Diclofenac inhibition of sodium currents in rat dorsal root ganglion neurons

Anesthetic- and Analgesic-Related Drugs Modulating Both Voltage-Gated Na+ and TRP Channels

Nociceptive information is transmitted by action potentials (APs) through primary afferent neurons from the periphery to the central nervous system. Voltage-gated Na+ channels are involved in this AP production, while transient receptor potential (TRP) channels, which are non-selective cation channels, are involved in receiving and transmitting nociceptive stimuli in the peripheral and central terminals of the primary afferent neurons. Peripheral terminal TRP vanilloid-1 (TRPV1), ankylin-1 (TRPA1) and melastatin-8 (TRPM8) activation produces APs, while central terminal TRP activation enhances the spontaneous release of L-glutamate from the terminal to spinal cord and brain stem lamina II neurons that play a pivotal role in modulating nociceptive transmission. There is much evidence demonstrating that chemical compounds involved in Na+ channel (or nerve AP conduction) inhibition modify TRP channel functions. Among these compounds are local anesthetics, anti-epileptics, α2-adrenoceptor agonists, antidepressants (all of which are used as analgesic adjuvants), general anesthetics, opioids, non-steroidal anti-inflammatory drugs and plant-derived compounds, many of which are involved in antinociception. This review mentions the modulation of Na+ channels and TRP channels including TRPV1, TRPA1 and TRPM8, both of which modulations are produced by pain-related compounds.

Ibuprofen modulates tetrodotoxin-resistant persistent Na+ currents at acidic pH in rat trigeminal ganglion neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used to relieve various symptoms such as headache, arthralgia, and dental pain. While the primary mechanism of NSAID-based pain relief is the inhibition of cyclooxygenase-2, several NSAIDs also modulate other molecular targets related to nociceptive transmission such as voltage-gated Na+ channels. In the present study, we examined the effects of NSAIDs on persistent Na+ current (INaP) mediated by tetrodotoxin-resistant (TTX-R) Na+ channels in small-to medium-sized trigeminal ganglion neurons using a whole-cell patch-clamp technique. At clinically relevant concentrations, all propionic acid derivatives tested (ibuprofen, naproxen, fenoprofen, and flurbiprofen) preferentially inhibited the TTX-R INaP. The inhibition was more potent at acidic extracellular pH (pH 6.5) than at normal pH (pH 7.4). Other NSAIDs, such as ketorolac, piroxicam, and aspirin, had a negligible effect on the TTX-R INaP. Ibuprofen both accelerated the onset of inactivation and retarded the recovery from inactivation of TTX-R Na+ channels at acidic extracellular pH. However, all NSAIDs tested in this study had minor effects on voltage-gated K+ currents, as well as hyperpolarization-activated and cyclic nucleotide-gated cation currents, at both acidic and normal extracellular pH. Under current-clamp conditions, ibuprofen decreased the number of action potentials elicited by depolarizing current stimuli at acidic (pH 6.5) extracellular pH. Considering that extracellular pH falls as low as 5.5 in inflamed tissues, TTX-R INaP inhibition could be a mechanism by which ibuprofen and propionic acid derivative NSAIDs modulate inflammatory pain.

Combinations of classical and non-classical voltage dependent potassium channel openers suppress nociceptor discharge and reverse chronic pain signs in a rat model of Gulf War illness

In a companion paper we examined whether combinations of K_v7 channel openers (Retigabine and Diclofenac; RET, DIC) could be effective modifiers of deep tissue nociceptor activity; and whether such combinations could then be optimized for use as safe analgesics for pain-like signs that developed in a rat model of GWI (Gulf War Illness) pain. In the present report, we examined the combinations of Retigabine/Meclofenamate (RET/MEC) and Meclofenamate/Diclofenac (MEC/DIC). Voltage clamp experiments were performed on deep tissue nociceptors isolated from rat DRG (dorsal root ganglion). In voltage clamp studies, a stepped voltage protocol was applied (-55 to -40 mV; V_h=-60 mV; 1500 msec) and K_v7 evoked currents were subsequently isolated by Linopirdine subtraction. MEC greatly enhanced voltage dependent conductance and produced exceptional maximum sustained currents of 6.01 ± 0.26 pA/pF (EC₅₀: 62.2 ± 8.99 μM). Combinations of RET/MEC, and MEC/DIC substantially amplified resting currents at low concentrations. MEC/DIC also greatly improved voltage dependent conductance. In current clamp experiments, a cholinergic challenge test (Oxotremorine-M, 10 μM; OXO), associated with our GWI rat model, produced powerful action potential (AP) bursts (85 APs). Optimized combinations of RET/MEC (5 and 0.5 μM) and MEC/DIC (0.5 and 2.5 μM) significantly reduced AP discharges to 3 and 7 Aps, respectively. Treatment of pain-like ambulatory behavior in our rat model with a RET/MEC combination (5 and 0.5 mg/kg) successfully rescued ambulation deficits, but could not be fully separated from the effect of RET alone. Further development of this approach is recommended.

Development and Challenges of Diclofenac-Based Novel Therapeutics: Targeting Cancer and Complex Diseases

Simple Summary

Diclofenac is a widely used drug for its anti-inflammatory and pain alleviating properties. This review summarizes the current understanding about the drug diclofenac. The potential applications of diclofenac beyond its well-known anti-inflammatory properties for other diseases such as cancer are discussed, along with existing limitations.

Abstract

Diclofenac is a highly prescribed non-steroidal anti-inflammatory drug (NSAID) that relieves inflammation, pain, fever, and aches, used at different doses depending on clinical conditions. This drug inhibits cyclooxygenase-1 and cyclooxygenase-2 enzymes, which are responsible for the generation of prostaglandin synthesis. To improve current diclofenac-based therapies, we require new molecular systematic therapeutic approaches to reduce complex multifactorial effects. However, the critical challenge that appears with diclofenac and other drugs of the same class is their side effects, such as signs of stomach injuries, kidney problems, cardiovascular issues, hepatic issues, and diarrhea. In this article, we discuss why defining diclofenac-based mechanisms, pharmacological features, and its medicinal properties are needed to direct future drug development against neurodegeneration and imperfect ageing and to improve cancer therapy. In addition, we describe various advance molecular mechanisms and fundamental aspects linked with diclofenac which can strengthen and enable the better designing of new derivatives of diclofenac to overcome critical challenges and improve their applications.

