NGF-evoked sensitization of muscle fascia nociceptors in humans

Movement Evoked Pain and Mechanical Hyperalgesia after Intramuscular Injection of Nerve Growth Factor: A Model of Sustained Elbow Pain

OBJECTIVE

Lateral epicondylalgia presents as lateral elbow pain provoked by upper limb tasks. An experimental model of elbow pain provoked by movement/muscle contraction and maintained over several days is required to better understand the mechanisms underlying sustained elbow pain. This study investigated the time course and pain location induced by nerve growth factor (NGF) injection into a wrist extensor muscle, and whether movement and muscle contraction/stretch provoked pain.

METHODS

On Day 0 twenty-six painfree volunteers were injected with NGF (N = 13) or isotonic saline (randomized) into the extensor carpi radialis brevis (ECRB) muscle of the dominant arm. On Day 2 pain was induced in all participants by hypertonic saline injection into ECRB. A Likert scale and patient-rated tennis elbow evaluation (PRTEE) was used to assess pain and functional limitation (Days 0-10). Pain intensity during contraction/stretch of ECRB, and pressure pain thresholds (PPTs) were recorded before and after injections on Days 0 and 2, and Days 4 and 10.

RESULTS

Compared with isotonic saline, NGF evoked: i) greater Likert pain ratings from 12 hours post-injection until Day 6, ii) greater PRTEE scores on Days 2 and 4, iii) greater pain during ECRB contraction/stretch on Day 2, and iv) lower PPTs on Day 4.

CONCLUSIONS

This article presents a novel experimental human pain model suitable to study the sustained effects of lateral elbow pain on sensorimotor function and to probe the mechanisms underlying persistent musculoskeletal pain.

Comparison of nerve growth factor-induced sensitization pattern in lumbar and tibial muscle and fascia

INTRODUCTION

Nerve growth factor (NGF) induces profound hyperalgesia. In this study we explored patterns of NGF sensitization in muscle and fascia of distal and paraspinal sites.

METHODS

We injected 1 µg of NGF into human (n = 8) tibialis anterior and erector spinae muscles and their fasciae. The spatial extent of pressure sensitization, pressure pain threshold, and mechanical hyperalgesia (150 kPa, 10 s) was assessed at days 0.25, 1, 3, 7, 14, and 21. Chemical sensitization was explored by acidic buffer injections (pH 4, 100 µl) at days 7 and 14.

RESULTS

The mechanical hyperalgesia area was larger in tibial fascia than in muscle. Pressure pain thresholds were lower, tonic pressure pain ratings, and citrate buffer evoked pain higher in fascia than in muscle.

CONCLUSIONS

Spatial mechanical sensitization differs between muscle and fascia. Thoracolumbar fasciae appear more sensitive than tibial fasciae and may be major contributors to low back pain, but the temporal sensitization profile is similar between paraspinal and distal sites. Muscle Nerve 52: 265-272, 2015.

Innervation changes induced by inflammation of the rat thoracolumbar fascia

Recently, the fascia innervation has become an important issue, particularly the existence of nociceptive fibers. Fascia can be a source of pain in several disorders such as fasciitis and non-specific low back pain. However, nothing is known about possible changes of the fascia innervation under pathological circumstances. This question is important, because theoretically pain from the fascia cannot only be due to increased nociceptor discharges, but also to a denser innervation of the fascia by nociceptive endings. In this histological study, an inflammation was induced in the thoracolumbar fascia (TLF) of rats and the innervation by various fiber types compared between the inflamed and intact TLF. Although the TLF is generally considered to have proprioceptive functions, no corpuscular proprioceptors (Pacini and Ruffini corpuscles) were found. To obtain quantitative data, the length of fibers and free nerve endings were determined in the three layers of the rat TLF: inner layer (IL, adjacent to the multifidus muscle), middle layer (ML) and outer layer (OL). The main results were that the overall innervation density showed little change; however, there were significant changes in some of the layers. The innervation density was significantly decreased in the OL, but this change was partly compensated for by an increase in the IL. The density of substance P (SP)-positive - presumably nociceptive - fibers was significantly increased. In contrast, the postganglionic sympathetic fibers were significantly decreased. In conclusion, the inflamed TLF showed an increase of presumably nociceptive fibers, which may explain the pain from a pathologically altered fascia. The meaning of the decreased innervation by sympathetic fibers is obscure at present. The lack of proprioceptive corpuscular receptors within the TLF does not preclude its role as a proprioceptive structure, because some of the free nerve endings may function as proprioceptors.

