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Capsaicin-induced central facilitation of a sympathetic vasoconstrictor response to painful stimulation in humans
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ELSEVIER
Neuroscience Letters 182 (1994) 163-166
N[UROSCIENC[ LETTERS
Capsaicin-induced central facilitation of a sympathetic vasoconstrictor response to painful stimulation in humans M a r i G r 6 n r o o s a, H a n n u
N a u k k a r i n e n b, A n t t i P e r t o v a a r a a'*
Departments of"Physiology and bPsychiatry, University of Helsinki, Helsinki, Finland Received 27 April 1994; Revised version received 5 October 1994; Accepted 7 October 1994
Abstract
The effect of capsaicin, a compound selectively activating nociceptive primary afferent fibers, on a centrally mediated autonomic (sympathetic) vasoconstriction response to painful peripheral stimulation was studied in healthy human volunteers. Capsaicin (1%) was applied topically to the dorsal forearm and the threshold for eliciting a vasoconstriction response in the contralateral forefinger to painful electrical or thermal stimulation of the forearm skin adjacent to or remote from the capsaicin-treated region was determined with Laser Doppler flowmetry. Capsaicin produced a significant decrease of the threshold for the vasoconstriction response to painful electrical stimulation of the area of central allodynia; i.e., the skin area located adjacent to the capsaicin-treated region in which a light touch evoked an unpleasant sensation. Capsaicin did not change the threshold for the vasoconsriction response to painful heat stimuli applied to the area of central allodynia or to painful electric stimulation applied outside the borders of the allodynic area. Heart rate, blood pressure and heart vagal tone were not modified by capsaicin. It is concluded that a selective activation of nociceptive primary afferent fibers of the skin by capsaicin produces a central submodality-dependent facilitation of an autonomic vasoconstriction response to noxious stimulation in humans. This central facilitation can be explained by segmental excitability changes of afferent interneurons at the spinal cord level.
Key words: Capsaicin; Sympathetic vasoconstrictor reflex; Nociception; Allodynia; Central facilitation
Tissue injury and the consequent injury discharge in nociceptive primary afferent fibers can produce a central facilitation of nociceptive flexor reflexes and marked changes in the response properties of nociceptive spinal dorsal horn neurons in animals [3,4,21]. Capsaicin, a compound selectively activating nociceptive primary afferent fibers [9,15], has been shown to produce central hyperalgesia and allodynia (= unpleasant sensations evoked by innocuous stimuli) in animals and in humans [10,11,13,14]. Electrophysiological reflections of the central hyperalgesia and allodynia in humans are the capsaicin-induced selective facilitation of the nociceptive flexor reflex and the enhancement of the somatosensoryevoked responses to electrical stimuli applied outside the capsaicin-treated skin region [6,7].
*Corresponding author. Address: Department of Physiology, PO Box 9, FIN-00014 University of Helsinki, Finland. Fax: (358) (0) 191-8681. 0304-3940/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-3940(94)00774-8
A tissue or nerve injury may also produce significant changes in central as well as peripheral mechanisms of vasomotor regulation as shown by previous experimental and clinical studies [2,8,17]. In the present study, we tried to find out if the vasomotor regulation of skin circulation is modulated also in the capsaicin model of central hyperalgesia. Therefore, we determined the effect of capsaicin on the sympathetically mediated vasoconstrictor response m painful stimulation of the skin [18]. Capsaicin was applied to a site different from that to which painful test stimuli eliciting the reflex were applied to exclude peripheral mechanisms as a cause of possible reflex changes. To study the submodality dependence of the capsaicin-induced changes in the vasocontrictor response, both painful electrical and heat stimuli were used to elicit the vasoconstriction reflex. 17 healthy humans (23-40 years; seven females and 10 males) volunteered for this study. The subjects were staff scientists and graduate students. An informed consent was obtained from the subjects before the experiments
164
M. GrOnroos et al./Neuroscience Letters 182 (1994) 163-166
