Background and purpose: Recent findings suggest that the noxious gas H2S is produced endogenously, and that physiological concentrations of H2S are able to modulate pain and inflammation in rodents. This study was undertaken to evaluate the ability of endogenous and exogenous H2S to modulate carrageenan-induced synovitis in the rat knee. Experimental approach: Synovitis was induced in Wistar rats by intra-articular injection of carrageenan into the knee joint. Sixty minutes prior to carrageenan injection, the rats were pretreated with indomethacin, an inhibitor of H2S formation (DL-propargylglycine) or an H2S donor [Lawesson's reagent (LR)]. Key results: Injection of carrageenan evoked knee inflammation, pain as characterized by impaired gait, secondary tactile allodynia of the ipsilateral hindpaw, joint swelling, histological changes, inflammatory cell infiltration, increased synovial myeloperoxidase, protein nitrotyrosine residues, inducible NOS (iNOS) activity and NO production. Pretreatment with LR or indomethacin significantly attenuated the pain responses, and all the inflammatory and biochemical changes, except for the increased iNOS activity, NO production and 3-NT. Propargylglycine pretreatment potentiated synovial iNOS activity (and NO production), and enhanced macrophage infiltration, but had no effect on other inflammatory parameters. Conclusions and implications:Whereas exogenous H2S delivered to the knee joint can produce a significant antiinflammatory and anti-nociceptive effect, locally produced H2S exerts little immunomodulatory effect. These data further support the development and use of H2S donors as potential alternatives (or complementary therapies) to the available anti-inflammatory compounds used for treatment of joint inflammation or relief of its symptoms.

BackgroundNon-steroidal antiinflammatory drugs (NSAIDs) are the most commonly prescribed agents for arthritic patients, although gastric effects limit their long-term use. Considering the reported gastric safety of hydrogen sulfide (H2S)-releasing NSAIDs, in addition to the anti-inflammatory effects of H2S administration to rats with synovitis, we decided to evaluate the effects of the H2S-releasing naproxen derivative ATB-346 in this animal model.MethodsMale Wistar rats were anesthetized with inhalatory halothane and pre-treated with equimolar oral doses of either naproxen (0.3, 1, 3 or 10 mg/kg) or ATB-346 (0.48, 1.6, 4.8, or 16 mg/kg) 30 min before the i.art. injection of 7.5 mg of carrageenan (CGN) into the right knee joint cavity. Joint swelling and pain score were assessed after 1, 3 and 5 h, and tactile allodynia after 2 and 4 h. After the last measurement, the joint cavity lavages were performed for counting of the recruited leukocytes. The drugs (at the highest doses) were also tested for their gastric effects by evaluating macroscopical damage score and neutrophil recruitment (measured as myeloperoxidase – MPO activity) in the stomachs 5 h after administration of the drugs. In addition, the serum naproxen pharmacokinetic profiles of both compounds, administered at the highest equimolar doses, were obtained during the first 6 h after dosing.ResultsAt the two highest tested doses, both naproxen and ATB-346 reduced edema and pain score (measured 3 and 5 h after CGN; P < 0.001). Tactile allodynia was similarly inhibited by ~45% 4 h after CGN by both naproxen (at 1, 3 and 10 mg/kg) and ATB-346 (at 1.6 and 4.8 mg/kg; P < 0.001), as well as leukocyte infiltration. Naproxen (but not ATB-346) induced significant gastric damage and, despite the increased gastric MPO activity by ~130% in the naproxen-, but not in the ATB-346-treated rats, this effect was of no statistical significance.ConclusionThe presence of a H2S-releasing moiety in the ATB-346 structure does not impair the antiinflammatory activity of the parent compound in rats with CGN-induced synovitis. In addition, released H2S may account for the absence of deleterious gastric effects, thus making of ATB-346 a potentially useful therapeutic alternative to traditional naproxen for treatment of patients with arthritis.

