Capsaicin inhibits responses of fine afferents from the knee joint of the cat to mechanical and chemical stimuli

A review of the cellular and molecular effects of extracorporeal shockwave therapy

Extracorporeal shockwave therapy (ESWT) is a novel therapeutic modality and its use in promoting connective tissue repair and analgesic effect has been advocated in the literature. It is convenient, cost-effective, and has negligible complications; it therefore bypasses many of the problems associated with surgical interventions. This paper reviews the proposed mechanisms of action in promoting tissue repair and regeneration as well as analysing its efficacy providing an analgesic effect in clinical applications. Further research will be required to not only identify the underlying mechanisms more precisely, but will also be critical for ensuring consistency across the literature so that the most beneficial treatment protocol can be developed. Extracorporeal shockwave therapy stands as a promising alternative modality in promoting tissue repair.

Mechanically sensitive Aδ nociceptors that innervate bone marrow respond to changes in intra-osseous pressure

KEY POINTS

Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a novel in vivo bone-nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra-osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra-osseous pressure stimulus and express the Piezo2 mechano-transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and that the Piezo2 mechano-transducer may be involved.

ABSTRACT

Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Aδ nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic-tonic units that appear to code for different intensities of intra-osseous pressure, or phasic units that code for the rate of change in intra-osseous pressure. Three different subclasses of mechanically sensitive Aδ units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic-tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak-to-peak action potential amplitude. The majority of small-diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Aδ mechano-nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and imply that Piezo2 signalling may be involved in mechano-transduction in these receptors.

Extracorporeal shockwave therapy as a novel and potential treatment for degenerative cartilage and bone disease: Osteoarthritis. A qualitative analysis of the literature

Osteoarthritis (OA) is characterized with pathological changes on articular cartilage and subchondral bone, with clinical symptoms of pain and motor dysfunction in affected joints. A growing number of investigations demonstrated the therapeutic effects of extracorporeal shockwave therapy (ESWT) on joints with OA. While the partial mechanisms of action are based on cellular mechanotransduction through cytoskeleton into nuclei to regulate gene expression and cause biophysical influences, the efficacy and exact mechanisms are still under exploration. At present, a summary of the evidence regarding effectiveness of ESWT on OA is not available. The purpose of this review is thus to offer an overview of ESWT in the management of OA in the aspects of cartilage, subchondral bone, pain sensation and motor function, in hopes of eliciting further multi-disciplinary scientific investigations into this promising application as an adjunct to other modalities or surgery. The optimal frequencies, impulses, energy intensity and protocols of ESWT in the management of OA continue to be elucidated. Further studies are required to reveal its exact mechanisms and biophysical effects on cells, animals and humans prior to the clinical application.

Sensory innervation of rat contracture shoulder model

BACKGROUND

To date, few studies have investigated the cause of pain experienced by patients with frozen shoulder. The purposes of this study were to establish a rat contracture model and clarify the innervation pattern of the glenohumeral (GH) joint and subacromial bursa (SAB) using immunohistochemistry in the dorsal root ganglion (DRG) neurons.

MATERIALS AND METHODS

The rat contracture models were made by tying the animal's humerus and scapula with No. 2-0 FiberWire (Arthrex, Naples, FL, USA). Contracture was confirmed on x-ray images taken 8 weeks after the operation. Subsequently, two kinds of neurotracers, Fluoro-Gold (FG) (Fluorochrome, Denver, CO, USA) and 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) (Molecular Probes, Eugene, OR, USA), were used to detect the GH joints and SAB separately. FG tracers were injected into GH joints, and DiI tracers were injected into the SAB. At 7 days after injection, DRGs were harvested between C1 and T1. Immunohistochemistry by use of calcitonin gene-related peptide (CGRP) was performed. CGRP is thought to be one of the causes of pain sensation in joint disease. We evaluated the percentages of FG-labeled CGRP-immunoreactive (CGRP-ir) neurons in the total number of FG-labeled neurons and of DiI-labeled CGRP-ir neurons in the total number of DiI-labeled neurons.

RESULTS

Abduction and total arc of the rotation were statistically significantly decreased in the contracture group. Furthermore, the percentage of CGRP-ir DRG neurons was significantly higher in the contracture group in both the GH joint and SAB.

CONCLUSION

These results show that pain sensation in rat shoulder contracture may be induced by the up-regulation of CGRP expression in DRG neurons.

Extracorporeal shock wave therapy improves motor dysfunction and pain originating from knee osteoarthritis in rats

OBJECTIVE

Although there have been several reports on the use of extracorporeal shock wave therapy (ESWT), the efficacy of ESWT for knee osteoarthritis (OA) has not been clarified. The aim of this study is to investigate the effect of ESWT on OA in a rat knee model.

METHODS

The rats were divided into three groups: (1) control, (2) OA, and (3) ESWT (knee OA+shock wave therapy). Behavioral analysis consisted of measuring the duration of walking on a treadmill. The expression of calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons innervating the knee using immunohistochemistry was examined in the three groups at their peak time point on the treadmill.

