N-palmitoylethanolamide in the anterior cingulate cortex attenuates inflammatory pain behaviour indirectly via a CB1 receptor-mediated mechanism

Treatment of Established Chemotherapy-Induced Neuropathy with N-Palmitoylethanolamide: A Randomized, Double-Blind Phase II Pilot Study

Background: Chemotherapy-induced peripheral neuropathy (CIPN) from oxaliplatin and taxane drugs is a bothersome toxicity. Palmitoylethanolamide (PEA) has been reported to improve myelinated nerve fiber function in patients experiencing painful CIPN. We conducted a double-blind, placebo-controlled, randomized trial of PEA in patients with established CIPN. Methods: Eligible patients were adults who had pain, numbness, tingling, or other symptoms of CIPN for at least three months following completion of paclitaxel, oxaliplatin, or cisplatin-based chemotherapy. Study patients were randomized to one of the two treatment groups (PEA versus placebo, both administered either once or twice daily). The CIPN20 questionnaire was assessed weekly. Results: A total of 17 males and 71 females participated in the study; most had neuropathy from paclitaxel. Most (85%) finished 8 weeks of treatment. There was no suggestion that either of the PEA arms did any better than the combined placebo arms. There was no signal of significant toxicity differences between the three study arms. Quality of life outcome measures were similar between the study arms, as were cognitive function evaluations. Discussion: PEA failed to improve established CIPN. Future trials might explore whether PEA may be effective in preventing CIPN or cognitive changes based on data that suggest it may be helpful in this situation. Conclusions: PEA failed to improve established chemotherapy-induced neuropathy.

Electroacupuncture Regulates Cannabinoid Receptor 1 Expression in a Mouse Fibromyalgia Model: Pharmacological and Chemogenetic Modulation

Fibromyalgia is a chronic illness usually accompanied by long-lasting, general pain throughout the body, often accompanied by anxiety, depression, fatigue, and sleep disruption. Meanwhile, doctors and scientists have not entirely discovered detailed mechanisms; patients always have an exaggerated sensation to pervasive pain without satisfied medical service. Given the lack of knowledge on its underlying mechanism, current treatments aim to provide pain and/or symptom relief. The present study aimed to clarify the role of cannabinoid receptor 1 (CB1) signaling in a mouse fibromyalgia pain model. To develop the mouse fibromyalgia model, mice were subjected to intermittent cold stress (ICS). Our results indicated that mechanical (2.09 ± 0.09 g) and thermal hyperalgesia (4.77 ± 0.29 s), which were evaluated by von Frey and Hargraves' tests, were induced by ICS, suggesting successful modeling. The hurting replies were then provoked by electroacupuncture (EA) but not for sham EA mice. Further, in a Western blot analysis, we found significantly decreased CB1 protein levels in the thalamus, somatosensory cortex, and anterior cingulate cortex. In addition, the levels of pain-related protein kinases and transcription factor were increased. Treatment with EA reliably increased CB1 expression in various brain regions sequentially alleviated by nociceptive mediators. Furthermore, the administration of a CB1 agonist significantly attenuated fibromyalgia pain, reversed EA analgesia by the CB1 antagonist, and further reversed the chemogenetic inhibition of SSC. Our innovative findings evidence the role of CB1 signaling in the interaction of EA and fibromyalgia, suggesting its potential for clinical trials and as a treatment target.

Electroacupuncture Exerts Analgesic Effects by Restoring Hyperactivity via Cannabinoid Type 1 Receptors in the Anterior Cingulate Cortex in Chronic Inflammatory Pain

