Comparison of two PBR ligands with classical antiinflammatory drugs in LPS-induced arthritis in rats

Targeting the soluble epoxide hydrolase pathway as a novel therapeutic approach for the treatment of pain

Chronic pain is a major burden and the complexities of chronic pain pathophysiology, including both peripheral and central sensitisation mechanisms, involves multiple cell types (neuronal, immune, neuroimmune, and vascular) which substantially complicates the development of new effective analgesic treatments. The epoxy fatty acids (EpFAs), including the epoxyeicosatrienoic acids (EETs), are derived from the metabolism of polyunsaturated fatty acids (PUFAs) via the cytochrome P450 enzymatic pathway and act to shut-down inflammatory signalling and provide analgesia. The EpFAs are rapidly metabolised by the enzyme soluble epoxide hydrolase (sEH) into their corresponding diol metabolites, which recent studies suggest are pro-inflammatory and pro-nociceptive. This review discusses clinical and mechanistic evidence for targeting the sEH pathway for the treatment of pain.

Clinical and Preclinical Evidence for Roles of Soluble Epoxide Hydrolase in Osteoarthritis Knee Pain

OBJECTIVE

Chronic pain due to osteoarthritis (OA) is a major clinical problem, and existing analgesics often have limited beneficial effects and/or adverse effects, necessitating the development of novel therapies. Epoxyeicosatrienoic acids (EETs) are endogenous antiinflammatory mediators, rapidly metabolized by soluble epoxide hydrolase (EH) to dihydroxyeicosatrienoic acids (DHETs). We undertook this study to assess whether soluble EH-driven metabolism of EETs to DHETs plays a critical role in chronic joint pain associated with OA and provides a new target for treatment.

METHODS

Potential associations of chronic knee pain with single-nucleotide polymorphisms (SNPs) in the gene-encoding soluble EH and with circulating levels of EETs and DHETs were investigated in human subjects. A surgically induced murine model of OA was used to determine the effects of both acute and chronic selective inhibition of soluble EH by N-[1-(1-oxopropy)-4-piperidinyl]-N'-(trifluoromethoxy)phenyl]-urea (TPPU) on weight-bearing asymmetry, hind paw withdrawal thresholds, joint histology, and circulating concentrations of EETs and DHETs.

RESULTS

In human subjects with chronic knee pain, 3 pain measures were associated with SNPs of the soluble EH gene EPHX2, and in 2 separate cohorts of subjects, circulating levels of EETs and DHETs were also associated with 3 pain measures. In the murine OA model, systemic administration of TPPU both acutely and chronically reversed established pain behaviors and decreased circulating levels of 8,9-DHET and 14,15-DHET. EET levels were unchanged by TPPU administration.

CONCLUSION

Our novel findings support a role of soluble EH in OA pain and suggest that inhibition of soluble EH and protection of endogenous EETs from catabolism represents a potential new therapeutic target for OA pain.

Imaging of carrageenan-induced local inflammation and adjuvant-induced systemic arthritis with [(11)C]PBR28 PET

INTRODUCTION

[(11)C] PBR28 binding to translocator protein (TSPO) was evaluated for imaging of acute and chronic inflammation using two established rat models.

METHODS

Acute inflammation was induced by local carrageenan injection into the paw of Fisher 344 rats (model A). T-cell mediated adjuvant arthritis was induced by heat-inactivated Mycobacterium butyricum injection in Lewis rats (model B). Micro-PET scan was performed after injection of approximately 35 MBq [(11)C]PBR28. In model A, volumes of interest (VOIs) were defined in the paw of Fisher 344 rats (n=6) with contralateral sham treatment as control. For model B, VOIs were defined in the tail, sacroiliac joints, hips, knees and thigh muscles of M. butyricum treated animals (n=8) and compared with sham-treated controls (n=4). The peak (11)C-PBR28 SUV (SUVpeak) and area under the curve (AUCSUV) of 60-minute time-activity data were calculated. Immunohistochemistry for CD68, a macrophage stain, was performed from paw tissues. In addition, the [(11)C]PBR28 cell uptake was measured in lipopolysaccharide (LPS)-stimulated and non-stimulated macrophage cultures.

