Experimental pain by ischaemic contractions compared with pain by intramuscular infusions of adenosine and hypertonic saline

Gait Adaptation to a Phase-Specific Nociceptive Electrical Stimulation Applied at the Ankle: A Model to Study Musculoskeletal-Like Pain

Introduction: Lower limb pain, whether induced experimentally or as a result of a musculoskeletal injury, can impair motor control, leading to gait adaptations such as increased muscle stiffness or modified load distribution around joints. These adaptations may initially reduce pain but can also lead to longer-term maladaptive plasticity and to the development of chronic pain. In humans, many current experimental musculoskeletal-like pain models are invasive, and most don’t accurately reproduce the movement-related characteristics of musculoskeletal pain. The main objective of this study was to measure pain adaptation strategies during gait of a musculoskeletal-like experimental pain protocol induced by phase-specific, non-invasive electrical stimulation.

Methods: Sixteen healthy participants walked on a treadmill at 4 km/h for three consecutive periods (BASELINE, PAIN, and POST-PAIN). Painful electrical stimulations were delivered at heel strike for the duration of heel contact (HC) using electrodes placed around the right lateral malleolus to mimic ankle sprains. Gait adaptations were quantified bilaterally using instrumented pressure-sensitive insoles. One-way ANOVAs and group time course analyses were performed to characterize the impact of electrical stimulation on heel and forefoot contact pressure and contact duration.

Results: During the first few painful strides, peak HC pressure decreased on the painful side (8.6 ± 1.0%, p < 0.0001) and increased on the non-stimulated side (11.9 ± 0.9%, p < 0.0001) while HC duration was significantly reduced bilaterally (painful: 12.1 ± 0.9%, p < 0.0001; non-stimulated: 4.8 ± 0.8%, p < 0.0001). No clinically meaningful modifications were observed for the forefoot. One minute after the onset of painful stimulation, perceived pain levels stabilized and peak HC pressure remained significantly decreased on the painful side, while the other gait adaptations returned to pre-stimulation values.

Discussion: These results demonstrate that a non-invasive, phase-specific pain can produce a stable painful gait pattern. Therefore, this protocol will be useful to study musculoskeletal pain locomotor adaptation strategies under controlled conditions.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

Repeated Injections of Low-Dose Nerve Growth Factor (NGF) in Healthy Humans Maintain Muscle Pain and Facilitate Ischemic Contraction-Evoked Pain

OBJECTIVE

Nerve growth factor (NGF) is essential for generating and potentiating pain responses. This double-blinded crossover study assessed NGF-evoked pain in healthy humans after repeated NGF injections in the tibialis anterior (TA) muscle compared with control injections of isotonic saline.

SUBJECTS

Twenty healthy subjects participated in two experimental phases; each consisted of seven sessions over 21 days.

METHODS

At day 0, day 2, and day 4, a low-dose NGF (1 µg) was injected. Data on daily self-reported muscle pain (using a Likert scale) were collected. Data on pressure pain thresholds (PPTs), pain evoked by nonischemic and ischemic muscle contractions (using a numerical rating scale [NRS]), pressure pain detection (PDT), and pain tolerance thresholds (PTTs) to cuff algometry were recorded before day 0 and at 1, 2, 4, 7, 10, and 21 days after the first injection. Temporal summation of pain (TSP) and conditioned pain modulation (CPM) were recorded to assess central pain mechanisms.

RESULTS

Likert scores remained elevated for 9 days after NGF injection (P<0.05). PPTs at the TA muscle were decreased at day 1 until day 7 after NGF injection compared with day 0 (P=0.05). In subjects presenting with NGF-induced muscle hyperalgesia, pain NRS scores evoked by nonischemic contractions were higher after NGF injection at day 4 and day 7 (P<0.04) compared with the control condition. At all time points, higher pain NRS scores were found with ischemic compared with nonischemic contractions (P<0.05). The pain NRS after ischemic contractions was elevated following prolonged NGF hyperalgesia at day 7 compared with the control condition and day 0 (P<0.04). The PDT, PTT, TSP, and CPM remained unchanged during the period of NGF-induced hyperalgesia.

CONCLUSIONS

Repeated low-dose NGF injections maintain muscle pain and potentiate pain evoked by ischemic contractions during prolonged NGF hyperalgesia.

Acute experimentally-induced pain replicates the distribution but not the quality or behaviour of clinical appendicular musculoskeletal pain. A systematic review

OBJECTIVES

Experimental pain is a commonly used method to draw conclusions about the motor response to clinical musculoskeletal pain. A systematic review was performed to determine if current models of acute experimental pain validly replicate the clinical experience of appendicular musculoskeletal pain with respect to the distribution and quality of pain and the pain response to provocation testing.

METHODS

A structured search of Medline, Scopus and Embase databases was conducted from database inception to August 2020 using the following key terms: "experimental muscle pain" OR "experimental pain" OR "pain induced" OR "induced pain" OR "muscle hyperalgesia" OR ("Pain model" AND "muscle"). Studies in English were included if investigators induced experimental musculoskeletal pain into a limb (including the sacroiliac joint) in humans, and if they measured and reported the distribution of pain, quality of pain or response to a provocation manoeuvre performed passively or actively. Studies were excluded if they involved prolonged or delayed experimental pain, if temporomandibular, orofacial, lumbar, thoracic or cervical spine pain were investigated, if a full text of the study was not available or if they were systematic reviews. Two investigators independently screened each title and abstract and each full text paper to determine inclusion in the review. Disagreements were resolved by consensus with a third investigator.

RESULTS

Data from 57 experimental pain studies were included in this review. Forty-six of these studies reported pain distribution, 41 reported pain quality and six detailed the pain response to provocation testing. Hypertonic saline injection was the most common mechanism used to induce pain with 43 studies employing this method. The next most common methods were capsaicin injection (5 studies) and electrical stimulation, injection of acidic solution and ischaemia with three studies each. The distribution of experimental pain was similar to the area of pain reported in clinical appendicular musculoskeletal conditions. The quality of appendicular musculoskeletal pain was not replicated with the affective component of the McGill Pain Questionnaire consistently lower than that typically reported by musculoskeletal pain patients. The response to provocation testing was rarely investigated following experimental pain induction. Based on the limited available data, the increase in pain experienced in clinical populations during provocative maneuvers was not consistently replicated.

CONCLUSIONS

Current acute experimental pain models replicate the distribution but not the quality of chronic clinical appendicular musculoskeletal pain. Limited evidence also indicates that experimentally induced acute pain does not consistently increase with tests known to provoke pain in patients with appendicular musculoskeletal pain. The results of this review question the validity of conclusions drawn from acute experimental pain studies regarding changes in muscle behaviour in response to pain in the clinical setting.

