Peripheral Morphine Research Articles

Peripheral Morphine Tolerance Involves Keratinocytes And PDGFR_ Signaling

Peripheral Morphine Tolerance Involves Keratinocytes And PDGFR_ Signaling

Stable isotope‐labelled morphine to study in vivo central and peripheral morphine glucuronidation and brain transport in tolerant mice

Chronic administration of medication can significantly affect metabolic enzymes leading to physiological adaptations. Morphine metabolism in the liver has been extensively studied following acute morphine treatment, but such metabolic processes in the CNS are poorly characterized. Long-term morphine treatment is limited by the development of tolerance, resulting in a decrease of its analgesic effect. Whether or not morphine analgesic tolerance affects in vivo brain morphine metabolism and blood-brain barrier (BBB) permeability remains a major question. Here, we have attempted to characterize the in vivo metabolism and BBB permeability of morphine after long-term treatment, at both central and peripheral levels. Male C57BL/6 mice were injected with morphine or saline solution for eight consecutive days in order to induce morphine analgesic tolerance. On the ninth day, both groups received a final injection of morphine (85%) and d3-morphine (morphine bearing three 2 H; 15%, w/w). Mice were then killed and blood, urine, brain and liver samples were collected. LC-MS/MS was used to quantify morphine, its metabolite morphine-3-glucuronide (M3G) and their respective d3-labelled forms. We found no significant differences in morphine CNS uptake and metabolism between control and tolerant mice. Interestingly, d3-morphine metabolism was decreased compared to morphine without any interference with our study. Our data suggests that tolerance to the analgesic effects of morphine is not linked to increased glucuronidation to M3G or to altered global BBB permeability of morphine.

A mimetic of the mSin3-binding helix of NRSF/REST ameliorates abnormal pain behavior in chronic pain models

The neuron-restrictive silencing factor NRSF/REST binds to neuron-restrictive silencing elements in neuronal genes and recruits corepressors such as mSin3 to inhibit epigenetically neuronal gene expression. Because dysregulation of NRSF/REST is related to neuropathic pain, here, we have designed compounds to target neuropathic pain based on the mSin3-binding helix structure of NRSF/REST and examined their ability to bind to mSin3 by NMR. One compound, mS-11, binds strongly to mSin3 with a binding mode similar to that of NRSF/REST. In a mouse model of neuropathic pain, mS-11 was found to ameliorate abnormal pain behavior and to reverse lost peripheral morphine analgesia. Furthermore, even in the less well epigenetically defined case of fibromyalgia, mS-11 ameliorated symptoms in a mouse model, suggesting that fibromyalgia is related to the dysfunction of NRSF/REST. Taken together, these findings show that the chemically optimized mimetic mS-11 can inhibit mSin3-NRSF/REST binding and successfully reverse lost peripheral and central morphine analgesia in mouse models of pain.

Peripheral morphine reduces acute pain in inflamed tissue after third molar extraction: A double-blind, randomized, active-controlled clinical trial

