Effect of etodolac, a COX-2 inhibitor, on neuropathic pain in a rat model

Pathophysiological Aspects of Muscle Atrophy and Osteopenia Induced by Chronic Constriction Injury (CCI) of the Sciatic Nerve in Rats

Skeletal muscle atrophy is a condition characterized by a loss of muscle mass and muscle strength caused by an imbalance between protein synthesis and protein degradation. Muscle atrophy is often associated with a loss of bone mass manifesting as osteoporosis. The aim of this study was to evaluate if chronic constriction injury (CCI) of the sciatic nerve in rats can be a valid model to study muscle atrophy and consequent osteoporosis. Body weight and body composition were assessed weekly. Magnetic resonance imaging (MRI) was performed on day zero before ligation and day 28 before sacrifice. Catabolic markers were assessed via Western blot and Quantitative Real-time PCR. After the sacrifice, a morphological analysis of the gastrocnemius muscle and Micro-Computed Tomography (Micro-CT) on the tibia bone were performed. Rats that underwent CCI had a lower body weight increase on day 28 compared to the naive group of rats (p < 0.001). Increases in lean body mass and fat mass were also significantly lower in the CCI group (p < 0.001). The weight of skeletal muscles was found to be significantly lower in the ipsilateral hindlimb compared to that of contralateral muscles; furthermore, the cross-sectional area of muscle fibers decreased significantly in the ipsilateral gastrocnemius. The CCI of the sciatic nerve induced a statistically significant increase in autophagic and UPS (Ubiquitin Proteasome System) markers and a statistically significant increase in Pax-7 (Paired Box-7) expression. Micro-CT showed a statistically significant decrease in the bone parameters of the ipsilateral tibial bone. Chronic nerve constriction appeared to be a valid model for inducing the condition of muscle atrophy, also causing changes in bone microstructure and leading to osteoporosis. Therefore, sciatic nerve constriction could be a valid approach to study muscle-bone crosstalk and to identify new strategies to prevent osteosarcopenia.

Antihyperalgesic/Antinociceptive Effects of Ceftriaxone and Its Synergistic Interactions with Different Analgesics in Inflammatory Pain in Rodents

Background:

The β-lactam antibiotic ceftriaxone stimulates glutamate transporter GLT-1 expression and is effective in neuropathic and visceral pain models. This study examined the effects of ceftriaxone and its interactions with different analgesics (ibuprofen, celecoxib, paracetamol, and levetiracetam) in somatic and visceral pain models in rodents.

Methods:

The effects of ceftriaxone (intraperitoneally/intraplantarly), analgesics (orally), and their combinations were examined in the carrageenan-induced paw inflammatory hyperalgesia model in rats (n = 6–12) and in the acetic acid-induced writhing test in mice (n = 6–10). The type of interaction between ceftriaxone and analgesics was determined by isobolographic analysis.

Results:

Pretreatment with intraperitoneally administered ceftriaxone (10–200 mg/kg per day) for 7 days produced a significant dose-dependent antihyperalgesia in the somatic inflammatory model. Acute administration of ceftriaxone, via either intraperitoneal (10–200 mg/kg) or intraplantar (0.05–0.2 mg per paw) routes, produced a significant and dose-dependent but less efficacious antihyperalgesia. In the visceral pain model, significant dose-dependent antinociception of ceftriaxone (25–200 mg/kg per day) was observed only after the 7-day pretreatment. Isobolographic analysis in the inflammatory hyperalgesia model revealed approximately 10-fold reduction of doses of both drugs in all examined combinations. In the visceral nociception model, more than 7- and 17-fold reduction of doses of both drugs was observed in combinations of ceftriaxone with ibuprofen/paracetamol and celecoxib/levetiracetam, respectively.

