Peripheral and spinal mechanisms of antinociceptive action of lumiracoxib

Synergistic interaction between B vitamins and statins to counter nociception in rats

Evidence suggests that the antinociceptive activity of various drugs can be increased when administered in combination with B vitamins (BVs). The aim of this study was to examine the potential interaction between statins and BVs to counter nociception, the latter measured by the formalin test. Rats were orally administered atorvastatin (1, 3, 10 and 30 mg/kg), pravastatin (1, 3, 10 and 30 mg/kg), rosuvastatin (1, 3, 10 and 30 mg/kg), BVs (31, 56, 100 and 180 mg/kg) or calculated combinations of BVs with each drug. The effective dose 30 (ED₃₀ ) was calculated for each statin and BVs and subjected to isobolographic analysis, thus finding the ED₃₀ of the combinations. The antinociceptive experimental ED₃₀ values for BVs administered with atorvastatin, pravastatin or rosuvastatin were 1.53 ± 0.38, 6.74 ± 0.04 and 4.26 ± 0.39, respectively, being lower (p < .05) than the corresponding theoretical ED₃₀ : 28.02 ± 2.20, 28.17 ± 2.20 and 29.86 ± 2.21. Since BVs likely boost the antinociceptive effect of statins, these combinations could possibly be advantageous in pain management.

Antinociceptive, antiedematous, and antiallodynic activity of 1H-pyrrolo[3,4-c]pyridine-1,3(2H)-dione derivatives in experimental models of pain

The aim of the presented study was to examine the potential antinociceptive, antiedematous (anti-inflammatory), and antiallodynic activities of two 1H-pyrrolo[3,4-c]pyridine-1,3(2H)-dione derivatives (DSZ 1 and DSZ 3) in various experimental models of pain. For this purpose, the hot plate test, the capsaicin test, the formalin test, the carrageenan model, and oxaliplatin-induced allodynia tests were performed. In the hot plate test, only DSZ 1 in the highest dose (20 mg/kg) was active but its effects appear to be due to sedatation rather than antinociceptiveness. In capsaicin-induced neurogenic pain model, both compounds displayed a significant antinociceptive activity. In the formalin test, DSZ 1 and DSZ 3 (5-20 mg/kg) revealed antinociceptive activity in both phases but it was more pronounced in the second phase of the test. In this test, pretreatment with caffeine, DPCPX reversed the antinociceptive effect of DSZ 3. On the other hand, pretreatment with L-NAME diminished the antinociceptive effect of DSZ 1. Pretreatment with naloxone did not affect antinociceptive activity of both compounds. Similar to ketoprofen, DSZ 1 and DSZ 3 showed antiedematous (antiinflammatory) and antihyperalgesic activity, and similar to lidocaine local anesthetic activity. Furthermore, both compounds (5 and 10 mg/kg) reduced tactile allodynia in acute and chronic phases of neuropathic pain. In the in vitro studies, DSZ 1 and DSZ 3 reduced the COX-2 level in LPS-activated RAW 264.7 cells, which suggests their anti-inflammatory activity. In conclusion, both DSZ 1 and DSZ 3 displayed broad spectrum of activity in several pain models, including neurogenic, tonic, inflammatory, and chemotherapy-induced peripheral neuropathic pain.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Electrophysiological evidence for voltage-gated calcium channel 2 (Cav2) modulation of mechano- and thermosensitive spinal neuronal responses in a rat model of osteoarthritis

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MIA-dependent antinociceptive effect of TROX-1 on neuronal activity.

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Alterations in Ca_v2.2 channel function contribute to osteoarthritic (OA) pain.

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Blocking Ca_v2.2 channels has therapeutic potential for treating OA pain.

Osteoarthritis (OA) remains one of the greatest healthcare burdens in western society, with chronic debilitating pain-dominating clinical presentation yet therapeutic strategies are inadequate in many patients. Development of better analgesics is contingent on improved understanding of the molecular mechanisms mediating OA pain. Voltage-gated calcium channels 2.2 (Ca_v2.2) play a critical role in spinal nociceptive transmission, therefore blocking Ca_v2.2 activity represents an attractive opportunity for OA pain treatment, but the only available licensed Ca_v2.2 antagonist ziconitide (PrilatTM) is of limited use. TROX-1 is an orally available, use dependent and state-selective Ca_v2 antagonist, exerting its analgesic effect primarily via Ca_v2.2 blockade, with an improved therapeutic window compared with ziconitide. Using a rat model of monosodium iodoacetate (MIA), 2 mg, induced OA we used in vivo electrophysiology to assess the effects of spinal or systemic administration of TROX-1 on the evoked activity of wide dynamic range spinal dorsal horn neurons in response to electrical, natural mechanical (dynamic brush and von Frey 2, 8, 26 and 6 g) and thermal (40, 45 and 45 °C) stimuli applied to the peripheral receptive field.

