NGF-evoked sensitization of muscle fascia nociceptors in humans

Neurochemical mechanism of muscular pain: Insight from the study on delayed onset muscle soreness

We reviewed fundamental studies on muscular pain, encompassing the characteristics of primary afferent fibers and neurons, spinal and thalamic projections, several muscular pain models, and possible neurochemical mechanisms of muscle pain. Most parts of this review were based on data obtained from animal experiments, and some researches on humans were also introduced. We focused on delayed-onset muscle soreness (DOMS) induced by lengthening contractions (LC), suitable for studying myofascial pain syndromes. The muscular mechanical withdrawal threshold (MMWT) decreased 1-3 days after LC in rats. Changing the speed and range of stretching showed that muscle injury seldom occurred, except in extreme conditions, and that DOMS occurred in parameters without muscle damage. The B2 bradykinin receptor-nerve growth factor (NGF) route and COX-2-glial cell line-derived neurotrophic factor (GDNF) route were involved in the development of DOMS. The interactions between these routes occurred at two levels. A repeated-bout effect was observed in MMWT and NGF upregulation, and this study showed that adaptation possibly occurred before B2 bradykinin receptor activation. We have also briefly discussed the prevention and treatment of DOMS.

Myofascial and discogenic origins of lumbar pain: A critical review

The purpose of this three-part narrative review is to examine the anatomy of, and the research which supports, either the lumbar myofascia or intervertebral disc (IVD) as principal sources of our patient's low back pain. A comprehensive understanding of anatomical lumbar pain generators in combination with the current treatment-based classification system will further improve and enhance clinical decision-making skills. Section I reviews the anatomy of the spinal myofascia, myofascial sources of lumbar pain, and imaging of myofascial tissues. Part II reviews the anatomy of the IVD, examines the IVD as a potential lumbar pain generator, and includes detailed discussion on Nerve Growth Factor, Inflammatory Cytokines, Vertebral End Plates and Modic change, Annular tears, and Discogenic instability. Part III looks at the history of myofascial pain, lab-based research and myofascial pain, and various levels of discogenic pain provocation research including animal, laboratory and human subjects. Our review concludes with author recommendations on developing a comprehensive understanding of altered stress concentrations affecting the posterior annulus fibrosis, neo-innervation of the IVD, inflammatory cytokines, discogenic instability, and how this knowledge can complement use of the Treatment-Based Classification System.

Persistent muscle hyperalgesia after adolescent stress is exacerbated by a mild-nociceptive input in adulthood and is associated with microglia activation

Non-specific low back pain (LBP) is a major global disease burden and childhood adversity predisposes to its development. The mechanisms are largely unknown. Here, we investigated if adversity in young rats augments mechanical hyperalgesia and how spinal cord microglia contribute to this. Adolescent rats underwent restraint stress, control animals were handled. In adulthood, all rats received two intramuscular injections of NGF/saline or both into the lumbar multifidus muscle. Stress induced in rats at adolescence lowered low back pressure pain threshold (PPT; p = 0.0001) and paw withdrawal threshold (PWT; p = 0.0007). The lowered muscle PPT persisted throughout adulthood (p = 0.012). A subsequent NGF in adulthood lowered only PPT (d = 0.87). Immunohistochemistry revealed changes in microglia morphology: stress followed by NGF induced a significant increase in ameboid state (p < 0.05). Repeated NGF injections without stress showed significantly increased cell size in surveilling and bushy states (p < 0.05). Thus, stress in adolescence induced persistent muscle hyperalgesia that can be enhanced by a mild-nociceptive input. The accompanying morphological changes in microglia differ between priming by adolescent stress and by nociceptive inputs. This novel rodent model shows that adolescent stress is a risk factor for the development of LBP in adulthood and that morphological changes in microglia are signs of spinal mechanisms involved.

Chronic musculoskeletal pain: traps and pitfalls in classification and management of a major global disease burden

Mary-Ann Fitzcharles et al. propose to introduce “regional fibromyalgia” as a new diagnosis. This commentary summarizes why this term is misleading but nonetheless the article may pave the way towards useful concepts for myofascial pains.

Demystifying the dual role of the angiotensin system in neuropathic pain

Neuropathic Pain is caused by damage to a nerve or disease of the somatosensory nervous system. Apart from the blood pressure regulating actions of angiotensin ligands, studies have shown that it also modulates neuropathic pain. In the animal models including surgical, chemotherapeutic, and retroviral-induced neuropathic pain, an increase in the levels of angiotensin II has been identified and it has been proposed that an increase in angiotensin II may participate in the induction of neuropathic pain. The pain-inducing actions of the angiotensin system are primarily due to the activation of AT₁ and AT₂ receptors, which trigger the diverse molecular mechanisms including the induction of neuroinflammation to initiate and maintain the state of neuropathic pain. On the other hand, the pain attenuating action of the angiotensin system has been attributed to decreasing in the levels of Ang_(1-7), and Ang IV and an increase in the levels of bradykinin. Ang_(1-7) may attenuate neuropathic pain via activation of the spinal Mas receptor. However, the detailed molecular mechanism involved in Ang_(1-7) and Ang IV-mediated pain attenuating actions needs to be explored. The present review discusses the dual role of angiotensin ligands in neuropathic pain along with the possible mechanisms involved in inducing or attenuating the state of neuropathic pain.

Fascial Innervation: A Systematic Review of the Literature

Currently, myofascial pain has become one of the main problems in healthcare systems. Research into its causes and the structures related to it may help to improve its management. Until some years ago, all the studies were focused on muscle alterations, as trigger points, but recently, fasciae are starting to be considered a new, possible source of pain. This systematic review has been conducted for the purpose of analyze the current evidence of the muscular/deep fasciae innervation from a histological and/or immunohistochemical point of view. A literature search published between 2000 and 2021 was made in PubMed and Google Scholar. Search terms included a combination of fascia, innervation, immunohistochemical, and different immunohistochemical markers. Of the 23 total studies included in the review, five studies were performed in rats, four in mice, two in horses, ten in humans, and two in both humans and rats. There were a great variety of immunohistochemical markers used to detect the innervation of the fasciae; the most used were Protein Gene Marker 9.5 (used in twelve studies), Calcitonin Gene-Related Peptide (ten studies), S100 (ten studies), substance P (seven studies), and tyrosine hydroxylase (six studies). Various areas have been studied, with the thoracolumbar fascia being the most observed. Besides, the papers highlighted diversity in the density and type of innervation in the various fasciae, going from free nerve endings to Pacini and Ruffini corpuscles. Finally, it has been observed that the innervation is increased in the pathological fasciae. From this review, it is evident that fasciae are well innerved, their innervation have a particular distribution and precise localization and is composed especially by proprioceptors and nociceptors, the latter being more numerous in pathological situations. This could contribute to a better comprehension and management of pain.

Mechanical sensitivity changes in pericranial muscles after local anesthesia and experimentally induced pain in the temporalis tendon: Implications for headache and facial pain

OBJECTIVES

To assess changes in mechanical sensitivity of the pericranial muscles in healthy individuals after a local anesthetic block of the temporalis tendon. In addition, to assess, if experimentally induced temporalis tendon pain, can lead to an increase in mechanical sensitivity of the pericranial muscles and reports of headache.