Conclusions:

Ceftriaxone exerts antihyperalgesia/antinociception in both somatic and visceral inflammatory pain. Its efficacy is higher after a 7-day pretreatment than after acute administration. The two-drug combinations of ceftriaxone and the nonsteroidal analgesics/levetiracetam have synergistic interactions in both pain models. These results suggest that ceftriaxone, particularly in combinations with ibuprofen, celecoxib, paracetamol, or levetiracetam, may provide useful approach to the clinical treatment of inflammation-related pain.

p300 exerts an epigenetic role in chronic neuropathic pain through its acetyltransferase activity in rats following chronic constriction injury (CCI)

BACKGROUND

Neuropathic pain is detrimental to human health; however, its pathogenesis still remains largely unknown. Overexpression of pain-associated genes and increased nociceptive somato-sensitivity are well observed in neuropathic pain. The importance of epigenetic mechanisms in regulating the expression of pro- or anti-nociceptive genes has been revealed by studies recently, and we hypothesize that the transcriptional coactivator and the histone acetyltransferase E1A binding protein p300 (p300), as a part of the epigenetic mechanisms of gene regulation, may be involved in the pathogenesis of neuropathic pain induced by chronic constriction injury (CCI). To test this hypothesis, two different approaches were used in this study: (I) down-regulating p300 with specific small hairpin RNA (shRNA) and (II) chemical inhibition of p300 acetyltransferase activity by a small molecule inhibitor, C646.

RESULTS

Using the CCI rat model, we found that the p300 expression was increased in the lumbar spinal cord on day 14 after CCI. The treatment with intrathecal p300 shRNA reversed CCI-induced mechanical allodynia and thermal hyperalgesia, and suppressed the expression of cyclooxygenase-2 (COX-2), a neuropathic pain-associated factor. Furthermore, C646, an inhibitor of p300 acetyltransferase, also attenuated mechanical allodynia and thermal hyperalgesia, accompanied by a suppressed COX-2 expression, in the spinal cord.

CONCLUSIONS

The results suggest that, through its acetyltransferase activity in the spinal cord after CCI, p300 epigenetically plays an important role in neuropathic pain. Inhibiting p300, using interfering RNA or C646, may be a promising approach to the development of new neuropathic pain therapies.

Ameliorative effect of combined administration of inducible nitric oxide synthase inhibitor with cyclooxygenase-2 inhibitors in neuropathic pain in rats

OBJECTIVES

The objective of this study was to examine the effects of rofecoxib, meloxicam, both cyclooxygenase-2 (COX-2) inhibitors and aminoguanidine hydrochloride, an inducible nitric oxide synthase (iNOS) inhibitor and their combinations in neuropathic pain in rats.

METHODS

Neuropathy was induced by chronic constriction injury (CCI) of right sciatic nerve under ketamine anesthesia in rats. Effect of ED(50) of aminoguanidine hydrochloride, rofecoxib and meloxicam administered orally was investigated using behavioral tests. Effect of combinations of aminoguanidine hydrochloride with rofecoxib and meloxicam was also investigated in neuropathic pain employing behavioral tests.

RESULTS

Behavioral tests, mechanical, thermal and cold stimuli confirmed the development of neuropathic pain after CCI. Aminoguanidine hydrochloride, rofecoxib and meloxicam when administered alone, produced significant increase in paw withdrawal threshold to mechanical stimuli at 6 h in ipsilateral hind paw after CCI. Co-administration of aminoguanidine hydrochloride (30 mg/kg) with rofecoxib (1.31 mg/kg) and meloxicam (1.34 mg/kg) was also found to produce significant increase in paw withdrawal latencies to mechanical stimuli at 6 h. Combined administration of aminoguanidine hydrochloride with meloxicam and rofecoxib produced significant rise in pain threshold for mechanical hyperalgesia in ipsilateral hind paw when compared with the groups treated with aminoguanidine hydrochloride, meloxicam and rofecoxib alone.

CONCLUSION

Co-administration of meloxicam and rofecoxib with aminoguanidine hydrochloride may be an alternative approach for the treatment of neuropathic pain.

