Long-term changes in discharge behaviour of cat dorsal horn neurones following noxious stimulation of deep tissues

Does Threat Enlarge Nociceptive Reflex Receptive Fields?

Threat-induced pain modulation can increase survival by amplifying physiological and behavioral reactions toward danger. Threat can also modulate spinal nociception, suggesting engagement of endogenous top-down circuitry. A unique method to assess spinal nociception is via reflex receptive fields (RRF) associated with the nociceptive withdrawal reflex (NWR, a protective spinally-mediated reflex). The size of nociceptive RRFs can be modulated by top-down circuitry with the enlargement of RRFs related to increased spinal nociception. Threat has been previously shown to enhance pain and spinal nociception, but the relationship between threat and RRFs has not been investigated. The present study investigated this issue in 25 healthy individuals. RRFs were determined from NWRs measured by electromyography of the tibialis anterior following electrocutaneous stimulations. RRFs and pain were assessed during periods in which participants were under threat of unpredictable painful abdominal stimulations and when they were not under threat. Results indicated that threat periods led to significantly higher pain, larger nociceptive RRFs and NWR magnitudes. These findings imply that threat produces changes in protective reflexes related to spinal nociceptive sensitivity and increased pain perception. This is likely mediated by top-down circuitry that enhances dorsal horn nociceptive neurons by enlarging RRFs and amplifying ascending pain signals. PERSPECTIVE: This article presents the enlargement of RRF during periods of threat. The results from this study may help clarify the mechanism underlining emotional modulation of spinal nociception.

Ultrasound-guided continuous block of median and ulnar nerves in horses: development of the technique

OBJECTIVE

To develop a technique for ultrasound-guided continuous median and ulnar peripheral nerve block in horses.

STUDY DESIGN

Anatomical and prospective experimental study.

ANIMALS

A total of 16 thoracic limbs from horse cadavers and 18 adult horses.

METHOD

This study was conducted in three phases. Phase 1: Dissection of median and ulnar nerves in the antebrachial region of two cadaver limbs to identify localizing landmarks. Description of sonoanatomy in 14 cadaver limbs using ultrasound-guided perineural infiltration of a combination of cellulose gel (5 mL), contrast medium (4 mL) and methylene blue (1 mL). Catheters were inserted between the perineural sheath and epineurium in six limbs, followed by computed tomography. Phase 2: Ultrasonographic images of the limbs of 18 healthy horses of different breeds were used to define an acoustic window and optimize the approach to nerves. Phase 3: Two case reports of horses with chronic pain of different etiologies. Catheters were inserted between the epineurium and paraneural sheath of the median and/or ulnar nerves guided by ultrasound, followed by continuous infusion of 0.4% ropivacaine.

RESULTS

Information from phase 1 was used to direct needle insertion, solution dispersion and catheter implantation in phase 2, which resulted in 100% technique accuracy. In response to the peripheral nerve block, pain reduction was apparent in the two clinical cases by increased weight bearing in affected limbs and decreased requirement for systemic analgesic medications. No local reactions were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

The ultrasound technique allowed real-time visualization of needle, catheter and drug dispersion and resulted in a high success rate for nerve blocks. The horses administered a median and ulnar nerve block exhibited no discomfort or signs of infection at the catheter insertion site. Further studies are warranted to validate the efficacy of this technique.

Scratching the surface: the processing of pain from deep tissues

Although most pain research focuses on skin, muscles, joints and viscerae are major sources of pain. We discuss the mechanisms of deep pains arising from somatic and visceral structures and how this can lead to widespread manifestations and chronification. We include how both altered peripheral and central sensory neurotransmission lead to deep pain states and comment on key areas such as top-down modulation where little is known. It is vital that the clinical characterization of deep pain in patients is improved to allow for back translation to preclinical models so that the missing links can be ascertained. The contribution of deeper somatic and visceral tissues to various chronic pain syndromes is common but there is much we need to know.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Quantification of local and referred pain in humans induced by intramuscular electrical stimulation

The basic knowledge related to referred muscle pain is limited. To study referred pain, an experimental model using intramuscular electrical stimulation has been developed. Four experiments were performed: (1) the thresholds for eliciting local (LPT) and referred pain (RPT) were determined; (2) stimulus-response functions relating stimulus intensity, pain intensity ratings and size of pain areas were determined; (3) inter- and intrasession variabilities were assessed; and (4) prolonged stimulations were given with a duration of 10 min to evaluate temporal aspects of the referred muscle pain. Intramuscular electrical stimulation of the tibialis anterior muscle elicited pain at the stimulation site in 94% of the subjects, and referred pain in 78% of the subjects. Referred pain was located in the anterior part of the ankle. The mean RPT was 72% higher than the mean LPT (p<0.01). Correlation was found between stimulus intensity, sensory/pain rating scores and size of pain areas (0.74< or =r< or =0.98,p<0.04). Size of pain areas and sensation/pain rating scores were correlated (0.86< or =r< or =0.97, p<0.01). Intersession variability showed that the LPTs were not significantly different (p>0.16), but the RPTs were disparate (p<0.02). Intrasession values revealed a significant difference between the five LPTs, RPTs, local and referred pain rating scores. The size of the local and referred pain areas remained constant. Prolonged stimulation at 150% of RPT showed that the onset (the first occurrence of pain) of referred pain occurred significantly later (43 s +/- 80 s) than at the local pain site (p<0.03). This study showed that local and referred muscle pain can be elicited by intramuscular electrical stimulation, and indicated that temporal and spatial summation may be involved in the elicitation of referred muscle pain.

