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Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans
Brain Research 774 Ž1997. 55–61
Research report
Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans A. Rossi ) , B. Decchi Laboratorio di Neurofisiologia, Istituto di Scienze Neurologiche dell’UniÕersita, ` Viale Bracci, 53100 Siena, Italy Accepted 29 July 1997
Abstract The effects of selective tonic cutaneous and muscle pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis to the soleus motoneurones were studied in five healthy human subjects. Tonic pain stimulation, monitored by a psychophysical method, was obtained by local injection of 60 mg levo-ascorbic acid ŽL-AS. in a volume of 0.3 ml. Nociceptive cutaneous and muscle stimulation of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to soleus motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. The time-course of these Ib changes strictly paralleled that of subjective pain sensation. On the contrary, when the same nociceptive stimuli were applied to the leg Žat pretibial level., the differences between cutaneous and muscle pain disappeared and similar short-lasting phases of facilitation and inhibition of Ib activity were observed. It is concluded that tonic discharge of cutaneous and muscle nociceptive afferents arising from the foot have specific and opposite effects on Ib inhibitory pathways to ankle extensor motoneurones. These interactions between muscle and cutaneous nociceptive and Ib pathways may be used to change muscle synergies, thus contributing to the establishment of appropriate adaptive locomotor strategies during pain. q 1997 Elsevier Science B.V. Keywords: Ib pathway; Muscle pain; Cutaneous pain; Motor control; Man
1. Introduction Activation of Ib interneurones results in widespread inhibition of homonymous and synergist motor nuclei and excitation of other motoneurones in the limb w5,14,18,19,21x. In addition to fibres arising from Golgi tendon organs Žcf. w15x., they receive extensive convergence from different modalities of primary high- and low-threshold cutaneous, joint and muscle afferents and descending systems Žw1,2,6,7,11,13,14x, and see w17x.. Because of this complexity, the role of Ib pathways must transcend the simple negative feedback circuit. Their main function is now considered to be that of co-ordinating the activity of muscles operating at different joints. Brain command for a movement may pre-select sub-populations of Ib interneurones, setting their sensitivity to a level appropriate for regulation of the particular movement. During the course of the movement, the ongoing segmental signals provide the final regulation of Ib interneuronal )
Corresponding author. Fax: q39 Ž577. 40-327; E-mail: rossiale@un-
isi.it 0006-8993r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 6 - 8 9 9 3 Ž 9 7 . 0 0 9 8 4 - 0
transmission to target motor nuclei. As mentioned above, high-threshold afferents also contribute to this regulation. For example, discharge of peripheral nociceptive fibres may facilitate Žby facilitatory interaction with Ib fibre discharge. correction of a movement when it reaches a painful stage. In addition to these transient changes, steady variations in Ib interneuronal transmission, induced by tonic pain activity, could result in steady alteration of motor strategies. For example, since activity from Golgi tendon organs in leg extensor muscles helps regulate the level of extensor activity during the stance phase of walking w4,9,24,25x, then a steady pain-induced change in Ib transmission to motoneurones Žmotoneurones. andror to the system of interneurones of the central rhythm generator, may alter locomotor activity. In a previous paper, we found that tonic nociceptive cutaneous input from the foot, evoked by local injection of levo-ascorbic acid ŽL-AS., produced long-lasting potentiation of the inhibitory activity of heteronymous Ib projections to lower limb extensor motoneurones w28x. In the present paper, we extended the study to muscle nociceptive input from the foot and leg and its effects on Ib transmis-
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sion compared with those of cutaneous nociceptive input coming from the same anatomical sites.
2. Materials and methods The study was performed with five healthy volunteers Ž22–39 years of age. from the laboratory staff, all of whom gave their informed consent to the experimental procedure, which was also approved by the local Ethical Committee. All subjects participating in this study were familiar with the experimental protocol and nociceptive stimulus. Each experiment was repeated four to five times in each subject. To avoid any sensitisation phenomena, the experiments were repeated in the same subject at an interval of at least 8 days. Fig. 1 illustrates schematically the experimental paradigm.
2.1. Test reflexes The size of the H-reflex from the soleus muscle ŽSol. was used to test the effects of conditioning stimulation applied to group I fibres and to nociceptive afferents. Test stimuli were rectangular pulses of 1 ms duration delivered every 5 s by a constant current stimulator. The active electrode used to stimulate the posterior tibial nerve was a ball Ž1.5 cm diameter. in the popliteal fossa. The reference electrode was fixed to the anterior aspect of the thigh, adjacent to the patella. The strength of the test stimulus was adjusted so that the size of the test H-reflex was about 50% of its maximum size. During conditioning nociceptive stimulation Žsee below., the size of the test reflex was maintained constant by adjusting the intensity of the test stimulus, so as to compensate for any excitability changes of the Sol monosynaptic arc. This was imperative to test
Fig. 1. Schematic drawing of the experimental paradigm for testing the effects of nociceptive stimulation on heteronymous Ib inhibition. The test H-reflex, evoked by a test stimulus ŽT. applied to group Ia afferents of the soleus muscle, was post-synaptically inhibited Žvia Ib interneurones: Ib int. by a conditioning stimulus ŽC1. applied to group I fibres of the gastrocnemious medialis muscle. Hence a fixed C1–T interval of 5 ms was used to study the effects of a tonic conditioning stimulation ŽC2. applied to high-threshold afferent Žh.t.a... The upper insert illustrates the mean curve of pain sensation Ževoked by L-AS injection in the extensor digitorum brevis muscle. obtained from four different experimental sessions in a subject. The lower insert shows the time-course of heteronymous Ib inhibition of the soleus H-reflex.
