Judith S. Walker

Experimental pain in healthy human subjects: gender differences in nociception and in response to ibuprofen.

We used electrically induced pain in healthy young subjects to study gender differences in nociception and the analgesic efficacy of ibuprofen.Cutaneous stimulation of the earlobe allowed measurement of pain detection thresholds and maximal pain tolerance. Drug and placebo were each administered twi

Hyperalgesia due to nerve damage: role of nerve growth factor.

The hypothesis that nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) contribute to hyperalgesia resulting from nerve damage was tested in rats in which the sciatic nerve was partially transected on one side. Administration of antisera raised against NGF and BDNF relieved mechanical and thermal hyperalgesia in these animals. It has been suggested that NGF may elicit hyperalgesia by inducing mast cells to release algesic agents such as serotonin (5-HT). We found that degranulation of mast cells with compound 48/80 relieved mechanical and thermal hyperalgesia produced by nerve damage. We also found that local injection of the 5-HT2A and 5-HT3 receptor antagonists ketanserin and ICS 205-930 into the affected hind paw relieved mechanical hyperalgesia in a dose-dependent fashion. These findings support the idea that in this rat model of hyperalgesia due to peripheral nerve damage, NGF acts on mast cells to induce release of 5-HT, which sensitizes nociceptors. Hyperalgesia due to nerve injury and hyperalgesia due to inflammation may share some common features.

Hyperalgesia due to nerve injury: role of prostaglandins.

The hypothesis that prostaglandins contribute to hyperalgesia resulting from nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. Subcutaneous injection of indomethacin (a classic inhibitor of cyclo-oxygenase) into the affected hindpaw relieved mechanical hyperalgesia for up to 10 days after injection. Subcutaneous injection of meloxicam or SC-58125 (selective inhibitors of cyclo-oxygenase-2) into the affected hindpaw also relieved mechanical hyperalgesia, but with a shorter time-course. Subcutaneous injection of SC-19220 (an EP1 prostaglandin receptor blocker) into the affected hindpaw produced significant relief of mechanical and thermal hyperalgesia. Comparable injections into the contralateral paw or abdomen had no effect on mechanical or thermal hyperalgesia, suggesting that the effects we observed were local rather than systemic. We conclude that prostaglandins, probably prostaglandin E1 or E2, contribute to the peripheral mechanisms underlying hyperalgesia following nerve injury. These data provide further evidence that inflammatory mediators contribute to neuropathic pain, and may warrant further study of peripherally administered non-steroidal anti-inflammatory drugs as a possible treatment for such pain in patients.

Effect of the peripherally selective κ‐opioid agonist, asimadoline, on adjuvant arthritis

Opioids, though widely used as analgesics, have not been seriously considered as therapy for rheumatoid arthritis. The present study evaluated the dose‐effect and time‐dependence relationships of a new peripherally selective κ agonist, asimadoline, in rats with adjuvant arthritis. The arthritis was assessed by a pooled severity index combining the comprehensive criteria of oedema, radiography and histological changes, in the hind limbs. Asimadoline was extremely effective in attenuating joint damage (by up to 80%) when administered parenterally (0.5 to 10 mg kg−1 day−1, i.p.) throughout the disease or during its early phase; treatment was less successful if confined to the latter stages. Ten fold higher doses were effective orally. Equimolar doses of a peripherally‐selective antagonist, naloxone methiodide, and the κ‐selective antagonist, MR2266, fully reversed the peripheral anti‐arthritic effects of asimadoline (5 mg kg−1 day−1), indicating that asimadoline acts through peripheral κ‐opioid receptors. However, an equivalent dose of MR2266 did not fully reverse the anti‐arthritic effects of the highest dose of asimadoline (40 mg kg−1 day−1), suggesting a loss of κ‐selectivity at this dose. Asimadoline also exhibited analgesic effects (mechanical nociceptive thresholds) in arthritic but not non‐arthritic rats, indicating that inflammation is necessary for asimadoline‐induced analgesia. These data confirm our previous findings that κ‐opioids possess anti‐arthritic properties and that these effects are mediated via peripheral κ‐receptors. The present results are new in showing that the peripherally acting κ‐opioid agonist, asimadoline, is a potent anti‐arthritic agent. Such novel drugs, essentially lacking central side effects, herald new treatments for rheumatoid arthritis.

