Janetti N. Francischi

Selective inhibitors of cyclo-oxygenase-2 (COX-2) induce hypoalgesia in a rat paw model of inflammation

It is well‐established that inhibitors of cyclo‐oxygenase (COX) and hence of prostaglandin (PG) biosynthesis reverse inflammatory hyperalgesia and oedema in both human and animal models of inflammatory pain. Paw oedema and hyperalgesia in rats were induced by injecting carrageenan (250 μg paw−1) into a hindpaw. Both inflammatory responses were followed for 24 h after the injection, measuring hyperalgesia by decreased pain threshold in the paws and oedema by plethysmography. Three selective inhibitors of cyclo‐oxygenase‐2 (COX‐2), celecoxib, rofecoxib and SC 236, given systemically in a range of doses, before the inflammatory stimulus, abolished carrageenan‐induced hyperalgesia with little reduction of oedema. These inhibitors also induced hypoalgesia, increasing nociceptive thresholds in the inflamed paw above normal, non‐inflamed levels. This hypoalgesia was lost at the higher doses of the selective inhibitors, although hyperalgesia was still prevented. In paws injected with saline only, celecoxib, given at the dose inducing the maximum hypoalgesia after carrageenan, did not alter the nociceptive thresholds. Two non‐selective inhibitors of COX‐2, indomethacin and piroxicam, abolished hyperalgesia and reduced oedema but did not induce hypoalgesia. Celecoxib given locally into the paw also abolished inflammatory hyperalgesia and induced hypoalgesia without reducing oedema. We conclude that hypoalgesia is expressed only over a critical range of COX‐2 inhibition and that concomitant inhibition of COX‐1 prevents expression of hypoalgesia, although hyperalgesia is still prevented. Our results suggest a novel anti‐nociceptive pathway mediating hypoalgesia, involving COX‐2 selectively and having a clear peripheral component. This peripheral component can be further explored for therapeutic purposes.

Anti-inflammatory and analgesic effects of the phosphodiesterase 4 inhibitor rolipram in a rat model of arthritis.

There has been much interest in strategies which modulate tumour necrosis factor-alpha (TNF-alpha) levels and/or function in rheumatoid arthritis. The elevation of intracellular levels of cyclic AMP in leukocytes by phosphodiesterase 4 inhibitors is accompanied by significant inhibition of the production of TNF-alpha. Nevertheless, these drugs may enhance the hyperalgesia induced by a range of inflammatory mediators, including TNF-alpha. In the present study, we examined the effects of the phosphodiesterase 4 inhibitor rolipram on the local inflammatory infiltrate and hyperalgesia in a rat model of adjuvant-induced arthritis. Rolipram (3 mg/kg) was administered by oral gavage from day 10 to 14 after disease induction. Pretreatment with rolipram abrogated oedema formation and significantly inhibited hyperalgesia. Histopathological analysis revealed a marked inhibition of cellular influx as well as bone and cartilage destruction. Serum and local TNF-alpha levels were suppressed in treated animals whereas there were little changes in interleukin-1beta levels. Although cyclic AMP elevating agents may affect nociceptor threshold to increase the hyperalgesic responses acutely, they also possess significant anti-inflammatory activity, which may hinder local mediator release and/or action. The anti-inflammatory effects of rolipram predominate during this chronic arthritis model in the rat.

Different mechanisms underlie the analgesic actions of paracetamol and dipyrone in a rat model of inflammatory pain

The analgesics, paracetamol and dipyrone are weak inhibitors of the cyclooxygenase isoforms 1 or 2 (COX‐1, COX‐2) but more potent on COX‐3. Both are also weak anti‐inflammatory agents, relative to their analgesic and antipyretic activities. In a model of inflammatory pain mediated by prostaglandins, both compounds were analgesic. We have analysed this shared effect further in order to elucidate the underlying mechanisms.