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the amount of Ib inhibition, since the susceptibility of a test H-reflex can change with its size w36x. This restoration ensured that the Ib inhibition changes were not due to a decreased number of motoneurones sensitive to Ib input in the test reflex Žsee also Section 4.. Only variations of the test reflex within 10% were considered acceptable. Because of the restoration, the test reflex could not be used to assess Sol H-reflex excitability changes due to the conditioning nociceptive stimulation. Therefore, a second Sol H-reflex ŽRef H-reflex., of the same size as the test reflex, was evoked alternately. 2.2. Conditioning stimuli Heteronymous Ib inhibition to Sol motoneurones was evoked by electrical stimulation of the gastrocnemius medialis ŽGM. nerve, 9–10 cm distally and 4–5 cm medially with respect to the point of the test stimulus to the tibial nerve. Single pulses of 1 ms duration were delivered through bipolar surface electrodes, with the cathode in proximal position. Stimulus intensity was kept at 0.90– 0.95-times the motor threshold Ž=Mth.. The effects of tonic pain conditioning stimulation were studied at a constant interstimulus interval of 5 ms, corresponding to the maximum of the Ib inhibitory phase w31x. Pain conditioning stimulation was delivered by local injection Žin 2–5 s. of 60 mg L-AS in a volume of 0.3 ml ŽpH 6.3 at 248C. w28x. Foot nociceptive stimulation was performed by: Ža. subcutaneous injection in the latero-dorsal aspect of the foot, 3–4 cm proximal to the metatarsophalangeal joint Žcutaneous stimulation., and Žb. deeply in the body of extensor digitorum brevis ŽEDB. Žmuscle stimulation.. Leg nociceptive stimulation was performed by Ža. subcutaneous injection in the lateral aspect of the leg Ž15–20 cm below the patella and 2 cm lateral to the tibia., and Žb. deeply in the body of tibialis anterior muscle ŽTA., just below the site of cutaneous stimulation. Since it is impossible to directly monitor peripheral nociceptive volley in man, we measured the time-course of subjective pain sensation using an 11-point box scale: 0 s absence of pain, 10 s worst possible pain w8,20x. After preliminary experiments to familiarise subjects with the stimulus Žquality, intensity and duration., a subjective reference curve of pain was plotted for each subject. When L-AS injection was used as conditioning nociceptive stimulation for Ib inhibition, the subjective pattern of the pain sensation was ascertained by asking the subject to score it verbally every minute. 2.3. General experimental procedure Subjects were seated in a reclining chair with the knee and ankle joints at 160–1708 and 100–1108, respectively. A foot was fixed to an immobile foot plate to avoid any change in skin contact. A sterile butterfly needle was inserted subcutaneously or in the muscle for the L-AS
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injection. After the disappearance of any local sensation due to the needle, the flexible connector and the syringe with L-AS were secured to the skin with sticking plaster. The control experimental phase consisted in measuring the inhibition of the test H-reflex after electrical conditioning stimuli applied to the GM nerve. Series of 10–15 unconditioned and conditioned H-reflexes, in random sequence, were recorded over a 10-min period. L-AS was then injected and Ref H-reflex, unconditioned and conditioned Žby Ib stimulation. test reflexes were evoked in a random sequence. The mean value of the Ref and conditioned test H-reflexes was computed every 2 min. Recording was interrupted 6–10 min after the end of the subjective pain sensation. Statistical analysis was performed by paired t-test. The grand mean value of the H-reflex inhibition Žinduced by a constant conditioning stimulation of the GM nerve. in the time-interval corresponding to its maximum potentiation or reduction after L-AS injection Žusually 4–8-min interval., was compared with the grand mean value of the H-reflex inhibition Žduring an analogous time-interval. before L-AS injection.
3. Results 3.1. SubjectiÕe pain sensation induced by muscle and cutaneous L-AS injection As discussed in a previous paper w28x, the nociceptive effect of L-AS is probably due to a drop in tissue pH. It is known, in fact, that myelinated and unmyelinated nociceptive fibres may be activated by an increase in tissue concentrations of hydrogen ions w10x. Subcutaneous injection of L-AS on the foot dorsum or the lateral aspect of the leg induced a very similar painful burning sensation localised at the site of stimulation Žabout 3 cm around the injection site.. No significant difference was found between the pain rating curve Žsee methods. obtained during foot and leg cutaneous stimulation. The pain reached maximum intensity Žmean s 6.75 " 0.8 S.D.. within 2 min after the injection and after a short plateau phase Žlasting about 2 min.. gradually decreased to zero within 20 min Žmean s 18.50 min " 2.1 S.D.. ŽFig. 4A,C.. As for cutaneous injection, pain rating curves during intramuscular injection of L-AS in the EDB and TA showed no significant difference. In both cases pain sensation reached its maximum Žmean s 8.10 " 0.4 S.D.. within 2 min after the injection. After a plateau lasting about 2 min, the sensation gradually decreased, reaching zero at about 20 min Žmean s 19.50 min " 1.4 S.D.. ŽFig. 4A,C.. Although quantitatively similar and typically described by the subjects as diffuse and difficult to localise, pain sensation evoked by TA and EDB stimulation exhibited, however, consistent differences. In the TA it was a deep, cramp-like pain involving the whole anterior loggia of the leg, whereas in the EDB it was a compression-like pain of
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94.0% in the 6–8-min interval after L-AS injection. Fig. 2B shows that this Ib disinhibition was paralleled by a depression of the Ref H-reflex. When the same amount of L-AS was injected subcutaneously in dorsal foot, just above the body of the EDB, opposite changes in the Ib inhibition and Ref H-reflex, with respect to those observed during muscle stimulation, were observed. About 2 min after the end of the injection, Ib inhibition increased Žfacilitation., peaking at 4–8 min and returning to control values in parallel with the subjective pain curve within 20 min ŽFig. 2D,F and Fig. 4A,B.. The grand mean value of Ib potentiation at 4–8 min interval was 14.1 " 6.8% S.D. Ž t s 4.2, P - 0.001.. A transient facilitation of the Sol H-reflex, lasting about 5 min, took place in parallel with Ib facilitation ŽFig. 2E.. These findings were consistent with those already published in a previous paper w28x, to which the reader is referred for more detailed analysis. The experiments with cutaneous nociceptive foot stimulation are replicated here to obtain a more direct comparison with the effects of foot muscle pain. Fig. 2. Effects of foot muscle and cutaneous nociceptive stimulation. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in extensor digitorum brevis muscle on: subjective pain sensation Ž4thorder regression curve. ŽA.; H-reflex response of the soleus ŽSol. muscle ŽB.; heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones ŽC.. Right: time-courses of effects induced by subcutaneous injection of L-AS in the dorsal foot. D, E, F as A, B, C. All curves are from the same subject. Each point is the mean of five measurements "S.E.M. expressed as a percentage of the size of the unconditioned Sol H-reflex Žhorizontal lines..
3.3. Effects of pretibial muscle and cutaneous nociceptiÕe stimulation on Ib inhibition Changes in heteronymous Ib inhibition to Sol motoneurones after L-AS injection in the ipsilateral TA or subcuta-
the foot and ankle, likened to joint dislocation or bone fracture. In all cases this was also accompanied by alteration of position sense of the foot. 3.2. Effects of foot muscle and cutaneous nociceptiÕe stimulation on Ib inhibition The grand mean value of the size of the Sol H-reflex conditioned by stimulation of the GM nerve Žat intensity of 0.90–0.95 = Mth and 5 ms conditioning-test interval. in the pre-injection period was 78.8 " 5.7% S.E.M. with respect to its unconditioned value. Fig. 2C and Fig. 4B show Ib changes after L-AS injection in the ipsilateral EDB. Heteronymous Ib inhibition from GM to Sol motoneurones underwent a marked decrease Ždisinhibition., reaching a maximum at about 5 min and returning to its control values by 20 min. The time-course of this Ib disinhibition was very similar to that of subjective pain sensation ŽFig. 2A,C and Fig. 4A,B.. The grand mean value of Ib disinhibition in the 6–8 min interval Žcorresponding to the maximum of Ib changes. was 15.2 " 6.5% S.D. Ž t s 5.2, P - 0.001.: i.e. the size of the Sol H-reflex after conditioning stimulation of the GM nerve, which corresponded to 78.8% in control condition, increased to
Fig. 3. Effects of leg muscle and cutaneous nociceptive stimulation. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in tibialis anterior muscle on: subjective pain sensation Ž4th-order regression curve. ŽA.; H-reflex response of the soleus ŽSol. muscle ŽB.; heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones ŽC.. Right: time-courses of effects induced by subcutaneos injection of L-AS in the pretibial site. D, E, F as A, B, C. Presentation as in Fig. 2.
