Daniela da Fonseca Pacheco

Central antinociception induced by µ-opioid receptor agonist morphine, but not δ- or κ-, is mediated by cannabinoid CB1 receptor

Background and purpose:  It has been demonstrated that cannabinoids evoke the release of endogenous opioids to produce antinociception; however, no information exists regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids are involved in central antinociception induced by activation of µ‐, δ‐ and κ‐opioid receptors.

Opioid receptor and NO/cGMP pathway as a mechanism of peripheral antinociceptive action of the cannabinoid receptor agonist anandamide.

AIMS In this study, we investigated whether the opioid system and the nitric oxide pathway were involved in the peripheral antinociception induced by a cannabinoid receptor agonist anandamide. MAIN METHODS Hyperalgesia was induced by a subcutaneous injection of carrageenan (250 microg) into the plantar surface of the rats hindpaw and measured by the paw pressure test 3h after injection. The weight in grams (g) required to elicit a nociceptive response, paw flexion, was determined as the nociceptive threshold. KEY FINDINGS Anandamide elicited a dose-dependent (50, 75, and 100 ng per paw) antinociceptive effect. The highest dose of anandamide did not produce antihyperalgesia in the contralateral paw, indicating a peripheral site of action. The CB(1) receptor antagonist AM251 (20, 40, 80 and 160mug per paw) antagonized peripheral antihyperalgesia induced by anandamide (100 ng), in a dose-dependent manner, suggesting CB(1) receptor activation. Anandamide-induced peripheral antihyperalgesia was reverted by blockers of the l-arginine/NO/cGMP pathway N(G)-nitro-l-arginine (NOARG; 24, 36 and 48 microg per paw) and 1H-[1,2,4] Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 25, 50 and 100 microg per paw), in a dose-dependent manner. Furthermore, opioid receptor antagonist naloxone (12.5, 25 and 50 microg per paw) antagonized the peripheral antihyperalgesia induced by anandamide. SIGNIFICANCE This study provides evidence that the peripheral antinociceptive effect of the cannabinoid receptor agonist anandamide may result from l-arginine/NO/cGMP pathway activation and that the opioid system is also involved.

Central antinociception induced by ketamine is mediated by endogenous opioids and μ- and δ-opioid receptors

It is generally believed that NMDA receptor antagonism accounts for most of the anesthetic and analgesic effects of ketamine, however, it interacts at multiple sites in the central nervous system, including NMDA and non-NMDA glutamate receptors, nicotinic and muscarinic cholinergic receptors, and adrenergic and opioid receptors. Interestingly, it was shown that at supraspinal sites, ketamine interacts with the μ-opioid system and causes supraspinal antinociception. In this study, we investigated the involvement of endogenous opioids in ketamine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Our results demonstrated that the opioid receptor antagonist naloxone, the μ-opioid receptor antagonist clocinnamox and the δ-opioid receptor antagonist naltrindole, but not the κ-opioid receptor antagonist nor-binaltorphimine, antagonized ketamine-induced central antinociception in a dose-dependent manner. Additionally, the administration of the aminopeptidase inhibitor bestatin significantly enhanced low-dose ketamine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ- and δ-opioid receptors in ketamine-induced central antinociception. In contrast, κ-opioid receptors not appear to be involved in this effect.

Endogenous cannabinoid receptor agonist anandamide induces peripheral antinociception by activation of ATP-sensitive K+ channels

AIMS The effects of several potassium (K(+)) channel blockers were studied to determine which K(+) channels are involved in peripheral antinociception induced by the cannabinoid receptor agonist, anandamide. MAIN METHODS Hyperalgesia was induced by subcutaneous injection of 250 μg carrageenan into the plantar surface of the hind paw of rats. The extent of hyperalgesia was measured using a paw pressure test 3 h following carrageenan injection. The weight in grams (g) that elicited a nociceptive response, paw flexion, during the paw pressure test was used as the nociceptive response threshold. KEY FINDINGS Doses of 50, 75, and 100 ng of anandamide elicited a dose-dependent antinociceptive effect. Following a 100 ng dose of anandamide no antinociception was observed in the paw that was contralateral to the anandamide injection site, which shows that anandamide has a peripheral site of action. Pretreatment with 20, 40 and 80 μg AM251, a CB(1) receptor antagonist, caused a dose-dependent decrease in anandamide-induced antinociception, suggesting that the CB(1) receptor is directly involved in anandamide effect. Treatment with 40, 80 and 160 μg glibenclamide, an ATP-sensitive K(+) channel blocker, caused a dose-dependent reversal of anandamide-induced peripheral antinociception. Treatment with other K(+) channel antagonists, tetraethylammonium (30 μg), paxilline (10 μg) and dequalinium (50 μg), had no effect on the induction of peripheral antinociception by anandamide. SIGNIFICANCE This study provides evidence that the peripheral antinociceptive effect of the cannabinoid receptor agonist, anandamide, is primarily caused by activation of ATP-sensitive K(+) channels and does not involve other potassium channels.

Xylazine induced central antinociception mediated by endogenous opioids and μ-opioid receptor, but not δ-or κ-opioid receptors

Endogenous opioids have been implicated in compound-induced antinociception, and our group previously suggested that xylazine induces peripheral antinociception by releasing endogenous opioids that act on their respective receptors. In this study, we investigated the involvement of endogenous opioids in α2-adrenoceptor agonist xylazine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Probabilities less than 5% (p<0.05) were considered to be statistically significant (ANOVA/Bonferronis test). Our results demonstrated that opioid receptor antagonist naloxone and μ-opioid receptor antagonist clocinnamox, but not δ-opioid receptor antagonist naltrindole and κ-opioid receptor antagonist nor-binaltorphimine, antagonized xylazine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ-opioid receptors in xylazine-induced central antinociception. In contrast, δ- and κ-opioid receptors do not appear to be involved in this effect. The results contribute to a greater understanding of the central antinociceptive mechanisms of a drug widely used in veterinary therapy.