Noxious Lingual Stimulation Influences the Excitability of the Face Primary Motor Cerebral Cortex (Face MI) in the Rat

The mechanisms whereby orofacial pain affects motor function are poorly understood. The aims were to determine whether 1) lingual algesic chemical stimulation affected face primary motor cerebral cortex (face MI) excitability defined by intracortical microstimulation (ICMS); and 2) any such effects were limited to the motor efferent MI zones driving muscles in the vicinity of the noxious stimulus. Ketamine-anesthetized Sprague–Dawley male rats were implanted with electromyographic (EMG) electrodes into anterior digastric, masseter, and genioglossus muscles. In 38 rats, three microelectrodes were located in left face MI at ICMS-defined sites for evoking digastric and/or genioglossus responses. ICMS thresholds for evoking EMG activity from each site were determined every 15 min for 1 h, then the right anterior tongue was infused (20 μl, 120 μl/h) with glutamate (1.0 M, n = 18) or isotonic saline ( n = 7). Subsequently, ICMS thresholds were determined every 15 min for 4 h. In intact control rats ( n = 13), ICMS thresholds were recorded over 5 h. Only left and right genioglossus ICMS thresholds were significantly increased (≤350%) in the glutamate infusion group compared with intact and isotonic saline groups ( P < 0.05). These dramatic effects of glutamate on ICMS-evoked genioglossus activity contrast with its weak effects only on right genioglossus activity evoked from the internal capsule or hypoglossal nucleus. This is the first documentation that intraoral noxious stimulation results in prolonged neuroplastic changes manifested as a decrease in face MI excitability. These changes appear to occur predominantly in those parts of face MI that provide motor output to the orofacial region receiving the noxious stimulation.

Prevalence and characteristics of allodynia in headache sufferers: a population study

OBJECTIVE

The authors estimated the prevalence and severity of cutaneous allodynia (CA) in individuals with primary headaches from the general population.

METHODS

We mailed questionnaires to a random sample of 24,000 headache sufferers previously identified from the population. The questionnaire included the validated Allodynia Symptom Checklist (ASC) as well as measures of headache features, disability, and comorbidities. We modeled allodynia as an outcome using headache diagnosis, frequency and severity of headaches, and disability as predictor variables in logistic regression. Covariates included demographic variables, comorbidities, use of preventive medication, and use of opioids.

RESULTS

Complete surveys were returned by 16,573 individuals. The prevalence of CA of any severity (ASC score >or=3) varied with headache type. Prevalence was significantly higher in transformed migraine (TM, 68.3%) than in episodic migraine (63.2%, p < 0.01) and significantly elevated in both of these groups compared with probable migraine (42.6%), other chronic daily headaches (36.8%), and severe episodic tension-type headache (36.7%). The prevalence of severe CA (ASC score >or=9) was also highest in TM (28.5%) followed by migraine (20.4%), probable migraine (12.3%), other chronic daily headaches (6.2%), and severe episodic tension-type headache (5.1%). In the migraine and TM groups, prevalence of CA was higher in women and increased with disability score. Among migraineurs, CA increased with headache frequency and body mass index. In all groups, ASC scores were higher in individuals with major depression.

CONCLUSIONS

Cutaneous allodynia (CA) is more common and more severe in transformed migraine and migraine than in other primary headaches. Among migraineurs, CA is associated with female sex, headache frequency, increased body mass index, disability, and depression.

Neurochemical Characterization of the TRPV1-Positive Nociceptive Primary Afferents Innervating Skeletal Muscles in the Rats

OBJECTIVE

Transient receptor potential vanilloid subfamily type 1 (TRPV1), a most specific marker of the nociceptive primary afferent, is expressed in peptidergic and non-pepetidergic primary afferents innervating skin and viscera. However, its expression in sensory fibers to skeletal muscle is not well known. In this study, we studied the neurochemical characteristics of TRPV1-positive primary afferents to skeletal muscles.

METHODS

Sprague-Dawley rats were injected with total 20 microl of 1% fast blue (FB) into the gastrocnemius and erector spinae muscle and animals were perfused 4 days after injection. FB-positive cells were traced in the L4-L5 (for gastrocnemius muscle) and L2-L4 (for erector spinae muscle) dorsal root ganglia. The neurochemical characteristics of the muscle afferents were studied with multiple immunofluorescence with TRPV1, calcitonin gene-related peptide (CGRP) and P2X(3). To identify spinal neurons responding to noxious stimulus to the skeletal muscle, 10% acetic acids were injected into the gastrocnemius and erector spinae muscles and expression of phospho extracellular signal-regulated kinase (pERK) in spinal cords were identified with immunohistochemical method.

RESULTS

TRPV1 was expressed in about 49% of muscle afferents traced from gastrocnemius and 40% of erector spinae. Sixty-five to 60% of TRPV1-positive muscles afferents also expressed CGRP. In contrast, expression of P2X(3) immnoreaction in TRPV1-positive muscle afferents were about 20%. TRPV1-positive primary afferents were contacted with spinal neurons expressing pERK after injection of acetic acid into the muscles.

CONCLUSION

It is consequently suggested that nociception from skeletal muscles are mediated by TRPV1-positive primary afferents and majority of them are also peptidergic.

Chronic pain

Physiatrists frequently see patients who have chronic pain, and the physiatric approach is highly relevant to pain management. This article is directed toward physiatrists who do not specialize in pain management. It discusses the epidemiology of pain in patient groups often treated by physiatrists, pathophysiologic processes underlying chronic pain, the assessment of chronic pain patients, and selected treatments for chronic pain.

Changes in the recruitment curve of the soleus H-reflex associated with chronic low back pain

OBJECTIVE

We investigated whether patients with chronic low back pain (CLBP) manifest changes in the excitability of the soleus H-reflex.

METHODS

H-reflex stimulus-response curve was studied in 14 CLBP patients and 14 age-matched healthy subjects. H-threshold, H-maximum size, H-steepness and H-latency were determined for both legs. Homosynaptic depression (HD), following a train of H-reflexes, and presynaptic inhibition (PI) from flexor afferents onto soleus Ia afferents were also evaluated.

RESULTS

H-threshold was significantly increased, H-size as a function of stimulus intensity was significantly different, and H-recruitment curve steepness was significantly lower in CLBP patients compared to healthy subjects. No significant difference in the amount of HD and PI of the H-reflex was found between the two groups. H-latency and Hmax/Mmax ratio was comparable between the subjects groups.