MIA injection into the knee joint resulted in mechanical hypersensitivity of the ipsilateral hind paw and weight-bearing asymmetry. Spinal administration of TROX-1 (0.1 and 1 μg/50 μl) produced a significant dose-related inhibition of dynamic brush, mechanical (von Frey filament (vF) 8, 26 and 60 g) and noxious thermal-(45 and 48 °C) evoked neuronal responses in MIA rats only. Systemic administration of TROX-1 produced a significant inhibition of the mechanical-(vF 8, 26 and 60 g) evoked neuronal responses in MIA rats. TROX-1 did not produce any significant effect on any neuronal measure in Sham controls. Our in vivo electrophysiological results demonstrate a pathological state-dependent effect of TROX-1, which suggests an increased functional role of Ca_v2, likely Ca_v2.2, channels in mediating OA pain.

Central and peripheral antinociceptive activity of 3-(2-oxopropyl)-3-hydroxy-2-oxindoles

Convolutamydine A has been shown to develop a significant antinociceptive effect. Here we demonstrated that new analogues (5-iodo-3-(2-oxopropyl)-3-hydroxy-2-oxindole (5-Iisa), 5-fluoro-3-(2-oxopropyl)-3-hydroxy-2-oxindole (5-Fisa), 5-chloro-3-(2-oxopropyl)-3-hydroxy-2-oxindole (5-Clisa) and 5-methyl-3-(2-oxopropyl)-3-hydroxy-2-oxindole (5-Meisa)), at 0.1-10mg/kg doses, have significant peripheral and central antinociceptive effects in thermal and chemical models of nociception. Oral administered analogues demonstrated more pronounced antinociceptive effects than that obtained with the classical opioid drug morphine (5mg/kg) in the first and second phases of formalin-induced licking. In the tail flick model, 5-Clisa and 5-Meisa antinociceptive effect was almost twice as that observed with the same dose of morphine. The concomitant administration of diverse antagonists and the analogues indicates that 5-Iisa effects involve the activation of opioid pathway. On the other hand, 5-Fisa and 5-Clisa have the participation of opioid, nitrergic, cholinergic adrenergic and serotoninergic pathways and 5-Meisa has the involvement of opioid, serotoninergic and cholinergic pathways. In conclusion, our results suggest that the new four analogues from Convolutamydine A have significant antinociceptive effects in thermal and chemical induced nociception and could be used in development of new drugs to be used in pain treatment with reduced side effects.

Antinociceptive and antidiarrheal effects of pioglitazone in a rat model of diarrhoea-predominant irritable bowel syndrome: role of nitric oxide

Irritable bowel syndrome (IBS) is a prevalent disease characterized by abdominal pain and abnormal bowel habits. Pioglitazone is a peroxisome proliferator-activated receptor (PPAR) γ agonist and, although it is mostly used as an antidiabetic agent, it has been reported to have analgesic effects. Nitric oxide (NO), a gaseous molecule that mediates many of the effects of pioglitazone, has been implicated in the pathophysiology of IBS. The aim of the present study was to investigate the effects of pioglitazone on symptoms in a rat model of diarrhoea-predominant IBS (D-IBS).and to determine the role of NO in these effects. Diarrhoea-predominant IBS was induced by intracolonic instillation of acetic acid. Pioglitazone (2 mg/kg, i.p.) was administered on Days 7, 9 and 11 after acetic acid instillation. To investigate the mechanism involved in pioglitazone action, rats were also administered either the PPARγ antagonist GW9662 (3 mg/kg, i.p.), the NO synthase (NOS) inhibitor N(G) -nitro-l-arginine methyl ester (l-NAME; 10 mg/kg, i.p.) or the NO precursor l-arginine (250 mg/kg, i.p.) along with pioglitazone. Visceral hypersensitivity, nociceptive thresholds, defecation frequency, stool form, serum and colon NO production and inducible (i) NOS activity were assessed 1 h after the final injection of pioglitazone or dimethylsulphoxide (used as the vehicle). Pioglitazone reduced visceral hypersensitivity and defecation frequency, increased nociceptive thresholds, NO production and iNOS activity and shifted stool form towards hard stools in D-IBS rats. These effects of pioglitazone were significantly reversed by l-NAME, but not GW9662. l-Arginine augmented the effects of pioglitazone. In conclusion, pioglitazone alleviates symptoms in a rat model of D-IBS through an NO-dependent mechanism.