METHODS

40 healthy participants were recruited for this randomized, double-blinded, controlled experiment, and were randomly injected with mepivacaine and isotonic saline into the dominant-side temporalis tendon in two different sessions, and either nerve growth factor (n = 20) or isotonic saline (n = 20) in a third session. Mechanical sensitivity was assessed in the temporalis, masseter, and trapezius muscles as well as in the temporalis tendon, on the dominant side, before and 10 minutes after each injection, and in a fourth session two days after the third session. Pain drawings and headache diaries were kept for 30 days after the final session to register any developing pain or headache.

RESULTS

Mepivacaine injection into the temporalis tendon caused a significant decrease in mechanical sensitivity in the temporal tendon (-54.5%) and the masseter (-15.4%) muscle (P < 0.05) but not the temporalis (-12.1%) and trapezius muscles (-12.7%) (P > 0.05). Nerve growth factor injection into the temporalis tendon caused a significant increase in mechanical sensitivity in the tendon (+15.4%) and masseter muscle (+14.4%) (P < 0.05) but not the temporalis (+2.8%) or trapezius muscles (+3.1%) (P > 0.05). A significant increase was found for headache frequency in the first seven days (P < 0.05) after nerve growth factor injection compared to after isotonic saline injection, but not intensity (P > 0.05).

CONCLUSION

These findings suggest that the therapeutic effect of temporalis tendon anesthetic injections on facial pain and headaches are most likely not only due to a direct effect of the local anesthetic on the temporalis tendon but rather to a more generalized block of the nerves in the area. In addition, the temporal tendon may contribute to the pathophysiological processes of headache.

The Deep Fascia and its Role in Chronic Pain & Pathological Conditions: A Review

INTRODUCTION

The deep fascia is a three-dimensional continuum of connective tissue surrounding the bones, muscles, nerves and blood vessels throughout our body. Its importance in chronically debilitating conditions has recently been brought to light. This work investigates changes in these tissues in pathological settings.

MATERIALS AND METHODS

A state-of-the-art review was conducted in PubMed and Google Scholar following a two-stage process. A first search was performed to identify main types of deep fasciae. A second search was performed to identify studies considering a deep fascia, common pathologies of this deep fascia and the associated alterations in tissue anatomy.

RESULTS

We find that five main deep fasciae pathologies are chronic low back pain, chronic neck pain, Dupuytren's disease, plantar fasciitis and iliotibial band syndrome. The corresponding fasciae are respectively the thoracolumbar fascia, the cervical fascia, the palmar fascia, the plantar fascia and the iliotibial tract. Pathological fascia is characterized by increased tissue stiffness along with alterations in myofibroblast activity and the extra-cellular matrix, both in terms of collagen and Matrix Metalloproteases (MMP) levels. Innervation changes such as increased density and sensitization of nociceptive nerve fibers are observed. Additionally, markers of inflammation such as pro-inflammatory cytokines and immune cells are documented. Pain originating from the deep fascia likely results from a combination of increased nerve density, sensitization and chronic nociceptive stimulation, whether physical or chemical.

CONCLUSIONS

The pathological fascia is characterized by changes in innervation, immunology and tissue contracture. Further investigation is required to best benefit both research opportunities and patient care.

TRPM3-mediated dynamic mitochondrial activity in NGF-induced latent sensitization of chronic low back pain

ABSTRACT

The transient receptor potential ion channel TRPM3 is highly prevalent on nociceptive dorsal root ganglion (DRG) neurons, but its functions in neuronal plasticity of chronic pain remain obscure. In an animal model of nonspecific low back pain (LBP), latent spinal sensitization known as nociceptive priming is induced by nerve growth factor (NGF) injection. Here we address the TRPM3-associated molecular basis of NGF-induced latent spinal sensitization at presynaptic level by studying TRPM3-mediated calcium transients in DRG neurons. By investigating TRPM3-expressing HEK cells, we further show the dynamic mitochondrial activity downstream of TRPM3 activation. NGF enhances TRPM3 function, attenuates TRPM3 tachyphylaxis, and slows intracellular calcium clearance; TRPM3 activation triggers more mitochondrial calcium loading than depolarization does, causing a steady-state mitochondrial calcium elevation and a delayed recovery of cytosolic calcium; mitochondrial calcium buffering accounts for approximately 40% of calcium influx subsequent to TRPM3 activation. TRPM3 activation provokes an outbreak of pulsatile superoxide production (mitoflash) that comes in the form of a surge in frequency being tunable. We suggest that mitoflash pulsations downstream of TRPM3 activation might be an early signaling event initiating pain sensitization. Tuning of mitoflash activity would be a novel bottom-up therapeutic strategy for chronic pain conditions such as LBP and beyond.

Somatosensory behavioral alterations in a NGF-induced persistent low back pain model

Low back pain (LBP) is a major global burden in part due to the underlying pathophysiological mechanisms being poorly understood. A LBP rat model involving two injections of nerve growth factor (NGF, an endogenous pain-related neurotrophin) into trunk musculature was recently developed. Additional behavioral work in this NGF-LBP rat model is required to better characterize local and remote somatosensory alterations related to NGF-induced peripheral and central sensitization. This work characterizes the time-dependent development of hypersensitivity to trunk and hindpaw cutaneous mechanical stimulation and deep muscle mechanical hyperalgesia in adult male Sprague-Dawley rats (n = 6/group). Behavioral assays were performed at baseline (Day 0, D0), D2, D5 (pre- and 4 h post-2nd NGF or control injection), D7, D10, and D14 in NGF and control groups. Trunk and hindpaw cutaneous mechanical hypersensitivity were tested using von Frey filaments. Deep trunk mechanical hyperalgesia was determined using a small animal algometer. NGF rats demonstrated increased cutaneous sensitivity to ipsilateral trunk mechanical stimuli at D7, D10, and D14. NGF rats also demonstrated ipsilateral deep mechanical hyperalgesia on D2, D5 + 4 h, D7, D10, and D14. Cutaneous hypersensitivity was delayed compared to deep hyperalgesia in NGF rats. No additional sensory changes were noted. Together, these results indicate that male mechanical somatosensory changes develop primarily locally in the ipsilateral trunk following unilateral NGF injections. These findings contrast with a previous report in female rats using this NGF-LBP model showing more widespread (bilateral) hyperalgesia and remote mechanical hypersensitivity. Future studies will examine potential sex-related pain behavioral differences in the NGF model.

Evaluating the Effectiveness of Soft Tissue Therapy in the Treatment of Disorders and Postoperative Conditions of the Knee Joint-A Systematic Review

The term "soft tissue therapy" (STT) refers to mechanical methods of treatment involving passive kneading, pressing and stretching of pathologically tense tissues in supporting the process of recovery after surgery or trauma to the musculoskeletal system. The objective of this study was to review current scientific reports evaluating the effectiveness of the use of STT in patients with diseases or after surgical procedures of the knee joint. A systematic search of the popular scientific databases PubMed, Scopus and Embase was performed from inception to 15 October 2021. Eight articles met eligibility criteria and were included in the review. Six papers were related to disorders of the knee joint, while the remaining two studies were related to dysfunctions associated with the conditions after surgical intervention. The findings presented confirmed the effectiveness of STT in orthopaedic patients who showed an increase in lower limb functional parameters. The research has shown that the use of various methods of STT has a significant impact on increasing muscle activity and flexibility as well as increasing the range of motion in the knee joint. The physiotherapeutic methods used had a significant impact on reducing pain and increasing physical function and quality of life. The techniques used reduced the time to descend stairs in patients with knee osteoarthritis. This review summarises the effectiveness of STT as an important form of treatment for orthopaedic patients with various knee joint dysfunctions.