Influence of nonsteroidal anti-inflammatory drugs on the antiplatelet effects of aspirin in rats

Low-dose aspirin acts by irreversibly acetylating internal cyclooxygenase-1 (COX-1) on platelets, thereby suppressing platelet aggregation. Because nonsteroidal anti-inflammatory drugs (NSAIDs) also inhibit COX-1, the antiplatelet effects of aspirin may be suppressed when it is co-administered with NSAIDs. In this study, the influences of ibuprofen, loxoprofen sodium and etodolac on the antiplatelet effects of aspirin were investigated in male Sprague-Dawley (SD) rats. Aspirin and/or NSAIDs were administered orally at single or multiple daily doses. Platelet aggregation (ADP and collagen were added as stimuli) and serum thromboxane B(2) (TXB(2)) concentrations were measured. The maximum inhibitions of aggregation in the aspirin before ibuprofen group were 41.0±7.8% for ADP and 38.7±5.4% for collagen at 6 h after administration; similar values were seen in the aspirin group; however, percent inhibitions in the aspirin before ibuprofen multiple administration group were lower than those in the aspirin group. Thus, the inhibitory effects of daily low-dose aspirin on platelets are competitively inhibited by the prolonged use of multiple daily doses of ibuprofen. In contrast, serum TXB(2) concentrations in all groups were lower than those in the control group (drug-free). This suggests that the relationship between the inhibition of platelet COX-1 and the suppression of platelet aggregation is nonlinear. When aspirin was administered with loxoprofen sodium, similar effects were observed; however, with etodolac, the antiplatelet effects in all groups were equal to those in the aspirin group. Accordingly, if co-administration with NSAIDs is necessary with low-dose aspirin, a selective COX-2 inhibitor, such as etodolac, should be used.

Statistical identification of predictors for peripheral neuropathy associated with administration of bortezomib, taxanes, oxaliplatin or vincristine using ordered logistic regression analysis

Chemotherapy-induced peripheral neuropathy (CIPN) is a major drug-induced adverse reaction that becomes a dose-limiting toxicity. However, effective strategies for preventing or treating CIPN are lacking. Accordingly, this study aimed to statistically identify predictors for CIPN. Retrospective analysis was carried out for 190 patients who had been treated with bortezomib (n=28), taxanes (paclitaxel or docetaxel; n=58), oxaliplatin (n=52) or vincristine (n=52) at our hospital between April 2005 and December 2008. The severity of CIPN was assessed at the time of chemotherapy completion, graded as grade 0-5 in accordance with the National Cancer Institute Common Terminology Criteria for Adverse Events v3.0. Multivariate ordered logistic regression analysis was used to investigate predictors for CIPN. Predictors for CIPN in patients that were administered bortezomib were no co-administration of dexamethasone [odds ratio (OR), 0.455; confidence interval (CI), 0.208-0.955; P=0.0376] and sex (male) (OR, 3.035; CI, 1.356-6.793; P=0.0069). For taxanes (paclitaxel or docetaxel), the predictor for CIPN was a large number of chemotherapy cycles (OR, 2.379; CI, 1.035-5.466; P=0.0412). For oxaliplatin, the predictors for CIPN were a large number of chemotherapy cycles (OR, 3.089; CI, 1.598-5.972; P=0.0008) and no co-administration of non-steroidal anti-inflammatory drugs (OR, 0.393; CI, 0.197-0.785; P=0.0082). For vincristine, predictors for CIPN were a large number of chemotherapy cycles (OR, 6.015; CI, 1.880-19.248; P=0.0025) and co-administration of an analgesic adjuvant (OR, 3.907; CI, 1.383-11.031; P=0.0101). In conclusion, our study indicates that CIPN will be alleviated by the co-administration of dexamethasone with bortezomib and non-steroidal anti-inflammatory drugs with oxaliplatin.