Mechanisms underlying the antinociceptive effect of mangiferin in the formalin test

The purpose of this study was to investigate the possible antinociceptive effect of mangiferin, a glucosylxanthone present in Mangifera indica L., in inflammatory pain. Furthermore, we sought to investigate the possible mechanisms action that contributes to these effects. The ipsilateral local peripheral (1-30 µg/paw), intrathecal (1-30 µg/rat) and oral (1-30 mg/kg) administration of mangiferin produced a dose-dependent reduction in formalin-induced nociception. The antinociceptive effect of this drug was similar to that induced by diclofenac after oral and local peripheral administration. Furthermore, mangiferin was also able to reduce 0.1% capsaicin- and serotonin-induced nociceptive behavior. The local peripheral antinociceptive effect of mangiferin in the formalin test was blocked by naloxone (50 μg/paw), naltrindole (1 μg/paw), 5-guanidinonaltrindole (5-GNTI, 1 μg/paw), N(G)-L-nitro-arginine methyl ester (L-NAME, 100 µg/paw), 1H-(1,2,4)-oxadiazolo [4,2-a]quinoxalin-1-one (ODQ, 50 µg/paw) and glibenclamide (50 μg/paw), but not by methiothepin (30 μg/paw). These results suggest that the antinociceptive effects induced by mangiferin are mediated by the peripheral opioidergic system involving the activation of δ, κ, and probably µ, receptors, but not serotonergic receptors. Data also suggests that mangiferin activates the NO-cyclic GMP-ATP-sensitive K(+) channels pathway in order to produce its local peripheral antinociceptive effect in the formalin test. Mangiferin may prove to be effective in treating inflammatory pain in humans.

Novel bisabolane derivative from "arnica-da-serra" (Vernonieae: Asteraceae) reduces pro-nociceptive cytokines levels in LPS-stimulated rat macrophages

ETHNOPHARMACOLOGY RELEVANCE

Hydro alcoholic leaves extracts (HALE) of Lychnophora ericoides Mart. ("false arnica" or "arnica-da-serra") had been popularly used against pain and inflammatory process.

AIM

The present work aimed to look for possible active volatile compounds that could be found in HALE of Lychnophora ericoides among the non volatile anti-inflammatory and analgesic compounds previously reported.

METHODS

Harvests were performed during the end of the wet summer season (April) when scented branches were instantly collected and frozen. HALE's were simulated at the lab by following the procedures lectured by the locals. Mass Spectrometry experiments suggested structural information when using both EI-MS and ESI-MS/MS. After isolation through classical thin layer chromatography (TLC) procedures, the NMR experiments and signals assignments were carried out. The effects on the cytokines or nitric oxide (NO) production were assessed at in vitro assays that had monitored the levels of these substances on the supernatant of LPS-stimulated macrophage primary cell culture.

RESULTS

The major metabolite from HALE was isolated from the essential oil and the major compound had its molecular formulae established by Mass Spectrometry (High Resolution) and its structure by NMR. Literature-based investigation enables us to define the structure of the new metabolite as 6-methyl-2-(4-methylcyclohex-4-enyl-2-acetyloxy) hept-5-en-2-ol and its name as orto-acetoxy-bisabolol. In vitro assay of interleukins release inhibition was carried out using rat peritoneal macrophages cultures. IL-1β and TNF-α levels were significantly reduced when cells were previously treated with low doses of orto-acetoxy-bisabolol, but neither IL-6 nor NO levels have their levels reduced. Results suggest that ethnical knowledge of anti-inflammatory and analgesic effects of the "arnica-da-serra" HALE may be associated to the orto-acetoxy-bisabolol ability on synthesis inhibition of the key inflammatory/hypernociceptive mediators.

CONCLUSIONS

Phytochemical investigation of the volatile active compounds in Lychnophora ericoides HALE allows us to isolate a new bisabolane derivative (orto-acetoxy-bisabolol) and to infer that this compound inhibits the synthesis of TNF-α and IL-1β, two important inflammatory mediators in the hypernociception. Our present data, in addition to literature's data, furnish scientific support to folk's use of Lychnophora ericoides as an endemic wound healer.

Peripheral antinociception of a chalcone, flavokawin B and possible involvement of the nitric oxide/cyclic guanosine monophosphate/potassium channels pathway

