Response characteristics of tooth pulp-driven postsynaptic neurons in the spinal trigeminal subnucleus interpolaris of the cat: comparison with primary afferent fiber, subnucleus caudalis, reflex, and sensory responses

Brain Research, 422 (1987) 205-217

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Response characteristics of tooth pulp-driven postsynaptic neurons in the spinal trigeminal subnucleus interpolaris of the cat: comparison with primary afferent fiber, subnucleus caudalis, reflex, and sensory responses Antti Pertovaara, Timo Huopaniemi, Synn6ve Carlson and Erkki Jyvfisj/irvi Department of Physiology, Universityof Helsinki, Helsinki (Finland) (Accepted 3 March 1987)

Key words: Tooth pulp; Nociception; Trigeminal subnucleus interpolaris; Cat; Conditioning stimulation; N aloxone

Tooth pulp-evoked single neuron responses were recorded in the spinal trigeminal subnucleus interpolaris of the cat. The thresholds to monopolar electric pulses of varying duration (0.2-20 msl were determined using a constant current stimulator. The thresholds were comparable with those of primary afferent A-fibers. although the most sensitive primary afferent fibers have lower thresholds. The thresholds and latencies showed that none of the interpolaris neurons received their input solely from intradental C-fibers. The most sensitive suhnucleus interpolaris neurons had lower thresholds than the respective subnucleus caudalis neurons studied in our previous work. The thresholds and strength-duration curves of the most sensitive interpolaris neurons and of the tooth pulp-elicited jaw-opening reflex are nearly similar, although the jaw reflex can be elicited at an intensity which is slightly lower than that needed to activate the most sensitive interpolaris neurons of the present sample. The most sensitive interpolaris neurons were activated at current intensities that were below the intensity needed to produce liminal dental pain in man, and the strength-duration curves of these neurons were flatter than the curve depicting liminal dental pain sensation in man. The relationship between stimulus intensity and response magnitude could be well described by power functions, the median exponent of which was 1.251. A conditioning stimulation of the tooth pulp at low intensity produced a short (<25 ms) enhancement of the response to the following test stimulus, whereas a high intensity conditioning stimulus produced a longer (>40 ms) suppression of the response to the following stimulus. The threshold of 33% of the neurons was elevated during a noxious tail pinch, and this elevation was not reversed by naloxone, an opioid antagonist The results indicate that in the tngeminal subnucleus interpolaris there are tooth pulp-driven neurons with an input from intradental A-fibers and that a considerable temporal summation of impulses from primary afferent fibers is needed to activate most of them. Human dental pain thresholds cannot be explained by the liminal response properties of the most sensitive interpolaris neurons, but they may be important in the mediation of near-threshold reflex events. It is possible, however, that the high-threshold interpolaris neurons may have a role in the mediation of sensory responses.

INTRODUCTION

ies at the p r i m a r y a f f e r e n t level, fibers with high cond u c t i o n v e l o c i t y 8"37 and low t h r e s h o l d to m e c h a n i c a l

T h e t o o t h p u l p has g e n e r a l l y b e e n c o n s i d e r e d a

stimulation 12 h a v e also b e e n r e p o r t e d , in a d d i t i o n to

p u r e l y n o c i c e p t i v e o r g a n . H o w e v e r . e v i d e n c e accu-

A 6 - and C-fibers. C o n s i s t e n t l y , a n a t o m i c a l studies

m u l a t i n g f r o m s e n s o r y studies indicates t h a t n o n -

h a v e d e m o n s t r a t e d t h e e x i s t e n c e of thick m y e l i n a t e d

n o x i o u s ' p r e p a i n ' s e n s a t i o n m a y also be elicited at w e a k c u r r e n t strengths a p p l i e d to t h e tooth6"34"48"56"57:

fibers in t h e t o o t h p u l p 19. T o o t h p u l p - e l i c i t e d reflex

n o n - n o x i o u s cold s e n s a t i o n to t h e r m a l s t i m u l a t i o n has b e e n r e p o r t e d t o o 17. I n e l e c t r o p h y s i o l o g i c a l stud-

d e p e n d e n t l y of t h e s i m u l t a n e o u s l y m e a s u r e d sensory r e s p o n s e u n d e r d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s 35.