Recent advances in the treatment of pain

Cancer pain and chronic non-malignant pain can be difficult to manage and may not respond satisfactorily to standard analgesics. Sequential empiric analgesic trials are usually done to manage individual patients. Experimental human pain models have helped to clarify mechanisms of opioid and adjuvant analgesic actions. Combinations of opioids and adjuvant analgesics better relieve pain than either opioids or adjuvant analgesics alone, as demonstrated in randomized controlled trials. The analgesic activity of antidepressants is largely dependent upon norepinephrine reuptake and activation of alpha 2 adrenergic receptors. Corticosteroids reduce postoperative orthopedic incident pain, which may allow patients to ambulate earlier and with less pain. Spinal corticosteroids reduce lower hemibody pain. Gabapentinoids as single high doses reduce postoperative pain and certain acute pain syndromes. Individuals who experience flares of pain while on spinal opioids benefit from intrathecal boluses of levobupivicaine or sublingual ketamine. Interventional approaches to pain management are often necessary due to the limitations of systemic analgesics. Electronics stimulators (peripheral, spinal and motor cortex) improve difficult to manage chronic pain syndromes. Pulsed radiofrequency reduces pain without tissue damage, which could be an advantage over chemical or radiofrequency neurotomy. Botulinum toxin A reduces focal neuropathic pain that is durable. Interventional related successes in relieving pain are operator dependent. Most reported benefits of systemic and regional analgesics and interventional approaches to pain relief are not based on randomized trials and are subject to selection bias, sampling error, and placebo responses, which may over-inflate reported benefits. Randomized controlled trials are needed to confirm reported benefits.

Effect of etanercept, a tumor necrosis factor-alpha inhibitor, on neuropathic pain in the rat chronic constriction injury model

STUDY DESIGN

The effects of a low, local dose of a tumor necrosis factor-alpha (TNF-alpha) inhibitor on neuropathic pain behaviors in a rat chronic constriction injury model were evaluated.

OBJECTIVE

To investigate whether a peripherally implanted polymer drug depot can deliver a dose of etanercept sufficient to reduce thermal hyperalgesia and mechanical allodynia in a rat model of neuropathic pain.

SUMMARY OF BACKGROUND DATA

TNF-alpha inhibitors reduce pain-associated behavior in experimental models of neuropathic pain. Moreover, systemic injections of TNF-alpha inhibitors have suggested some efficacy in treating sciatic pain in limited, off-label clinical studies. Improvements in these results may be obtained by optimal dosing via targeted, sustained delivery at the site of disc-induced inflammation.

METHODS

Unilateral chronic constriction injury was applied to the sciatic nerve of 56 male, Wistar rats. Four groups of animals (n = 7) received 0.5 mL phosphate-buffered saline every 3 days, 0.3 or 3 mg/kg etanercept every 3 days, or gabapentin (60 mg/kg) 1 hour before each behavioral test all via subcutaneous injection. Two groups of animals received 1.5 or 3.0 microg/h etanercept delivered by poly(lactic-co-glycolic acid) (PLGA) millicylinders (1 mm diameter x 10 mm long) implanted near the injured sciatic nerve. One group received a PLGA millicylinder implanted near the injured sciatic nerve. The final group received 3.0 microg/h etanercept via PLGA millicylinder implanted next to the uninjured, contralateral sciatic nerve.

RESULTS

A low, local dose of etanercept (approximately 3 microg/h) delivered by a polymer depot significantly reduced (P < 0.05) thermal hyperalgesia for 57 days as compared to polymer depot without drug or an etanercept-loaded depot implanted near the contralateral sciatic nerve, and equivalent to a 10-fold higher dose delivered by repeat subcutaneous injection.

CONCLUSION

This preclinical study indicates that delivering TNF-alpha inhibitors by means of a locally administered polymeric formulation provides long-lasting analgesia in an inflammatory neuropathic pain model.

