Nitric oxide and potassium channels but not opioid and cannabinoid receptors mediate tramadol-induced peripheral antinociception in rat model of paw pressure withdrawal.

Tramadol, an analgesic classified as an "atypical opioid", exhibits both opioid and non-opioid mechanisms of action. This study aimed to explore these mechanisms, specifically the opioid-, cannabinoid-, nitric oxide-, and potassium channel-based mechanisms, which contribute to the peripheral antinociception effect of tramadol, in an experimental rat model. The nociceptive threshold was determined using paw pressure withdrawal. To examine the mechanisms of action, several substances were administered intraplantarly: naloxone, a non-selective opioid antagonist (50 µg/paw); AM251 (80 µg/paw) and AM630 (100 µg/paw) as the selective antagonists for types 1 and 2 cannabinoid receptors, respectively; nitric oxide synthase inhibitors L-NOArg, L-NIO, L-NPA, and L-NIL (24 µg/paw); and the enzyme inhibitors of guanylatocyclase and phosphodiesterase of cGMP, ODQ, and zaprinast. Additionally, potassium channel blockers glibenclamide, tetraethylammonium, dequalinium, and paxillin were used. The results showed that opioid and cannabinoid receptor antagonists did not reverse tramadol's effects. L-NOarg, L-NIO, and L-NPA partially reversed antinociception, while ODQ completely reversed, and zaprinast enhanced tramadol's antinociception effect. Notably, glibenclamide blocked tramadol's antinociception in a dose-dependent manner. These findings suggest that tramadol's peripheral antinociception effect is likely mediated by the nitrergic pathway and sensitive ATP potassium channels, rather than the opioid and cannabinoid pathways.

The antipsychotic aripiprazole induces peripheral antinociceptive effects through PI3Kγ/NO/cGMP/KATP pathway activation.

BCKGROUND: Aripiprazole is an antipsychotic drug used to treat schizophrenia and bipolar disorder. Recently, its peripheral analgesic component was evaluated, however, the mechanism involved in this effect is not fully established. Therefore, the aim of the study was to obtain pharmacological evidence for the involvement of the nitric oxide system in the peripheral antinociceptive effect induced by aripiprazole. METHODS: The hyperalgesia was induced via intraplantar injection of prostaglandin E2 in mice and the nociceptive thresholds were evaluated using the paw pressure test. All drugs were injected locally into the right hind paw. RESULTS: The PI3K inhibitor (AS605240), but not rapamycin (mTOR kinase inhibitor), reversed the peripheral antinociceptive effect induced by Aripiprazole. Antinociception was antagonized by the non-selective inhibitor of the nitric oxide synthase (L-NOarg). The same response was observed using the selective iNOS, but not with the selective nNOS inhibitors. The selective guanylyl cyclase enzyme inhibitor (ODQ) and the non-selective potassium channel blocker tetraethylammonium were able to reverse the antinociceptive effect of aripiprazole. The same was seen using glibenclamide, an ATP-dependent K+ channel blocker. However, calcium-activated potassium channel blockers of small and high conductance, dequalinium chloride and paxilline, respectively, did not reverse this effect. The injection of cGMP-specific phosphodiesterase type 5 inhibitor zaprinast, potentiated the antinociceptive effect induced by a low dose of aripiprazole. CONCLUSION: The results provide evidence that aripiprazole induces peripheral antinociceptive effects via PI3K/NO/cGMP/KATP pathway activation.

Endogenous opioid and cannabinoid systems modulate the muscle pain: A pharmacological study into the peripheral site.