r e s p o n s e s in h u m a n s h a v e b e e n s h o w n to b e h a v e in-

Correspondence: A. Pertovaara, Department of Physiology, University of Helsinki. Siltavuorenpenger 20 J. SF-00170 Hetsinki. Finland. 0006-8993/87/$03.50 C~)1987 Elsevier Science Publishers B.V. (Biomedical Division)

206 Thus, although stimulation of the tooth pulp mainly produces pain and pain-related neural activity, limp nal stimulus intensities are capable of eliciting nonnociceptive responses too. The afferent fibers from the tooth pulp project to the sensory trigeminal nuclear complex which consists of 4 different subnuclei: the mesencephalic nucleus principalis, and the spinal subnucleus oralis, interpolaris and caudalis 13'2s. The tooth pulp fibers have been shown to project to all these structures 2. The role of these different subnuclei in relaying signals from the teeth is controversial. Classically, the subnucleus caudalis has been considered to have an important role in the mediation of nociceptive signals from the facial area 5~, including the teeth. However, this concept has been challenged by some recent lesion studies which suggest that pain from oral and perioral regions is relayed by more rostral subnuclei 4'59"64'65. According to some investigations, the thresholds and response characteristics of tooth pulpdriven neurons in the oral and caudal subnuclei are identical 6, which finding does not favor the concept that the subnucleus caudalis has a more important role than other trigeminal subnuclei in the mediation of dental pain. In contrast, others have reported that the tooth pulp-driven neurons in the subnucleus caudalis have higher thresholds than those in the subnucleus oralis 39'40. Furthermore, recent electrophysiological recordings indicate that subnucleus caudalis neurons have some unique response properties: in contrast to oralis neurons, they can be activated by natural noxious stimuli applied to the tooth 2°. This finding supports the concept that the subnucleus caudalis has an important role in relaying pain of dental origin. A third alternative hypothesis claims that the neural output of the trigeminal complex as a whole and not that of any subdivision per se is crucial for the mediation of trigeminal pain1°,6°; this hypothesis, however, was challenged in a recent lesion study 33. Most investigators have proposed that tooth pulpdriven neurons in the subnucleus oralis and principalis relay reflex responses, which proposal has gained support from lesion studies 1 and from studies indicating that neurons in the oral parts of the trigeminal tract have lower thresholds than neurons in the subnucleus caudalis 39'4°. In most investigations on tooth pulp-driven spinal neurons the main emphasis has been on the spinal tri-

geminal subnucleus caudalis or oralis, and only in a few studies the tooth pulp-driven subnucleus interpolaris neurons have been recorded 2"1s. Thus, only little is known about the physiological response properties of the tooth pulp-driven subnucleus interpolaris neurons, and about their possible role in relaying sensory or reflex responses. In the current study we attempted to determine the response characteristics of tooth pulp-driven spinal trigeminal subnucleus interpolaris neurons of the cat, and to compare the results with those obtained earlier using similar stimuli in recordings of primary afferent fiber 36'37's6'58, subnucleus caudalis 44, and reflex 38'43 responses in the cat, and of sensory responses in man 2s's6's7. Furthermore, the effect of remote noxious conditioning stimulation was determined to find out whether or not subnucleus interpolaris neurons in this respect behave similarly to subnucleus caudalis neurons 44 or the presumed pain-relay cells of the spinal dorsal horn 3°. MATERIALS AND METHODS The experiments were performed on 5 adult cats. After an i.p. injection of pentobarbital (36 mg/kg) the animal was placed in a frame with conventional stereotaxical supports. The size of the pupilla, general muscle tone, and autonomic and other reflex reactions to a noxious pinch were observed throughout the experiments to assess the depth of the anesthesia. The anesthesia was maintained by subsequent i.v. injections of pentobarbital as required. The region of the spinal trigeminal nucleus was exposed by removing part of the atlas and part of the occipital region of the skull. A tracheostomy was performed and the animal could be artificially ventilated as required to hold the end tidalpCO 2 at 3 - 4 % . However, artificial ventilation was not needed in any of the experiments. Extracellular impulse activity was recorded from individual neurons in the spinal trigeminal subnucleus interpolaris with glass-coated tungsten microelectrodes (1-4 Mr2 at 1 kHz) made according to the method described by Wilska 62. Several criteria were used to identify postsynaptic cell discharge: the distance along the penetration over which the activity of a single neuron could be recorded, latency variation exceeding 150/~s to brief electric pulses applied at supraliminal intensities to the tooth pulp (Fig. 1A), and

