Collaborative action between noradrenergic and serotoninergic systems in peripheral antinociception in mice.

Noradrenaline (NA) and serotonin (5-HT) induce nociception and antinociception. This antagonistic effect can be explained by the dose and type of activated receptors. We investigated the existence of synergism between the noradrenergic and serotonergic systems during peripheral antinociception. The paw pressure test was performed in mice that had increased sensitivity by intraplantar injection of prostaglandin E2 (PGE2). Noradrenaline (80 ng) administered intraplantarly induced an antinociceptive effect, that was reversed by the administration of selective antagonists of serotoninergic receptors 5-HT1B isamoltan, 5-HT1D BRL15572, 5-HT2A ketanserin, 5-HT3 ondansetron, but not by selective receptor antagonist 5-HT7 SB-269970. The administration of escitalopram, a serotonin reuptake inhibitor, potentiated the antinociceptive effect at a submaximal dose of NA. These results, indicate the existence of synergism between the noradrenergic and serotonergic systems in peripheral antinociception in mice.

Endogenous Cholinergic System Involved in Peripheral Analgesic Control in Mice Is Activated by TNF-α, CXCL-1, and IL-1ß.

INTRODUCTION: Tissue injury results in the release of inflammatory mediators, including a cascade of algogenic substances, which contribute to the development of hyperalgesia. During this process, endogenous analgesic substances are peripherally released to counterbalance hyperalgesia. The present study aimed to investigate whether inflammatory mediators TNF-α, IL-1ß, CXCL1, norepinephrine (NE), and prostaglandin E2 (PGE2) may be involved in the deflagration of peripheral endogenous modulation of inflammatory pain by activation of the cholinergic system. METHODS: Male Swiss mice were subjected to paw withdrawal test. All the substances were injected via the intraplantar route. RESULTS: The main findings of this study were as follows: (1) carrageenan (Cg), TNF-α, CXCL-1, IL1-ß, NE, and PGE2 induced hyperalgesia; (2) the acetylcholinesterase enzyme inhibitor, neostigmine, reversed the hyperalgesia observed after Cg, TNF-α, CXCL-1, and IL1-ß injection; (3) the non-selective muscarinic receptor antagonist, atropine, and the selective muscarinic type 1 receptor (m1AChr) antagonist, telenzepine, potentiated the hyperalgesia induced by Cg and CXCL-1; (4) mecamylamine, a non-selective nicotinic receptor antagonist, potentiated the hyperalgesia induced by Cg, TNF-α, CXCL-1, and IL1-ß; (5) Cg, CXCL-1, and PGE2 increased the expression of the m1AChr and nicotinic receptor subunit α4protein. CONCLUSION: These results suggest that the cholinergic system may modulate the inflammatory pain induced by Cg, PGE2, TNF-α, CXCL-1, and IL1-ß.

Cannabidiol induces systemic analgesia through activation of the PI3Kγ/nNOS/NO/KATP signaling pathway in neuropathic mice. A KATP channel S-nitrosylation-dependent mechanism.

BACKGROUND: Cannabidiol (CBD) is the second most abundant pharmacologically active component present in Cannabis sp. Unlike Δ-9-tetrahydrocannabinol (THC), it has no psychotomimetic effects and has recently received significant interest from the scientific community due to its potential to treat anxiety and epilepsy. CBD has excellent anti-inflammatory potential and can be used to treat some types of inflammatory and neuropathic pain. In this context, the present study aimed to evaluate the analgesic mechanism of cannabidiol administered systemically for the treatment of neuropathic pain and determine the endogenous mechanisms involved with this analgesia. METHODS: Neuropathic pain was induced by sciatic nerve constriction surgery, and the nociceptive threshold was measured using the paw compression test in mice. RESULTS: CBD produced dose-dependent antinociception after intraperitoneal injection. Selective inhibition of PI3Kγ dose-dependently reversed CBD-induced antinociception. Selective inhibition of nNOS enzymes reversed the antinociception induced by CBD, while selective inhibition of iNOS and eNOS did not alter this antinociception. However, the inhibition of cGMP production by guanylyl cyclase did not alter CBD-mediated antinociception, but selective blockade of ATP-sensitive K+ channels dose-dependently reversed CBD-induced antinociception. Inhibition of S-nitrosylation dose-dependently and completely reversed CBD-mediated antinociception. CONCLUSION: Cannabidiol has an antinociceptive effect when administered systemically and this effect is mediated by the activation of PI3Kγ as well as by nitric oxide and subsequent direct S-nitrosylation of KATP channels on peripheral nociceptors.