according to the ethical principles of the Helsinki convention. Capsaicin (1%; Fluka) dissolved in 70% ethanol was applied in a plaster topically to a 4-cm 2 area of the dorsal forearm. Application time was 40 min. In control experiments, only 70% ethanol was applied topically to the dorsal forearm. The interval between capsaicin sessions was at least 1 week and capsaicin was never applied to the same skin area in the consecutive sessions. The intensity of the burning pain sensation elicited by capsaicin was rated verbally from 0 to 10 (0 = no pain, 10 = the worst imaginable pain from the skin area studied). A Laser Doppler flowmeter (Periflux PF3; Perimed, Stockholm, Sweden) was used to provide continuous records of blood flow changes in the skin. The analogue output of this equipment gives no absolute values but relative changes of cutaneous blood flow. The maximum output of the gain level used was taken as 100%. A detailed discussion of the method is presented elsewhere [12]. The electrical test stimuli for eliciting the vasoconstriction response in the contralateral forefinger were applied into two places: the right forearm skin at the wrist ( = 6-8 cm outside the borders of the allodynic area) and - 6 - 8 cm proximal to the wrist (the area of capsaicin-induced central allodynia adjacent to the capsaicin-treated skin region). The electrical test stimuli were delivered using a couple of surface electrodes placed on the degreased skin overlying the skin 2 cm apart. Each electrical stimulus consisted of a train of constant current pulses (train duration 40 ms, each pulse of 1 ms duration at the frequency of 100 Hz). Autonomic reflex responses were recorded by the Laser Doppler flowmeter from the {A}
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Fig. 1. A: the threshold for eliciting the contralateral vasoconstriction response to painful electrical stimulation of the skin in different experimental conditions, a (100%) = the control threshold before application of capsaicin/dissolvent, b = the threshold following application of capsaicin adjacent to the stimulus electrodes (stimuli applied within the area of central allodynia), c = the threshold following application of capsaicin in an area remote from the stimulus electrodes (stimuli applied 6-8 cm outside the the borders of the allodynic skin area), d = the threshold following application of dissolvent (70% ethanol) alone adjacent to the stimulus electrodes. The various conditions (b~l) were tested in separate sessions with an interval ->1 week. B: the threshold for eliciting the contralateral vasoconstriction response to painful heat stimuli before (pre) and following the application of capsaicin (caps) adjacent to the heat stimulus. The vertical error bars represent S.E.M. (in A n = 5, in B n = 4). ** = P < 0.005 (reference: the corresponding precapsaicin threshold; paired t test).
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Fig. 2. A: vagal tone over six subjects with capsaicin or with dissolvent (ethanol) applied at t = 0 min. B: a representative example of the original Laser Doppler flow recording before (the lower trace) and following (the upper trace) application of capsaicin. The electric stimuli were applied to the forearm contralateral to the finger from which the blood flow was measured. Capsaicin was applied adjacent to the stimulus electrodes. The stimulus intensities (mA) producing a vasoconstriction are indicated by arrows.
palmar skin of the forefinger contralaterally to the site of painful test stimulation. When determining the threshold for eliciting the vasoconstriction reflex, the electrical stimuli were applied at three to five different amplitudes in a random order at 45-60-s intervals. Each amplitude was presented 3-4 times. The amplitude at which there was a vasoconstriction to 50% of stimuli was defined as the threshold and it was determined before capsaicin and 40 min after capsaicin application. Only a blood flow change exceeding by 100% the basal blood flow variability and starting within a couple of seconds following the stimulation was considered to represent a vasomotor response to stimulation. The threshold for eliciting vasoconstriction response to painful heat stimuli was determined in a separate session. Heat stimuli (duration 5 s) were delivered using a commercially available feedback-controlled contact thermostimulator [20], composed of Peltier elements with a stimulating surface of 11.8 cm 2. The rate of temperature change was 6.0°C/s. The adaptation temperature was thermoneutral 35°C. The stimuli were usually delivered at four different temperatures (49-52°C). To avoid skin sensitization or damage, there were four test sites around the capsaicin-treated area. These sites were stimulated consecutively so that there was only one stimulation at each site. Each stimulus was presented only once before and during capsaicin. The heat stimuli were applied to the allodynic area adjacent to the capsaicintreated region and the interval between successive heat stimulus presentations was 1 min. Heart rate and blood pressure were measured at the wrist contralateral to the blood flow measurements using an automatic blood pressure cuff (Omron HEM-601). The mean blood pressure and heart rate before capsaicin application were compared with the corresponding values following capsaicin application. Vagal tone was measured in a separate experiment
