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.     

Additive antinociceptive effect of the combination of diazoxide, an activator of ATP-sensitive K+ channels, and sodium nitroprusside and dibutyryl-cGMP

Using the rat paw pressure test, in which increased sensitivity is induced by intraplantar injection of prostaglandin E2, we assessed the antinociceptive effect of the ATP-sensitive K+ channel opener diazoxide and the large-conductance Ca(2+)-activated K+ channel opener NS-1619 (1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl) phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one) on the peripheral hyperalgesia induced by prostaglandin E2. Diazoxide, administered locally into the right hindpaw (20, 38, 75, 150, 300 and 600 microg), elicited a dose-dependent antinociceptive effect on prostaglandin E2-induced hyperalgesia (2 microg/paw). The effect of diazoxide at the dose of 300 microg/paw was shown to be local since it did not produce any effect when administered in the contralateral paw. The action of diazoxide (300 microg/paw) as an ATP-sensitive K+ channel opener seems to be specific, since its effect was antagonized in a dose-dependent manner by glibenclamide (40, 80 and 160 microg/paw), a specific blocker of these channels, while tetraethylammonium (7.5, 15 and 30 microg/paw), dequalinium (12.5, 25 and 50 microg/paw) or charybdotoxin (0.5, 1 and 2 microg/paw), blockers of voltage-dependent K+ channels and of small- and large-conductance Ca(2+)-activated K+ channels, respectively, were not able to abolish the antinociception induced by diazoxide. The peripheral antinociceptive effect of diazoxide was not prevented by prior administration of naloxone (12.5, 25 and 50 microg/paw), an opioid receptor antagonist, or methylene blue (75, 125 and 300 microg/paw), an agent that inhibits the activation of guanylate cyclase by nitric oxide. A low dose of diazoxide (20 microg/paw) administered together with a low dose of sodium nitroprusside (125 microg/paw) or dibutyryl cGMP (db-cGMP, 50 microg/paw) induced a marked antinociceptive effect similar to that observed when each drug was administered alone. NS1619 (75, 150 and 300 microg/paw), a specific opener of large-conductance Ca(2+)-activated K+ channels, had no antinociceptive action on prostaglandin E2-induced hyperalgesia. This series of experiments provides evidence for a peripheral antinociceptive action of diazoxide and supports the suggestion that the activation of ATP-sensitive K+ channels could be the mechanism by which sodium nitroprusside and db-cGMP induce peripheral antinociception, excluding the involvement of large-contuctance Ca(2+)-activated K+ channels in the process.     

Dibutyryl-cyclic GMP induces peripheral antinociception via activation of ATP-sensitive K(+) channels in the rat PGE2-induced hyperalgesic paw

1. Using the rat paw pressure test, in which increased sensitivity is induced by intraplantar injection of prostaglandin E2, we studied the action of several K(+) channel blockers in order to determine what types of K(+) channels could be involved in the peripheral antinociception induced by dibutyrylguanosine 3 : 5'-cyclic monophosphate (DbcGMP), a membrane permeable analogue of cyclic GMP. 2. DbcGMP elicited a dose-dependent (50, 75, 100 and 200 microg paw(-1)) peripheral antinociceptive effect. The effect of the 100 microg dose of DbcGMP was considered to be local since only a higher dose (300 microg paw(-1)) produced antinociception in the contralateral paw. 3. The antinociceptive effect of DbcGMP (100 microg paw(-1)) was dose-dependently antagonized by intraplantar administration of the sulphonylureas tolbutamide (20, 40 and 160 microg) and glibenclamide (40, 80 and 160 microg), selective blockers of ATP-sensitive K(+) channels. 4. Charybdotoxin (2 microg paw(-1)), a selective blocker of high conductance Ca(2+)-activated K(+) channels, and apamin (10 microg paw(-1)), a selective blocker of low conductance Ca(2+)-activated K(+) channels, did not modify the peripheral antinociception induced by DbcGMP. 5. Tetraethylammonium (2 mg paw(-1)), 4-aminopyridine (200 microg paw(-1)) and cesium (800 paw(-1)), non-selective voltage-gated potassium channel blockers, also had no effect. 6. Based on this experimental evidence, we conclude that the activation of ATP-sensitive K(+) channels could be the mechanism by which DbcGMP induces peripheral antinociception, and that Ca(2+)-activated K(+) channels and voltage-dependent K(+) channels appear not to be involved in the process.     