Development of KVO treatment strategies for chronic pain in a rat model of Gulf War Illness

We examined whether combinations of K_v7 channel openers could be effective modifiers of deep tissue nociceptor activity; and whether such combinations could then be optimized for use as safe analgesics for pain-like signs that developed in a rat model of GWI (Gulf War Illness) pain. Voltage clamp experiments were performed on subclassified nociceptors isolated from rat DRG (dorsal root ganglion). A stepped voltage protocol was applied (-55 to -40 mV; V_h = -60 mV; 1500 ms) and K_v7 evoked currents were subsequently isolated by linopirdine subtraction. Directly activated and voltage activated K⁺ currents were characterized in the presence and absence of Retigabine (5-100 μM) and/or Diclofenac (50-140 μM). Retigabine produced substantial voltage dependent effects and a maximal sustained current of 1.14 pA/pF ± 0.15 (ED₅₀: 62.7 ± 3.18 μM). Diclofenac produced weak voltage dependent effects but a similar maximum sustained current of 1.01 ± 0.26 pA/pF (ED₅₀: 93.2 ± 8.99 μM). Combinations of Retigabine and Diclofenac substantially amplified resting currents but had little effect on voltage dependence. Using a cholinergic challenge test (Oxotremorine, 10 μM) associated with our GWI rat model, combinations of Retigabine (5 uM) and Diclofenac (2.5, 20 and 50 μM) substantially reduced or totally abrogated action potential discharge to the cholinergic challenge. When combinations of Retigabine and Diclofenac were used to relieve pain-signs in our rat model of GWI, only those combinations associated with serious subacute side effects could relieve pain-like behaviors.

Hazardous impact of diclofenac on mammalian system: Mitigation strategy through green remediation approach

Diclofenac is an anti-inflammatory drug used as an analgesic. It is often detected in various environmental sources around the world and is considered as one of the emerging contaminants (ECs). This paper reviews the distribution of diclofenac at high concentrations in diverse environments and its adverse ecological impact. Recent studies observed strong evidence of the hazardous effect of diclofenac on mammals, including humans. Diclofenac could cause gastrointestinal complications, neurotoxicity, cardiotoxicity, hepatotoxicity, nephrotoxicity, hematotoxicity, genotoxicity, teratogenicity, bone fractures, and skin allergy in mammals even at a low concentration. Collectively, this comprehensive review relates the mode of toxicity, level of exposure, and route of administration as a unique approach for addressing the destructive consequence of diclofenac in mammalian systems. Finally, the mitigation strategy to eradicate the diclofenac toxicity through green remediation is critically discussed. This review will undoubtedly shed light on the toxic effects of pseudo-persistent diclofenac on mammals as well as frame stringent guidelines against its common usage.

Spinal Actions of the NSAID Diclofenac on Nociceptive Transmission in Comparison to the Kv7 Channel Opener Flupirtine

NSAIDs are the drugs most commonly used to alleviate pain. Despite being a heterogeneous group of compounds, all of them share a mechanism of action based on blockade of COXs enzymes, which confers them anti-inflammatory and analgesic properties. Diclofenac is a NSAID with preferred activity on COX-2 isozymes, but additionally, other targets may be implicated in its analgesic activity. Among them, diclofenac may facilitate the activity of K_v7 channels, that have been previously recognized as potential therapeutic targets in analgesia. In this study, the antinociceptive actions of diclofenac acting at the spinal level and the role of K_v7 channels in its effects were evaluated. Electrophysiological recordings of spinal reflexes and responses of dorsal horn neurons were obtained using in vitro spinal cord preparations from neonatal mice. Diclofenac, applied at clinically relevant concentrations to the entire preparation, depressed wind-up of spinal reflexes with a pattern similar to that of flupirtine, an analgesic with activity as K_v7 channel opener. Depressant actions of both compounds were strongly reduced after K_v7 channel blockade with XE-991, indicating the implication of these channels in the observed effects. Flupirtine, but not diclofenac, also reduced action potential firing of dorsal horn neurons in response to electrical activation of nociceptive afferents, suggesting differences in the actions of both compounds on K_v7 channel configurations present in sensory areas of the cord. Results demonstrate previously unknown central actions of diclofenac on K_v7 channels located in spinal circuits, expanding the knowledge about its pharmacological actions.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

Fenamates Inhibit Human Sodium Channel Nav1.2 and Protect Glutamate-Induced Injury in SH-SY5Y Cells

Voltage-gated sodium channels are crucial mediators of neuronal damage in ischemic and excitotoxicity disease models. Fenamates have been reported to have anti-inflammatory properties following a decrease in prostaglandin synthesis. Several researches showed that fenamates appear to be ion channel modulators and potential neuroprotectants. In this study, the neuroprotective effects of tolfenamic acid, flufenamic acid, and mefenamic acid were tested by glutamate-induced injury in SH-SY5Y cells. Following this, fenamates' effects were examined on both the expression level and the function of hNav1.1 and hNav1.2, which were closely associated with neuroprotection, using Western blot and patch clamp. Finally, the effect of fenamates on the expression of apoptosis-related proteins in SH-SY5Y cells was examined. The results showed that both flufenamic acid and mefenamic acid exhibited neuroprotective effects against glutamate-induced injury in SH-SY5Y cells. They inhibited peak currents of both hNav1.1 and hNav1.2. However, fenamates exhibited decreased inhibitory effects on hNav1.1 when compared to hNav1.2. Correspondingly, the inhibitory effect of fenamates was found to be consistent with the level of neuroprotective effects in vitro. Fenamates inhibited glutamate-induced apoptosis through the modulation of the Bcl-2/Bax-dependent cell death pathways. Taken together, Nav1.2 might play a part in fenamates' neuroprotection mechanism. Nav1.2 and NMDAR might take part in the neuroprotection mechanism of the fenamates. The fenamates inhibited glutamate-induced apoptosis through modulation of the Bcl-2/Bax-dependent cell death pathways.