A unifying neuro-fasciagenic model of somatic dysfunction - underlying mechanisms and treatment - Part I

This paper offers an extensive review of the main fascia-mediated mechanisms underlying various dysfunctional and pathophysiological processes of clinical relevance for manual therapy. The concept of somatic dysfunction is revisited in light of the diverse fascial influences that may come into play in its genesis and maintenance. A change in perspective is thus proposed: from a nociceptive model that for decades has viewed somatic dysfunction as a neurologically-mediated phenomenon, to a unifying fascial model that integrates neural influences into a multifactorial and multidimensional interpretation of dysfunctional process as being partially, if not entirely, mediated by the fascia.

Role of nerve growth factor in pain

Nerve growth factor (NGF) was first identified as a substance that is essential for the development of nociceptive primary neurons and later found to have a role in inflammatory hyperalgesia in adults. Involvement of NGF in conditions with no apparent inflammatory signs has also been demonstrated. In this review we look at the hyperalgesic effects of exogenously injected NGF into different tissues, both human and animal, with special emphasis on the time course of these effects. The roles of NGF in inflammatory and neuropathic conditions as well as cancer pain are then reviewed. The role of NGF in delayed onset muscle soreness is described in more detail than its other roles based on the authors' recent observations. Acute effects are considered to be peripherally mediated, and accordingly, sensitization of nociceptors by NGF to heat and mechanical stimulation has been reported. Changes in the conductive properties of axons have also been reported. The intracellular mechanisms so far proposed for heat sensitization are direct phosphorylation and membrane trafficking of TRPV1 by TrkA. Little investigation has been done on the mechanism of mechanical sensitization, and it is still unclear whether mechanisms similar to those for heat sensitization work in mechanical sensitization. Long-lasting sensitizing effects are mediated both by changed expression of neuropeptides and ion channels (Na channels, ASIC, TRPV1) in primary afferents and by spinal NMDA receptors. Therapeutic perspectives are briefly discussed at the end of the chapter.

Therapeutic potential of inhibitors of endocannabinoid degradation for the treatment of stress-related hyperalgesia in an animal model of chronic pain

The occurrence of chronic stress, depression, and anxiety can increase nociception in humans and may facilitate the transition from localized to chronic widespread pain. The mechanisms underlying chronic widespread pain are still unknown, hindering the development of effective pharmacological therapies. Here, we exposed C57BL/6J mice to chronic unpredictable stress (CUS) to investigate how persistent stress affects nociception. Next, mice were treated with multiple intramuscular nerve growth factor (NGF) injections, which induced chronic widespread nociception. Thus, combination of CUS and NGF served as a model where psychophysiological impairment coexists with long-lasting hyperalgesia. We found that CUS increased anxiety- and depression-like behavior and enhanced basal nociception in mice. When co-applied with repeated NGF injections, CUS elicited a sustained long-lasting widespread hyperalgesia. In order to evaluate a potential therapeutic strategy for the treatment of chronic pain associated with stress, we hypothesized that the endocannabinoid system (ECS) may represent a target signaling system. We found that URB597, an inhibitor of the anandamide-degrading enzyme fatty acid amide hydrolase (FAAH), and JZL184, an inhibitor of the 2-arachidonoyl glycerol-degrading enzyme monoacylglycerol lipase (MAGL), increased eCB levels in the brain and periphery and were both effective in reducing CUS-induced anxiety measured by the light-dark test and CUS-induced thermal hyperalgesia. Remarkably, the long-lasting widespread hyperalgesia induced by combining CUS and NGF was effectively reduced by URB597, but not by JZL184. Simultaneous inhibition of FAAH and MAGL did not improve the overall therapeutic response. Therefore, our findings indicate that enhancement of anandamide signaling with URB597 is a promising pharmacological approach for the alleviation of chronic widespread nociception in stress-exposed mice, and thus, it could represent a potential treatment strategy for chronic pain associated with neuropsychiatric disorders in humans.