Possible activation of the NO-cyclic GMP-protein kinase G-K+ channels pathway by gabapentin on the formalin test

The effect of modulators of the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway on the local peripheral antinociceptive action induced by gabapentin was assessed in the rat 1% formalin test. Local peripheral administration of gabapentin produced a dose-dependent antinociception in the second phase of the test. Gabapentin-induced antinociception was due to a local action as its administration in the contralateral paw was ineffective. Local peripheral pretreatment of the paws with NG-L-nitro-arginine methyl ester (L-NAME, a nitric oxide synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) and KT-5823 (a protein kinase G inhibitor) dose-dependently reduced gabapentin-induced antinociception. Likewise, glibenclamide or tolbutamide (ATP-sensitive K+ channel inhibitors), 4-aminopyridine or tetraethylammonium (non-selective inward rectifier K+ channel inhibitors) or charybdotoxin (large-conductance Ca2+-activated-K+ channel blocker), but not apamin (small-conductance Ca2+-activated-K+ channel blocker) or naloxone (opioid receptor antagonist), reduced the antinociception induced by gabapentin. Our data suggest that gabapentin could activate the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway in order to produce its peripheral antinociceptive effect in the rat 1% formalin test.     

Delta-opioid receptor agonist SNC80 induces peripheral antinociception via activation of ATP-sensitive K+ channels

We investigated the effect of several K+ channel blockers on the antinociception induced by delta-opioid receptor agonist SNC80 using the paw pressure test, in which pain sensitivity is increased by an intraplantar injection (2 microg) of prostaglandin E2 (PGE2). Administration of SNC80 (20, 40 and 80 microg/paw) caused a decrease in the hyperalgesia induced by PGE2, in a dose-dependent manner. The possibility of higher dose of SNC80 (80 microg) causing a central or systemic effect was excluded since administration of the drug into the contralateral paw did not elicit antinociception in the right paw. Specific blockers of ATP-sensitive K+ channels, glibenclamide (20, 40 and 80 microg/paw) and tolbutamide (40, 80 and 160 microg/paw), antagonized the peripheral antinociception induced by SNC80 (80 microg). On the other hand, charybdotoxin (2 microg/paw), a large-conductance blocker of Ca(2+)-activated K+ channels, and dequalinium (50 microg/paw), a small conductance selective blocker of Ca(2+)-activated K+ channels, did not modify the effect of SNC80. This effect also remained unaffected by intraplantar administration of the voltage-dependent K+ channel blockers tetraethylammonium (30 microg/paw) and 4-aminopyridine (10 microg/paw), and of a non-specific K+ channel blocker, cesium (500 microg/paw). This study provides evidence that the peripheral antinociceptive effect of SNC80 result from the activation of ATP-sensitive K+ channels, and the other K+ channels are not involved.     

The peripheral antinociceptive effect induced by morphine is associated with ATP-sensitive K(+) channels