Inhibition by non-steroidal anti-inflammatory drugs of compound action potentials in frog sciatic nerve fibers

AIMS

Although antinociception produced by non-steroidal anti-inflammatory drugs (NSAIDs) is partly attributed to nerve conduction inhibition, this has not been thoroughly examined yet. The aim of the present study was to reveal quantitatively how various types of NSAIDs affect compound action potentials (CAPs), a measure of nerve conduction.

MAIN METHODS

CAPs were recorded from the frog sciatic nerve by using the air-gap method.

KEY FINDINGS

Soaking the sciatic nerve with acetic acid-based NSAIDs (diclofenac and aceclofenac) reduced the peak amplitude of CAP in a concentration-dependent manner; their IC₅₀ values were 0.94 and 0.47 mM, respectively. Other acetic acid-based NSAIDs (indomethacin, acemetacin and etodolac) also inhibited CAPs [the extent of inhibition: some 40% (1 mM), 40% (0.5 mM) and 15% (1 mM), respectively], except for sulindac and felbinac at 1 mM that had no effects on CAP peak amplitudes. A similar inhibition was produced by fenamic acid-based NSAIDs [tolfenamic acid (IC₅₀ = 0.29 mM), meclofenamic acid (0.19 mM), flufenamic acid (0.22 mM) and mefenamic acid] which are similar in chemical structure to diclofenac and aceclofenac; their derivatives (2,6-dichlorodiphenylamine and N-phenylanthranilic acid) also inhibited. On the other hand, salicylic acid-based (aspirin), propionic acid-based (ketoprofen, naproxen, ibuprofen, loxoprofen and flurbiprofen) and enolic acid-based (meloxicam and piroxicam) NSAIDs had no effects on CAP peak amplitudes.

SIGNIFICANCE

At least a part of antinociception produced by NSAIDs used as a dermatological drug to alleviate pain may be attributed to their inhibitory effects on nerve conduction, which depend on the chemical structures of NSAIDs.

Peripheral nerve and diclofenac sodium: Molecular and clinical approaches

Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most frequently prescribed medications worldwide. Diclofenac sodium (DS), one of these NSAIDs, has a high specificity for arachidonic acid-degrading cyclooxygenase (COX)-2 enzymes. This drug can be used to relieve neuropathic pain. In this review, we examine the relevant researches, including in vivo, animal, and clinical human studies, with the aim of understanding the effect of DS on the peripheral nerves. In injured nerves, COX-2 is potently upregulated around the injury site. When a nerve is damaged, both COX-1 and COX-2 expression is increased in macrophages and Schwann cells. In addition, COX inhibitors can promote axonal outgrowth in cultured neurons. Neuropathic pain occurs after injury and leads to dysfunction of the peripheral nervous system. NSAIDs can modulate the nociceptive and inflammatory pain pathways and control neuropathic pain. DS may accelerate nerve regeneration and its effects on healing, as well as causing deleterious effects in the developing nerves. DS teratogenicity disrupts myelin sheath thickness and axon structure. Understanding the possible benefits and limitations of DS and specific conditions such as prenatal use will be of benefit in clinical practice.

Diclofenac, a nonsteroidal anti-inflammatory drug, is an antagonist of human TRPM3 isoforms

The effects of diclofenac (Dic), an acetic acid derivative-type nonsteroidal anti-inflammatory drug, were examined on the function of transient receptor potential (TRP) melastatin (TRPM) 3 (TRPM3) in human embryonic kidney 293 cell-line (HEK293) cells with recombinant human TRPM3 isoforms (TRPM31325, TRPM3-3, TRPM3-9, and TRPM3-S) and in a neuroblastoma cell line human neuroblastoma IMR-32 cells (IMR-32 cells) derived from human peripheral neurons. TRPM3 responses evoked by pregnenolone sulfate (PregS) were effectively inhibited by Dic in a concentration-dependent manner in Ca(2+) measurement and electrophysiological assays. The apparent IC 50 for PregS-induced Ca(2+) response of TRPM31325, TRPM3-3, and TRPM3-9 was calculated to be 18.8, 42.5, and 7.1 μmol/L, respectively. The TRPM3-dependent Ca(2+) responses evoked by nifedipine, another TRPM3 agonist, were also significantly inhibited by Dic. In contrast, aceclofenac, an acetoxymethyl analog of Dic, had no effects on PregS-induced TRPM3 responses. Constitutive channel activity of TRPM3-S without TRPM3 agonists was substantially inhibited by Dic, ruling out the possibility of interaction of Dic against TRPM3 agonists to the channel binding sites. Moreover, Dic reversibly inhibited TRPM3 single-channel activity recorded in excised outside-out patches without affecting the channel conductance. In differentiated neuronal IMR-32 cells with endogenous TRPM3, Dic inhibited PregS-evoked Ca(2+) responses with an apparent IC 50 of 17.1 μmol/L. Taken together, our findings demonstrate that Dic inhibits human TRPM3 without interacting with the channel pore.