Dry needling increases antioxidant activity and grip force in a rat model of muscle pain

BACKGROUND

Muscle pain syndromes (MPS) are one of the main causes of functional, structural and metabolic problems, being associated with tissue oxidative damage. Although dry needling is widely used in the treatment of MPS, there is little scientific evidence of its efficacy and underlying mechanisms of action.

OBJECTIVES

To investigate the effects of different dry needling techniques on thermal and mechanical hyperalgesia, locomotor and functional activity, and oxidative stress markers in a rat model of muscle pain.

METHODS

A total of 48 male Wistar rats underwent injection of the gastrocnemius muscle with control neutral saline (pH 7) and remained untreated (Saline group), or acidic saline (pH 4) and remained untreated (ASA group) or received pregabalin (PG group), deep needling (DN group), superficial needling (SN group) or twitch needling (TN group) with n = 8 rats per group. Mechanical (von Frey test) and thermal hyperalgesia (acetone test), muscle edema (assessed with a caliper), strength and muscle function (grip force evaluation), surface thermography and locomotor and exploratory activities (open field test) were evaluated. The animals were then euthanized, and the gastrocnemius muscle was excised for assessment of oxidative analyses of lipid peroxidation with thiobarbituric acid reactive species (TBA-RS) and total glutathione (GSH) levels.

RESULTS

All treatments significantly improved muscle strength and function when compared to the AS group (p < 0.05). Pregabalin reduced locomotor and exploratory activities, while the TN intervention increased the antioxidant response (p < 0.05).

CONCLUSION

Dry needling improved strength, functionality and locomotor activity in a rat model of muscle pain. Twitch needling induced an antioxidant effect.

Deep fascia as a potential source of pain: A narrative review

BACKGROUND

The fascial component of the myofascial pain syndrome and the contribution of the deep fascia to various painful conditions has not been well-described and is still less understood.

OBJECTIVES

The aims of this study were to evaluate the possible role of the deep fascia on musculoskeletal pain, focusing on findings from histological and experimental studies; and to assess the nociceptive and associated responses of the deep fascia to experimentally-induced irritation.

METHODS

Narrative review of the English scientific literature.

RESULTS AND CONCLUSIONS

Different components of the deep fascia, both in humans and animals are richly innervated, with some differences between body segments. These fascial components usually exhibit dense innervation, encompassing amongst others, nociceptive afferents. The application of different types of stimuli, i.e., electrical, mechanical, and chemical to these fascial components produces long-lasting pain responses. In some cases, the intensity and severity of pain produced by the stimulation of fascia were higher than ones produced by the stimulation of the related muscular tissue. These observations may denote that the deep fascia and its various components could be a source of pain in different pathologies and various pain syndromes.

Shedding Light on the Pharmacological Interactions between μ-Opioid Analgesics and Angiotensin Receptor Modulators: A New Option for Treating Chronic Pain

The current protocols for neuropathic pain management include µ-opioid receptor (MOR) analgesics alongside other drugs; however, there is debate on the effectiveness of opioids. Nevertheless, dose escalation is required to maintain their analgesia, which, in turn, contributes to a further increase in opioid side effects. Finding novel approaches to effectively control chronic pain, particularly neuropathic pain, is a great challenge clinically. Literature data related to pain transmission reveal that angiotensin and its receptors (the AT1R, AT2R, and MAS receptors) could affect the nociception both in the periphery and CNS. The MOR and angiotensin receptors or drugs interacting with these receptors have been independently investigated in relation to analgesia. However, the interaction between the MOR and angiotensin receptors has not been excessively studied in chronic pain, particularly neuropathy. This review aims to shed light on existing literature information in relation to the analgesic action of AT1R and AT2R or MASR ligands in neuropathic pain conditions. Finally, based on literature data, we can hypothesize that combining MOR agonists with AT1R or AT2R antagonists might improve analgesia.

A physiatrist's understanding and application of the current literature on chronic pelvic pain: a narrative review

Understanding the complex, multifactorial nature of chronic pelvic pain can help physicians determine the pain's etiology and thus refer specialists to include in the multidisciplinary treatment required.

Chronic pelvic pain (CPP) is a highly prevalent condition which is underdiagnosed and poorly understood. The purpose of this review is to outline the various aspects of the nature of CPP, including its etiologies, clinical presentation, and nonoperative treatment options. For data collection, a PubMed search was conducted using indexing terms such as chronic pelvic pain and pelvic pain. Literature reviews and studies focusing on etiologies, clinical presentation, and/or the diagnosis of CPP were compiled for review by a team of 3 physiatrists. Studies investigating conservative treatments, medications, and interventional procedures for CPP and related conditions with comparable etiologies were also included. Of the 502 articles retrieved, 116 were deemed suitable by the team for this study. Although CPP is a complex, multifaceted condition, a particular susceptibility to nociceptive stimuli was demonstrated as an underlying theme in its evolution. There are many treatment options currently used; however, more robust evidence, such as randomized controlled trials, are needed before creating comprehensive guidelines for treating CPP.

Is "Delayed Onset Muscle Soreness" a False Friend? The Potential Implication of the Fascial Connective Tissue in Post-Exercise Discomfort

Strenuous and unaccustomed exercise frequently lead to what has been coined “delayed onset muscle soreness” (DOMS). As implied by this term, it has been proposed that the associated pain and stiffness stem from micro-lesions, inflammation, or metabolite accumulation within the skeletal muscle. However, recent research points towards a strong involvement of the connective tissue. First, according to anatomical studies, the deep fascia displays an intimate structural relationship with the underlying skeletal muscle and may therefore be damaged during excessive loading. Second, histological and experimental studies suggest a rich supply of algogenic nociceptors whose stimulation evokes stronger pain responses than muscle irritation. Taken together, the findings support the hypothesis that DOMS originates in the muscle-associated connective tissue rather than in the muscle itself. Sports and fitness professionals designing exercise programs should hence consider fascia-oriented methods and techniques (e.g., foam rolling, collagen supplementation) when aiming to treat or prevent DOMS.

Tenderness of the Skin after Chemical Stimulation of Underlying Temporal and Thoracolumbar Fasciae Reveals Somatosensory Crosstalk between Superficial and Deep Tissues

Musculoskeletal pain is often associated with pain referred to adjacent areas or skin. So far, no study has analyzed the somatosensory changes of the skin after the stimulation of different underlying fasciae. The current study aimed to investigate heterotopic somatosensory crosstalk between deep tissue (muscle or fascia) and superficial tissue (skin) using two established models of deep tissue pain (namely focal high frequency electrical stimulation (HFS) (100 pulses of constant current electrical stimulation at 10× detection threshold) or the injection of hypertonic saline in stimulus locations as verified using ultrasound). In a methodological pilot experiment in the TLF, different injection volumes of hypertonic saline (50–800 µL) revealed that small injection volumes were most suitable, as they elicited sufficient pain but avoided the complication of the numbing pinprick sensitivity encountered after the injection of a very large volume (800 µL), particularly following muscle injections. The testing of fascia at different body sites revealed that 100 µL of hypertonic saline in the temporal fascia and TLF elicited significant pinprick hyperalgesia in the overlying skin (–26.2% and –23.5% adjusted threshold reduction, p < 0.001 and p < 0.05, respectively), but not the trapezius fascia or iliotibial band. Notably, both estimates of hyperalgesia were significantly correlated (r = 0.61, p < 0.005). Comprehensive somatosensory testing (DFNS standard) revealed that no test parameter was changed significantly following electrical HFS. The experiments demonstrated that fascia stimulation at a sufficient stimulus intensity elicited significant across-tissue facilitation to pinprick stimulation (referred hyperalgesia), a hallmark sign of nociceptive central sensitization.

