Structural pathology in a rodent model of osteoarthritis is associated with neuropathic pain: increased expression of ATF-3 and pharmacological characterisation

Pharmacological validation of early and late phase of rat mono-iodoacetate model using the Tekscan system

BACKGROUND

Previous pharmacological validations of the rat mono-iodoacetate (MIA)-induced chronic joint pain model were mostly performed by measuring weight-bearing (WB) deficit with an incapacitance tester. However, conventional incapacitance testers have several drawbacks including restrain stress on animal and sole use of hind limbs WB.

OBJECTIVES

The aim of the present study was to compare pharmacological sensitivity of the early (up to 1 week after MIA) versus late (between 2 and 4 weeks after MIA) phase of the rat MIA model using a highly sensitive tactile pressure measurement system (Tekscan(®)), which can measure weight borne by all four limbs and the tail in a non-restrained animal.

METHODS

The Tekscan(®) WB measurement system was used in MIA rats to examine the acute and chronic dosing effects of drugs that targeted different mechanisms. Electrophysiological recordings from joint afferents and biochemical analysis of synovial fluid were also performed.

RESULTS

Dexamethasone, duloxetine and morphine significantly alleviated WB deficits in the Tekscan(®) system during both early and late phase of the MIA model while celecoxib and naproxen alleviated WB deficit only during the early phase. Similarly, naproxen was able to inhibit spontaneous neuronal activity from MIA joint afferents only during the early phase. Finally, concentrations of prostaglandin E(2) in synovial fluid were elevated only during the early phase of the rat MIA model.

CONCLUSIONS

Our pharmacological validation studies using the Tekscan(®) system along with electrophysiological and biochemical results suggest different mechanisms for early and late phase of MIA-induced chronic joint pain in rat.

Identification of the pathogenic pathways in osteoarthritic hip cartilage: commonality and discord between hip and knee OA

OBJECTIVE

To define for the first time the transcriptomes of normal and end-stage osteoarthritis (OA) hip cartilage.

MATERIALS AND METHODS

RNA was isolated from cartilage within 2h of joint replacement surgery. Gene expression was analyzed using Agilent GeneSpring GX 11 following hybridization to Illumina Human HT-12 V3 microarrays. Real-time reverse-transcription polymerase chain reaction (RT-PCR) was used to validate the expression of six genes identified by microarray as differentially expressed. Gene Set Enrichment Analysis (GSEA) and Ingenuity Pathway Analysis (IPA) were used to investigate enriched functions or canonical pathways amongst differentially expressed genes respectively.

RESULTS

In total we identified 998 differentially expressed genes (fold change ≥ ±1.5, P-value ≤ 0.01) between neck of femur fracture (NOF) (n = 10) and OA hip (n = 9) patient cartilage. These differentially expressed genes were enriched within 71 canonical pathways. A comparison between a comparable knee dataset(20) only identified 229 genes similarly differentially expressed although remarkably 34 canonical pathways overlapped between experiments.

CONCLUSIONS

This study is the first to report a comprehensive gene expression analysis of human hip OA cartilage compared to control (NOF) cartilage at the whole-genome level. Our differential gene expression dataset shows excellent correlation with similar defined studies using comparable tissue but reveals discord between hip and knee OA at the individual gene status but with commonality with regards the molecular pathways involved.

Local ASIC3 modulates pain and disease progression in a rat model of osteoarthritis

Background

Recent data have suggested a relationship between acute arthritic pain and acid sensing ion channel 3 (ASIC3) on primary afferent fibers innervating joints. The purpose of this study was to clarify the role of ASIC3 in a rat model of osteoarthritis (OA) which is considered a degenerative rather than an inflammatory disease.

Methods

We induced OA via intra-articular mono-iodoacetate (MIA) injection, and evaluated pain-related behaviors including weight bearing measured with an incapacitance tester and paw withdrawal threshold in a von Frey hair test, histology of affected knee joint, and immunohistochemistry of knee joint afferents. We also assessed the effect of ASIC3 selective peptide blocker (APETx2) on pain behavior, disease progression, and ASIC3 expression in knee joint afferents.

Results

OA rats showed not only weight-bearing pain but also mechanical hyperalgesia outside the knee joint (secondary hyperalgesia). ASIC3 expression in knee joint afferents was significantly upregulated approximately twofold at Day 14. Continuous intra-articular injections of APETx2 inhibited weight distribution asymmetry and secondary hyperalgesia by attenuating ASIC3 upregulation in knee joint afferents. Histology of ipsilateral knee joint showed APETx2 worked chondroprotectively if administered in the early, but not late phase.

Conclusions

Local ASIC3 immunoreactive nerve is strongly associated with weight-bearing pain and secondary hyperalgesia in MIA-induced OA model. APETx2 inhibited ASIC3 upregulation in knee joint afferents regardless of the time-point of administration. Furthermore, early administration of APETx2 prevented cartilage damage. APETx2 is a novel, promising drug for OA by relieving pain and inhibiting disease progression.

Dose-dependent expression of neuronal injury markers during experimental osteoarthritis induced by monoiodoacetate in the rat

Background

It was recently reported that the mono-iodoacetate (MIA) experimental model of osteoarthritis (OA) courses with changes of neurons innervating the affected joints that are commonly interpreted as a neuronal response to axonal injury. To better characterize these changes, we evaluated the expression of two markers of neuronal damage, ATF-3 and NPY, and the growth associated protein GAP-43, in primary afferent neurons of OA animals injected with three different doses of MIA (0.3, 1 or 2 mg). Measurements were performed at days 3, 7, 14, 21 and 31 post-MIA injection.

Results

OA animals showed the characteristic histopathological changes of the joints and the accompanying nociceptive behaviour, evaluated by the Knee-Bed and CatWalk tests. An increase of ATF-3 expression was detected in the DRG of OA animals as early as 3 days after the injection of 1 or 2 mg of MIA and 7 days after the injection of 0.3 mg. NPY expression was increased in animals injected with 1 or 2 mg of MIA, at day 3 or in all time-points, respectively. From day 7 onwards there was a massive increase of GAP-43 expression in ATF-3 cells.

Conclusions

The expression of the neuronal injury markers ATF-3 and NPY as well as an up-regulation of GAP-43 expression, indicative of peripheral fibre regeneration, suggests that axonal injury and a regeneration response may be happening in this model of OA. This opens new perspectives in the unravelling of the physiopathology of the human disease.

