Peripheral antinociception induced by ketamine is mediated by the endogenous opioid system.

Ketamine is a drug largely used in clinical practice as an anesthetic and it can also be used as an analgesic to manage chronic pain symptoms. Despite its interactions with several other signaling systems such as cholinergic, serotoninergic and adrenergic, it is accepted that NMDA receptor antagonism is the main mechanism of action of this drug. In this study we investigated the actions of endogenous opioids in the mechanism of peripheral analgesia induced by ketamine. The nociceptive threshold for mechanical stimuli was measured in Swiss mice using the Randall and Selitto test. The drugs used in this study were administered via intraplantar injection. Our results demonstrated that non selective opioid receptor antagonism (naloxone), selective µ- and δ-opioid receptors antagonism (clocinamox and naltrindole, respectively) but not κ-opioid receptor antagonism (nor-binaltorphimine NORBNI) antagonized ketamine-induced peripheral antinociception in a dose-dependent manner. In addition, administration of aminopeptidase inhibitor bestatin significantly potentiated ketamine-induced peripheral antinociception. Ketamine injection in the right hind paw induced ß-endorphine synthesis in the epithelial tissue of the hindpaw. Together these results indicate a role for µ- and δ-opioid receptors and for the endogenous opioid ß-endorphine increased synthesis in ketamine-induced peripheral analgesia mechanism of action.

Epidemiological investigation and molecular typing of dermatophytosis caused by Microsporum canis in dogs and cats.

The objective of the present study was investigate the prevalence of dermatophytes in dogs, cats and environment floor through molecular epidemiology tools to identify the genetic profile of these infectious agents. This was an observational study with cross-sectional surveys design. Sample were collected from the hair and skin of 52 dogs and cats with the clinical suspicion of dermatophytosis, over a period of one year in Maringá, in the state of Paraná, Brazil. Household samples (carpets and floor), were collected from animals that were positive for dermatophytosis by morphological colonies characteristics, and samples of dogs or cats living in the same household as with the positive animals were also collected. After mycological confirmation, molecular typing was performed by random amplified polymorphic DNA (RAPD). Microsporum canis was the unic dermatophyto isolated whose prevalence was 26.9% (14/52) in animals with the clinical suspicion of dermatophytosis and four other animals that lived with positive animals. As some animals had more than one lesion site, there were 22 total positive cultures from samples from animals and another ten from abiotic sources. The majority of the animals that provided positive cultures for M. canis were aged up to five months (77.8%) and were female (66.7%). Molecular typing using the P1 primer revealed genetically distinct profiles in the symptomatic, asymptomatic and environmental animal samples, or the same animal, furthermore, showed that M. canis could have microevolution.

Dual effects of Rho-kinase inhibitors on a rat model of inflammatory pain.

BACKGROUND: Rho-kinases (ROCKs), a family of small GTP-dependent enzymes, are involved in a range of pain models, and their inhibition typically leads to antinociceptive effects. OBJECTIVES: To study the effects of inhibiting ROCKs using two known inhibitors, Y27632 and HA1077 (fasudil), administered locally, on nociception and paw edema in rats. METHODS: A range of doses of Y27632 or HA1077 (2.5 µg to 1000 µg) were injected locally into rat paws alone or in combination with carrageenan, a known proinflammatory stimulus. Nociceptive responses to mechanical stimuli and increased paw volume, reflecting edema formation, were measured at 2 h and 3 h, using a Randall-Selitto apparatus and a hydroplethysmometer, respectively. RESULTS: Animals treated with either ROCK inhibitor showed biphasic nociceptive effects, with lower doses being associated with pronociceptive, and higher doses with antinociceptive responses. In contrast, a monophasic dose-dependent increase in edema was observed in the same animals. Local injection of 8-bromo-cyclic (c)GMP, an activator of the nitric oxide/cGMP/protein kinase G pathway, also produced biphasic effects on nociceptive responses in rat paws; however, low doses were antinociceptive and high doses were pronociceptive. Local administration of cytochalasin B, an inhibitor of actin polymerization and a downstream mediator of ROCK activity, reversed the antinociceptive effect of Y27632. CONCLUSIONS: The results of the present study suggest that ROCKs participate in the local mechanisms associated with nociception/antinociception and inflammation, with a possible involvement of the nitric oxide/cGMP/protein kinase G pathway. Also, drug effects following local administration may differ markedly from the effects following systemic administration. Finally, separate treatment of pain and edema may be needed to maximize clinical benefit in inflammatory pain.

Crucial involvement of actin filaments in celecoxib and morphine analgesia in a model of inflammatory pain.

BACKGROUND: Celecoxib exerted analgesic effects (hypoalgesia) reversed by opioid receptor antagonists in a model of inflammatory pain. To analyze this celecoxib-induced hypoalgesia further, we assessed the effects of several disruptors of cytoskeletal components in our model of inflammation. METHODS: Hyperalgesia to mechanical stimuli was induced in rat hind paws by intraplantar injection of carrageenan and measured using the Randall-Selitto method over the next 8 hours. The effects of systemic pretreatment with celecoxib and a range of cytoskeletal disruptors (colchicine, nocodazole, cytochalasin B, latrunculin B, acrylamide, each given by intraplantar injection) on carrageenan-induced hyperalgesia were similarly investigated. Morphine and other selective cyclo-oxygenase 1 (SC-560), cyclo-oxygenase 2 (SC-236), and nonselective cyclo-oxygenase (indomethacin) inhibitors were also tested under similar conditions. RESULTS: None of the cytoskeletal disruptors affected the peak intensity of carrageenan-induced hyperalgesia, and its duration was increased only by nocodazole and colchicine. Pretreatment with celecoxib 30 minutes before carrageenan reversed the hyperalgesia and raised the nociceptive threshold (hypoalgesia). All analgesic effects of celecoxib were blocked by nocodazole, colchicine, cytochalasin B, and latrunculin B. Pretreatment with morphine also induced hypoalgesia in carrageenan-inflamed paws, an effect reversed by colchicine and cytochalasin B. However, the analgesic effects of indomethacin were not reversed by disruption of actin filaments with cytochalasin B or latrunculin B. CONCLUSION: These data strengthen the correlation between cytoskeletal structures and the processes of pain and analgesia.