M. Griinroos et aL / Neuroscienee Letters 182 (1994) 163-166
with a Vagal Tone Monitor (Delta-Biometrics, Bethesda, MD) which calculates a measure of cardiac vagal tone by quantifying the amplitude of respiratory sinus arrhythmia from the heart rate pattern; i.e., variance of heart rate within the frequencies associated with breathing. A detailed discussion of the method is presented elsewhere [1]. Vagal tone was continuously calculated over each 30-s period and in this study vagal tone was measured as the mean over each 5-min periods. Vagal tone was determined before capsaicin and during the 40-min application time of capsaicin in 5-min periods. Paired t test (two-tailed) was used in statistical evaluation of the data. P < 0.05 was considered to represent a significant difference. Capsaicin produced a burning pain sensation in all subjects. The mean pain rating was 4.1 + 0.8 (+ S.E.M., n = 6). The latency to the onset of the spontaneous capsaicin-induced pain varied from 20 to 40 min. Capsaicin also produced allodynia (= unpleasant sensation evoked by innocuous stimulation) to light mechanical stroking of the skin with a cotton swab in all subjects. The borders of the allodynic area extended a few cm from the capsaicin-treated region. The dissolvent (70% ethanol) alone did not produce any sensory symptoms. Only five of the 14 subjects tested had a consistent vasoconstrictor response to repetitive painful electrical stimulation in preliminary sessions. In further studies on the capsaicin-induced modulation of vasoconstriction reflex, only these five subjects were tested. The rest of the subjects had to be discarded because they did not have a blood flow change induced by painful test stimulation at any of the stimulus intensities applied to the contralateral forearm (n = 5), the basal blood flow was extremely labile (n = 1), the vasoconstrictor response habituated (/disappeared) following a few painful stimuli independent of the interstimulus interval (45 s to 5 rain; n = 2), or painful stimulation induced a blood flow increase in the contralateral finger independent of the adapting skin temperature (n = 1). In a control experiment, the mean threshold for eliciting the contralateral vasoconstrictor response by electrical stimulation was 13.9 + 4.9 mA (+ S.E.M., n = 5) before the application of dissolvent (70% ethanol) and the corresponding threshold was 14.2 + 4.7 mA following the application of dissolvent alone (Fig. 1A). The mean threshold for eliciting a perception was 0.36 + 0.1 mA and the mean threshold for eliciting a sensation of pain was 7.0 + 1.9 mA. Capsaicin induced a marked decrease of the threshold for eliciting a contralateral vasoconstrictor response to electrical stimulation of the skin area of central allodynia (= adjacent to the capsaicin-treated region) in all subjects (n = 5; Figs. 1A, 2B). The mean threshold before capsaicin was 13.1 + 1.8 and 7.2 + 1.6 mA following the application of capsaicin in the immediate vicinity of the stimulus electrodes (P = 0.0038, paired t test). When the elec-
165
trical test stimulation was applied 6-8 cm outside the borders of the allodynic area, there was no capsaicininduced change in the threshold for eliciting a contralateral vasoconstriction response (Fig. 1A). Capsaicin did not change the threshold for the vasoconstriction reflex evoked by heat stimuli applied to the allodynic area (50.1 + 0.9 vs. 50.3 + 0.9°C, n = 4; Fig. 1B). In separate experiments, the effects of capsaicin vs. the dissolvent alone were determined on heart rate, blood pressure and vagal tone. No general changes in the sympathetic and parasympathetic system were induced by capsaicin. The mean vagal tone was not significantly changed by capsaicin during the 40-min observation period when compared with the effect of dissolvent alone (Fig. 2A). The main finding of this study was that capsaicin produced a tonic threshold decrease for eliciting a centrally (sympathetically) mediated vasoconstriction response to painful electrical stimuli in humans. This threshold decrease induced by capsaicin was due to central mechanisms since capsaicin was applied to a site different from that to which