Rat dorsal horn neurons primed by stress develop a long-lasting manifest sensitization after a short-lasting nociceptive low back input

Background

A single injection of nerve growth factor (NGF) into a low back muscle induces a latent sensitization of rat dorsal horn neurons (DHNs) that primes for a manifest sensitization by a subsequent second NGF injection. Repeated restraint stress also causes a latent DHN sensitization.

Objective

In this study, we investigated whether repeated restraint stress followed by a single NGF injection causes a manifest sensitization of DHNs.

Methods

Rats were stressed repeatedly in a narrow plastic restrainer (1 hour on 12 consecutive days). Control animals were handled but not restrained. Two days after stress paradigm, behavioral tests and electrophysiological in vivo recordings from single DHNs were performed. Mild nociceptive low back input was induced by a single NGF injection into the lumbar multifidus muscle just before the recording started.

Results

Restraint stress slightly lowered the low back pressure pain threshold (Cohen d = 0.83). Subsequent NGF injection increased the proportion of neurons responsive to deep low back input (control + NGF: 14%, stress + NGF: 39%; P = 0.041), mostly for neurons with input from outside the low back (7% vs 26%; P = 0.081). There was an increased proportion of neurons with resting activity (28% vs 55%; P = 0.039), especially in neurons having deep input (0% vs 26%; P = 0.004).

Conclusions

The results indicate that stress followed by a short-lasting nociceptive input causes manifest sensitization of DHNs to deep input, mainly from tissue outside the low back associated with an increased resting activity. These findings on neuronal mechanisms in our rodent model suggest how stress might predispose to radiating pain in patients.

Spinal cord fractalkine (CX3CL1) signaling is critical for neuronal sensitization in experimental nonspecific, myofascial low back pain

Neuroactive substances released by activated microglia contribute to hyperexcitability of spinal dorsal horn neurons in many animal models of chronic pain. An important feedback loop mechanism is via release of fractalkine (CX3CL1) from primary afferent terminals and dorsal horn neurons and binding to CX3CR1 receptors on microglial cells. We studied the involvement of fractalkine signaling in latent and manifest spinal sensitization induced by two injections of nerve growth factor (NGF) into the lumbar multifidus muscle as a model for myofascial low back pain. Single dorsal horn neurons were recorded in vivo to study their receptive fields and spontaneous activity. Under intrathecal vehicle application, the two NGF injections led to an increased proportion of neurons responding to stimulation of deep tissues (41%), to receptive field expansion into the hindlimb (15%), and to resting activity (53%). Blocking fractalkine signaling by continuous intrathecal administration of neutralizing antibodies completely prevented these signs of spinal sensitization to a similar extent as in a previous study with the microglia inhibitor minocycline. Reversely, fractalkine itself induced similar sensitization in a dose-dependent manner (for 200 ng/mL: 45% deep tissue responses, 24% receptive field expansion, and 45% resting activity) as repeated nociceptive stimulation by intramuscular NGF injections. A subsequent single NGF injection did not have an additive effect. Our data suggest that neuron-to-microglia signaling via the CX3CL1-CX3CR1 pathway is critically involved in the initiation of nonspecific, myofascial low back pain through repetitive nociceptive stimuli.NEW & NOTEWORTHY Blocking fractalkine signaling by neutralizing antibodies completely prevented spinal sensitization induced by repetitive mild nociceptive input [2 nerve growth factor (NGF) injections into the multifidus muscle] Conversely, fractalkine given intrathecally caused the same pattern of spinal sensitization as the nociceptive NGF injections. Fractalkine signaling is critically involved in sensitization of dorsal horn neurons induced by repeated nociceptive low back muscle stimulation and may hence be a potential target for the prevention of nonspecific, myofascial low back pain.

The morphological and microscopical characteristics of posterior layer of human thoracolumbar fascia; A potential source of low back pain

BACKGROUND

Posterior layer of thoracolumbar fascia (PTLF) is the deep fascia of back of the trunk, which connects the trunk, upper limb and lower limb muscles. Very few cadaveric studies of posterior layer of thoracolumbar fascia (PTLF) are found in the literature, which mention the presence of nerve receptors in it but, quantification of the nerve receptors where not found. Providing the morphological and morphometrical data of PTLF may help the exercise physiologists, sports physicians, occupational health assistants and, physiotherapists to modify or invent new protocol of treatment to help the society.

METHODS

In this study, twenty formalin embalmed human cadavers were used and we have documented the orientation of the PTLF and quantified the number of peripheral nerve endings at the different vertebral levels.

RESULTS

Mean distance of PTLF from vertebral spines to the musculofascial junction was at thoracic region 3.38cm and 3.34cm; at lumbar region, it was 7.4cm and 7.36cm and at sacral region it was 2.98cm and 2.96cm on right and left side, respectively. The angulation of PTLF varies from 18-110 degrees at different vertebral levels. The microscopic data shows the thickness of PTLF and number of nerve endings in the sacral level is increased compared to that of thoracic vertebral levels.

CONCLUSIONS

We have contributed the novel morphological and microscopical details to the limited existing data on PTLF. We also have provided the quantitative data of nerve fibers, which are possible nociceptors of PTLF.

Nerve growth factor sensitizes nociceptors to C-fibre selective supra-threshold electrical stimuli in human skin

BACKGROUND

Intradermal injection of 1 µg nerve growth factor (NGF) causes sustained nociceptor sensitization. Slowly depolarizing electrical current preferentially activates C-nociceptors.

METHODS

We explored the differential contribution of A-delta and C-nociceptors in NGF-sensitized skin using slowly depolarizing transcutaneous electrical current stimuli, CO2 laser heat, mechanical impact, and A-fibre compression block. In 14 healthy volunteers, pain rating was recorded on a numeric scale at days 1-14 after NGF treatment. Ratings during A-fibre conduction block were investigated at days 3 and 7 post-NGF.

RESULTS

Pain ratings to electrical, CO2 heat and mechanical impact stimuli were enhanced (>30%, p < .0005, ANOVA) at NGF-injection sites. Axon reflex erythema evoked by electrical stimulation was also larger at NGF-injection sites (p < .02, ANOVA). Diminution of pain during continuous (1 min) sinusoidal current stimulation at 4 Hz was less pronounced after NGF (p < .05, ANOVA). Pain ratings to electrical sinusoidal and mechanical impact stimuli during A-fibre conduction block were significantly elevated at the NGF sites compared to NaCl-treated skin (p < .05, ANOVA).

CONCLUSIONS

NGF-induced sensitization of human skin to electrical and mechanical stimuli is primarily driven by C-nociceptors with little contribution from A-delta fibres. Less-pronounced accommodation during ongoing sinusoidal stimulation suggests that NGF could facilitate axonal spike generation and conduction in primary afferent nociceptors in humans. Further studies using this sinusoidal electrical stimulation profile to investigate patients with chronic inflammatory pain may allow localized assessment of skin C-nociceptors and their putative excitability changes under pathologic conditions.