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4. Discussion
Fig. 4. Grand mean of effects of foot and leg muscle and cutaneous nociceptive stimulation from all subjects. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in the dorsal foot. A: subjective pain curves Ž4th-order regression curves. during nociceptive stimulation of extensor digitorum brevis ŽEDB. Žsolid line. and dorsal surface of foot Ždashed line.. B: time-courses of heteronymous Ib inhibition from gastrocnemius medialis nerve to soleus ŽSol. motoneurones after EDB muscle L-AS injection Žfilled points. and dorsal foot subcutaneous L-AS injection Žopen points.. Right: time-courses of effects induced by injection of L-AS in the leg. C: subjective pain curves Ž4th-order regression curves. after pretibial Žtibialis anterior. muscle Žsolid line. and skin Ždashed line. nociceptive stimulation. D: time-courses of heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones after pretibial muscle Žfilled points. and skin Žopen points. nociceptive stimulation. Each point is the mean of 50–75 measurements"S.E.M.
neously Žabove the site of intramuscular injection. were very similar. These consisted of a transient increase in Ib inhibition Žfacilitation. at 2–4 min, followed by a shortlasting disinhibition 6–8 min after the end of the L-AS injection ŽFig. 3C,F and Fig. 4D.. In some cases, the latter phase appeared as a relative disinhibition superimposed on a long-lasting inhibition ŽFig. 3E,F.. The grand mean of the initial Ib potentiation after muscle and cutaneous nociceptive stimulation, both measured at 4 min, was 14.3 " 5.7% S.D. Ž t s 4.3, P - 0.001. and 8.0 " 7.6% S.D. Ž t s 1.8, not significant. respectively. The subsequent disinhibition phase at 6 min corresponded to 9.8 " 7.5% S.D. for muscle Ž t s 2.7, P - 0.01. and 4.7 " 8.6% S.D. Ž t s 1.1, not significant. for cutaneous stimulation, respectively. These changes were only weakly linked to the subjective pain curve and die out about 10 min before the end of nociceptive sensation ŽFig. 3 and Fig. 4C,D.. In addition, unlike the response to painful stimulation of the dorsal foot, the changes in the Ref H-reflex during pretibial pain showed no apparent relation with Ib inhibition variations: a prevalent, long-lasting inhibition and facilitation were usually observed during muscle and cutaneous pain stimulation, respectively.
In a previous paper it was shown that local injection of L-AS is an effective and safe method of evoking long-lasting, fully reversible pain activation in humans w28x. In the present study, this method was used to activate cutaneous and muscle nociceptive afferents from foot and leg and to study their interactions with the heteronymous Ib pathway from GM to Sol motoneurones Žgroup I non-reciprocal inhibition.. The main finding of this investigation is that muscle and cutaneous tonic nociceptive stimulation of the ispilateral dorsal foot produced opposite, long-lasting effects on the Sol Ib heteronymous inhibition. Cutaneous stimulation, by local injection of 0.3 mg L-AS, produced an increase in inhibition, whereas the same stimulation applied in the body of EDB, depressed it. The most immediate interpretation is that cutaneous nociceptive discharge arising from the dorsal foot facilitated Ib inhibitory pathways to Sol, while muscle nociceptive discharge from the same site inhibited it. This interpretation, however, should be adopted with caution, since nociceptive stimulation modified the excitability of the Sol H-reflex, by which Ib activity was indirectly analysed. Although the size of the test reflex was kept constant Ži.e. each variation induced by nociceptive stimulation was systematically compensated.; this, however, could not compensate for possible changes in the distribution of Ia excitation in the motoneurone pool. Any such changes could result in a incorrect estimation of Ib activity if the Sol motoneurones responsible for the test reflex in control conditions were different Žand with a different sensitivity to Ib inhibition. from those responsible for the same reflex during pain stimulation. Three main observations suggest, however, that Ib variations were not a function of the test reflex excitability changes: Ž1. during cutaneous nociceptive foot stimulation, Ib and H-reflex facilitations usually exhibited different time-courses, the former lasting many minutes longer than the latter Žcf. w28x.; Ž2. cutaneous and muscle nociceptive stimulation applied at pretibial level produced very similar changes in Ib inhibition but different, roughly opposite, effects on the H-reflex; finally, Ž3. the observation that tonic pain volley does not distort the recruitment curve of the Sol H-reflex Žw28x and cf. w30x. is an additional argument against the possibility of pain-induced changes in distribution of Ia excitation within the Sol motoneurone pool. Our conclusion is therefore that changes in Ib inhibitory activity during pain stimulation were actually due to interactions between nociceptive discharge and Ib heteronymous activity. The time-course of these pain-induced Ib changes after foot stimulation closely paralleled that of subjective pain sensation. During muscle and cutaneous nociceptive stimulation, Ib changes reached their maximum at the highest values of the subjective pain rating curve, then they progressively reduced in parallel with the decay phase of the pain curve. This close relationship between pain sensation
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and changes in Ib excitability strongly suggests that the latter were a function of the former. This is not to say, however, that there was a linear relationship between Ib changes and peripheral nociceptive discharge. Pain sensation curves provide, in fact, an estimation of centrally activated pain mechanisms, and it is well known that the relation between the discharge of peripheral nociceptors and subjective pain sensation is much more complex than a linear relationship w34x. Interaction Žvia segmental andror supraspinal pathways. between foot nociceptive and Ib activity may take place at interneuronal or presynaptic level. Post-synaptic potentials on Ib interneurones after activation of cutaneous and muscle high-threshold afferents w11x, as well as primary afferent depolarisation of group I fibre terminals ŽIa and Ib. by activation of high-threshold cutaneous afferents w3,16,33,35x have, in fact, been described in animals. In addition to Ib pre- or post-synaptic effects, the possibility of pain-induced presynaptic changes in Ia terminals Žcontacting both Sol motoneurones and Ib interneurones. deserves special attention. Assuming that muscle pain from the dorsal foot increases Ia presynaptic inhibition, than a depression in Ib inhibition Ždue to a decrease in the Ia excitatory drive on Ib interneurones, see w12x. and in the Sol H-reflex, as we actually observed, is expected to occur. On the other hand, the potentiation of Ib inhibition and the facilitation of the H-reflex observed during cutaneous nociceptive stimulation could be explained by a reduction of presynaptic inhibition acting on the