Existence of a neuropathic pain component in patients with osteoarthritis of the knee

PURPOSE

Pain from osteoarthritis (OA) is generally classified as nociceptive (inflammatory). Animal models of knee OA have shown that sensory nerve fibers innervating the knee are significantly damaged with destruction of subchondral bone junction, and induce neuropathic pain (NP). Our objective was to examine NP in the knees of OA patients using painDETECT (an NP questionnaire) and to evaluate the relationship between NP, pain intensity, and stage of OA.

MATERIALS AND METHODS

Ninety-two knee OA patients were evaluated in this study. Pain scores using Visual Analogue Scales (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), painDETECT, duration of symptoms, severity of OA using the Kellgren-Lawrence (KL) system, and amount of joint fluid were evaluated and compared using a Spearman's correlation coefficient by rank test.

RESULTS

Our study identified at least 5.4% of our knee OA patients as likely to have NP and 15.2% as possibly having NP. The painDETECT score was significantly correlated with the VAS and WOMAC pain severity. Compared with the painDETECT score, there was a tendency for positive correlation with the KL grade, and tendency for negative correlation with the existence and amount of joint fluid, but these correlations were not significant.

CONCLUSION

PainDETECT scores classified 5.4% of pain from knee OA as NP. NP tended to be seen in patients with less joint fluid and increased KL grade, both of which corresponded to late stages of OA. It is important to consider the existence of NP in the treatment of knee OA pain.

Spontaneous firing in C-fibers and increased mechanical sensitivity in A-fibers of knee joint-associated mechanoreceptive primary afferent neurones during MIA-induced osteoarthritis in the rat

OBJECTIVE

Osteoarthritis (OA) pain mechanisms are poorly understood. We used the monosodium iodoacetate (MIA) model of knee OA to characterize changes in excitability during the course of OA in different classes of mechanosensitive afferents projecting to joint-associated tissues, and examine whether these afferent responses and pain behavior are correlated.

METHODS

Rats were injected intra-articularly with MIA (1mg in 50 μl). Hind-limb weight bearing was studied 3 (MIA3) and 14 (MIA14) days after MIA, followed by deep anesthesia and teased-nerve-fiber recordings. Spontaneous activity (SA) and mechanically evoked responses of A- and C-mechanosensitive fibers (AM and CM respectively, probably nociceptive) innervating tissues associated with the ipsilateral knee joint were examined.

RESULTS

MIA3 and MIA14 rats exhibited reduced ipsilateral weight bearing. SA (>0.02 impulses/s) occurred in ∼50% of CMs from MIA rats vs 0% in normals. SA firing rates in CMs were significantly higher than normal; decreased weight bearing was correlated with increased CM SA rates. Neither percentages of AMs with SA (20%) nor their firing rates (0-0.01 impulses/s) significantly increased after MIA. In contrast, in MIA rats AMs, but not CMs, exhibited decreased mechanical thresholds and increased firing rates in response to suprathreshold mechanical stimulation.

CONCLUSIONS

These findings of increased SA firing rate in CMs but not AMs and increased mechanical sensitivity of AMs, but not CMs, have not previously been reported. These are two distinct important physiological mechanisms that may underpin spontaneous pain (CMs) and stimulus-evoked pain (AMs) in OA. Our data contribute to a mechanism-based understanding of OA pain.

Characterisation of a peripheral neuropathic component of the rat monoiodoacetate model of osteoarthritis

Joint degeneration observed in the rat monoiodoacetate (MIA) model of osteoarthritis shares many histological features with the clinical condition. The accompanying pain phenotype has seen the model widely used to investigate the pathophysiology of osteoarthritis pain, and for preclinical screening of analgesic compounds. We have investigated the pathophysiological sequellae of MIA used at low (1 mg) or high (2 mg) dose. Intra-articular 2 mg MIA induced expression of ATF-3, a sensitive marker for peripheral neuron stress/injury, in small and large diameter DRG cell profiles principally at levels L4 and 5 (levels predominated by neurones innervating the hindpaw) rather than L3. At the 7 day timepoint, ATF-3 signal was significantly smaller in 1 mg MIA treated animals than in the 2 mg treated group. 2 mg, but not 1 mg, intra-articular MIA was also associated with a significant reduction in intra-epidermal nerve fibre density in plantar hindpaw skin, and produced spinal cord dorsal and ventral horn microgliosis. The 2 mg treatment evoked mechanical pain-related hypersensitivity of the hindpaw that was significantly greater than the 1 mg treatment. MIA treatment produced weight bearing asymmetry and cold hypersensitivity which was similar at both doses. Additionally, while pregabalin significantly reduced deep dorsal horn evoked neuronal responses in animals treated with 2 mg MIA, this effect was much reduced or absent in the 1 mg or sham treated groups. These data demonstrate that intra-articular 2 mg MIA not only produces joint degeneration, but also evokes significant axonal injury to DRG cells including those innervating targets outside of the knee joint such as hindpaw skin. This significant neuropathic component needs to be taken into account when interpreting studies using this model, particularly at doses greater than 1 mg MIA.

Afferent drive elicits ongoing pain in a model of advanced osteoarthritis

Osteoarthritis (OA) is a chronic condition characterized by pain during joint movement. Additionally, patients with advanced disease experience pain at rest (ie, ongoing pain) that is generally resistant to nonsteroidal antiinflammatory drugs. Injection of monosodium iodoacetate (MIA) into the intraarticular space of the rodent knee is a well-established model of OA that elicits weight-bearing asymmetry and referred tactile and thermal hypersensitivity. Whether ongoing pain is present in this model is unknown. Additionally, the possible relationship of ongoing pain to MIA dose is not known. MIA produced weight asymmetry, joint osteolysis, and cartilage erosion across a range of doses (1, 3, and 4.8 mg). However, only rats treated with the highest dose of MIA showed conditioned place preference to a context paired with intraarticular lidocaine, indicating relief from ongoing pain. Diclofenac blocked the MIA-induced weight asymmetry but failed to block MIA-induced ongoing pain. Systemic AMG9810, a transient receptor potential V1 channel (TRPV1) antagonist, effectively blocked thermal hypersensitivity, but failed to block high-dose MIA-induced weight asymmetry or ongoing pain. Additionally, systemic or intraarticular HC030031, a TRPA1 antagonist, failed to block high-dose MIA-induced weight asymmetry or ongoing pain. Our studies suggest that a high dose of intraarticular MIA induces ongoing pain originating from the site of injury that is dependent on afferent fiber activity but apparently independent of TRPV1 or TRPA1 activation. Identification of mechanisms driving ongoing pain may enable development of improved treatments for patients with severe OA pain and diminish the need for joint replacement surgery.