Midazolam-induced hyperalgesia in rats: modulation via GABAA receptors at supraspinal level

The effect of benzodiazepines on the nociceptive threshold was studied in rats using the tail-flick and the formalin tests. Systemic injection of midazolam (10 mg/kg, i.p.) induced a significant decrease of the tail-flick latency and produced a long-lasting nociceptive effect in the formalin test, thus characterising a hyperalgesic state. The hyperalgesia induced by midazolam in the tail-flick test was blocked by flumazenil, a specific antagonist for benzodiazepine sites associated with GABA(A) receptors. Picrotoxin, a Cl- channel blocker, inhibited midazolam-induced hyperalgesia in both tests. Midazolam caused hyperalgesia when administered intracerebroventricularly (i.c.v.; 25 microg) but not intrathecally (i.t.; 75 microg). I.c.v. but not i.t. (5 microg) injection of flumazenil suppressed the hyperalgesia induced by midazolam (10 mg/kg, i.p.). Combination of non-hyperalgesic doses of diazepam (10 mg/kg, i.p.) or ethanol (0.48 g/kg, oral) with midazolam (5 mg/kg, i.p.) also induced hyperalgesia. Our results demonstrate that midazolam and diazepam alone or in combination with ethanol can produce hyperalgesia by interacting with GABA(A) receptors at the supraspinal level in rats. The risk of hyperalgesia should be taken in account when these drugs are used in combination in humans.

ANALGESIC AND ANTIINFLAMMATORY EFFECTS OF DIPYRONE IN RAT ADJUVANT ARTHRITIS MODEL

Dipyrone, a pirazolone derivative, is a known analgesic drag with minor toxic effects associated with its administration. The aim of the present study was to determine the analgesic and antiinflammatory effects of dipyrone in a model of chronic inflammation (adjuvant-induced arthritis in rats). Hind-paw hyperalgesia was detected in arthritic rats from the 10th to the 16th day of observation. Edema development was maximum (twofold increase) at the 14th day of observation compared to control animals and reduced at the 16th day of observation. Dipyrone (1–50 mg/kg) dose-dependently reduced both hind-paw hyperalgesia and edema from arthritic rats. However, it was shown to be more potent as analgesic than antiinflammatory in the present model. In contrast, indomethacin (2 mg/kg) and dexamethasone (0.4 mg/kg) completely inhibited hind-paw hyperalgesia and edema development. Our results indicate that dipyrone reduced the hyperalgesia and edema in arthritic rats by a mechanism not involving release of prostaglandin-like substances. The possibility of dipyrone inducing analgesia in arthritic rats through a peripheral action supports the use of dipyrone as an alternative choice drug for the treatment of pain associated with arthritislike diseases in selected cases.

Central antinociception induced by µ-opioid receptor agonist morphine, but not δ- or κ-, is mediated by cannabinoid CB1 receptor

Background and purpose:  It has been demonstrated that cannabinoids evoke the release of endogenous opioids to produce antinociception; however, no information exists regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids are involved in central antinociception induced by activation of µ‐, δ‐ and κ‐opioid receptors.

Differential involvement of cyclooxygenase isoforms in neutrophil migration in vivo and in vitro

Pretreatment using celecoxib, a cyclooxygenase (COX) 2 inhibitor, or indomethacin, a nonselective COX inhibitor, reduced lypopolyssaccharide (LPS)-induced leukocyte migration to the rat peritoneal cavity. The effect of celecoxib (12 mg/kg) or indomethacin (2 mg/kg) on neutrophil chemotaxis induced by formyl-methionyl-leucyl-phenylalanine (FMLP) in an in vitro chemotactic assay (Boyden chamber) was investigated. Celecoxib and indomethacin inhibited chemotaxis induced by FMLP (Control=26.6+/-1.45, Celecoxib=12.8+/-3.04, Indomethacin=6.26+/-2.19 cells/field). When observed under intravital microscopy, a mouse cremaster preparation was used to assess the microvasculature to further investigate which step of cell recruitment was affected by these drugs. Celecoxib and indomethacin inhibited leukocyte migration induced by 0.05 microg/kg LPS injected into the cremaster muscle. However, the effect of celecoxib was associated with reduced cell rolling and adhesion, whereas indomethacin was only effective at inhibiting cell adhesion. Furthermore, SC560 pretreatment (a COX-1 selective inhibitor) of normal or LPS-challenged tissues did not alter leukocyte migration or cell adhesion, but it did enhance leukocyte rolling activity in both cases. Taken together, these results indicate that: 1) COX-1 activity is mainly related to leukocyte traffic under physiological conditions, and 2) COX-2 activity is mainly related to cell traffic under inflammatory conditions in vascular beds, suggesting a possible effect of selective COX-2 inhibitors on the expression of adhesion molecules.