Participation of the cannabinoid system and the NO/cGMP/KATP pathway in serotonin-induced peripheral antinociception.

It has already been shown that serotonin can release endocannabinoids at the spinal cord level, culminating in inhibition of the dorsal horn. At the peripheral level, cannabinoid receptors modulate primary afferent neurons by inhibiting calcium conductance and increasing potassium conductance. Studies have shown that after the activation of opioid receptors and cannabinoids, there is also the activation of the NO/cGMP/KATP pathway, inducing cellular hyperpolarization. In this study, we evaluated the participation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway in the peripheral antinociceptive effect of serotonin. The paw pressure test of mice was used in animals that had their sensitivity to pain increased due to an intraplantar injection of PGE2 (2 µg). Serotonin (250 ng/paw), administered locally in the right hind paw, induced antinociceptive effect. CB1 and CB2 cannabinoid receptors antagonists, AM251 (20, 40 and 80 µg) and AM630 (25, 50 and 100 µg), respectively, reversed the serotonin-induced antinociceptive effect. MAFP (0.5 µg), an inhibitor of the FAAH enzyme that degrades anandamide, and JZL184 (3.75 µg), an inhibitor of the enzyme MAGL that degrades 2-AG, as well as the VDM11 (2.5 µg) inhibitor of anandamide reuptake, potentiated the antinociceptive effect induced by a low dose (62. 5 ng) of serotonin. In the evaluation of the participation of the NO/cGMP/KATP pathway, the antinociceptive effect of serotonin was reversed by the administration of the non-selective inhibitor of NOS isoforms L-NOarg (12.5, 25 and 50 µg) and by the selective inhibitor for the neuronal isoform LNPA (24 µg), as well as by the soluble guanylate cyclase inhibitor ODQ (25, 50 and 100 µg). Among potassium channel blockers, only Glibenclamide (20, 40 and 80 µg), an ATP-sensitive potassium channel blocker, reversed the effect of serotonin. In addition, intraplantar administration of serotonin (250 ng) was shown to induce a significant increase in nitrite levels in the homogenate of the plantar surface of the paw of mice. Taken together, these data suggest that the antinociceptive effect of serotonin occurs by activation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway.

Anandamide reduces the migration of lymphocytes to the intestine by CB2 activation and reduces TNF-α in the target organs, protecting mice from graft-versus-host disease.

Graft-versus-host disease (GVHD) is a serious inflammatory illness that often occurs as a secondary complication of bone marrow transplantation. Current therapies have limited effectiveness and fail to achieve a balance between inflammation and the graft-versus-tumor effect. In this study, we investigate the effects of the endocannabinoid anandamide on the complex pathology of GVHD. We assess the effects of an irreversible inhibitor of fatty acid amine hydrolase or exogenous anandamide and find that they increase survival and reduce clinical signs in GVHD mice. In the intestine of GVHD mice, treatment with exogenous anandamide also leads to a reduction in the number of CD3+, CD3+CD4+, and CD3+CD8+ cells, which reduces the activation of CD3+CD4+ and CD3+CD8+ cells, as assessed by enhanced CD28 expression, a T cell co-stimulatory molecule. Exogenous AEA was also able to reduce TNF-α and increase IL-10 in the intestine of GVHD mice. In the liver, exogenous AEA reduces injury, TNF-α levels, and the number of CD3+CD8+ cells. Interestingly, anandamide reduces Mac-1α, which lowers the adhesion of transplanted cells in mesenteric veins. These effects are mimicked by JWH133-a CB2 selective agonist-and abolished by treatment with a CB2 antagonist. Furthermore, the effects caused by anandamide treatment on survival were related to the CB2 receptor, as the CB2 antagonist abolished it. This study shows the critical role of the CB2 receptor in the modulation of the inflammatory response of GVHD by treatment with anandamide, the most prominent endocannabinoid.