Activating transcription factor 3 and the nervous system

Activating transcription factor 3 (ATF3) belongs to the ATF/cyclic AMP responsive element binding family of transcription factors and is often described as an adaptive response gene whose activity is usually regulated by stressful stimuli. Although expressed in a number of splice variants and generally recognized as a transcriptional repressor, ATF3 has the ability to interact with a number of other transcription factors including c-Jun to form complexes which not only repress, but can also activate various genes. ATF3 expression is modulated mainly at the transcriptional level and has markedly different effects in different types of cell. The levels of ATF3 mRNA and protein are normally very low in neurons and glia but their expression is rapidly upregulated in response to injury. ATF3 expression in neurons is closely linked to their survival and the regeneration of their axons following axotomy, and that in peripheral nerves correlates with the generation of a Schwann cell phenotype that is conducive to axonal regeneration. ATF3 is also induced by Toll-like receptor (TLR) ligands but acts as a negative regulator of TLR signaling, suppressing the innate immune response which is involved in immuno-surveillance and can enhance or reduce the survival of injured neurons and promote the regeneration of their axons.

Antinociceptive effects of eugenol evaluated in a monoiodoacetate-induced osteoarthritis rat model

The aim of the present study was to evaluate whether eugenol, the main constituent of clove oil, has the capacity to provide analgesia in the monoiodoacetate-induced rat model of osteoarthritis. Animals (n = 6/group) received either eugenol (20 or 40 mg/kg) or a vehicle by gavage. Daily administrations were initiated 2 days post osteoarthritis induction and continued for the duration of the study (4 weeks). Gait analysis was performed using the CatWalk method and secondary mechanical allodynia was assessed with von Frey filaments. Selected spinal cord peptides (substance P, calcitonin gene-related peptide and dynorphin) were quantified by mass spectrometry. Significant changes were identified in dynamic gait parameters (swing speed, swing phase duration and duty cycle) of the affected limb following 40 mg/kg eugenol treatment compared with the vehicle (p < 0.05). Von Frey results revealed significant differences between the 40 mg/kg treatment and the vehicle group during the first and the third week of the study (p < 0.02). Spinal pain-related peptide analysis revealed a decreased content of substance P and CGRP accompanied by an increase of dynorphin in animals treated with 40 mg/kg eugenol. These results suggest a therapeutic potential of eugenol to alleviate osteoarthritis-related pain.

Determination of specific neuropeptides modulation time course in a rat model of osteoarthritis pain by liquid chromatography ion trap mass spectrometry

Animal models are useful to evaluate pharmacological therapies to alleviate joint pain. The present study characterized central neuropeptides modulation in the monoiodoacetate (MIA) rat model. Animals receiving a single 3mg MIA injection were euthanized at 3, 7, 14, 21 and 28 days post injection. Spinal cords were analyzed by liquid chromatography ion trap mass spectrometry. Up-regulations of the calcitonin gene-related peptide and substance P were observed starting on days 7 and 28 respectively, whereas big dynorphin(₁₋₃₂) content decreased significantly on day 14 in comparison to control animals (P<0.05). Preclinical drug evaluations using this model should be conducted between 7 and 21 days post injection when the lesions resemble most to human osteoarthritis.

The contribution of spinal glial cells to chronic pain behaviour in the monosodium iodoacetate model of osteoarthritic pain

Background

Clinical studies of osteoarthritis (OA) suggest central sensitization may contribute to the chronic pain experienced. This preclinical study used the monosodium iodoacetate (MIA) model of OA joint pain to investigate the potential contribution of spinal sensitization, in particular spinal glial cell activation, to pain behaviour in this model. Experimental OA was induced in the rat by the intra-articular injection of MIA and pain behaviour (change in weight bearing and distal allodynia) was assessed. Spinal cord microglia (Iba1 staining) and astrocyte (GFAP immunofluorescence) activation were measured at 7, 14 and 28 days post MIA-treatment. The effects of two known inhibitors of glial activation, nimesulide and minocycline, on pain behaviour and activation of microglia and astrocytes were assessed.

Results

Seven days following intra-articular injection of MIA, microglia in the ipsilateral spinal cord were activated (p < 0.05, compared to contralateral levels and compared to saline controls). Levels of activated microglia were significantly elevated at day 14 and 21 post MIA-injection. At day 28, microglia activation was significantly correlated with distal allodynia (p < 0.05). Ipsilateral spinal GFAP immunofluorescence was significantly (p < 0.01) increased at day 28, but not at earlier timepoints, in the MIA model, compared to saline controls. Repeated oral dosing (days 14-20) with nimesulide attenuated pain behaviour and the activation of microglia in the ipsilateral spinal cord at day 21. This dosing regimen also significantly attenuated distal allodynia (p < 0.001) and numbers of activated microglia (p < 0.05) and GFAP immunofluorescence (p < 0.001) one week later in MIA-treated rats, compared to vehicle-treated rats. Repeated administration of minocycline also significantly attenuated pain behaviour and reduced the number of activated microglia and decreased GFAP immunofluorescence in ipsilateral spinal cord of MIA treated rats.

Conclusions

Here we provide evidence for a contribution of spinal glial cells to pain behaviour, in particular distal allodynia, in this model of osteoarthritic pain. Our data suggest there is a potential role of glial cells in the central sensitization associated with OA, which may provide a novel analgesic target for the treatment of OA pain.

Neuronal injury marker ATF-3 is induced in primary afferent neurons of monoarthritic rats

Activating transcription factor 3 (ATF-3) expression has been associated with several signaling pathways implicated in cellular stress response in many cell types and is usually regarded as a neuronal damage marker in dorsal root ganglia (DRG). We investigated ATF-3 expression in primary afferents in the monoarthritic (MA) model of chronic inflammatory joint pain. Immunohistochemistry revealed that ATF-3 is highly induced mainly in small and medium neurons, especially at 2 and 4 days of MA in L(5) DRGs. Colocalization with calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4) demonstrated that ATF-3-immunoreactive cells are mainly peptidergic. The lack of significant differences in ATF-3 and pAkt colocalization indicated that ATF-3 is probably not involved in a pAkt-mediated survival pathway. Anti-inflammatory (ketoprofen) administration failed to reverse ATF-3 induction in MA rats, but significantly increased CGRP expression. These data suggest that ATF-3 expression is definitely involved in MA, actually marking injured neurons. Some degree of neuronal damage seems to occur right from the first days of disease, mainly affecting small-to-medium peptidergic neurons. The intra-articular injection of complete Freund's adjuvant and the generation of a neuroinflammatory environment seem to be the plausible explanation for the local nerve damage.