Intradermal Injection with Nerve Growth Factor: A Reproducible Model to Induce Experimental Allodynia and Hyperalgesia

BACKGROUND

Nerve growth factor (NGF) plays a pivotal role in survival, growth, and differentiation of the nervous system. Increased levels of NGF have been reported in human pain disorders. Experimental injection of NGF in humans is known to evoke long-lasting mechanical sensitization and subsequent allodynia and hyperalgesia.

METHODS

Reproducibility of intradermal injection of NGF was investigated. Twenty healthy male volunteers were included (mean age 24 years, range 19 to 31). The experiment consisted of 3 identical treatment periods with period 1 stimulating the right arm, period 2 the left arm, and period 3 stimulating the right arm again (period one and three were separated by at least 21 days). Pain intensity was assessed in response to several phasic stimuli in 3 adjacent sites of the volar forearm: pressure; pinprick; brush; and heat before and after NGF injection. Additionally, areas of allodynia and secondary hyperalgesia were assessed. Rekindling with pressure was performed 1 hour and 24 hours after injection. Reproducibility was assessed with intraclass correlation coefficient (ICC 3,1).

RESULTS

ICC values > 0.6 for all phasic stimuli and for the area of hyperalgesia. After NGF injection, pressure pain (P < 0.001) and heat pain (P < 0.01) sensitivity increased significantly. After rekindling, the area of hyperalgesia (von Frey 26 g) was significantly increased (P = 0.03) and sensitivity to pinprick was increased (P < 0.02).

CONCLUSION

Intradermal NGF injection is capable of inducing reproducible allodynia and hyperalgesia, and the model is recommended for future experimental and pharmacological pain studies.

Pro-neurotrophins, sortilin, and nociception

Nerve growth factor (NGF) signaling is important in the development and functional maintenance of nociceptors, but it also plays a central role in initiating and sustaining heat and mechanical hyperalgesia following inflammation. NGF signaling in pain has traditionally been thought of as primarily engaging the classic high-affinity receptor tyrosine kinase receptor TrkA to initiate sensitization events. However, the discovery that secreted proforms of nerve NGF have biological functions distinct from the processed mature factors raised the possibility that these proneurotrophins (proNTs) may have distinct function in painful conditions. ProNTs engage a novel receptor system that is distinct from that of mature neurotrophins, consisting of sortilin, a type I membrane protein belonging to the VPS10p family, and its co-receptor, the classic low-affinity neurotrophin receptor p75NTR. Here, we review how this new receptor system may itself function with or independently of the classic TrkA system in regulating inflammatory or neuropathic pain.

Clinical and symptomatological reflections: the fascial system

Every body structure is wrapped in connective tissue, or fascia, creating a structural continuity that gives form and function to every tissue and organ. Currently, there is still little information on the functions and interactions between the fascial continuum and the body system; unfortunately, in medical literature there are few texts explaining how fascial stasis or altered movement of the various connective layers can generate a clinical problem. Certainly, the fascia plays a significant role in conveying mechanical tension, in order to control an inflammatory environment. The fascial continuum is essential for transmitting muscle force, for correct motor coordination, and for preserving the organs in their site; the fascia is a vital instrument that enables the individual to communicate and live independently. This article considers what the literature offers on symptoms related to the fascial system, trying to connect the existing information on the continuity of the connective tissue and symptoms that are not always clearly defined. In our opinion, knowing and understanding this complex system of fascial layers is essential for the clinician and other health practitioners in finding the best treatment strategy for the patient.