Nerve growth factor-induced muscle hyperalgesia facilitates ischaemic contraction-evoked pain

BACKGROUND

Intramuscular injection of Nerve Growth Factor (NGF) may influence the responsiveness of active chemo-sensitive channels affecting muscle pain sensitivity. This double-blinded crossover study in healthy humans assessed contraction-evoked pain responses and pain sensitivity during acute ischaemia in the tibialis anterior (TA) muscle before and 24 hr after five distributed NGF injections (1 µg, 4 cm interval) compared with control injections (isotonic-saline).

METHODS

Twenty-one subjects participated in two experimental phases, each including five sessions over 7 days, with a gap of 4 weeks in-between. Muscle pain intensity evoked with daily functional tasks (Likert scale score) was collected using a paper diary. Pain intensity evoked by ischaemic and non-ischaemic contractions numerical rating scale (NRS) was collected at Day0 and Day1. Pressure pain thresholds (PPTs) on the TA were recorded before (Day0), 3 hr, 1, 3, and 7 days post-injection, and after the ischaemic-contractions and post-cuff deflation at Day0 and Day1.

RESULTS

Increased Likert scores of pain were present for 7 days after NGF compared to control injections (p < .05). Higher NRS pain scores of ischaemic-contractions were seen when contracting the muscle injected with NGF compared to baseline (p = .003) and control (p = .012). Pain during non-ischaemic contractions was not significantly affected by NGF injections. Decreased PPTs were found at 3 hr, Day1 and Day3 post-injection (p < .05) in both conditions. Compared with pre-contractions, PPTs were increased following ischaemic contractions at Day0 (p < .05) and Day1 (p < .05) in both conditions.

CONCLUSION

This study showed that ischaemic contraction-evoked pain was facilitated in an NGF-sensitized muscle.

SIGNIFICANCE

Acidification of the muscle environment may affect muscle nociceptors and pain by different mechanisms, including activation of ASIC₃ and TRPV1. In this study, pain evoked following ischaemic contractions was increased in the Nerve Growth Factor (NGF)-sensitized muscle compared with non-ischaemic contractions and in the non-sensitized muscle. These findings illustrate that responses of peripheral afferents under ischaemic conditions are altered by a pre-sensitized muscle. This highlights the role of growth factors, including NGF, in peripheral muscle sensitization with clinical implications for ischaemic myalgia.

Delayed-onset muscle soreness in human masticatory muscles increases inhibitory jaw reflex responses

The effects of masticatory muscles' overloading on jaw-motor control are not yet fully clarified. Therefore, it was tested whether eccentric and concentric exercises of the human masticatory muscles would influence inhibitory jaw reflex responses. Eleven participants (6 males, 5 females) performed 6, 5-minutes bouts of eccentric-concentric contractions. Before, immediately after, 24 hours, 48 hours and 1 week afterwards, visual analogue scale (VAS) scores for jaw muscle fatigue and pain, maximum voluntary bite force (MVBF) and inhibitory jaw reflexes were recorded. Reflex data were analysed with the cumulative sum control chart error box method. Immediate and delayed masticatory muscle fatigue and pain were provoked. Further, 24 hours after the exercises, MVBF tended to decrease (P = .056), suggesting that delayed-onset muscle soreness (DOMS) was provoked in the masticatory muscles. In addition, the inhibitory jaw reflex showed a delayed increase in size 24 hours after the exercise (P < .05). In conclusion, DOMS provoked in the masticatory muscles alters jaw motor control by inducing a delayed increase in the size of the inhibitory jaw reflex.

Pain Neuroimaging in Humans: A Primer for Beginners and Non-Imagers

Human pain neuroimaging has exploded in the past 2 decades. During this time, the broader neuroimaging community has continued to investigate and refine methods. Another key to progress is exchange with clinicians and pain scientists working with other model systems and approaches. These collaborative efforts require that non-imagers be able to evaluate and assess the evidence provided in these reports. Likewise, new trainees must design rigorous and reliable pain imaging experiments. In this article we provide a guideline for designing, reading, evaluating, analyzing, and reporting results of a pain neuroimaging experiment, with a focus on functional and structural magnetic resonance imaging. We focus in particular on considerations that are unique to neuroimaging studies of pain in humans, including study design and analysis, inferences that can be drawn from these studies, and the strengths and limitations of the approach.

No effect of a pulsed magnetic field on induced ischemic muscle pain. A double-blind, randomized, placebo-controlled trial

Empirical evidence supporting the effectiveness of pulsed (electro)magnetic field (PEMF) therapy on chronic and acute pain is equivocal. In the current randomized, double-blind, placebo-controlled experiment, impact of a commercially available whole-body PEMF mat on acute ischemic muscle pain induced in the forearm was studied with the participation of 70 healthy volunteers. The device emitted a pulsed magnetic field with a repetition frequency of 2.05Hz and a maximum flux density of 25.3μT (rms). The highest dB/dt was 48mT/s. No differences between the groups receiving actual and sham PEMF were found in terms of pain threshold, pain tolerance, heart rate, and perceived decrease of pain. However, participants' expectation concerning the effectiveness of the intervention improved pain tolerance and affected perceived change of pain. In conclusion, the used PEMF device had no specific effect on acute ischemic muscle pain, while the contribution of the placebo effect was considerable.

Effects of fatigue on corticospinal excitability of the human knee extensors

NEW FINDINGS

What is the central question of this study? Do group III and IV muscle afferents act at the spinal or cortical level to affect the ability of the central nervous system to drive quadriceps muscles during fatiguing exercise? What is the main finding and its importance? The excitability of the motoneurone pool of vastus lateralis was unchanged by feedback from group III and IV muscle afferents. In contrast, feedback from these afferents may contribute to inhibition at the cortex. However, the excitability of the corticospinal pathway was not directly affected by feedback from these afferents. These findings are important for understanding neural processes during fatiguing exercise. In upper limb muscles, changes in afferent feedback, motoneurone excitability, and motor cortical output can contribute to failure of the central nervous system to recruit muscles fully during fatigue. It is not known whether similar changes occur with fatigue of muscles in the lower limb. We assessed the corticospinal pathway to vastus lateralis during fatiguing sustained maximal voluntary contractions (MVCs) of the knee extensors and during firing of fatigue-sensitive group III/IV muscle afferents maintained by postexercise ischaemia after fatiguing MVCs of the knee extensors and, separately, the flexors. In two experiments, subjects (n = 9) performed brief knee extensor MVCs before and after 2-min sustained MVCs of the knee extensors (experiment 1) or knee flexors (experiment 2). During MVCs, motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation over the motor cortex and thoracic motor evoked potentials (TMEPs) by electrical stimulation over the thoracic spine. During the 2-min extensor contraction, the size of vastus lateralis MEPs normalized to the maximal M-wave increased (P < 0.05), but normalized TMEPs were unchanged (P = 0.16). After the 2-min MVC, maintained firing of group III/IV muscle afferents had no effect on vastus lateralis MEPs or TMEPs (P = 0.18 and P = 0.50, respectively). Likewise, after the 2-min knee flexor MVC, maintained firing of these afferents showed no effect on vastus lateralis MEPs or TMEPs (P = 0.69 and P = 0.34, respectively). Motoneurones of vastus lateralis do not become less excitable during fatiguing isometric MVCs. Moreover, fatigue-sensitive group III/IV muscle afferents fail to affect the overall excitability of vastus lateralis motoneurones during MVCs.