207 failure to follow trains of electric stimuli applied at high frequency (300-500 Hz) and intensity to the receptive field 44'45. Only the recordings from neurons that were considered non-primary afferents were included in the study. All neurons considered in this paper were recorded as single units with a stable response to tooth pulp stimulation. In 50% of the cases it was impossible to ascertain whether a tooth pulp neuron also had input, either tactile and/or noxious, from facial tissues because outburst of multicellular activity occurred whenever the face was touched. In 50% of the cases the possible existence and type of converging input to the tooth pulp-driven neuron could be reliably determined. The stimuli used in the classification of the convergent input included airpuffs, gentle tactile stroking or tapping of the skin and oral mucosa and teeth with blunt probes, and noxious cutaneous mechanical stimuli (pinprick, pinch with forceps) that evoked painful sensations when applied to the experimenter's skin. The neurons were classified into 5 groups according to the previously used criteria 18'2°'21 depending on the type of converging input: (1) low-threshold mechanoreceptive neurons which responded to light touch or hair movement. Their discharge rate did not increase with more intense stimuli. (2) Wide-dynamic range neurons which responded to low-intensity cutaneous mechanical stimuli and which increased their discharge rate as the mechanical stimulus intensity passed into the noxious range (Fig. 10). (3) Nociceptive specific neurons which did not respond to low-intensity mechanical stimuli but required high intensity stimuli (heavy pressure and/or pinch) to excite them. (4) Corneal input neurons which were activated by light stroking of the cornea. (5) Neurons that had input from the tooth pulp only with no convergence from the oral-facial region. The monopolar cathodal stimulation of the tooth and the constant current stimulator have been described in detail earlier 37'3s'43. The electrode was positioned on the surface of a carefully dried tooth. The stimulator had a built-in device for measuring the electrode resistance, which indicated whether there was any short-circuiting to the periodontal tissue. The postsynaptic responses to the dental stimulation were not due to the activation of periodontal primary afferent fibers: even at the maximal intensity

of the stimulator {200 ~A), application of the snmulus current to the periodontine did not activate the tooth pulp-driven neurons (Fig. 1B), which finding is consistent with previous recordings at the primary afferent level 37 The test stimuli used for searching single unit responses were applied at 0.25 Hz and with an intensity of 30-50/~A (pulse duration: 5 ms), which stimulus intensity is high enough to activate intradental C-fibers 3~'58. When a single unit was isolated, its threshold was determined with stimulus pulses of different durations {0.2-20 ms). Thus, a strength-duration curve of the unit was obtained, Then stimulus-response functions, i.e. the pulse amplitude needed to produce 1, 2, 3 etc. impulses per stimulus, were determined for single pulses of 5 ms duration. The effect of distant noxious conditioning stimulation v3°-32475° on the tooth pulp-evoked responses was determined by comparing the thresholds before. during, and after a noxious tail pinch. A threshold elevation of 3(1~ or more (reference: the threshold before the tail pinch} that lasted to the end of the pinch was considered a positive response to the pinch. The intensity of the pinch was high enough to produce a behavioral response in the anesthetized animals le,g. increased muscle tone and increased the rate of ventilation). For some recording sites electrolytic DC lesions were created along the microelectrode track (electrode anode: 20 s. 20 ~tA), and after the recording the medulla was sectioned to expose the lesion sites. No more than one recording site per animal was marked. No attempt was made to expose the laminar substructures of the recording sites: only the locations of the recording sites in the spinal trigeminal subnucleus mterpolaris were verified. RESULTS Altogether 32 tooth pulp-driven single units were quantitatively studied. The neurons were located in the subnucleus mterpolaris and the adjacent reticular formation. The recording sites were located in an area extending from 2.5 to 5 mm rostral to the obex. and 3.5-4.0 mm lateral to the midtine. Tooth pulpdriven neurons were found at all recording depths in the subnucleus interpolaris. For 16 units the possible existence of converging inpul could be tested. Six of