CONCLUSIONS

In CLBP there is a reduced excitability of group Ia afferent fibres from the soleus muscle to which presynaptic factors do not seem to contribute and that presumably depend on changes in the peripheral sensory input.

SIGNIFICANCE

Changes in H-reflex excitability may underlie a decrease in the gain of a peripheral sensor in CLBP. Estimation of soleus H-threshold and H-recruitment curve may contribute to the diagnostic evaluation of CLBP and may be used to monitor the efficacy of treatment.

Mechanisms underlying development of spatially distributed chronic pain (fibromyalgia)

Chronic fibromyalgia (FM) pain is prevalent (estimated as high as 13%), predominantly affects women, and is associated with a variety of focal pain conditions. Ongoing FM pain is referred to deep tissues and is described as widespread but usually is maximally located within a restricted region such as the shoulders. Palpation of deep tissues reveals an enhanced nociceptive sensitivity that is not restricted to regions of clinical pain. Similarly, psychophysical testing reveals allodynia and hyperalgesia for cutaneous stimulation at locations beyond regions of clinical pain referral. The combination of widely distributed clinical pain and generalized hypersensitivity is highly disabling, but no satisfactory treatment is regularly prescribed. A thorough understanding of mechanisms will likely be required to develop and document adequate therapies. The generalized hypersensitivity associated with FM has focused considerable interest on central (CNS) mechanisms for the disorder. These include central sensitization, central disinhibition and a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis. However, the central effects associated with FM can be produced by a peripheral source of pain. Chronic nociceptive input induces central sensitization, magnifying pain, and it activates the HPA and the sympathetic nervous system. Chronic sympathetic activation indirectly sensitizes peripheral nociceptors and sets up a vicious cycle. Thus, it appears that central mechanisms of FM pain are dependent on abnormal peripheral input(s) for development and maintenance of this condition. A substantial literature defines peripheral-CNS-peripheral interactions that are integral to FM pain. These reciprocal actions and related phenomena of relevance to FM pain are reviewed here, leading to suggestions for testing of therapeutic approaches.

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.

Characterization of a method for measuring primary hyperalgesia of deep somatic tissue

Measuring primary hyperalgesia from deep somatic tissue (ie, muscle and joint) is difficult in laboratory animals but clinically important. In this study, we modified a newly developed method to measure primary hyperalgesia of muscle in rats and compared this with primary hyperalgesia from the knee. Compression withdrawal thresholds of the gastrocnemius muscle or the knee joint were measured with a device consisting of strain gauges attached to forceps. Compression of the muscle or joint with the forceps results in hind limb withdrawal, and thresholds were measured before and 4 hours after induction of inflammation by 3% carrageenan injected into the gastrocnemius muscle or 3% kaolin-carrageenan injected into the knee joint. Compression thresholds were significantly decreased 4 hours after induction of inflammation in the muscle or knee joint compared with thresholds before inflammation. Surprisingly, in animals with muscle inflammation, compression thresholds were also significantly decreased on the contralateral hind limb. Systemic morphine (5 mg/kg, intraperitoneal) or lidocaine (2%) injected into the inflamed tissue reversed the decreased compression threshold induced by deep tissue inflammation. However, local anesthetic applied to the skin overlying the muscle or knee joint did not affect the decreased threshold. Thus, we report a consistent and convenient method to measure primary hyperalgesia in deep tissues of rats. The measured hyperalgesia originates in the inflamed tissues and has no measurable contribution from skin.

PERSPECTIVE

The current method measures primary hyperalgesia directly from injured deep somatic tissues. Thus it is relevant to painful clinical conditions that are aggravated by mechanical pressure at the site of injury. As such, it might serve as a model for basic mechanistic studies as well as drug discovery for musculoskeletal pain syndromes.

Reduced cold pressor pain tolerance in non-recovered whiplash patients: a 1-year prospective study

Whiplash injury and chronic whiplash syndrome represent major health problems in certain western communities, pain being the main symptom. Sensitization of the nociceptive system may play a role for non-recovery after whiplash injury.

AIMS

This study examined if tolerance to endure pain stimuli may predict outcome in whiplash injury. In a prospective fashion, 141 acute whiplash patients exposed to rear-end car collision (WAD grade 1-3) and 40 ankle-injured controls were followed and exposed to a cold pressor test, respectively, 1 week, 1, 3, 6 and 12 months after the injury. VAS score of pain and discomfort was obtained before, during and after immersion of the dominant hand into cold water for 2 min. The McGill Pain Questionnaire showed that ankle-injured controls had higher initial pain scores than the corresponding whiplash group, while whiplash-injured subjects had higher scores at 6 months; pain scores being similar at other time points. No difference was found in cold pressor pain between recovered whiplash patients and ankle-injured subjects. Non-recovery was only encountered in whiplash injury. Eleven non-recovered whiplash patients (defined as: handicap after 1 year) showed reduced time to peak pain from 1 week to 3 months (P<0.001), 6 months (P<0.01), but not 12 months after the injury. A larger pain area was seen in non-recovered vs. recovered whiplash-injured subjects during the entire observation period (P<0.001). Non-recovery after whiplash was associated with initially reduced cold pressor pain endurance and increased peak pain, suggesting that dysfunction of central pain modulating control systems plays a role in chronic pain after acute whiplash injury.

Innocuous jaw movements increase c-fos expression in trigeminal sensory nuclei produced by masseter muscle inflammation

Muscle tenderness and pain during movements are prominent symptoms associated with persistent jaw muscle pain. However, there is virtually no information on how trigeminal neurons respond to jaw movements (JM) or muscle palpation in the presence of muscle tissue injury or myositis. In this study, we investigated the effects of innocuous JM in the presence of acute masseteric inflammation on postsynaptic responses in the trigeminal brainstem nuclei by examining the expression of c-fos. In one group of rats, unilateral injections of an inflammatory substance, mustard oil (MO: 20%, 25 microl) were made into a masseter muscle. In another group, controlled and systematic JM were provided following MO injection. Three additional groups of rats were used to control for anesthetic, JM, and injection procedure. MO injected in the masseter muscle induced a high level of Fos protein expression in four principal trigeminal regions: the subnucleus caudalis (Vc), the ventral and dorsal regions of the Vc/Vi (subnucleus interpolaris) transition zone, and the paratrigeminal nucleus (PTN). Movements following MO injection consistently produced a significantly greater level of Fos expression in all these areas, especially in the Vc/Vi transition region and caudal Vc on the ipsilateral side. Importantly, movements also induced a significantly greater level of Fos expression in the caudal Vc on the contralateral side. The present results provide the first documentation that innocuous JM in the presence of muscle inflammation significantly increase the MO-induced c-fos expression in the trigeminal brainstem nuclei, which may explain the greater pain experienced during movement of inflamed or injured muscles.