The effect of several K(+) channel blockers such as glibenclamide, tolbutamide, charybdotoxin (ChTX), apamin, tetraethylammonium (TEA), 4-aminopyridine (4-AP) and cesium on the peripheral antinociceptive effect of morphine was evaluated by the paw pressure test in Wistar rats. The intraplantar administration of a carrageenan suspension (250 microg) resulted in an acute inflammatory response and a decreased threshold to noxious pressure. Morphine administered locally into the paw (25, 50, 100 and 200 microg) elicited a dose-dependent antinociceptive effect which was demonstrated to be mediated by a peripheral site up to the 100 microg dose. The selective blockers of ATP-sensitive K(+) channels glibenclamide (20, 40 and 80 microg paw(-1)) and tolbutamide (40, 80 and 160 microg paw(-1)) antagonized the peripheral antinociception induced by morphine (100 microg paw(-1)). This effect was unaffected by ChTX (0. 5, 1.0 and 2.0 microg paw(-1)), a large conductance Ca(2+)-activated K(+) channel blocker, or by apamin (2.5, 5.0 and 10.0 microg paw(-1)), a selective blocker of a small conductance Ca(2+)-activated K(+) channel. Intraplantar administration of the non-specific K(+) channel blockers TEA (160, 320 and 640 microg), 4-AP (10, 50 and 100 microg) and cesium (125, 250 and 500 microg) also did not modify the peripheral antinociceptive effect of morphine. These results suggest that the peripheral antinociceptive effect of morphine may result from activation of ATP-sensitive K(+) channels, which may cause a hyperpolarization of peripheral terminals of primary afferents, leading to a decrease in action potential generation. In contrast, large conductance Ca(2+)-activated K(+) channels, small conductance Ca(2+)-activated K(+) channels as well as voltage-dependent K(+) channels appear not to be involved in this transduction pathway. British Journal of Pharmacology (2000) 129, 110 - 114     

Pharmacological evidence for the activation of Ca2+-activated K+ channels by meloxicam in the formalin test

The possible participation of K+ channels in the antinociceptive action of meloxicam was assessed in the 1% formalin test. Local peripheral administration of meloxicam produced a dose-dependent antinociception only during the second phase of the formalin test. K+ channel blockers alone did not modify formalin-induced nociceptive behavior. However, local peripheral pretreatment of the paw with charybdotoxin and apamin (large- and small-conductance Ca2+-activated K+ channel inhibitors, respectively), 4-aminopyridine and tetraethylammonium (non-selective voltage-dependent K+ channel inhibitors), but not glibenclamide or tolbutamide (ATP-sensitive K+ channel inhibitors), dose-dependently prevented meloxicam-induced antinociception. It is concluded that meloxicam could open large- and small-conductance Ca2+-activated K+ channels, but not ATP-sensitive K+ channels, in order to produce its peripheral antinociceptive effect in the formalin test. The participation of voltage-dependent K+ channels was also suggested, but since non-selective inhibitors were used the data await further confirmation.     

Possible involvement of potassium channels in peripheral antinociception induced by metamizol: lack of participation of ATP-sensitive K+ channels

The present work assessed the possible participation of K+ channels in the peripheral antinociceptive action of metamizol in the 1% formalin test. Ipsilateral, but not contralateral, local peripheral administration of metamizol produced a dose-dependent antinociception only during the second phase of the formalin test. K+ channel blockers alone did not modify formalin-induced nociceptive behavior. However, local peripheral pretreatment of the paw with charybdotoxin and apamin (large- and small-conductance Ca(2+)-activated K+ channel blockers, respectively), 4-aminopyridine and tetraethylammonium (voltage-dependent K+ channel inhibitors), but not glibenclamide or tolbutamide (ATP-sensitive K+ channel inhibitors), dose-dependently prevented metamizol-induced antinociception. The above results suggest that metamizol could open large- and small-conductance Ca(2+)-activated K+ channels, but not ATP-sensitive K+ channels, in order to produce its peripheral antinociceptive effect in the formalin test. The participation of voltage-dependent K+ channels was also suggested, but since nonselective inhibitors were used, the data await further confirmation.     

Involvement of ATP-sensitive K(+) channels in the peripheral antinociceptive effect induced by dipyrone