Cytokine expression and synovial pathology in the initiation and spontaneous resolution phases of adjuvant arthritis: Interleukin-17 expression is upregulated in early disease

The aim of this study was to understand the immune processes controlling the initiation and spontaneous resolution of adjuvant arthritis (AA). We investigated synovial T‐cell recruitment and mRNA expression of IL‐17 and other important disease related cytokines, IFN‐γ, IL‐2, IL‐4, TNF and TGF‐β in inguinal lymph node (ILN) and synovial membrane (SM). Arthritis severity was assessed by a numerical rating score and rats were sacrificed every 3–4 days postadjuvant induction. Further assessment involved quantitative radiology and histology of the ankle joints on each day, and the ILN and SM were removed for RNA extraction. Cytokine mRNA expression was measured using RT‐PCR and densitometry. Paraffin sections of rat ankle joints were stained for T‐cells (CD3) by immunohistochemistry. In the ILN, there was an increase in IL‐17, TNF and IFN‐γ expression in the early stages of disease, with a secondary sustained increase in IFN‐γ expression. In the SM, there was expression of T‐cell cytokines in early arthritis (day 13), and prolonged TNF and TGF‐β expression, which reflected disease progression. IL‐4 mRNA expression increased in the later stages of AA. Synovial T‐cell numbers transiently increased at day 6, and remained high from days 13–28. Increased pro‐inflammatory cytokine expression, including IL‐17, in the ILN reflects the initiating events in the early stage of disease. IL‐17 may therefore play an important role in the pathogenesis of AA. The increase in IL‐4 (an anti‐inflammatory cytokine) in the SM in the later stages of AA suggests that IL‐4 is involved in the spontaneous resolution of AA. The initial increase in IFN‐γ in the ILN may reflect a pro‐inflammatory response, while the prolonged secondary increase may indicate activation of regulatory T‐cells.

The site of anti-arthritic action of the K-opioid, U-50,488H, in adjuvant arthritis: importance of local administration

1 . Currently available pharmacological therapies treat arthritis inadequately. We have previously found that the kappa (K)‐opioid, U‐50,488H (trans‐(±)‐3,4‐dichloro‐N‐methyl‐N‐[2‐(1‐pyrrolidinyl) cyclohexyl]‐benzene‐acetamide methane sulphonate), possesses anti‐arthritic effects. In light of the finding that opioid receptors in the periphery are upregulated during inflammation, K‐opioids may represent a novel therapy for arthritis. The primary aim and unique feature of the present study is to investigate whether opioids exert their anti‐arthritic effects in the periphery. Thus, the dose‐effect relationship of a K‐opioid agonist, U‐50,488H was compared after both local and distant administration. Further, we tested whether the anti‐arthritic effects of this drug are stereospecific and receptor‐mediated by use of opioid antagonists. 2 . Using an adjuvant model of arthritis in male Lewis rats, arthritis was judged by oedema, radiography and histological changes in the contralateral ankle of the hind limb. Treatment with (±)‐U‐50,488H for 3 days during disease onset and 3 days during established disease significantly attenuated arthritis, but the effects of (±)‐U‐50,488H on radiology and histology varied according to treatment time. Administration of (±)‐U‐50,488H during disease onset had a more marked effect on radiography, suggesting that treatment with that drug should be started early to prevent progressive joint destruction. Further, it was found that (±)‐U‐50,488H, administered for 3 days during the disease onset, either by direct subcutaneous injection into the inflamed paw or at a more distant site into the back of the neck, dose‐dependently attenuated arthritic damage as measured by an index which pooled all three variables. More importantly however, (±)‐U‐50,488H was approximately fourfold more potent as an ‘anti‐arthritic’ agent after local compared to distant subcutaneous injection (ED50; local vs distant: 5.8 ± 1.6 vs 19.5 ± 0.8 mg kg−1). 3 . Equivalent doses of the (−)−enantiomer (20 mg kg−1 day−1) and the racemate (±) of U‐50,488H (40 mg kg−1 day−1), elicited a similar attenuation of arthritic parameters while the (+)‐enantiomer exacerbated arthritis, suggesting that the anti‐arthritic activity lies solely with the (−)−enantiomer. 4 . Both the peripherally selective antagonist, naloxone methiodide, and the K‐selective antagonist, MR2266 ((−)−5, 9α‐diethyl‐2‐(3‐furylmethyl)‐2′‐hydroxy‐6,7‐benzomorphan), were able to reverse fully the peripheral anti‐arthritic effects of U‐50,488H, indicating that it exerts its effects through peripheral K‐opioid receptors. 5 . Taken together, these results not only confirm our previous findings that demonstrate anti‐arthritic effects of U‐50,488H but they indicate that the opioid attenuation of experimental arthritis is mediated via peripheral K‐receptors in the arthritic joint. Peripherally acting K‐opioid agonists should lead to new therapies for arthritis.