Indomethacin inhibits tetrodotoxin-resistant Na(+) channels at acidic pH in rat nociceptive neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are well-known inhibitors of cyclooxygenases (COXs) and are widely used for the treatment of inflammatory pain; however several NSAIDs display COX-independent analgesic action including the inhibition of voltage-gated Na(+) channels expressed in primary afferent neurons. In the present study, we examined whether NSAIDs modulate tetrodotoxin-resistant (TTX-R) Na(+) channels and if this modulation depends on the extracellular pH. The TTX-R Na(+) currents were recorded from small-sized trigeminal ganglion neurons by using a whole-cell patch clamp technique. Among eight NSAIDs tested in this study, several drugs, including aspirin and ibuprofen, did not affect TTX-R Na(+) channels either at pH 7.4 or at pH 6.0. However, we found that indomethacin, and, to a lesser extent, ibuprofen and naproxen potently inhibited the peak amplitude of TTX-R Na(+) currents at pH 6.0. The indomethacin-induced inhibition of TTX-R Na(+) channels was more potent at depolarized membrane potentials. Indomethacin significantly shifted both the voltage-activation and voltage-inactivation relationships to depolarizing potentials at pH 6.0. Indomethacin accelerated the development of inactivation and retarded the recovery from inactivation of TTX-R Na(+) channels at pH 6.0. Given that indomethacin and several other NSAIDs could further suppress local nociceptive signals by inhibiting TTX-R Na(+) channels at an acidic pH in addition to the classical COX inhibition, these drugs could be particularly useful for the treatment of inflammatory pain.

Flufenamic acid prevents behavioral manifestations of salicylate-induced tinnitus in the rat

INTRODUCTION

Tinnitus is defined as a phantom auditory sensation, the perception of sound in the absence of external acoustic stimulation. Given that flufenamic acid (FFA) blocks TRPM2 cation channels, resulting in reduced neuronal excitability, we aimed to investigate whether FFA suppresses the behavioral manifestation of sodium salicylate (SSA)-induced tinnitus in rats.

MATERIAL AND METHODS

Tinnitus was evaluated using a conditioned lick suppression model of behavioral testing. Thirty-one Wistar rats, randomly divided into four treatment groups, were trained and tested in the behavioral experiment: (1) control group: DMSO + saline (n = 6), (2) SSA group: DMSO + SSA (n = 6), (3) FFA group: FFA (66 mg/kg bw) + saline (n = 9), (4) FFA + SSA group: FFA (66 mg/kg bw) + SSA (400 mg/kg bw) (n = 10). Localization of TRPM2 to the plasma membrane of cochlear nucleus neurons was demonstrated by confocal microscopy.

RESULTS

Pavlovian training resulted in strong suppression of licking, having a mean value of 0.05 ±0.03 on extinction day 1, which is below the suppression training criterion level of 0.20 in control tinnitus animals. The suppression rate for rats having both FFA (66 mg/kg bw) and SSA (400 mg/kg bw) injections was significantly lower than that for the rats having SSA injections (p < 0.01).

CONCLUSIONS

We suggest that SSA-induced tinnitus could possibly be prevented by administration of a TRPM2 ion channel antagonist, FFA at 66 mg/kg bw.

pH-dependent inhibition of tetrodotoxin-resistant Na(+) channels by diclofenac in rat nociceptive neurons

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for the treatment of inflammatory pain. It is well established that NSAIDs exert their analgesic effects by inhibiting cyclooxygenase to prevent the production of prostaglandins; however, several NSAIDs including diclofenac also modulate other ion channels expressed in nociceptive neurons. In this study, we investigated the pH-dependent effects of diclofenac on tetrodotoxin-resistant (TTX-R) Na(+) channels in rat trigeminal sensory neurons by using the whole-cell patch clamp technique. Diclofenac decreased the peak amplitude of TTX-R Na(+) currents (INa) in a concentration dependent manner. While diclofenac had little effect on the voltage-activation relationship, it significantly shifted the steady-state fast inactivation relationship toward hyperpolarized potentials. Diclofenac increased the extent of use-dependent inhibition of TTX-R Na(+) currents. Diclofenac also significantly accelerated the development of inactivation and retarded the recovery from inactivation of TTX-R Na(+) channels. The effects of diclofenac on TTX-R Na(+) channels were stronger at pH 6.0 than at pH7.4 for most of the parameters tested. Considering that the extracellular pH falls in inflamed tissues, and that TTX-R Na(+) channels expressed on nociceptive neurons are implicated in the prostaglandin-mediated development and maintenance of inflammatory hyperalgesia, our findings could provide an additional analgesic effect of diclofenac under acidic pH conditions.

Evaluation of neuroprotection by melatonin against adverse effects of prenatal exposure to a nonsteroidal anti-inflammatory drug during peripheral nerve development

The potential ability of melatonin to protect against impairment of the fetal peripheral nerve system due to maternal consumption of diclofenac sodium (DS) was investigated. Eighty-four pregnant rats were divided into seven groups: control (CONT), saline administered (PS), DS administered (DS), DS with low-dose melatonin administered (DS+MLT10), DS with high-dose melatonin administered (DS+MLT50), low-dose melatonin administered (MLT10), and high-dose melatonin administered (MLT50). After the pregnancy, six male newborn rats from each group were sacrificed at 4 and 20 weeks of age. Their right sciatic nerves were harvested, and nerve fibers were evaluated using stereological techniques. Mean numbers of myelinated axons, axon cross-section areas and the mean thickness of the myelin sheet were estimated. Four-week-old prenatally DS-exposed rats had significantly fewer axons, a smaller myelinated axonal area, and a thinner myelin sheath compared to CONT group (p<0.05). Although melatonin at both doses significantly increased axon numbers, only a high dose of melatonin increased the diameter of those axons (p<0.05). At 20-weeks of age, myelinated axon number in the DS group was not only significantly lower than all other groups (p<0.05) but also the cross-sectional area of these axons was smaller than all other groups (p<0.05). There were no differences between the groups regarding the mean thickness of the myelin sheet. The current study indicates that prenatal exposure to DS decreases the number and the diameter of sciatic nerve axons and that melatonin prophylaxis can prevent these effects.