RESULTS

LPS-stimulated macrophages displayed dose-dependent increased [(11)C]PBR28 uptake, which was blocked by non-labeled PBR28. In both models, radiotracer uptake of treated lesions increased rapidly within minutes and displayed overall accumulative kinetics. The SUVpeak and AUCSUV of carrageenan-treated paws was significantly increased compared to controls. Also, the [(11)C]PBR28 uptake ratio of carrageenan-treated vs. sham-treated paw correlated significantly with CD68 staining ratios of the same animals. In adjuvant arthritis, significantly increased [(11)C]PBR28 SUVpeak and AUCSUV values were identified at the tail, knees, and sacroiliac joints, while no significant differences were identified in the lumbar spine and hips.

CONCLUSIONS

Based on our initial data, [(11)C]PBR28 PET appears to have potential for imaging of various inflammatory processes involving macrophage activation.

Evidências de que um protocolo de atividade física pode reduzir a contagem de leucócitos sinoviais de ratos artríticos

INTRODUÇÃO: O exercício físico apresenta potenciais benefícios na artrite, retardando a incapacidade funcional e melhorando a função das articulações. Estudos in vivo utilizando modelos experimentais de artrite podem fornecer informações úteis sobre estes benefícios. OBJETIVO: O propósito deste estudo foi avaliar os efeitos do exercício de baixa intensidade em um modelo de artrite induzida por CFA em ratos. MÉTODOS: A incapacitação articular foi mensurada pelo tempo de elevação da pata em uma deambulação estimulada no período de um minuto. O edema foi avaliado pela medida do diâmetro articular do joelho. O exsudato inflamatório foi coletado após dez dias para contagem de leucócitos. O protocolo de exercício consistiu de dois minutos de deambulação no primeiro dia, dez minutos de deambulação no segundo dia e 20 minutos de deambulação do terceiro ao décimo dia. O grupo controle foi submetido a um minuto de deambulação uma vez ao dia através de dez dias. O envolvimento de corticosteroide foi avaliado pelo tratamento dos animais por aminoglutetimida. RESULTADOS: O protocolo de exercício produziu uma pequena, mas sustentada redução da incapacitação e do edema articulares, associada a uma grande redução na contagem de leucócitos sinoviais. A aminoglutetimida preveniu apenas o efeito na contagem de leucócitos sinoviais. CONCLUSÃO: Esses resultados sugerem que uma atividade física de baixa intensidade não agrava a sintomatologia dos animais artríticos, de fato apresentando leve melhora, e ainda pode reduzir acentuadamente a migração de leucócitos para o espaço sinovial.

Spinal translocator protein (TSPO) modulates pain behavior in rats with CFA-induced monoarthritis

Translocator protein 18 kDa (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is predominantly located in the mitochondrial outer membrane and plays an important role in steroidogenesis, immunomodulation, cell survival and proliferation. Previous studies have shown an increased expression of TSPO centrally in neuropathology, as well as in injured nerves. TSPO has also been implicated in modulation of nociception. In the present study, we examined the hypothesis that TSPO is involved in the initiation and maintenance of inflammatory pain using a rat model of Complete Freund's Adjuvant (CFA)-induced monoarthritis of the tibio-tarsal joint. Immunohistochemistry was performed using Iba-1 (microglia), NeuN (neurons), anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) and anti-PBR (TSPO) on Days 1, 7 and 14 after CFA-induced arthritis. Rats with CFA-induced monoarthritis showed mechanical allodynia and thermal hyperalgesia on the ipsilateral hindpaw, which correlated with the increased TSPO expression in ipsilateral laminae I-II on all experimental days. Iba-1 expression in the ipsilateral dorsal horn was also increased on Days 7 and 14. Moreover, TSPO was colocalized with Iba-1, GFAP and NeuN within the spinal cord dorsal horn. The TSPO agonist Ro5-4864, given intrathecally, dose-dependently retarded or prevented the development of mechanical allodynia and thermal hyperalgesia in rats with CFA-induced monoarthritis. These findings provide evidence that spinal TSPO is involved in the development and maintenance of inflammatory pain behaviors in rats. Thus, spinal TSPO may present a central target as a complementary therapy to reduce inflammatory pain.