electrical stimuli eliciting the reflex were applied. It seems that this central sensitization of the vasoconstriction reflex was due to an action on the afferent side of the reflex arch since the contralateral vasoconstrictor response to painful heat stimuli was not facilitated although the efferent side of the reflex arch should be the same in both stimulation conditions. The lack of central sensitization of the vasoconstrictor response to heat stimuli is in line with the sensory studies showing a lack of heat hyperalgesia in the area of central allodynia [10,13,16]. In the capsaicin-treated region, hyperalgesia to heat has been described [5] which can be explained by peripheral mechanisms (sensitization of nociceptive primary afferent fibers). Capsaicin did not produce systemic changes in the autonomic nervous system as indicated by the lack of effect on heart rate, blood pressure and cardiac vagal tone. This finding together with the spatially restricted facilitatory effect of capsaicin suggests that the sensitization of the vasoconstriction reflex was due to a segmental spinal facilitation of the afferent interneurons of the sympathetic reflex arch. The spinal segmental mechanism as a cause of the capsaicin-induced reflex facilitation is supported by the recent finding that noxious conditioning stimulation did not change or minimally increased the central thermoregulatory threshold for vasoconstriction during general anesthesia in humans [19]. Interestingly, the reflex vasoconstriction response to painful stimulation could be reliably elicited and studied only in a minority of the subjects which limits its use to a subpopulation of subjects. The reason for this is not clear. Anyhow, the difficulty in eliciting the reflex and the tendency of the reflex to habituate further emphasize the significance of the capsaicin-induced facilitation. The results of this study support the evidence indicat-
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M. Gr6nroos et al./Neuroscience Letters 182 (1994) 163-166
ing that activation of nociceptive primary afferent fibers may lead to central excitability changes in nociceptive systems [3,4,14,16,21 ]. These central excitability changes may also influence vasomotor regulation as well as somatic reflexes [6] and they are potentially important for explaining pain-related clinical symptoms, including abnormal functions of the autonomic nervous system encountered in clinical patients with a neuropathic pain condition [2]. The capsaicin-induced central facilitation of the autonomic reflex may provide a fruitful model to study experimentally the underlying mechanisms and new therapeutic applications to these conditions in human subjects. This study was supported by grants from the Paulo Foundation and the Sigrid Juselius Foundation, Helsinki, and the Academy of Finland. [1] Adinoff, B., Mefford, I., Waxman, R. and Linnoila, M., Vagal tone decreases following intravenous diazepam, Psychiatry Res., 41 (1992) 89-97. [2] Blumberg, H., A new clinical approach for diagnosing reflex sympathetic dystrophy. In M.R. Bond et al. (Eds.), Proc. Vlth World Congr. Pain, Elsevier, Amsterdam, The Netherlands, 1991, pp. 399-407. [3] Coderre, T.J. and Melzack, R., Increased pain sensitivity following heat injury involves a central mechanism, Behav. Brain Res., 15 (1985) 259-262. [4] Cook, A.J., Woolf, C.J., Wall, P.D. and McMahon, S.B., Dynamic receptive field plasticity in rat spinal cord dorsal horn following C-primary afferent fiber input, Nature (London), 325 (1987) 151 153. [51 Culp, W.J., Ochoa, J., Cline, M. and Dotson, R., Heat and mechanical hyperalgesia induced by capsaicin, Brain, 112 (1989) 1317-1331. [6] Gr6nroos, M. and Pertovaara, A., Capsaicin-induced central facilitation of a nociceptive flexion reflex in humans, Neurosci. Lett., 159 (1993) 215-218. [7] Gr6nroos, M., Huttunen, J. and Pertovaara, A., Capsaicin-induced central facilitation of the somatosensory evoked potential in humans, Neurosci. Res. Commun., 14 (1994) 71 74.