same Ia terminals. Although data concerning muscle nociceptive afferents have yet to emerge, evidence of Ia presynaptic modulation after high-intensity stimulation of the sural nerve has been obtained recently in man w27x. Unlike the opposite, monotonic Ib changes observed during cutaneous and muscle pain stimulation of the dorsal foot, nociceptive stimulation of the skin and muscle at pretibial level resulted in very similar Ib modifications. They mainly consisted of a short-lasting phase of Ib facilitation followed by a transient disinhibition, exhausting many minutes before the end of pain sensation. It recalled a mixture of excitatory and inhibitory post-synaptic potentials, as already observed in the cat after stimulation of high-threshold cutaneous and muscle afferents on some Ib interneurones w11x. To sum up, tonic nociceptive stimulation arising from skin and muscle of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to Sol motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. On the contrary, when the same nociceptive stimulation was applied to the leg, differences between cutaneous and muscle pain were no longer observed and similar shortlasting phases of facilitation, and inhibition of Ib activity occurred. Thus, as already observed for low-threshold afferents w26,29x, nociceptive activity from foot appears to have stronger and more specific interactions with Ib path-
way to Sol motoneurones than that from leg. We cannot exclude, however, that cutaneous and muscle nociceptive afferents from pretibial site have more specific influences on Ib pathways projecting to other leg motor nuclei. Although the precise functional significance of the effects of tonic pain activity on Ib pathways cannot be established from the present experiments, it is conceivable that they may influence locomotor activity Žw25x, and see Section 1.. If, as shown in animals, the action of group Ib activity is reversed from inhibitory to excitatory when locomotion is initiated w24x, then an important task for the future will be to explore the interaction between nociceptive and Ib pathways during motor activity. A final point to consider is concerning the psychophysical effects induced by local L-AS injection. Differently from cutaneous Žwhich produced the same qualitative pain sensation when applied to leg or foot., intramuscular L-AS stimulation evoked different nociceptive sensations when applied in TA or EDB. In the TA, L-AS produced cramplike pain, recalling what described after nociceptive electrical stimulation of muscle afferents w22,23x or injection of carrageenin in the artery of the muscle w32x. On the other hand, injection in the EDB produced a clear-cut sensation of painful compression of the foot, mainly referred to bone. These differences in the subjective sensation could be due to the following. Ž1. A different nociceptive innervation of the two muscles. Although quantitative studies are missing, the available experimental data do not support this assumption w22x. Ž2. The different ratio between activated nociceptors and muscle size. Presumably almost all the EDB nociceptors were activated by the L-AS injection, but only a small fraction in the TA. This has to be seriously considered, since progressive spreading of pain sensation to tendons and bone has been observed as a result of increasing electrical recruitment of human nociceptive muscle afferents w22x. Finally, Ž3. the diffusion of L-AS to deep tissues other than muscle fibres, which could occur for the EDB but hardly for TA. Whatever mechanism, it has, however, to underline that these qualitative differences in the muscle pain sensation Žduring EDB and TA stimulation. cannot, per se, explain the different effects observed on Ib activity. In fact, clear-cut differences in Ib activity were also observed during cutaneous leg and foot stimulation, which produced identical pain sensations.
Acknowledgements We wish to thank all the volunteers who participated in this study. This work was supported by a research grant from Italian MURST Ž60%. and by a grant from the Istituto di Riabilitazione Fisiomedica Loretana ŽCB, Italy. to B.D.
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References w1x E. Brink, P.J. Harrison, E. Jankowska, D. McCrea, B. Skoog, Postsynaptic potentials in a population of motoneurones following activity of single interneurones in the cat, J. Physiol. 343 Ž1983. 341–359. w2x E. Brink, E. Jankowska, D. McCrea, B. Skoog, Inhibitory interactions between interneurones in reflex pathways from group Ia afferents in the cat, J. Physiol. 343 Ž1983. 361–379. w3x R.E. Burke, P. Rudomin, L. Vyklicky, F.E. Zajac III, Primary afferent depolarization and flexion reflexes produced by radiant heat stimulation of the skin, J. Physiol. 213 Ž1971. 185–214. w4x B.A. Conway, H. Hultborn, O. Kiehn, Proprioceptive input reset central locomotor rhythm in the spinal cat, Exp. Brain Res. 68 Ž1987. 643–656. w5x J.C. Eccles, R.M. Eccles, A. Lundberg, Synaptic actions in motoneurones caused by impulses in Golgi tendon afferents, J. Physiol. 138 Ž1957. 227–252. w6x J.C. Eccles, A. Lundberg, Synaptic actions in motoneurones by afferents which may evoke the flexion reflex, Arch. Ital. Biol. 97 Ž1959. 199–221. w7x J.C. Eccles, A. Lundberg, Supraspinal control of interneurones mediating spinal reflexes, J. Physiol. 147 Ž1959. 565–584. w8x A. Ekblom, P. Hansson, Pain intensity measurements in patients with acute pain receiving afferent stimulation, J. Neurol. Neurosurg. Psychiatry 51 Ž1988. 481–486. w9x J.P. Gossard, H. Hultborn, The organization of the spinal rhythm generation in locomotion, in: A. Wernig ŽEd.., Plasticity of Motoneuronal Connection, Elsevier Science, Amsterdam, 1991, pp. 385–404. w10x H.O. Handwerker, G. Kobal, Psychophysiology of experimentally induced pain, Physiol. Rev. 73 Ž1993. 639–671. w11x P.J. Harrison, E. Jankowska, Sources of input to interneurones mediating group I non- reciprocal inhibition of motoneurones in the cat, J. Physiol. 361 Ž1985. 379–401. w12x P.J. Harrison, E. Jankowska, T. Johannisson, Shared reflex pathways of group I afferents of different cat hind-limb muscles, J. Physiol. 338 Ž1983. 113–128. w13x T. Hongo, E. Jankowska, A. Lundberg, The rubro-spinal tract I. Effects on alpha motoneurones innervating hind limb muscles in cats, Exp. Brain Res. 7 Ž1969. 344–364. w14x T. Hongo, E. Jankowska, A. Lundberg, The rubro-spinal tract II. Facilitation of interneuronal transmission in reflex path to motoneurones, Exp. Brain Res. 7 Ž1969. 365–391. w15x L. Jami, Golgi tendon organs in mammalian skeletal muscle: functional properties and central actions, Physiol. Rev. 72 Ž1992. 623– 666. w16x W. Janig, M. Zimmermann, Presynaptic depolarization of myeli¨ nated afferent fibres evoked by stimulation of cutaneous C fibres, J. Physiol. 214 Ž1971. 29–50. w17x E. Jankowska, Interneuronal relay in spinal pathways from proprioceptors, Prog. Neurobiol. 38 Ž1992. 335–378. w18x E. Jankowska, D. McCrea, R. Mackel, Pattern of ‘non-reciprocal’ inhibition of motoneurones by impulses in group Ia muscle spindle afferents, J. Physiol. 316 Ž1981. 393–409.