Reduced Maximal Force during Acute Anterior Knee Pain Is Associated with Deficits in Voluntary Muscle Activation

Although maximal voluntary contraction (MVC) force is reduced during pain, studies using interpolated twitch show no consistent reduction of voluntary muscle drive. The present study aimed to test if the reduction in MVC force during acute experimental pain could be explained by increased activation of antagonist muscles, weak voluntary activation at baseline, or changes in force direction. Twenty-two healthy volunteers performed maximal voluntary isometric knee extensions before, during, and after the effects of hypertonic (pain) and isotonic (control) saline injections into the infrapatellar fat pad. The MVC force, voluntary activation, electromyographic (EMG) activity of agonist, antagonist, and auxiliary (hip) muscles, and pain cognition and anxiety scores were recorded. MVC force was 9.3% lower during pain than baseline (p < 0.001), but there was no systematic change in voluntary activation. Reduced MVC force during pain was variable between participants (SD: 14%), and was correlated with reduced voluntary activation (r = 0.90), baseline voluntary activation (r = − 0.62), and reduced EMG amplitude of agonist and antagonist muscles (all r > 0.52), but not with changes in force direction, pain or anxiety scores. Hence, reduced MVC force during acute pain was mainly explained by deficits in maximal voluntary drive.

Fatigue-related firing of muscle nociceptors reduces voluntary activation of ipsilateral but not contralateral lower limb muscles

During fatiguing upper limb exercise, maintained firing of group III/IV muscle afferents can limit voluntary drive to muscles within the same limb. It is not known if this effect occurs in the lower limb. We investigated the effects of group III/IV muscle afferent firing from fatigued ipsilateral and contralateral extensor muscles and ipsilateral flexor muscles of the knee on voluntary activation of the knee extensors. In three experiments, we examined voluntary activation of the knee extensors by measuring changes in superimposed twitches evoked by femoral nerve stimulation. Subjects attended on 2 days for each experiment. On one day a sphygmomanometer cuff occluded blood flow of the fatigued muscles to maintain firing of group III/IV muscle afferents. After a 2-min extensor contraction (experiment 1; n = 9), mean voluntary activation was lower with than without maintained ischemia (47 ± 19% vs. 87 ± 8%, respectively; P < 0.001). After a 2-min knee flexor maximal voluntary contraction (MVC) (experiment 2; n = 8), mean voluntary activation was also lower with than without ischemia (59 ± 21% vs. 79 ± 9%; P < 0.01). After the contralateral (left) MVC (experiment 3; n = 8), mean voluntary activation of the right leg was similar with or without ischemia (92 ± 6% vs. 93 ± 4%; P = 0.65). After fatiguing exercise, activity in group III/IV muscle afferents reduces voluntary activation of the fatigued muscle and nonfatigued antagonist muscles in the same leg. However, group III/IV muscle afferents from the fatigued left leg had no effect on the unfatigued right leg. This suggests that any "crossover" of central fatigue in the lower limbs is not mediated by group III/IV muscle afferents.

Spinal μ-opioid receptor-sensitive lower limb muscle afferents determine corticospinal responsiveness and promote central fatigue in upper limb muscle

We investigated the influence of group III/IV lower limb muscle afferents on the development of supraspinal fatigue and the responsiveness of corticospinal projections to an arm muscle. Eight males performed constant-load leg cycling exercise (80% peak power output) for 30 s (non-fatiguing) and to exhaustion (∼9 min; fatiguing) both under control conditions and with lumbar intrathecal fentanyl impairing feedback from μ-opioid receptor-sensitive lower limb muscle afferents. Voluntary activation (VA) of elbow flexors was assessed via transcranial magnetic stimulation (TMS) during maximum voluntary contraction (MVC) and corticospinal responsiveness was monitored via TMS-evoked potentials (MEPs) during a 25% MVC. Accompanied by a significant 5 ± 1% reduction in VA from pre- to post-exercise, elbow flexor MVC progressively decreased during the fatiguing trial (P < 0.05). By contrast, with attenuated feedback from locomotor muscle afferents, MVC and VA remained unchanged during fatiguing exercise (P > 0.3). MEPs decreased by 36 ± 6% (P < 0.05) from the start of exercise to exhaustion under control conditions, but this reduction was prevented with fentanyl blockade. Furthermore, fentanyl blockade prevented the significant increase in elbow flexor MEP observed from rest to non-fatiguing exercise under control conditions and resulted in a 14% lower corticospinal responsiveness during this short bout (P < 0.05). Taken together, in the absence of locomotor muscle fatigue, group III/IV-mediated leg muscle afferents facilitate responsiveness of the motor pathway to upper limb flexor muscles. By contrast, in the presence of cycling-induced leg fatigue, group III/IV locomotor muscle afferents facilitate supraspinal fatigue in remote muscle not involved in the exercise and disfacilitate, or inhibit, the responsiveness of corticospinal projections to upper limb muscles.

Is myofascial pain in temporomandibular disorder patients a manifestation of delayed-onset muscle soreness?

OBJECTIVE

In a study to the possible role of overuse of the jaw muscles in the pathogenesis of jaw muscle pain, we used a protocol involving concentric and eccentric muscle contractions to provoke a state of delayed-onset muscle soreness (DOMS) in the jaw muscles of healthy individuals. We tested whether the accompanying signs and symptoms would yield the temporary diagnosis of myofascial pain according to the research diagnostic criteria for temporomandibular disorders (RDC/TMD) in these individuals.

METHODS

Forty persons (mean age±SD=27.7±7.5 y) performed six, 5-minute bouts of eccentric and concentric jaw muscle contractions. Before and immediately after the exercise, and 24 hours, 48 hours, and 1 week later, self-reported muscle fatigue and pain, pain-free maximum mouth opening, pressure-pain thresholds, and the number of painful jaw muscle palpation sites were recorded.

RESULTS

Significant signs and symptoms of DOMS in the jaw muscles were found, which all had resolved after 1 week. In 31 (77.5%) of the participants, these signs and symptoms also gave rise to a temporary diagnosis of myofascial pain according to the RDC/TMD.