BackgroundThe clinical use of peripheral analgesic effects of opioids has been investigated in numerous controlled clinical trials. The majorities of these have tested the local, intra-articular administration of morphine in knee surgery and have demonstrated marginal postoperative analgesia. ObjectiveWe examined direct morphine infiltration of the surgical site in a clinical model of tooth pain under two different conditions. Eighty-eight patients undergoing surgical tooth removal entered into the two prospective, parallel, randomized, double-blind studies. MethodsPatients undergoing surgical tooth removal received a standard local anaesthetic solution (articaine plus epinephrine) before surgery.Patients were assigned to an injection of peripheral 2mg morphine either into non-inflamed (Trial I) or inflamed (Trial II) submucous tissue before the surgery. Patients who received an intramuscular morphine in the upper arm were concomitantly given 1ml isotonic saline (NaCl) as a submucous injection. Patients who received a submucous injection of morphine peripherally were concomitantly given an intramuscular injection (IM) of 1ml of NaCl in the upper arm. Postoperative pain intensity was assessed by the numeric rating scale every 30min for the first 2h and then every hour for the next 8h after surgery. In addition, patients recorded the occurrence of side effects and the supplemental consumption of ibuprophen and codeine+paracetamol combination tablets. ResultsOf the eighty-eight original participants, nine patients (4 patients in Trial I and 5 patients in Trial II) were withdrawn for protocol noncompliance and loss at follow-up. Thirty-one patients in trial I and forty-eight patients in trial II were analyzed. Patients receiving 2mg morphine into non-inflamed tissue did not show any further reduction in pain scores and pain medication consumption compared to IM morphine group (Trial I).In patients receiving 2mg morphine into inflamed tissue, pain scores at rest were reduced to a similar extent in both groups at all measurement times up to 10h in the follow-up (Trial II). At the same time, in the area under the curves pain scores on swallowing between 2 and 6h in the peripheral morphine group (5.2±5.6) were significantly lower than in the IM morphine group (9.3±7.3, p=0.03), demonstrating the marginal analgesic efficacy of additional morphine. Sedation scores were significantly higher in the peripheral morphine group only 1h after surgery in Trial I (p=0.008). The time to first analgesic intake was similar between groups. No serious side effects were reported. ConclusionsOur results showed in patients undergoing surgical tooth removal that injection of 2mg of morphine into inflamed tissue results in significantly lower pain scores on swallowing in the early postoperative state while administration into non-inflamed tissue is not effective. ImplicationsOur studies indicate that the peripheral administration of opioids, at the doses and conditions set out for these two studies, produces significant analgesia by a pharmacologically specific mechanism that is active in chronically, but not acutely, inflamed tissue. Thus, consistent with preclinical experimental studies, the requirement of an inflammatory process for the occurrence of the peripheral opioid effects is also found in the clinical setting.

Histone deacetylase inhibitors relieve morphine resistance in neuropathic pain after peripheral nerve injury

Neuropathic pain is often insensitive to morphine. Our previous study has demonstrated that neuron-restrictive silencer factor represses mu opioid receptor (MOP) gene expression in the dorsal root ganglion (DRG) via histone hypoacetylation-mediated mechanisms after peripheral nerve injury, thereby causing loss of peripheral morphine analgesia. Here, we showed that histone deacetylase (HDAC) inhibitors, such as trichostatin A and valproic acid, restored peripheral and systemic morphine analgesia in neuropathic pain. Also, these agents blocked nerve injury-induced MOP down-regulation in the DRG. These results suggest that HDAC inhibitors could serve as adjuvant analgesics to morphine for the management of neuropathic pain.

The effects of morphine and methylnaltrexone on gastrointestinal pain in healthy male participants.

Opioid antagonists are increasingly used to abolish the gastrointestinal side effects of opioids. However, they can potentially interfere with local analgesia exerted via opioid receptors in the gut. Thus, in the current study we aimed to explore the effect of rectal morphine before and after blocking opioid receptors outside the central nervous system with methylnaltrexone (MNTX). In this randomized, placebo controlled, cross-over study 15 healthy male participants received the following drugs at three separate sessions: (i) placebo, (ii) 30 mg morphine administered per rectum, or (iii) 12 mg MNTX given subcutaneously before 30 mg rectal morphine. At baseline and after drug administration peripheral and central effects of the drugs were assessed by experimental pain to the skin, muscle, rectum and pupillometry. Compared to placebo there was no local effect of morphine on mechanical rectal distension. In contrast, an increase in tolerated volume was seen following MNTX/morphine administration (p < 0.001), starting 7 min after dosing. Both morphine and MNTX/morphine had a central effect manifested as an increase in mechanical muscle pressure thresholds (both p < 0.001) and a decrease in pupil diameter (both p < 0.001). These effects occurred 30 min after dosing. No peripheral analgesic effect of morphine was found. Methodological shortcomings may have contributed to the lack of peripheral analgesia and thus, a peripheral morphine effect on rectal pain cannot be excluded. On the other hand, the combination of MNTX and morphine exerted a local effect on rectal distensions and seems to improve analgesia.