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w19x E. Jankowska, D. McCrea, R. Mackel, Oligosynaptic excitation of motoneurones by impulses in group Ia muscle spindle afferents, J. Physiol. 316 Ž1981. 411–425. w20x M.P. Jensen, J.A. Turner, J.M. Romano, What is the maximum number of levels needed in pain intensity measurement?, Pain 58 Ž1994. 387–392. w21x A. Lundberg, K. Malmgren, E.D. Schomburg, Role of joint afferents in motor control exemplified by effects on reflex pathways from Ib afferents, J. Physiol. 284 Ž1978. 327–343. w22x P. Marchettini, D.A. Simone, G. Caputi, J.L. Ochoa, Pain from excitation of identified muscle nociceptors in humans, Brain Res. 740 Ž1996. 109–116. w23x J.L. Ochoa, H.E. Torebjork, ¨ Pain from skin and muscle, Pain 1 ŽSuppl.. Ž1981. 87. w24x K.G. Pearson, D.F. Collins, Reversal of influence of group Ib afferents from plantaris on activity in medial gastrocnemius muscle during locomotor activity, J. Neurophysiol. 70 Ž1993. 1009–1017. w25x K.G. Pearson, J.M. Ramirez, W. Jiang, Entrainment of the locomotor rhythm by group Ib afferents from ankle extensor muscles in spinal cats, Exp. Brain Res. 90 Ž1992. 557–566. w26x E. Pierrot-Deseilligny, C. Bergego, R. Katz, C. Morin, Cutaneous depression of Ib reflex pathways to motoneurones in man, Exp. Brain Res. 42 Ž1981. 351–361. w27x A. Roby-Bramy, B. Bussel, Effects of flexor reflex afferent stimulation on the soleus H reflex in patients with a complete spinal cord lesion: evidence for presynaptic inhibition of Ia transmission, Exp. Brain Res. 61 Ž1990. 593–601. w28x A. Rossi, B. Decchi, Cutaneous nociceptive facilitation of Ib heteronymous pathways to lower limb motoneurones in humans, Brain Res. 700 Ž1995. 164–172. w29x A. Rossi, R. Mazzocchio, Cutaneous control of group I pathways from ankle flexor to extensor in man, Exp. Brain Res. 73 Ž1988. 8–14. w30x A. Rossi, A. Zalaffi, B. Decchi, Interaction of nociceptive and non-nociceptive cutaneous afferents from foot sole in common reflex pathways to tibialis anterior motoneurones in humans, Brian Res. 714 Ž1996. 76–86. w31x A. Rossi, A. Zalaffi, B. Decchi, Heteronymous recurrent inhibition from gastrocnemius muscle to soleus motoneurones in humans, Neurosci. Lett. 169 Ž1994. 141–144. w32x D.A. Simone, P. Marchettini, G. Caputi, J.L. Ochoa, Identification of muscle afferents subserving sensation of deep pain in humans, J. Neurophysiol. 72 Ž1994. 883–889. w33x H. Steffens, E.D. Schomburg, Convergence in segmental reflex pathways from nociceptive and non-nociceptive afferents to alphamotoneurones in the cat, J. Physiol. 466 Ž1993. 191–211. w34x P.D. Wall, S.B. McMahon, The relationship of perceived pain to afferent nerve impulses, Trends Neurosci. 6 Ž1986. 254–255. w35x D. Whitehorn, P.R. Burgess, Changes in polarization of central branches of myelinated mechanoreceptor and nociceptor fibres during noxious and innocuous stimulation of the skin, J. Neurophysiol. 36 Ž1973. 226–237. w36x S. Yanagawa, M. Shindo, S. Nakagawa, Increase in Ib inhibition by antagonistic voluntary contraction in man, J. Physiol. 440 Ž1991. 311–323.
Research report
Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans A. Rossi ) , B. Decchi Laboratorio di Neurofisiologia, Istituto di Scienze Neurologiche dell’UniÕersita, ` Viale Bracci, 53100 Siena, Italy Accepted 29 July 1997
Abstract The effects of selective tonic cutaneous and muscle pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis to the soleus motoneurones were studied in five healthy human subjects. Tonic pain stimulation, monitored by a psychophysical method, was obtained by local injection of 60 mg levo-ascorbic acid ŽL-AS. in a volume of 0.3 ml. Nociceptive cutaneous and muscle stimulation of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to soleus motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. The time-course of these Ib changes strictly paralleled that of subjective pain sensation. On the contrary, when the same nociceptive stimuli were applied to the leg Žat pretibial level., the differences between cutaneous and muscle pain disappeared and similar short-lasting phases of facilitation and inhibition of Ib activity were observed. It is concluded that tonic discharge of cutaneous and muscle nociceptive afferents arising from the foot have specific and opposite effects on Ib inhibitory pathways to ankle extensor motoneurones. These interactions between muscle and cutaneous nociceptive and Ib pathways may be used to change muscle synergies, thus contributing to the establishment of appropriate adaptive locomotor strategies during pain. q 1997 Elsevier Science B.V. Keywords: Ib pathway; Muscle pain; Cutaneous pain; Motor control; Man
1. Introduction Activation of Ib interneurones results in widespread inhibition of homonymous and synergist motor nuclei and excitation of other motoneurones in the limb w5,14,18,19,21x. In addition to fibres arising from Golgi tendon organs Žcf. w15x., they receive extensive convergence from different modalities of primary high- and low-threshold cutaneous, joint and muscle afferents and descending systems Žw1,2,6,7,11,13,14x, and see w17x.. Because of this complexity, the role of Ib pathways must transcend the simple negative feedback circuit. Their main function is now considered to be that of co-ordinating the activity of muscles operating at different joints. Brain command for a movement may pre-select sub-populations of Ib interneurones, setting their sensitivity to a level appropriate for regulation of the particular movement. During the course of the movement, the ongoing segmental signals provide the final regulation of Ib interneuronal )
Corresponding author. Fax: q39 Ž577. 40-327; E-mail: rossiale@un-
isi.it 0006-8993r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 6 - 8 9 9 3 Ž 9 7 . 0 0 9 8 4 - 0
transmission to target motor nuclei. As mentioned above, high-threshold afferents also contribute to this regulation. For example, discharge of peripheral nociceptive fibres may facilitate Žby facilitatory interaction with Ib fibre discharge. correction of a movement when it reaches a painful stage. In addition to these transient changes, steady variations in Ib interneuronal transmission, induced by tonic pain activity, could result in steady alteration of motor strategies. For example, since activity from Golgi tendon organs in leg extensor muscles helps regulate the level of extensor activity during the stance phase of walking w4,9,24,25x, then a steady pain-induced change in Ib transmission to motoneurones Žmotoneurones. andror to the system of interneurones of the central rhythm generator, may alter locomotor activity. In a previous paper, we found that tonic nociceptive cutaneous input from the foot, evoked by local injection of levo-ascorbic acid ŽL-AS., produced long-lasting potentiation of the inhibitory activity of heteronymous Ib projections to lower limb extensor motoneurones w28x. In the present paper, we extended the study to muscle nociceptive input from the foot and leg and its effects on Ib transmis-
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sion compared with those of cutaneous nociceptive input coming from the same anatomical sites.