CONCLUSIONS

The results of this study demonstrate that an experimental protocol involving concentric and eccentric muscle contractions can provoke DOMS in the jaw muscles and the temporary diagnosis of myofascial pain according to the RDC/TMD. The results observed strengthen the supposition that the myofascial pain in TMD patients may be a manifestation of DOMS in the jaw muscles.

Insight into motor adaptation to pain from between-leg compensation

PURPOSE

Although it appears obvious that we change movement behaviors to unload the painful region, non-systematic motor adaptations observed in simple experimental tasks with pain question this theory. We investigated the effect of unilateral pain on performance of a bilateral plantarflexion task. This experimental task clearly allowed for stress on painful tissue to be reduced by modification of load sharing between legs.

METHODS

Fourteen participants performed a bilateral plantarflexion at 10, 30, 50 and 70 % of their MVC during 5 conditions (Baseline, Saline-1, Washout-1, Saline-2, Washout-2). For Saline-1 and -2, either isotonic saline (Iso) or hypertonic saline (Pain) was injected into the left soleus.

RESULTS

The force produced by the painful leg was less during Pain than Baseline (range -52.6 % at 10 % of MVC to -20.1 % at 70 % of MVC; P < 0.003). This was compensated by more force produced by the non-painful leg (range 18.4 % at 70 % of MVC to 70.2 % at 10 % of MVC; P < 0.001). The reduction in plantarflexion force was not accompanied by a significant decrease in soleus electromyographic activity at 10 and 30 % of MVC. Further, no significant linear relationship was found between changes in soleus electromyographic activity and change in plantarflexion force for the painful leg (with the exception of a weak relationship at 10 % of MVC, i.e., R (2) = 0.31).

CONCLUSION

These results show that when the nervous system is presented with an obvious solution to decrease stress on irritated tissue, this option is selected. However, this was not strongly related to a decrease in soleus (painful muscle) activity level.

Exogenously applied muscle metabolites synergistically evoke sensations of muscle fatigue and pain in human subjects

NEW FINDINGS

What is the central question of this study? Can physiological concentrations of metabolite combinations evoke sensations of fatigue and pain when injected into skeletal muscle? If so, what sensations are evoked? What is the main finding and its importance? Low concentrations of protons, lactate and ATP evoked sensations related to fatigue. Higher concentrations of these metabolites evoked pain. Single metabolites evoked no sensations. This suggests that the combination of an ASIC receptor and a purinergic P2X receptor is required for signalling fatigue and pain. The results also suggest that two types of sensory neurons encode metabolites; one detects low concentrations of metabolites and signals sensations of fatigue, whereas the other detects higher levels of metabolites and signals ache and hot. The perception of fatigue is common in many disease states; however, the mechanisms of sensory muscle fatigue are not understood. In mice, rats and cats, muscle afferents signal metabolite production in skeletal muscle using a complex of ASIC, P2X and TRPV1 receptors. Endogenous muscle agonists for these receptors are combinations of protons, lactate and ATP. Here we applied physiological concentrations of these agonists to muscle interstitium in human subjects to determine whether this combination could activate sensations and, if so, to determine how the subjects described these sensations. Ten volunteers received infusions (0.2 ml over 30 s) containing protons, lactate and ATP under the fascia of a thumb muscle, abductor pollicis brevis. Infusion of individual metabolites at maximal amounts evoked no fatigue or pain. Metabolite combinations found in resting muscles (pH 7.4 + 300 nm ATP + 1 mm lactate) also evoked no sensation. The infusion of a metabolite combination found in muscle during moderate endurance exercise (pH 7.3 + 400 nm ATP + 5 mm lactate) produced significant fatigue sensations. Infusion of a metabolite combination associated with vigorous exercise (pH 7.2 + 500 nm ATP + 10 mm lactate) produced stronger sensations of fatigue and some ache. Higher levels of metabolites (as found with ischaemic exercise) caused more ache but no additional fatigue sensation. Thus, in a dose-dependent manner, intramuscular infusion of combinations of protons, lactate and ATP leads to fatigue sensation and eventually pain, probably through activation of ASIC, P2X and TRPV1 receptors. This is the first demonstration in humans that metabolites normally produced by exercise act in combination to activate sensory neurons that signal sensations of fatigue and muscle pain.

Wound hypoxia in deep tissue after incision in rats

Our previous studies using rat models of incisional pain have shown that tissue lactate levels increase and pH decreases for several days after incision, suggesting the presence of an ischemic-like condition. The purpose of this study was to evaluate the time course and the extent of tissue hypoxia that develops in incised muscle and skin. We directly measured oxygen tension at several time points after incisions of the gastrocnemius muscle, the paraspinal skin, and the plantar hindpaw in anesthetized rats using an oxygen-sensitive microelectrode. In vivo hypoxia of the incised tissues was also evaluated immunohistochemically using a hypoxia marker, pimonidazole hydrochloride. To minimize intersubject variability, unincised contralateral tissues were used as a control. Tissue oxygen tension was decreased in both skeletal muscle and skin compared with control, for several days after incision. When measured directly, oxygen tension decreased immediately and remained low for several days after incisions. Pimonidazole immunostaining revealed hypoxic areas in incised muscle and skin for several days. By postoperative day 10, tissue oxygen tension recovered to that of control tissue. These results support the evidence that a hypoxic condition is present in deep tissue after incisions and that an ischemic-like mechanism may contribute to postoperative pain.

Firing of antagonist small-diameter muscle afferents reduces voluntary activation and torque of elbow flexors

During muscle fatigue, firing of small-diameter muscle afferents can decrease voluntary activation of the fatigued muscle. However, these afferents may have a more widespread effect on other muscles in the exercising limb. We examined if the firing of fatigue-sensitive afferents from elbow extensor muscles in the same arm reduces torque production and voluntary activation of elbow flexors. In nine subjects we examined voluntary activation of elbow flexors by measuring changes in superimposed twitches evoked by transcranial magnetic stimulation of the motor cortex during brief (2-3 s) maximal voluntary contractions (MVC). Inflation of a blood pressure cuff following a 2-min sustained MVC blocked blood flow to the fatigued muscle and maintained firing of small-diameter afferents. After a fatiguing elbow flexion contraction, maximal flexion torque was lower (26.0 ± 4.4% versus 67.9 ± 5.2% of initial maximal torque; means ± s.d.; P < 0.001) and superimposed twitches were larger (4.1 ± 1.1% versus 1.8 ± 0.2% ongoing MVC, P = 0.01) with than without ischaemia. After a fatiguing elbow extensor contraction, maximal flexion torque was also reduced (82.2 ± 4.9% versus 91.4 ± 2.3% of initial maximal torque; P = 0.007), superimposed twitches were larger (2.7 ± 0.7% versus 1.3 ± 0.2% ongoing MVC; P = 0.02) and voluntary activation lower (81.6 ± 8.2% versus 95.5 ± 6.9%; P = 0.04) with than without ischaemia. After a fatiguing contraction, voluntary drive to the fatigued muscles is reduced with continued input from small-diameter muscle afferents. Furthermore, fatigue of the elbow extensor muscles decreases voluntary drive to unfatigued elbow flexors of the same arm. Therefore, firing of small-diameter muscle afferents from one muscle can affect voluntary activation and hence torque generation of another muscle in the same limb.