The participation of the peripheral opioid and cannabinoid endogenous systems in modulating muscle pain and inflammation has not been fully explored. Thus, the aim of this study was to investigate the involvement of these endogenous systems during muscular-tissue hyperalgesia induced by inflammation. Hyperalgesia was induced by carrageenan injection into the tibialis anterior muscles of male Wistar rats. We padronized an available Randal-Sellito test adaptation to evaluate nociceptive behavior elicited by mechanical insult in muscles. Western blot analysis was performed to evaluate the expression levels of opioid and cannabinoid receptors in the dorsal root ganglia. The non-selective opioid peptide receptor antagonist (naloxone) and the selective mu opioid receptor MOP (clocinnamox) and kappa opioid receptor KOP (nor-binaltorphimine) antagonists were able to intensify carrageenan-induced muscular hyperalgesia. On the other hand, the selective delta opioid receptor (DOP) antagonist (naltrindole) did not present any effect on nociceptive behavior. Moreover, the selective inhibitor of aminopeptidases (Bestatin) provoked considerable dose-dependent analgesia when intramuscularly injected into the hyperalgesic muscle. The CB1 receptor antagonist (AM251), but not the CB2 receptor antagonist (AM630), intensified muscle hyperalgesia. All irreversible inhibitors of anandamide hydrolase (MAFP), the inhibitor for monoacylglycerol lipase (JZL184) and the anandamide reuptake inhibitor (VDM11) decreased carrageenan-induced hyperalgesia in muscular tissue. Lastly, MOP, KOP and CB1 expression levels in DRG were baseline even after muscular injection with carrageenan. The endogenous opioid and cannabinoid systems participate in peripheral muscle pain control through the activation of MOP, KOP and CB1 receptors.

Role of peripheral 5-HT1D, 5-HT3 and 5-HT7 receptors in the mechanical allodynia induced by serotonin in mice.

Serotonin (5-HT) acts as a neurotransmitter in the central nervous system (CNS) and as a mediator released by enterochromaffin cells to regulate intestinal motility. However, this amine also plays an important role as an inflammatory mediator and induces phenotypic changes of nociceptors. Despite the wide knowledge of the role of 5-HT in nociception, most studies have focused on its role in the CNS, while a clear information about its role in peripheral tissues is still lacking. In the present study, we investigated the role of peripheral 5-HT receptors in the nociceptive response induced by 5-HT or carrageenan in mice by using antagonists that target different 5-HT receptors. Mechanical nociceptive threshold was measured with an analgesimeter and evaluated after intraplantar (i.pl.) injection of 5-HT or carrageenan. 5-HT antagonists were injected via the i.pl. route. 5-HT (10, 20, 40 or 80 µg/paw) or carrageenan (100 µg/paw) induced mechanical allodynia. Pretreatment with isamoltane (5 µg; 5-HT1B antagonist) or ketanserine (1 µg; 5-HT2A antagonist) did not affect the mechanical allodynia induced by 5-HT. This response was inhibited by BRL 15572 (10 µg; 5-HT1D antagonist) or SB 269970 (25 µg; 5-HT7 antagonist). On the other hand, mechanical allodynia induced by 5-HT or carrageenan was exacerbated by ondansetron (10, 20 or 40 µg; 5-HT3 antagonist). The results indicate that activation of 5-HT1D and 5-HT7 receptors plays a role in the mechanical allodynia induced by 5-HT in mice. This study also demonstrates the inhibitory role of peripheral 5-HT3 receptors in the nociceptive response induced by 5-HT or carrageenan.

Role of Endocannabinoid System in the Peripheral Antinociceptive Action of Aripiprazole.

BACKGROUND: Recently, we demonstrated that the antipsychotic dopaminergic and serotoninergic agonist aripiprazole induced peripheral antinociception. However, the mechanism underlying this effect has not been fully established. Here, our aim was to identify possible relationships between this action of aripiprazole and the endocannabinoid system. METHODS: All drugs were given locally into the right hind paw of male Swiss mice weighing 30-35 g in a volume of 20 µL. The hyperalgesia was induced by intraplantar injection of prostaglandin E2 (2 µg). Aripiprazole was injected 10 minutes before the measurement, and an irreversible inhibitor of anandamide hydrolase (MAFP), an inhibitor for monoacylglycerol lipase (JZL184), and an anandamide reuptake inhibitor (VDM11) were given 10 minutes before the aripiprazole. Nociceptive thresholds were measured using an algesimetric apparatus in the third hour after prostaglandin E2 injection. Data were analyzed by ANOVA and Bonferroni tests. RESULTS: The antinociceptive effect induced by aripiprazole (100 µg) was blocked by cannabinoid 1 or 2 receptor antagonists AM251 (40 µg [P < .01], 80 µg [P < .0001], and 160 µg [P < .0001]) and AM630 (100 µg [P < .0001], 200 µg [P < .0001], and 400 µg [P < .0001]), respectively. The peripheral antinociception induced by aripiprazole (25 µg) was enhanced by administration of the inhibitor of fatty acid amide hydrolase (MAFP, 0.5 µg [P < .0001]) or monoacylglycerol lipase (JZL184, 4 µg [P < .0001]). Moreover, a similar enhancement was observed with the anandamide reuptake inhibitor (VDM11, 2.5 µg [P < .0001]). CONCLUSIONS: These results provide evidence for the involvement of the endocannabinoid system in peripheral antinociception induced by aripiprazole treatment.