RESULTS

Walking duration was significantly extended 4, 7 and 14 days after ESWT in rats with knee OA (peak time point: 4 days), again decreasing by days 21 and 28. Immunohistochemical studies revealed that the OA group had significantly higher percentages of CGRP positive neurons in the DRG than were found in the control group. In addition, ESWT reduced the ratio of CGRP positive DRG neurons in the OA model.

CONCLUSION

The improvement in walking ability and the reduction of CGRP positive neurons in DRG indicates that ESWT is a useful treatment for knee OA.

Regulation of calcitonin gene-related peptide and TRPV1 in a rat model of osteoarthritis

Pain in osteoarthritis (OA) remains an intractable problem in a majority of patients, with many of the commonly prescribed analgesics providing insufficient relief and considerable side effects. However, the structural or mechanistic cause of OA pain is still unknown. Animal models to address this issue have only recently been established, with much of the research to date focused on tissue pathology rather than pain. We have previously compared the surgically induced partial medial meniscectomy and chemically induced intra-articular iodoacetate injection rat models of OA in the rat, with reference to pain behaviour. This demonstrated relevant tissue pathology in both models, but greater evidence of pain related behaviour in the iodoacetate induced model. Here we further investigate the iodoacetate model using Fast Blue backlabelling from the articular joint space to identify the cell bodies of primary sensory afferents from the knee at the L4 dorsal root ganglion. Expression of calcitonin gene-related peptide (CGRP) and the vanilloid receptor TRPV1 was quantified in these backlabelled cells and was enriched in the knee afferents in all animals studied, compared to the expression in neurons across the whole dorsal root ganglia (DRG). Analysis of the backlabelled population in the osteoarthritis model and controls showed an increase in both CGRP and TRPV1 expression in the iodoacetate model compared with control animals. Therefore, there is a potential role for CGRP and TRPV1 in the manifestation of pain behaviour accompanied by OA changes in the knee in the iodoacetate induced model.

Ginger--an herbal medicinal product with broad anti-inflammatory actions

The anti-inflammatory properties of ginger have been known and valued for centuries. During the past 25 years, many laboratories have provided scientific support for the long-held belief that ginger contains constituents with antiinflammatory properties. The original discovery of ginger's inhibitory effects on prostaglandin biosynthesis in the early 1970s has been repeatedly confirmed. This discovery identified ginger as an herbal medicinal product that shares pharmacological properties with non-steroidal anti-inflammatory drugs. Ginger suppresses prostaglandin synthesis through inhibition of cyclooxygenase-1 and cyclooxygenase-2. An important extension of this early work was the observation that ginger also suppresses leukotriene biosynthesis by inhibiting 5-lipoxygenase. This pharmacological property distinguishes ginger from nonsteroidal anti-inflammatory drugs. This discovery preceded the observation that dual inhibitors of cyclooxygenase and 5-lipoxygenase may have a better therapeutic profile and have fewer side effects than non-steroidal anti-inflammatory drugs. The characterization of the pharmacological properties of ginger entered a new phase with the discovery that a ginger extract (EV.EXT.77) derived from Zingiber officinale (family Zingiberaceae) and Alpina galanga (family Zingiberaceae) inhibits the induction of several genes involved in the inflammatory response. These include genes encoding cytokines, chemokines, and the inducible enzyme cyclooxygenase-2. This discovery provided the first evidence that ginger modulates biochemical pathways activated in chronic inflammation. Identification of the molecular targets of individual ginger constituents provides an opportunity to optimize and standardize ginger products with respect to their effects on specific biomarkers of inflammation. Such preparations will be useful for studies in experimental animals and humans.

The proportion of TRPV1 protein-positive lumbar DRG neurones does not increase in the course of acute and chronic antigen-induced arthritis in the knee joint of the rat

The TRPV1 receptor, previously called VR1 receptor, is a non-selective cation channel gated by capsaicin, noxious heat, protons and anandamide. The TRPV1 receptor is essential for the development of thermal hyperalgesia. The present study investigated whether the proportion of neurones with TRPV1 receptor increases in lumbar DRG neurones in the course of an antigen-induced arthritis (AIA) of one knee joint in the rat. In control rats 38.1+/-2.3% of the neurones from sections of the L1-L5 ganglia showed TRPV1-like immunoreactivity. Neither in the acute (3 days) nor chronic phase (21 days) of AIA in the knee joint the proportion of TRPV1-like immunoreactive profiles showed significant changes. Thus AIA in the knee joint is not associated with an up-regulation of the TRPV1 receptor in the lumbar DRG neurones.

Meloxicam and selective COX-2 inhibitors in the management of pain in the palliative care population

This paper discusses the treatment of pain in the palliative care patient, specifically the use of meloxicam and recent advances in agents with cyclooxygenase-2 (COX-2) selectivity. Meloxicam is a nonsteroidal anti-inflammatory drug (NSAID) that preferentially inhibits COX-2 more than cyclooxygenase-1 (COX-1), especially at low doses, thereby offering advantages over traditional nonselective NSAIDs. New COX-2 selective agents are discussed, including valdecoxib, parecoxib, etoricoxib, and COX-189.