As one of the commonly used therapies for pain-related diseases in clinical practice, electroacupuncture (EA) has been proven to be effective. In chronic pain, neurons in the anterior cingulate cortex (ACC) have been reported to be hyperactive, while the mechanism by which cannabinoid type 1 receptors (CB1Rs) in the ACC are involved in EA-mediated analgesic mechanisms remains to be elucidated. In this study, we investigated the potential central mechanism of EA analgesia. A combination of techniques was used to detect the expression and function of CB1R, including quantitative real-time PCR (q-PCR), western blot (WB), immunofluorescence (IF), enzyme-linked immunosorbent assay (ELISA), and in vivo multichannel optical fibre recording, and neuronal activity was examined by in vivo two-photon imaging and in vivo electrophysiological recording. We found that the hyperactivity of pyramidal neurons in the ACC during chronic inflammatory pain is associated with impairment of the endocannabinoid system. EA at the Zusanli acupoint (ST36) can reduce the hyperactivity of pyramidal neurons and exert analgesic effects by increasing the endocannabinoid ligands anandamide (AEA), 2-arachidonoylglycerol (2-AG) and CB1R. More importantly, CB1R in the ACC is one of the necessary conditions for the EA-mediated analgesia effect, which may be related to the negative regulation of the N-methyl-D-aspartate receptor (NMDAR) by the activation of CB1R downregulating NR1 subunits of NMDAR (NR1) via histidine triad nucleotide-binding protein 1 (HINT1). Our study suggested that the endocannabinoid system in the ACC plays an important role in acupuncture analgesia and provides evidence for a central mechanism of EA-mediated analgesia.

Accurate quantification of endogenous N-acylethanolamides by chemical isotope labeling coupled with liquid chromatography-tandem mass spectrometry

N-acylethanolamides (NAEs) are a class of naturally occurring lipid molecules with pleiotropic activities ranging from energy homeostasis to analgesic functioning. However, the comprehensive quantitation of endogenous NAEs is challenged by the sub-trace level (nM) in complex biological samples and the limited availability of stable isotope labeled internal standards (SIL-IS). Herein, a sensitive method was developed to accurately determine 20 NAEs in biological samples by chemical isotope labeling strategy coupled with liquid chromatography - tandem mass spectrometry (LC-MS/MS). A pair of efficient derivatization reagents, acetyl chloride-d₀ (ACC-d₀) and acetyl chloride-d₃ (ACC-d₃), were used to label NAEs in biological samples and NAE standard mixture, respectively. The heavily labeled NAE derivatives of the standard substances were used as one-to-one internal standards to minimize the matrix effects and potential ion suppression in MS analysis. Although no chemical moiety with high ionization capability was introduced, the detection sensitivity of the derivatized NAEs were substantially enhanced, as evidenced by 6- to 170-fold increase in LOQs, compared to non-derivatized NAEs. The derivatized NAEs provided the stable and abundant specific product ions in MS/MS spectrum, which were used as the quantitation ions for multiple reaction monitoring (MRM) analysis. The validated LC-MS/MS method was also successfully applied to determine NAEs in serum samples and liver tissues from control and alcohol-fed mice, which shown its practicability in the analysis of endogenous NAE in biological samples. Collectively, the proposed method offers a sensitive and accurate quantification of endogenous NAEs, which may facilitate the understanding of NAE metabolisms and their functions in the physiological and pathological processes.

Cannabidiol modulation of oxidative stress and signalling

Cannabidiol (CBD), one of the primary non-euphoric components in the Cannabis sativa L. plant, has undergone clinical development over the last number of years as a therapeutic for patients with Lennox-Gastaut syndrome and Dravet syndromes. This phytocannabinoid demonstrates functional and pharmacological diversity, and research data indicate that CBD is a comparable antioxidant to common antioxidants. This review gathers the latest knowledge regarding the impact of CBD on oxidative signalling, with focus on the proclivity of CBD to regulate antioxidants and control the production of reactive oxygen species. CBD is considered an attractive therapeutic agent for neuroimmune disorders, and a body of literature indicates that CBD can regulate redox function at multiple levels, with a range of downstream effects on cells and tissues. However, pro-oxidant capacity of CBD has also been reported, and hence caution must be applied when considering CBD from a therapeutic standpoint. Such pro- and antioxidant functions of CBD may be cell- and model-dependent and may also be influenced by CBD dose, the duration of CBD treatment and the underlying pathology.