Physiology of chronic spinal pain syndromes: from animal models to biomechanics

STUDY DESIGN

The literature and current research related to spinal pain mechanisms were reviewed, as well as animal models related to its study.

OBJECTIVES

To provide a pragmatic discussion of spinal pain that both reviews relevant research and coherently synthesizes the existing body of literature related to pain, nociception, animal modeling, and injury biomechanics.

SUMMARY OF BACKGROUND DATA

A detailed body of literature suggests that spinal pain mechanisms are quite complicated and involve a host of different processes (e.g., genetics, gender, neurophysiology, and biomechanics) that may contribute to clinical manifestations and symptoms.

METHODS

Both a review of the literature and a presentation of current and ongoing laboratory research are presented. Specific findings from the authors' laboratory using a rodent model of lumbar radiculopathy are presented to elucidate the role of local nerve root biomechanics in initiating and maintaining behavioral symptoms of nociception and pain.

RESULTS

For an understanding of chronic pain, a bidirectional-translational approach that incorporates cross-disciplinary methods such as in vivo biomechanical techniques is required. A conceptual model of chronic spine pain is proposed that details the dynamic and integrated roles of injury, biomechanics, and nociceptive physiology.

CONCLUSIONS

Areas of continued research are highlighted that may help guide the management of painful spine symptoms and syndromes.

Effects of localization and intensity of experimental muscle pain on ankle joint proprioception

Accurate proprioceptive input is a prerequisite for balance control and coordination of movement. The present study investigated whether experimental muscle pain induced in healthy human subjects disturbed movement sense (detection of movement) or position sense (recognition of a reference position). Muscle pain was produced by infusion of 6% hypertonic saline simultaneously in m. tibialis anterior (TA) and m. soleus (experiment 1), by infusion of 6% hypertonic saline in TA (experiment 2) and by infusion of 9% hypertonic saline in TA (experiment 3). Control measurements were done with infusions of 0.9% isotonic saline. All infusions of 6% and 9% saline produced pain intensities significantly higher than the corresponding control infusions. Only infusion of 6% saline in two muscles (visual analogue scale=4-5) produced an elevation in movement detection thresholds which was significantly higher, compared with before infusion. No other significant changes in movement and position sense were found during the painful or control infusions. Pain of relatively high intensity in two antagonist muscles is necessary to disturb the movement detection threshold. The ability to recognize a reference position is not disturbed by experimentally induced muscle pain. Whether the disturbed movement sense is caused by sensitivity changes in muscle spindle afferents or altered processing of proprioceptive input cannot be answered. The present findings indicate that human ankle proprioception is rather robust to muscle pain.

The whole body receptive field of dorsal horn multireceptive neurones

Multireceptive neurones are found in the spinal dorsal horn and may be projection neurones and/or interneurones for polysynaptic reflexes. The cutaneous receptive field of a multireceptive neurone exhibits a gradient of sensitivity with the centre responding to any mechanical stimulus, including hair movements and light touch, while the periphery responds only to noxious stimuli. These neurones also receive signals from viscera, muscles and joints. This convergence of inputs means that multireceptive neurones are continuously capturing all the information from both the interface with the external environment (the skin) and the internal milieu (the viscera, muscles, etc.). This information constitutes a 'basic somaesthetic activity' that could help the somatosensory system build a 'global representation of the body'. In addition to be seen as a global entity, the output of multireceptive neurones should be understood in dynamic terms since the size of the peripheral fields of the individual neurones may change, as a result of the plasticity of both excitatory and inhibitory segmental processes. Furthermore, the activity of these neurones can be inhibited from most of the remaining parts of the body via supraspinal mechanisms. These diffuse noxious inhibitory controls (DNIC) are triggered by peripheral A delta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. A painful focus that both activates a segmental subset of neurones and inhibits the remaining population can seriously disrupt this basic activity, resulting in the distortion of the body representation in favour of the painful focus, which becomes pre-eminent and (relatively) oversized.

Regulation of afferent connectivity in the adult spinal cord by nerve growth factor

During development, nerve growth factor (NGF) regulates the density and character of peripheral target innervation (Barde, Neuron, 2, 1525 - 1534, 1989; Ritter et al., Soc. Neurosci. Abstr., 17, 546.2, 1991); its role in adult animals is less well defined. Here we have asked if the availability of growth factors such as NGF in peripheral tissues can influence the pattern of primary afferent connections in the CNS. Using osmotic minipumps, we raised the levels of NGF in rat skeletal muscle in vivo, a tissue where the levels of this factor are normally very low (Korsching and Thoenen, Proc. Natl. Acad. Sci. USA, 80, 3513 - 3516, 1983; Shelton and Reichardt, Proc. Natl. Acad. Sci. USA, 81, 7951 - 7955, 1984; Goedert et al., Mol. Brain Res., 1, 85 - 92, 1986). After 2 weeks of treatment we asked if the sensory neurons innervating this tissue showed an altered strength and distribution of connections with dorsal horn neurons. The contralateral (vehicle-treated) muscle, and totally untreated animals, served as controls. In normal and vehicle-treated animals, electrical stimulation of muscle afferents excited relatively few neurons in the dorsal horn, and these generally showed only weak responses. In contrast, on the NGF-treated side many more dorsal horn neurons in the lumbar enlargement of the spinal cord were excited by muscle afferents. The increased responsiveness could not be explained by a generalized increase in dorsal horn excitability, since spontaneous activity was not enhanced, nor by a change in A-fibre-mediated inhibitions from the treated afferents. Thus, these afferents appeared to establish new synaptic connections or strengthened previously weak ones as a result of increased neurotrophic factor availability. The data suggest that, in the adult rat, the levels of growth factors in peripheral targets may be used to regulate an appropriate degree of afferent connectivity within the central nervous system.