Inflammation Mobilizes Local Resources to Control Hyperalgesia: The Role of Endogenous Opioid Peptides

The aim of the present study was to investigate the mechanisms underlying the endogenous control of nociception at a peripheral level during inflammation. Using a pharmacological approach and the rat paw pressure test, we assessed the effect of an intraplantar injection of naloxone, an opioid receptor antagonist, and bestatin, an aminopeptidase inhibitor, on hyperalgesia induced by carrageenan, which mimics an inflammatory process, or prostaglandin E2 (PGE2), which directly sensitizes nociceptors. Naloxone induced a significant and dose-dependent (25, 50 or 100 µg) increase in carrageenan-induced hyperalgesia, but not PGE2-induced hyperalgesia. Bestatin (400 µg/paw) significantly counteracted carrageenan-induced hyperalgesia, inducing an increase in the nociceptive threshold compared to control, but it did not modify hyperalgesia induced by PGE2 injection into the rat paw. Positive β-endorphin immunoreactivity was increased in paw inflammation induced by carrageenan in comparison with the control group. However, PGE2 did not significantly alter the immunostained area. These results provide evidence for activation of the endogenous opioidergic system during inflammation and indicate that this system regulates hyperalgesia through a negative feedback mechanism, modulating it at a peripheral level.

PnPP-19, a spider toxin peptide, induces peripheral antinociception through opioid and cannabinoid receptors and inhibition of neutral endopeptidase.

The synthetic peptide PnPP‐19 has been studied as a new drug candidate to treat erectile dysfunction. However, PnTx2–6, the spider toxin from which the peptide was designed, induces hyperalgesia. Therefore, we intended to investigate the role of PnPP‐19 in the nociceptive pathway.

Peripheral antinociception induced by δ-opioid receptors activation, but not μ- or κ-, is mediated by Ca2 +-activated Cl− channels

Studies have demonstrated that the L-arginine/NO/cGMP pathway and the potassium and calcium channels are involved in the mechanisms underlying opioid receptor activation. As additional pathways may participate in the observed antinociceptive effects following opioid exposure, the aim of our study was to determine whether Ca(2+)-activated Cl(-) channels (CaCCs) are involved in peripheral antinociception induced by μ-, δ- and κ-opioid receptor activation. Hyperalgesia was induced by intraplantar injection of prostaglandin E(2) (PGE(2), 2 μg). Nociceptive thresholds to pressure (grams) were measured using an algesimetric apparatus 3h following injection. The μ-opioid receptor agonist morphine (200 μg), δ-opioid receptor agonist (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80, 80 μg), κ-opioid receptor agonist bremazocine (50 μg), CaCCs blocker niflumic acid (8-64 μg), CaCCs blocker 5-Nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 32-128 μg), nitric oxide donor sodium nitroprusside (SNP, 500 μg) and cGMP exogenous analogs dibutyryl cGMP (db-cGMP, 100 μg) were also administered into the paw. The CaCCs blocker niflumic acid and NPPB partially reversed the peripheral antinociception induced by exposure to the SNC80 in a dose-dependent manner. In contrast, niflumic acid did not modify the antinociceptive effect observed following exposure to morphine or bremazocine. Additionally, the peripheral antinociception induced by the NO donor SNP or by db-cGMP was not inhibited by niflumic acid. These results provide evidence for the involvement of CaCCs in the peripheral antinociception induced by SNC80. CaCCs activation does not appear to be involved when μ- and κ-opioid receptors are activated. In addition, we did not observe a link between CaCCs and the L-arginine/NO/GMPc pathway.

Probable involvement of Ca2 +-activated Cl− channels (CaCCs) in the activation of CB1 cannabinoid receptors

AIMS Recently, we demonstrated that peripheral antinociception induced by δ opioid receptor is dependent of Ca(2+)-activated Cl(-) channels (CaCCs). Because opioid and cannabinoid receptors share some common mechanisms of action, our objective was to identify a possible relationship between CaCCs and the endocannabinoid system. MAIN METHODS To induce hyperalgesia, rat paws were treated with intraplantar prostaglandin E2 (PGE2, 2μg). Nociceptive thresholds to pressure (grams) were measured using an algesimetric apparatus 3h following injection. Probabilities were calculated using ANOVA/Bonferronis test, and values that were less than 5% were considered to be statistically significant. KEY FINDINGS Administration of the cannabinoid agonist CB1 anandamide (12.5, 25 and 50μg/paw) and the cannabinoid agonist CB2 PEA (5, 10 and 20μg/paw) decreased the PGE2-induced hyperalgesia in a dose-dependent manner. The possibility of the higher doses of anandamide (50μg) and PEA (20μg) having a central or systemic effect was excluded because the administration of the drug into the contralateral paw did not elicit antinociception in the right paw. As expected, the antinociceptive effects induced by anandamide and PEA were blocked by the CB1 and CB2 receptor antagonists AM251 and AM630, respectively. The peripheral antinociception was induced by anandamide but not PEA and was dose-dependently inhibited by the CaCC blocker niflumic acid (8, 16 and 32μg). SIGNIFICANCE These results provide the first evidence for the involvement of CaCCs in the peripheral antinociception induced by activation of the CB1 cannabinoid receptor.

δ-Opioid receptor agonist SNC80 induces central antinociception mediated by Ca2+-activated Cl- channels

Objectives  The aim of this study was to determine whether Ca2+‐activated Cl‐ channels (CaCCs) are involved in central antinociception induced by the activation of µ‐, δ‐ and κ‐opioid receptors.