[8] J~nig, W. and Koltzenburg, M., Sympathetic reflex activity and neuroeffector transmission change after chronic nerve lesions. In M.R. Bond et al. (Eds.), Proc. Vlth World Congr. Pain, Elsevier, Amsterdam, The Netherlands, 1991, pp. 365-371. [9] Kenins, P., Responses of single nerve fibers to capsaicin applied to the skin, Neurosci. Lett., 29 (1982) 83-88. [10] Koltzenburg, M., Lundberg, L.E.R. and Torebj6rk, H.E., Dynamic and static components of mechanical hyperalgesia in human hairy skin, Pain, 51 (1992) 207-219. [I1] LaMotte, R.H., Lundberg, L.E.R. and Torebj6rk, H.E., Pain, hyperalgesia and activity in nociceptive C units in humans after intradermal injection of capsaicin, J. Physiol. (London), 448 (1992) 749-764. [12] Oberg, P./k., Laser-Doppler flowmetry, Crit. Rev. Biomed. Eng., 18 (1990) 125-163. [13] Simone, D.A., Baumann, T.K. and LaMotte, R.H., Dose-dependent pain and mechanical hyperalgesia in humans after intradermal injection of capsaicin, Pain, 38 (1989) 99-107. [14] Simone, D.A., Sorkin, L.S., Oh, U., Chung, J.M., Owens, C., LaMotte, R.H. and Willis, W.D., Neurogenic hyperalgesia: Central neural correlates in responses of spinothalamic tract neurons, J. Neurophysiol., 66 (1991) 228-246. [15] Szolcsanyi, J., Anton, F., Reeh, P.W. and Handwerker, H.O., Selective excitation by capsaicin of mechano-heat sensitive nociceptors in rat skin, Brain Res., 446 (1988) 262-268. [16] Treede, R.-D., Meyer, R.A., Raja, S.N. and Campbell, J.N., Peripheral and central mechanisms of cutaneous hyperalgesia, Prog. Neurobiol., 38 (1992) 397-421. [17] Wakisaka, S., Kajander, K.C. and Bennett, G.J., Abnormal skin temperature and abnormal sympathetic vasomotor innervation in an experimental painful peripheral neuropathy, Pain, 46 (1991) 299-313. [18] Wallin, B.G., Neural control of human skin blood flow, J. Auton. Nerv. Syst., 30 (1990) 185-190. [19] Washington, D.E., Sessler, D.I., McGuire, J., Hynson, J., Schroeder, M. and Moayeri, A., Painful stimulation minimally increases the thermoregulatory threshold for for vasoconstriction during enflurane anesthesia in humans, Anesthesiology, 77 (1992) 286290. [20] Wilcox, G.L. and Giesler, G.J., An instrument using a multiplelayer of Peltier device to change skin temperature rapidly, Brain Res. Bull., 12 (1984) 143-146. [21] Woolf, C.J., Evidence for a central component of post-injury pain hypersensitivity, Nature (London), 306 (1983) 686~88.
'
,'
,
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ELSEVIER
Neuroscience Letters 182 (1994) 163-166
N[UROSCIENC[ LETTERS
Capsaicin-induced central facilitation of a sympathetic vasoconstrictor response to painful stimulation in humans M a r i G r 6 n r o o s a, H a n n u
N a u k k a r i n e n b, A n t t i P e r t o v a a r a a'*
Departments of"Physiology and bPsychiatry, University of Helsinki, Helsinki, Finland Received 27 April 1994; Revised version received 5 October 1994; Accepted 7 October 1994
Abstract
The effect of capsaicin, a compound selectively activating nociceptive primary afferent fibers, on a centrally mediated autonomic (sympathetic) vasoconstriction response to painful peripheral stimulation was studied in healthy human volunteers. Capsaicin (1%) was applied topically to the dorsal forearm and the threshold for eliciting a vasoconstriction response in the contralateral forefinger to painful electrical or thermal stimulation of the forearm skin adjacent to or remote from the capsaicin-treated region was determined with Laser Doppler flowmetry. Capsaicin produced a significant decrease of the threshold for the vasoconstriction response to painful electrical stimulation of the area of central allodynia; i.e., the skin area located adjacent to the capsaicin-treated region in which a light touch evoked an unpleasant sensation. Capsaicin did not change the threshold for the vasoconsriction response to painful heat stimuli applied to the area of central allodynia or to painful electric stimulation applied outside the borders of the allodynic area. Heart rate, blood pressure and heart vagal tone were not modified by capsaicin. It is concluded that a selective activation of nociceptive primary afferent fibers of the skin by capsaicin produces a central submodality-dependent facilitation of an autonomic vasoconstriction response to noxious stimulation in humans. This central facilitation can be explained by segmental excitability changes of afferent interneurons at the spinal cord level.
Key words: Capsaicin; Sympathetic vasoconstrictor reflex; Nociception; Allodynia; Central facilitation
Tissue injury and the consequent injury discharge in nociceptive primary afferent fibers can produce a central facilitation of nociceptive flexor reflexes and marked changes in the response properties of nociceptive spinal dorsal horn neurons in animals [3,4,21]. Capsaicin, a compound selectively activating nociceptive primary afferent fibers [9,15], has been shown to produce central hyperalgesia and allodynia (= unpleasant sensations evoked by innocuous stimuli) in animals and in humans [10,11,13,14]. Electrophysiological reflections of the central hyperalgesia and allodynia in humans are the capsaicin-induced selective facilitation of the nociceptive flexor reflex and the enhancement of the somatosensoryevoked responses to electrical stimuli applied outside the capsaicin-treated skin region [6,7].