Background and purpose:  The effects of centrally administered cannabinoids on body core temperature (Tc) and the contribution of endogenous cannabinoids to thermoregulation and fever induced by lipopolysaccharide (LPS) (Sigma Chem. Co., St. Louis, MO, USA) were investigated. Experimental approach:  Drug‐induced changes in Tc of male Wistar rats were recorded over 6 h using a thermistor probe (Yellow Springs Instruments 402, Dayton, OH, USA) inserted into the rectum. Key results:  Injection of anandamide [(arachidonoylethanolamide (AEA); Tocris, Ellisville, MO, USA], 0.01–1 µg i.c.v. or 0.1–100 ng intra‐hypothalamic (i.h.), induced graded increases in Tc (peaks 1.5 and 1.6°C at 4 h after 1 µg i.c.v. or 10 ng i.h.). The effect of AEA (1 µg, i.c.v.) was preceded by decreases in tail skin temperature and heat loss index (values at 1.5 h: vehicle 0.62, AEA 0.48). Bell‐shaped curves were obtained for the increase in Tc induced by the fatty acid amide hydrolase inhibitor [3‐(3‐carbamoylphenyl)phenyl] N‐cyclohexylcarbamate (Cayman Chemical Co., Ann Arbor, MI, USA) (0.001–1 ng i.c.v.; peak 1.9°C at 5 h after 0.1 ng) and arachidonyl‐2‐chloroethylamide (ACEA; Tocris) (selective CB1 agonist; 0.001–1 µg i.c.v.; peak 1.4°C 5 h after 0.01 µg), but (R,S)‐(+)‐(2‐Iodo‐5‐nitrobenzoyl)‐[1‐(1‐methyl‐piperidin‐2‐ylmethyl)‐1H‐indole‐3‐yl] methanone (Tocris) (selective CB2 agonist) had no effect on Tc. AEA‐induced fever was unaffected by i.c.v. pretreatment with 6‐Iodo‐2‐methyl‐1‐[2‐(4‐morpholinyl)ethyl]‐1H‐indole‐3‐yl](4‐methoxyphenyl) methanone (Tocris) (selective CB2 antagonist), but reduced by i.c.v. pretreatment with N‐(piperidin‐1‐yl)‐5‐(4‐iodophenyl)‐1‐(2,4‐dichlorophenyl)‐4‐methyl‐1H‐pyrazole‐3‐carboxamide (AM251; Tocris) (selective CB1 antagonist). AM251 also reduced the fever induced by ACEA or LPS. Conclusions and implications:  The endogenous cannabinoid AEA induces an integrated febrile response through activation of CB1 receptors. Endocannabinoids participate in the development of the febrile response to LPS constituting a target for antipyretic therapy.

We investigated the potential of secretory phospholipase A(2) (sPLA(2))-induced pancreatitis to promote abdominal hyperalgesia, as well as to depolarize sensory fibres in vitro using a grease-gap technique. Pancreatitis was induced by the injection of sPLA(2) from Crotalus durissus terrificus (sPLA(2)Cdt, 300μgkg(-1)) venom into the common bile duct of rats. Pancreatic inflammatory signs, serum amylase levels and abdominal hyperalgesia were evaluated in rats treated or not with SR140333, a tachykinin NK(1) receptor antagonist. Injection of sPLA(2)Cdt caused pancreatic oedema formation and increased pancreatic neutrophil infiltration and serum amylase at 4h, which returned to normality by 24h, except for the neutrophil infiltration, which was still increased at this time point. Animals injected with sPLA(2) exhibited a lower withdrawal threshold to electronic von Frey stimulation in the upper abdominal region at 4h, but not 24h, post-injection when compared with saline-injected rats. Pre-treatment of animals with SR140333 significantly reduced the sPLA(2)Cdt-induced abdominal hyperalgesia, without affecting the other parameters. Neither sPLA(2)Cdt nor sPLA(2) from Naja mocambique mocambique venom depolarized capsaicin-sensitive sensory fibres from rat vagus nerve, but they decreased the propagated compound action potentials in both A and C fibres. These data show for the first time that NK(1) receptors play an important role in the early abdominal hyperalgesia in a rat model of sPLA(2)-induced pancreatitis, suggesting that these receptors are of importance in the development of pain in the pancreatitis condition. We also provide evidence that sPLA(2)s do not directly depolarize sensory fibres in vitro.

Cannabinoid system is a potential target for pain control. Cannabinoid receptor 1 (CB) activation play a role in the analgesic effect of cannabinoids once it is expressed in primary afferent neurons. This study investigates whether the anti-hyperalgesic effect of CB receptor activation involves P2X receptor in primary afferent neurons. Mechanical hyperalgesia was evaluated by electronic von Frey test. Cannabinoid effect was evaluated using anandamide or ACEA, a non-selective or a selective CB receptor agonists, respectively; AM251, a CB receptor antagonist, and antisense ODN for CB receptor. Calcium imaging assay was performed to evaluated α,β-meATP-responsive cultured DRG neurons pretreated with ACEA. Anandamide or ACEA administered in peripheral tissue reduced the carrageenan-induced mechanical hyperalgesia. The reduction in the carrageenan-induced hyperalgesia induced by ACEA was completely reversed by administration of AM251 as well as by the intrathecal treatment with antisense ODN for CB receptor. Also, ACEA reduced the mechanical hyperalgesia induced by bradykinin and by α,β-meATP, a P2X receptor non-selective agonist, but not by tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and chemokine-induced chemoattractant-1 (CINC-1). Finally, CB receptors are co-localized with P2X receptors in DRG small-diameter neurons and the treatment with ACEA reduced the number of α,β-meATP-responsive cultured DRG neurons. Our data suggest that the analgesic effect of CB receptor activation is mediated by a negative modulation of the P2X receptor in the primary afferent neurons.