EFFECT OF MU -OPIOIDS MORPHINE AND BUPRENORPHINE ON THE DEVELOPMENT OF ADJUVANT ARTHRITIS IN RATS

Objective and Design: On the basis that endogenous opioids play a role in the physiological response to inflammation, this study tests the antiarthritic effects of a μ-opioid agonist, morphine and the partial μ-agonist, buprenorphine.Material: Male Lewis rats were used.Treatment: Rats were innoculated subcutaneously with 0.05 ml of Freunds complete adjuvant (5 mg/ml) into the right hind paw to produce adjuvant arthritis. Morphine (either 10 to 60 mg/kg/day s.c. bolus or 60 mg/kg/day s.c. infusion) and buprenorphine (0.65±0.06 mg/kg/day, orally), respectively, were administered for 3 days during the primary inflammatory phase of adjuvant arthritis.Methods: The progression of adjuvant arthritis was monitored every three days by body weight change and hind limb oedema (ipsilateral and contralateral). On day 21 the animals were sacrificed and histology and radiography of the contralateral limb were performed. In rats receiving Freunds adjuvant and no drug treatment, the incidence of arthritis was 89%. Effect was expressed as the pooled severity index (PSI) derived from the arithmetic average of the volume, histology and radiography scores in the contralateral hind limb.Results: Buprenorphine had no effect on experimental arthritis (PSI control vs treated: 242±28 vs 253±28%). In contrast, morphine by subcutaneous injection twice daily (10 to 60 mg/kg/day) but not by subcutaneous infusion (60 mg/kg/day) was found to attenuate the progression of adjuvant arthritis in a dose-dependent manner. This indicates that the anti-arthritic effects of morphine are opioid receptor mediated (ED50, 58±9 mg/kg) and suggests that the local concentration reached effective levels only after subcutaneous injection. It is also possible that the high doses of morphine were anti-inflammatory through effects at the kappa receptor. However, these high doses of morphine produced death in one third of the rats, the calculated lethal dose (LD50, 63±2 mg/kg) being close to the effective dose.Conclusion: Anti-arthritic effects of morphine are opioid receptor mediated but morphine use for this indication is restricted by its adverse effects.

Involvement of substance P in the anti-inflammatory effects of the peripherally selective kappa-opioid asimadoline and the NK1 antagonist GR205171.