Peripherally injected canabidiol reduces neuropathic pain in mice: Role of the 5-HT1A and TRPV1 receptors.

Cannabidiol (CBD) is the most abundant non-psychoactive component found in plants of the genus Cannabis. Its analgesic effect for the treatment of neuropathy has been widely studied. However, little is known about its effects in the acute treatment when Cannabidiol is administered peripherally. Because of that, this research was aimed to evaluate the antinociceptive effects of the CBD when administered peripherally for the treatment of acute neuropathic pain and check the involvement of the 5-HT1A and the TRPV1 receptors in this event. Neuropathic pain was induced with the constriction of the sciatic nerve while the nociceptive threshold was measured using the pressure test of the mouse paw. The technique used proved to be efficient to induce neuropathy, and the CBD (5, 10 and 30 µg/paw) induced the antinociception in a dosage-dependent manner. The dosage used that induced a more potent effect (30 µg/paw), did not induce a systemic response, as demonstrated by both the motor coordination assessment test (RotaRod) and the antinociceptive effect restricted to the paw treated with CBD. The administration of NAN-190 (10 µg/paw), a selective 5-HT1A receptor antagonist, and SB-366791 (16 µg/paw), a selective TRPV1 antagonist, partially reversed the CBD-induced antinociception. The results of the research suggest that the CBD produces the peripheral antinociception during the acute treatment of the neuropathic pain and it partially involved the participation of the 5-HT1A and TRPV1 receptors.

Bradykinin induces peripheral antinociception in PGE2-induced hyperalgesia in mice.

BACKGROUND: Bradykinin (BK) is an endogenous peptide involved in vascular permeability and inflammation. It has opposite effects (inducing hyperalgesia or antinociception) when administered directly in the central nervous system. The aim of this study was to evaluate whether BK may also present this dual effect when injected peripherally in a PGE2-induced nociceptive pain model, as well as to investigate the possible mechanisms of action involved in this event in mice. METHODS: Male Swiss and C57BL/6 knockout mice for B1 or B2 bradykinin receptors were submitted to a mechanical paw pressure test and hyperalgesia was induced by intraplantar prostaglandin E2 (2 µg/paw) injection. RESULTS: Bradykinin (20, 40 and 80 ng/paw) produced dose-dependent peripheral antinociception against PGE2-induced hyperalgesia. This effect was antagonized by bradyzide (8, 16 and 32 µg/paw), naloxone (12.5, 25 and 50 µg/paw), nor-binaltorphimine (50, 100 and 200 µg/paw) and AM251 (20, 40 and 80 µg/paw). Bestatin (400 µg/paw), MAFP (0.5 µg/paw) and VDM11 (2.5 µg/paw) potentiated the antinociception of a lower 20 ng BK dose. The knockout of B1 or B2 bradykinin receptors partially abolished the antinociceptive action of BK (80 ng/paw), bremazocine (1 µg/paw) and anandamide (40 ng/paw) when compared with wild-type animals, which show complete antinociception with the same dose of each drug. CONCLUSION: The present study is the first to demonstrate BK-induced antinociception in peripheral tissues against PGE2-induced nociception in mice and the involvement of κ-opioid and CB1 cannabinoid receptors in this effect.

Curcumin induces peripheral antinociception by opioidergic and cannabinoidergic mechanism: Pharmacological evidence.