In vitro morphine metabolism by rat microglia

Morphine is mainly transformed to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) in the liver. Glucuronidation is also performed by rat brain homogenates and UDP-glucuronosyltransferases (UGTs) are present in the brain. Here we investigated the possibility that microglia transforms morphine into its metabolites M3G and M6G. Primary cultures of neonatal rat microglia were incubated for different intervals of time in basal conditions or with different concentrations of morphine. The following measures were performed on these cultures and/or in the medium: (i) morphine as well as M3G and M6G concentrations; (ii) levels of mRNA coding for UGT1A1, UGT1A6, UGT1A7, and UGT2B1 as well as their protein levels; (iii) released prostaglandin (PG)E2 and nitrite concentrations. Results show that in basal conditions morphine and M3G are produced by microglia; accordingly, these cells expressed UGT1A1, UGT1A6 and UGT1A7, but not UGT2B1. When cultures were exposed to different concentrations of exogenous morphine, M6G was also synthesized. This shift in the glucuronidation was associated with variations in the expression of UGT isozymes. In particular, UGT1A7 expression was rapidly upregulated and this event was translated into enhanced protein levels of UGT1A7; lesser effects were exerted on UGT1A1 and UGT1A6. Upon prolonged exposure to morphine, microglial cell UGT expression returned to baseline conditions or even to reduced levels of expression. Morphine exposure did not affect the synthesis of both PGE2 and nitrites, ruling out a generalized priming of microglia by morphine. In conclusion, this study suggests that morphine glucuronides found in the cerebrospinal liquor upon peripheral morphine administration may at least in part be brain-born, reconciling the conceptual gap between the high hydrophilic features of morphine glucuronides and their presence beyond the blood–brain barrier.

Potentiation of morphine-induced mechanical antinociception by σ1 receptor inhibition: Role of peripheral σ1 receptors

We studied the modulation of morphine-induced mechanical antinociception and side effects by σ1 receptor inhibition. Both wild-type (WT) and σ1 receptor knockout (σ1-KO) mice showed similar responses to paw pressure (100–600 g). The systemic (subcutaneous) or local (intraplantar) administration of σ1 antagonists (BD-1063, BD-1047, NE-100 and S1RA) was devoid of antinociceptive effects in WT mice. However, σ1-KO mice exhibited an enhanced mechanical antinociception in response to systemic morphine (1–16 mg/kg). Similarly, systemic treatment of WT mice with σ1 antagonists markedly potentiated morphine-induced antinociception, and its effects were reversed by the selective σ1 agonist PRE-084. Although the local administration of morphine (50–200 μg) was devoid of antinociceptive effects in WT mice, it induced dose-dependent antinociception in σ1-KO mice. This effect was limited to the injected paw. Enhancement of peripheral morphine antinociception was replicated in WT mice locally co-administered with σ1 antagonists and the opioid. None of the σ1 antagonists tested enhanced morphine-antinociception in σ1-KO mice, confirming a σ1-mediated action. Morphine-induced side-effects (hyperlocomotion and inhibition of gastrointestinal transit) were unaltered in σ1-KO mice. These results cannot be explained by a direct interaction of σ1 ligands with μ-opioid receptors or adaptive changes of μ-receptors in σ1-KO mice, given that [3H]DAMGO binding in forebrain, spinal cord, and hind-paw skin membranes was unaltered in mutant mice, and none of the σ1 drugs tested bound to μ-opioid receptors. These results show that σ1 receptor inhibition potentiates morphine-induced mechanical analgesia but not its acute side effects, and that this enhanced analgesia can be induced at peripheral level.

A comparison of methods of post-operative analgesia in children undergoing limb reconstruction with circular frames

This retrospective study compared post-operative epidural analgesia (E), continuous peripheral nerve blocks (CPNB) and morphine infusion (M) in 68 children undergoing limb reconstruction with circular frames. The data collected included episodes of severe pain, post-operative duration of analgesia, requirement for top-up analgesia, number of osteotomies, side effects and complications. There was a significant difference between the number of episodes of severe pain in patients receiving a morphine infusion and those receiving epidurals or CPNB (M vs E, p < 0.0001; M vs CPNB, p = 0.018). The CPNB group was associated with the lowest incidence of episodes of severe pain and top-up analgesia. Epidural analgesia was associated with significantly more nausea and vomiting than morphine infusion (p = 0.053) and CPNB (p = 0.023). It also had a significantly higher incidence of motor blockade than CPNB (p < 0.01). We found that the most effective method of post-operative analgesia for children undergoing lower limb reconstruction was sciatic nerve catheterisation and continuous infusion.

