Jorge E Torres-López

Pharmacological evidence for the activation of K+ channels by diclofenac

The involvement of K(+) channels in the antinociceptive action of diclofenac was assessed in the formalin test. Local administration of diclofenac produced a dose-dependent antinociceptive effect due to a local action because drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with glibenclamide and tolbutamide (ATP-sensitive K(+) channel inhibitors), charybdotoxin and apamin (large- and small-conductance Ca(2+)-activated K(+) channel blockers, respectively), 4-aminopyridine or tetraethylammonium (voltage-dependent K(+) channel inhibitors) prevented diclofenac-induced antinociception. Given alone, K(+) channel inhibitors did not modify formalin-induced nociceptive behavior. Pinacidil (an ATP-sensitive K(+) channel opener) also produced antinociception which was blocked by glibenclamide. The peripheral antinociceptive effect of morphine (positive control) was blocked by glibenclamide and 4-aminopyridine but not by charybdotoxin or apamin. The results suggest that the peripheral antinociceptive effect of diclofenac may result from the activation of several types of K(+) channels, which may cause hyperpolarization of peripheral terminals of primary afferents.

Participation of the nitric oxide–cyclic GMP–ATP-sensitive K+ channel pathway in the antinociceptive action of ketorolac

The involvement of nitric oxide (NO), cyclic GMP and ATP-sensitive K(+) channels in the antinociceptive effect of ketorolac was assessed using the formalin test in the rat. Local administration of ketorolac in a formalin-injured paw produced a dose-dependent antinociceptive effect due to a local action, as drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with N(G)-L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) or glibenclamide (an ATP-sensitive K(+) channel blocker) prevented ketorolac-induced antinociception. However, pretreatment with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not block antinociception. Local administration of S-nitroso-N-acetylpenicillamine (SNAP, an NO donor) was inactive by itself, but increased the effect of ketorolac. The present results suggest that the antinociceptive effect of ketorolac involves activation of the NO-cyclic GMP pathway, followed by an opening of ATP-sensitive K(+) channels at the peripheral level.

Comparison of the antinociceptive effect of celecoxib, diclofenac and resveratrol in the formalin test

The peripheral antinociceptive effect of the selective COX-2 inhibitor celecoxib in the formalin-induced inflammatory pain was compared with that of resveratrol (COX-1 inhibitor) and diclofenac (non-selective COX inhibitor). Rats received local pretreatment with saline, celecoxib, diclofenac or resveratrol followed by 50 microl of either 1% or 5% formalin. Peripheral administration of celecoxib did not produce antinociception at either formalin concentration. In contrast, diclofenac and resveratrol produced a dose-dependent antinociceptive effect in the second phase of both 1% and 5% formalin test. The peripheral antinociception produced by diclofenac or resveratrol was due to a local action, as drug administration in the contralateral paw was ineffective. Results indicate that the selective COX-2 inhibitor celecoxib does not produce peripheral antinociception in formalin-induced inflammatory pain. In contrast, selective COX-1 and non-selective COX inhibitors (resveratrol and diclofenac, respectively) are effective drugs in this model of pain.

Sildenafil increases diclofenac antinociception in the formalin test

The antinociceptive activity of an inhibitor of phosphodiesterase 5, alone or combined with diclofenac, was assessed in the formalin test. Local administration of diclofenac produced a significant antinociception in both phases of the formalin test in female Wistar rats. In contrast, 1-[4-ethoxy-3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo [3,4-d]pyrimidin-5-yl)phenylsulfonyl]-4-methyl piperazine (sildenafil, an inhibitor of phosphodiesterase 5) produced significant antinociception, only during the second phase of the formalin test. Non-effective doses of sildenafil (25-100 microg/paw) significantly increased the antinociceptive effect of an inactive dose of diclofenac (25 microg) in both phases of the test. The antinociception produced by the drugs alone or the combination was due to a local action, as its administration in the contralateral paw was ineffective. Since sildenafil is a potent and selective inhibitor of phosphodiesterase 5, our results suggest that this drug produced its antinociceptive activity, and increased that of diclofenac, probably through the inhibition of cyclic GMP degradation.

Peripheral Antinociceptive Action of Morphine and the Synergistic Interaction with Lamotrigine

Background Lamotrigine inhibits glutamate release through the preferential blockade of voltage-dependent Na+ channels. In contrast, morphine reduces release of excitatory amino acids through the activation of opioid receptors and also inhibits tetrodotoxin-resistant Na+ channels on peripheral afferent neurons. The current study was designed to investigate the antinociceptive effects of locally administered morphine and lamotrigine. The interaction between morphine and lamotrigine at the periphery was also examined. Methods Morphine, lamotrigine, or a combination of morphine and lamotrigine was administered locally to female Wistar rats, and the antinociceptive effect was determined in the formalin test. Isobolographic analyses were used to define the nature of the functional interactions between morphine and lamotrigine. Results Peripheral administration of either morphine or lamotrigine produced a dose-related antinociceptive effect. Isobolographic analyses revealed that peripheral morphine and lamotrigine interacted synergistically in the formalin test. Conclusions The study shows a functional interaction between lamotrigine and morphine at the peripheral level.