Cannabidiol effectively reverses mechanical and thermal allodynia, hyperalgesia, and anxious behaviors in a neuropathic pain model: Possible role of CB1 and TRPV1 receptors

The incidence of chronic pain is high in the general population and it is closely related to anxiety disorders, which promote negative effects on the quality of life. The cannabinoid system has essential participation in the pain sensitivity circuit. In this perspective, cannabidiol (CBD) is considered a promising strategy for treating neuropathic pain. Our study aimed to evaluate the effects of sub-chronic systemic treatment with CBD (0.3, 3, 10, or 30 mg/kg, i.p.) in male in rats submitted to chronic constriction injury of the sciatic nerve (CCI) or not (SHAM) and assessed in nociceptive tests (von Frey, acetone, and hot plate, three days CBD's treatment) and in the open field test (OFT, two days CBD's treatment). We performed a screening immunoreactivity of CB1 and TRPV1 receptors in cortical and limbic regions tissues, which were collected after 1.5 h of behavioral tests on the 24th experimental day. This study presents a dose-response curve to understand better the effects of low doses (3 mg/kg) on CBD's antiallodynic and anxiolytic effects. Also, low doses of CBD were able to (1) reverse mechanical and thermal allodynia (cold) and hyperalgesia, (2) reverse anxious behaviors (reduction of the % of grooming and freezing time, and increase of the % of center time in the OFT) induced by chronic pain. The peripheral neuropathy promoted the increase in the expression of CB1 and TRPV1 receptors in the anterior cingulate cortex (ACC), anterior insular cortex (AIC), basolateral amygdala (BLA), dorsal hippocampus (DH), and ventral hippocampus (VH). CBD potentiated this effect in the ACC, AIC, BLA, DH, and VH regions. These results provide substantial evidence of the role of the ACC-AIC-BLA corticolimbic circuit, and BLA-VH for pain regulation. These results can be clinically relevant since they contribute to the evidence of CBD's beneficial effects on treating chronic pain and associated comorbidities such as anxiety.

Pharmacological Blockade of PPARα Exacerbates Inflammatory Pain-Related Impairment of Spatial Memory in Rats

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors that exist in three isoforms: PPARα, PPARβ/δ and PPARγ. Studies suggest that the PPAR signalling system may modulate pain, anxiety and cognition. The aim of the present study was to investigate whether endogenous signalling via PPARs differentially modulates innate anxiety responses and mnemonic function in the presence and absence of inflammatory pain. We examined the effects of intraperitoneal administration of GW6471 (PPARα antagonist), GSK0660 (PPARβ/δ antagonist), GW9662 (PPARγ antagonist), and N-palmitoylethanolamide (PEA) on rat behaviour in the elevated plus maze (EPM), open field (OF), light-dark box (LDB), and novel object recognition (NOR) tests in the presence or absence of chronic inflammatory pain. Complete Freund’s Adjuvant (CFA)-injected rats exhibited impaired recognition and spatial mnemonic performance in the NOR test and pharmacological blockade of PPARα further impaired spatial memory in CFA-treated rats. N-oleoylethanolamide (OEA) levels were higher in the dorsal hippocampus in CFA-injected animals compared to their counterparts. The results suggest a modulatory effect of CFA-induced chronic inflammatory pain on cognitive processing, but not on innate anxiety-related responses. Increased OEA-PPARα signalling may act as a compensatory mechanism to preserve spatial memory function following CFA injection.

Chronic Pain in Dogs and Cats: Is There Place for Dietary Intervention with Micro-Palmitoylethanolamide?