Recent advances in the biology and medicinal chemistry of TRPA1

The transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is a nonselective cation channel that is highly expressed in small-diameter sensory neurons, where it functions as a polymodal receptor, responsible for detecting potentially harmful chemicals, mechanical forces and temperatures. TRPA1 is also activated and/or sensitized by multiple endogenous inflammatory mediators. As such, TRPA1 likely mediates the pain and neurogenic inflammation caused by exposure to reactive chemicals. In addition, it is also possible that this channel may mediate some of the symptoms of chronic inflammatory conditions such as asthma. We review recent advances in the biology of TRPA1 and summarize the evidence for TRPA1 as a therapeutic drug target. In addition, we provide an update on TRPA1 medicinal chemistry and the progress in the search for novel TRPA1 antagonists.

Pain-related sensory innervation in monoiodoacetate-induced osteoarthritis in rat knees that gradually develops neuronal injury in addition to inflammatory pain

Background

The exact mechanism of knee osteoarthritis (OA)-associated pain is unclear, whereas mixed evidence of inflammatory pain and neuropathic pain has been noted. We aimed to investigate pain-related sensory innervation in a monoiodoacetate (MIA)-induced model of OA.

Methods

Sixty of seventy female Sprague Dawley rats of six week-old underwent intra-articular MIA and fluorogold (FG) retrograde neurotracer injection into their right (ipsilateral) knee, while their left knees were treated with FG in saline as a control (contralateral knee). Other rats were treated with FG only bilaterally, and used as controls. Rats were evaluated for tactile allodynia using von Frey hairs. Proinflammatory mediators in the knee soft tissues, including tumor necrosis factor (TNF)-α, interleukin (IL)-6, and nerve growth factor (NGF), were quantified using ELISAs to evaluate inflammation in the knee after 1, 4, 7,14,21, and 28 days post injection:. Dorsal root ganglia (DRG) were immunostained for three molecules after 7,14,21, and 28 days post injection: calcitonin gene-related peptide (CGRP), a marker of inflammatory pain; and activating transcription factor-3 (ATF3) and growth associated protein-43 (GAP43), as markers for nerve injury and regenerating axons. The distribution of microglia in the spinal cord were also evaluated, because they have been reported to increase in neuropathic pain states. These evaluations were performed up to 28 days postinjection. P < 0.05 was considered significant.

Results

Progressive tactile allodynia and elevated cytokine concentrations were observed in ipsilateral knees. CGRP-immunoreactive (-ir) ipsilateral DRG neurons significantly increased, peaking at 14 days postinjection, while expression of FG-labeled ATF3-ir or ATF3-ir GAP43-ir DRG neurons significantly increased in a time-dependent manner. Significant proliferation of microglia were found with time in the ipsilateral dorsal horn.

Conclusions

Pain-related characteristics in a MIA-induced rat OA model can originate from an inflammatory pain state induced by the local inflammation initiated by inflammatory cytokines, and that state will be followed by gradual initiation of neuronal injury, which may induce the neuropathic pain state.

Monosodium iodoacetate-induced joint pain is associated with increased phosphorylation of mitogen activated protein kinases in the rat spinal cord

BACKGROUND

Intra-articular injection of monosodium iodoacetate (MIA) in the knee joint of rats disrupts chondrocyte metabolism resulting in cartilage degeneration and subsequent nociceptive behavior that has been described as a model of osteoarthritis (OA) pain. Central sensitization through activation of mitogen activated protein kinases (MAPKs) is recognized as a pathogenic mechanism in chronic pain. In the present studies, induction of central sensitization as indicated by spinal dorsal horn MAPK activation, specifically ERK and p38 phosphorylation, was assessed in the MIA-OA model.

RESULTS

Behaviorally, MIA-injected rats displayed reduced hind limb grip force 1, 2, and 3 weeks post-MIA treatment. In the same animals, activation of phospho ERK1/2 was gradually increased, reaching a significant level at post injection week 3. Conversely, phosphorylation of p38 MAPK was enhanced maximally at post injection week 1 and decreased, but remained elevated, thereafter. Double labeling from 3-wk MIA rats demonstrated spinal pERK1/2 expression in neurons, but not glia. In contrast, p-p38 was expressed by microglia and a subpopulation of neurons, but not astrocytes. Additionally, there was increased ipsilateral expression of microglia, but not astrocytes, in 3-wk MIA-OA rats. Consistent with increased MAPK immunoreactivity in the contralateral dorsal horn, mechanical allodynia to the contralateral hind-limb was observed 3-wk following MIA. Finally, intrathecal injection of the MEK1 inhibitor PD98059 blocked both reduced hind-limb grip force and pERK1/2 induction in MIA-OA rats.

CONCLUSION

Results of these studies support the role of MAPK activation in the progression and maintenance of central sensitization in the MIA-OA experimental pain model.

Tonic modulation of spinal hyperexcitability by the endocannabinoid receptor system in a rat model of osteoarthritis pain

Objective

To investigate the impact of an experimental model of osteoarthritis (OA) on spinal nociceptive processing and the role of the inhibitory endocannabinoid system in regulating sensory processing at the spinal level.

Methods

Experimental OA was induced in rats by intraarticular injection of sodium mono-iodoacetate (MIA), and the development of pain behavior was assessed. Extracellular single-unit recordings of wide dynamic range (WDR) neurons in the dorsal horn were obtained in MIA-treated rats and saline-treated rats. The levels of endocannabinoids and the protein and messenger RNA levels of the main synthetic enzymes for the endocannabinoids (N-acyl phosphatidylethanolamine phospholipase D [NAPE-PLD] and diacylglycerol lipase α [DAGLα]) in the spinal cord were measured.