SIGNIFICANCE

The application of novel slowly depolarizing electrical stimuli demonstrated a predominant C-nociceptor sensitization in NGF-treated skin. Increased pain ratings, larger axon reflex erythema and less accommodation of C-fibres to ongoing sinusoidal stimulation all indicated an enhanced nociceptor discharge after NGF. A-fibre conduction block had little effect on electrical and mechanical hyperalgesia skin in NGF-treated compared to NaCl-treated skin. This electrical stimulus profile may be applicable for patients with chronic inflammatory pain, allowing localized assessment of skin C-nociceptors and their putative excitability changes under pathologic conditions.

Transcranial Direct Current Stimulation Accelerates The Onset of Exercise-Induced Hypoalgesia: A Randomized Controlled Study

Exercise-induced hypoalgesia (EIH) describes acute reductions in pain that occur following exercise. Current evidence suggests that the magnitude of EIH is small-to-moderate at best, warranting exploration of novel avenues to bolster these effects. Transcranial direct current stimulation (tDCS) has been shown to relieve pain and represents a promising intervention that may enhance EIH. This study aimed to determine whether anodal tDCS of the primary motor cortex (M1) can augment EIH in healthy individuals experiencing experimentally-induced musculoskeletal pain. Twenty-four healthy subjects attended 2 experimental sessions ("Day 0" and "Day 2"). On Day 0, subjects were injected with nerve growth factor into their right extensor carpi radialis brevis to induce persistent elbow pain. On Day 2, each subject received active or sham tDCS over M1 followed by an isometric grip exercise. Pain intensity, muscle soreness, sensitivity (pressure pain thresholds), and conditioned pain modulation were assessed prior to the nerve growth factor injection, on Day 2 before tDCS, immediately post-exercise, and 15 minutes post-exercise. Active tDCS expedited the onset of EIH, inducing immediate reductions in pain intensity that were not present until 15 minutes post-exercise in the sham group. However, active tDCS did not reduce muscle soreness or sensitivity when compared to sham tDCS. PERSPECTIVE: These findings suggest that active tDCS accelerates the onset of EIH in healthy individuals experiencing experimentally-induced pain. This may represent a promising means of enhancing adherence to exercise protocols. However, larger randomised controlled trials in persistent pain populations are required to confirm the clinical impact of these findings.

Nerve Growth Factor Signaling and Its Contribution to Pain

Nerve growth factor (NGF) is a neurotrophic protein essential for the growth, differentiation, and survival of sympathetic and sensory afferent neurons during development. A substantial body of evidence, based on both animal and human studies, demonstrates that NGF plays a pivotal role in modulation of nociception in adulthood. This has spurred development of a variety of novel analgesics that target the NGF signaling pathway. Here, we present a narrative review designed to summarize how NGF receptor activation and downstream signaling alters nociception through direct sensitization of nociceptors at the site of injury and changes in gene expression in the dorsal root ganglion that collectively increase nociceptive signaling from the periphery to the central nervous system. This review illustrates that NGF has a well-known and multifunctional role in nociceptive processing, although the precise signaling pathways downstream of NGF receptor activation that mediate nociception are complex and not completely understood. Additionally, much of the existing knowledge derives from studies performed in animal models and may not accurately represent the human condition. However, available data establish a role for NGF in the modulation of nociception through effects on the release of inflammatory mediators, nociceptive ion channel/receptor activity, nociceptive gene expression, and local neuronal sprouting. The role of NGF in nociception and the generation and/or maintenance of chronic pain has led to it becoming a novel and attractive target of pain therapeutics for the treatment of chronic pain conditions.

Spinal Mobilization Prevents NGF-Induced Trunk Mechanical Hyperalgesia and Attenuates Expression of CGRP

Introduction

Low back pain (LBP) is a complex and growing global health problem in need of more effective pain management strategies. Spinal mobilization (SM) is a non-pharmacological approach recommended by most clinical guidelines for LBP, but greater utilization and treatment optimization are hampered by a lack of mechanistic knowledge underlying its hypoalgesic clinical effects.

Methods

Groups of female Sprague-Dawley rats received unilateral trunk (L5 vertebral level) injections (50 μl) of either vehicle (phosphate-buffer solution, PBS; VEH) or nerve growth factor (NGF; 0.8 μM) on Days 0 and 5 with or without daily L5 SM (VEH, NGF, VEH + SM, VEH + SM). Daily passive SM (10 min) was delivered by a feedback motor (1.2 Hz, 0.9N) from Days 1 to 12. Changes in pain assays were determined for mechanical and thermal reflexive behavior, exploratory behavior (open field events) and spontaneous pain behavior (rat grimace scale). On Day 12, lumbar (L1–L6) dorsal root ganglia (DRG) were harvested bilaterally and calcitonin gene-related peptide (CGRP) positive immunoreactive neurons were quantified from 3 animals (1 DRG tissue section per segmental level) per experimental group.

Results

NGF induced bilateral trunk (left P = 0.006, right P = 0.001) mechanical hyperalgesia and unilateral hindpaw allodynia (P = 0.006) compared to the vehicle group by Day 12. Additionally, we found for the first time that NGF animals demonstrated decreased exploratory behaviors (total distance traveled) and increased grimace scale scoring compared to the VEH group. Passive SM prevented this development of local (trunk) mechanical hyperalgesia and distant (hindpaw) allodynia, and normalized grimace scale scores. NGF increased CGRP positive immunoreactive neurons in ipsilateral lumbar DRGs compared to the VEH group ([L1]P = 0.02; [L2]P = 0.007) and SM effectively negated this increase in pain-related neuropeptide CGRP expression.

Conclusion

SM prevents the development of local (trunk) NGF-induced mechanical hyperalgesia and distant (hindpaw) allodynia, in part, through attenuation of CGRP expression in lumbar DRG sensory neurons. NGF decreases rat exploratory behavior and increases spontaneous pain for which passive SM acts to mitigate these pain-related behavioral changes. These initial study findings suggest that beginning daily SM soon after injury onset might act to minimize or prevent the development of LBP by reducing production of pain-related neuropeptides.

A dual role for peripheral GDNF signaling in nociception and cardiovascular reflexes in the mouse

Group III/IV muscle afferents transduce nociceptive signals and modulate exercise pressor reflexes (EPRs). However, the mechanisms governing afferent responsiveness to dually modulate these processes are not well characterized. We and others have shown that ischemic injury can induce both nociception-related behaviors and exacerbated EPRs in the same mice. This correlated with primary muscle afferent sensitization and increased expression of glial cell line-derived neurotrophic factor (GDNF) in injured muscle and increased expression of GDNF family receptor α1 (GFRα1) in dorsal root ganglia (DRG). Here, we report that increased GDNF/GFRα1 signaling to sensory neurons from ischemia/reperfusion-affected muscle directly modulated nociceptive-like behaviors and increased exercise-mediated reflexes and group III/IV muscle afferent sensitization. This appeared to have taken effect through increased cyclic adenosine monophosphate (cAMP) response element binding (CREB)/CREB binding protein-mediated expression of the purinergic receptor P2X5 in the DRGs. Muscle GDNF signaling to neurons may, therefore, play an important dual role in nociception and sympathetic reflexes and could provide a therapeutic target for treating complications from ischemic injuries.