*Corresponding author. Address: Department of Physiology, PO Box 9, FIN-00014 University of Helsinki, Finland. Fax: (358) (0) 191-8681. 0304-3940/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-3940(94)00774-8
A tissue or nerve injury may also produce significant changes in central as well as peripheral mechanisms of vasomotor regulation as shown by previous experimental and clinical studies [2,8,17]. In the present study, we tried to find out if the vasomotor regulation of skin circulation is modulated also in the capsaicin model of central hyperalgesia. Therefore, we determined the effect of capsaicin on the sympathetically mediated vasoconstrictor response m painful stimulation of the skin [18]. Capsaicin was applied to a site different from that to which painful test stimuli eliciting the reflex were applied to exclude peripheral mechanisms as a cause of possible reflex changes. To study the submodality dependence of the capsaicin-induced changes in the vasocontrictor response, both painful electrical and heat stimuli were used to elicit the vasoconstriction reflex. 17 healthy humans (23-40 years; seven females and 10 males) volunteered for this study. The subjects were staff scientists and graduate students. An informed consent was obtained from the subjects before the experiments
164
M. GrOnroos et al./Neuroscience Letters 182 (1994) 163-166
according to the ethical principles of the Helsinki convention. Capsaicin (1%; Fluka) dissolved in 70% ethanol was applied in a plaster topically to a 4-cm 2 area of the dorsal forearm. Application time was 40 min. In control experiments, only 70% ethanol was applied topically to the dorsal forearm. The interval between capsaicin sessions was at least 1 week and capsaicin was never applied to the same skin area in the consecutive sessions. The intensity of the burning pain sensation elicited by capsaicin was rated verbally from 0 to 10 (0 = no pain, 10 = the worst imaginable pain from the skin area studied). A Laser Doppler flowmeter (Periflux PF3; Perimed, Stockholm, Sweden) was used to provide continuous records of blood flow changes in the skin. The analogue output of this equipment gives no absolute values but relative changes of cutaneous blood flow. The maximum output of the gain level used was taken as 100%. A detailed discussion of the method is presented elsewhere [12]. The electrical test stimuli for eliciting the vasoconstriction response in the contralateral forefinger were applied into two places: the right forearm skin at the wrist ( = 6-8 cm outside the borders of the allodynic area) and - 6 - 8 cm proximal to the wrist (the area of capsaicin-induced central allodynia adjacent to the capsaicin-treated skin region). The electrical test stimuli were delivered using a couple of surface electrodes placed on the degreased skin overlying the skin 2 cm apart. Each electrical stimulus consisted of a train of constant current pulses (train duration 40 ms, each pulse of 1 ms duration at the frequency of 100 Hz). Autonomic reflex responses were recorded by the Laser Doppler flowmeter from the {A}
(B)
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Fig. 1. A: the threshold for eliciting the contralateral vasoconstriction response to painful electrical stimulation of the skin in different experimental conditions, a (100%) = the control threshold before application of capsaicin/dissolvent, b = the threshold following application of capsaicin adjacent to the stimulus electrodes (stimuli applied within the area of central allodynia), c = the threshold following application of capsaicin in an area remote from the stimulus electrodes (stimuli applied 6-8 cm outside the the borders of the allodynic skin area), d = the threshold following application of dissolvent (70% ethanol) alone adjacent to the stimulus electrodes. The various conditions (b~l) were tested in separate sessions with an interval ->1 week. B: the threshold for eliciting the contralateral vasoconstriction response to painful heat stimuli before (pre) and following the application of capsaicin (caps) adjacent to the heat stimulus. The vertical error bars represent S.E.M. (in A n = 5, in B n = 4). ** = P < 0.005 (reference: the corresponding precapsaicin threshold; paired t test).
(A)
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Fig. 2. A: vagal tone over six subjects with capsaicin or with dissolvent (ethanol) applied at t = 0 min. B: a representative example of the original Laser Doppler flow recording before (the lower trace) and following (the upper trace) application of capsaicin. The electric stimuli were applied to the forearm contralateral to the finger from which the blood flow was measured. Capsaicin was applied adjacent to the stimulus electrodes. The stimulus intensities (mA) producing a vasoconstriction are indicated by arrows.