2. Materials and methods The study was performed with five healthy volunteers Ž22–39 years of age. from the laboratory staff, all of whom gave their informed consent to the experimental procedure, which was also approved by the local Ethical Committee. All subjects participating in this study were familiar with the experimental protocol and nociceptive stimulus. Each experiment was repeated four to five times in each subject. To avoid any sensitisation phenomena, the experiments were repeated in the same subject at an interval of at least 8 days. Fig. 1 illustrates schematically the experimental paradigm.
2.1. Test reflexes The size of the H-reflex from the soleus muscle ŽSol. was used to test the effects of conditioning stimulation applied to group I fibres and to nociceptive afferents. Test stimuli were rectangular pulses of 1 ms duration delivered every 5 s by a constant current stimulator. The active electrode used to stimulate the posterior tibial nerve was a ball Ž1.5 cm diameter. in the popliteal fossa. The reference electrode was fixed to the anterior aspect of the thigh, adjacent to the patella. The strength of the test stimulus was adjusted so that the size of the test H-reflex was about 50% of its maximum size. During conditioning nociceptive stimulation Žsee below., the size of the test reflex was maintained constant by adjusting the intensity of the test stimulus, so as to compensate for any excitability changes of the Sol monosynaptic arc. This was imperative to test
Fig. 1. Schematic drawing of the experimental paradigm for testing the effects of nociceptive stimulation on heteronymous Ib inhibition. The test H-reflex, evoked by a test stimulus ŽT. applied to group Ia afferents of the soleus muscle, was post-synaptically inhibited Žvia Ib interneurones: Ib int. by a conditioning stimulus ŽC1. applied to group I fibres of the gastrocnemious medialis muscle. Hence a fixed C1–T interval of 5 ms was used to study the effects of a tonic conditioning stimulation ŽC2. applied to high-threshold afferent Žh.t.a... The upper insert illustrates the mean curve of pain sensation Ževoked by L-AS injection in the extensor digitorum brevis muscle. obtained from four different experimental sessions in a subject. The lower insert shows the time-course of heteronymous Ib inhibition of the soleus H-reflex.
A. Rossi, B. Decchir Brain Research 774 (1997) 55–61
the amount of Ib inhibition, since the susceptibility of a test H-reflex can change with its size w36x. This restoration ensured that the Ib inhibition changes were not due to a decreased number of motoneurones sensitive to Ib input in the test reflex Žsee also Section 4.. Only variations of the test reflex within 10% were considered acceptable. Because of the restoration, the test reflex could not be used to assess Sol H-reflex excitability changes due to the conditioning nociceptive stimulation. Therefore, a second Sol H-reflex ŽRef H-reflex., of the same size as the test reflex, was evoked alternately. 2.2. Conditioning stimuli Heteronymous Ib inhibition to Sol motoneurones was evoked by electrical stimulation of the gastrocnemius medialis ŽGM. nerve, 9–10 cm distally and 4–5 cm medially with respect to the point of the test stimulus to the tibial nerve. Single pulses of 1 ms duration were delivered through bipolar surface electrodes, with the cathode in proximal position. Stimulus intensity was kept at 0.90– 0.95-times the motor threshold Ž=Mth.. The effects of tonic pain conditioning stimulation were studied at a constant interstimulus interval of 5 ms, corresponding to the maximum of the Ib inhibitory phase w31x. Pain conditioning stimulation was delivered by local injection Žin 2–5 s. of 60 mg L-AS in a volume of 0.3 ml ŽpH 6.3 at 248C. w28x. Foot nociceptive stimulation was performed by: Ža. subcutaneous injection in the latero-dorsal aspect of the foot, 3–4 cm proximal to the metatarsophalangeal joint Žcutaneous stimulation., and Žb. deeply in the body of extensor digitorum brevis ŽEDB. Žmuscle stimulation.. Leg nociceptive stimulation was performed by Ža. subcutaneous injection in the lateral aspect of the leg Ž15–20 cm below the patella and 2 cm lateral to the tibia., and Žb. deeply in the body of tibialis anterior muscle ŽTA., just below the site of cutaneous stimulation. Since it is impossible to directly monitor peripheral nociceptive volley in man, we measured the time-course of subjective pain sensation using an 11-point box scale: 0 s absence of pain, 10 s worst possible pain w8,20x. After preliminary experiments to familiarise subjects with the stimulus Žquality, intensity and duration., a subjective reference curve of pain was plotted for each subject. When L-AS injection was used as conditioning nociceptive stimulation for Ib inhibition, the subjective pattern of the pain sensation was ascertained by asking the subject to score it verbally every minute. 2.3. General experimental procedure Subjects were seated in a reclining chair with the knee and ankle joints at 160–1708 and 100–1108, respectively. A foot was fixed to an immobile foot plate to avoid any change in skin contact. A sterile butterfly needle was inserted subcutaneously or in the muscle for the L-AS
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injection. After the disappearance of any local sensation due to the needle, the flexible connector and the syringe with L-AS were secured to the skin with sticking plaster. The control experimental phase consisted in measuring the inhibition of the test H-reflex after electrical conditioning stimuli applied to the GM nerve. Series of 10–15 unconditioned and conditioned H-reflexes, in random sequence, were recorded over a 10-min period. L-AS was then injected and Ref H-reflex, unconditioned and conditioned Žby Ib stimulation. test reflexes were evoked in a random sequence. The mean value of the Ref and conditioned test H-reflexes was computed every 2 min. Recording was interrupted 6–10 min after the end of the subjective pain sensation. Statistical analysis was performed by paired t-test. The grand mean value of the H-reflex inhibition Žinduced by a constant conditioning stimulation of the GM nerve. in the time-interval corresponding to its maximum potentiation or reduction after L-AS injection Žusually 4–8-min interval., was compared with the grand mean value of the H-reflex inhibition Žduring an analogous time-interval. before L-AS injection.