Role of fasciae in nonspecific low back pain

More and more evidences show how the thoracolumbar fascia is involved with nonspecific low back pain. Additionally, recent studies about anatomy have shown the presence of a continuity between the thoracolumbar fascia and the deep fascia of the limbs; but actually, a dysfunction of just the thoracolumbar fascia or of the tightly contiguous myofascial tissue is generally recognized as possible cause of nonspecific low back pain. Five patients among those affected by nonspecific low back pain were manipulated just on those fascial spots that were painful, when palpated, and located in other areas of the body than the low back one. Each patient reported a clinically significant reduction of the painful symptoms (a Pain Numerical Rating Scale score difference ≥ 2) straight after the manipulation. A dysfunction of the myofascial tissue that is not tightly contiguous with the symptomatic area is then suggested to be taken into consideration among the causes of nonspecific low back-pain.

Painful neurotrophins and their role in visceral pain

Beyond their well-known role in embryonic development of the central and peripheral nervous system, neurotrophins, particularly nerve growth factor and brain-derived neurotrophic factor, exert an essential role in pain production and sensitization. This has mainly been studied within the framework of somatic pain, and even antibodies (tanezumab and fasinumab) have recently been developed for their use in chronic somatic painful conditions, such as osteoarthritis or low back pain. However, data suggest that neurotrophins also exert an important role in the occurrence of visceral pain and visceral sensitization. Visceral pain is a distressing symptom that prompts many consultations and is typically encountered in both 'organic' (generally inflammatory) and 'functional' (displaying no obvious structural changes in routine clinical evaluations) disorders of the gut, such as inflammatory bowel disease and irritable bowel syndrome, respectively. The present review provides a summary of neurotrophins as a molecular family and their role in pain in general and addresses recent investigations of the involvement of nerve growth factor and brain-derived neurotrophic factor in visceral pain, particularly that associated with inflammatory bowel disease and irritable bowel syndrome.

Educational avenues for promoting dialog on fascia

If your healthcare professional students have not heard about the importance of fascia they definitely should, and if your residents have not heard about the manifestations of fascia health they definitely will from their patients. While fascia may not be the sexiest of organ systems, it is one of the most influential. Fascia is gaining interest from researchers, physicians, and many subdivisions of manual medicine including massage therapists. The fascial system is now being recognized with roles in pathology, fluid movement, and proprioception. It is also important in skeletal muscle movement, perception of pain, protein regulation and expression, cell signaling, neoplastic growth, and hormone distribution in our body. It can be the reason why we feel chronic pain or why we feel tightness after physical activity. The primary responsibility of fascia is to connect systems so that the body works as a whole, which is what permits this topic to be easily embedded anywhere in our health curricula. Whether you teach students in schools of medical, veterinary, dental, physical therapy, physician assistant studies, or occupational therapy, fascia matters. Whether you teach in an integrated curriculum or a curriculum that is designed for problem-based learning or a classical discipline-based curriculum, connective tissue has a place in academia. So, in our cramped curriculum how do we make sure that our current undergraduate and graduate students understand the complexity of fascia without adding additional time to coursework? To answer this question, this article demonstrates how fascia can fit anywhere in the curriculum because it is found everywhere. Clin. Anat. 32:871-876, 2019. © 2019 Wiley Periodicals, Inc.

Resolution of a Greater Than 50-year History of Severe, Chronic Low Back Pain Following an Ultrasound-guided Platelet-rich Plasma Infiltration of the Thoracolumbar Fascia

The diagnosis of chronic low back pain is a scourge of society that does not take into account the pathoanatomical cause of pain. This case describes a six-year search for the pinpoint pathoanatomical diagnosis of a patient's 50-plus year history of debilitating chronic low back pain after he failed the standard nonoperative and operative treatment modalities. Ultrasound-guided diagnostic blocks identified a potential space within the thoracolumbar fascia. This was treated with platelet-rich plasma, yielding a complete resolution of his pain and a full return to normal activities of daily living for three years since the procedure.

Topical thermal therapy with hot packs suppresses physical inactivity-induced mechanical hyperalgesia and up-regulation of NGF

We focused on the analgesic effect of hot packs for mechanical hyperalgesia in physically inactive rats. Male Wistar rats were randomly divided into four groups: control, physical inactivity (PI), PI + sham treatment (PI + sham), and PI + hot pack treatment (PI + hot pack) groups. Physical inactivity rats wore casts on both hind limbs in full plantar flexed position for 4 weeks. Hot pack treatment was performed for 20 min a day, 5 days a week. Although mechanical hyperalgesia and the up-regulation of NGF in the plantar skin and gastrocnemius muscle were observed in the PI and the PI + sham groups, these changes were significantly suppressed in the PI + hot pack group. The present results clearly demonstrated that hot pack treatment was effective in reducing physical inactivity-induced mechanical hyperalgesia and up-regulation of NGF in plantar skin and gastrocnemius muscle.

Differential contributions of peripheral and central mechanisms to pain in a rodent model of osteoarthritis

The mechanisms underlying the transition from acute nociceptive pain to centrally maintained chronic pain are not clear. We have studied the contributions of the peripheral and central nervous systems during the development of osteoarthritis (OA) pain. Male Sprague-Dawley rats received unilateral intra-articular injections of monosodium iodoacetate (MIA 1 mg) or saline, and weight-bearing (WB) asymmetry and distal allodynia measured. Subgroups of rats received intra-articular injections of, QX-314 (membrane impermeable local anaesthetic) + capsaicin, QX-314, capsaicin or vehicle on days 7, 14 or 28 post-MIA and WB and PWT remeasured. On days 7&14 post-MIA, but not day 28, QX-314 + capsaicin signficantly attenuated changes in WB induced by MIA, illustrating a crucial role for TRPV1 expressing nociceptors in early OA pain. The role of top-down control of spinal excitability was investigated. The mu-opioid receptor agonist DAMGO was microinjected into the rostroventral medulla, to activate endogenous pain modulatory systems, in MIA and control rats and reflex excitability measured using electromyography. DAMGO (3 ng) had a significantly larger inhibitory effect in MIA treated rats than in controls. These data show distinct temporal contribtuions of TRPV1 expressing nociceptors and opioidergic pain control systems at later timepoints.

Peripheral Mechanisms Contributing to Osteoarthritis Pain

PURPOSE OF REVIEW

Osteoarthritis (OA) is the most common form of arthritis and a major source of pain and disability worldwide. OA-associated pain is usually refractory to classically used analgesics, and disease-modifying therapies are still lacking. Therefore, a better understanding of mechanisms and mediators contributing to the generation and maintenance of OA pain is critical for the development of efficient and safe pain-relieving therapies.

RECENT FINDINGS

Both peripheral and central mechanisms contribute to OA pain. Clinical evidence suggests that a strong peripheral nociceptive drive from the affected joint maintains pain and central sensitization associated with OA. Mediators present in the OA joint, including nerve growth factor, chemokines, cytokines, and inflammatory cells can contribute to sensitization. Furthermore, structural alterations in joint innervation and nerve damage occur in the course of OA. Several interrelated pathological processes, including joint damage, structural reorganization of joint afferents, low-grade inflammation, neuroplasticity, and nerve damage all contribute to the pain observed in OA. It can be anticipated that elucidating exactly how these mechanisms are operational in the course of progressive OA may lead to the identification of novel targets for intervention.