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Fig. 1. A: variation in latency of the responses to 3 successive electric tooth pulp stimuli. The triangle indicates the stimulus artifact. B: the lower trace shows the response of two tooth pulp-driven neurons to an electric tooth pulp stimulus of 10pA. The upper trace shows that neither of these neurons was activated by a 200-pA stimulus applied to periodontal tissue. The triangle indicates the stimulus artifact. C: a tooth pulp-driven neuron with converging input from the cornea. In the upper trace the response of the neuron to an electric tooth pulp stimulus. In the lower trace the response of the same neuron to a mechanical stimulus applied to the cornea. The triangle indicates the electric stimulus artifact. D: a tooth pulp-driven neuron whose converging cutaneous input from the facial skin was classified as wide-dynamic range. R e s p o n s e s to cutaneous stimulation of different intensities shown in the figure, n, response to noxious mechanical skin stimulaton; m, response to innocuous moderate pressure; 1, response to a weak airpuff. The horizontal calibration bar represents in A 20 ms, in B 10 ms, in C 20 ms for the upper trace and 0.1 s for the lower trace, and in D 5 s.

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these units had converging input from low-threshold mechanoreceptors innervating the facial skin. Based on the response characteristics of the converging cutaneous input, one unit was classified as a wide-dynamic range neuron. Two units received converging input from specific nociceptors of the facial skin. Moreover, 5 of the tooth pulp-driven units could be activated with corneal stimulation (Fig. 1C). Two of the 16 units could not be activated from the facial skin, cornea, or extraputpal tissues of the oral cavity.

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For 16 of the 32 tooth pulp-driven units the possible existence of converging input could not be reliably tested because outbursts of multicellutar activity occurred whenever the face was touched. Fig. 2 shows the strength-duration curves for all the neurons studied. With the longer stimulus pulses (10-20 ms) the thresholds varied from 3 to 100/~A. The median threshold with a 10-ms stimulus pulse was 8.5/~A; the interquartile range was from 4.5 to 26/~A. All the neurons with a converging input from cutaneous low-threshold mechanoreceptors had thresholds below the median threshold o f the whole

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population. The mean threshold of the 10 most sensitive interpolaris neurons was 3.9 _+ 0.3 ~ A ( + S.E.M.; 10-ms stimulus pulse), which is significantly lower than the respective threshold of the subnucleus

caudalis neurons found in our earlier study 44 (6.6 _+ 1 . 3 ~ A ; P < 0.05, M a n n - W h i t n e y U-test). In 75% of the units the threshold was steeply elevated with decreasing duration of the stimulus pulse. The threshold elevation with decreasing stimulus pulse duration was not as steep in the most sensitive units as in the units with higher thresholds. The form of the s t r e n g t h - d u r a t i o n curve was not related to the type of the converging cutaneous input in the current sample. Fig. 3 shows the s t r e n g t h - d u r a t i o n curves for a unit which could be activated from both the u p p e r and lower canine. The threshold of this unit was lower for the stimulation of the upper canine, but in other respects the s t r e n g t h - d u r a t i o n curves d e t e r m i n e d

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Fig. 7. The thresholds of spinal trigeminal subnucleus interpolaris neurons during conditioning with electric stimuli. Both the conditioning and test stimulus was applied to the same tooth, and they were of equal intensity and duration (2 ms). The threshold to the test stimulus during conditioning is expressed as percentage of the threshold without conditioning (100% = the threshold obtained with a single stimulus pulse of 2 ms duration applied at a frequency of 0.25 Hz).