Previous studies have shown that systemic administration of 6'-hydroxy-2',4'-dimethoxychalcone (flavokawin B, FKB) exerts significant peripheral and central antinociceptive effects in laboratory animals. However, the mechanisms underlying these peripheral and central antinociceptive effects have yet to be elucidated. Therefore, the objective of the present study was to evaluate the participation of nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)/potassium (K⁺) channels pathway in the peripheral antinociception induced by FKB. It was demonstrated that intraplantar (i.pl.) administration of FKB (150, 250, 375 and 500 µg/paw) resulted in dose-dependent peripheral antinociception against mechanical hyperalgesia in carrageenan-induced hyperalgesia test model in rats. The possibility of FKB having either a central or a systemic effect was excluded since administration of FKB into the right paw did not elicit antinociception in the contralateral paw. Furthermore, peripheral antinociception induced by FKB (500 µg/paw) was significantly reduced when l-arginine (25 µg/paw, i.pl.), Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 50 µg/paw, i.pl.), glibenclamide (300 µg/paw, i.pl.), tetraethylammonium (300 µg/paw, i.pl.) and charybdotoxin (3 µg/paw, i.pl.) were injected before treatment. Taken together, our present data suggest that FKB elicits peripheral antinociception when assessed in the mechanical hyperalgesia induced by carrageenan. In addition, it was also demonstrated that this effect was mediated through interaction of the NO/cGMP/K⁺ channels signaling pathway.

Evidence for the Participation of ATP-sensitive Potassium Channels in the Antinociceptive Effect of Curcumin

Background

It has been reported that curcumin, the main active compound of Curcuma longa, also known as turmeric, exhibits antinociceptive properties. The aim of this study was to examine the participation of ATP-sensitive potassium channels (K_ATP channels) and, in particular, that of the L-arginine-nitric oxide-cyclic GMP-K_ATP channel pathway, in the antinociceptive effect of curcumin.

Methods

Pain was induced by the intraplantar injection of 1% formalin in the right hind paw of Wistar rats. Formalin-induced flinching behavior was interpreted as an expression of nociception. The antinociceptive effect of oral curcumin was explored in the presence and absence of local pretreatment with L-NAME, an inhibitor of nitric oxide synthase, ODQ, an inhibitor of soluble guanylyl cyclase, and glibenclamide, a blocker of K_ATP channels.

Results

Oral curcumin produced a dose-dependent antinociceptive effect in the 1% formalin test. Curcumin-induced antinociception was not altered by local L-NAME or ODQ, but was significantly impaired by glibenclamide.

Conclusions

Our results confirm that curcumin is an effective antinociceptive agent. Curcumin-induced antinociception appears to involve the participation of K_ATP channels at the peripheral level, as local injection of glibenclamide prevented its effect. Activation of K_ATP channels, however, does not occur by activation of the L-arginine-nitric oxide-cGMP-K_ATP channel pathway.

Role of ATP-sensitive K+ channels in the antinociception induced by non-steroidal anti-inflammatory drugs in streptozotocin-diabetic and non-diabetic rats

There is evidence that systemic sulfonylureas block diclofenac-induced antinociception in normal rat, suggesting that diclofenac activates ATP-sensitive K(+) channels. However, there is no evidence for the systemic interaction between different non-steroidal anti-inflammatory drugs (NSAIDs) and sulfonylureas in streptozotocin (STZ)-diabetic rats. Therefore, this work was undertaken to determine whether two sulfonylureas, glibenclamide and glipizide, have any effect on the systemic antinociception that is induced by diclofenac (30 mg/kg), lumiracoxib (56 mg/kg), meloxicam (30 mg/kg), metamizol (56 mg/kg) and indomethacin (30 mg/kg) using the non-diabetic and STZ-diabetic rat formalin test. Systemic injections of NSAIDs produced dose-dependent antinociception during the second phase of the test in both non-diabetic and STZ-diabetic rats. Systemic pretreatment with glibenclamide (10 mg/kg) and glipizide (10 mg/kg) blocked diclofenac-induced systemic antinociception in the second phase of the test (P<0.05) in both non-diabetic and STZ-diabetic rats. In contrast, pretreatment with glibenclamide or glipizide did not block lumiracoxib-, meloxicam-, metamizol-, and indomethacin-induced systemic antinociception (P>0.05) in both groups. Results showed that systemic NSAIDs are able to produce antinociception in STZ-diabetic rats. Likewise, data suggest that diclofenac, but not other NSAIDs, activated K(+) channels to induce its systemic antinociceptive effect in the non-diabetic and STZ-diabetic rat formalin test.

Insights into the effects of diclofenac and other non-steroidal anti-inflammatory agents on ion channels

OBJECTIVES

Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are widely used in the treatment of inflammation and pain. Most effects of NSAIDs are attributed to the inhibition of cyclooxygenases (COX). However, many NSAIDs may have other effects not related to COX, including the modulation of various ion channels. The clinical implications of the effects on channels are not fully understood. This review outlines the effects of NSAIDs, with special attention to diclofenac, on ion channels and highlights the possible underlying mechanisms.

KEY FINDINGS

NSAIDs have effects on channels such as inhibition, activation or changes in expression patterns. The channels affected include voltage-gated Na(+) , Ca(2+) , or K(+) channels, ligand-gated K(+) channels, transient receptor potential and other cation channels as well as chloride channels in several types of cells. The mechanisms of drug actions not related to COX inhibition may involve drug-channel interactions, interference with the generation of second messengers, changes in channel expression, or synergistic/antagonist interactions with other channel modulators.