Mechanisms of pain in arthritis

Inflammation in the joint causes peripheral sensitization (increase of sensitivity of nociceptive primary afferent neurons) and central sensitization (hyperexcitability of nociceptive neurons in the central nervous system). The processes of sensitization are thought to be the basis of arthritic pain that appears as spontaneous pain (joints at rest) and hyperalgesia (augmented pain response on noxious stimulation and pain on normally nonpainful stimulation). Sensitization also facilitates efferent neuronal processes through which the nervous system influences the inflammatory process. Peripheral sensitization is produced by the action of inflammatory mediators such as bradykinin, prostaglandins, neuropeptides, and cytokines which activate corresponding receptors in proportions of nerve fibers. In addition, the expression of receptors, for example, bradykinin and neurokinin 1 receptors, is upregulated during inflammation. The development of hyperexcitability of spinal cord neurons is produced by various transmitter/receptor systems that constitute and modulate synaptic activation of the neurons. The key transmitter is glutamate that activates N-methyl-d-aspartate (NMDA) and non-NMDA receptors on spinal cord neurons. Blockade of these receptors prevents and reduces central sensitization. Excitatory neuropeptides (substance P and calcitonin gene-related peptide) further central sensitization. Central sensitization also is facilitated by mediators that have complex actions (e.g., prostaglandin E(2)). Spinal PGE(2) binds to receptors at presynaptic endings of primary afferent neurons (thus influencing synaptic release) and to receptors on postsynaptic spinal cord neurons. The administration of PGE(2) to the spinal cord surface produces changes of responsiveness of spinal neurons similar to peripheral inflammation, and spinal indomethacin to the spinal cord attenuates development of hyperexcitability significantly.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

The effects of capsaicin, bradykinin, PGE2 and cicaprost on the discharge of articular sensory receptors in vitro

The responses of articular sensory receptors to capsaicin, bradykinin, PGE2, and the selective IP-receptor agonist cicaprost were studied in a rat isolated hindlimb in vitro preparation. Long-term maintenance of normal sensory receptor function was achieved in vitro under conditions of combined superfusion and slow perfusion. Response characteristics to mechanical or chemical stimuli on articular sensory receptors identified in this study did not differ to those reported in vivo. This preparation lacks complex effects mediated via spinal or central reflex mechanisms and allows greater control over the physiological environment of the receptors being studied. These results support the conclusion that the effects of capsaicin, bradykinin and the prostanoids are mediated by distinct pharmacological receptors associated with articular sensory nerve endings. The potent potentiating effects of cicaprost on bradykinin-induced excitation suggests that these actions are mediated via IP-receptors.

Activation of normal and inflamed fine articular afferent units by serotonin

In cats anesthetized with alpha-chloralose, extracellular recordings were made from fine afferent units belonging to the medial articular nerve (MAN) of the knee joint. The excitatory and sensitizing effects on articular afferents of serotonin (5-HT) applied intra-arterially close to the joint were examined. The joints were either normal or an experimental arthritis had been induced some hours before the recording session. Bolus injections of 1.35-135 micrograms 5-HT excited about 43% of group III (CV: 2.5-20 m/sec) and 73% of group IV units (CV: less than 2.5 m/sec) from normal joints. The latency was usually between 10 and 30 sec, and the duration and size of the responses were dose-dependent. Fast group III units (CV: greater than 16 m/sec) and group II units (CV: greater than 20 m/sec) were never excited by 5-HT. Repetitive administration led to pronounced tachyphylaxis of the 5-HT response. Inflammation induced an enhanced sensitivity of group III articular afferent units to close intra-arterial application of 5-HT. In particular the total duration of each response was considerably prolonged (4-10 min against 1-2 min under normal conditions). At the same time the tachyphylaxis seen under normal conditions was greatly reduced. In contrast, group IV articular afferent units did not become sensitized to 5-HT in the course of inflammation. In normal joints 5-HT did not sensitize fine afferent units for movement-induced responses. However, after inflammation, a distinct sensitization to such movements by 5-HT application could be observed both in group III and group IV fiber ranges. The sensitization had a short time course not exceeding 7 min. The tonic component of the movement-induced response was more enhanced than the phasic one. The bolus application of 5-HT led to temporary vasoconstriction of the knee joint vessels. This vasoconstriction was especially pronounced in inflamed joints and impeded the access of subsequently applied substances to the terminal regions of the afferent units under observation. It is concluded that the present results support the notion that 5-HT may participate in the mediation of pain from inflamed tissue such as an arthritic joint by exciting and sensitizing fine afferent units. During inflammation group III units are particularly sensitive to 5-HT and, thus, may carry the bulk of the 5-HT-induced nociceptive messages.