Soluble epoxide hydrolase and epoxyeicosatrienoic acids modulate two distinct analgesic pathways

During inflammation, a large amount of arachidonic acid (AA) is released into the cellular milieu and cyclooxygenase enzymes convert this AA to prostaglandins that in turn sensitize pain pathways. However, AA is also converted to natural epoxyeicosatrienoic acids (EETs) by cytochrome P450 enzymes. EET levels are typically regulated by soluble epoxide hydrolase (sEH), the major enzyme degrading EETs. Here we demonstrate that EETs or inhibition of sEH lead to antihyperalgesia by at least 2 spinal mechanisms, first by repressing the induction of the COX2 gene and second by rapidly up-regulating an acute neurosteroid-producing gene, StARD1, which requires the synchronized presence of elevated cAMP and EET levels. The analgesic activities of neurosteroids are well known; however, here we describe a clear course toward augmenting the levels of these molecules. Redirecting the flow of pronociceptive intracellular cAMP toward up-regulation of StARD1 mRNA by concomitantly elevating EETs is a novel path to accomplish pain relief in both inflammatory and neuropathic pain states.

Ro5-4864 promotes neonatal motor neuron survival and nerve regeneration in adult rats

The translocator protein (18 kDa; TSPO), formerly known as the peripheral benzodiazepine receptor, is an outer mitochondrial membrane protein that associates with the mitochondrial permeability transition pore to regulate both steroidogenesis and apoptosis. TSPO expression is induced in adult dorsal root ganglion (DRG) sensory neurons after peripheral nerve injury and a TSPO receptor ligand, Ro5-4864, enhances DRG neurite growth in vitro and axonal regeneration in vivo. We have now found that TSPO is induced in neonatal motor neurons after peripheral nerve injury and have evaluated its involvement in neonatal and adult sensory and motor neuron survival, and in adult motor neuron regeneration. The TSPO ligand Ro5-4864 rescued cultured neonatal DRG neurons from nerve growth factor withdrawal-induced apoptosis and protected neonatal spinal cord motor neurons from death due to sciatic nerve axotomy. However, Ro5-4864 had only a small neuroprotective effect on adult facial motor neurons after axotomy, did not delay onset or prolong survival in SOD1 mutant mice, and failed to protect adult DRG neurons from sciatic nerve injury-induced death. In contrast, Ro5-4864 substantially enhanced adult facial motor neuron nerve regeneration and restoration of function after facial nerve axotomy. These data indicate a selective sensitivity of neonatal sensory and motor neurons to survival in response to Ro5-4864, which highlights that survival in injured immature neurons cannot necessarily predict success in adults. Furthermore, although Ro5-4864 is only a very weak promoter of survival in adult neurons, it significantly enhances regeneration and functional recovery in adults.

Role of cyclooxygenases in oedema-forming activity of bothropic venoms

The venoms of Bothrops asper (BaV) and Bothrops jararaca (BjV), two of the most medically important poisonous snakes of Latin America, cause pronounced oedema in the victims through poorly understood mechanisms. In the present study, we examined the possible role of cyclooxygenases (COX) in the genesis of mouse paw oedema caused by BaV and BjV injections. BaV at 2.5 microg/paw and BjV at 0.75 microg/paw induced significant oedema that persisted for up to 6h following subplantar injection. Treatment with indomethacin (2 mg/kg), rofecoxib, (10 mg/kg), or dexamethasone (2 mg/kg) significantly reduced the BaV- and BjV-induced oedema formation. Treatment with SC-560 (30 mg/kg) significantly reduced the oedema formation induced by BjV but had no effect on that induced by BaV. Both venoms induced significant increases in the levels of prostaglandin E(2) (PGE(2)) and the expression of COX-1 and COX-2 in paw tissue. The peak of oedema formation and PGE(2) release correlated with marked expression of COX-2 in the paw tissue. These results demonstrate that injection of BaV and BjV results in a rapid increase in oedema formation that is, at least partially, mediated by arachidonic acid metabolites formed by COX-2. In the case of BjV, COX-1-derived prostanoids also appear to contribute significantly to the inflammatory changes.