Effect of plantar local anesthetic injection on dorsal horn neuron activity and pain behaviors caused by incision

Hypersensitivity after tissue injury is an expression of neuronal plasticity in the central nervous system. This has been explored most extensively using in vitro preparations and animal models of inflammatory pain and chemical irritation. For pain after surgery, a similar process has been proposed. In the present study, we examined dorsal horn neuron (DHN) sensitization using the plantar incision model for post-operative pain. In behavioral experiments, the effect of a local anesthetic injection (or saline vehicle) 15 min before plantar incision on pain behaviors several days after incision was studied. Bupivacaine injection before incision prevented pain behaviors until 4 h afterwards; injection after incision produced the same effect. One day after incision, pain behaviors were not different between rats injected with saline or bupivacaine. In neurophysiologic experiments, however, bupivacaine injection blocked activation of DHNs during incision. One hour after incision, expansion of receptive fields (RFs) to pinch and increased background activity occurred in 14 of 16 neurons in the saline group but only in two of 22 neurons in the bupivacaine group. The difference was not due to a systemic effect of bupivacaine. Ten sensitized neurons were studied using the injection of bupivacaine 90 min after incision. Increased background activity (n=7) and expanded RFs (n=7) were reversed by bupivacaine. Sensitization was re-established in seven of eight neurons 2 h after injection as the local anesthetic dissipated. These results indicate that activation of DHNs during plantar incision and sensitization 1 h later are not necessary for subsequent pain behaviors. Because sensitization was reversed 90 min after plantar incision and then re-established as the local anesthetic effect diminished, enhanced responsiveness of DHN requires ongoing afferent input during the first day after incision.

c-fos Expression and NADPH-d reactivity in spinal neurons after fatiguing stimulation of hindlimb muscles in the rat

The distribution of Fos-immunoreactive (Fos-ir) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-reactive neurons in the rat lumbar spinal cord was examined following muscle fatigue caused by intermittent high-rate (100 s(-1)) electrical stimulation of the triceps surae muscle or the ventral root L5 (VRL5) for 30 min. Following both types of stimulation, the fatigue-related c-fos gene expression was more extensive in the L2-L5 segments on the stimulated side, and the majority of Fos-ir neurons were concentrated in the dorsal horn. After direct muscle stimulation, the highest number of Fos-ir neurons were detected in two regions: layer 5, and superficial layers (1 and 2(o)), although many labeled cells were also found in layers 3, 4, 6, and 7. In response to VRL5 stimulation, the maximal density of Fos-ir neurons was detected in the middle and lateral parts of layers 1 and 2(o), the zone of termination of high-threshold muscle afferents(.) Statistically significant prevalence of Fos-ir cell number was also found in layers 5 and 7 on the stimulated side. A few Fos-ir neurons were detected in the ventral horn (layer 8 and area 10) on both sides. The lamellar distribution of NADPH-d-reactive neurons was similar over all experimental groups of animals. In the L3-L6 segments, such reactive cells were arranged in two distinct regions: dorsal horn (layers 2(i), 3, and 5) and area 10; in the L1 and L2 segments, an additional cluster of NADPH-d positive cells was found in the intermediolateral cell column (IML). Double-labeled cells were not detected. We suggest that c-fos expression in response to muscle fatigue reveals activity of functionally different types of spinal neurons which could operate together with NOS-containing cells in pre-motoneuronal networks to modulate the motoneuron output.

Intensity coding by TMJ-responsive neurons in superficial laminae of caudal medullary dorsal horn of the rat

Temporomandibular disorders (TMD) represent a family of recurrent conditions that often cause pain in the temporomandibular joint (TMJ) region and muscles of mastication. To determine if TMJ-responsive neurons encoded the intensity of pro-inflammatory chemical signals, dose-effect relationships were assessed after direct injection bradykinin into the joint space and compared with responses after injection of glutamate or saline. Neurons were recorded from superficial laminae of the trigeminal subnucleus caudalis/upper cervical cord junction region (Vc/C(2)) and identified by palpation of the TMJ region in barbiturate-anesthetized male rats. The majority (62 of 84) of units received convergent input from facial skin, while 26% were driven only by deep input from the TMJ region. Conduction-velocity based on the latency to firing after electrical stimulation of the TMJ region indicated 64% of units were driven by A-delta fiber input only. Bradykinin (0.1-10 microM) excited 69% of neurons tested, and 70% (19 of 27) of these units were activated by the lowest dose (0.1 microM). Glutamate (50-200 mM) excited 27% of units; however, when tested after bradykinin, 58% of units were activated by glutamate. Some TMJ units (17%) were excited by saline injection alone and not enhanced further by bradykinin or glutamate. Most (88%) TMJ units were activated by injection of the small fiber excitant, mustard oil (20% solution), into the TMJ region. Units responsive to bradykinin or glutamate were not restricted to particular classes [e.g., wide dynamic range (WDR), nociceptive specific (NS), deep only]. A small percentage of TMJ units (approximately 15%) were activated antidromically from the contralateral posterior thalamus. In parallel studies using c-fos immunocytochemistry, bradykinin (1 microM) injection into the TMJ region produced a greater number of Fos-positive neurons at the Vc/C(2) region than glutamate (200 mM) or saline. These results revealed two broad classes of TMJ units that encoded the intensity of pro-inflammatory chemical stimuli applied to the TMJ region, units that received convergent nociceptive input from facial skin (i.e., WDR and NS units) and units that responded only to deep input from the TMJ region. On the basis of encoding properties and efferent projection status, it is concluded that activation of TMJ units within the superficial laminae at the Vc/C(2) region contribute to the diffuse and spreading nature of TMD pain sensation.