Celecoxib induces tolerance in a model of peripheral inflammatory pain in rats.

Celecoxib is a non-steroidal anti-inflammatory drug (NSAID) that selectively inhibits cyclooxygenase-2 (COX-2). Like most NSAIDs, celecoxib exhibits analgesic effects in models of inflammatory pain but these appear to be dependent on endogenous opioid release. Therefore, this study has assessed the ability of celecoxib to induce tolerance in rats, comparable to that induced by morphine. Rats were injected subcutaneously (s.c.) twice daily with divided doses of celecoxib, morphine or indomethacin. Inflammation was induced in one hind paw of rats by injecting prostaglandin E(2) (PGE(2); 200 ng) 30 min after drug administration, on days 1, 3, 5 and 6 or 7. Nociceptive thresholds to mechanical stimulation were measured 3 h after PGE(2) injection, on the same days. On days 6 or 7, analgesic effects of the full doses of test drugs were assessed. Celecoxib-induced tolerance, as did morphine, an effect not shown by another NSAID, indomethacin. Cross-tolerance between celecoxib and morphine was observed as they did not induce analgesia when animals were chronically treated with morphine or celecoxib, respectively. In addition, tolerance to celecoxib's analgesic effects persisted for at least two days after the end of the chronic treatment with celecoxib. Naltrexone prevented induction of tolerance to morphine or celecoxib. The present results strengthen the possibility that celecoxib has also mechanisms of analgesia unrelated to COX inhibition but dependent on endogenous opioid release. Our results also imply the existence of a new class of analgesics without the deleterious effects of COX inhibitors.

Is the sulphonamide radical in the celecoxib molecule essential for its analgesic activity?

In a model of peripherally induced inflammatory pain in rats, selective inhibitors of cyclooxygenase (COX)-2 raised nociceptive thresholds above basal values, an effect referred to as "hypoalgesia". However other, non-selective, inhibitors of COX (indomethacin, piroxicam) or a selective inhibitor of COX-1 did not induce hypoalgesia in this model, implying that COX inhibition was not causally related to the hypoalgesic effect. Here, we have assessed whether other COX-2 inhibitors or other sulphonamides, apart from celecoxib could exhibit hypoalgesia in our model of inflammatory pain. Inflammation was induced in one hind paw of rats by intraplantar injection of carrageenan (250 µg). Nociceptive thresholds to mechanical stimulation were measured in the inflamed and contralateral paws for 6 h after carrageenan. Three sulphonamides, celecoxib itself, furosemide (a loop diuretic), acetazolamide (a carbonic anhydrase inhibitor), or a selective COX-2 inhibitor lacking the sulphonamide group, lumiracoxib, were injected s.c., 30 min before the pro-inflammatory stimulus. Naltrexone, a non-selective opioid antagonist was also administered s.c., 30 min before test drugs. Furosemide and acetazolamide dose-dependently induced hypoalgesia in the inflamed paw, as did celecoxib. However, lumiracoxib only produced anti-hyperalgesia. Pre-treatment with naltrexone completely prevented the hypoalgesia induced by the sulphonamides, but only partially prevented the anti-hyperalgesic effect of lumiracoxib. Taken together, our results suggest that the sulphonamide group in the structure of celecoxib is more critical for the development of hypoalgesia in our model than its ability to inhibit COX-2. Further, other sulphonamides lacking significant COX inhibition were also able to exhibit hypoalgesic effects, mediated by the endogenous opioid system.

Peripheral mu-, kappa- and delta-opioid receptors mediate the hypoalgesic effect of celecoxib in a rat model of thermal hyperalgesia.

AIMS: The endogenous opioids mediate the analgesic effects of celecoxib in a model of mechanical hyperalgesia in rats. As responses to thermal stimuli may differ from those to mechanical stimuli, we have here assessed celecoxib in a rat model of thermal hyperalgesia and the possible involvement of endogenous opioids and their corresponding receptors in these effects. MAIN METHODS: Injection of carrageenan (CG) into one hind paw induced a dose-related hyperalgesia (decreased time for paw withdrawal) to thermal stimuli (infra-red light beam), over 6h. KEY FINDINGS: Celecoxib (sc) 30 min before CG (250 microg per paw) induced a dose-dependent reversal of hyperalgesia, with withdrawal times well above basal levels, characterizing development of hypoalgesia. Indomethacin (sc) reversed CG-induced hyperalgesia only to basal levels (an anti-hyperalgesic effect). Naltrexone (sc) prevented hypoalgesia after celecoxib but did not change the response to indomethacin. Local (intraplantar) injection of either a selective antagonist of mu-(beta-funaltrexamine), kappa-(nor-binaltorphimine) or of delta-(naltrindole) opioid receptors also reversed the hypoalgesic effects of celecoxib, without modifying the hyperalgesia due to CG or affecting the nociceptive thresholds in the non-injected paw. SIGNIFICANCE: Our data show that celecoxib, unlike indomethacin, was hypoalgesic in this model of thermal hyperalgesia, and that this effect was mediated by peripheral mu-, kappa- and delta-opioid receptors.