The management of chronic pain is an integral challenge of small animal veterinary practitioners. Multiple pharmacological agents are usually employed to treat maladaptive pain including opiates, non-steroidal anti-inflammatory drugs, anticonvulsants, antidepressants, and others. In order to limit adverse effects and tolerance development, they are often combined with non-pharmacologic measures such as acupuncture and dietary interventions. Accumulating evidence suggests that non-neuronal cells such as mast cells and microglia play active roles in the pathogenesis of maladaptive pain. Accordingly, these cells are currently viewed as potential new targets for managing chronic pain. Palmitoylethanolamide is an endocannabinoid-like compound found in several food sources and considered a body's own analgesic. The receptor-dependent control of non-neuronal cells mediates the pain-relieving effect of palmitoylethanolamide. Accumulating evidence shows the anti-hyperalgesic effect of supplemented palmitoylethanolamide, especially in the micronized and co-micronized formulations (i.e., micro-palmitoylethanolamide), which allow for higher bioavailability. In the present paper, the role of non-neuronal cells in pain signaling is discussed and a large number of studies on the effect of palmitoylethanolamide in inflammatory and neuropathic chronic pain are reviewed. Overall, available evidence suggests that there is place for micro-palmitoylethanolamide in the dietary management of chronic pain in dogs and cats.

Effects of inflammatory pain on CB1 receptor in the midbrain periaqueductal gray

Western blot and GTPγS analyses reveal inflammatory pain–induced adaptations in the midbrain periaqueductal gray, which is critically involved in descending pain modulation. Pain upregulates the expression of the CB1 receptor and increases G-protein coupling in the periaqueductal gray.

Introduction:

The periaqueductal gray (PAG) mediates the antinociceptive properties of analgesics, including opioids and cannabinoids. Administration of either opioids or cannabinoids into the PAG induces antinociception. However, most studies characterizing the antinociceptive properties of cannabinoids in the PAG have been conducted in naive animals. Few studies have reported on the role of CB1 receptors in the PAG during conditions which would prompt the administration of analgesics, namely, during pain states.

Objectives:

To examine inflammatory pain-induced changes in CB1 receptor expression and function in the midbrain periaqueductal gray.

Methods:

In this study, we used the Complete Freund Adjuvant model to characterize CB1 receptor expression and G-protein coupling during persistent inflammatory pain.

Results:

Inflammatory pain induced an upregulation in the expression of synaptic CB1 receptors in the PAG. Despite this pain-induced change in CB1 expression, there was no corresponding upregulation of CB1 mRNA after the induction of inflammatory pain, suggesting a pain-induced recruitment of CB1 receptors to the synaptic sites within PAG neurons or increased coupling efficiency between the receptor and effector systems. Inflammatory pain also enhanced ventrolateral PAG CB1 receptor activity, as there was an increase in CP55,940-stimulated G-protein activation compared with pain-naïve control animals.

Conclusion:

These findings complement a growing body of evidence which demonstrate pain-induced changes in brain regions that are responsible for both the analgesic and rewarding properties of analgesic pharmacotherapies. Because much of our understanding of the pharmacology of cannabinoids is based on studies which use largely pain-naïve male animals, this work fills in important gaps in the knowledge base by incorporating pain-induced adaptations and cannabinoid pharmacology in females.

The Basal Pharmacology of Palmitoylethanolamide

Palmitoylethanolamide (PEA, N-hexadecanoylethanolamide) is an endogenous compound belonging to the family of N-acylethanolamines. PEA has anti-inflammatory and analgesic properties and is very well tolerated in humans. In the present article, the basal pharmacology of PEA is reviewed. In terms of its pharmacokinetic properties, most work has been undertaken upon designing formulations for its absorption and upon characterising the enzymes involved in its metabolism, but little is known about its bioavailability, tissue distribution, and excretion pathways. PEA exerts most of its biological effects in the body secondary to the activation of peroxisome proliferator-activated receptor-α (PPAR-α), but PPAR-α-independent pathways involving other receptors (Transient Receptor Potential Vanilloid 1 (TRPV1), GPR55) have also been identified. Given the potential clinical utility of PEA, not least for the treatment of pain where there is a clear need for new well-tolerated drugs, we conclude that the gaps in our knowledge, in particular those relating to the pharmacokinetic properties of the compound, need to be filled.