BACKGROUND: Curcumin is one of the compounds present in plants of the genus Curcuma sp., being very used not only as condiment but also with medicinal purposes. As an analgesic, papers highlight the efficacy of curcumin in the treatment of various types of pain. AIMS: In this study we evaluated the peripheral antinociceptive effect of curcumin and by which mechanisms this effect is induced. MAIN METHODS: The mice paw pressure test was used on animals which had increased pain sensitivity by intraplantar injection of carrageenan. All the drugs were administered in the right hind paw. KEY FINDINGS: Curcumin was administered to the right hind paw animals induced antinociceptive effect. Non -selective antagonist of opioid receptors naloxone reverted the antinociceptive effect induced by curcumin. Selective antagonists for µ, δ and κ opioid receptors clocinnamox, naltrindole and nor- binaltorphimine, respectively, reverted the antinociceptive effect induced by curcumin. Bestatin, enkephalinases inhibitor that degrade peptides opioids, did not change the nociceptive response. Selective antagonists for CB1 and CB2 cannabinoid receptors, AM251 and AM630, respectively, reversed the antinociceptive effect induced by curcumin. The MAFP inhibitor of the enzyme FAAH which breaks down anandamide, JZL184, enzyme inhibitor MAGL which breaks down the 2-AG, as well as the VDM11 anandamide reuptake inhibitor potentiated the antinociceptive effect of curcumin. SIGNIFICANCE: These results suggest that curcumin possibly peripheral antinociception induced by opioid and cannabinoid systems activation and possibly for endocannabinoids and opioids release.

Cannabidiol Enhances Intestinal Cannabinoid Receptor Type 2 Receptor Expression and Activation Increasing Regulatory T Cells and Reduces Murine Acute Graft-versus-Host Disease without Interfering with the Graft-versus-Leukemia Response.

Cannabidiol (CBD) is a highly lipidic phytocannabinoid with remarkable anti-inflammatory effects. The aim of this study was to evaluate CBD's effects and mechanisms of action in the treatment of mice subjected to acute graft-versus-host disease (aGVHD). aGVHD was induced by the transplantation of bone marrow cells and splenocytes from C57BL-6j to Balb-c mice. The recipient mice were treated daily with CBD, and the treatment reduced mouse mortality by decreasing inflammation and injury and promoting immune regulation in the jejunum, ileum, and liver. Analysis of the jejunum and ileum showed that CBD treatment reduced the levels of C-C motif chemokine ligand (CCL) 2, CCL3, CCL5, tumor necrosis factor α, and interferon γ (IFNγ). CCL3 and IFNγ levels were also decreased in the liver. Mechanistically, CBD also increased the number of cannabinoid receptor type 2 (CB2) receptors on CD4+ and forkhead box P3+ cells in the intestine, which may explain the reduction in proinflammatory cytokines and chemokines. Antagonists of the CB2 receptor reduced the survival rates of CBD-treated mice, suggesting the participation of this receptor in the effects of CBD. Furthermore, treatment with CBD did not interfere with the graft-versus-leukemia response. CBD treatment appears to protect aGVHD mice by anti-inflammatory and immunomodulatory effects partially mediated by CB2 receptor interaction. Altogether, our study suggests that CBD represents an interesting approach in the treatment of aGVHD, with potential therapeutic applications in patients undergoing bone marrow transplantation. SIGNIFICANCE STATEMENT: This study provides for the first time a mechanism by which cannabidiol, a phytocannabinoid with no psychoactive effect, induces immunomodulation in the graft-versus-host disease. Enhancing intestinal cannabinoid receptor type 2 (CB2) receptor expression on CD4+ and forkhead box P3+ cells and increasing the number of these regulatory cells, cannabidiol decreases proinflammatory cytokines and increases graft-versus-host disease mice survival. This effect is dependent of CB2 receptor activation. Besides, cannabidiol did not interfere with graft-versus-leukemia response, a central response to avoid primary disease relapse.

α2-Adrenoceptor agonist induces peripheral antinociception via the endocannabinoid system.