Participation of cannabinoid receptors in peripheral nociception induced by some NSAIDs

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been used extensively to control inflammatory pain. Several peripheral antinociceptive mechanisms have been described, such as opioid system and NO/cGMP/KATP pathway activation. There is evidence that the cannabinoid system can also contribute to the in vivo pharmacological effects of ibuprofen and indomethacin. However, there is no evidence of the involvement of the endocannabinoid system in the peripheral antinociception induced by NSAIDs. Thus, the aim of this study was to investigate the participation of the endocannabinoid system in the peripheral antinociceptive effect of NSAIDs. All experiments were performed on male Wistar rats (160-200 g; N = 4 per group). Hyperalgesia was induced by a subcutaneous intraplantar (ipl) injection of prostaglandin E2 (PGE2, 2 μg/paw) in the rat's hindpaw and measured by the paw pressure test 3 h after injection. The weight in grams required to elicit a nociceptive response, paw flexion, was determined as the nociceptive threshold. The hyperalgesia was calculated as the difference between the measurements made before and after PGE2, which induced hyperalgesia (mean = 83.3 ± 4.505 g). AM-251 (80 μg/paw) and AM-630 (100 μg/paw) were used as CB1 and CB2 cannabinoid receptor antagonists, respectively. Ipl injection of 40 μg dipyrone (mean = 5.825 ± 2.842 g), 20 μg diclofenac (mean = 4.825 ± 3.850 g) and 40 μg indomethacin (mean = 6.650 ± 3.611 g) elicited a local peripheral antinociceptive effect. This effect was not antagonized by ipl CB1 cannabinoid antagonist to dipyrone (mean = 5.00 ± 0.9815 g), diclofenac (mean = 2.50 ± 0.8337 g) and indomethacin (mean = 6.650 ± 4.069 g) or CB2 cannabinoid antagonist to dipyrone (mean = 1.050 ± 6.436 g), diclofenac (mean = 6.675 ± 1.368 g) and indomethacin (mean = 2.85 ± 5.01 g). Thus, cannabinoid receptors do not seem to be involved in the peripheral antinociceptive mechanism of the NSAIDs dipyrone, diclofenac and indomethacin.

Regulation of TRPC6 Channels by Non-Steroidal Anti-Inflammatory Drugs

Family focal segmental glomerulosclerosis (FSGS) is characterized by sclerosis and hyalinosis of particular loops of glomeruli and is one of the causes of the nephrotic syndrome. Certain mutations in the structure of TRPC6 channels are the genetic impetus for FSGS development resulting in podocytes functional abnormalities and various nephropathies. We have recently demonstrated that non-steroid anti-inflammatory drugs (NSAID) ibuprofen and diclofenac decrease the activity of endogenous TRPC like cal cium channels in the podocytes of the freshly isolated rat glomeruli. It has also been shown that TRPC6 chan nels are expressed in the podocytes. In the current study we have functionally reconstituted TRPC6 channels in mammalian cells to investigate the effects of diclofenac on the activity of wild type TRPC6 channel and TRPC6^P112Q channel containing a mutation in the N-terminus that was described in FSGS patients. Intracellular calcium level measurements in transfected cells revealed a more intensive carbachol induced increase of calcium concentration in HEK 293 cells expressing TRPC6^P112Q versus the cells expressing wild-type TRPC6. We also performed patch-clamp experiments to study TRPC6 channels reconstituted in Chinese hamster ovary (CHO) cell line and found that application of diclofenac (500 μM) acutely reduced single channel activity. Preincubation with diclofenac (100 μM) also decreased the whole cell current in CHO cells overexpressing TRPC6^P112Q. Therefore, our previously published data on the effects of NSAID on TRPC-like channels in the isolated rat glomeruli, along with this current investigation on the cultured overexpressed mammalian cells, allows hypothesizing that TRPC6 channels may be a target for NSAID that can be impor tant in the treatment of FSGS.

Intramuscular ketorolac inhibits activation of rat peripheral NMDA receptors

The nonsteroidal anti-inflammatory drug (NSAID) diclofenac has local anesthetic-like and peripheral N-methyl-d-aspartate (NMDA) receptor antagonist characteristics when administered at higher concentrations to masticatory muscle. It is not known if the ability to inhibit NMDA receptors is unique to diclofenac or shared by other NSAIDs. This study was undertaken to determine whether intramuscular injection of ketorolac or naproxen at concentrations that do not induce local anesthetic-like effects could attenuate jaw-closer muscle nociceptor discharge in anesthetized Sprague-Dawley rats. It was found that ketorolac (5 mM) inhibited hypertonic saline-evoked nociceptor discharge, which suggests that at this concentration, ketorolac has local anesthetic-like properties. A lower concentration of ketorolac (0.5 mM), which did not affect hypertonic saline-evoked discharge, did inhibit nociceptor discharge evoked by NMDA. In contrast, naproxen (5 mM) did not alter hypertonic saline- or NMDA-evoked nociceptor discharge. Subsequent experiments revealed that ketorolac (0.5 mM) had no effect on nociceptor discharge evoked by αβ-methylene ATP, 5-hydroxytryptamine, or AMPA. The inhibitory effect of ketorolac did not appear to be related to cyclooxygenase inhibition, because the concentration of prostaglandin E(2) in the masticatory muscles 10 min after injection of either NSAID was not significantly decreased. The present study indicates that in vivo, ketorolac, but not naproxen, can antagonize NMDA-evoked nociceptor discharge similarly to diclofenac. We speculate that structural similarities between ketorolac and diclofenac could account for the ability of these NSAIDs to inhibit NMDA-evoked nociceptor discharge. These properties may partly explain the analgesic effect of intramuscularly injected ketorolac in the clinic.

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.

Neurophysiological markers of central sensitisation in the trigeminal pathway and their modulation by the cyclo-oxygenase inhibitor ketorolac

Central sensitisation is a key mechanism of migraine; understanding its modulation by anti-migraine drugs is essential for rationalising treatment. We used an animal model of central trigeminal sensitisation to investigate neuronal responses to dural electrical stimulation as a putative electrophysiological marker of sensitisation and its modulation by ketorolac. In anaesthetised rats, responses of single convergent wide-dynamic range neurons of the spinal trigeminal nucleus to dural electrical simulation were recorded in parallel to their ongoing activity and responses to facial mechanical stimulation before and after a short-term dural application of an IS. Both ongoing activity and responses to dural electrical stimuli were enhanced by the inflammatory challenge, whereas neuronal thresholds to mechanical skin stimulation were reduced ( p < .05, N = 12). Intravenous ketorolac (2 mg/kg, N = 6) reduced ongoing activity and responses to dural electrical stimulation, and increased mechanical thresholds versus vehicle controls ( p < .05, N = 6). We conclude that neuronal responses to dural electrical stimulation can serve as a suitable marker which together with admitted electrophysiological signs can objectively detect central trigeminal sensitisation and its modulation by anti-migraine treatments in this preclinical model of migraine.