The Involvement of the Endocannabinoid System in the Peripheral Antinociceptive Action of Ketamine.

Ketamine has been widely used as an analgesic and produces dissociative anesthetic effects. The antinociceptive effects of ketamine have been studied, but the involvement of endocannabinoids in these effects has not yet been investigated. In this study, we evaluated the involvement of the endocannabinoid system in the peripheral antinociceptive effects induced by ketamine. All drugs were administered via the intraplantar route. To induce hyperalgesia, rat paws were injected with prostaglandin E2 (2 µg per paw). The nociceptive threshold for mechanical stimulation was measured in the right hind paw of Wistar rats using the Randall-Selitto test. The tissue levels of anandamide (AEA), 2-arachidonoylglycerol, palmitoylethanolamide, and oleoylethanolamide were measured using liquid chromatography coupled to single quadrupole mass spectrometry. The administration of the cannabinoid receptor type 1 (CB1) antagonist, N(piperidine-1yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl 1 pyrazolcarboxamide (20, 40, and 80 µg per paw), but not the cannabinoid receptor type 2 antagonist, 6-iodo-2-methyl-1-(2-morpholinoethyl)-1H-indol-3-yl) (4-methoxyphenyl) methanone (100 µg per paw), antagonized the ketamine-induced peripheral antinociception in a dose-dependent manner. Additionally, the administration of the endocannabinoid metabolizing enzyme inhibitor (.5 µg per paw) or an AEA reuptake inhibitor, (5Z,8Z,11Z,14Z)N(4Hydroxy2methylphenyl)5,8,11,14 eicosatetraenamide (2.5 µg per paw) significantly enhanced low-dose ketamine-induced peripheral antinociception. AEA paw levels were increased only after ketamine administration to prostaglandin E2-injected paws. These data suggest that ketamine, in the presence of a nociceptive stimulus, induces a selective release of AEA levels and subsequent CB1 cannabinoid activation at the peripheral level. PERSPECTIVE: This study suggests that ketamine antinociception depends at least in part on AEA release and CB1 cannabinoid receptor activation in inflammatory conditions. This study could potentially help clinicians in the use of ketamine as a peripheral analgesic for inflammatory pain.

Serotonin induces peripheral mechanical antihyperalgesic effects in mice.

The role of serotonin (5-HT) in nociception will vary according to the subtypes of receptors activated. When administered peripherally, it induces pain in humans and in rats by activation of 5-HT1, 5-HT2 and 5-HT3 receptors. In addition, endogenous 5-HT produced in situ, is involved in the nociceptive response induced by formalin in rat's paw inflammation, possibly via 5-HT3 receptors. Moreover, it has been shown that 5-HT released in the dorsal horn of the spinal cord by stimulation of the periaqueductal gray causes activation of inhibitory interneurons, resulting in inhibition of spinal neurons. In the present study we evaluated the effect of serotonin and its receptors at peripheral antinociception. The mice paw pressure test was used in animals that had increased sensitivity by an intraplantar injection of PGE2 (2 µg). We used selective antagonists of serotonin receptors (isamoltan 5-HT1B, BRL 15572 5-HT1D, ketanserin 5-HT2A, ondansetron 5-HT3 and SB-269970 5-HT7). Administration of serotonin into the right hind paw (62.5, 125, 250 and 500 ng and 1 µg) produced a dose-dependent peripheral mechanical antihyperalgesic effect of serotonin in mice. Selective antagonists for 5-HT1B, 5-HT2A, 5-HT3 receptors at doses of 0.1, 1 and 10 µg, reversed the antihyperalgesic effect induced by 250 ng serotonin. In contrast, selective antagonists for 5-HT1D and 5-HT7 receptors were unable to reverse the antihyperalgesic effect induced by serotonin. These results demonstrated for the first time, the peripheral mechanical antihyperalgesic effect of serotonin, and participation of 5-HT1B, 5-HT2A and 5-HT3 receptors in this event.