Peripheral benzodiazepine receptors on platelets of fibromyalgic patients

OBJECTIVE

The aim of the present study was to analyze if alterations of peripheral-type benzodiazepine receptor (PBR) characteristics occurred in platelet membranes of patients affected by primary fibromyalgia (FM).

DESIGN AND METHODS

Platelets were obtained from 30 patients with FM. Evaluation of kinetic parameters of PBR was performed using [(3)H] PK11195 as specific radioligand compared with 16 healthy volunteers.

RESULTS

The results showed a significant increase of PBR binding sites value in platelet membranes from FM patients (B(max) was 5366+/-188 fmol/mg vs. controls, 4193+/-341 fmol/mg, mean+/-SEM) (**p<0.01) but not for affinity value (K(d) was 4.90+/-0.39 nM vs. controls, 4.74+/-0.39 nM, mean+/-SEM) (p>0.05). Symptom severity scores (pain and tiredness) were positively correlated with B(max).

CONCLUSIONS

Our results showed an up-regulation of PBR in platelets of FM patients, and this seems to be related to the severity of fibromyalgic symptoms.

Axonal injury-dependent induction of the peripheral benzodiazepine receptor in small-diameter adult rat primary sensory neurons

The peripheral benzodiazepine receptor (PBR), a benzodiazepine but not gamma-aminobutyric acid-binding mitochondrial membrane protein, has roles in steroid production, energy metabolism, cell survival and growth. PBR expression in the nervous system has been reported in non-neuronal glial and immune cells. We now show expression of both PBR mRNA and protein, and the appearance of binding of a synthetic ligand, [(3)H]PK11195, in dorsal root ganglion (DRG) neurons following injury to the sciatic nerve. In naïve animals, PBR mRNA, protein expression and ligand binding are undetectable in the DRG. Three days after sciatic nerve transection, however, PBR mRNA begins to be expressed in injured neurons, and 4 weeks after the injury, expression and ligand binding are present in 35% of L4 DRG neurons. PBR ligand binding also appears after injury in the superficial dorsal horn of the spinal cord. The PBR expression in the DRG is restricted to small and medium-sized neurons and returns to naïve levels if the injured peripheral axons are allowed to regrow and reinnervate targets. No non-neuronal PBR expression is detected, unlike its putative endogenous ligand the diazepam binding inhibitor (DBI), which is expressed only in non-neuronal cells, including the satellite cells that surround DRG neurons. DBI expression does not change with sciatic nerve transection. PBR acting on small-calibre neurons could play a role in the adaptive survival and growth responses of these cells to injury of their axons.

Involvement of steroids in anti-inflammatory effects of peripheral benzodiazepine receptor ligands

Mouse paw oedema induced by carrageenan is used to determine if glucocorticoids are involved in the anti-inflammatory effects of peripheral benzodiazepine receptor ligands. The anti-inflammatory responses elicited by i.p. treatment with 1-(2-chlorophenyl)-N-methyl-N (1-methyl-propyl)-3-isoquinoline carboxamide (PK11195) and 7-chloro-5-(4-chlorophenyl)-1,3-dihydro-1-methyl-2-H-1, 4-benzodiazepin-2 (Ro5-4864) were reversed by aminoglutethimide, an inhibitor of steroidal synthesis. Intraplantar injection into the ipsilateral paw of Ro5-4864, but not PK11195, inhibited the formation of paw oedema and this effect was reversed by aminoglutethimide. These results suggest that glucocorticoids are involved in the systemic and local anti-inflammatory effects of Ro5-4864 and only in the systemic response to PK11195.