Normalization of widespread hyperesthesia and facilitated spatial summation of deep-tissue pain in knee osteoarthritis patients after knee replacement

OBJECTIVE

The modest association between radiographic joint damage and pain in osteoarthritis (OA) has led to the suggestion of facilitated central pain processing. This study evaluated the importance of ongoing tissue pathology in the maintenance of enhanced central pain processing.

METHODS

Pain assessment was performed on 48 patients with symptomatic knee OA and 21 sex- and age-matched pain-free healthy control subjects. Twenty of the OA patients subsequently underwent total knee replacement surgery and were reassessed. Pressure-pain thresholds (PPTs) were recorded using a pressure algometer (both over and distant from the knee) and a double-chamber inflatable cuff mounted around the calf. Spatial summation was assessed by relating PPTs using the dual- and single-chamber cuff. Conditioned pain modulation (CPM) was assessed by recording the increase in PPT in response to experimental arm pain.

RESULTS

PPTs at the knee and at sites away from the knee were reduced in OA patients as compared with healthy pain-free control subjects (P < 0.0001). Cuff PPTs were decreased in OA patients as compared with the healthy controls (P < 0.05), who also exhibited a greater degree of spatial summation (P < 0.05). Whereas an elevation of PPTs was noted in the healthy controls in response to experimental arm pain (P < 0.0001), no such CPM was observed in the OA patients. Following joint replacement in the OA patients, there was a reduction in the widespread mechanical hyperesthesia, along with normalization of spatial summation ratios and restoration of CPM.

CONCLUSION

The widespread hyperesthesia and enhanced spatial summation observed in OA patients imply sensitized central pain mechanisms together with the loss of CPM. Normalization of the results following joint replacement implies that these central pain processes are maintained by peripheral input.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Effect of experimental muscle pain on maximal voluntary activation of human biceps brachii muscle

Muscle pain has widespread effects on motor performance, but the effect of pain on voluntary activation, which is the level of neural drive to contracting muscle, is not known. To determine whether induced muscle pain reduces voluntary activation during maximal voluntary contractions, voluntary activation of elbow flexors was assessed with both motor-point stimulation and transcranial magnetic stimulation over the motor cortex. In addition, we performed a psychophysical experiment to investigate the effect of induced muscle pain across a wide range of submaximal efforts (5–75% maximum). In all studies, elbow flexion torque was recorded before, during, and after experimental muscle pain by injection of 1 ml of 5% hypertonic saline into biceps. Injection of hypertonic saline evoked deep pain in the muscle (pain rating ∼5 on a scale from 0 to 10). Experimental muscle pain caused a small (∼5%) but significant reduction of maximal voluntary torque in the motor-point and motor cortical studies ( P < 0.001 and P = 0.045, respectively; n = 7). By contrast, experimental muscle pain had no significant effect on voluntary activation when assessed with motor-point and motor cortical stimulation although voluntary activation tested with motor-point stimulation was reduced by ∼2% in contractions after pain had resolved ( P = 0.003). Furthermore, induced muscle pain had no significant effect on torque output during submaximal efforts ( P > 0.05; n = 6), which suggests that muscle pain did not alter the relationship between the sense of effort and production of voluntary torque. Hence, the present study suggests that transient experimental muscle pain in biceps brachii has a limited effect on central motor pathways.

Silk fibroin encapsulated powder reservoirs for sustained release of adenosine

Due to its unique properties, silk fibroin was studied as a biodegradable polymer vehicle for sustained, local delivery of the anticonvulsant adenosine from encapsulated reservoirs. Silk is a biologically derived protein polymer that is biocompatible, mechanically strong and degrades to non-toxic products in vivo. To achieve local, sustained, controlled adenosine release from fully degradable implants, solid adenosine powder reservoirs were coated with silk fibroin. Material properties of the silk coating including thickness, crystallinity and morphology were investigated to assess the relationships between silk coating biomaterial features and adenosine release from silk encapsulated reservoirs. Reservoir coating thickness was varied through manipulation of the silk coating solution concentration and number of coatings applied. Release studies were also performed in proteinase type XIV to model the effects of degradation. Increasing the barrier to diffusion, either by increasing coating thickness or crystallinity was found to delay adenosine burst, decrease average daily release rate, and increase duration of release. In the case of encapsulated reservoirs coated with eight layers of 8% (w/v) silk, a linear release profile was observed and adenosine release was sustained for 14days. The ability to achieve nearly constant release for 2weeks for adenosine via control of the silk coating suggests these encapsulated reservoirs represent a novel system for delivering adenosine. We anticipate that this approach could also be extended to other implant needs and small-molecule drugs to treat a range of clinical needs.

Gender and site of injection do not influence intensity of hypertonic saline-induced muscle pain in healthy volunteers

The aim of the study was to determine whether the same stimulus to different muscles results in comparable pain and whether gender has any influence on the pain. We compared the quality and intensity of muscle pain induced by a hypertonic saline injection into the tibialis anterior (leg) muscle to that after an injection into the lumbar erector spinae (back) muscle in both male (n=10) and female (n=10) volunteers. Hypertonic or isotonic saline was injected into the leg and back muscles and pain intensity (visual analogue scale, VAS) and pain quality (McGill Pain Questionnaire) were measured. Pressure pain tolerance around the site of injection and on the contralateral side was measured. Hypertonic saline injection induced significant muscle pain in the back and leg compared to isotonic saline (P<0.05, ANOVA). The site of injection did not influence the quality of pain but there was a gender bias in the descriptive words chosen (chi(2) test, P<0.05) and female subjects were more sensitive to pressure than male subjects. Experimentally induced muscle pain is equivalent in intensity and quality in the leg and back muscle. Gender does not influence muscle pain intensity but does influence sensitivity to pressure and the description of pain.

Open experience with a new myorelaxant agent for low back pain

INTRODUCTION

Eperisone hydrochloride has been recently proposed as a muscle relaxant for the treatment of muscle contracture and chronic low back pain (LBP) as it is devoid of clinically relevant sedative effects on the central nervous system (CNS). We tested this hypothesis by performing a study of patients with LBP and muscle contracture who were treated with full-dose eperisone.