with stimuli applied to the upper or lower canine were similar. S t i m u l u s - r e s p o n s e functions were d e t e r m i n e d for 9 neurons with single stimulus pulses of 5 ms duration. Responses of all these neurons increased with increasing intensity of the dental stimulus. The stimulus i n t e n s i t y - r e s p o n s e magnitude relationships could be well described with p o w e r functions, the exponents of which varied from 0.364 to 3.456. The median exponent was 1.251 (Fig. 4) The correlation coefficients of the p o w e r functions varied from 0.867 to 0.999 (median: 0.993). No a p p a r e n t correlation between the s t i m u l u s - r e s p o n s e functions and the type of converging input was o b s e r v e d in the current sample of neurons. The minimal latencies of the units to test stimuli of

211 i ms duration varied from 8 to 50 ms. The median latency was 25 ms, and the interquartile range was from 13 to 40 ms. With longer stimulus pulses the latency was shorter. The latency correlated positively w i t h the threshold (Fig. 5). The latency was not strictly related to the type of converging input but short- and long-latency neurons were found among all neuron populations. The latency was dependent on the stimulus intensity: the increase in stimulus intensity from the threshold level to a value about 3 times the t h r e s h o l d level produced a latency decrease of a few milliseconds (Fig. 6). After a plateau was

reached, a further increase in the stimulus intensity did not produce any latency decrease. T h e maximal frequency of t h e t o o t h pulp stimuli that the units could follow continuously by discharging one or more impulses per stimulus pulse varied from 0.5 to 15 Hz (median: 2 Hz). When two identical brief electric stimuli were applied to the same tooth pulp, the unit could be activated with a markedly lower intensity than when using a single stimulus, especially if the interval between the two stimuli was less than 25 ms (Fig. 7). Another way of studying the interaction between two

Fig. 8. A photographic record of responses during conditioning with electric stimuli of low (upper row) or high (lower row) intensity. Both the conditioning and test stimulus were applied to the same tooth, and they were of equal intensity and duration. A: the response to a single stimulus pulse of low intensity (reference). B: the response to two successive stimuli of low intensity exceeds the response produced by two single pulses (see A). The interstimuhis interval is 4 ms. C: the same as in B but the interstimuhis interval is 20 ms. I! can be seen that it is the response to the latter (test) stimulus that is enhanced. D: the response to a single stimulus of high intensity (reference). E: the response to two successive stimuli of high intensity is tess than that produced by two single stimuli (see D). The interstimulus interval is 4 ms. F: the same as E except that the interstimulus interval is 30 ms. It can be seen that it isthe response to the latter (test) stimulus that is suppressed. The arrows indicate the stimulus artifact of the electric tooth pulp stimulus. The horizontal calibration bar represents 10 ms.

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successive tooth pulp stimuli was to measure the response magnitude (number of impulses) elicited by them. The response magnitude produced by a single stimulus was used as a reference (Fig. 8A,D). If the response to a double pulse stimulation exceeded the response evoked by two single pulses, it would be an indication of a facilitatory interaction. If the response to a double pulse stimulus were less than the response produced by two single pulses, it would indicate an inhibitory interaction. The results indicated that when the intensity of the double pulse stimulus was at near-threshold levels, there was an enhancement of the response (Fig. 8B), and that it was the response to the latter stimulus of the pair that was enhanced (Fig. 8C). The facilitation between two successive stimuli usually took place at interstimulus intervals below 25 ms (Fig. 9). When the interstimulus interval was longer (40 ms) or the intensity of the double pulse stimulus was high, there was an inhibitory interaction between the stimuli (Fig. BE). It was the response to the latter stimulus of the pair that was suppressed (Fig. 8F). The inhibitory interaction between two successive tooth pulp stimuli had a longer time