palmar skin of the forefinger contralaterally to the site of painful test stimulation. When determining the threshold for eliciting the vasoconstriction reflex, the electrical stimuli were applied at three to five different amplitudes in a random order at 45-60-s intervals. Each amplitude was presented 3-4 times. The amplitude at which there was a vasoconstriction to 50% of stimuli was defined as the threshold and it was determined before capsaicin and 40 min after capsaicin application. Only a blood flow change exceeding by 100% the basal blood flow variability and starting within a couple of seconds following the stimulation was considered to represent a vasomotor response to stimulation. The threshold for eliciting vasoconstriction response to painful heat stimuli was determined in a separate session. Heat stimuli (duration 5 s) were delivered using a commercially available feedback-controlled contact thermostimulator [20], composed of Peltier elements with a stimulating surface of 11.8 cm 2. The rate of temperature change was 6.0°C/s. The adaptation temperature was thermoneutral 35°C. The stimuli were usually delivered at four different temperatures (49-52°C). To avoid skin sensitization or damage, there were four test sites around the capsaicin-treated area. These sites were stimulated consecutively so that there was only one stimulation at each site. Each stimulus was presented only once before and during capsaicin. The heat stimuli were applied to the allodynic area adjacent to the capsaicintreated region and the interval between successive heat stimulus presentations was 1 min. Heart rate and blood pressure were measured at the wrist contralateral to the blood flow measurements using an automatic blood pressure cuff (Omron HEM-601). The mean blood pressure and heart rate before capsaicin application were compared with the corresponding values following capsaicin application. Vagal tone was measured in a separate experiment
M. Griinroos et aL / Neuroscienee Letters 182 (1994) 163-166
with a Vagal Tone Monitor (Delta-Biometrics, Bethesda, MD) which calculates a measure of cardiac vagal tone by quantifying the amplitude of respiratory sinus arrhythmia from the heart rate pattern; i.e., variance of heart rate within the frequencies associated with breathing. A detailed discussion of the method is presented elsewhere [1]. Vagal tone was continuously calculated over each 30-s period and in this study vagal tone was measured as the mean over each 5-min periods. Vagal tone was determined before capsaicin and during the 40-min application time of capsaicin in 5-min periods. Paired t test (two-tailed) was used in statistical evaluation of the data. P < 0.05 was considered to represent a significant difference. Capsaicin produced a burning pain sensation in all subjects. The mean pain rating was 4.1 + 0.8 (+ S.E.M., n = 6). The latency to the onset of the spontaneous capsaicin-induced pain varied from 20 to 40 min. Capsaicin also produced allodynia (= unpleasant sensation evoked by innocuous stimulation) to light mechanical stroking of the skin with a cotton swab in all subjects. The borders of the allodynic area extended a few cm from the capsaicin-treated region. The dissolvent (70% ethanol) alone did not produce any sensory symptoms. Only five of the 14 subjects tested had a consistent vasoconstrictor response to repetitive painful electrical stimulation in preliminary sessions. In further studies on the capsaicin-induced modulation of vasoconstriction reflex, only these five subjects were tested. The rest of the subjects had to be discarded because they did not have a blood flow change induced by painful test stimulation at any of the stimulus intensities applied to the contralateral forearm (n = 5), the basal blood flow was extremely labile (n = 1), the vasoconstrictor response habituated (/disappeared) following a few painful stimuli independent of the interstimulus interval (45 s to 5 rain; n = 2), or painful stimulation induced a blood flow increase in the contralateral finger independent of the adapting skin temperature (n = 1). In a control experiment, the mean threshold for eliciting the contralateral vasoconstrictor response by electrical stimulation was 13.9 + 4.9 mA (+ S.E.M., n = 5) before the application of dissolvent (70% ethanol) and the corresponding threshold was 14.2 + 4.7 mA following the application of dissolvent alone (Fig. 1A). The mean threshold for eliciting a perception was 0.36 + 0.1 mA and the mean threshold for eliciting a sensation of pain was 7.0 + 1.9 mA. Capsaicin induced a marked decrease of the threshold for eliciting a contralateral vasoconstrictor response to electrical stimulation of the skin area of central allodynia (= adjacent to the capsaicin-treated region) in all subjects (n = 5; Figs. 1A, 2B). The mean threshold before capsaicin was 13.1 + 1.8 and 7.2 + 1.6 mA following the application of capsaicin in the immediate vicinity of the stimulus electrodes (P = 0.0038, paired t test). When the elec-