Results

Low-weight (10 gm) mechanically evoked responses of WDR neurons were significantly (P < 0.05) facilitated 28 days after MIA injection compared with the responses in saline-treated rats, and spinal cord levels of anandamide and 2-arachidonoyl glycerol (2-AG) were increased in MIA-treated rats. Protein levels of NAPE-PLD and DAGLα, which synthesize anandamide and 2-AG, respectively, were elevated in the spinal cords of MIA-treated rats. The functional role of endocannabinoids in the spinal cords of MIA-treated rats was increased via activation of cannabinoid 1 (CB₁) and CB₂ receptors, and blockade of the catabolism of anandamide had significantly greater inhibitory effects in MIA-treated rats compared with control rats.

Conclusion

Our findings provide new evidence for altered spinal nociceptive processing indicative of central sensitization and for adaptive changes in the spinal cord endocannabinoid system in an experimental model of OA. The novel control of spinal cord neuronal responses by spinal cord CB₂ receptors suggests that this receptor system may be an important target for the modulation of pain in OA.

Phenotypic alterations of neurons that innervate osteoarthritic joints in rats

OBJECTIVE

Pain is a prominent feature of osteoarthritis (OA). To further understand the primary mechanisms of nociception in OA, we studied the expression of the phenotype markers calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), and neurofilament 200 (NF200) in sensory neurons innervating the OA knee joint in rats.

METHODS

OA was induced in rats by intraarticular injection of 2 mg of mono-iodoacetate (MIA) into the knee. Neurons innervating the joint were identified by retrograde labeling with fluorogold in dorsal root ganglia (DRG) and colocalized with neurochemical markers by immunofluorescence. The total number of DRG cells was determined by stereologic methods in Nissl-stained sections.

RESULTS

A 37% decrease in the number of fluorogold-backlabeled cells was observed in rats with OA when compared with control rats, even though no decrease in the total number of cells was observed. However, an increase in the number of medium/large cell bodies and a decrease in the number of the smallest cells were observed, suggesting the occurrence of perikarya hypertrophy. The percentage of CGRP-positive cells increased significantly, predominantly in medium/large cells, suggesting the occurrence of a phenotypic switch. Colocalization of CGRP and NF200 revealed no significant changes in the percentage of double-labeled cells, but an increase in the number of medium/large double-labeled cells was observed. No differences in the expression of either IB4 or NF200 were observed in fluorogold-backlabeled cells.

CONCLUSION

These results indicate that MIA-induced OA causes an up-regulation of CGRP in different subpopulations of primary afferent neurons in DRG due to a phenotypic switch and/or cell hypertrophy which may be functionally relevant in terms of the onset of pain in this pathologic condition.

Ongoing pain in the MIA model of osteoarthritis

Osteoarthritis (OA) is a chronic pain condition characterized by pain during joint use as well as pain at rest (i.e., ongoing pain). Although injection of monosodium iodoacetate (MIA) into the intra-articular space of the rodent knee is a well established model of OA pain that is characterized by changes in weight bearing and hypersensitivity to tactile and thermal stimuli, it is not known if this procedure elicits ongoing pain. Further, the time-course and possible underlying mechanisms of these components of pain remain poorly understood. In these studies, we demonstrated the presence of ongoing pain in addition to changes in weight bearing and evoked hypersensitivity. Twenty-eight days following MIA injection, spinal clonidine blocked changes in weight bearing and thermal hypersensitivity and produced place preference indicating that MIA induces ongoing and evoked pain. These findings demonstrate the presence of ongoing pain in this model that is present at a late-time point after MIA allowing for mechanistic investigation.

Gait analysis and pain response of two rodent models of osteoarthritis

The purpose of this study was to compare the gait parameters recorded on the CatWalk and the mechanical sensitivity with von Frey filaments of two putative models of osteoarthritis over a one month period, and to evaluate the effect of celecoxib on these parameters. Animals underwent either a surgical sectioning of the anterior cruciate ligament with partial medial menisectomy (ACLT+pMMx) to create a joint instability model or received an intra-articular injection of monoiodoacetate (MIA) as a putative inflammatory joint pain model. Animals were assessed for four consecutive weeks and knee joints were then evaluated histologically. Spinal cord lumbar enlargements were harvested for selected neuropeptide analysis (substance P (SP) and calcitonin gene related peptide (CGRP)). With the MIA model, significant changes persisted in selected dynamic gait parameters throughout the study in the injured limb as well as with the von Frey filaments. The ACLT+pMMx model in contrast showed no clear differential response between both hind limb for both gait parameters and pain-related behavior with von Frey filaments occurred only on the last day of the study. Neuropeptide analysis of spinal cord lumbar enlargements revealed a significant increase in CGRP concentration in both models and an increase in SP concentration only in the MIA model. Histological evaluation confirmed the presence of articular cartilage lesions in both models, but they were much more severe in the MIA model. Celecoxib had an effect on all selected gait parameters at the very beginning of the study and had an important alleviating effect on mechanical allodynia. These results suggest that the MIA model may be more appropriate for the evaluation of short term pain studies and that celecoxib may modulate mechanical allodynia through central sensitization mechanisms.

Alteration of sensory neurons and spinal response to an experimental osteoarthritis pain model

OBJECTIVE

To verify the biologic links between progressive cellular and structural alterations within knee joint components and development of symptomatic chronic pain that are characteristic of osteoarthritis (OA), and to investigate the molecular basis of alterations in nociceptive pathways caused by OA-induced pain.

METHODS

An animal model of knee joint OA pain was generated by intraarticular injection of mono-iodoacetate (MIA) in Sprague-Dawley rats, and symptomatic pain behavior tests were performed. Relationships between development of OA with accompanying pain responses and gradual alterations in cellular and structural knee joint components (i.e., cartilage, synovium, meniscus, subchondral bone) were examined by histologic and immunohistologic analysis, microscopic examination, and microfocal computed tomography. Progressive changes in the dynamic interrelationships between peripheral knee joint tissue and central components of nociceptive pathways caused by OA-induced pain were examined by investigating cytokine production and expression in sensory neurons of the dorsal root ganglion and spinal cord.

RESULTS

We observed that structural changes in components of the peripheral knee joint correlate with alterations in the central compartments (dorsal root ganglia and the spinal cord) and symptomatic pain assessed by behavioral hyperalgesia. Our comparative gene expression studies revealed that the pain pathways in MIA-induced knee OA may overlap, at least in part, with neuropathic pain mechanisms. Similar results were also observed upon destabilization of the knee joint in the anterior cruciate ligament transection and destabilization of the medial meniscus models of OA.