course than the facilitatory interaction. With the longest interstimulus interval used in the current study (40 ms) there was still a marked suppression of the response to a double pulse stimulus of high intensity (Fig. 9). The effect of a noxious tail pinch was tested for 15 single units. Five of these units had markedly elevated thresholds during the pinch. The threshold rise varied between 150 and 1060% (reference: the threshold before the pinch = 100%). The threshold of these 5 units remained elevated as long as the pinch continued (up to 3 min). The threshold recovered to the prepinch value within a few minutes and the effect could be obtained repeatedly. The effect of i.v. naloxone, an opioid antagonist, was tested for 3 neurons whose threshold was markedly elevated during the tail pinch. Naloxone (0.5 mg/kg) did not influence the prepinch threshold or the threshold elevation during the pinch in any of these neurons. The units with a marked threshold elevation lasting the whole pinch period received converging cutaneous input classified as nociceptive-specific (n = 2), wide-dynamic range (n = 1), tooth pulp only (n = 1),

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or undefined (n = 1). Seven of the 15 neurons tested with the pinch had a less marked threshold elevation during the first 10-15 s of the pinch; after this the threshold recovered to the prepinch value in spite of the continuing remote noxious stimulation. Of the neurons in which the tail pinch produced a short-lasting threshold elevation 3 had a converging input from the cornea, 3 from the low-threshold mechanoreceptors of the skin; for one unit the possible converging input could not be reliably tested. Three of the 15 neurons in which the effect of the tail pinch was tested had no threshold change during the noxious conditioning stimulation: two of these units had a converging input from low-threshold mechanoreceptors of the skin and for one of these neurons the possible convergence could not be determined. The units whose thresholds were uninfluenced by the tail pinch had identical supraliminal responses to tooth pulp stimuli before, during and after the pinch (Fig. 10). DISCUSSION Tooth pulp-driven postsynaptic neurons could be found in the spinal trigeminal subnucleus interpolaris of the cat; this finding is in agreement with earlier reports 2A8. The thresholds varied in a wide range. However, the thresholds of the 10 most sensitive interpolaris neurons were significantly lower than the

thresholds of the respective caudalis neurons of our earlier study 44. Earlier it has also been reported that the sensitivity of the tooth pulp-driven neurons in the spinal trigeminal tract increases rostratwards (caudalis vs oralis) 39'4°. although in some investigations no difference has been found between the thresholds of the caudalis and oralis neurons 6. In the majority ot the interpolaris neurons the thresholds were steeply elevated with decreasing duration of the stimulus pulse: a similar result was obtained for the subnucleus caudalis n e u r o n s t o o 44. At the primary afferent level the threshold for repetitive firing (e.g, 3 impulses/stimulus) behaves simitarl~ to the absolute threshold (one impulse per stimulus) of the postsynaptic neurons 57. whereas the absolute threshold of the primary afferent A-fibers innervating the tooth pulp is only little influenced by the duration of the stimulus pulse 56'58 Thus. the results obtained at the primary afferent level using stimuli similar to those of the present study are in line with the concept that a considerable temporal summation of impulses from primary afferent fibers is needed to activate the postsynaptic spinal trigeminal neurons 6"~t~'~6"44 The current findings on temporal summation, and the similar findings reported earlier 644. indicate that to avoid sampling bias m investigations on tooth pulp-driven spinal neurons, the stimulus parameters must be carefully selected. With bipolar electrode coupling and short stimulus pulses probably only the most sensitive intradental nerve units are activated 56. In studies on dental nociception sufficiently long and strong search stimuli should be used. because the most sensitive tooth pulp-innervating neurons may be non-nociceptive12. The average threshold of the intradental primary afferent A-fibers 56'58 and that of the tooth pulp-elicited law-opening reflex 38'43 are in the same range as the thresholds of the most sensitive interpolaris neurons. However. a response in the most sensitive primary afferent A-fibers 5658 and the liminal jaw-opening reflex 38'43 can be elicited even at current intensities that are below the intensity needed to activate the most sensitive interpolaris neurons of the present sample. Concerning the effect of stimulus pulse duration. the strength-duration curves of the most sensitive interpolaris neurons resembled that of the limihal jaw-opening reflex 38. Thus. the most sensitive mterpolaris neurons might contribute to the mediation