Role of the cysteine protease cathepsin S in neuropathic hyperalgesia

Using a gene expression analysis approach we found that the mRNA encoding the lysosomal cysteine protease cathepsin S (CatS) was up-regulated in rat dorsal root ganglia (DRG) following peripheral nerve injury. CatS protein was expressed in infiltrating macrophages in DRG and near the site of injury. At both sites CatS expression progressively increased from day 3 to day 14 after injury. In naïve rats, intraplantar injection of activated rat recombinant (rr) CatS (0.3, 1 microg/rat) induced a mechanical hyperalgesia that developed within half-an-hour, diminished by 3h and was absent after 24h. Activated rrCathepsin B (CatB) and non-activated rrCatS injected intraplantarly at the same or higher doses than activated rrCatS had no effect on rat nociceptive thresholds. In nerve-injured rats, mechanical hyperalgesia, but not allodynia, was significantly reversed for up to 3h by systemic administration of a non-brain penetrant, irreversible CatS inhibitor (LHVS, 3-30 mg/kg s.c.). Depletion of peripheral macrophages by intravenous injection of liposome encapsulate clodronate (1ml, 5 mg/ml) partially reduced established mechanical hyperalgesia but not allodynia, and abolished the anti-hyperalgesic effect of LHVS. Our results demonstrate a pro-nociceptive effect of CatS and indicate that endogenous CatS released by peripheral macrophages contributes to the maintenance of neuropathic hyperalgesia following nerve injury.

GW406381, a novel COX-2 inhibitor, attenuates spontaneous ectopic discharge in sural nerves of rats following chronic constriction injury

There are several lines of evidence to suggest that cyclooxygenase-2 (COX-2) plays an important role in the generation and maintenance of neuropathic pain states following peripheral nerve injury. However, COX-2 inhibitors are generally ineffective in reversing mechanical allodynia and hyperalgesia in models of neuropathic hypersensitivity. Here, we have investigated the effects of GW406381, a novel COX-2 inhibitor, on mechanical allodynia, hyperalgesia and generation of spontaneous ectopic discharge in rats following chronic constriction injury (CCI) of the sciatic nerve and compared it with rofecoxib. GW406381 (5mg/kg, 5 days of treatment) significantly reversed the CCI-induced decrease in paw withdrawal thresholds (PWTs), assessed using both von Frey hair and paw pressure tests, whereas an equi-effective dose of rofecoxib (5mg/kg, 5 days of treatment) in inflammatory pain models was ineffective. In rats treated with GW406381, the proportion of fibres showing spontaneous activity was significantly lower (15.58%) than that in the vehicle (32.67%)- and rofecoxib (39.66%)-treated rats. Ibuprofen, a non-selective COX inhibitor, at 5mg/kg, orally dosed three times a day for 5 days did not significantly affect the PWTs in CCI rats. In naïve rats, GW406381 did not significantly change the PWTs. These results illustrate that COX-2 may indeed play an important role in the maintenance of neuropathic pain following nerve injury, but that only certain COX-2 inhibitors, such as GW406381, are effective in this paradigm. Whilst the mechanisms underlying this differential effect of GW406381 are not clear, differences in drug/enzyme kinetic interactions may be a key contributing factor.

Surgically induced osteoarthritis in the rat results in the development of both osteoarthritis-like joint pain and secondary hyperalgesia

OBJECTIVE

In the present study, we sought to develop/characterize the pain profile of a rat model of surgically induced osteoarthritis (OA).

METHODS

OA was surgically induced in male Lewis rats (200-225 g) by transection of the medial collateral ligament and medial meniscus of the femoro-tibial joint. In order to characterize the pain profile, animals were assessed for a change in hind paw weight distribution (HPWD), development of mechanical allodynia, and the presence of thermal and mechanical hyperalgesia. Rofecoxib and gabapentin were examined for their ability to decrease change in weight distribution and tactile allodynia.

RESULTS

Transection of the medial collateral ligament and medial meniscus of male Lewis rats resulted in rapid (<3 days) changes in hind paw weight bearing and the development of tactile allodynia (secondary hyperalgesia). There was, however, no appreciable effect on thermal hyperalgesia or mechanical hyperalgesia. Treatment with a single dose of rofecoxib (10 mg/kg, PO, day 21 post surgery) or gabapentin (100mg/kg, PO, day 21 post surgery) significantly attenuated the change in HPWD, however, only gabapentin significantly decreased tactile allodynia.