Maternal presence or absence alters nociceptive responding and cortical anandamide levels in juvenile female rats

The influence of parental support on child pain experiences is well recognised. Accordingly, animal studies have revealed both short- and long-term effects of early life stress on nociceptive responding and neural substrates such as endocannabinoids. The endocannabinoid system plays an important role in mediating and modulating stress, social interaction, and nociception. This study examined the effects of maternal support or acute isolation on nociceptive responding of female rats to a range of stimuli during the juvenile pre-adolescent period and accompanying changes in the endocannabinoid system. The data revealed that juvenile female Sprague Dawley rats (PND21-24) isolated from the dam for 1 h prior to nociceptive testing exhibited increased latency to withdraw in the hot plate test and increased mechanical withdrawal threshold in the Von Frey test, compared to rats tested in the presence of the dam. Furthermore, isolated rats exhibited reduced latency to respond in the acetone drop test and enhanced nociceptive responding in the formalin test when compared to dam-paired counterparts. Anandamide, but not 2-AG, levels were reduced in the prefrontal cortex of dam-paired, but not isolated, juvenile rats following nociceptive testing. There was no change in the expression of CB1, FAAH or MAGL; however, CB2 receptor expression was reduced in both dam-paired and isolated rats following nociceptive testing. Taken together the data demonstrate that brief social isolation or the presence of the dam modulates nociceptive responding of juvenile rat pups in a modality specific manner, and suggest a possible role for the endocannabinoid system in the prefrontal cortex in sociobehavioural pain responses during early life.

Pharmacological Blockade of PPAR Isoforms Increases Conditioned Fear Responding in the Presence of Nociceptive Tone

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors with three isoforms (PPARα, PPARβ/δ, PPARγ) and can regulate pain, anxiety, and cognition. However, their role in conditioned fear and pain-fear interactions has not yet been investigated. Here, we investigated the effects of systemically administered PPAR antagonists on formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA), and conditioned fear in the presence of nociceptive tone in rats. Twenty-three and a half hours following fear conditioning to context, male Sprague-Dawley rats received an intraplantar injection of formalin and intraperitoneal administration of vehicle, PPARα (GW6471), PPARβ/δ (GSK0660) or PPARγ (GW9662) antagonists, and 30 min later were re-exposed to the conditioning arena for 15 min. The PPAR antagonists did not alter nociceptive behaviour or fear-conditioned analgesia. The PPARα and PPARβ/δ antagonists prolonged context-induced freezing in the presence of nociceptive tone without affecting its initial expression. The PPARγ antagonist potentiated freezing over the entire trial. In conclusion, pharmacological blockade of PPARα and PPARβ/δ in the presence of formalin-evoked nociceptive tone, impaired short-term, within-trial fear-extinction in rats without affecting pain response, while blockade of PPARγ potentiated conditioned fear responding. These results suggest that endogenous signalling through these three PPAR isoforms may reduce the expression of conditioned fear in the presence of nociceptive tone.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain.

Results: This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂ receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Palmitoylethanolamide as adjunctive therapy for autism: Efficacy and safety results from a randomized controlled trial

Inflammation as well as glutamate excitotoxicity have been proposed to participate in the propagation of autism. Palmitoylethanolamide (PEA) is an endocannabinoid proven to prevent glutamatergic toxicity and inhibit inflammatory responses simultaneously. The present randomized, parallel group, double-blind placebo-controlled trial is the first study depicted to probe the efficacy of co-treatment with risperidone and PEA over 10 weeks in children with autism. Seventy children (aged 4-12 years) with autism and moderate to severe symptoms of irritability were randomly assigned to two treatment regimens. The study outcomes were measured using the Aberrant Behavior Checklist-Community Edition (ABC-C). At trial endpoint (week 10), combination of PEA and risperidone had superior efficacy in ameliorating the ABC-irritability and hyperactivity/noncompliance symptoms (Cohen's d, 95% confidence interval (CI) = 0.94, 0.41 to 1.46, p = 0.001) compared with a risperidone plus placebo regimen. Interestingly, effect of combination treatment on hyperactivity symptoms was also observed at trial midpoint (week 5) but with a smaller effect size (d = 0.53, p = 0.04) than that at the endpoint (d = 0.94, p = 0.001). Meanwhile, there was a trend toward significance for superior effect of risperidone plus PEA over risperidone plus placebo on inappropriate speech at trial endpoint (d = 0.51, p = 0.051). No significant differences existed between the two treatment groups for the other two ABC-C subscales (lethargy/social withdrawal and stereotypic behavior). The findings suggest that PEA may augment therapeutic effects of risperidone on autism-related irritability and hyperactivity. Future studies are warranted to investigate whether PEA can serve as a stand-alone treatment for autism.