Diclofenac, a Non-steroidal Anti-inflammatory Drug, Inhibits L-type Ca Channels in Neonatal Rat Ventricular Cardiomyocytes

A non-steroidal anti-inflammatory drug (NSAID) has many adverse effects including cardiovascular (CV) risk. Diclofenac among the nonselective NSAIDs has the highest CV risk such as congestive heart failure, which resulted commonly from the impaired cardiac pumping due to a disrupted excitation-contraction (E-C) coupling. We investigated the effects of diclofenac on the L-type calcium channels which are essential to the E-C coupling at the level of single ventricular myocytes isolated from neonatal rat heart, using the whole-cell voltage-clamp technique. Only diclofenac of three NSAIDs, including naproxen and ibuprofen, significantly reduced inward whole cell currents. At concentrations higher than 3 microM, diclofenac inhibited reversibly the Na(+) current and did irreversibly the L-type Ca(2+) channels-mediated inward current (IC(50)=12.89+/-0.43 microM) in a dose-dependent manner. However, nifedipine, a well-known L-type channel blocker, effectively inhibited the L-type Ca(2+) currents but not the Na(+) current. Our finding may explain that diclofenac causes the CV risk by the inhibition of L-type Ca(2+) channel, leading to the impairment of E-C coupling in cardiac myocytes.

Flufenamic acid suppresses epileptiform activity in hippocampus by reducing excitatory synaptic transmission and neuronal excitability

PURPOSE

In this study, we explore the antiepileptic effects of flufenamic acid (FFA) in order to identify the cellular mechanisms that underlie the potential anticonvulsant properties of this nonsteroidal antiinflammatory compound.

METHODS

The mechanisms of FFA action were analyzed using an in vitro model in which epileptiform activity was induced in hippocampal slices by perfusion with 100 microm 4-aminopyridine (4-AP) added to a modified Mg(2+)-free solution. The activity of CA1 pyramidal neurons as well as the synaptic connection between CA3 and CA1 was monitored using extracellular and patch-clamp recordings.

RESULTS

Epileptiform activity was suppressed in hippocampal neurons by FFA at concentrations between 50 and 200 microm. Glutamatergic excitatory synaptic transmission was diminished by FFA without modifying recurrent gamma-aminobutyric acid (GABA)ergic synaptic inhibition. Several lines of evidence indicated that FFA did not decrease neurotransmitter release probability, implicating a postsynaptic mechanism of action. FFA also potently reduced neuronal excitability, but did not alter the amplitude, duration, or undershoot of action potentials.

CONCLUSIONS

Our results suggest that FFA exerts an anticonvulsive effect on hippocampal pyramidal neurons by simultaneously decreasing glutamatergic excitatory synaptic activity and reducing neuronal excitability. Therefore, our study provides experimental evidence that FFA may represent an effective pharmacologic agent in the treatment of epilepsy in the mammalian central nervous system.

Sensitization of meningeal nociceptors: inhibition by naproxen

Migraine attacks associated with throbbing (manifestation of peripheral sensitization) and cutaneous allodynia (manifestation of central sensitization) are readily terminated by intravenous administration of a non-selective cyclooxygenase (COX) inhibitor. Evidence that sensitization of rat central trigeminovascular neurons was also terminated in vivo by non-selective COX inhibition has led us to propose that COX inhibitors may act centrally in the dorsal horn. In the present study, we examined whether COX inhibition can also suppress peripheral sensitization in meningeal nociceptors. Using single-unit recording in the trigeminal ganglion in vivo, we found that intravenous infusion of naproxen, a non-selective COX inhibitor, reversed measures of sensitization induced in meningeal nociceptors by prior exposure of the dura to inflammatory soup (IS): ongoing activity of Adelta- and C-units and their response magnitude to mechanical stimulation of the dura, which were enhanced after IS, returned to baseline after naproxen infusion. Topical application of naproxen or the selective COX-2 inhibitor N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS-398) onto the dural receptive field of Adelta- and C-unit nociceptors also reversed the neuronal hyper-responsiveness to mechanical stimulation of the dura. The findings suggest that local COX activity in the dura could mediate the peripheral sensitization that underlies migraine headache.

Mechanisms of non-steroid anti-inflammatory drugs action on ASICs expressed in hippocampal interneurons

The inhibitory action of non-steroid anti-inflammatory drugs was investigated on acid-sensing ionic channels (ASIC) in isolated hippocampal interneurons and on recombinant ASICs expressed in Chinese hamster ovary (CHO) cells. Diclofenac and ibuprofen inhibited proton-induced currents in hippocampal interneurons (IC(50) were 622 +/- 34 muM and 3.42 +/- 0.50 mM, respectively). This non-competitive effect was fast and fully reversible for both drugs. Aspirin and salicylic acid at 500 muM were ineffective. Diclofenac and ibuprofen decreased the amplitude of proton-evoked currents and slowed the rates of current decay with a good correlation between these effects. Simultaneous application of acid solution and diclofenac was required for its inhibitory effect. Unlike amiloride, the action of diclofenac was voltage-independent and no competition between two drugs was found. Analysis of the action of diclofenac and ibuprofen on activation and desensitization of ASICs showed that diclofenac but not ibuprofen shifted the steady-state desensitization curve to more alkaline pH values. The reason for this shift was slowing down the recovery from desensitization of ASICs. Thus, diclofenac may serve as a neuroprotective agent during pathological conditions associated with acidification.