214 of near-threshold reflex events elicited by dental stimulation, but apparently the more rostral parts of the trigeminal tract are the main structures in such mediation, as also suggested by lesion studies 1'59 Although some of the interpolaris neurons had thresholds as high and strength-duration curves as steep as the primary afferent C-fibers of the tooth pulp 56"58, none of the postsynaptic interpolaris neurons had latencies in the C-fiber range (>50 ms) 2°. Thus it seems that in the current sample there were no neurons with solely C-fiber input. However, this interpretation should be made with some caution, because it is possible that intradental C-fibers become myelinated somewhere along their path to the spinal trigeminal nuclei which could explain the shorter latencies. Human perception and pain thresholds are considerably higher 25"56"57 than the thresholds of the most sensitive interpolaris neurons. Furthermore, the strength-duration curves of the liminal perception and pain response in man 56's7 are influenced more by the stimulus pulse duration than those of the most sensitive interpolaris neurons. Thus, it seems that the perception threshold or liminal dental pain sensation cannot be explained by the liminal response properties of the most sensitive interpolaris neurons. This role can rather be attributed to the most sensitive caudalis neurons. However, there still remains the possibility that supraliminal activation of the most sensitive interpolaris neurons or liminal activation of the high-threshold interpolaris neurons could contribute to the production of dental pain sensation. Furthermore, Hu and Sessle 2° found that tooth pulpdriven neurons in the subnucleus caudalis, in contrast to those in the subnucleus oralis, can be activated by natural noxious stimulation of the tooth pulp, which finding supports the concept that subnucleus caudalis neurons have a significant role in the mediation of dental pain. In the current study, the responses of all tooth pulp-driven neurons in the subnucleus interpolaris of the cat increased with increasing stimulus intensity. The relationship between stimulus intensity and response magnitude can be expressed as a power function with a median exponent of 1.251. In a previous study in rats 22it was shown that the mean power function exponent of 4.4 described the relationship between the intensity of electric tooth pulp stimuli and the magnitude of unit responses in the trigeminal sub-

nucleus caudalis. Thus, stimulus-response functions of caudalis neurons might be steeper than those of interpolaris neurons. According to Stevens 52 stimulus-response functions are steeper in the nociceptive than non-nociceptive systems, which concept together with the steeper stimulus-response functions of subnucleus caudalis neurons 2z, fits the hypothesis that tooth pulp-driven subnucleus caudalis neurons are more involved in the mediation of dental pain than neurons of the subnucleus interpolaris. However, the concept proposed by Stevens has been challenged in more recent psychophysical studies demonstrating that the stimulus-response functions of nonnociceptive and nociceptive sensations need not be different 3. Furthermore, one should be careful when comparing the results obtained by electrical stimulation of the tooth pulp of the cat and rat, because due to anatomical reasons it is difficult to activate intradental nerve fibers selectively in rodents 14"53. Interestingly, Woolston et al. 63 showed that in the trigeminal subnucleus interpolaris of the rat the responses of whisker-driven neurons changed non-monotonically with increasing stimulus velocity, which is in contrast with the behavior of the tooth pulp-driven neurons of the current study, i.e. their responses increased with increasing stimulus intensity. It has been reported that conditioning stimulation of the tooth pulp can either suppress > ' ~ or enhance a1'44'49 the responses to the following stimulus applied to the same tooth as has been demonstrated by recordings of spinal trigeminal neurons. The resuits of the current study indicate that the intensity of a conditioning tooth pulp stimulus determines its effect on the responses to the following tooth pulp stimulus: weak conditioning stimuli produce an enhancement and strong ones a suppression. The enhancement and suppression seem to have a different time course because facilitation seldom occurred if the interstimulus interval was more than 25 ms. The spatial features of this intensity-dependent in-field interaction still remain to be settled. In human psychophysical studies Laskin and Spencer 29 have described a similar phenomenon. They found that a weak conditioning airpuff stimulus to the skin produced an enhancement of the cutaneous sensitivity to the following airpuff stimulus applied to the same site, whereas a strong airpuff suppressed the sensitivity to the following stimulus. More recently this kind of interac-