Role of NHE1 in Nociception

Intracellular pH is a fundamental parameter to cell function that requires tight homeostasis. In the absence of any regulation, excessive acidification of the cytosol would have the tendency to produce cellular damage. Mammalian Na+/H+ exchangers (NHEs) are electroneutral Na+-dependent proteins that exchange extracellular Na+ for intracellular H+. To date, there are 9 identified NHE isoforms where NHE1 is the most ubiquitous member, known as the housekeeping exchanger. NHE1 seems to have a protective role in the ischemia-reperfusion injury and other inflammatory diseases. In nociception, NHE1 is found in neurons along nociceptive pathways, and its pharmacological inhibition increases nociceptive behavior in acute pain models at peripheral and central levels. Electrophysiological studies also show that NHE modulates electrical activity of primary nociceptive terminals. However, its role in neuropathic pain still remains controversial. In humans, NHE1 may be responsible for inflammatory bowel diseases since its expression is reduced in Crohns disease and ulcerative colitis. The purpose of this work is to provide a review of the evidence about participation of NHE1 in the nociceptive processing.

Peripheral and spinal TRPA1 channels contribute to formalin-induced long-lasting mechanical hypersensitivity

Background Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel expressed by a subset of nociceptive neurons that acts as a multimodal receptor. Its activity contributes to modulate nociceptive transmission in acute inflammatory pain. However, the role of this channel in chronic pain has been less studied. The purpose of this study was to investigate the local peripheral and spinal participation of TRPA1 channels in formalin-induced long-lasting hypersensitivity. Materials and methods Formalin (1%)-induced chronic hypersensitivity was determined by the application of von Frey filaments to ipsilateral and contralateral paws and through pharmacological testing using a selective TRPA1 blocker (A-967079). TRPA1 expression in the dorsal root ganglion (DRG) and spinal cord was analyzed by Western blotting. Results Formalin (1%) injection produced acute flinching behavior (1 h) as well as secondary allodynia and hyperalgesia (12 days). Local peripheral pretreatment (10 min before) or posttreatment (6 days later) with A-967079 (1–100 µM) partially prevented and reversed, respectively, in a dose-dependent manner, long-lasting secondary mechanical allodynia and hyperalgesia evoked by 1% formalin. Likewise, intrathecal pretreatment or posttreatment with A-967079 partially prevented and reversed, respectively, formalin-induced long-lasting hypersensitivity. A-967079 (100 µM) completely abolished the pro-nociceptive effect of formalin (adjusted to pH 7.4). Finally, formalin injection increased TRPA1 protein expression in the DRG and spinal cord. Conclusion Results indicate that TRPA1 expressed in the DRG and spinal cord plays a relevant role in formalin-induced long-lasting secondary nociceptive hypersensitivity.

Pharmacokinetic–pharmacodynamic modeling considering spinal and peripheral actions of nonsteroidal antiinflammatory drugs to optimize the treatment of inflammation-induced pain

It has been documented that nonsteroidal antiinflammatory drugs (NSAIDs) exhibit pharmacological actions at both the peripheral and central levels. However, the actual participation of the mechanisms of action elicited at different anatomical sites after systemic NSAID administration is not clear. To gain further knowledge on this issue, the aim of the present work is to study the pharmacodynamics and pharmacokinetics of diclofenac, a NSAID prototype, using an integrative approach. n nWe have previously documented that local diclofenac administration at the site of injury induces antinociception in the formalin test, as well as in the pain-induced functional impairment model in the rat. This effect is reduced by nitric oxide (NO) and cyclic GMP synthesis inhibitors, as well as by potassium channel blockers [1,2]. We therefore assayed the effect of oral (systemic) diclofenac in the formalin test after pretreatment with either NG-nitro-L-arginine Methyl ester (L-NAME), a NO synthesis blocker, or glibenclamide, a potassium channel blocker, given by two routes of administration: locally at the site of injury and intrathecally. L-NAME and glibenclamide given by these two routes significantly reduced oral diclofenac antinociception. These results suggest that, after systemic administration, effective diclofenac concentrations are achieved at the site of injury as well as at the spinal cord, and that in these two sites of action there is a participation of the L-arginine–NO–cyclic GMP–potassium channel pathway. n nIn a second series of experiments, diclofenac was administered locally at the site of injury (peripheral location), intrathecally, and simultaneously at the site of injury, intrathecally. Isobolographic analysis showed that there is an additive interaction between the effects at the peripheral and spinal levels. It is then likely that, after systemic administration, the observed antinociceptive effect is the result of the sum of peripheral and central mechanisms. Thus, effective diclofenac concentrations at central sites achieved after systemic administration probably are considerably lower that those required to observe an antinociceptive effect after direct injection, due to the interaction of mechanisms elicited at different anatomical locations. Therefore, from a pharmacokinetic point of view, the various central and peripheral sites of action can be considered as the affected compartment. n nIn a third experimental series, the pharmacokinetics and antinociceptive effect of diclofenac was assayed in the pain-induced functional impairment model in the rat, which allows a simultaneous determination of antinociception and blood drug concentration [3]. It appeared that there was no direct relationship between diclofenac in blood and antinociception. Antinociception, however, was significantly related to affect compartment concentrations estimated by pharmacokinetic–pharmacodynamic modeling, consistently with the earlier described pharmacodynamic results. Computer simulations using the derived pharmacokinetic–pharmacodynamic model showed that an oral formulation of diclofenac yielding a very fast absorption, and hence a high blood concentration peak, can optimize drug transfer to the affected compartment, resulting in a fast-onset, long-lasting antinociceptive response. n nOur results show that the study of NSAID pharmacodynamics and pharmacokinetics using an integrative approach is useful for the characterization of the mechanisms of action involved after systemic drug administration, yielding information that allows the optimization of dosing regimens.