We have previously shown that kappa‐opioids have antiarthritic properties. In this study, using two differently acting drugs (the peripherally selective κ‐agonist, asimadoline, and the NK1‐antagonist, GR205171), we have examined possible roles of the neuropeptide substance P (SP) in the pathogenesis and maintenance of experimental arthritis in rats. The anti‐inflammatory actions and the time dependence of these drugs were compared, and concentrations of SP determined in joint tissue. In untreated animals, SP levels in ankle joint tissue increased late in the disease (by day 21) but substantially lagged behind development of clinical disease. Prolonged (days 1–21 or days 12–18) but not early, short‐term (days 1–3) treatment with the NK1‐antagonist GR205171 (1 mg/kg/day i.p.) significantly attenuated joint damage; SP levels showed multiphasic dose dependence over the 21‐day treatment. The data suggest that GR205171 antagonizes the action of SP by presynaptic as well as postsynaptic mechanisms. Treatment with asimadoline (5 mg/kg/day i.p.) produced marked (and sustained) attenuation of the disease with all three time regimes. The effect of asimadoline on SP levels was time dependent: reduction of SP content after 3 days but an increase after 12 or 21 days treatment, paradoxically with clinical improvement in each case. Drug‐induced changes in SP content could follow from changed release or synthesis from either neural or immune cells. The results suggest that both drugs have potential therapeutic value at different stages of inflammatory joint disease.

Lack of cross-tolerance between the antinociceptive effects of systemic morphine and asimadoline, a peripherally-selective κ-opioid agonist, in CCI-neuropathic rats

The development of tolerance following repeated doses of morphine hinders the treatment of clinical pain. We have previously shown that morphine tolerance develops in neuropathic rats without cross-tolerance to a systemic kappa-opioid agonist; in the current work, using paw-pressure vocalization thresholds, we studied the antinociceptive effect of the peripherally-selective kappa (kappa)-opioid agonist, asimadoline, in both morphine-tolerant and opioid-naïve rats 2 weeks after sciatic nerve injury. In naïve rats, intraplantar (i.pl.) injection of asimadoline into the nerve-injured paw, at doses of 10, 15 and 20 (but not 30) microg, dose-dependently relieved the mechanical allodynia-like behaviour. The kappa-opioid antagonist, norbinaltorphimine, (30 microg, i.pl.) reversed this action; injection of asimadoline (15 microg) into the contralateral paw (i.pl.) or i.v., however, had no effect. These results confirm that at low doses, asimadoline exerts its action only in the periphery. In morphine-tolerant rats (after 10 mg/kg s.c. , twice daily for 4 days) and naïve, saline-pretreated rats, asimadoline (15 microg, i.pl.) relieved the mechanical allodynia-like behaviour to the same extent, indicating no cross-tolerance between morphine and the peripherally-selective drug. Our findings show promise for the treatment of neuropathic pain with low doses of peripherally-selective kappa-opioids.

Facilitation of enkephalins catabolism inhibitor-induced antinociception by drugs classically used in pain management

The aim of this study was to investigate the facilitatory effects of subanalgesic or low doses of different drugs (acetylsalicylic acid, ibuprofen and morphine) on the antinociceptive responses induced by the endogenous opioid peptides, enkephalins, protected from their catabolism by the dual enkephalin-degrading enzymes inhibitor RB101. According to the analgesic profile of the three studied compounds different antinociceptive assays were used: the hot plate and formalin tests in mice, and the tail flick and paw pressure tests on inflamed paws in rats and polyarthritic rats. Facilitatory effects of subanalgesic doses of acetylsalicylic acid and ibuprofen on RB101-induced antinociceptive responses were observed in the early and late phases of the formalin test, respectively. In the hot plate, tail flick and paw pressure tests, the dose-dependent analgesic effects of RB101 were strongly potentiated by subanalgesic doses of morphine (0.5 mg/kg), while in these tests, acetylsalicylic acid and ibuprofen were unable to modify the RB101-induced antinociceptive responses. The synergism in antinociceptive effects observed with the combination of RB101 and morphine supported by isobolographic analysis, may have interesting clinical implications, considering both the lack of opiate drawbacks observed with RB101 and the high potentiation of its antinociceptive effects with very low doses of morphine.