate that pain is the only sensation that can be elicited from the dentine and pulp, providing that the stimulus does not spread to gingival and periodontal tissues. Mumford and Bowsher (14), however, revealed that threshold electrical activation of the teeth in man elicited a spectrum of sensations that varied from pain to vibration, tingling, or warmth. In the absence of a “perfect” pain stimulus, the dental pulp stimulation appears to be one of the better experimental pain inputs. Intrapulpal stimulation was reported to evoke neuronal activities in the marginal zone of the subnucleus caudalis, in its magnocellular region, and in the adjacent reticular formation (16, 17,21,25,26). According to Dubner et crl. (8). the rostra1 trigeminal nuclei (subnucleus oralis-main sensory nucleus) appeared to receive a larger representation from the dental pulp, at least electrophysiologically, than the subnucleus caudalis (6, 20-24), suggesting that the subnucleus oralis may be related to the localization of the pulpal stimuli, Keller rf ul. (13) showed that cortical potentials evoked in the somatic cortex by tooth pulp stimulation were abolished by the destruction of the subnucleus oralis. Thus, the subnucleus oralis may be an important relay nucleus for the centripetal transmission of dental pain signals. It follows that the oralis potential evoked by dental pulp stimulation may be a good experimental index of dental pain. The powerful NRGC-elicited suppression of the dental pulp-induced
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oralis potential in this and a previous (4) study may be interpreted to indicate that this reticular nucleus is actively participating in the analgesic processes against dentalgia. Pearl and Anderson (18) found a good number of gigantocellular neurons responded to dental pulp stimulation and suggested that the NRGC is involved in the processing of nociceptive information. Subjected to further verification, the parallel time courses of the negative spinal trigeminal tract potential and the inhibition of the oralis potential, elicited by the same NRGC stimulation, implied that presynaptic inhibitory mechanism may, at least in part, underlie this process. However, because the suppression of oralis potential outlasted the negative potential, it is possible that other synaptic mechanisms may also be involved. REFERENCES 1. ANDERSON, D. J., A. G. HANNAM, AND B. MATTHEWS. 1970. Sensory mechanisms in mammalian teeth and their supporting structures. Physiol. Rev. 50: 171-195. 2. BROOKHART, J. M., W. K. LIVINGSTONE, AND F. P. HAUGEN. 1953. Functional characteristics of afferent fibers from tooth pulp of cat. J. Neurophysiol. 16: 634-642. 3. CHAN, S. H. H. 1979. Participation of the nucleus reticularis gigantocellularis in the morphine suppression of jaw-opening reflex in cats. Bruin Res. 160: 377-380. 4. CHAN. S. H. H. 1979. Suppression ofdental pulp-evoked trigeminalresponses by nucleus reticularis gigantocellularis in the cat. Exp. Neural. 66: 356-364. 5. CHAN, S. H. H., AND C. D. BARNES. 1972. A presynaptic mechanism evoked from brain stem reticular formation in the lumbar cord and its temporal significance. Bruin Res. 45: 101-114. 6. DAVIES. W. I. R., D. SCOTT, JR., K. VESTERSTRBM. AND L. VYKLICK?. 1971. Depolarization of the tooth pulp afferent terminals in the brain stem of the cat. J. Physiol. (London) 218: 515-532. 7. DUBNER, R., AND B. J. SESSLE. 1971. Presynaptic excitability changes ofprimary afferent and cortifugal fibers projecting to trigeminal brain serm nuclei. Exp. Neural. 30: 223-238. 8. DUBNER, R., B. J. SESSLE, AND A. T. STOREY. 1979. The NeuralBasis oj‘Oralund Facial Function. Plenum, New York. 9. ECCLES, J. C. 1964. Presynaptic inhibition in the spinal cord. Pages 65-80in J. C. ECCLES AND J. P. SCHAD~, Eds., Physiology ofspinal Neurons, Progress in Brain Research. Elsevier, Amsterdam. 10. GOBEL, S., AND M. B. PURVIS. 1972. Anatomical studies ofthe organization of the spinal V nucleus: the deep bundles and the spinal V tract. Brain Res. 48: 27-44. 11. HAMMER, B.. R. TARNECKI, L. VYKLICK-?., AND M. WIESENDANGER. 1966. Cortifugal control of presynaptic inhibition in the spinal trigeminal complex of the cat. Brian Res. 2: 216-218. 12. Hu, J. W., J. 0. DOSTROVSKY, AND B. J. SESSLE. 1978. Primary afferent depolarization of tooth pulp afferents is not affected by naloxone. Nurure (London) 276: 283-284. 13. KELLER, O., S. M. BUTKHUZI, L. VYKLICK~, AND G. BROZEK. 1974. Cortical response evoked by stimulation of tooth pulp afferents in the cat. Physiol. Bohemoslov. 23: 45-54.