BACKGROUND: Xylazine is an α2 adrenoceptor agonist that is extensively used in veterinary medicine and animal experimentation procedures to produce analgesia, sedation and muscle relaxation without causing general anesthesia. Considering the lack of knowledge of the mechanisms involved in peripheral antinociception induced by xylazine and the potential interactions between the adrenergic and endocannabinoid systems, the present study investigated the contribution of the latter system in the mechanism of xylazine. METHODS: The rat paw pressure test, in which hyperalgesia was induced by the intraplantar injection of prostaglandin E2, was performed. RESULTS: Xylazine administered via an intraplantar injection (25, 50 and 100 µg) induced a peripheral antinociceptive effect against prostaglandin E2 (2 µg)-induced hyperalgesia. This effect was blocked by treatment with the selective CB1 cannabinoid antagonist AM251 (20, 40 and 80 µg) but not by the selective CB2 cannabinoid antagonist AM630 (100 µg). The anandamide reuptake inhibitor VDM11 (2.5 µg) intensified the peripheral antinociceptive effect of a submaximal dose of xylazine (25 µg), and the inhibitor of endocannabinoid enzymatic hydrolysis, MAFP (0.5 µg), showed a tendency towards this same effect. In addition, liquid-chromatography mass spectrometric analysis indicated that xylazine (100 µg) treatment was associated with an increase in anandamide levels in the rat paws treated with PGE2. CONCLUSIONS: The present results provides evidence that the peripheral antinociceptive effect of the α2 adrenoceptor agonist xylazine probably results from anandamide release and subsequent CB1 cannabinoid receptor activation.

Evidence for the involvement of opioid and cannabinoid systems in the peripheral antinociception mediated by resveratrol.

Despite all the development of modern medicine, around 100 compounds derived from natural products were undergoing clinical trials only at the end of 2013. Among these natural substances in clinical trials, we found the resveratrol (RES), a pharmacological multi-target drug. RES analgesic properties have been demonstrated, although the bases of these mechanisms have not been fully elucidated. The aim of this study was to evaluate the involvement of opioid and cannabinoid systems in RES-induced peripheral antinociception. Paw withdrawal method was used and hyperalgesia was induced by carrageenan (200 µg/paw). All drugs were given by intraplantar injection in male Swiss mice (n = 5). RES (100 µg/paw) administered in the right hind paw induced local antinociception that was antagonized by naloxone, non-selective opioid receptor antagonist, and clocinnamox, µOR selective antagonist. Naltrindole and nor-binaltorfimine, selective antagonists for δOR and kOR, respectively, did not reverse RES-induced peripheral antinociception. CB1R antagonist AM251, but not CB2R antagonist AM630, antagonized RES-induced peripheral antinociception. Peripheral antinociception of RES intermediate-dose (50 µg/paw) was increased by: (i) bestatin, inhibitor of endogenous opioid degradation involved-enzymes; (ii) MAFP, inhibitor of anandamide amidase; (iii) JZL184, inhibitor of 2-arachidonoylglycerol degradation involved-enzyme; (iv) VDM11, endocannabinoid reuptake inhibitor. Acute and peripheral administration of RES failed to affect the amount of µOR, CB1R and CB2R. Experimental data suggest that RES induces peripheral antinociception through µOR and CB1R activation by endogenous opioid and endocannabinoid releasing.

Ketamine induces central antinociception mediated by endogenous cannabinoids and activation of CB1 receptors.

The participation of endocannabinoids in central and peripheral antinociception induced by several compounds has been shown by our group. In this study, we investigated the effect of endocannabinoids on the central antinociception induced by ketamine. The nociceptive threshold for thermal stimulation was measured using the tail-flick test in Swiss mice. The drugs were administered intracerebroventricularly. Probabilities less than 5% (p < 0.05) were considered to be statistically significant (Two-way ANOVA/Bonferroni's test). The CB1-selective cannabinoid receptor antagonist AM251 (2 and 4 µg) completely reversed the central antinociception induced by ketamine (4 µg) in a dose-dependent manner. In contrast, the CB2-selective cannabinoid receptor antagonist AM630 (2 and 4 µg) did not antagonize this effect. Additionally, the administration of the anandamide amidase inhibitor MAFP (0.2 µg) and anandamide uptake inhibitor VDM11 (4 µg) significantly enhanced the antinociception induced by a low dose of ketamine (2 µg). It was concluded that central antinociception induced by ketamine involves the activation of CB1 cannabinoid receptors. Mobilization of cannabinoids might be required for the activation of those receptors, since inhibitors of the endogenous cannabinoids potentiate the effect of Ketamine.