215 lion between two innocuous cutaneous stimuli has been demonstrated with scalp-recorded evoked potential techniques in humans by Gandevia et al. 15, Thus, the intensity-dependent change from in-field facilitation to in-field inhibition is not a submodalityspecific but a more general phenomenon, which according to current results is based on spinal mechanisms. The activation of facilitatory mechanisms by low-intensity stimuli can help to detect weak environmental cues. The activation of inhibitory mechanisms by stronger stimuli resembles the earlier described in-field inhibition in the lemniscal system 24, and it could be beneficial in the discrimination of temporal features of the environment. It has been suggested that distant noxious conditioning stimulation mainly inhibits concurrent pain 46'61, pain-related behavior 27, and activity in the presumably pain-mediating convergent neurons of the spinal dorsal horn 3° and its trigeminal equivalent H'32. Tactile sensitivity 42'46, activity in the nonconverging low-threshold mechanoreceptive neurons of the spinal dorsal horn 3~'55, and activity in the dorsal column system mechanoreceptors 45 (however 23) is only little, if at all, influenced by noxious conditioning stimulation. Remote conditioning has been found to suppress tooth pulp-evoked jaw reflexes 7,47,50,53, although depending on the stimulus conditions (e.g. timing of the stimuli) no effect or even facilitation has been reported in the same studies. In our previous study the threshold for the tooth pulp-elicited jaw-opening reflex in cats was uninfluenced by a noxious tail pinch 43, although the simultaneously measured threshold for the tooth pulpdriven subnucleus caudalis neurons was markedly elevated 44. In the current study a noxious tail pinch produced a threshold elevation lasting as long as the pinch in 33% (5/15) of the tooth pulp-driven subnucleus interpolaris neurons, whereas in two earlier studies remote noxious conditioning stimulation produced a threshold elevation in 100% (3/3) 11 and 62% (8/13) 44 of tooth pulp-driven subnucleus caudalis neurons in the cat. Thus, it seems that noxious conditioning stimulation may be more effective in suppressing tooth pulp-elicited activity in the subnucleus caudalis than in the subnucleus interpolaris, although a larger sample of neurons is needed to confirm this conclusion. Naloxone, a specific opioid antagonist, did not re-

verse the tail pinch-induced suppression of neural responses to dental stimulation. Apparently naloxonesensitive endogenous opioids do not contribute to the inhibition of tooth pulp-driven neurons. However. tt should be borne in mind that there are several opioiddependent systems, and only some of them are naloxone-sensitive 54. Interestingly, the suppression of responses to noxious skin and visceral stimuli during remote noxious conditioning stimulation has been reversed by naioxone 32. This suggests that the multisegmental suppression of pain-related neural activity during a tail pinch is partially based on different mechanisms in the pathways mediating dental sensations than in the pathways mediating somatic or visceral sensations. The dental pain threshold elevation produced by remote noxious conditioning stimulation in humans was naloxone-insensitive too ~'. It has been shown that some of the brainstem inhibitorv systems attenuate only the gain of the ascending somatosensory signals without affecting the threshold 9'5°. The interpolaris neurons that had no threshold elevation during a tail pinch could not have had attenuated supraliminal responses because the stimulus-response functions were identical before, during, and after the tail pinch. The comparison of the response properties of the tooth pulp-driven neurons in the subnucleus interpolaris and in the subnucleus caudalis 44 suggests that the most sensitive subnucleus caudalis neurons are more probable candidates for the mediation of dental pain than the most sensitive interpolaris neurons However, among interpolaris neurons there are many high-threshold neurons too. and their role in the mediation of pain cannot be excluded. Concerning tooth pulp-elicited reflex responses, it seems that the subnucleus caudalis neurons 44 are not capable of mediating liminal reflex responses. The interpolaris neurons might mediate near-threshold reflex events but apparently the more rostral subdivisions of the trigeminal tract contribute more to the relaying of liminal reflex events. ACKNOWLEDGEMENTS This study was supported by grants from the Paulo Foundation, Helsinki, Finland. We wish to thank I. Linnankoski, B.A., for his assistance in completing the manuscript.

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