CONCLUSION

The rat medial meniscal tear (MMT) model mimics both nociceptive and neuropathic OA pain and is responsive to both a selective cylooxygenase-2 (COX-2) inhibitor commonly utilized for OA pain (rofecoxib) and a widely prescribed drug for neuropathic pain (gabapentin). The rat MMT model may therefore represent a predictive tool for the development of pharmacologic interventions for the treatment of the symptoms associated with OA.

Antihyperalgesic effects of local injections of anandamide, ibuprofen, rofecoxib and their combinations in a model of neuropathic pain

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit fatty acid amidohydrolase (FAAH), the enzyme responsible for the metabolism of anandamide, an endocannabinoid. The analgesic interactions between anandamide (0.01 microg), ibuprofen (0.1 microg) and rofecoxib (0.1 microg) or their combinations administered locally in the hind paw of neuropathic rats were investigated together with the effects of specific antagonists for the cannabinoid CB(1) (AM251; 80 microg) and CB(2) (AM630; 25 microg) receptors. Mechanical allodynia and thermal hyperalgesia were evaluated in 108 Wistar rats allocated to: (1-4) NaCl 0.9%; anandamide; ibuprofen; rofecoxib; (5-6) anandamide+ibuprofen or rofecoxib; (7-8) AM251 or AM630; (9-10) anandamide+AM251 or AM630; (11-12) ibuprofen+AM251 or AM630; (13-14) rofecoxib+AM251 or AM630; (15-16) anandamide+ibuprofen+AM251 or AM630; (17-18) anandamide+rofecoxib+AM251 or AM630. Drugs were given subcutaneously in the hind paw 15min before pain tests. Anandamide, ibuprofen, rofecoxib and their combinations significantly decreased mechanical allodynia and thermal hyperalgesia with an ED(50) of 1.6+/-0.68ng and 1.1+/-1.09 ng for anandamide, respectively. The effects of NSAIDs were not antagonized by AM251 or AM630 but those of anandamide were inhibited by AM251 but not by AM630. In conclusion, locally injected anandamide, ibuprofen, rofecoxib and their combinations decreased pain behavior in neuropathic animals. Local use of endocannabinoids to treat neuropathic pain may be an interesting way to treat this condition without having the deleterious central effects of systemic cannabinoids.

Neuropathy-induced osteopenia in rats is not due to a reduction in weight born on the affected limb

Changes in bone mineral density (BMD) are associated with clinical neuropathies. Following nerve injury in the rat, there is a loss of BMD, which may be related to nerve injury or reduced mechanical loading. The purpose of this study was to investigate if altered mechanical loading is solely responsible for the observed loss of BMD in neuropathic pain models. In addition, we sought to study the action of chronic bisphosphonate treatment on both neuropathy-induced osteopenia and pain. We therefore had two hypotheses: firstly, that nerve injuries can have variable effects on hind limb bone loss in rats which are not attributable to differences in the extent of hind limb disuse and, secondly, that bisphosphonate treatment can reverse bone loss in a rat mononeuropathy model, and this is not attributable to bisphosphonate effects on nociception or hind paw unweighting. Male Sprague-Dawley rats were subject to chronic constriction injury (CCI), partial sciatic nerve ligation (PSN) or L5 + L6 spinal nerve ligation (SNL). Loss of BMD, defined as a numerically lower BMD as compared to control animals, was extreme following CCI (maximum ipsilateral/contralateral difference of 0.023 +/- 0.011); BMD loss following either PSN or SNL in the rat was subtle (0.010 +/- 0.002 and 0.013 +/- 0.012 g/cm2, respectively), significant only at early time points and had resolved by 7 weeks post-surgery. Chronic bisphosphonate treatment significantly inhibited CCI-induced osteopenia in the rat without inhibiting the reduction in weight-bearing tactile allodynia or mechanical hyperalgesia. Loss of BMD is observed in rats in a variety of neuropathic pain models. Lack of correlation between neuropathy-induced bone loss and weight bearing demonstrates that the bone loss is not simply a function of reduced mechanical loading and suggests that altered bone-nerve signaling is involved. Furthermore, chronic bisphosphonate treatment inhibits neuropathy-induced osteopenia without affecting behavioral measurements of neuropathic pain. This indicates that osteopenia is not directly related to neuropathic pain behaviors.