3. Results 3.1. SubjectiÕe pain sensation induced by muscle and cutaneous L-AS injection As discussed in a previous paper w28x, the nociceptive effect of L-AS is probably due to a drop in tissue pH. It is known, in fact, that myelinated and unmyelinated nociceptive fibres may be activated by an increase in tissue concentrations of hydrogen ions w10x. Subcutaneous injection of L-AS on the foot dorsum or the lateral aspect of the leg induced a very similar painful burning sensation localised at the site of stimulation Žabout 3 cm around the injection site.. No significant difference was found between the pain rating curve Žsee methods. obtained during foot and leg cutaneous stimulation. The pain reached maximum intensity Žmean s 6.75 " 0.8 S.D.. within 2 min after the injection and after a short plateau phase Žlasting about 2 min.. gradually decreased to zero within 20 min Žmean s 18.50 min " 2.1 S.D.. ŽFig. 4A,C.. As for cutaneous injection, pain rating curves during intramuscular injection of L-AS in the EDB and TA showed no significant difference. In both cases pain sensation reached its maximum Žmean s 8.10 " 0.4 S.D.. within 2 min after the injection. After a plateau lasting about 2 min, the sensation gradually decreased, reaching zero at about 20 min Žmean s 19.50 min " 1.4 S.D.. ŽFig. 4A,C.. Although quantitatively similar and typically described by the subjects as diffuse and difficult to localise, pain sensation evoked by TA and EDB stimulation exhibited, however, consistent differences. In the TA it was a deep, cramp-like pain involving the whole anterior loggia of the leg, whereas in the EDB it was a compression-like pain of
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94.0% in the 6–8-min interval after L-AS injection. Fig. 2B shows that this Ib disinhibition was paralleled by a depression of the Ref H-reflex. When the same amount of L-AS was injected subcutaneously in dorsal foot, just above the body of the EDB, opposite changes in the Ib inhibition and Ref H-reflex, with respect to those observed during muscle stimulation, were observed. About 2 min after the end of the injection, Ib inhibition increased Žfacilitation., peaking at 4–8 min and returning to control values in parallel with the subjective pain curve within 20 min ŽFig. 2D,F and Fig. 4A,B.. The grand mean value of Ib potentiation at 4–8 min interval was 14.1 " 6.8% S.D. Ž t s 4.2, P - 0.001.. A transient facilitation of the Sol H-reflex, lasting about 5 min, took place in parallel with Ib facilitation ŽFig. 2E.. These findings were consistent with those already published in a previous paper w28x, to which the reader is referred for more detailed analysis. The experiments with cutaneous nociceptive foot stimulation are replicated here to obtain a more direct comparison with the effects of foot muscle pain. Fig. 2. Effects of foot muscle and cutaneous nociceptive stimulation. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in extensor digitorum brevis muscle on: subjective pain sensation Ž4thorder regression curve. ŽA.; H-reflex response of the soleus ŽSol. muscle ŽB.; heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones ŽC.. Right: time-courses of effects induced by subcutaneous injection of L-AS in the dorsal foot. D, E, F as A, B, C. All curves are from the same subject. Each point is the mean of five measurements "S.E.M. expressed as a percentage of the size of the unconditioned Sol H-reflex Žhorizontal lines..
3.3. Effects of pretibial muscle and cutaneous nociceptiÕe stimulation on Ib inhibition Changes in heteronymous Ib inhibition to Sol motoneurones after L-AS injection in the ipsilateral TA or subcuta-
the foot and ankle, likened to joint dislocation or bone fracture. In all cases this was also accompanied by alteration of position sense of the foot. 3.2. Effects of foot muscle and cutaneous nociceptiÕe stimulation on Ib inhibition The grand mean value of the size of the Sol H-reflex conditioned by stimulation of the GM nerve Žat intensity of 0.90–0.95 = Mth and 5 ms conditioning-test interval. in the pre-injection period was 78.8 " 5.7% S.E.M. with respect to its unconditioned value. Fig. 2C and Fig. 4B show Ib changes after L-AS injection in the ipsilateral EDB. Heteronymous Ib inhibition from GM to Sol motoneurones underwent a marked decrease Ždisinhibition., reaching a maximum at about 5 min and returning to its control values by 20 min. The time-course of this Ib disinhibition was very similar to that of subjective pain sensation ŽFig. 2A,C and Fig. 4A,B.. The grand mean value of Ib disinhibition in the 6–8 min interval Žcorresponding to the maximum of Ib changes. was 15.2 " 6.5% S.D. Ž t s 5.2, P - 0.001.: i.e. the size of the Sol H-reflex after conditioning stimulation of the GM nerve, which corresponded to 78.8% in control condition, increased to
Fig. 3. Effects of leg muscle and cutaneous nociceptive stimulation. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in tibialis anterior muscle on: subjective pain sensation Ž4th-order regression curve. ŽA.; H-reflex response of the soleus ŽSol. muscle ŽB.; heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones ŽC.. Right: time-courses of effects induced by subcutaneos injection of L-AS in the pretibial site. D, E, F as A, B, C. Presentation as in Fig. 2.
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4. Discussion
Fig. 4. Grand mean of effects of foot and leg muscle and cutaneous nociceptive stimulation from all subjects. Left: time-courses of effects induced by injection of levo-ascorbic acid ŽL-AS. in the dorsal foot. A: subjective pain curves Ž4th-order regression curves. during nociceptive stimulation of extensor digitorum brevis ŽEDB. Žsolid line. and dorsal surface of foot Ždashed line.. B: time-courses of heteronymous Ib inhibition from gastrocnemius medialis nerve to soleus ŽSol. motoneurones after EDB muscle L-AS injection Žfilled points. and dorsal foot subcutaneous L-AS injection Žopen points.. Right: time-courses of effects induced by injection of L-AS in the leg. C: subjective pain curves Ž4th-order regression curves. after pretibial Žtibialis anterior. muscle Žsolid line. and skin Ždashed line. nociceptive stimulation. D: time-courses of heteronymous Ib inhibition from gastrocnemius medialis nerve to Sol motoneurones after pretibial muscle Žfilled points. and skin Žopen points. nociceptive stimulation. Each point is the mean of 50–75 measurements"S.E.M.
neously Žabove the site of intramuscular injection. were very similar. These consisted of a transient increase in Ib inhibition Žfacilitation. at 2–4 min, followed by a shortlasting disinhibition 6–8 min after the end of the L-AS injection ŽFig. 3C,F and Fig. 4D.. In some cases, the latter phase appeared as a relative disinhibition superimposed on a long-lasting inhibition ŽFig. 3E,F.. The grand mean of the initial Ib potentiation after muscle and cutaneous nociceptive stimulation, both measured at 4 min, was 14.3 " 5.7% S.D. Ž t s 4.3, P - 0.001. and 8.0 " 7.6% S.D. Ž t s 1.8, not significant. respectively. The subsequent disinhibition phase at 6 min corresponded to 9.8 " 7.5% S.D. for muscle Ž t s 2.7, P - 0.01. and 4.7 " 8.6% S.D. Ž t s 1.1, not significant. for cutaneous stimulation, respectively. These changes were only weakly linked to the subjective pain curve and die out about 10 min before the end of nociceptive sensation ŽFig. 3 and Fig. 4C,D.. In addition, unlike the response to painful stimulation of the dorsal foot, the changes in the Ref H-reflex during pretibial pain showed no apparent relation with Ib inhibition variations: a prevalent, long-lasting inhibition and facilitation were usually observed during muscle and cutaneous pain stimulation, respectively.