Endogenous opioids mediate the hypoalgesia induced by selective inhibitors of cyclo-oxygenase 2 in rat paws treated with carrageenan.

Mechanical hyperalgesia induced in rat paws by carrageenan (250microg) was modified by pre-treatment with three selective inhibitors of cyclo-oxygenase-2 (COX-2); celecoxib, rofecoxib and SC236. These inhibitors raised the nociceptive threshold above the normal, non-inflamed, level, inducing a state of hypoalgesia. Such hypoalgesia was observed in different strains of rat (Holtzman, Wistar and Sprague-Dawley) and after different modes of administration of the COX-2 inhibitor (locally, in the paw, or systemically). A selective inhibitor of COX-1 (SC 560; 1-10mg kg(-1)) decreased hyperalgesia but did not induce hypoalgesia. Pre-treatment with naltrexone (3mg kg(-1)), an opioid receptor antagonist, did not affect carrageenan-induced hyperalgesia but abolished the hypoalgesic effects of COX-2 inhibitors, without diminishing the anti-hyperalgesic effect of indomethacin. In rats made tolerant to the anti-nociceptive effects of morphine, all anti-nociceptive effects of SC236 were abolished but the anti-hyperalgesic effects of indomethacin or SC 560 were unaffected. We conclude that, in our model of inflammatory hyperalgesia, the anti-nociceptive effect of selective COX-2 inhibitors involved the participation of endogenous opioids.

The analgesic actions of centrally administered celecoxib are mediated by endogenous opioids

ABSTRACT Celecoxib is a selective inhibitor of cyclooxygenase‐2 (COX‐2) and blocks prostaglandin (PG) biosynthesis associated with inflammatory conditions. In a model of peripherally induced inflammatory pain in rats, celecoxib, given systemically, induced a state of hypoalgesia where the nociceptive threshold was raised above basal values, an effect not observed after treatment with non‐selective inhibitors of COX (indomethacin, piroxicam). Here, we have assessed the possibility that these atypical effects of celecoxib could be mediated by action at a site in the CNS. Inflammation and hyperalgesia were induced in one hind paw of rats by intraplantar injection of carrageenan (250 μg). Nociceptive thresholds to mechanical stimulation were measured in the inflammed and contralateral paws for 6 h after carrageenan injection. Celecoxib, SC236 (selective COX‐2 inhibitors), indomethacin (non‐selective COX inhibitor), SC560 (selective COX‐1 inhibitor) or morphine were given by i.c.v. injection, 30 min before carrageenan. Celecoxib, SC236 or morphine‐induced hypoalgesia whereas, after indomethacin or SC 560, the nociceptive threshold only returned to basal values. Naltrexone, also given i.c.v., reversed the hypoalgesia after celecoxib or morphine. Bestatin, an inhibitor of metabolism of endogenous opioid peptides, given i.c.v., potentiated the analgesic effects of a low dose of celecoxib. Taken together, these data indicate that celecoxib could act centrally after systemic administration to produce its characteristic profile of analgesia in this model of peripheral inflammatory pain. Moreover, this atypical analgesia appeared to be mediated by endogenous opioids rather than by inhibition of PG biosynthesis.

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To investigate whether selective COX 2 inhibitors (celecoxib, rofecoxib) would play a role in a model of leukocyte migration in rats. Bacterial endotoxin (Escherichia coli LPS) was intraperitoneally injected at time zero in rats that were previously treated with unspecific and selective cyclooxygenase inhibitors. LPS induced a dose and time-dependent increase in leukocyte number, which was predominantly related to the presence of PMN neutrophils. Only rats treated with selective COX 2 inhibitors and indomethacin showed a significant reduction in leukocyte numbers following LPS administration. Prostaglandins E2 and % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaaca % qGgbWaaSbaaSqaaiaaikdacqaHXoqyaeqaaaaa!3AE0!