Prostanoid receptor EP1 and Cox-2 in injured human nerves and a rat model of nerve injury: a time-course study

Background

Recent studies show that inflammatory processes may contribute to neuropathic pain. Cyclooxygenase-2 (Cox-2) is an inducible enzyme responsible for production of prostanoids, which may sensitise sensory neurones via the EP1 receptor. We have recently reported that while macrophages infiltrate injured nerves within days of injury, they express increased Cox-2-immunoreactivity (Cox-2-IR) from 2 to 3 weeks after injury. We have now investigated the time course of EP1 and Cox-2 changes in injured human nerves and dorsal root ganglia (DRG), and the chronic constriction nerve injury (CCI) model in the rat.

Methods

Tissue sections were immunostained with specific antibodies to EP1, Cox-2, CD68 (human macrophage marker) or OX42 (rat microglial marker), and neurofilaments (NF), prior to image analysis, from the following: human brachial plexus nerves (21 to 196 days post-injury), painful neuromas (9 days to 12 years post-injury), avulsion injured DRG, control nerves and DRG, and rat CCI model tissues. EP1 and NF-immunoreactive nerve fibres were quantified by image analysis.

Results

EP1:NF ratio was significantly increased in human brachial plexus nerve fibres, both proximal and distal to injury, in comparison with uninjured nerves. Sensory neurones in injured human DRG showed a significant acute increase of EP1-IR intensity. While there was a rapid increase in EP1-fibres and CD-68 positive macrophages, Cox-2 increase was apparent later, but was persistent in human painful neuromas for years. A similar time-course of changes was found in the rat CCI model with the above markers, both in the injured nerves and ipsilateral dorsal spinal cord.

Conclusion

Different stages of infiltration and activation of macrophages may be observed in the peripheral and central nervous system following peripheral nerve injury. EP1 receptor level increase in sensory neurones, and macrophage infiltration, appears to precede increased Cox-2 expression by macrophages. However, other methods for detecting Cox-2 levels and activity are required. EP1 antagonists may show therapeutic effects in acute and chronic neuropathic pain, in addition to inflammatory pain.

Neonatal immune challenge alters nociception in the adult rat

Intense pain or intense peripheral inflammation experienced during development can have pronounced effects upon adult pain sensation. However, little is known about the more commonly encountered mild systemic inflammation, such as that experienced with mild illness. Neonatal exposure to lipopolysaccharide (LPS), an established model of immune system activation, has been shown to affect febrile and cyclooxygenase-2 (COX-2) responses to a similar exposure in adulthood. Adult LPS also elicits a range of sickness behaviours, including enhanced responses to painful stimuli. We, therefore, hypothesized that adult sensation and pain responses could be affected by a neonatal LPS challenge. Male and female Sprague-Dawley rats were administered LPS at postnatal day 14 and were tested in adulthood for nociceptive responses to thermal and mechanical stimuli using, respectively, a plantar test apparatus and von Frey filaments, before and after adult LPS. Expression of dorsal root ganglion and lumbar spinal cord COX-2 was also examined. Animals treated as neonates with saline showed the expected hypersensitivity to painful stimuli after adult LPS as well as enhanced spinal cord COX-2. Neonatally LPS-treated rats, however, showed a significantly different profile. They displayed enhanced baseline nociception and elevated basal spinal cord COX-2 compared with neonatally saline-treated rats. Also, rather than the expected hyperalgesia after adult LPS, no changes in nociceptive responses and a reduction in spinal cord COX-2 expression were observed. These findings have important implications for the understanding of pain and its management and highlight the importance of the neonatal period in the development of pain pathways.