Screening the role of pronociceptive molecules in a rodent model of endometriosis pain

Chronic pain is a major symptom in patients with endometriosis, a common gynecologic condition affecting women in their reproductive years. Although many proalgesic substances are produced by endometriosis lesions, experimental evidence supporting their relative roles is still lacking. Furthermore, it is unclear whether these proalgesic agents directly activate nociceptors to induce endometriosis pain. To determine their relative contribution to pain associated with endometriosis, we evaluated the intrathecal administration of oligodeoxynucleotides (ODNs) antisense to messenger RNA for receptors for 3 pronociceptive mediators known to be produced by the ectopic endometrium. Two weeks after the implant of autologous uterine tissue onto the gastrocnemius muscle, local mechanical hyperalgesia was observed in operated rats. Intrathecal antisense ODN targeting messenger RNA for the interleukin 6 receptor-signaling complex subunit glycoprotein 130 and the nerve growth factor tyrosine kinase receptor A, but not their mismatch ODNs, reversibly attenuated mechanical hyperalgesia at the implant site. In contrast, intrathecal antisense ODN targeting the tumor necrosis factor receptor 1, at a dose that markedly inhibited intramuscularly injected tumor necrosis factor alpha, had only a small antihyperalgesic effect in this model. These results indicate the relative contribution of pronociceptive mediators produced by ectopic endometrial tissue to endometriosis pain. The experimental approach presented here provides a novel method to evaluate for the differential contribution of mediators produced by other painful lesions as well as endometriosis lesions as targets for novel treatment of pain syndromes.

PERSPECTIVE

This article presents evidence for the relative contribution of proalgesic mediators to primary hyperalgesia displayed by rats submitted to a model of endometriosis pain. This approach can be used to identify potential targets for the treatment of endometriosis pain.

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.

Fascial components of the myofascial pain syndrome

Myofascial pain syndrome (MPS) is described as the muscle, sensory, motor, and autonomic nervous system symptoms caused by stimulation of myofascial trigger points (MTP). The participation of fascia in this syndrome has often been neglected. Several manual and physical approaches have been proposed to improve myofascial function after traumatic injuries, but the processes that induce pathological modifications of myofascial tissue after trauma remain unclear. Alterations in collagen fiber composition, in fibroblasts or in extracellular matrix composition have been postulated. We summarize here recent developments in the biology of fascia, and in particular, its associated hyaluronan (HA)-rich matrix that address the issue of MPS.

Nerve growth factor and nociception: from experimental embryology to new analgesic therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.

[Ultrasound in interventional pain therapy]

Peripheral nerve blocks are currently performed relatively blind even in the most complex anatomical structures and physicians mostly rely on palpable anatomical landmarks on the surface. Ultrasound has become an indispensable part of the modern medical world and has long since found its way into almost all medical professions. More and more this trend also reaches interventional pain physicians as it is possible to accurately target structures, to track the needle course during the intervention and to visualize the spread of the local anesthetic. Another advantage compared to other radiological techniques is the profound radiation safety for patients as well as for personnel performing the intervention. A deep understanding of anatomy and its correlate in ultrasound images is one of the most important requirements for the successful use of these interventional techniques. Moreover, the safe performance of the procedure depends on the simultaneous hand-eye coordination. Nevertheless, despite the euphoria ultrasound technology should only be used in pain management with sufficient indications.

Augmented pain behavioural responses to intra-articular injection of nerve growth factor in two animal models of osteoarthritis

OBJECTIVES

Nerve growth factor (NGF) is a promising analgesic target, particularly in osteoarthritis (OA) where existing therapies are inadequate. We hypothesised that pain responses to NGF are increased in OA joints. Here, NGF-evoked pain behaviour was compared in two rodent models of OA, and possible mechanisms underlying altered pain responses were examined.