CONCLUSION

Our results indicate that MIA-induced joint degeneration in rats generates an animal model that is suitable for mechanistic and pharmacologic studies on nociceptive pain pathways caused by OA, and provide key in vivo evidence that OA pain is caused by central sensitization through communication between peripheral OA nociceptors and the central sensory system. Furthermore, our data suggest a mechanistic overlap between OA-induced pain and neuropathic pain.

Differential ATF3 expression in dorsal root ganglion neurons reveals the profile of primary afferents engaged by diverse noxious chemical stimuli

Although transgenic and knockout mice have helped delineate the mechanisms of action of diverse noxious compounds, it is still difficult to determine unequivocally the subpopulations of primary afferent nociceptor that these molecules engage. As most noxious stimuli lead to tissue and/or nerve injury, here we used induction of activating transcription factor 3 (ATF3), a reliable marker of nerve injury, to assess the populations of primary afferent fibers that are activated after peripheral administration of noxious chemical stimuli. In wild-type mice, hindpaw injections of capsaicin, formalin, mustard oil or menthol induce expression of ATF3 in distinct subpopulations of sensory neurons. Interestingly, even though these noxious chemicals are thought to act through subtypes of transient receptor potential (TRP) channels, all compounds also induced ATF3 in neurons that appear not to express the expected TRP channel subtypes. On the other hand, capsaicin failed to induce ATF3 in mice lacking TRPV1, indicating that TRPV1 is required for both the direct and indirect induction of ATF3 in sensory neurons. By contrast, only low doses of formalin or mustard oil failed to induce ATF3 in TRPA1 null mice, indicating that injections of high doses (>0.5%) of formalin or mustard oil recruit both TRPA1- and non-TRPA1 expressing primary afferent fibers. Finally, peripheral injection of menthol, a TRPM8 receptor agonist, induced ATF3 in a wide variety of sensory neurons, but in a TRPM8-independent manner. We conclude that purportedly selective agonists can activate a heterogeneous population of sensory neurons, which ultimately could contribute to the behavioral responses evoked.

Characterization of the acute and persistent pain state present in K/BxN serum transfer arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune arthritis that affects approximately 1% of the population. Synovial inflammation cannot fully explain the level of pain reported by patients and facilitation of pain processing at the spinal level has been implicated. We characterized the K/BxN serum transfer arthritis model as a model of joint inflammation-induced pain and examined pharmacologic responsiveness and spinal glia activation. Mechanical allodynia developed congruently with joint swelling. Surprisingly, allodynia persisted after resolution of inflammation. At the peak of joint inflammation (days 4-10), hypersensitivity was attenuated with i.p. etanercept, gabapentin, and ketorolac. Following resolution of synovial inflammation (days 19-23), only gabapentin relieved allodynia. The superficial dorsal horn of arthritic mice displayed increased staining of microglia at early and late time points, but astrocyte staining increased only during the inflammatory phase. ATF3, a marker of nerve injury, was significantly increased in the lumbar dorsal root ganglia during the late phase (day 28). Hence, serum transfer in the K/BxN serum transfer arthritis model produces a persistent pain state, where the allodynia during the inflammatory state is attenuated by TNF and prostaglandin inhibitors, and the pharmacology and histochemistry data suggest a transition from an inflammatory state to a state that resembles a neuropathic condition over time. Therefore, the K/BxN serum transfer model represents a multifaceted model for studies exploring pain mechanisms in conditions of joint inflammation and may serve as a platform for exploring novel treatment strategies for pain in human arthritic conditions.

Changes in Abeta non-nociceptive primary sensory neurons in a rat model of osteoarthritis pain

Background

Pain is a major debilitating factor in osteoarthritis (OA), yet few mechanism-based therapies are available. To address the need to understand underlying mechanisms the aim of the present study was to determine changes in sensory neurons in an animal model of OA pain.

Results

The model displayed typical osteoarthritis pathology characterized by cartilage degeneration in the knee joint and also manifested knee pathophysiology (edema and increased vasculature permeability of the joint) and altered nociception of the affected limb (hind paw tenderness and knee articulation-evoked reduction in the tail flick latency). Neurons included in this report innervated regions throughout the entire hind limb. Aβ-fiber low threshold mechanoreceptors exhibited a slowing of the dynamics of action potential (AP) genesis, including wider AP duration and slower maximum rising rate, and muscle spindle neurons were the most affected subgroup. Only minor AP configuration changes were observed in either C- or Aδ-fiber nociceptors.

Conclusion

Thus, at one month after induction of the OA model Aβ-fiber low threshold mechanoreceptors but not C- or Aδ-fiber nociceptors had undergone changes in electrophysiological properties. If these changes reflect a change in functional role of these neurons in primary afferent sensory processing, then Aβ-fiber non-nociceptive primary sensory neurons may be involved in the pathogenesis of OA pain. Further, it is important to point out that the patterns of the changes we observed are consistent with observations in models of peripheral neuropathy but not models of peripheral inflammation.

Skin incision induces expression of axonal regeneration-related genes in adult rat spinal sensory neurons

Skin incision and nerve injury both induce painful conditions. Incisional and postsurgical pain is believed to arise primarily from inflammation of tissue and the subsequent sensitization of peripheral and central neurons. The role of axonal regeneration-related processes in development of pain has only been considered when there has been injury to the peripheral nerve itself, even though tissue damage likely induces injury of resident axons. We sought to determine if skin incision would affect expression of regeneration-related genes such as activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) neurons. ATF3 is absent from DRG neurons of the normal adult rodent, but is induced by injury of peripheral nerves and modulates the regenerative capacity of axons. Image analysis of immunolabeled DRG sections revealed that skin incision led to an increase in the number of DRG neurons expressing ATF3. RT-PCR indicated that other regeneration-associated genes (galanin, GAP-43, Gadd45a) were also increased, further suggesting an injury-like response in DRG neurons. Our finding that injury of skin can induce expression of neuronal injury/regeneration-associated genes may impact how clinical postsurgical pain is investigated and treated.

PERSPECTIVE

Tissue injury, even without direct nerve injury, may induce a state of enhanced growth capacity in sensory neurons. Axonal regeneration-associated processes should be considered alongside nerve signal conduction and inflammatory/sensitization processes as possible mechanisms contributing to pain, particularly the transition from acute to chronic pain.

Beta2-adrenergic agonist-induced hypertrophy of the quadriceps skeletal muscle does not modulate disease severity in the rodent meniscectomy model of osteoarthritis

OBJECTIVE

To examine whether beta2-adrenergic agonist-induced hypertrophy of the quadriceps skeletal muscle can modulate the severity of osteoarthritis (OA) in the rodent meniscectomy (MNX) model.