Flufenamic acid affects multiple currents and causes intracellular Ca2+ release in Aplysia bag cell neurons

Flufenamic acid (FFA) is a nonsteroidal antiinflammatory agent, commonly used to block nonselective cation channels. We previously reported that FFA potentiated, rather than inhibited, a cation current in Aplysia bag cell neurons. Prompted by this paradoxical result, the present study examined the effects of FFA on membrane currents and intracellular Ca2+ in cultured bag cell neurons. Under whole cell voltage clamp, FFA evoked either outward (I out) or inward (I in) currents. I out had a rapid onset, was inhibited by the K+ channel blocker, tetraethylammonium, and was associated with both an increase in membrane conductance and a negative shift in the whole cell current reversal potential. I in developed more slowly, was inhibited by the cation channel blocker, Gd3+, and was concomitant with both an increased conductance and positive shift in reversal potential. FFA also enhanced the use-dependent inactivation and caused a positive-shift in the activation curve of the voltage-dependent Ca2+ current. Furthermore, as measured by ratiometric imaging, FFA produced a rise in intracellular Ca2+ that persisted in the absence of extracellular Ca2+ and was reduced by depleting either the endoplasmic reticulum and/or mitochondrial stores. Ca2+ appeared to be involved in the activation of I in, as strong intracellular Ca2+ buffering effectively eliminated I in but did not alter I out. Finally, the effects of FFA were likely not due to block of cyclooxygenase given that the general cyclooxygenase inhibitor, indomethacin, failed to evoke either current. That FFA influences a number of neuronal properties needs to be taken into consideration when employing it as a cation channel antagonist.

Calcium-dependent fast depolarizing afterpotentials in vasopressin neurons in the rat supraoptic nucleus

Oxytocin (OT) and vasopressin (VP) synthesizing magnocellular cells (MNCs) in the supraoptic nucleus (SON) display distinct firing patterns during the physiological demands for these hormones. Depolarizing afterpotentials (DAPs) in these neurons are involved in controlling phasic bursting in VP neurons. Our whole cell recordings demonstrated a Cs(+)-resistant fast DAP (fDAP; decay tau = approximately 200 ms), which has not been previously reported, in addition to the well-known Cs(+)-sensitive slower DAP (sDAP; decay tau = approximately 2 s). Immunoidentification of recorded neurons revealed that all VP neurons, but only 20% of OT neurons, expressed the fDAP. The activation of the fDAP required influx of Ca(2+) through voltage-gated Ca(2+) channels as it was strongly suppressed in Ca(2+)-free extracellular solution or by bath application of Cd(2+). Additionally, the current underlying the fDAP (I(fDAP)) is a Ca(2+)-activated current rather than a Ca(2+) current per se as it was abolished by strongly buffering intracellular Ca(2+) with BAPTA. The I-V relationship of the I(fDAP) was linear at potentials less than -60 mV but showed pronounced outward rectification near -50 mV. I(fDAP) is sensitive to changes in extracellular Na(+) and K(+) but not Cl(-). A blocker of Ca(2+)-activated nonselective cation (CAN) currents, flufenamic acid, blocked the fDAP, suggesting the involvement of a CAN current in the generation of fDAP in VP neurons. We speculate that the two DAPs have different roles in generating after burst discharges and could play important roles in determining the distinct firing properties of VP neurons in the SON neurons.

Celecoxib inhibits Na+ currents in rat dorsal root ganglion neurons

Celecoxib is a selective cyclooxygenase-2 (COX-2) inhibitor used in the treatment of osteoarthritis and rheumatoid arthritis with fewer gastrointestinal toxicities compared to traditional non-steroidal anti-inflammatory drugs. Voltage-gated Na(+) channels in primary sensory neurons play an important role in the pathogenesis of various pain conditions. We examined the effects of celecoxib on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(+) currents in acutely dissociated rat dorsal root ganglion neurons. Celecoxib suppressed both currents in dose- and frequency-dependent manner. The apparent dissociation constants (K(d)) for TTX-S and TTX-R Na(+) currents measured at 0 mV from a holding potential of -80 mV were estimated to be 5.6 and 19.5 microM, respectively. Celecoxib slightly slowed inactivation kinetics of TTX-S Na(+) current, but made it much faster in TTX-R Na(+) current. Celecoxib shifted the activation voltage of TTX-S Na(+) current to a depolarizing direction, but not that of TTX-R Na(+) current. Celecoxib caused a hyperpolarizing shift of the steady-state inactivation curve in both Na(+) currents to a great extent. In addition celecoxib reduced the maximal availability of both Na(+) channels. Thus celecoxib appears to bind to both inactivated and resting Na(+) channels. Celecoxib slowed the recovery of both Na(+) channels from inactivation. All these effects combined would suppress the excitability of sensory neurons. Thus, beside COX-2 inhibition, the Na(+) channel inhibition is considered to contribute to celecoxib analgesia.

TRPM2

TRPM2 is a cation channel enabling influx of Na+ and Ca2+, leading to depolarization and increases in the cytosolic Ca2+ concentration ([Ca2+]i). It is widely expressed, e.g. in many neurons, blood cells and the endocrine pancreas. Channel gating is induced by ADP-ribose (ADPR) that binds to a Nudix box motif in the cytosolic C-terminus of the channel. Endogenous ADPR concentrations in leucocytes are sufficiently high to activate TRPM2 in the presence of an increased [Ca2+]i but probably not at resting [Ca2+]i. Another channel activator is oxidative stress, especially hydrogen peroxide (H2O2) that may act through ADPR after ADPR polymers have been formed by poly(ADP-ribose) polymerases (PARPs) and hydolysed by glycohydrolases. H2O2-stimulated TRPM2 channels essentially contribute to insulin secretion in pancreatic beta-cells and alloxan-induced diabetes mellitus. Inhibition of TRPM2 channels may be achieved by channel blockers such as flufenamic acid or the anti-fungal agents clotrimazole or econazole. Selective blockers of TRPM2 are not yet available; those would be valuable for a characterization of biological roles of TRPM2 in various tissues and as potential drugs directed against oxidative cell damage, reperfusion injury or leucocyte activation. Activation of TRPM2 may be prevented by anti-oxidants, PARP inhibitors and glycohydrolase inhibitors. In future, binding of ADPR to the Nudix box may be targeted. In light of the wide-spread expression and growing list of cellular functions of TRPM2, useful therapeutic applications are expected for future drugs that block TRPM2 channels or inhibit their activation.