Anti-inflammatory and antinociceptive properties of the hydroalcoholic fractions from the leaves of Annona crassiflora Mart. in mice.

BACKGROUND: Annona crassiflora Mart., popularly known as "Araticum", is a native tree of the Brazilian Cerrado used in folk medicine for treatment of pain and inflammatory diseases. We proposed to analyze analgesic and anti-inflammatory properties of the filtrate (F1) and the precipitate (F2) of the hydroalcoholic fraction from the leaves of Annona crassiflora Mart. in mice. MATERIALS AND METHODS: Swiss mice were submitted to formalin-induced nociception test and tail-flick reflex test, to assess antinociceptive properties, and to the rota-rod test, for motor performance analyses. To evaluate anti-inflammatory properties, F1 and F2 were orally administered 1 h prior to the intrathoracic injection of carrageenan, zymosan, LPS, CXCL8, or vehicle in Balb/c mice and neutrophil infiltration was evaluated 4 h after injection. RESULTS: F1 and F2 reduced the licking time in the second phase of formalin-induced nociception test, but only F2 showed a dose-dependent response. Neither F1 nor F2 reduced the latency time in the tail-flick reflex test. In addition, motor performance alteration was not observed in F1- or F2-treated mice. F2 treatment significantly inhibited the neutrophilia induced by carrageenan, LPS, or CXCL8, but not zymosan. CONCLUSIONS: The experimental data demonstrated that hydroalcoholic fractions of Annona crassiflora Mart. leaves have remarkable anti-inflammatory and antinociceptive activities.

TNF-α mediated upregulation of NaV1.7 currents in rat dorsal root ganglion neurons is independent of CRMP2 SUMOylation.

Clinical and preclinical studies have shown that patients with Diabetic Neuropathy Pain (DNP) present with increased tumor necrosis factor alpha (TNF-α) serum concentration, whereas studies with diabetic animals have shown that TNF-α induces an increase in NaV1.7 sodium channel expression. This is expected to result in sensitization of nociceptor neuron terminals, and therefore the development of DNP. For further study of this mechanism, dissociated dorsal root ganglion (DRG) neurons were exposed to TNF-α for 6 h, at a concentration equivalent to that measured in STZ-induced diabetic rats that developed hyperalgesia. Tetrodotoxin sensitive (TTXs), resistant (TTXr) and total sodium current was studied in these DRG neurons. Total sodium current was also studied in DRG neurons expressing the collapsin response mediator protein 2 (CRMP2) SUMO-incompetent mutant protein (CRMP2-K374A), which causes a significant reduction in NaV1.7 membrane cell expression levels. Our results show that TNF-α exposure increased the density of the total, TTXs and TTXr sodium current in DRG neurons. Furthermore, TNF-α shifted the steady state activation and inactivation curves of the total and TTXs sodium current. DRG neurons expressing the CRMP2-K374A mutant also exhibited total sodium current increases after exposure to TNF-α, indicating that these effects were independent of SUMOylation of CRMP2. In conclusion, TNF-α sensitizes DRG neurons via augmentation of whole cell sodium current. This may underlie the pronociceptive effects of TNF-α and suggests a molecular mechanism responsible for pain hypersensitivity in diabetic neuropathy patients.

Noradrenaline induces peripheral antinociception by endogenous opioid release.