Non-noxious A fiber afferent input enhances capsaicin-induced mechanical hyperalgesia in the rat

Intradermal injection of capsaicin induces primary hyperalgesia at the injection site and secondary hyperalgesia in the surrounding undamaged skin. The secondary hyperalgesia is thought to be due to central sensitization of the dorsal horn neurons while primary hyperalgesia is caused by sensitization of nociceptors in the damaged skin. In this study, we asked if additional non-noxious afferent input from the undamaged skin influences the already developed secondary hyperalgesia, which follows an intradermal injection of capsaicin. Capsaicin dissolved in olive oil was injected into the middle of the hind paw of male Sprague-Dawley rats (250-300 g) under gaseous anesthesia. This produced a decrease in the mechanical threshold at the base of the toes for hind limb withdrawals lasting for 1-2h, thus showing a short-lasting (hours) secondary hyperalgesia. When the capsaicin injection was immediately followed by repeated non-noxious mechanical stimuli or weak electrical stimuli (A fiber strength) applied to the area of secondary hyperalgesia (toes) for 30 min, the reduction of the mechanical threshold lasted longer than 24h. These results suggest that non-noxious A fiber afferent input can powerfully modulate central sensitization in the spinal dorsal horn, causing the duration of the secondary hyperalgesia to be greatly extended.

Central pain mechanisms: A new horizon in manual therapy

Central pain mechanisms are deeply embodied in the psychophysical problem of pain. They are located in the brain and spinal cord and are becoming increasingly recognised as playing a major role in the generation and maintenance of pain and disability associated with neuromusculoskeletal problems. Central mechanisms participate in all pain states, acute and chronic. They are universally influenced by psychological and physical factors, whether or not a specific pathology can be identified. Misconceptions are that manual therapy operates on peripheral mechanisms without influencing the central ones and that when a central problem exists, psychological management is preferable. In reality, as key players in the healing process, central mechanisms are profoundly affected by manual therapy even when it is directed at a peripheral problem. Treatment of peripheral mechanisms can be performed through central techniques because both peripheral and central mechanisms are always part of the same clinical problem. Consequently, manual therapy must change its mindset from a peripheral standpoint and integrate central mechanisms into clinical practice as a means of improving therapeutic efficacy and to prevent the descent of acute pain into chronic.

Inhibition of motor system excitability at cortical and spinal level by tonic muscle pain

OBJECTIVE

To assess whether the motor system excitability can be modified by experimental tonic pain induced either in muscles or in subcutis.

METHODS

Transcranial magnetic stimulation of the left primary motor cortex was used to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) muscle. Recordings were made before, during and after experimental pain induced by (1) injection of hypertonic (5%) saline into the right ADM, the right first dorsal interosseum (FDI) and the left ADM muscles, and (2) injection of hypertonic saline in the subcutaneous region of the right ADM. Both MEPs and H-reflex were recorded also from the right flexor carpi radialis (FCR) before, during and after muscle pain.

RESULTS

MEPs recorded from the ADM muscle were significantly reduced in amplitude during pain induced in the right ADM and right FDI muscles, but not during pain in the left ADM muscle or during subcutaneous pain. This inhibitory effect was observed during the peak-pain and persisted also after the disappearance of the pain sensation. In the FCR muscle, the MEP inhibition was observed during the peak-pain, while a significant reduction of the H-reflex's amplitude was observed starting 1 min after the peak-pain.

CONCLUSIONS

Tonic muscle pain can inhibit the motor system. The motor cortex inhibition observed at an early phase is followed by a reduction of the excitability of both cortical and spinal motoneurones.

Sex-related differences in human pain and rat afferent discharge evoked by injection of glutamate into the masseter muscle

Animal studies have suggested that tissue injury-related increased levels of glutamate may be involved in peripheral nociceptive mechanisms in deep craniofacial tissues. Indeed, injection of glutamate (0.1-1 M, 10 microl) into the temporomandibular region evokes reflex jaw muscle responses through activation of peripheral excitatory amino acid receptors. It has recently been found that this glutamate-evoked reflex muscle activity is significantly greater in female than male rats. However, it is not known whether peripheral administration of glutamate, in the same concentrations that evoke jaw muscle activity in rats, causes pain in humans or activates deep craniofacial nociceptive afferents. Therefore we examined whether injection of glutamate into the masseter muscle induces pain in male and female volunteers and, since masseter afferent recordings were not feasible in humans, whether glutamate excites putative nociceptive afferents supplying the masseter muscle of male and female rats. Injection of glutamate (0.5 M or 1.0 M, 0.2 ml) into the masseter muscle of both men and women caused significantly higher levels of peak pain, duration of pain, and overall pain than injection of isotonic saline (0.2 ml). In addition, glutamate-evoked peak and overall muscle pain in women was significantly greater than in men. In rats of both sexes, glutamate (10 microl, 0.5 M) evoked activity in a subpopulation of masseter muscle afferents (n = 36) that projected to the subnucleus caudalis, an important relay of noxious input from the craniofacial region. The largest responses to glutamate were recorded in muscle afferents with the slowest conduction velocities (2.5-5 m/s). Further, glutamate-evoked masseter muscle afferent activity was significantly greater in female than in male rats. These results indicate that glutamate injection into the masseter muscle evokes pain responses that are greater in women than men and that one possible mechanism for this difference may be a greater sensitivity to glutamate of masseter muscle afferents in females. These sex-related differences in acute experimental masseter muscle pain are particularly interesting given the higher prevalence of many chronic muscle pain conditions in women.