Antipropulsive effects of central and peripheral morphine in the rat gastrointestinal tract.

The antipropulsive effects of centrally or peripherally administered morphine have been examined at three levels of the rat gastrointestinal tract. Adult male rats were anaesthetized with pentobarbitone (50 mg kg-1 i.p.) and were implanted with an intraluminal catheter in either the proximal duodenum or mid-jejunum. Other animals were also implanted with a cannula in the right lateral cerebral ventricle. Gastric emptying and transit were determined selectively by measuring the progression of a radioactive chromium (CR-51) solution, given intragastrically for tests of gastric emptying or instilled into the proximal or distal intestinal catheter for determination of intestinal transit, 30 min after administration of morphine or saline given either subcutaneously (s.c.) or intracerebroventricularly (i.c.v.). Morphine given either s.c. (5 mg kg-1) or i.c.v. (30 micrograms, total dose) significantly inhibited gastric emptying and transit through bulk portions of the small intestine indicating that by either route it inhibits propulsion at all three levels of the gastrointestinal tract.

Morphine infusions into the rostrolateral periaqueductal gray affect maternal behaviors

It is well established that morphine inhibits maternal behaviors. Previous studies by our group have shown activation of the rostrolateral periaqueductal gray (rlPAG) upon inhibition-intended subcutaneous injections of morphine. In this context, we demonstrated that a single naloxone infusion into the rlPAG, following this opioid-induced inhibition, reactivated maternal behaviors. Since these data were obtained by using peripheral morphine injections, the present study was designed to test whether morphine injected directly into the rlPAG would affect maternal behaviors. Our hypothesis that morphine acting through the rlPAG would disrupt maternal behaviors was confirmed with a local infusion of morphine. The mothers showed shorter latency for locomotor behavior to explore the home cage (P = 0.049). Inhibition was especially evident regarding retrieving (P = 0.002), nest building (P = 0.05) and full maternal behavior (P = 0.023). These results support the view that opioidergic transmission plays a behaviorally meaningful inhibitory role in the rostrolateral PAG.

Peripheral chronic inflammatory pain and morphine analgesia: Involvement of interleukin-1

Peripheral chronic inflammatory pain and morphine analgesia: Involvement of interleukin-1

Epigenetic Gene Silencing Underlies C-Fiber Dysfunctions in Neuropathic Pain

Peripheral nerve injury causes neuropathic pain, which is characterized by the paradoxical sensations of positive and negative symptoms. Clinically, negative signs are frequently observed; however, their underlying molecular mechanisms are largely unknown. Dysfunction of C-fibers is assumed to underlie negative symptoms and is accompanied by long-lasting downregulation of Na(v)1.8 sodium channel and mu-opioid receptor (MOP) in the dorsal root ganglion (DRG). In the present study, we found that nerve injury upregulates neuron-restrictive silencer factor (NRSF) expression in the DRG neurons mediated through epigenetic mechanisms. In addition, chromatin immunoprecipitation analysis revealed that nerve injury promotes NRSF binding to the neuron-restrictive silencer element within MOP and Na(v)1.8 genes, thereby causing epigenetic silencing. Furthermore, NRSF knockdown significantly blocked nerve injury-induced downregulations of MOP and Na(v)1.8 gene expressions, C-fiber hypoesthesia, and the losses of peripheral morphine analgesia and Na(v)1.8-selective blocker-induced hypoesthesia. Together, these data suggest that NRSF causes pathological and pharmacological dysfunction of C-fibers, which underlies the negative symptoms in neuropathic pain.