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trical test stimulation was applied 6-8 cm outside the borders of the allodynic area, there was no capsaicininduced change in the threshold for eliciting a contralateral vasoconstriction response (Fig. 1A). Capsaicin did not change the threshold for the vasoconstriction reflex evoked by heat stimuli applied to the allodynic area (50.1 + 0.9 vs. 50.3 + 0.9°C, n = 4; Fig. 1B). In separate experiments, the effects of capsaicin vs. the dissolvent alone were determined on heart rate, blood pressure and vagal tone. No general changes in the sympathetic and parasympathetic system were induced by capsaicin. The mean vagal tone was not significantly changed by capsaicin during the 40-min observation period when compared with the effect of dissolvent alone (Fig. 2A). The main finding of this study was that capsaicin produced a tonic threshold decrease for eliciting a centrally (sympathetically) mediated vasoconstriction response to painful electrical stimuli in humans. This threshold decrease induced by capsaicin was due to central mechanisms since capsaicin was applied to a site different from that to which electrical stimuli eliciting the reflex were applied. It seems that this central sensitization of the vasoconstriction reflex was due to an action on the afferent side of the reflex arch since the contralateral vasoconstrictor response to painful heat stimuli was not facilitated although the efferent side of the reflex arch should be the same in both stimulation conditions. The lack of central sensitization of the vasoconstrictor response to heat stimuli is in line with the sensory studies showing a lack of heat hyperalgesia in the area of central allodynia [10,13,16]. In the capsaicin-treated region, hyperalgesia to heat has been described [5] which can be explained by peripheral mechanisms (sensitization of nociceptive primary afferent fibers). Capsaicin did not produce systemic changes in the autonomic nervous system as indicated by the lack of effect on heart rate, blood pressure and cardiac vagal tone. This finding together with the spatially restricted facilitatory effect of capsaicin suggests that the sensitization of the vasoconstriction reflex was due to a segmental spinal facilitation of the afferent interneurons of the sympathetic reflex arch. The spinal segmental mechanism as a cause of the capsaicin-induced reflex facilitation is supported by the recent finding that noxious conditioning stimulation did not change or minimally increased the central thermoregulatory threshold for vasoconstriction during general anesthesia in humans [19]. Interestingly, the reflex vasoconstriction response to painful stimulation could be reliably elicited and studied only in a minority of the subjects which limits its use to a subpopulation of subjects. The reason for this is not clear. Anyhow, the difficulty in eliciting the reflex and the tendency of the reflex to habituate further emphasize the significance of the capsaicin-induced facilitation. The results of this study support the evidence indicat-
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M. Gr6nroos et al./Neuroscience Letters 182 (1994) 163-166
ing that activation of nociceptive primary afferent fibers may lead to central excitability changes in nociceptive systems [3,4,14,16,21 ]. These central excitability changes may also influence vasomotor regulation as well as somatic reflexes [6] and they are potentially important for explaining pain-related clinical symptoms, including abnormal functions of the autonomic nervous system encountered in clinical patients with a neuropathic pain condition [2]. The capsaicin-induced central facilitation of the autonomic reflex may provide a fruitful model to study experimentally the underlying mechanisms and new therapeutic applications to these conditions in human subjects. This study was supported by grants from the Paulo Foundation and the Sigrid Juselius Foundation, Helsinki, and the Academy of Finland. [1] Adinoff, B., Mefford, I., Waxman, R. and Linnoila, M., Vagal tone decreases following intravenous diazepam, Psychiatry Res., 41 (1992) 89-97. [2] Blumberg, H., A new clinical approach for diagnosing reflex sympathetic dystrophy. In M.R. Bond et al. (Eds.), Proc. Vlth World Congr. Pain, Elsevier, Amsterdam, The Netherlands, 1991, pp. 399-407. [3] Coderre, T.J. and Melzack, R., Increased pain sensitivity following heat injury involves a central mechanism, Behav. Brain Res., 15 (1985) 259-262. [4] Cook, A.J., Woolf, C.J., Wall, P.D. and McMahon, S.B., Dynamic receptive field plasticity in rat spinal cord dorsal horn following C-primary afferent fiber input, Nature (London), 325 (1987) 151 153. [51 Culp, W.J., Ochoa, J., Cline, M. and Dotson, R., Heat and mechanical hyperalgesia induced by capsaicin, Brain, 112 (1989) 1317-1331. [6] Gr6nroos, M. and Pertovaara, A., Capsaicin-induced central facilitation of a nociceptive flexion reflex in humans, Neurosci. Lett., 159 (1993) 215-218. [7] Gr6nroos, M., Huttunen, J. and Pertovaara, A., Capsaicin-induced central facilitation of the somatosensory evoked potential in humans, Neurosci. Res. Commun., 14 (1994) 71 74.
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