We evaluated the effect of several K(+) channel blockers on the peripheral antinociception induced by dipyrone using the rat paw pressure test, in which sensitivity is increased by intraplantar injection (2 micro g) of prostaglandin E(2). Dipyrone administered locally into the right hindpaw (50, 100 and 200 micro g) elicited a dose-dependent antinociceptive effect which was demonstrated to be local, since only higher doses produced an effect when injected in the contralateral paw. The specific blockers of ATP-sensitive K(+) channels glibenclamide (40, 80 and 160 micro g/paw) and tolbutamide (80, 160 and 320 micro g/paw) antagonized the peripheral antinociception induced by dipyrone (200 micro g/paw). Charybdotoxin (2 micro g/ paw), a blocker of large conductance Ca(2+)-activated K(+) channels, and dequalinium (50 micro g/paw), a selective blocker of small conductance Ca(2+)-activated K(+) channels, did not modify the effect of dipyrone. This effect was also unaffected neither by intraplantar administration of non-specific voltage-dependent K(+) channel blockers tetraethylammonium (1700 micro g) and 4-aminopyridine (100 micro g) nor cesium (500 micro g), a non-specific K(+) channel blocker. These results suggest that the peripheral antinociceptive effect of dipyrone may result from activation of ATP-sensitive K(+) channels, while other K(+) channels appear not to be involved in the process.     

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.     

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.     

Activation of ATP-sensitive K(+) channels: mechanism of peripheral antinociceptive action of the nitric oxide donor, sodium nitroprusside

Using the rat paw pressure test, in which sensitivity is increased by intraplantar injection of prostaglandin E(2) (PGE(2)), we conducted a study using several K(+) channel blockers. The objective was to determine what types of K(+) channels could be involved in the peripheral antinociceptive action of the nitric oxide donor sodium nitroprusside (SNP). SNP elicited a dose-dependent (250 and 500 microgram/paw) peripheral antinociceptive effect, which was considered local, since only higher doses produced an effect in the contralateral paw. The effect of SNP (500 microgram/paw) was dose-dependently antagonized by intraplantar administration of the sulfonylureas tolbutamide (20, 40 and 160 microgram) and glibenclamide (40, 80 and 160 microgram), selective blockers of ATP-sensitive K(+) channels. Charybdotoxin (2 microgram/paw), a selective blocker of high conductance Ca(2+)-activated K(+) channels, and apamin (10 microgram/paw), a selective blocker of low conductance Ca(2+)-activated K(+) channels, did not modify the peripheral antinociception induced by SNP. Tetraethylammonium (2 mg/paw), 4-aminopyridine (200 microgram/paw) and cesium (800 microgram/paw) also had no effect. Based on this experimental evidence, we conclude that the activation of ATP-sensitive K(+) channels could be the mechanism by which nitric oxide, donated by SNP, induces peripheral antinociception, and that Ca(2+)-activated K(+) channels and voltage-dependent K(+) channels appear not to be involved in the process.     

Study of the involvement of K+ channels in the peripheral antinociception of the kappa-opioid receptor agonist bremazocine

The involvement of the nitric oxide (NO)/cyclic GMP pathway in the molecular mechanisms of antinociceptive drugs like morphine has been previously shown by our group. Additionally, it is known that the desensitisation of nociceptors by K(+) channel opening should be the final target for several analgesic drugs including nitric oxide donors and exogenous micro-opioid receptor agonists. In our previous study, we demonstrated that bremazocine, a kappa-opioid receptor agonist, induces peripheral antinociception by activating nitric oxide/cyclic GMP pathway. In the current study, we assessed whether bremazocine is capable to activate K(+) channels eliciting antinociception. Bremazocine (20, 40 and 50 microg) dose-dependently reversed the hyperalgesia induced in the rat paw by local injection of carrageenan (250 microg) or prostaglandin E(2) (2 microg), measured by the paw pressure test. Using the selective kappa-opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 200 microg/paw), it was confirmed that bremazocine (50 microg/paw) acts specifically on the kappa-opioid receptors present at peripheral sites. Prior treatment with the ATP-sensitive K(+) channel blockers glibenclamide (40, 80 and 160 microg) and tolbutamide (40, 80 and 160 microg) did not antagonise the antinociceptive effect of bremazocine (50 microg). The same results were obtained when we used prostaglandin E(2) (2 microg) as the hyperalgesic stimulus. The supposed participation of other types of K(+) channels was tested using the Ca(2+)-activated K(+) channel blockers dequalinium (12.5, 25 and 50 microg) and charybdotoxin (0.5, 1 and 2 microg) and different types of the non-selective K(+) channel blockers tetraethylammonium (25, 50 and 100 microg) and 4-aminopyridine (10, 25 and 50 microg). None of the K(+) channel blockers reversed the antinociceptive effect of bremazocine. On the basis of these results, we suggest that K(+) channels are not involved in the peripheral antinociceptive effect of bremazocine, although this opioid receptor agonist induces nitric oxide/cGMP pathway activation.     