Deletion of the inducible nitric oxide synthase gene reduces peripheral morphine tolerance in a mouse model of chronic inflammation

The implication of inducible nitric-oxide synthase (iNOS) on peripheral tolerance to morphine was evaluated in wild-type (WT) and iNOS knockout mice. Chronic inflammation was induced by subplantar (s.p.) injection of Complete Freund's Adjuvant (CFA), and morphine tolerance by subcutaneous implantation of a 75 mg morphine-pellet. Withdrawal was assessed after the intraperitoneal injection of 2 mg/kg naloxone. Antinociception was assessed (Randall-Selitto test) 5 min after a fixed dose of s.p. morphine (16 microg). In the absence of inflammation, s.p. morphine did not induce antinociception, while during CFA-inflammation produced 47.4 +/- 0.8 and 38.8 +/- 2.7% inhibitions respectively, in each genotype (P < 0.05). In morphine-tolerant mice with CFA-inflammation, no antinociception could be elicited in WT mice (2.4 +/- 0.3% inhibition); however, iNOS knockout mice showed significant antinociception (33.1 +/- 0.9%) (P < 0.001). Thus, iNOS gene deletion partially prevented tolerance to the peripheral effects of morphine, and significantly attenuated withdrawal-induced hyperactivity.

Development of a behavior model of pain induced by experimental tooth movement in rats

The mechanism of orthodontic pain and discomfort is poorly understood partly because of the limited number of animal behavioral models for pain assessment. This study aimed to develop a behavioral model for assessment of tooth-movement pain in rats using directed face-grooming activity. Male Sprague-Dawley rats weighing 200-300 g were used. They were videotaped on days 1, 3, 5, 7, and 14 after experimental tooth movement and their directed face-grooming behavior was evaluated. In addition, we also evaluated behavioral responses to the application of a progressively higher magnitude force and to multiple applications of an equal magnitude force. Finally, the effects of peripherally and systemically administered morphine and of the N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, on the behavioral responses were evaluated. The results indicated that time spent on directed face-grooming activity increased dramatically after initiating experimental tooth movement. The change concurred with the initial orthodontic pain response. This behavioral change was reproducible and was related to force magnitude. Application of both systemic and peripheral morphine and MK-801 could exert an analgesic effect on this pain model. These results suggest that directed face-grooming behavior can be a reliable measure for tooth-movement pain in rats, which could be widely used in investigating the orthodontic pain mechanism.

Intramuscular administration of morphine reduces mustard oil-induced craniofacial muscle pain behavior in lightly anesthetized rats

The present study investigated the role of peripheral opioid receptors in mustard oil-induced nociceptive behavior and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague–Dawley rats weighing between 300 and 400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for the intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 μl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle. Intramuscularly-administered morphine significantly reduced shaking behavior but not MO-induced inflammation. Intramuscular pretreatment with naloxone, an opioid receptor antagonist, reversed antinociception produced by intramuscularly-administered morphine, while intracisternal administration of naloxone did not affect the antinociception of peripheral morphine. Pretreatment with d-Pen-Cys-Tyr- d-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a μ opioid receptor antagonist, but not naltrindole, a δ opioid receptor antagonist, nor norbinaltorphimine (nor-BNI), a κ opioid receptor antagonist, reversed intramuscularly-administered morphine-induced antinociception. These results indicate that intramuscularly-administered morphine produces antinociception in craniofacial muscle nociception and that this intramuscularly-administered morphine-induced antinociception is mediated by a peripheral μ opioid receptor. Our observations further support the clinical approach of administering opioids in the periphery for the treatment of craniofacial muscle nociception.

Conformation State-sensitive Antibodies to G-protein-coupled Receptors

A growing body of evidence indicates that G-protein-coupled receptors undergo complex conformational changes upon agonist activation. It is likely that the extracellular region, including the N terminus, undergoes activation-dependent conformational changes. We examined this by generating antibodies to regions within the N terminus of micro-opioid receptors. We find that antibodies to the midportion of the N-terminal tail exhibit enhanced recognition of activated receptors, whereas those to the distal regions do not. The enhanced recognition is abolished upon treatment with agents that block G-protein coupling or deglycosylate the receptor. This suggests that the N-terminal region of mu receptors undergoes conformational changes following receptor activation that can be selectively detected by these region-specific antibodies. We used these antibodies to characterize micro receptor type-specific ligands and find that the antibodies accurately differentiate ligands with varying efficacies. Next, we examined if these antibodies can be used to investigate the extent and duration of activation of endogenous receptors. We find that peripheral morphine administration leads to a time-dependent increase in antibody binding in the striatum and prefrontal cortex with a peak at about 30 min, indicating that these antibodies can be used to probe the spatio-temporal dynamics of native mu receptors. Finally, we show that this strategy of targeting the N-terminal region to generate receptor conformation-specific antisera can be applied to other G(alpha)(i)-coupled (delta-opioid, CB1 cannabinoid, alpha(2A)-adrenergic) as well as G(alpha)(s)-(beta(2)-adrenergic) and G(alpha)(q)-coupled (AT1 angiotensin) receptors. Taken together, these studies describe antisera as tools that allow, for the first time, studies probing differential conformation states of G-protein-coupled receptors, which could be used to identify molecules of therapeutic interest.