METHODS

Patients with moderate to severe, acute, or relapsing LBP received eperisone 100 mg three times daily for 10 consecutive days. Assessments included: spontaneous pain, pain on movement, resistance to passive movement, antalgic rigidity, and tolerability.

RESULTS

In total, 100 patients were enrolled into the study. The treatment achieved a consistent analgesic and muscle relaxant activity across all patients. Both spontaneous pain and pain on movement were significantly decreased, as was resistance encountered by the investigator to passive movements, antalgic rigidity, and muscle contracture. As a consequence, treatment with eperisone resulted in a lower rigidity of the lower back and an improved motility for patients. Only seven adverse reactions were reported, including light-headedness (1), occasional vertigo and/or loss of equilibrium (3), mild somnolence (2), and epigastric pain (1). In almost all cases, there was no need to interrupt the treatment and the adverse reaction resolved spontaneously.

CONCLUSIONS

Eperisone had an analgesic and muscle relaxant effect in patients with LBP. It should be noted that while it is common practice in rheumatology to combine a pain killer with a muscle relaxant in order to achieve a satisfactory result on both symptoms, the present results with eperisone were achieved with a single drug. With an improved tolerability profile compared with nonsteroidal anti-inflammatory drugs, and a lack of significant adverse effects on the CNS, eperisone hydrochloride represents a valuable alternative to traditional analgesics and muscle relaxants for the treatment of LBP.

Effect of pulsed magnetic field therapy on pain reported by human volunteers in a laboratory model of acute pain †

BACKGROUND

Pulsed magnetic field therapy (PMFT) is a non-invasive, simple technique used extensively for the treatment of muscle pain. However, evidence to support its use from well-designed, clinical, or experimental studies is sparse.

METHODS

We have utilized an acute pain model to perform a randomized, double-blinded, placebo-controlled, crossover-study on 10 male (18-40 yr) volunteers. Pain was elicited by infusion of hypertonic saline 5% into the brachioradialis muscle of the non-dominant arm on two occasions, at least 1 week apart. Subjects received active or sham PMFT for 30 min in a randomized order delivered by two identical, commercially available machines (PulsePack 6000, Quantum Techniks). The active machine delivered a M-wave magnetic pulse (1.25 Hz, 3 ms width, 600 Gauss); the sham device was deactivated and delivered no magnetic energy. Pain was assessed at 15-s intervals, and area under the visual analogue score (VAS) pain curve (AUCp) was calculated using the trapezoid method.

RESULTS

There were no significant differences in mean VAS pain scores between the two machines at any time. In addition, there were no significant differences with respect to mean (sem) maximum pain score [sham 60 (8), active 63 (9) mm; P = 0.66, 95% CI -18 to 12 mm] or AUCp [sham 463 (50), active 499 (90); P = 0.64, 95% CI -201 to 129].

CONCLUSIONS

We conclude that, using the electromagnetic characteristics of the machine in this study, the PMFT had no effect on pain in our experimental model. More work is required to provide an evidence base in support of the use of this technique for pain.

New models for visceral pain

PURPOSE OF REVIEW

After the initial use of unspecific stimuli, scientists have recently put a lot of effort into the development of new visceral pain models. These models will help to study physiological and pharmacological questions closely related to clinically treated visceral pain. The aim of this review is to provide an overview of the new models published most recently and encourage further studies in this field.

RECENT FINDINGS

Four rodent and three human models have been introduced. Each of these models addresses specific types of visceral pain, related to the urogenital tract (n=3), to the gastrointestinal tract (n=3), and to visceral organs (n=1). Mechanical distension as a brief stimulus was used mainly, but chemical stimuli resulting in intermediate to prolonged duration of pain were also investigated.

SUMMARY

Developments in the scientific visceral pain field are encouraging. Growing interest in studying aetiology, physiology and pharmacology of visceral pain proves the clinical importance of this pain entity. However, these new models need further evaluation. Future studies will show which of these new models will become a standard scientific tool.

Pathophysiological mechanisms in chronic musculoskeletal pain (fibromyalgia): the role of central and peripheral sensitization and pain disinhibition

Chronic musculoskeletal pain has biological, psychological and social components. This review deals with the biological factors, with emphasis on the fibromyalgia syndrome (FMS). Studies on central sensitization of pain-transmitting neurons, changes in endogenous pain modulation that give rise to pain disinhibition, referred pain, pain-related decrease in muscle strength and endurance, and pain generators in deep tissues are reviewed. In FMS there is strong scientific support for the statement that the biological part of the syndrome is a longstanding or permanent change in the function of the nociceptive nervous system that can be equated with a disease. Further research is necessary in order to determine which methods are best for diagnosis of the pain hypersensitivity in clinical practice. FMS may be the far end of a continuum that starts with chronic localized/regional musculoskeletal pain and ends with widespread chronic disabling pain.

Pressure pain threshold changes after repeated mechano-nociceptive stimulation of the trapezius muscle: possible influence of previous pain experience

We examined the relation between repeated noxious pressure over the trapezius muscle and changes in pressure pain thresholds (PPTs) in a before-after trial design. A conditioning series of 30 mechano-nociceptive stimuli was applied manually with a handheld algometer probe, and PPTs were measured over 1 trapezius muscle (skin anaesthetized) in 27 healthy women before and after the intervention. With a mean stimulation rate of 0.40 Hz and a mean nociceptive stimulation intensity of 1.78 x Threshold, subjects were found to systematically react with a change in PPT, either a decrease or an increase. Normalized data, transformed into mean unidirectional PPT differences, showed statistically highly significant changes after intervention. The relative risk of reacting with lowered PPTs on noxious stimulation was 3.7 times higher for subjects who had not given birth to children than for subjects who had given birth to 1 or several children (P<.046). When 11 subjects were tested at a second session, a clear correlation of PPT reactions (r=0.527; P<.001) was found. In summary, repetitive mechano-nociceptive stimulation of the trapezius muscle in healthy females evokes moderate and temporary changes in PPT that last for at least 35 minutes after cessation of stimulation.

PERSPECTIVE

A possible development of the response with transiently decreased PPTs into a model for human muscle pain is an intriguing possibility, since other models usually involve the introduction of chemical or thermal agents in the muscle, but this must await further research.