Involvement of endogenous opioid peptides in the peripheral antinociceptive effect induced by the coffee specific diterpene kahweol.

BACKGROUND: Kahweol is a diterpene present in the oil derived from coffee beans. Although several pharmacological activities of kahweol are already well described in the literature, no study was done in order to assess the analgesic activity of this substance. Thus, the aim of this study was to investigate the possible peripheral antinociceptive effect of kahweol. Considering that the opioid peptides have been implicated in peripheral antinociception induced by non-opioidergic compounds, the present study also evaluated the endogenous opioids involvement in this effect. METHODS: The rat paw pressure test was used, and hyperalgesia was induced by intraplantar injection of prostaglandin E2 (2µg/paw). All drugs were administered subcutaneously in the hindpaws of male Wistar rats. The expression of ß-endorphin was examined by immunohistochemistry in the skin tissue samples of the plantar surface of rat right hindpaws. RESULTS: Intraplantar injection of kahweol (40 and 80µg) induced significant peripheral antinociception. The antinociceptive effect of kahweol was due to a local peripheral action because the higher dose (80µg/paw) did not produce any effect in the contralateral paw. The opioid receptor antagonist naloxone (50 and 100µg/paw) prevented action of kahweol (80µg/paw) and the aminopeptidases inhibitor bestatin (400µg/paw) potentiated the antinociceptive effect of kahweol (40µg/paw). Furthermore, kahweol treatment increased the intensity of ß-endorphin immunoreactivity in the epithelium of rat paws. CONCLUSIONS: The results discussed here provide evidence that kahweol treatment has peripheral antinociceptive effect and suggest that this effect is mediated by the release of endogenous opioids.

Angiotensin-(1-7) through Mas receptor activation induces peripheral antinociception by interaction with adrenoreceptors.

Angiotensin-(1-7) [Ang-(1-7)] develops its functions interacting with Mas receptor. Mas receptor was recently identified in the DRG and its activation by Ang-(1-7) resulted in peripheral antinociception against PGE2 hyperalgesia in an opioid-independent pathway. Nevertheless, the mechanism by which Ang-(1-7) induce peripheral antinociception was not yet elucidated. Considering that endogenous noradrenaline could induce antinociceptive effects by activation of the adrenoceptors the aim of this study was verify if the Ang-(1-7) is able to induce peripheral antinociception by interacting with the endogenous noradrenergic system. Hyperalgesia was induced by intraplantar injection of prostaglandin E2 (2µg). Ang-(1-7) was administered locally into the right hindpaw alone and after either agents, α2-adrenoceptor antagonist, yohimbine (5, 10 and 20 µg/paw), α2C-adrenoceptor antagonist rauwolscine (10, 15 and 20 µg/paw), α1-adrenoceptor antagonist prazosin (0.5, 1 and 2 µg/paw), ß-adrenoceptor antagonist propranolol (150, 300 and 600 ng/paw). Noradrenaline (NA) reuptake inhibitor reboxetine (30 µg/paw) was administered prior to Ang-(1-7) low dose (20 ng) and guanetidine 3 days prior to experiment (30 mg/kg/animal, once a day), depleting NA storage. Intraplantar Ang-(1-7) induced peripheral antinociception against hyperalgesia induced by PGE2. This effect was reversed, in dose dependent manner, by intraplantar injection of yohimbine, rauwolscine, prazosin and propranolol. Reboxetine intensified the antinociceptive effects of low-dose of Ang-(1-7) and guanethidine, which depletes peripheral sympathomimetic amines, reversed almost 70% the Ang-(1-7)-induced peripheral antinociception. Then, this study provides evidence that Ang-(1-7) induce peripheral antinociception stimulating an endogenous noradrenaline release that activates peripheral adrenoceptors inducing antinociception.

Participation of AT1 and Mas receptors in the modulation of inflammatory pain.

We investigated the mechanisms underlying the endogenous control of nociception at the peripheral level during inflammation. We hypothesized that angiotensin receptors could modulate pain at the peripheral level via endogenous processes because angiotensin receptors are present in peripheral nerve terminals. We evaluated the role of the angiotensin receptors system (RAS) in the modulation of inflammatory and neuropathic pain states. Mas receptor KO mice exhibited major inflammatory pain compared to wild-type mice. Similar results were observed when rats were injected with the Mas receptor antagonist A779 or the AT1 receptor antagonist, losartan after inflammatory stimulation by carrageenan. However, these antagonists were not effective in animals with neuropathic-induced pain (e.g., sciatic nerve constriction). Therefore, RAS seems to play an important role in inflammatory but not neuropathic pain.