SUMMARY

The effects on ion channels may account for novel therapeutic actions of NSAIDs or for adverse effects. Among the NSAIDs, diclofenac may serve as a template for developing new channel modulators and as a tool for investigating the actions of other drugs.

Examination of the interaction between peripheral lumiracoxib and opioids on the 1% formalin test in rats

It has been shown that the association of non-steroidal anti-inflammatory drugs (NSAIDs) with opioid analgesic agents can increase their antinociceptive activity, allowing the use of lower doses and thus limiting side effects. Therefore, the aim of the present study was to examine the possible pharmacological interaction between lumiracoxib and codeine or nalbuphine at the local peripheral level in the rat using the 1% formalin test and isobolographic analysis. Lumiracoxib, codeine, nalbuphine or fixed-dose ratios lumiracoxib-codeine or lumiracoxib-nalbuphine combinations were administrated locally in the formalin-injured paw and the antinociceptive effect was evaluated using the 1% formalin test. All treatments produced a dose-dependent antinociceptive effect. ED(40) values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical ED(40)'s for the lumiracoxib-codeine and lumiracoxib-nalbuphine combinations were 423.4+/-31.3 microg/paw and 310.9+/-24.2 microg/paw, respectively, being significantly higher than the actually observed experimental ED(40) values, 233.2+/-30.9 microg/paw and 132.7+/-11.6 microg/paw, respectively. These results correspond to a synergistic interaction between lumiracoxib and opioids at the local peripheral level, potency being about two times higher with regard to that expected from the addition of the effects of the individual drugs. Data suggest that low doses of the lumiracoxib-opioids combination can interact synergistically at the peripheral level and therefore this drug association may represent a therapeutic advantage for the clinical treatment of inflammatory pain.

Involvement of cholinergic system in suppression of formalin-induced inflammatory pain by cobratoxin

AIM

To investigate the analgesic effect of cobratoxin (CTX), a long-chain α-neurotoxin from Thailand cobra venom, in a rat model of formalin-induced inflammatory pain.

METHODS

Inflammatory pain was induced in SD rats via injecting 5% formalin (50 μL) into the plantar surface of their right hind paw. CTX and other agents were ip administered before formalin injection. The time that the animals spent for licking the injected paw was counted every 5 min for 1 h.

RESULTS

CTX (25, 34, and 45 μg/kg) exhibited a dose-dependent analgesic effect during the phase 1 (0-15 min) and phase 2 (20-60 min) response induced by formalin. Pretreatment with naloxone (0.5 or 2.5 mg/kg) did not block the analgesic effect of CTX. Pretreatment with atropine at 5 mg/kg, but not at 2.5 mg/kg, antagonized the analgesic effect of CTX. Treatment with the nonselective nAChR antagonist mecamylamine (3 mg/kg) inhibited the analgesic effects of CTX in Phase 1 and Phase 2 responses, while with the selective α7-nAChR antagonist methyllycaconitine (3 mg/kg) antagonized the effect of CTX only in the Phase 1 response. Treatment with the α7-nAChR agonist PNU282987 (3 mg/kg) significantly reduced the formalin-induced phase 2 pain response, but only slightly reduced the Phase 1 pain response.

CONCLUSION

The results suggest that CTX exerts an antinociceptive effect in formalin-induced inflammatory pain, which appears to be mediated by mAChR and α7-nAChR.

Spinal 5-HT receptors that contribute to the pain-relieving effects of spinal cord stimulation in a rat model of neuropathy

Spinal cord stimulation (SCS) is extensively employed in the management of neuropathic pain, but the underlying mechanisms are only partially understood. Recently, we demonstrated that the pain-relieving effect of SCS appears to involve the spinal serotonin system, and the present study aimed at identifying the types of the spinal serotonin receptors involved. Experiments were performed on rats with neuropathy produced by partial ligation of the sciatic nerve. Tactile sensitivity was assessed using von Frey filaments, and cold and heat sensitivity with cold spray and radiant heat, respectively. Selective 5-HT receptor antagonists, methiothepin (5-HT(1,6,7)), ketanserin tartrate (5-HT(2A)), TICM (5-HT(3)), SDZ-205,557 (5-HT(4)), as well as receptor agonists, α-m-5-HT (5-HT(2)), m-CPBG (5-HT(3)) in per se ineffective doses, or vehicle, were administrated intrathecally 5 minutes prior to the application of SCS. Ketanserin and SDZ-205,557 significantly attenuated the suppressive effect of SCS on tactile hypersensitivity, while methiothepin and TICM were ineffective. The suppressive effect on cold hypersensitivity of SCS was counteracted by ketanserin only. None of the 5-HT receptor antagonists attenuated the suppressive effect on heat hyperalgesia of SCS. Subeffective doses of α-m-5-HT and m-CPBG enhanced the suppressive effect of SCS on tactile hypersensitivity. The enhancing effect of m-CPBG was abolished by a γ-aminobutyric acid (GABA)(A) or GABA(B) antagonist intrathecally. These results suggest that the activation of 5-HT(2A), 5-HT(3), and 5-HT(4) receptors plays an important role in SCS-induced relief of neuropathic pain. The activation of 5-HT(3) receptors appears to operate via spinal GABAergic interneurons.