METHODS

OA was induced in rat knees by meniscal transection (MNX) or intra-articular monosodium iodoacetate injection (MIA). Once OA pathology was fully established (day 20), we assessed pain behaviour (hindlimb weight-bearing asymmetry and hindpaw mechanical withdrawal thresholds) evoked by intra-articular injection of NGF (10 µg). Possible mechanisms underlying alterations in NGF-induced pain behaviour were explored using indomethacin pretreatment, histopathological evaluation of synovitis, and rtPCR for NGF receptor (tropomyosin receptor kinase (Trk)-A) expression in dorsal root ganglia (DRG).

RESULTS

Both the MIA and MNX models of OA displayed reduced ipsilateral weight bearing and hindpaw mechanical withdrawal thresholds, mild synovitis and increased TrkA expression in DRG. NGF injection into OA knees produced a prolonged augmentation of weight-bearing asymmetry, compared to NGF injection in non-osteoarthritic knees. However, hindpaw mechanical withdrawal thresholds were not further decreased by NGF. Pretreatment with indomethacin attenuated NGF-facilitated weight-bearing asymmetry and reversed OA-induced ipsilateral TrkA mRNA up-regulation.

CONCLUSIONS

OA knees were more sensitive to NGF-induced pain behaviour compared to non-osteoarthritic knees. Cyclo-oxygenase products may contribute to increased TrkA expression during OA development, and the subsequent increased NGF sensitivity. Treatments that reduce sensitivity to NGF have potential to improve OA pain.

[Ultrasound in interventional pain therapy]

Peripheral nerve blocks are currently performed relatively blind even in the most complex anatomical structures and physicians mostly rely on palpable anatomical landmarks on the surface. Ultrasound has become an indispensable part of the modern medical world and has long since found its way into almost all medical professions. More and more this trend also reaches interventional pain physicians as it is possible to accurately target structures, to track the needle course during the intervention and to visualize the spread of the local anesthetic. Another advantage compared to other radiological techniques is the profound radiation safety for patients as well as for personnel performing the intervention. A deep understanding of anatomy and its correlate in ultrasound images is one of the most important requirements for the successful use of these interventional techniques. Moreover, the safe performance of the procedure depends on the simultaneous hand-eye coordination. Nevertheless, despite the euphoria ultrasound technology should only be used in pain management with sufficient indications.

Sciatic nerve regeneration is not inhibited by anti-NGF antibody treatment in the adult rat

Elevated nerve growth factor (NGF) is believed to play a role in many types of pain. An NGF-blocking antibody (muMab 911) has been shown to reduce pain and hyperalgesia in pain models, suggesting a novel therapeutic approach for pain management. Since NGF also plays important roles in peripheral nervous system development and sensory nerve outgrowth, we asked whether anti-NGF antibodies would adversely impact peripheral nerve regeneration. Adult rats underwent a unilateral sciatic nerve crush to transect axons and were subcutaneously dosed weekly for 8weeks with muMab 911 or vehicle beginning 1day prior to injury. Plasma levels of muMab 911 were assessed from blood samples and foot print analysis was used to assess functional recovery. At 8-weeks post-nerve injury, sciatic nerves were prepared for light and electron microscopy. In a separate group, Fluro-Gold was injected subcutaneously at the ankle prior to perfusion, and counts and sizes of retrogradely labeled and unlabeled dorsal root ganglion neurons were obtained. There was no difference in the time course of gait recovery in antibody-treated and vehicle-treated animals. The number of myelinated and nonmyelinated axons was the same in the muMab 911-treated crushed nerves and intact nerves, consistent with observed complete recovery. Treatment with muMab 911 did however result in a small decrease in average cell body size on both the intact and injured sides. These results indicate that muMab 911 did not impair functional recovery or nerve regeneration after nerve injury in adult rats.