Proteinase-activated receptor 2 blockade impairs CCL11- or allergen-induced eosinophil recruitment in experimental pleurisy.

Although proteinase-activated receptor (PAR)-2 has been implicated in inflammatory diseases, its role in regulating eosinophil recruitment in response to chemoattractants remains unclear. Here, we investigated the role of PAR-2 and PAR-2-activating Mast Cell (MC) tryptase on chemokine C-C motif ligand (CCL)11- and antigen-induced eosinophil recruitment to the pleural cavity of BALB/c mice. The PAR-2-activating peptide H-Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SLIGRL-NH2) induced eosinophil recruitment whereas PAR-2 blockade inhibited ovalbumin (OVA)- or CCL11-induced eosinophil recruitment. Moreover, OVA and CCL11 induced PAR-2 expression in pleural leukocytes, and the MC tryptase inhibitor APC 366 ([N-(1-hydroxy-2-napthoyl)-l-arginyl-l-prolinamide hydrochloride]) abolished CCL11-induced eosinophil recruitment. These results suggest a pro inflammatory effect of PAR-2 and support a role for MC tryptase mediating eosinophil migration via PAR-2 signaling. Taken together, our results suggest that PAR-2 activation through endogenous MC tryptase activity could be required, at least partially, to mediate CCL11-induced eosinophil migration.

Mast cell tryptase induces eosinophil recruitment in the pleural cavity of mice via proteinase-activated receptor 2.

Proteinase-activated receptor (PAR) 2 has been implicated in eosinophil migration. Mast cell (MC) tryptase has been similarly implicated in allergic diseases through the activation of PAR-2, but the role of this receptor in MC tryptase-induced inflammation is not well elucidated. This study aims to investigate the ability of MC tryptase or PAR-2 activating peptide (SLIGRL-NH2) to induce eosinophil recruitment to the pleural cavity of mice. Mast cell tryptase-injected mice were pretreated with PAR-2 antagonist ENMD-1068. Mice injected with SLIGRL-NH2 were pretreated with mast cell tryptase inhibitor APC 366, and eosinophil migration into the pleural cavity and PAR-2 expression was analyzed after 24 or 48 h. SLIGRL-NH2-induced eosinophil recruitment was inhibited by APC 366, and MC tryptase-induced eosinophil recruitment was abolished by ENMD-1068. MC tryptase induced PAR-2 expression on pleural eosinophils. Our results demonstrate a key role for PAR-2 in mediating eosinophil recruitment in MC tryptase-induced pleurisy in mice. The ability of MC tryptase to inducing PAR-2 expression on eosinophils corroborates the relevance of MC tryptase and PAR-2 on modulating eosinophil migration.

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

OBJECTIVE: To investigate the contribution of K(+) channels on peripheral antinociception induced by ketamine. STUDY DESIGN: Prospective experimental study. ANIMALS: 110 male Wistar rats weighing 160-200 g. METHODS: The paw pressure required to elicit limb flexion was designated as the nociceptive threshold. Hyperalgesia was induced by intraplantar injection of prostaglandin E2 . All drugs were administered locally into the right hind paw of rats. Ketamine was administered into the right hind paw 2 hours and 55 minutes after local injection of PGE2 . Tetraethylammonium was administered 30 minutes prior to ketamine and the other K(+) channel blockers, glibenclamide, dequalinium and paxilline, were administered 5 minutes prior to ketamine. RESULTS: Prostaglandin E2 (2 µg per paw) induced hyperalgesia. Ketamine (10, 20, 40 and 80 µg per paw) elicited a local peripheral antinociceptive effect that was antagonized by a specific blocker of ATP-sensitive K(+) channels, glibenclamide (20, 40 and 80 µg per paw). In another experiment, the non-selective voltage-dependent K(+) channel blocker tetraethylammonium (30 µg per paw) and small and large conductance blockers of Ca(2+) -activated K(+) channels, dequalinium (50 µg per paw) and paxilline (20 µg per paw), were ineffective at blocking the effect of a local ketamine injection. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of these results provides evidence that ketamine, may in part, induce peripheral antinociceptive effects by ATP-sensitive K(+) channel pathway activation.