Mechanisms of non-opioid analgesics beyond cyclooxygenase enzyme inhibition

Non-opioid analgesics including both selective and non-selective cyclooxygenase (COX) inhibitors and acetaminophen are the most widely used treatments for pain. Inhibition of COX is thought to be largely responsible for both the therapeutic and adverse effects of this class of drugs. Accumulating evidence over the past two decades has demonstrated effects of non-opioids beyond the inhibition of COX and prostaglandin synthesis that might also explain their therapeutic and adverse effects. These include their interaction with endocannabinoids, nitric oxide, monoaminergic, and cholinergic systems. Moreover, the recent development of microarray technology that allows the study of human gene expression suggests multiple pathways that may be related to the analgesic and anti-inflammatory effects of non-opioids. The present review will discuss the multiple actions of non-opioids and their interactions with these systems during inflammation and pain, suggesting that COX inhibition is an incomplete explanation for the actions of non-opioids and proposes the involvement of multiple selective targets for their analgesic, as well as, their adverse effects.

Peripheral and spinal antihyperalgesic activity of najanalgesin isolated from Naja naja atra in a rat experimental model of neuropathic pain

Snake venoms are a rich source of various compounds that have applications in medicine and biochemistry. Recently, it has been demonstrated that najanalgesin isolated from the venom of Naja naja atra exerts analgesic effects on acute pain in mice. The objective of this study was to evaluate the antinociceptive effect of najanalgesin in a rat model of neuropathic pain, induced by L5 spinal nerve ligation and transaction. We observed that intraperitoneal (i.p.) administration of najanalgesin produced significant increase in hind paw withdrawal latency (HWL) in response to both mechanical and thermal stimulation. Moreover, a single dose of najanalgesin was able to induce antinociceptive activity that lasted for 1 week. Intrathecal injection of najanalgesin increased the HWL in response to mechanical stimuli. The antinociceptive effect of najanalgesin administered intrathecally was partly inhibited by intrathecal injection of naloxone or atropine. These results demonstrate that najanalgesin has antinociceptive effects on the central and peripheral system in the rat neuropathic pain model. The opioid receptor and muscatinic receptor are involved in najanalgesin-induced antinociception in the spinal cord. This research supports the possibility of using najanalgesin as a novel pharmacotherapeutic agent for neuropathic pain.

Role of nitric oxide/cyclic GMP/K(+) channel pathways in the antinociceptive effect caused by 2,3-bis(mesitylseleno)propenol

The present study examined the antinociceptive effects induced by 2,3-bis(mesitylseleno)propenol, a bis-selenide alkene derivate, given orally, in chemical models of pain in rats and mice. Selenide administered orally (p.o.) into the rats caused antinociception against the first and second phases of the formalin test, with mean ID(50) values of 28.17 and 39.68 mg/kg, respectively. The antinociceptive effect caused by selenide (50 mg/kg, p.o.) on the formalin test was reversed by pretreatment with N(G)-L-nitro-arginine methyl ester (L-NAME, a nitric oxide (NO) synthase inhibitor), methylene blue (a non-specific NO/guanylyl cyclase inhibitor) and glibenclamide (an ATP-sensitive K(+) channel inhibitor), but not by atropine (a muscarinic antagonist). Given orally selenide in mice produced an inhibition of glutamate-, histamine- and compound 48/80-induced nociception with mean ID(50) values of 27.58, 36.18 and 44.53 mg/kg, respectively. Moreover, oral treatment with selenide in mice decreased licking -- induced by serotonin (mean ID(50) value of >50 mg/kg). The data show that selenide exerts pronounced systemic antinociception in chemical (formalin, glutamate, histamine, compound 48/80 and serotonin-induced pain) models of nociception. Taken together, these results suggest that the antinociceptive effect of selenide on the formalin test involves the participation of nitric oxide/cyclic GMP/K(+) channel pathways in rats.