METHODS

Male Lewis rats were subcutaneously administered with 1.5 mg/kg/day clenbuterol hydrochloride (n=15) or saline vehicle (n=20) for 14 days. Following pre-treatment, five animals from each group were sacrificed to assess the immediate effects of clenbuterol. The remaining animals underwent either invasive knee surgery (clenbuterol pre-treated n=10; saline pre-treated n=10) or a sham control surgical procedure (saline pre-treated n=5). During disease initiation and progression, weight bearing was assessed by hindlimb loading. Myosin heavy chain (MHC) protein isoforms were quantified by silver stained SDS PAGE. OA severity was graded by assessment of toluidine blue stained step coronal sections of the total knee joint.

RESULTS

Clenbuterol treatment resulted in an increase in total bodyweight, growth rate and in quadriceps skeletal muscle mass. Meniscal surgery resulted in the development of OA-like lesions, changes to weight bearing, and changes in MHC protein expression in the quadriceps. Clenbuterol-induced skeletal muscle hypertrophy had no effect on either weight bearing or articular pathology following MNX surgery.

CONCLUSIONS

Our data reveal that clenbuterol-induced skeletal muscle hypertrophy is unable to mimic the beneficial clinical effects of increased musculature derived through targeted strength training in humans, in a rodent model of MNX-induced OA. In addition we observed fibre-type switching to "slow twitch" in the quadriceps muscle during the induction of OA that warrants further investigation as to its relationship to joint stability.

Chemotherapy-induced peripheral neuropathy as a predictor of neuropathic pain in breast cancer patients previously treated with paclitaxel

Neuropathic pain (NP) remains difficult to control for a significant number of patients with cancer. Chemotherapy-induced peripheral neuropathy (CIPN) has been postulated as an initial stage in the development of NP. To assess whether CIPN (defined as National Cancer Institute Common Toxicity Criteria grade 2 or higher) was associated with NP, we conducted a survey of breast cancer patients who had participated in clinical trials of paclitaxel. Of the 430 potential respondents, 240 responded to the survey. Results showed that 64% experienced CIPN during paclitaxel treatment. Follow-up survey data revealed that 27% of those with CIPN were subsequently diagnosed with NP. Logistic regression analyses showed that those who had experienced CIPN were 3 times more likely to develop NP (95% confidence interval = 1.2-7.2; P < .001), which persisted in the multivariate logistic model. In addition, NP patients reported twice as many visits to their health care provider (P = .02) and had taken more prescription (50% vs 19%; P = .001) and over-the-counter medications (62.5% versus 45%; P = .08) for pain than those without NP. The results of this study confirm that CIPN is a predictor of NP, suggesting that survivors treated with paclitaxel should be regularly monitored for NP beyond treatment.

PERSPECTIVE

The survival rates of breast cancer patients have steadily improved over recent years; thus, research into symptoms that persist after treatment is important. We found CIPN as a predictor of NP. Understanding the epidemiology of NP in breast cancer patients has high clinical and public health significance.

Descending serotonergic facilitation and the antinociceptive effects of pregabalin in a rat model of osteoarthritic pain

Background

Descending facilitation, from the brainstem, promotes spinal neuronal hyperexcitability and behavioural hypersensitivity in many chronic pain states. We have previously demonstrated enhanced descending facilitation onto dorsal horn neurones in a neuropathic pain model, and shown this to enable the analgesic effectiveness of gabapentin. Here we have tested if this hypothesis applies to other pain states by using a combination of approaches in a rat model of osteoarthritis (OA) to ascertain if 1) a role for descending 5HT mediated facilitation exists, and 2) if pregabalin (a newer analogue of gabapentin) is an effective antinociceptive agent in this model. Further, quantitative-PCR experiments were undertaken to analyse the α₂δ-1 and 5-HT3A subunit mRNA levels in L3–6 DRG in order to assess whether changes in these molecular substrates have a bearing on the pharmacological effects of ondansetron and pregabalin in OA.

Results

Osteoarthritis was induced via intra-articular injection of monosodium iodoacetate (MIA) into the knee joint. Control animals were injected with 0.9% saline. Two weeks later in vivo electrophysiology was performed, comparing the effects of spinal ondansetron (10–100 μg/50 μl) or systemic pregabalin (0.3 – 10 mg/kg) on evoked responses of dorsal horn neurones to electrical, mechanical and thermal stimuli in MIA or control rats. In MIA rats, ondansetron significantly inhibited the evoked responses to both innocuous and noxious natural evoked neuronal responses, whereas only inhibition of noxious evoked responses was seen in controls. Pregabalin significantly inhibited neuronal responses in the MIA rats only; this effect was blocked by a pre-administration of spinal ondansetron. Analysis of α₂δ-1 and 5-HT3A subunit mRNA levels in L3–6 DRG revealed a significant increase in α₂δ-1 levels in ipsilateral L3&4 DRG in MIA rats. 5-HT3A subunit mRNA levels were unchanged.

Conclusion

These data suggest descending serotonergic facilitation plays a role in mediating the brush and innocuous mechanical punctate evoked neuronal responses in MIA rats, suggesting an adaptive change in the excitatory serotonergic drive modulating low threshold evoked neuronal responses in MIA-induced OA pain. This alteration in excitatory serotonergic drive, alongside an increase in α₂δ-1 mRNA levels, may underlie pregabalin's state dependent effects in this model of chronic pain.

Cellular and histopathological changes in the infrapatellar fat pad in the monoiodoacetate model of osteoarthritis pain

OBJECTIVE

The infrapatellar fat pad (IPFP) has been identified as a source of anterior knee pain. Fibrosis and marked inflammatory infiltrate in the IPFP of patients with arthritis of the knee and reduction in pain post knee replacement in patients following resection of the IPFP have been observed. We have investigated changes in the IPFP of rats undergoing the monoiodoacetate (MIA) model of degenerative joint disease, a model that exhibits some histopathological similarities to osteoarthritis (OA).

METHODS

Rats were injected intra-articularly with MIA and the development of weight bearing asymmetry was followed for 21 days as compared to vehicle-injected animals. In addition, IPFPs were removed from both ipsilateral and contralateral joints. Both inflammatory infiltrate and histopathological changes were analysed.