CB1 and CB2 cannabinoid receptor agonists induce peripheral antinociception by activation of the endogenous noradrenergic system.

BACKGROUND: Cannabinoid agonists induce norepinephrine release in central, spinal, and peripheral sites. Previous studies suggest an interaction between the cannabinoid and adrenergic systems on antinociception. In this study, we sought to verify whether the CB1 and CB2 cannabinoid receptor agonists anandamide and N-palmitoyl-ethanolamine (PEA), respectively, are able to induce peripheral antinociception via an adrenergic mechanism. METHODS: All drugs were administered locally into the right hindpaw of male Wistar rats. The rat paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E2 (2 µg). RESULTS: Anandamide, 12.5 ng/paw, 25 ng/paw, and 50 ng/paw elicited a local peripheral antinociceptive effect that was antagonized by CB1 cannabinoid receptor antagonist AM251, 20 µg/paw, 40 µg/paw, and 80 µg/paw, but not by CB2 cannabinoid receptor antagonist AM630, 100 µg/paw. PEA, 5 µg/paw, 10 µg/paw, and 20 µg/paw, elicited a local peripheral antinociceptive effect that was antagonized by AM630, 25 µg/paw, 50 µg/paw, and 100 µg/paw, but not by AM251, 80 µg/paw. Antinociception induced by anandamide or PEA was antagonized by the nonselective α2 adrenoceptor antagonist yohimbine, 05 µg/paw, 10 µg/paw, and 20 µg/paw, and by the selective α2C adrenoceptor antagonist rauwolscine, 10 µg/paw, 15 µg/paw, and 20 µg/paw, but not by the selective antagonists for α2A, α2B, and α2D adrenoceptor subtypes, 20 µg/paw. The antinociceptive effect of the cannabinoids was also antagonized by the nonselective α1 adrenoceptor antagonist prazosin, 0.5 µg/paw, 1 µg/paw, and 2 µg/paw, and by the nonselective ß adrenoceptor antagonist propranolol, 150 ng/paw, 300 ng/paw, and 600 ng/paw. Guanethidine, which depletes peripheral sympathomimetic amines (30 mg/kg/animal, once a day for 3 days), restored approximately 70% the anandamide-induced and PEA-induced peripheral antinociception. Furthermore, acute injection of the norepinephrine reuptake inhibitor reboxetine, 30 µg/paw, intensified the antinociceptive effects of low-dose anandamide, 12.5 ng/paw, and PEA, 5 µg/paw. CONCLUSIONS: This study provides evidence that anandamide and PEA induce peripheral antinociception activating CB1 and CB2 cannabinoid receptors, respectively, stimulating an endogenous norepinephrine release that activates peripheral adrenoceptors inducing antinociception.

NO/cGMP production is important for the endogenous peripheral control of hyperalgesia during inflammation.

Various studies have demonstrated the role of the nitric oxide (NO)/cGMP pathway in pain processing. Our group has also shown that this system participates in opioid-induced antinociception during peripheral inflammation. We have previously observed that inflammation mobilizes an endogenous opioidergic system to control hyperalgesia. Here, we investigated whether the NO/cGMP pathway underlies peripheral endogenous nociception control during inflammation. In this study, a pharmacological approach was used in conjunction with the rat paw pressure test to assess the effects of intraplantar NO synthase inhibitor NG-Nitro-l-arginine (NOArg), guanylyl cyclase inhibitor methylene blue (MB), phosphodiesterase-5 inhibitor zaprinast (ZP), or NO precursor l-arginine injection on carrageenan-induced hyperalgesia, which mimics an inflammatory process, or by prostaglandin E(2) (PGE(2)), which directly sensitizes nociceptors. Intraplantar carrageenan (62.5, 125, 250 or 500µg) or PGE(2) (0.1, 0.5 or 2µg) administration produced hyperalgesia, which manifested as a reduction in the rat nociceptive threshold to mechanical stimuli. NOArg (25, 50 or 100µg/paw) and MB (125, 250 or 500µg/paw) induced significant and dose-dependent reductions in the nociceptive threshold of carrageenan-induced (125µg/paw) hyperalgesia, but not PGE(2)-induced (0.5µg/paw) hyperalgesia. This was a local effect because it did not produce any modifications in the contralateral paw. Both Zaprinast (100, 200 or 400µg/paw) and l-arginine (100, 200 or 400µg/paw) significantly counteracted carrageenan-induced hyperalgesia (250µg/paw), yielding an increase in the nociceptive threshold compared with the control. Zaprinast (200µg/paw) or l-arginine (400µg/paw) did not produce an antinociceptive effect in the contralateral paw, indicating local action. In addition, at the same dose that was able to modify carrageenan-induced hyperalgesia, neither zaprinast nor l-arginine modified PGE(2) (2µg) injection-induced hyperalgesia of the rat paw. Taken together, these results indicate that the l-arginine/NO/cGMP pathway functions as an endogenous modulator of peripheral inflammatory hyperalgesia.