Synergistic depression of NMDA receptor-mediated transmission by ketamine, ketoprofen and L-NAME combinations in neonatal rat spinal cords in vitro

BACKGROUND AND PURPOSE

Spinal N-methyl-D-aspartate (NMDA) receptor/cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways play a major role in nociceptive processing, and influencing them simultaneously may induce synergistic analgesia. This study determined the spinal antinociceptive interactions between ketamine (NMDA receptor channel blocker), ketoprofen (COX inhibitor) and L-NAME (NOS inhibitor) combinations.

EXPERIMENTAL APPROACH

Using an in vitro neonatal rat spinal cord preparation, two A-fibre-mediated reflexes, the monosynaptic reflex (MSR) and the low-intensity excitatory postsynaptic potential (epsp), and one C-fibre-mediated reflex, the high-intensity epsp, were evoked electrically. The effect of drugs and drug combinations on these reflexes was assessed and the type of interaction determined by isobolographic analysis.

KEY RESULTS

Infusion of ketamine alone decreased all three reflexes. That of ketoprofen decreased both the low and the high-intensity epsp only. Infusion of L-NAME alone produced no significant effects. Co-infusion of fixed ratios of IC(40) fractions of both (ketamine+ketoprofen) and (ketamine+L-NAME) were synergistic for depressing the low and the high-intensity epsps. The interaction was sub-additive for both combinations on the MSR. The only significant effect for the (ketoprofen+L-NAME) combination was synergism on the high-intensity epsp.

CONCLUSIONS AND IMPLICATIONS

All three combinations synergistically depressed nociceptive spinal transmission, and both ketamine and ketoprofen and ketamine and L-NAME combinations did so with potentially decreased motor side effects. If such combination profiles also occur in vivo, the present findings raise the possibility of ultimate therapeutic exploitation of increased analgesia with fewer side effects.

Lumiracoxib in the management of osteoarthritis and acute pain

Lumiracoxib is a highly selective COX-2 inhibitor with a novel chemical structure and a relatively short plasma half-life. It has been approved in > 40 countries for the symptomatic treatment of osteoarthritis and/or acute pain related to primary dysmenorrhoea and dental or orthopaedic surgery. In these conditions, lumiracoxib has proved to be as effective as standard doses of conventional NSAIDs and other COX-2 selective inhibitors (coxibs). According to the Therapeutic Arthritis Research Gastrointestinal Trial, which enrolled 18,325 patients with osteoarthritis, lumiracoxib 400 mg/day (four times its recommended dosage) was associated with a significant decrease in the risk of ulcer complications compared with naproxen 1000 mg/day and ibuprofen 2400 mg/day, at least in the population not taking low-dose aspirin. The atherothrombotic potential of NSAIDs, especially coxibs, has been much debated. In this respect, available data do not suggest that lumiracoxib may be associated with an increased hazard of cardiovascular events compared with non-selective NSAIDs. Finally, lumiracoxib may be an effective and safe drug provided both physicians and patients will comply with its approved indications and contraindications.

Subcutaneous, intrathecal and periaqueductal grey administration of asimadoline and ICI-204448 reduces tactile allodynia in the rat

The purpose of this study was to assess the possible antiallodynic effect of asimadoline ([N-methyl-N-[1S)-1-phenyl)-2-(13S))-3-hydroxypyrrolidine-1-yl)-ethyl]-2,2-diphenylacetamide HCl]) and ICI-20448 ([2-[3-(1-(3,4-Dichlorophenyl-N-methylacetamido)-2-pyrrolidinoethyl)-phenoxy]acetic acid HCl]), two peripheral selective kappa opioid receptor agonists, after subcutaneous, spinal and periaqueductal grey administration to neuropathic rats. Twelve days after spinal nerve ligation tactile allodynia was observed, along with an increase in kappa opioid receptor mRNA expression in dorsal root ganglion and dorsal horn spinal cord. A non-significant increase in periaqueductal grey was also seen. Subcutaneous (s.c.) administration of asimadoline and ICI-204448 (1-30 mg/kg) dose-dependently reduced tactile allodynia. This effect was partially blocked by s.c., but not intrathecal, naloxone. Moreover, intrathecal administration of asimadoline or ICI-204448 (1-30 mug) reduced tactile allodynia in a dose-dependent manner and this effect was completely blocked by intrathecal naloxone. Microinjection of both kappa opioid receptor agonists (3-30 mug) into periaqueductal grey also produced a naloxone-sensitive antiallodynic effect in rats. Our results indicate that systemic, intrathecal and periaqueductal grey administration of asimadoline and ICI-204448 reduces tactile allodynia. This effect may be a consequence of an increase in kappa opioid receptor mRNA expression in dorsal root ganglion, dorsal horn spinal cord and, to some extent, in periaqueductal grey. Finally, our data suggest that these drugs could be useful to treat neuropathic pain in human beings.