Osteoarthritis and its association with muscle hyperalgesia: an experimental controlled study

Hypertonic saline effectively excites muscle nociceptors. Muscle hyperalgesia was assessed in osteoarthritis (OA) by intramuscular infusion of 0.5 ml hypertonic saline (6%) into the tibialis anterior muscle in humans. Patients (n=14) with OA in the lower extremities were compared with an equal number of age- and sex-matched healthy controls. Ten of the 14 OA patients had pain in the knee joint as the most common presenting complaint. Visual analogue scale (VAS) pain intensity and assessment of pain areas were recorded before infusion and immediately, 2, 5, 10 and 20 min after infusion, and then every 10 min, until the pain vanished. The mean pain offset time in OA patients (11.3+/-7.9 min) was larger as compared with the control subjects (6.04+/-2.1 min) (P=0.025). OA patients had increased pain intensity VAS after the infusion in the right leg compared with controls (P<0.05). Referred and radiating pain areas at 2 min post-infusion increased in OA patients and not in controls as compared with the local pain areas (P<0.05). It is concluded that muscle hyperalgesia and extended pain areas might be due to central sensitization caused by painful osteoarthritis.

Pain thresholds and tenderness in neck and head following acute whiplash injury: a prospective study

OBJECTIVE OF THE INVESTIGATION: In a 6-month prospective study of 141 consecutive acute whiplash-injured participants, and 40 acute, ankle-injured controls, pain and tenderness in the neck/head, and at a distant control site, were measured.

BASIC PROCEDURES

Muscle palpation and pressure algometry in five head/neck muscle-pairs were performed after 1 week and 1, 3 and 6 months after injury. Algometry was performed at a distant control site.

MAIN FINDINGS

Whiplash-injured patients had lowered pressure-pain-detection thresholds and higher palpation-score initially in the neck/head, but the groups were similar after 6 months, and the control site was not sensitized.

PRINCIPAL CONCLUSION

Focal, but not generalized, sensitization to musculoskeletal structure is present until 3 months, but not 6 months, after whiplash injury, and probably does not play a major role in the development of late whiplash syndrome. Pressure algometry and palpation are useful clinical tools in the evaluation of neck and jaw pain in acute whiplash injury.

Long-lasting effect evoked by tonic muscle pain on parietal EEG activity in humans

OBJECTIVE

To explore EEG changes evoked by tonic experimental muscle pain compared to a non-painful vibratory stimulus.

METHODS

Thirty-one EEG channels were recorded before, during and after painful and non-painful stimulation. Pain was induced in the left brachioradialis muscle by injection of hypertonic (5%) saline. The vibratory stimulus was applied to the skin area overlying the brachioradialis muscle. The power of the major frequency components of the EEG activity (FFT, fast Fourier transform) was quantified and t-maps between the different experimental conditions were evaluated in frequency domain.

RESULTS

The main effect of muscle pain, compared to non-painful stimulation, was a significant and long-lasting increase of delta (1-3 Hz) power and an alpha-1 (9-11 Hz) power increase over the contralateral parietal locus. This finding could suggest a decreased excitability of the primary somatosensory cortex during muscle pain. The main effect of vibration, compared to its unstimulated baseline, consisted in an increase of beta-1 (14-20 Hz) power in the right frontal region.

CONCLUSIONS

Our data demonstrate significant and specific topographic EEG changes during tonic muscle pain. Since these modifications differ from those produced by an unstimulated baseline and during non-painful tonic stimulation, they might reflect mechanisms involved in the processing of nociceptive and adverse tonic stimuli.

The role of the motor system in spinal pain: implications for rehabilitation of the athlete following lower back pain

The purpose of this review is to consider the role of the motor system in spinal pain. It is well accepted that spinal stability is dependent on the contribution of the muscular system. However, the ability of this system to satisfy the requirements of stability is dependent on its controller--the central nervous system (CNS). The CNS must predict the outcome of movements to plan appropriate strategies of muscle activity to meet the demands of internal and external forces, and initiate appropriate responses to unexpected disturbances. In addition, this complex control of stability must occur in conjunction with control of the trunk muscles for other functions, such as respiration. For the CNS to cope with athletic performance the coordination of these parameters must be streamlined. Yet evidence suggests that when spinal pain is present the strategies used by the CNS to control trunk muscles may be altered. The mechanism for these changes is poorly understood but may be due to changes at many levels of the CNS. For rehabilitation of the athlete with spinal pain it is critical that the motor control of stability is optimised. Furthermore, this must be coordinated with the multiple other functions of trunk muscles, including respiration.

Injection of hypertonic saline into musculus infraspinatus resulted in referred pain and sensory disturbances in the ipsilateral upper arm

A confounding factor in the analysis of chronic pain patients is the finding of somatosensory disturbances not only in neuropathic pain patients, but also in a subgroup of patients with musculoskeletal pain. The purpose of the study was to examine if referred pain, induced by intramuscular injections of hypertonic saline (5% NaCl) into the left musculus infraspinatus, resulted in somatosensory alterations. Thermal sensitivity, pressure pain sensitivity, as well as low threshold mechanoreceptive function, were assessed in the referred pain area and the homologous contralateral site before, during and following the injections. In 10 out of 12 subjects the procedure induced only referred pain localized in the dorsolateral part of the ipsilateral proximal upper arm. In this referred pain area there was a significantly decreased sensitivity to light touch, as tested with von Frey filaments, during the pain period and the post-injection period compared to the contralateral side (p<0.004 and p<0.009, respectively). A trend for thermal hypoaesthesia, which was only demonstrable in the sum of warm and cold thresholds, was found in the referred pain area, but not contralaterally, during the pain period compared to the pre-injection period. Significantly increased sensitivity to threshold and suprathreshold heat pain was found bilaterally during post-injection assessments (p<0.02 and p<0.006, respectively). There were no statistically significant changes in sensitivity to innocuous thermal stimuli when assessing the two percepts separately, or to pressure pain or brush-evoked touch. In conclusion, intramuscular injections of hypertonic saline resulted in referred pain and tactile hypoaesthesia in the referred pain area.