The non-steroidal anti-inflammatory drug, diclofenac, inhibits Na+ current in rat myoblasts

The inhibitory effect of diclofenac, a non-steroidal anti-inflammatory drug (NSAID), on the voltage-gated inward Na+ current (I(Na)) in cultured rat myoblasts was investigated using the whole-cell voltage-clamp technique. At concentrations of 10 nM-100 microM, diclofenac produced a dose-dependent and reversible inhibition of I(Na) with an IC50 of 8.51 microM, without modulating the fast activation and inactivation process. The inhibitory effect of diclofenac took place at resting channels and increased with more depolarizing holding potential. In addition to inhibiting the Na+ current amplitude, diclofenac significantly modulated the steady-state inactivation properties of the Na+ channels, but did not alter the steady-state activation. The steady-state inactivation curve was significantly shifted towards the hyperpolarizing potential in the presence of diclofenac. Furthermore, diclofenac treatment resulted in a fairly slow recovery from inactivation of the Na+ channel. The inhibitory effect of diclofenac was enhanced by repetitive pulses and was inflected by changing frequency; the blocking effect at higher frequency was significantly greater than at lower frequency. Both intracellular and extracellular application of diclofenac could inhibit I(Na), indicating that diclofenac may exert its channel inhibitory action both inside and outside the channel sites. Our data directly demonstrate that diclofenac can inhibit the inward Na+ channels in rat myoblasts. Some different inhibitory mechanisms from that in neuronal Na+ channels are discussed.

COX-dependent mechanisms involved in the antinociceptive action of NSAIDs at central and peripheral sites

Despite the diverse chemical structure of aspirin-like drugs, the antinociceptive effect of NSAIDs is mainly due to their common property of inhibiting cyclooxygenases involved in the formation of prostaglandins. Prostaglandins are potent hyperalgesic mediators which modulate multiple sites along the nociceptive pathway and enhance both transduction (peripheral sensitizing effect) and transmission (central sensitizing effect) of nociceptive information. Inhibition of the formation of prostaglandins at peripheral and central sites by NSAIDs thus leads to the normalisation of the increased pain threshold associated with inflammation. The contribution of peripheral and central mechanisms to the overall antinociceptive action of NSAIDs depends on several factors including the location of the targets of drug action, the site of drug delivery and the uptake and distribution to the site of action. The present work reviews the data on the regulation and location of cyclooxygenases at central and peripheral sites of the nociceptive pathway and focuses on the role of COX in the generation and maintenance of pain hypersensitivity. Experimental and clinical evidences are used to evaluate the significance of the peripheral and central antihyperalgesic effects of NSAIDs.

Diclofenac, a nonsteroidal anti-inflammatory drug, activates the transient outward K+ current in rat cerebellar granule cells

Diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), has been widely investigated in terms of its pharmacological action, but less is known about its direct effect on ion channels. Here, the effect of diclofenac on voltage-dependent transient outward K+ currents (I(A)) in cultured rat cerebellar granule cells was investigated using the whole-cell voltage-clamp technique. At concentrations of 10(-5)-10(-3) M, diclofenac reversibly increased the I(A) amplitude in a dose-dependent manner and significantly modulated the steady-state inactivation properties of the I(A) channels, but did not alter the steady-state activation properties. Furthermore, diclofenac treatment resulted in a slightly accelerated recovery from I(A) channel inactivation. Intracellular application of diclofenac could mimic the effects induced by extracellular application, although once the intracellular response reached a plateau, extracellular application of diclofenac could induce further increases in the current. These observations indicate that diclofenac might exert its effects on the channel protein at both the inner and outer sides of the cell membrane. Our data provide the first evidence that diclofenac is able to activate transient outward potassium channels in neurons. Although further work will be necessary to define the exact mechanism of diclofenac-induced I(A) channel activation, this study provides evidence that the nonsteroidal anti-inflammatory drug, diclofenac, may play a novel neuronal role that is worthy of future study.

Flufenamic acid is a pH-dependent antagonist of TRPM2 channels

Like a number of other TRP channels, TRPM2 is a Ca(2+)-permeable non-selective cation channel, the activity of which is regulated by intracellular and extracellular Ca(2+). A unique feature of TRPM2 is its activation by ADP-ribose and chemical species that arise during oxidative stress, for example, NAD(+) and H(2)O(2). These properties have lead to proposals that this channel may play a role in the cell death produced by pathological redox states. The lack of known antagonists of this channel have made these hypotheses difficult to test. Here, we demonstrate, using patch clamp electrophysiology, that the non-steroidal anti-inflammatory compound flufenamic acid (FFA) inhibits recombinant human TRPM2 (hTRPM2) as well as currents activated by intracellular ADP-ribose in the CRI-G1 rat insulinoma cell line. All concentrations tested in a range from 50 to 1000 microM produced complete inhibition of the TRPM2-mediated current. Following FFA removal, a small (typically 10-15%) component of current was rapidly recovered (time constant approximately 3 s), considerably longer periods in the absence of FFA produced no further current recovery. Reapplication of FFA re-antagonised the recovered current and subsequent FFA washout produced recovery of only a small percentage of the reblocked current. Decreasing extracellular pH accelerated FFA inhibition of TRPM2. Additional experiments indicated hTRPM2 activation was required for FFA antagonism to occur and that the generation of irreversible antagonism was preceded by a reversible component of block. FFA inhibition could not be induced by intracellular application of FFA. ADP-ribose activated currents in the rat insulinoma cell line CRI-G1 were also antagonised by FFA with concentration- and pH-dependent kinetics. In contrast to the observations made with hTRPM2, antagonism of ADP-ribose activated currents in CRI-G1 cells could be fully reversed following FFA removal. These experiments suggest that FFA may be a useful tool antagonist for studies of TRPM2 function.