Synergistic antinociceptive effects of anandamide, an endocannabinoid, and nonsteroidal anti-inflammatory drugs in peripheral tissue: a role for endogenous fatty-acid ethanolamides?

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit fatty-acid amide hydrolase (FAAH), the enzyme responsible for the metabolism of anandamide, an endocannabinoid. It has been suggested that the mechanisms of action of NSAIDs could be due to inhibition of cyclooxygenase (COX) and also to an increase in endocannabinoid concentrations. In a previous study we have demonstrated that the local analgesic interaction between anandamide and ibuprofen (a non-specific COX inhibitor) was synergistic for the acute and inflammatory phases of the formalin test. To test this hypothesis further, we repeated similar experiments with rofecoxib (a selective COX-2 inhibitor) and also measured the local concentrations of anandamide, and of two fatty-acid amides, oleoylethanolamide and palmitoylethanolamide. We established the ED(50) for anandamide (34.52 pmol+/-17.26) and rofecoxib (381.72 pmol+/-190.86) and showed that the analgesic effect of the combination was synergistic. We also found that paw tissue levels of anandamide, oleoylethanolamide and palmitoylethanolamide were significantly higher when anandamide was combined with NSAIDs and that this effect was greater with rofecoxib. In conclusion, local injection of anandamide or rofecoxib was antinociceptive in a test of acute and inflammatory pain and the combination of anandamide with rofecoxib was synergistic. Finally, locally injected anandamide with either NSAID (ibuprofen or rofecoxib) generates higher amount of fatty-acid ethanolamides. The exact comprehension of the mechanisms involved needs further investigation.

Pharmacological evidence for the participation of NO-cyclic GMP-PKG-K+ channel pathway in the antiallodynic action of resveratrol

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG)-K+ channels pathway in the antiallodynic action of resveratrol and YC-1 in spinal nerve injured rats was assessed. Ligation of L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal administration of resveratrol (100-600 microg) and 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (0.1-2.7 microg, YC-1, a soluble guanylyl cyclase activator) decreased tactile allodynia induced by ligation of L5/L6 spinal nerves. Intrathecal treatment with NG-L-nitro-arginine methyl ester (10-100 microg, L-NAME, a NO synthase inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (1-10 microg, ODQ, a soluble guanylyl cyclase inhibitor), KT-5823 (5-500 ng, a PKG inhibitor) and iberiotoxin (5-500 ng, a large-conductance Ca2+ -activated K+ channel blocker), but not NG-D-nitro-arginine methyl ester (100 microg, D-NAME, an inactive isomer of L-NAME), glibenclamide (12.5-50 microg, ATP-sensitive K+ channel blocker) or vehicle, significantly diminished resveratrol (300 microg)- and YC-1 (2.7 microg)-induced spinal antiallodynia. These effects were independent of prostaglandin synthesis inhibition as indomethacin did not affect resveratrol-induced antiallodynia. Results suggest that resveratrol and YC-1 could activate the proteins of the NO-cyclic GMP-PKG spinal pathway or large-conductance Ca2+ -activated, but not ATP-sensitive, K+ channels at the spinal cord in order to produce at least part of their antiallodynic effect in this model of neuropathy.

Effect of diabetes on the mechanisms of intrathecal antinociception of sildenafil in rats

The mechanism of intrathecal antinociceptive action of the phosphodiesterase 5 inhibitor sildenafil was assessed in diabetic rats using the formalin test. Intrathecal administration of sildenafil (12.5-50 microg) produced a dose-related antinociception during both phases of the formalin test in non-diabetic and diabetic rats. Intrathecal pretreatment with N-L-nitro-arginine methyl ester (L-NAME, nitric oxide (NO) synthase inhibitor, 1-50 microg), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor, 1-10 microg), KT5823 (protein kinase G (PKG) inhibitor, 5-500 ng), charybdotoxin (large-conductance Ca2+-activated K+ channel blocker, 0.01-1 ng), apamin (small-conductance Ca2+-activated K+ channel blocker, 0.1-3 ng) and glibenclamide (ATP-sensitive K+ channel blocker, 12.5-50 microg), but not N-D-nitro-arginine methyl ester (D-NAME, 50 microg) or saline, significantly diminished sildenafil (50 microg)-induced antinociception in non-diabetic rats. Intrathecal administration of ODQ, KT5823, apamin and glibenclamide, but not L-NAME nor charybdotoxin, reversed intrathecal antinociception induced by sildenafil in diabetic rats. Results suggest that sildenafil produces its intrathecal antinociceptive effect via activation of NO-cyclic GMP-PKG-K+ channels pathway in non-diabetic rats. Data suggest that diabetes leads to a dysfunction in NO and large-conductance Ca2+-activated K+ channels. Sildenafil could have a role in the pharmacotherapy of diabetes-associated pain.