Delayed onset muscle soreness at tendon-bone junction and muscle tissue is associated with facilitated referred pain

Delayed onset muscle soreness (DOMS) involves central and peripheral pain mechanisms. Referred pain patterns following stimulation of DOMS affected tissue have not been fully described. Referred pain may provide information on how central mechanisms are involved in DOMS, as referred pain is a central mechanism. Further, tendon tissue involvement in DOMS is not clear. This study assessed pressure pain threshold (PPT) sensitivity at the tendon, tendon-bone junction (TBJ) and muscle belly sites of tibialis anterior pre- and during DOMS in 45 subjects (34 males, 11 females). Furthermore, pain and referred pain areas at these three sites in response to hypertonic saline injection (n = 15 per injection site) were investigated pre- and during DOMS. DOMS was induced using controlled plantarflexion from a platform (bodyweight as resistance) causing eccentric contraction of the tibialis anterior muscle. DOMS induced PPT decrease was found at the TBJ and muscle belly sites only (P < 0.001). No mechanical effect was found in the unexercised limb. Maximal pain intensity induced by hypertonic saline given pre-DOMS was significantly higher for the tendon and TBJ injections compared to intramuscular injections (P < 0.05). Significantly higher referred pain frequency and enlarged pain areas were found at the muscle belly and TBJ sites following injection during DOMS compared to pre-DOMS. The results indicate that muscle belly and TBJ sites are sensitised while tendon tissue per se is unaffected by DOMS. Central sensitivity changes caused by DOMS may explain the increase in referred pain frequency and enlarged pain areas.

Pain perception in major depression depends on pain modality

One frequently described feature of depression is an increased vulnerability to pain complaints, and chronic pain is frequently accompanied by symptoms of depression. In contrast to this, a decreased sensitivity to experimental pain has been described in major depression. The physiological basis of this phenomenon is yet elusive. We investigated 30 patients suffering from a major depressive disorder and matched controls. Pain testing (threshold and tolerance) was performed on both sides of the body and included assessment of thermal, electrical and ischemic pain. While confirming hypoalgesia to heat and electrical pain in comparison to controls, we found hyperalgesia to ischemic muscle pain. Furthermore, thermal pain tolerance and electrical pain tolerance were significantly increased on the right hand side confirming previous results of a lateralized perception of pain in depression. Our main finding suggests that painful stimuli are processed differentially depending on the localization of pain induction in depression. This knowledge may enable us to understand and ultimately treat pain complaints more appropriately in depressed patients.

Different types of variant muscle nociception after intermittent and continuous neuromuscular stimulation in rats

Critical assessment of experimental muscle-pain models resulting from maximal muscle contraction may provide a means of assessing hypersensitivity and the central nociceptive mechanisms involved in diffused muscle pain. The aim of the present study was, therefore, to investigate the patterns of nociceptive behavior and neuronal changes in the rat spinal cord after two modes of maximal muscle contraction. The gastrocnemius muscle of adult male Sprague-Dawley rats was subjected to continuous (10 min) or intermittent (60 min, 10/50 s on/off ratio) premodulated electrical stimulation of median frequency. Similar peak forces but different patterns of contraction output were generated by these two stimulation modes. Nociceptive behavioral scores and hind-leg oedema were significantly greater in the continuous group compared to the controls; however, significant difference was not demonstrated for either parameter comparing the intermittent and control groups. The sensory threshold was slightly reduced after the intermittent stimulation, and elevated after the continuous modality. The elevation of sensory threshold could be reversed by naloxone administration. More Fos-labeled nuclei were noted for both of the stimulation groups relative to the controls. The Fos-labeled nuclei were larger for the intermittent group than for the continuous and control analogs. The results of the present study suggest that prolonged contraction from continuous stimulation results in specific nociceptive neuron activation, muscle lesion and endogenous opioid release causing exaggerated nociception.

Referred pain and hyperalgesia in human tendon and muscle belly tissue

The sensitivity of tendon and tendon-bone junction is not fully described although these tissues have high clinical impacts. This study assessed (1) pain intensity and referred pain caused by hypertonic saline injection to the proximal tendon-bone junction (PTBJ), tendon and muscle belly sites of tibialis anterior muscle and (2) pressure pain sensitivity, pre, during and post hypertonic saline injections. Eighteen subjects (14 males and 4 females) participated. Subjects also had constant mechanical stimulation for 120s at 130% of baseline pressure pain threshold (PPT) during which VAS parameters were recorded. VAS parameters after hypertonic saline for PTBJ (VAS area, VAS peak), and tendon sites (VAS area, duration and time to maximum VAS) were significantly (P < 0.05) higher than muscle belly. During hypertonic saline pain all three sites displayed local and frequently enlarged and referred pain areas. Hypertonic saline pain at the PTBJ and tendon transiently increased pressure sensitivity at these sites (P < 0.05). When referred pain was caused by mechanical stimulation it occurred predominantly at the PTBJ and tendon sites (86% cases). Constant mechanical stimulation caused steadily increasing pain (summation of pain) at all sites. Hypertonic saline pain at the tendon and PTBJ caused significantly higher (P < 0.001) final VAS scores compared to the muscle belly site. The results indicate the PTBJ and tendon sites are more sensitive and susceptible to sensitisation by hypertonic saline than muscle belly. Furthermore, there may be site specific central changes reflected by the differences in the results regarding sensitivity and summation over time.

Relationship between the thickness and hemodynamics of the erector spinae muscles in various lumbar curvatures

BACKGROUND

There is little information about the relationship between the changes of hemodynamics and the morphologic changes of the erector spinae muscle.

METHODS

Fifty healthy male volunteers participated. Ultrasonography was used to measure muscle thickness, and near-infrared spectroscopy was used to measure tissue blood volume and its oxygenation in the erector spinae muscle at L3 in six different relaxed trunk postures (flexed 20 degrees , flexed 40 degrees, flexed maximum, neutral posture, extended 20 degrees, and extended maximum of the lumbar spine). We also evaluated the reproducibility of the near-infrared spectroscopy measurements.

FINDINGS

Near-infrared spectroscopy gave highly reproducible measurements. The thickness of the erector spinae muscle and the total and oxygenated hemoglobin were simultaneously increased during relaxed extension and decreased during relaxed flexion. Changes in the thickness of the erector spinae muscle with various lumbar curvature were similar in pattern to the changes in tissue blood volume and its oxygenation.

INTERPRETATION

The erector spinae muscles' thickness, tissue blood volume, and its oxygenation are simultaneously increased during relaxed extension and decreased during relaxed flexion, as demonstrated by non-invasive near-infrared spectroscopy and ultrasonography. These findings might afford a better understanding of the pathomechanics of posture-related back symptoms.

RELEVANCE

The erector spinae muscles' thickness, tissue blood volume, and its oxygenation are simultaneously increased during relaxed extension and decreased during relaxed flexion, as shown by non-invasive near-infrared spectroscopy and ultrasonography. Changes in hemodynamics and morphology of the erector spinae muscles in asymptomatic subjects are given for further research on the pathomechanism of back pain.