The peripheral antinociceptive effect of resveratrol is associated with activation of potassium channels

The possible participation of K(+) channels in the antinociceptive action induced by resveratrol was assessed in the 1% formalin test. Local administration of resveratrol produced a dose-dependent antinociception in the second phase of the test. The antinociception produced by resveratrol was due to a local action as its administration in the contralateral paw was not active. Local pretreatment of the injured paw with glibenclamide, tolbutamide or glipizide (ATP-sensitive K(+) channel inhibitors) did not modify resveratrol-induced antinociception. In contrast, charybdotoxin and apamin (large and small conductance Ca(2+) activated-K(+) channel blockers, respectively), 4-aminopyridine or tetraethylammonium (voltage-dependent K(+) channel inhibitors) dose-dependently prevented resveratrol-induced antinociception. Local peripheral administration of glibenclamide, but not charybdotoxin or apamin, significantly reduced the antinociceptive effect produced by peripheral morphine (positive control). At the highest effective doses, none of the drugs used induced behavioral side effects as revealed by the evaluation of stepping, righting, corneal and pinna reflexes. In addition, when given alone, none of the inhibitors modified the nociceptive behavior induced by 1% formalin. The results suggest that resveratrol opens large and small conductance Ca(2+)-activated K(+) channels, but not ATP-sensitive K(+) channels, in order to produce its peripheral antinociceptive effect in the formalin test. The participation of voltage-dependent K(+) channels was also suggested, but since non-selective inhibitors were used the data awaits further confirmation.     

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.     

N-palmitoyl-ethanolamine (PEA) induces peripheral antinociceptive effect by ATP-sensitive K+-channel activation

Although the antinociceptive effects of N-palmitoyl-ethanolamine (PEA) were first characterized nearly 50 years ago, the identity of the mechanism that mediates these actions has not been elucidated. The present study investigated the contribution of K(+) channels on peripheral antinociception induced by the CB(2) agonist PEA. Nociceptive thresholds to mechanical paw stimulation of Wistar rats treated with intraplantar prostaglandin E(2) to induce hyperalgesia were measured, and other agents were also given by local injection. PEA (5, 10, and 20 μg/paw) elicited a local peripheral antinociceptive effect. This effect was antagonized by glibenclamide, a selective blocker of ATP-sensitive K(+) channels (20, 40, and 80 μg/paw). In addition, neither the voltage-dependent K(+) channel-specific blocker tetraethylammonium (30 μg/paw) nor the small and large conductance blockers of Ca(2+)-activated K(+) channels, dequalinium (50 μg/paw) and paxilline (20 μg/paw), respectively, were able to block the local antinociceptive effect of PEA. These results indicate that the activation of ATP-sensitive K(+) channels could be the mechanism that induces peripheral antinociception by PEA and that voltage-dependent K(+) channels and small and large conductance Ca(2+)-activated K(+) channels do not appear to be involved in this mechanism.     

Sildenafil,a phosphodiesterase-5 inhibitor,enhances the antinociceptive effect of morphine