BACKGROUND: The aim of this study was to investigate this involvement in not inflammatory model of pain and which opioid receptor subtype mediates noradrenaline-induced peripheral antinociception. Noradrenaline is involved in the intrinsic control of pain-inducing pro-nociceptive effects in the primary afferent nociceptors. However, inflammation can induce various plastic changes in the central and peripheral noradrenergic system that, upon interaction with the immune system, may contribute, in part, to peripheral antinociception. METHODS: Hyperalgesia was induced by intraplantar injection of prostaglandin E2 (PGE2, 2µg) into the plantar surface of the right hind paw and the paw pressure test to evaluated the hyperalgesia was used. Noradrenaline (NA) was administered locally into right hind paw of Wistar rat (160-200g) alone and after either agents, α2-adrenoceptor antagonist yohimbine, α1-adrenoceptor antagonist prazosin, ß-adrenoceptor antagonist propranolol, µ-opioid antagonist clocinnamox, δ-opioid antagonist naltrindole and κ-opioid antagonist nor-binaltorfimina. In addition, the enkephalinase inhibitor bestatin was administered prior to NA low dose. RESULTS: Intraplantar injection of NA induced peripheral antinociception against hyperalgesia induced by PGE2. This effect was reversed, in dose dependent manner, by intraplantar injection of yohimbine, prazosin, propranolol, clocinnamox and naltrindole. However, injection of nor-binaltorfimina did not alter antinociception of NA after PGE2 hyperalgesia. Bestatin intensified the antinociceptive effects of low-dose of NA. CONCLUSION: Besides the α2-adrenoceptor, the present data provide evidence that, in absence of inflammation, NA activating α1 and ß-adrenoceptor induce endogenous opioid release to produce peripheral antinociceptive effect by µ and δ opioid receptors.

Serotonin induces peripheral antinociception via the opioidergic system.

PURPOSE: Studies conducted since 1969 have shown that the release of serotonin (5-HT) in the dorsal horn of the spinal cord contributes to opioid analgesia. In the present study, the participation of the opioidergic system in antinociceptive effect serotonin at the peripheral level was examined. METHODS: The paw pressure test was used with mice (Swiss, males from 35 g) which had increased pain sensitivity by intraplantar injection of PGE2 (2 µg). Serotonin (250 ng), administered locally to the right paw of animals, produces antinociception in this model. RESULTS: The selective antagonists for mu, delta and kappa opioid receptors, clocinnamox clocinnamox (40 µg), naltrindole (60 µg) and nor-binaltorfimina (200 µg), respectively, inhibited the antinociceptive effect induced by serotonin. Additionally, bestatin (400 µg), an inhibitor of enkephalinases that degrade peptides opioids, enhanced the antinociceptive effect induced by serotonin (low dose of 62.5 ng). CONCLUSIONS: These results suggest that serotonin possibly induce peripheral antinociception through the release of endogenous opioid peptides, possible from immune cells or keratinocytes.

Peripheral Antinociception Induced by Aripiprazole Is Mediated by the Opioid System.

BACKGROUND: Aripiprazole is an antipsychotic drug used to treat schizophrenia and related disorders. Our previous study showed that this compound also induces antinociceptive effects. The present study aimed to assess the participation of the opioid system in this effect. METHODS: Male Swiss mice were submitted to paw pressure test and hyperalgesia was induced by intraplantar injection of prostaglandin E2 (PGE2, 2 µg). Aripiprazole was injected 10 min before the measurement. Naloxone, clocinnamox, naltrindole, nor-binaltorphimine, and bestatin were given 30 min before aripiprazole. Nociceptive thresholds were measured in the 3rd hour after PGE2 injection. RESULTS: Aripiprazole (100 µg/paw) injected locally into the right hind paw induced an antinociceptive effect that was blocked by naloxone (50 µg/paw), a nonselective opioid receptor antagonist. The role of µ-, δ-, and κ-opioid receptors was investigated using the selective antagonists, clocinnamox (40 µg/paw), naltrindole (15, 30, and 60 µg/paw), and nor-binaltorphimine (200 µg/paw), respectively. The data indicated that only the δ-opioid receptor antagonist inhibited the peripheral antinociception induced by aripiprazole. Bestatin (400 µg), an aminopeptidase-N inhibitor, significantly enhanced low-dose (25 µg/paw) aripiprazole-induced peripheral antinociception. CONCLUSION: The results suggest the participation of the opioid system via δ-opioid receptor in the peripheral antinociceptive effect induced by aripiprazole.

Peltatoside Isolated from Annona crassiflora Induces Peripheral Antinociception by Activation of the Cannabinoid System.