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14. MUMFORD, .I., AND D. BOWSHER. 1976. Pain and protopathic sensibility. A review with particular reference to the teeth. Pain 2: 223-243. 15. NAKAMURA, Y..T. MURAKAMI. M. KIKUCHI,~. KUBO. AND% ISHIMINE. 1977. Primary afferent depolarization in the trigeminal spinal nucleus of cats. E?rp. Brain Res. 29: 45-56. 16. NORD, S. G. 1976. Responses of neurons in rostra1 and caudal trigeminal nuclei to tooth pulp stimulation. Brain Res. Bull. 1: 489-492. 17. NORD, S. G. 1976. Bilateral projection of the canine tooth pulp to bulbar trigeminal neurons. Brain Res. 113: 517-523. 18. PEARL, G. S.. AND K. V. ANDERSON. 1978. Response patterns ofcells in the feline caudal nucleus reticularis gigantocellularis after noxious trigeminal and spinal stimulation. Exp. Neural. 58: 231-241. 19. SESSLE, B. J., AND L. F. GREENWOOD. 1974. Influence oftrigeminal nucleus caudalis on the responses of cat trigeminal brain stem neurons with orofacial mechanoreceptive fields. Bruin Res. 67: 330-333. 20. SESSLE. B. J., AND L. F. GREENWOOD. 1976. Inputs to trigeminal brain stem neurons from facial, oral, tooth pulp and pharyngolaryngeal tissues: I. Responses to innocuous and noxious stimuli. Brain Res. 117: 221-226. 21. SESSLE, B. J., R. DUBNER, L. F. GREENWOOD, AND G. E. LUCIER. 1976. Descending influences of periaqueductal gray matter and somatosensory cerebral cortex on neurons in trigeminal brain stem nuclei. Can. J. Physiol. Phnrmacol. 54: 66-69. 22. TAMAROVA. Z. A., A. I. SHAPOVALOV. AND L. VYKLICK~. 1973. Projection oftooth pulp afferents in the brain stem of rhesus monkey. Brain Res. 64: 442-445. 23. VYKLICKP, L.. AND 0. KELLER. 1973. Central projection of tooth pulp primary afferents in the cat. Acta Neurohid. Exp. 33: 803-809. 24. WODA, A., J. AZERAD. AND D. ALB~-FESSARD. 1977. Mapping of the trigeminal sensory complex of the cat. Characterization of its neurons by stimulations of peripheral field, dental pulp afferents and thalamic projections. J Phpsiol. (Paris) 73: 367-378. 25. YOKOTA, T. 1975. Excitation of units in marginal rim of trigeminal subnucleus caudalis elicited by tooth pulp stimulation. Brain Res. 95: 154- 158. 26. YOKOTA. T., AND S. HASHIMOTO. 1976. Periaqueductal gray and tooth pulp afferent interaction on units in caudal medulla oblongata. Bruin Res. 117: 508-512. 27. YOUNG, R. F., AND R. B. KING. 1972. Excitability changes in trigeminal primary afferent fibers in response to noxious and nonnoxious stimuli, J. Neurophysio[. 35: 87-95. 28. Yu, H. H., AND J. K. AVERY. 1974. Primary afferent depolarization: direct evidence in the trigeminal system. Brain Res. 75: 328-333.