Various evidence has demonstrated a role of the nitric oxide (NO)/cGMP signaling pathway in the processing of nociception. The exact role of phosphodiesterase-5 (PDE-5) via the NO/cGMP pathway is not fully understood in pain response. The aim of the present study was to investigate the possible peripheral interaction between a PDE-5 inhibitor (sildenafil) and morphine. Carrageenan-induced hyperalgesia in rats and the acetic-acid-induced writhing test in mice were used as animal models. Local administration of sildenafil (50-200 microg/paw, i.pl.) exhibited a dose-dependent antinociceptive effect against the paw pressure test. Sildenafil also demonstrated an antinociceptive effect (1-10 mg/kg, i.p.) against in the writhing test. Co-administration of sildenafil (100 microg/paw, i.pl. and 2 mg/kg, i.p.) significantly enhanced the antinociceptive effect of morphine (2 microg/ paw, i.pl. and 2 mg/kg, i.p respectively). The antinociception produced by the drugs alone or combined was due to a local action, as its administration in the contralateral paws was ineffective. Pretreatment with N(G)-nitro-L-arginine methyl ester (an NO synthesis inhibitor), methylene blue (gunalyl cyclase inhibitor) or naloxone (opioid receptor antagonist) blocked the effect of a sildenafil-morphine combination in both tests. The results suggest that opioid receptor (NO and cGMP) mechanisms are involved in the combined antinociceptive effect. Further, sildenafil produced antinociception per se and increased the response of morphine, probably through the inhibition of cGMP degradation.     

Baclofen, an agonist at peripheral GABAB receptors, induces antinociception via activation of TEA-sensitive potassium channels

BACKGROUND AND PURPOSE: Central anti-nociceptive actions of baclofen involve activation of K+ channels. Here we assessed what types of K+ channel might participate in the peripheral anti-nociception induced by baclofen.Experimental approach:Nociceptive thresholds to mechanical stimulation in rat paws treated with intraplantar prostaglandin E2.(PGE2) to induce hyperalgesia were measured 3 h after PGE2 injection. Other agents were also given by intraplantar injection. KEY RESULTS: Baclofen elicited a dose-dependent (15 - 240 microg per paw) anti-nociceptive effect. An intermediate dose of baclofen (60 microg) did not produce antinociception in the contralateral paw, showing its peripheral site of action. The GABAB receptor antagonist saclofen (12.5 - 100 microg per paw) antagonized, in a dose-dependent manner, peripheral antinociception induced by baclofen (60 microg), suggesting a specific effect. This antinociceptive action of baclofen was unaffected by bicuculline, GABAA receptor antagonist (80 microg per paw), or by (1,2,5,6 tetrahydropyridin-4-yl) methylphosphinic acid, GABAC receptor antagonist (20 microg per paw). The peripheral antinociception induced by baclofen (60 microg) was reversed, in a dose-dependent manner, by the voltage-dependent K+ channel blockers tetraethylammonium (7.5 - 30 microg per paw) and 4-aminopyridine (2.5 - 10 microg per paw). The blockers of other K+ channels, glibenclamide (160 microg), tolbutamide (320 microg), charybdotoxin (2 microg), dequalinium (50 microg) and caesium (500 microg) had no effect. CONCLUSIONS AND IMPLICATIONS: This study provides evidence that the peripheral antinociceptive effect of the GABAB receptor agonist baclofen results from the activation of tetraethylammonium-sensitive K+ channels. Other K+ channels appear not to be involved.     

Additive interaction between peripheral and central mechanisms involved in the antinociceptive effect of diclofenac in the formalin test in rats

It has been proposed that the antinociception of systemic diclofenac is the outcome of peripheral and central actions. Hence, our purpose was to examine if systemic diclofenac is able to achieve effective concentrations at local and spinal sites and to characterize the interaction between its local and spinal actions. Pain was produced in the rat using the formalin test. Oral diclofenac (1-10 mg/kg) reduced formalin-induced pain. The antinociceptive effect of oral diclofenac (10 mg/kg) was abolished by local or spinal administration of either L-NAME (1-100 microg and 1-50 microg) or glibenclamide (12.5-100 microg and 25-75 microg). These results suggest that oral diclofenac achieves effective concentrations producing an antinociceptive effect involving participation of the NO-potassium channel pathway at both, the local and spinal levels. In an additional experimental series, diclofenac was administered either locally (25-200 mug) or spinally (12.5-100 mug), yielding an antinociceptive effect by both routes. Then, diclofenac was given simultaneously by these two routes in a fixed-ratio, and antinociception was assayed. Isobolographic analysis revealed an additive interaction between the local and spinal effects of diclofenac. Hence, our results provide evidence that the overall antinociceptive effect induced by systemic diclofenac is the outcome of central and peripheral mechanisms.     

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.