In a previous paper it was shown that local injection of L-AS is an effective and safe method of evoking long-lasting, fully reversible pain activation in humans w28x. In the present study, this method was used to activate cutaneous and muscle nociceptive afferents from foot and leg and to study their interactions with the heteronymous Ib pathway from GM to Sol motoneurones Žgroup I non-reciprocal inhibition.. The main finding of this investigation is that muscle and cutaneous tonic nociceptive stimulation of the ispilateral dorsal foot produced opposite, long-lasting effects on the Sol Ib heteronymous inhibition. Cutaneous stimulation, by local injection of 0.3 mg L-AS, produced an increase in inhibition, whereas the same stimulation applied in the body of EDB, depressed it. The most immediate interpretation is that cutaneous nociceptive discharge arising from the dorsal foot facilitated Ib inhibitory pathways to Sol, while muscle nociceptive discharge from the same site inhibited it. This interpretation, however, should be adopted with caution, since nociceptive stimulation modified the excitability of the Sol H-reflex, by which Ib activity was indirectly analysed. Although the size of the test reflex was kept constant Ži.e. each variation induced by nociceptive stimulation was systematically compensated.; this, however, could not compensate for possible changes in the distribution of Ia excitation in the motoneurone pool. Any such changes could result in a incorrect estimation of Ib activity if the Sol motoneurones responsible for the test reflex in control conditions were different Žand with a different sensitivity to Ib inhibition. from those responsible for the same reflex during pain stimulation. Three main observations suggest, however, that Ib variations were not a function of the test reflex excitability changes: Ž1. during cutaneous nociceptive foot stimulation, Ib and H-reflex facilitations usually exhibited different time-courses, the former lasting many minutes longer than the latter Žcf. w28x.; Ž2. cutaneous and muscle nociceptive stimulation applied at pretibial level produced very similar changes in Ib inhibition but different, roughly opposite, effects on the H-reflex; finally, Ž3. the observation that tonic pain volley does not distort the recruitment curve of the Sol H-reflex Žw28x and cf. w30x. is an additional argument against the possibility of pain-induced changes in distribution of Ia excitation within the Sol motoneurone pool. Our conclusion is therefore that changes in Ib inhibitory activity during pain stimulation were actually due to interactions between nociceptive discharge and Ib heteronymous activity. The time-course of these pain-induced Ib changes after foot stimulation closely paralleled that of subjective pain sensation. During muscle and cutaneous nociceptive stimulation, Ib changes reached their maximum at the highest values of the subjective pain rating curve, then they progressively reduced in parallel with the decay phase of the pain curve. This close relationship between pain sensation
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and changes in Ib excitability strongly suggests that the latter were a function of the former. This is not to say, however, that there was a linear relationship between Ib changes and peripheral nociceptive discharge. Pain sensation curves provide, in fact, an estimation of centrally activated pain mechanisms, and it is well known that the relation between the discharge of peripheral nociceptors and subjective pain sensation is much more complex than a linear relationship w34x. Interaction Žvia segmental andror supraspinal pathways. between foot nociceptive and Ib activity may take place at interneuronal or presynaptic level. Post-synaptic potentials on Ib interneurones after activation of cutaneous and muscle high-threshold afferents w11x, as well as primary afferent depolarisation of group I fibre terminals ŽIa and Ib. by activation of high-threshold cutaneous afferents w3,16,33,35x have, in fact, been described in animals. In addition to Ib pre- or post-synaptic effects, the possibility of pain-induced presynaptic changes in Ia terminals Žcontacting both Sol motoneurones and Ib interneurones. deserves special attention. Assuming that muscle pain from the dorsal foot increases Ia presynaptic inhibition, than a depression in Ib inhibition Ždue to a decrease in the Ia excitatory drive on Ib interneurones, see w12x. and in the Sol H-reflex, as we actually observed, is expected to occur. On the other hand, the potentiation of Ib inhibition and the facilitation of the H-reflex observed during cutaneous nociceptive stimulation could be explained by a reduction of presynaptic inhibition acting on the same Ia terminals. Although data concerning muscle nociceptive afferents have yet to emerge, evidence of Ia presynaptic modulation after high-intensity stimulation of the sural nerve has been obtained recently in man w27x. Unlike the opposite, monotonic Ib changes observed during cutaneous and muscle pain stimulation of the dorsal foot, nociceptive stimulation of the skin and muscle at pretibial level resulted in very similar Ib modifications. They mainly consisted of a short-lasting phase of Ib facilitation followed by a transient disinhibition, exhausting many minutes before the end of pain sensation. It recalled a mixture of excitatory and inhibitory post-synaptic potentials, as already observed in the cat after stimulation of high-threshold cutaneous and muscle afferents on some Ib interneurones w11x. To sum up, tonic nociceptive stimulation arising from skin and muscle of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to Sol motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. On the contrary, when the same nociceptive stimulation was applied to the leg, differences between cutaneous and muscle pain were no longer observed and similar shortlasting phases of facilitation, and inhibition of Ib activity occurred. Thus, as already observed for low-threshold afferents w26,29x, nociceptive activity from foot appears to have stronger and more specific interactions with Ib path-
way to Sol motoneurones than that from leg. We cannot exclude, however, that cutaneous and muscle nociceptive afferents from pretibial site have more specific influences on Ib pathways projecting to other leg motor nuclei. Although the precise functional significance of the effects of tonic pain activity on Ib pathways cannot be established from the present experiments, it is conceivable that they may influence locomotor activity Žw25x, and see Section 1.. If, as shown in animals, the action of group Ib activity is reversed from inhibitory to excitatory when locomotion is initiated w24x, then an important task for the future will be to explore the interaction between nociceptive and Ib pathways during motor activity. A final point to consider is concerning the psychophysical effects induced by local L-AS injection. Differently from cutaneous Žwhich produced the same qualitative pain sensation when applied to leg or foot., intramuscular L-AS stimulation evoked different nociceptive sensations when applied in TA or EDB. In the TA, L-AS produced cramplike pain, recalling what described after nociceptive electrical stimulation of muscle afferents w22,23x or injection of carrageenin in the artery of the muscle w32x. On the other hand, injection in the EDB produced a clear-cut sensation of painful compression of the foot, mainly referred to bone. These differences in the subjective sensation could be due to the following. Ž1. A different nociceptive innervation of the two muscles. Although quantitative studies are missing, the available experimental data do not support this assumption w22x. Ž2. The different ratio between activated nociceptors and muscle size. Presumably almost all the EDB nociceptors were activated by the L-AS injection, but only a small fraction in the TA. This has to be seriously considered, since progressive spreading of pain sensation to tendons and bone has been observed as a result of increasing electrical recruitment of human nociceptive muscle afferents w22x. Finally, Ž3. the diffusion of L-AS to deep tissues other than muscle fibres, which could occur for the EDB but hardly for TA. Whatever mechanism, it has, however, to underline that these qualitative differences in the muscle pain sensation Žduring EDB and TA stimulation. cannot, per se, explain the different effects observed on Ib activity. In fact, clear-cut differences in Ib activity were also observed during cutaneous leg and foot stimulation, which produced identical pain sensations.
Acknowledgements We wish to thank all the volunteers who participated in this study. This work was supported by a research grant from Italian MURST Ž60%. and by a grant from the Istituto di Riabilitazione Fisiomedica Loretana ŽCB, Italy. to B.D.
A. Rossi, B. Decchir Brain Research 774 (1997) 55–61
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