RESULTS

MIA injection caused marked weight bearing asymmetry. Ipsilateral IPFP wet weights were significantly increased on days 1 and 3 in MIA-treated animals. MIA treatment also resulted in significant increases in IPFP total white blood cells and monocytes on days 1, 3, and 7 and neutrophils on days 1 and 3. This was supported by histopathological findings at early time points which progressed to adipocyte necrosis, IPFP fibrosis, patellar cartilage and subchondral bone necrosis with synovial hyperplasia at later timepoints.

CONCLUSIONS

The current study clearly demonstrated that marked inflammatory changes in the IPFP occur during the early stage of the MIA model of OA which may contribute to the pain observed at this early stage. The role of the IPFP in later stages of the model needs to be further explored.

Behavioural and electrophysiological characterisation of experimentally induced osteoarthritis and neuropathy in C57Bl/6 mice

Background

Osteoarthritis is a widespread condition affecting the elderly where ~70–90% of over 75 year olds are affected, representing one of the largest cost burdens to healthcare in the western world. The monosodium iodoacetate (MIA) osteoarthritis model has been well described in the rat especially in terms of the pathological progression of the disease and more recently pain behaviour. In this study, we characterise, for the first time, MIA induced osteoarthritis in mice and compare it with nerve-injured mice (partial sciatic nerve injury), using both behavioural and in vivo electrophysiological measurements. These approaches uniquely allow the threshold and suprathreshold measures to many modalities to be quantified and so form a basis for improving and expanding transgenic studies.

Results

Significant mechanical hypersensitivity was observed in the ipsilateral hindpaw in MIA injected mice at all observed time points following infrapetellar MIA injection (p < 0.05). The mechanical hypersensitivity exhibited a partial biphasic temporal pattern, but thermal hypersensitivity was absent. Electrically-evoked dorsal horn neuronal responses in MIA injected mice were significantly elevated (p < 0.05) with respect to A- and C-fibre firing, input, pinch and noxious von Frey (26 and 60 g). No significant changes in A- or C-fibre thresholds were observed. Nerve-injured mice displayed significant behavioural thermal and mechanical hypersensitivity (p < 0.05) and evoked dorsal horn responses were significantly increased with respect to C-fibre firing, pinch and wind-up (p < 0.05).

Conclusion

The MIA model of osteoarthritic pain in mice displays behavioural characteristics similar to those observed in rats. Changes in both behavioural measures and neuronal activity from the paw, suggest that central changes are involved in this pain state, although a role for peripheral drives is also likely. Moreover, the behavioural and neuronal measures in these two pain models showed overlapping alterations in terms of certain neuronal measures and mechanical sensitivity despite their very different pathologies and a loss of input in neuropathy, suggesting some commonalities in the central processing of different peripheral pain states. This murine model of osteoarthritis will allow the exploitation of knock out animals to better understand underlying mechanisms and identify novel molecular targets.

New advances in musculoskeletal pain

Non-malignant musculoskeletal pain is the most common clinical symptom that causes patients to seek medical attention and is a major cause of disability in the world. Musculoskeletal pain can arise from a variety of common conditions including osteoarthritis, rheumatoid arthritis, osteoporosis, surgery, low back pain and bone fracture. A major problem in designing new therapies to treat musculoskeletal pain is that the underlying mechanisms driving musculoskeletal pain are not well understood. This lack of knowledge is largely due to the scarcity of animal models that closely mirror the human condition which would allow the development of a mechanistic understanding and novel therapies to treat this pain. To begin to develop a mechanism-based understanding of the factors involved in generating musculoskeletal pain, in this review we present recent advances in preclinical models of osteoarthritis, post-surgical pain and bone fracture pain. The models discussed appear to offer an attractive platform for understanding the factors that drive this pain and the preclinical screening of novel therapies to treat musculoskeletal pain. Developing both an understanding of the mechanisms that drive persistent musculoskeletal pain and novel mechanism-based therapies to treat these unique pain states would address a major unmet clinical need and have significant clinical, economic and societal benefits.

Differential analgesic effects of morphine and gabapentin on behavioural measures of pain and disability in a model of osteoarthritis pain in rats

Osteoarthritis (OA) is associated with chronic debilitating joint pain. Pain is the result of an emotional and sensory experience and preclinical models of OA can thus be useful to better understand the underlying mechanisms of the disease and test new therapeutic options. We induced unilateral knee OA in Sprague-Dawley rats using monosodium iodoacetate (MIA), a glycolysis inhibitor and assessed the effects of acute and chronic morphine and gabapentin using a battery of quantitative behavioural outcome measures of pain and disability. Animals received a single intra-articular injection of 2mg MIA in 25 microl saline, causing inflammation and progressive cartilage degradation. Mechanical and thermal sensitivity as well as ambulatory-evoked pain were then monitored using von Frey hairs, acetone and a rotarod. Once maximum nociceptive responses were reached, chronic bi-daily morphine (3mg/kg s.c.) or gabapentin (30 mg/kg s.c.) were administered for 5 days. We observed a marked biphasic mechanical hypersensitivity that increased and reached a plateau from day 14 (317.6% of control response, p<0.01, with von Frey 6g). Moreover we found a marked cooling hypersensitivity, and validated a novel ambulatory-evoked pain score. These measures were significantly reduced after both acute (13.3% of sham response, p<0.01, von Frey 6g) and chronic (38.3%, p<0.05) morphine whilst only chronic gabapentin (37.0%, p<0.05) had an effect. We show the reliability of the model in terms of mechanical hypersensitivity and demonstrate cooling hypersensitivity and ambulatory-evoked pain. In terms of translational research, the effects of morphine and gabapentin validate the model and suggest trials of these therapeutic approaches in OA patients.

Arthritis and pain. Future targets to control osteoarthritis pain

Clinical presentation of osteoarthritis (OA) is dominated by pain during joint use and at rest. OA pain is caused by aberrant functioning of a pathologically altered nervous system with key mechanistic drivers from peripheral nerves and central pain pathways. This review focuses on symptomatic pain therapy exemplified by molecular targets that alter sensitization and hyperexcitability of the nervous system, for example, opioids and cannabinoids. We highlight opportunities for targeting inflammatory mediators and their key receptors (for example, prostanoids, kinins, cytokines and chemokines), ion channels (for example, NaV1.8, NaV1.7 and CaV2.2) and neurotrophins (for example, nerve growth factor), noting evidence that relates to their participation in OA etiology and treatment. Future neurological treatments of pain appear optimistic but will require the systematic evaluation of emerging opportunities.