Neutrophil recruitment is inhibited by nicotinamide in experimental pleurisy in mice.

Several emerging lines of evidence support an anti-inflammatory role for nicotinamide and other vitamin B components. However, the mechanisms underlying their activity remain unclear. In the present study, we investigated the ability of nicotinamide to inhibit both neutrophil recruitment in IL-8-, LTB(4)- or carrageenan-induced pleurisy in mice and the rolling and adherence of neutrophils. Nicotinamide inhibited IL-8-, LTB(4)- and carrageenan-induced neutrophil migration, KC production and carrageenan-induced neutrophil rolling and adherence. We propose that the effects of nicotinamide in inhibiting neutrophil recruitment in carrageenan-induced pleurisy may be due to the ability of nicotinamide to inhibit the action of IL-8 and LTB(4), decrease KC production, and inhibit early events that regulate leukocyte migration from blood vessels into tissue.

Mu, delta, and kappa opioid receptor agonists induce peripheral antinociception by activation of endogenous noradrenergic system.

Opioid receptor agonists induce noradrenaline release in the supraspinal, spinal, and peripheral sites. Endogenous noradrenaline release can induce an antinociceptive effect by activation of the α(2) adrenoceptor. This interaction between the opioid and the adrenergic systems could be the alternative mechanism by which opioid receptor agonists mediate peripheral antinociception. Therefore, the aim of the present study was to verify whether peripheral antinociception induced by the µ, δ, and κ opioid receptor agonists DAMGO, SNC80, and bremazocine, respectively, through the endogenous noradrenergic system. All drugs were administered locally into the right hind paw of male Wistar rats. The rat paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E(2). DAMGO, SNC80, or bremazocine elicited local dose-dependent peripheral antinociception. This peripheral effect was antagonized by the nonselective α(2) adrenoceptor antagonist yohimbine and by the selective α(2C) adrenoceptor antagonist rauwolscine but not by the selective antagonists for α(2A), α(2B), and α(2D) adrenoceptor subtypes (BRL 44 480, imiloxan, and RX 821002, respectively). The opioid-induced effect was antagonized by the nonselective α(1) adrenoceptor antagonist prazosin and by the nonselective ß adrenoceptor antagonist propranolol. Guanethidine, a depletor of peripheral sympathomimetic amines, restored approximately 50-60% of the opioid-induced peripheral antinociception. Furthermore, acute injection of the noradrenaline reuptake inhibitor reboxetine intensified the antinociceptive effects of low-dose DAMGO, SNC80, or bremazocine. This study provides evidence that DAMGO, SNC80, or bremazocine induces peripheral antinociception by noradrenaline release and interaction with adrenoceptors.

Involvement of the L-arginine/nitric oxide/cyclic guanosine monophosphate pathway in peripheral antinociception induced by N-palmitoyl-ethanolamine in rats.