An expansion of Simons' integrated hypothesis of trigger point formation

Simons' integrated hypothesis proposed a model of trigger point (TrP) activation to explain known TrP phenomena, particularly endplate noise. We propose an expansion of this hypothesis to account for new experimental data and established muscle pathophysiology.

Acidic pH and capsaicin activate mechanosensitive group IV muscle receptors in the rat

Strenuous exercise of muscle as well as inflammation and ischaemia are associated with tissue acidosis. However, so far, nothing is known about the sensitivity to hydrogen ions of slowly conducting muscle afferent units. The study investigated if acid-sensing ion channels, e.g. the polymodal acid/capsaicin-sensitive vanilloid receptors, were present on unmyelinated (group IV) muscle afferent units of the gastrocnemius-soleus muscle of the rat. Intramuscular injections of acidic phosphate buffer pH 6 excited 56.0% of the group IV units. A similar proportion (54.29%) was activated by capsaicin (655 microM). Tests of the same unit with both adenosine triphosphate at neutral pH (ATP, 7.6 mM; pH 7.4) and acidic phosphate demonstrated that most acid-sensitive units were also excited by ATP at neutral pH. The data show that (1) a high proportion of group IV muscle receptors are responsive to an increased extracellular hydrogen ion concentration, and (2) a subpopulation of these units are sensitive to both acidic pH and ATP.

Experimentally induced ischaemic pain in healthy humans is attenuated by the adenosine receptor antagonist theophylline

AIMS

Endogenous adenosine is considered a prominent pain mediator in ischaemia. In contrast, it has been shown that exogenous adenosine can reduce tourniquet induced ischaemic pain in healthy volunteers. The aim of this study was to investigate if pharmacological antagonism of endogenous adenosine actions with an intravenous infusion of theophylline could attenuate experimentally induced ischaemic pain.

METHODS

Nineteen healthy volunteers, 11 males, eight female, received theophylline 7 mg kg-1 or placebo intravenously, in a randomized, double blind and crossover fashion, prior to a sub-maximum effort forearm tourniquet test. Experiments were carried out with 1-week intervals to avoid pre-conditioning. Pain scores [visual analogue scale (VAS), 0-100] were assessed every minute up to a maximum of 30 min.

RESULTS

The sum of pain scores (accumulation of VAS scores) was attenuated by theophylline, 691 [200-1550 (median and 25-75% percentiles)], compared with placebo, 1231 (545-1675), P < 0.001. Also, peak VAS pain was lower during theophylline treatment, 48 +/- 38 (mean +/- SD), compared with placebo, 74 +/- 27, P < 0.001. Blood pressure increased during the experiment with no difference between treatments. Heart rate was not affected by tourniquet or drug treatment.

CONCLUSIONS

It is concluded that the adenosine receptor antagonist theophylline is able to attenuate the development of ischaemia pain during experimental ischaemia in humans. This implies a role for adenosine as both facilitatory mediator and a modulator of ischaemia skeletal muscle pain.

Endothelins contribute towards nociception induced by antigen in ovalbumin-sensitised mice

1. The contribution of endogenous endothelins to nociceptive responses elicited by ovalbumin (OVA) in the hind-paw of mice sensitised to this antigen (50 microg OVA+5 mg Al(OH)(3), s.c., 14 days beforehand) was investigated. 2. Sensitised mice exhibited greater nocifensive responsiveness to intraplantar (i.pl.) OVA (total licking time over first 30 min: 85.2+/-14.6 s at 0.3 microg; 152.6+/-35.6 s at 1 microg) than nonsensitised animals (29.3+/-7.4 s at 1 microg). Nocifensive responses of sensitised mice to 0.3 microg OVA were inhibited by morphine (3 mg kg(-1), s.c.) or local depletion of mast cells (four daily i.pl. injections of compound 48/80). 3. Pretreatment with i.v. bosentan (mixed ET(A)/ET(B) receptor antagonist; 52 micromol kg(-1)) or A-122722.5 (selective ET(A) receptor antagonist; 6 micromol kg(-1)) reduced OVA-induced licking from 124.8+/-20.6 s to 45.7+/-13.0 s and 64.2+/-12.1 s, respectively, whereas A-192621.1 (selective ET(B) receptor antagonist; 25 micromol kg(-1)) enhanced them to 259.2+/-39.6 s. 4. Local i.pl. pretreatment with BQ-123 or BQ-788 (selective ET(A) or ET(B) receptor antagonists, respectively, each at 3 nmol) reduced OVA-induced licking (from 106.2+/-15.2 to 57.0+/-9.4 s and from 118.6+/-10.5 to 76.8+/-14.7 s, respectively). Sarafotoxin S6c (selective ETB receptor agonist, 30 pmol, i.pl., 30 min after OVA) induced nocifensive responses in OVA-sensitised, but not in nonsensitised, animals. 5. Compound 48/80 (0.3 microg, i.pl.) induced nocifensive responses per se and potentiated those induced by i.pl. capsaicin (0.1 microg). Treatment with BQ-123 (3 nmol, i.pl.) reduced only the hyperalgesic effect of compound 48/80, whereas BQ-788 (3 nmol) was ineffective. 6. Thus, immune-mediated Type I hypersensitivity reactions elicit mast cell- and endothelin-dependent nociception in the mouse hind-paw, which are mediated locally by both ET(A) and ET(B) receptors. The nocifensive response to antigen is amenable to blockade by systemic treatment with dual ET(A)/ET(B) or selective ET(A) receptor antagonists, but is sharply potentiated by systemic selective ET(B) receptor antagonist treatment. The apparently distinct roles played by ET(B) receptors in this phenomenon at local and other sites remain to be characterised.

Induction and assessment of muscle pain, referred pain, and muscular hyperalgesia

Muscle pain can be induced and assessed experimentally by a variety of methods. Ischemic and exercise-induced muscle pain are typical endogenous pain models; external stimulation with mechanical, electrical, and chemical modalities constitute the exogenous models. These models are a good basis to study the muscle sensitivity, muscle pain responses under normal and pathophysiologic conditions, and drug efficacy on specific muscle pain mechanisms. When evaluating muscle pain in clinical or experimental settings, it is important to assess parameters related to the pain intensity, pain quality, referred and local distribution, and the deep tissue sensitivity in local and referred areas. The experimental test paradigm must include different stimulation modalities (multimodal) to obtain sufficiently advanced and differentiated information about the human nociceptive system under normal and pathophysiologic conditions because the different stimuli activate different receptors, pathways, and mechanisms. This may be a useful approach in future mechanism-based classification and treatment of muscle pain. Similarly, the multimodal approach is important in clinical studies to provide evidence for which specific muscle pain modalities and mechanisms are affected and how they are modulated by pharmacologic approaches.