Peltatoside is a natural compound isolated from leaves of Annona crassiflora Mart., a plant widely used in folk medicine. This substance is an analogue of quercetin, a flavonoid extensively studied because of its diverse biological activities, including analgesic effects. Besides, a previous study suggested, by computer structure analyses, a possible quercetin-CB1 cannabinoid receptor interaction. Thus, the aim of this work was to assess the antinociceptive effect of peltatoside and analyze the cannabinoid system involvement in this action. The mouse paw pressure test was used and hyperalgesia was induced by intraplantar injection of carrageenan (200 µg/paw). All used drugs were administered by intraplantar administration in Swiss male mice (n = 6). Peltatoside (100 µg/paw) elicited a local inhibition of hyperalgesia. The peripheral antinociceptive action of peltatoside was antagonized by the CB1 cannabinoid antagonist AM251 (160 µg/paw), but not by CB2 cannabinoid antagonist AM630 (100 µg/paw). In order to assess the role of endocannabinoids in this peripheral antinociceptive effect, we used (i) [5Z,8Z,11Z,14Z]-5,8,11,14-eicosatetraenyl-methyl ester phosphonofluoridic acid, an inhibitor of anandamide amidase; (ii) JZL184, an inhibitor for monoacylglycerol lipase, the primary enzyme responsible for degrading the endocannabinoid 2-arachidonoylglycerol; and (iii) VDM11, an endocannabinoid reuptake inhibitor. MAFP, JZL184, and VDM11 did not induce antinociception, respectively, at the doses 0.5, 3.8, and 2.5 µg/paw, however, these three drugs were able to potentiate the peripheral antinociceptive effect of peltatoside at an intermediary dose (50 µg/paw). Our results suggest that this natural substance is capable of inducing analgesia through the activation of peripheral CB1 receptors, involving endocannabinoids in this process.

Endocannabinoid mechanism for orofacial antinociception induced by electroacupuncture in acupoint St36 in rats.

INTRODUCTION: This study was conducted with the aim of evaluating whether electroacupuncture (EA) at acupoint St36 could produce antinociception through the activation of an endocannabinoid mechanism. METHODS: Male Wistar rats were divided into experimental groups. Heat was applied to the faces of rats, and the latency to withdraw the face was measured. Furthermore, the influence of electrical stimulation (100HzP) of acupoint St36, at a 0.5mA intensity, was investigated in the facial withdrawal threshold. RESULTS: The EA produced antinociception, which lasted for 180min. This effect was antagonized by the pre-injection of AM 251, a CB1 cannabinoid receptor antagonist, but not by AM 630, a CB2 cannabinoid receptor antagonist. Additionally, pretreatment with an endocannabinoid metabolizing enzyme inhibitor (MAFP) and an anandamide reuptake inhibitor (VDM11) prolonged and intensified the antinociceptive effect produced by EA. CONCLUSION: This study demonstrated for the first time that the CB1 cannabinoid receptor participates in the antinociceptive effect induced by EA.

κ-Opioid receptor participates of NSAIDs peripheral antinociception.

NSAIDs represent some of the most widely prescribed drugs for relief of short-term fever, pain and inflammation. The participation of the opioid system in the peripheral is poorly understood. The aim of this study was evaluate the role of opioid system in the peripheral antinociception by diclofenac and dipyrone. To test this hypothesis, opioid receptor antagonists were evaluated using the rat paw pressure test, in which pain sensitivity is increased by intraplantar injection of prostaglandin E2 (PGE2, 2µg). Diclofenac (20µg/paw) and Dipyrone (40µg/paw) administered locally into the right paw elicited an antinociceptive effect. It was used naloxone (50µg/paw), a non-selective opioid receptor antagonist, which antagonized peripheral antinociception induced by diclofenac and dipyrone. Selectively, it was evaluated the µ-, δ- and κ-opioid receptor antagonists, respectively, clocinnamox (40µg/paw), naltrindole (50µg/paw) and nor-binaltorphimine (20, 40 and 80µg/paw). Our data indicated that only the κ-opioid antagonist was capable to reverse the peripheral antinociception by NSAIDs. The present results provide evidence that the opioid system participated in the diclofenac and dipyrone-induced peripheral antinociception by indirect activation of κ-opioid receptor probable by release of endogenous opioids such as dynorphins.