The Influence of Site of Collection on Postmortem Morphine Concentrations in Heroin Overdose Victims

When assaying for postmortem morphine concentration, significant site sampling variability exists between central and peripheral sampling sites and even within sampling regions of the body. To study the variation, 76 suspected heroin overdoses were identified. Each had femoral artery (FA) and vein (FV), left and right ventricle and pooled heart blood samples obtained at autopsy. Forty-four tested positive for morphine. Morphine concentrations were determined by gas chromatography/mass spectrometry, with sampling site differences reported as log-transformed ratios and compared by signed rank test. The mean FA to FV ratio for total morphine was 1.2 (range 0-4.5). The ratio for left heart to right heart total morphine was 1.1 (range 0.4-3.2). Left ventricular to FV total morphine ratio was 2.0 (range 0.6-6.9). In these opioid overdose deaths, FA and FV morphine concentrations are usually similar, although up to 4.5-fold differences were noted. Centrally obtained morphine concentrations are on average twice as high compared with peripheral morphine concentrations. Left and right ventricular morphine concentrations were usually similar, although up to 3.2-fold differences were noted (left side higher).

Loss of Morphine–induced Suppression of NK Cell activity and T-cell Functions in μ-Opioid Receptor Knockout Mice

In vivo administration of µ-opioid receptor selective agonists to various species is known to suppress lymphocyte, NK cell, and macrophage functions, in addition to mediate pain relief and euphoria. Using a mouse model in which the µ-opioid receptor gene was disrupted by targeted homologous recombination, we explored the involvement of this receptor in natural killer (NK) cell activity and T lymphocyte function. Following peripheral morphine administration, NK cell activity was not affected in homozygous µ-opioid receptor knockout mice, heterozygous animals marginally responded to the immunosuppressive effects of the drug, while wild-type animals were significantly suppressed. In addition, splenic T-cell proliferative responses to concanavalin A, phytohemaglutinin and an antibody to T-cell receptorabαβ (TCR) plus interleukin-2 were not affected by morphine treatment in µ-opioid receptor knockout homozygous and heterozygous mice, whereas morphine significantly suppressed T-cell proliferation in wild-type mice. Taken together, these results suggest a role of the µ-opioid receptor in immunoregulation.

Morphine Directly Inhibits Nociceptors in Inflamed Skin

Peripherally delivered opiates attenuate mechanical and thermal hyperalgesia in experimental models of inflammation, suggesting that activation of peripheral opioid receptors decreases the excitability of nociceptors in inflamed tissues. The current study examines the effects of peripheral morphine sulfate on response properties of sensory neurons in healthy and inflamed skin. Afferent units (185) were isolated from tibial nerve of rats using an in vitro glabrous skin-nerve teased-fiber preparation. Of these, 107 units were from normal healthy skin, and 78 were from inflamed skin 18 h after intraplantar injection of complete Freund's adjuvant. As a population, C-fiber units innervating inflamed skin exhibited properties characteristic of sensitization when compared with units innervating healthy control skin. Mechanical thresholds were lowered, responses to noxious mechanical and thermal stimuli were elevated, a greater proportion of units was spontaneously active, and the average rate of spontaneous discharge was higher. Response properties in other conduction velocity groups remained unchanged. Fifty-eight percent of C and C/Adelta nociceptors innervating inflamed skin were opiate-sensitive, and their excitability was attenuated by direct application of morphine to their receptive fields. All morphine-sensitive units were nociceptors from inflamed skin with conduction velocities <1.3 m/s. Morphine effects were concentration-dependent and naloxone-sensitive, indicating that the effects were receptor-mediated. These findings provide direct evidence that morphine acts through peripheral opioid receptors to inhibit the activity of cutaneous nociceptors under conditions of inflammation.