N-palmitoyl-ethanolamine (PEA) is an endogenous substance that was first identified in lipid tissue extracts. It has been classified as a CB(2) receptor agonist. Exogenous PEA has the potential to become a valid treatment for neuropathic and inflammatory pain. In spite of the well-demonstrated antiinflammatory properties of PEA, its involvement in controlling pain pathways remains poorly characterized. The participation of the L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway in peripheral antinociception has been established by our group to the µ-, κ- or δ-opioid receptor agonists, nonsteroidal analgesics, α(2C) -adrenoceptor agonists, and even nonpharmacological electroacupuncture. The aim of this study was to verify whether the peripheral antinociception effects of PEA involve the activation of this pathway. All drugs were locally administered to the right hind paw of male Wistar rats. The paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E(2) . PEA elicited a local peripheral antinociceptive effect that was antagonized by the nonselective NO synthase (NOS) inhibitor L-NOARG and the selective neuronal NOS (nNOS) inhibitor L-NPA. Selective inhibition of endothelial (eNOS) and inducible (iNOS) NOS via L-NIO and L-NIL, respectively, was ineffective at blocking the effects of a local PEA injection. In addition, the dosage of nitrite in the homogenized paw, as determined by colorimetric assay, indicated that exogenous PEA is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ antagonized the PEA effect, whereas the cGMP-phosphodiesterase inhibitor zaprinast potentiated the antinociceptive effect of low-dose PEA. This study provides evidence that PEA activates nNOS, thus initiating the NO/cGMP pathway and inducing peripheral antinociceptive effects.

Angiotensin-(1-7) induces peripheral antinociception through mas receptor activation in an opioid-independent pathway.

The G protein-coupled receptor Mas was recently described as an angiotensin-(1-7) [Ang-(1-7)] receptor. In the present study, we demonstrate an antinociceptive effect of Ang-(1-7) for the first time. Additionally, we evaluated the anatomical localization of Mas in the dorsal root ganglia using immunofluorescence. This is the first evidence indicating that this receptor is present in sensitive neurons. The antinociceptive effect was demonstrated using the rat paw pressure test. For this test, sensitivity is increased by intraplantar injection of prostaglandin E(2). Ang-(1-7) administered locally into the right hind paw elicited a dose-dependent antinociceptive effect. Because the higher dose of Ang-(1-7) did not produce an effect when injected into the contralateral paw, this effect was considered local. The specific antagonist for the Mas receptor, A-779, inhibited the peripheral antinociception induced by exposure to 4 µg/paw Ang-(1-7) in a dose-dependent manner. The highest dose completely reversed the antinociceptive effect induced by Ang-(1-7), suggesting that the Mas receptor is an obligatory component in this process and that other angiotensin receptors may not be involved. When injected alone, the antagonist was unable to induce hyperalgesia or antinociception. Alternatively, naloxone was unable to inhibit the antinociceptive effect induced by Ang-(1-7), suggesting that endogenous opioid peptides may not be involved in this response. These data provide the first anatomical basis for the physiological role of Ang-(1-7) in the modulation of pain perception via Mas receptor activation in an opioid-independent pathway. Taken together, these results provide new perspectives for the development of a new class of analgesic drugs.

Noradrenaline activates the NO/cGMP/ATP-sensitive K(+) channels pathway to induce peripheral antinociception in rats.

Despite the classical peripheral pronociceptive effect of noradrenaline (NA), recently studies showed the involvement of NA in antinociceptive effect under immune system interaction. In addition, the participation of the NO/cGMP/KATP pathway in the peripheral antinociception has been established by our group as the molecular mechanism of another adrenoceptor agonist xylazine. Thus the aim of this study was to obtain pharmacological evidences for the involvement of the NO/cGMP/KATP pathway in the peripheral antinociceptive effect induced by exogenous noradrenaline. The rat paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E(2) (2µg/paw). All drugs were locally administered into the right hind paw of male Wistar rats. NA (5, 20 and 80ng/paw) elicited a local inhibition of hyperalgesia. The non-selective NO synthase inhibitor l-NOarg (12, 18 and 24µg/paw) antagonized the antinociception effect induced by the highest dose of NA. The soluble guanylyl cyclase inhibitor ODQ (25, 50 and 100µg/paw) antagonized the NA-induced effect; and cGMP-phosphodiesterase inhibitor zaprinast (50µg/paw) potentiated the antinociceptive effect of NA low dose (5ng/paw). In addition, the local effect of NA was antagonized by a selective blocker of an ATP-sensitive K(+) channel, glibenclamide (20, 40 and 80µg/paw). On the other hand, the specifically voltage-dependent K(+) channel blocker, tetraethylammonium (30µg/paw), Ca(2+)-activated K(+) channel blockers of small and large conductance types dequalinium (50µg/paw) and paxilline (20µg/paw), respectively, were not able to block local antinociceptive effect of NA. The results provide evidences that NA probably induces peripheral antinociceptive effects by activation of the NO/cGMP/KATP pathway.