Prevention and reversal of latent sensitization of dorsal horn neurons by glial blockers in a model of low back pain in male rats

In an animal model of nonspecific low back pain, recordings from dorsal horn neurons were made to investigate the influence of glial cells in the central sensitization process. To induce a latent sensitization of the neurons, nerve growth factor (NGF) was injected into the multifidus muscle; the manifest sensitization to a second NGF injection 5 days later was used as a read-out. The sensitization manifested in increased resting activity and in an increased proportion of neurons responding to stimulation of deep somatic tissues. To block microglial activation, minocycline was continuously administered intrathecally starting 1 day before or 2 days after the first NGF injection. The glia inhibitor fluorocitrate that also blocks astrocyte activation was administrated 2 days after the first injection. Minocycline applied before the first NGF injection reduced the manifest sensitization after the second NGF injection to control values. The proportion of neurons responsive to stimulation of deep tissues was reduced from 50% to 17.7% (P < 0.01). No significant changes occurred when minocycline was applied after the first injection. In contrast, fluorocitrate administrated after the first NGF injection reduced significantly the proportion of neurons with deep input (15.8%, P < 0.01). A block of glia activation had no significant effect on the increased resting activity. The data suggest that blocking microglial activation prevented the NGF-induced latent spinal sensitization, whereas blocking astrocyte activation reversed it. The induction of spinal neuronal sensitization in this pain model appears to depend on microglia activation, whereas its maintenance is regulated by activated astrocytes.NEW & NOTEWORTHY Activated microglia and astrocytes mediate the latent sensitization induced by nerve growth factor in dorsal horn neurons that receive input from deep tissues of the low back. These processes may contribute to nonspecific low back pain.

The Lumbodorsal Fascia as a Potential Source of Low Back Pain: A Narrative Review

The lumbodorsal fascia (LF) has been proposed to represent a possible source of idiopathic low back pain. In fact, histological studies have demonstrated the presence of nociceptive free nerve endings within the LF, which, furthermore, appear to exhibit morphological changes in patients with chronic low back pain. However, it is unclear how these characteristics relate to the aetiology of the pain. In vivo elicitation of back pain via experimental stimulation of the LF suggests that dorsal horn neurons react by increasing their excitability. Such sensitization of fascia-related dorsal horn neurons, in turn, could be related to microinjuries and/or inflammation in the LF. Despite available data point towards a significant role of the LF in low back pain, further studies are needed to better understand the involved neurophysiological dynamics.

Brain-derived neurotrophic factor plasma levels are increased in older women after an acute episode of low back pain

BACKGROUND

Low back pain (LBP) is a growing public health problem in old age, and it is associated with disabling pain and depressive disorders. We compared brain-derived neurotrophic factor (BDNF) plasma levels, a key neurotrophin in pain modulation, between older women after an acute episode of LBP and age-matched pain-free controls, and investigated potential differences in BDNF levels between controls and LBP subgroups based on pain severity, presence of depressive symptoms and use of analgesic and antidepressant drugs.

METHODS

A total of 221 participants (154 with LBP and 67 pain-free) were studied. A comprehensive assessment of sociodemographic and clinical variables was conducted including pain severity (11-point NRS), depressive symptoms (GDS-15), age, body mass index, physical activity and total number of comorbidities and medications in use.

RESULTS

BDNF levels in LBP group were significantly higher than controls (7515.9±3021.2; Md=7116.0 vs 6331.8±3364.0; Md=5897.5pg/mL, P=0.005). LBP subgroups exhibited higher BDNF levels than controls, regardless of pain severity, presence of depressive symptoms and use of analgesic drugs. BDNF levels were significantly higher in LBP subgroup without use of antidepressant drugs compared to both controls and LBP subgroup with use of antidepressant drugs.

DISCUSSION

This study provides evidence that older women with acute low back pain exhibit higher BDNF plasma levels compared to pain-free controls. Subgroup comparisons suggest that use of pain-relief drugs may influence BDNF levels. The study results offer a novel target for research on mechanisms of back pain in older adults.

The interaction between NGF-induced hyperalgesia and acid-provoked pain in the infrapatellar fat pad and tibialis anterior muscle of healthy volunteers

BACKGROUND

Tissue pH is lowered in inflamed tissues, and the increased proton concentration activates acid-sensing ion channels (ASICs), contributing to pain and hyperalgesia. ASICs can be upregulated by nerve growth factor (NGF). The aim of this study was to investigate two new human experimental pain models combining NGF- and acid-induced pain in a randomized, controlled, double-blind study.

METHODS

In experiment 1, volunteers (N = 16) received an injection of either NGF or isotonic saline in each infrapatellar fat pad (IFP). One day after 5 mL of phosphate-buffered acidic saline was infused into each IFP at a rate of 20 mL/h. In experiment 2, the tibialis anterior (TA) muscle of additional volunteers (N = 16) was examined, following the same procedure except that the volume and infusion rate of acid were different (10 mL, 30 mL/h). Continuous pain ratings were recorded during and after acid infusions. In addition, soreness scores on a Likert scale and pressure pain thresholds (PPTs) were assessed.

RESULTS

The PPT of the IFP was significantly decreased at the NGF injection site on day 1, but acid-provoked pain ratings and the change in PPT from pre- to postinfusion between the knees were similar. In the muscle pain model, local mechanical hyperalgesia developed 3 h after the NGF injection and a significant additional decrease in PPT was found after acid infusion compared to preinfusion.

CONCLUSIONS

NGF sensitization in the IFP was not facilitated by acid, whereas an acid-provoked enhancement of muscle hyperalgesia was found. NGF sensitization of adipose tissue responds differently to acid provocation compared to muscle tissue.

SIGNIFICANCE

Quantification of two novel pain models combining NGF and acid. Hyperalgesia developed after NGF injection in the infrapatellar fat pad, but it was not facilitated by acid provocation. Contrary, NGF-induced hyperalgesia in muscle tissue was enhanced by acid.

Spontaneous activity in C-fibres after partial damage to the saphenous nerve in mice: Effects of retigabine

BACKGROUND

Spontaneous pain is the most devastating positive symptom in neuropathic pain patients. Recent data show a direct relationship between spontaneous discharges in C-fibres and spontaneous pain in neuropathic patients. Unfortunately, to date there is a lack of experimental animal models for drug testing.

METHODS

We recorded afferent fibres from a new experimental model in vitro. The preparation contains a neuroma formed in a peripheral branch of the saphenous nerve together with the undamaged branches, which maintain intact terminals in a skin flap.

RESULTS

Fibres with stable rates of ectopic spontaneous discharges were found among axotomized (5 A- and 18 C-fibres, mean discharge 0.48 ± 0.08 Hz) and 'putative intact' fibres (12 C-fibres, mean discharge 0.28 ± 0.08 Hz). A proportion (~9%) of axotomized fibres had mechanical receptive fields in the skin far beyond the site of injury. Collision experiments demonstrated that action potentials evoked from neuroma and skin travelled by the same fibre, indicating functional cross-talk between neuromatose and putative intact fibres. Retigabine, the specific Kv7 channel opener, depressed spontaneous discharges by 70% in 15/18 units tested. In contrast, responses to mechanical stimulation of the skin were unaltered by retigabine.

CONCLUSIONS

Partial damage to a peripheral nerve may increase the incidence of spontaneous activity in C-fibres. Retigabine reduced spontaneous activity but not stimulus-evoked activity, suggesting an important role for ion channels in the control of spontaneous pain and demonstrating the utility of the model for the testing of compounds in clinically relevant variables. WHAT DOES THIS STUDY ADD?: Our in vitro experimental model of peripheral neuropathy allows for pharmacological characterization of spontaneously active fibres. Using this model, we show that retigabine inhibits aberrant spontaneous discharges without altering physiological responses in primary afferents.