Ketamine reduces muscle pain, temporal summation, and referred pain in fibromyalgia patients

Central mechanisms related to referred muscle pain and temporal summation of muscular nociceptive activity are facilitated in fibromyalgia syndrome (FMS) patients. The present study assessed the effects of an NMDA-antagonist (ketamine) on these central mechanisms. FMS patients received either i.v. placebo or ketamine (0.3 mg/kg, Ketalar((R))50% decrease in pain intensity at rest by active drug on two consecutive VAS assessments). Fifteen out of 17 ketamine-responders were included in the second part of the study. Before and after ketamine or placebo, experimental local and referred pain was induced by intramuscular (i.m.) infusion of hypertonic saline (0.7 ml, 5%) into the tibialis anterior (TA) muscle. The saline-induced pain intensity was assessed on an electronic VAS, and the distribution of pain drawn by the subject. In addition, the pain threshold (PT) to i.m. electrical stimulation was determined for single stimulus and five repeated (2 Hz, temporal summation) stimuli. The pressure PT of the TA muscle was determined, and the pressure PT and pressure pain tolerance threshold were determined at three bilaterally located tenderpoints (knee, epicondyle, and mid upper trapezius). VAS scores of pain at rest were progressively reduced during ketamine infusion compared with placebo infusion. Pain intensity (area under the VAS curve) to the post-drug infusion of hypertonic saline was reduced by ketamine (-18. 4+/-0.3% of pre-drug VAS area) compared with placebo (29.9+/-18.8%, P<0.02). Local and referred pain areas were reduced by ketamine (-12. 0+/-14.6% of pre-drug pain areas) compared with placebo (126.3+/-83. 2%, P<0.03). Ketamine had no significant effect on the PT to single i.m. electrical stimulation. However, the span between the PT to single and repeated i.m. stimuli was significantly decreased by the ketamine (-42.3+/-15.0% of pre-drug PT) compared with placebo (50. 5+/-49.2%, P<0.03) indicating a predominant effect on temporal summation. Mean pressure pain tolerance from the three paired tenderpoints was increased by ketamine (16.6+/-6.2% of pre-drug thresholds) compared with placebo (-2.3+/-4.9%, P<0.009). The pressure PT at the TA muscle was increased after ketamine (42.4+/-9. 2% of pre-drug PT) compared with placebo (7.0+/-6.6%, P<0.011). The present study showed that mechanisms involved in referred pain, temporal summation, muscular hyperalgesia, and muscle pain at rest were attenuated by the NMDA-antagonist in FMS patients. It suggested a link between central hyperexcitability and the mechanisms for facilitated referred pain and temporal summation in a sub-group of the fibromyalgia syndrome patients. Whether this is specific for FMS patients or a general phenomena in painful musculoskeletal disorders is not known.

Intramuscular and intradermal injection of capsaicin: a comparison of local and referred pain

The present study compared capsaicin-induced muscle and skin pain in humans. Twelve healthy subjects received, in a randomised, balanced order, 3 intramuscular (i.m.) injections into the brachioradial muscle: capsaicin 100 microg/1 ml, capsaicin 100 microg/20 microl or 1 ml solvent (Tween 80), and one intradermal injection (i.d.): capsaicin 100 microg/20 microl. Local and referred pain intensities and areas were assessed from 0 to 60 min after injection. Intradermal capsaicin produced more intense local pain than i.m. capsaicin in the first min (skin: 68+/-6, muscle: 51+/-6 mm VASxmin, P<0.05). In contrast, the local pain offset was later (muscle: 38+/-5, skin: 23+/-5 min, P<0.05) and referred pain was more frequent (muscle: 9/12, skin: 1/12 subjects, P<0.01) following i.m. capsaicin compared with i.d. capsaicin. Capsaicin (1 ml) produced significantly more pain than 20 microl i.m. (pain in the first min: 1 ml: 71+/-6, 20 microl: 51+/-6 VASxmin, P<0.05, offset: 1 ml: 50+/-4, 20 microl: 38+/-5 min, P<0.05). The different local and referred pain following identical noxious stimulation of muscle and skin indicates that the neurophysiological mechanisms underlying skin and muscle pain differs. The model with identical noxious stimulation of muscle and skin may be suitable for the study of differences in deep and superficial pain as seen in the clinic.

Duration and distribution of experimental muscle hyperalgesia in humans following combined infusions of serotonin and bradykinin

The present study examined distribution and duration of muscle hyperalgesia to pressure stimuli after intramuscular bolus-infusions of serotonin (5-HT, 20 nmol) and bradykinin (BKN, 10 nmol) in 10 volunteers. Infusions were given into the tibialis anterior (TA) muscle over 20 s with an inter-infusions interval of 3 min. Infusions of isotonic saline (NaCl, 0.9%) were given as control. Pain intensity was continuously scored on a visual analogue scale (VAS), and subjects drew the distribution of the pain areas on an anatomical map. Pressure pain thresholds (PPTs) were assessed with an electronic algometer at the injection site (10 cm below the patella), 2, 5, and 10 cm distal from the injection site, and at the ankle. Control assessments of PPTs were done at the contralateral TA and ankle. Skin sensibility was assessed with a Von Frey hair at the same sites. All measurements were done before and 5, 20, 40, and 60 min after infusions. The VAS-peak after BKN was significantly higher (P<0.05) compared with 5-HT and the second infusion of NaCl. The duration of the increase in VAS after 5-HT+BKN was significantly longer (P<0.05) compared with the infusions of NaCl. The local pain area after infusion of BKN was significantly larger (P<0.05) compared with 5-HT and control infusions. Cutaneous sensibility to tactile stimuli was not affected by any of the combinations. PPTs at the injection site and 2 cm (5, 20, and 40 min) were significantly decreased (P<0.05) after 5-HT+BKN compared with baseline and isotonic saline. In addition, PPTs were significantly decreased (P<0.05) after 5-HT+BKN at 5 cm (5 and 20 min) and 10 cm (5 min). Serotonin may enhance the effect of bradykinin in producing experimental muscle pain and muscle hyperalgesia to mechanical stimuli. The combination of serotonin and bradykinin can produce muscle hyperalgesia, lasted for up to 40 min and located within the muscle. No widespread hyperalgesia to the ankle and other leg (tested at 10 cm below the patella and ankle) was observed suggesting a predominant peripheral origin of the experimentally induced hyperalgesic stage.