PG110, A Humanized Anti-NGF Antibody, Reverses Established Pain Hypersensitivity in Persistent Inflammatory Pain, but not Peripheral Neuropathic Pain, Rat Models

BACKGROUND

Chronic inflammatory and peripheral neuropathic pain (PNP) is a major health problem for which effective drug treatment is lacking. The pathophysiology of these debilitating conditions is incompletely understood, but nerve growth factor (NGF) is believed to play a major role. NGF-antagonism has previously been shown to prevent pain hypersensitivity in rodent models of acute inflammatory pain and PNP, but most of those animal studies did not address the more clinically relevant issue of whether NGF-antagonism provides relief of established chronic pain behavior. Therefore, the aim of this study was to investigate whether blocking NGF actions with a humanized anti-NGF monoclonal antibody (PG110) would reverse/attenuate established pain hypersensitivity in rat models of chronic/persistent inflammatory pain and PNP.

METHODS

The complete Freund's adjuvant (CFA) rat model of persistent inflammatory pain, and the L5 spinal nerve axotomy (SNA) model of PNP, were used in the present study. The effect of a single intravenous injection (10, 30, and 300 µg/kg) of an anti-NGF antibody PG110 on heat and mechanical hypersensitivity was assessed 5 and 7 days after CFA and SNA, respectively.

RESULTS

Compared to vehicle treated group, PG110 dose dependently attenuated established heat and mechanical hypersensitivity induced by CFA, but not that induced by SNA. The anti-allodynic and anti-hyperalgesic effects of PG110 in the CFA model were similar to those of the positive control naproxen (30 mg/kg, i.v.).

CONCLUSION

These findings suggest that therapies that target NGF or its receptors may be effective for treatment of persistent/chronic inflammatory pain, but probably not PNP.

Evidence for the existence of nociceptors in rat thoracolumbar fascia

Recently, the existence of nociceptive fibers in fascia tissue has attracted much interest. Fascia can be a source of pain in several disorders such as fasciitis and non-specific low back pain. However, little is known about the properties of fascia nociceptors and possible changes of the fascia innervation by nociceptors under pathological circumstances. In this histologic study, the density of presumably nociceptive fibers and free nerve endings was determined in the three layers of the rat TLF: inner layer (IL, covering the multifidus muscle), middle layer (ML) and outer layer (OL). As markers for nociceptive fibers, antibodies to the neuropeptides CGRP and SP as well as to the transient receptor potential vanilloid 1 (TRPV1) were used. As a pathological state, inflammation of the TLF was induced with injection of complete Freund's adjuvant. The density of CGRP- and SP-positive fibers was significantly increased in the inner and outer layer of the inflamed fascia. In the thick middle layer, no inflammation-induced change occurred. In additional experiments, a neurogenic inflammation was induced in the fascia by electrical stimulation of dorsal roots. In these experiments, plasma extravasation was visible in the TLF, which is clear functional evidence for the existence of fascia nociceptors. The presence of nociceptors in the TLF and the increased density of presumably nociceptive fibers under chronic painful circumstances may explain the pain from a pathologically altered fascia. The fascia nociceptors probably contribute also to the pain in non-specific low back pain.

Interaction between ultraviolet B-induced cutaneous hyperalgesia and nerve growth factor-induced muscle hyperalgesia

BACKGROUNDS AND OBJECTIVES

Clinical observations indicate that cutaneous hyperalgesia may arise from pain located in deep structures. The objective of this study was to investigate whether combined sensitization of deep and superficial somatic tissues facilitates skin hyperalgesia.

METHODS

The interaction between muscle and cutaneous hyperalgesia was investigated in 16 healthy volunteers. Skin sensitization was induced unilaterally on the same randomly selected part of the body by ultraviolet B (UVB) irradiation above the upper trapezius and low back muscles. The next day, muscle hyperalgesia was induced bilaterally in low back muscles by injections of nerve growth factor (NGF). Thus, 1 day after irradiation there was skin sensitization, whereas after 2 days both skin and muscle sensitizations were present. Cutaneous blood flow, pin-prick thresholds, pressure pain thresholds (PPTs), temporal summation to repetitive painful pressure stimulation, and stimulus-response functions of graded pressure stimulations and pain intensity were assessed within the irradiated skin area and in the surrounding area before and 1, 2 and 3 days after irradiation.

RESULTS

Comparing baseline with 1 day after irradiation, UVB and UVB+NGF locations demonstrated: (1) Increased superficial blood flow inside the irradiated area (p < 0.01); (2) Reduced pin-prick (p < 0.01) and PPTs (p < 0.05) within the irradiated area and in the surrounding area; (3) Left-shifted pressure stimulus-response function within the irradiated area (p < 0.01); (4) Facilitated temporal summation inside the irradiated area (p < 0.01).

CONCLUSIONS

Using skin and deep tissue pain sensitization models simultaneously, no significant synergistic effects were found within the 3-day investigation suggesting little integration between the two phenomena in this period.

Inflammation of the thoracolumbar fascia excites and sensitizes rat dorsal horn neurons

BACKGROUND

Recent data show that the thoracolumbar fascia can be a source of pain. However, the spinal neuronal mechanisms underlying pain from a pathologically altered fascia are unknown. The present study aimed at finding out how dorsal horn neurons react to input from a chronically inflamed thoracolumbar fascia.

METHODS

Recordings from rat dorsal horn neurons were made in the spinal segment L3. Twelve days before the recordings, the thoracolumbar fascia was inflamed by injection of complete Freund's adjuvant. Control animals received an injection of isotonic saline. In addition, behavioural experiments were carried out.

RESULTS

Neurons in the spinal segment L3 do not normally receive input from the fascia, but 11.1% of the neurons did when the fascia was inflamed. Compared with control, the proportion of neurons having input from all deep somatic tissues rose from 10.8% to 33.3% (p < 0.02). Moreover, many neurons acquired new deep receptive fields, most of which were located in the hindlimb (p < 0.04). Surprisingly, the pressure pain threshold of the inflamed rats did not change, but they showed a reduction in exploratory activity.

CONCLUSIONS

One of the prominent findings was the appearance of new receptive fields in deep tissues of the hindlimb. Together with the expansion of the spinal target region of fascia afferents into the segment L3, the appearance of new receptive fields is a possible explanation for the spread of pain in patients with non-specific low back pain.

Fascia and Primo Vascular System

The anatomical basis for the concept of acupuncture points/meridians in traditional Chinese medicine (TCM) has not been resolved. This paper reviews the fascia research progress and the relationship among acupuncture points/meridians, primo vascular system (PVS), and fascia. Fascia is as a covering, with common origins of layers of the fascial system despite diverse names for individual parts. Fascia assists gliding and fluid flow and holds memory and is highly innervated. Fascia is intimately involved with nourishment of all cells of the body, including those of disease and cancer. The human body's fascia network may be the physical substrate represented by the meridians of TCM. The PVS is a newly found circulatory system; recent increased interest has led to new research and new discoveries in the anatomical and functional aspects of the PVS. The fasciology theory provides new insights into the physiological effects of acupuncture needling on basic cellular mechanisms including connective tissue mechanotransduction and regeneration. This view represents a theoretical basis and means for applying modern biomedical research to examining TCM principles and therapies, and it favors a holistic approach to diagnosis and treatment.