The prefrontal cortical endocannabinoid system modulates fear-pain interactions in a subregion-specific manner

BACKGROUND AND PURPOSE

The emotional processing and coordination of top-down responses to noxious and conditioned aversive stimuli involves the medial prefrontal cortex (mPFC). Evidence suggests that subregions of the mPFC [infralimbic (IfL), prelimbic (PrL) and anterior cingulate (ACC) cortices] differentially alter the expression of contextually induced fear and nociceptive behaviour. We investigated the role of the endocannabinoid system in the IfL, PrL and ACC in formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA) and conditioned fear in the presence of nociceptive tone.

EXPERIMENTAL APPROACH

FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-mPFC administration of AM251 [cannabinoid type 1 (CB₁ ) receptor antagonist/inverse agonist], URB597 [fatty acid amide hydrolase (FAAH) inhibitor] or URB597 + AM251 on FCA and freezing behaviour were assessed.

KEY RESULTS

AM251 attenuated FCA when injected into the IfL or PrL and reduced contextually induced freezing behaviour when injected intra-IfL but not intra-PrL or intra-ACC. Intra-ACC administration of AM251 alone or in combination with URB597 had no effect on FCA or freezing. URB597 attenuated FCA and freezing behaviour when injected intra-IfL, prolonged the expression of FCA when injected intra-PrL and had no effect on these behaviours when injected intra-ACC.

CONCLUSIONS AND IMPLICATIONS

These results suggest important and differing roles for FAAH substrates or CB₁ receptors in the PrL, IfL and ACC in the expression of FCA and conditioned fear in the presence of nociceptive tone.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Cannabinoid Receptor Type 1 in the Brain Regulates the Affective Component of Visceral Pain in Mice

Endocannabinoids acting through cannabinoid receptor type 1 (CB1) are major modulators of peripheral somatic and visceral nociception. Although only partially studied, some evidence suggests a particular role of CB1 within the brain in nociceptive processes. As the endocannabinoid system regulates affect and emotional behaviors, we hypothesized that cerebral CB1 influences affective processing of visceral pain-related behaviors in laboratory animals. To study nocifensive responses modulated by supraspinal CB1, we used conditional knock-out mice lacking CB1 either in cortical glutamatergic neurons (Glu-CB1-KO), or in forebrain GABAergic neurons (GABA-CB1-KO), or in principal neurons of the forebrain (CaMK-CB1-KO). These mutant mice and mice treated with the CB1 antagonist SR141716 were tested for different pain-related behaviors. In an acetic acid-induced abdominal constriction test, supraspinal CB1 deletions did not affect nocifensive responses. In the cerulein-model of acute pancreatitis, mechanical allodynia or hyperalgesia were not changed, but Glu-CB1- and CaMK-CB1-KO mice showed significantly increased facial grimacing scores indicating increased affective responses to this noxious visceral stimulus. Similarly, these brain-specific CB1 KO mice also showed significantly changed thermal nociception in a hot-plate test. These results reveal a novel, and important role of CB1 expressed by cortical glutamatergic neurons in the affective component of visceral nociception.

PPARs and pain

Chronic pain is a common cause of disability worldwide and remains a global health and socio-economic challenge. Current analgesics are either ineffective in a significant proportion of patients with chronic pain or associated with significant adverse side effects. The PPARs, a family of nuclear hormone transcription factors, have emerged as important modulators of pain in preclinical studies and therefore a potential therapeutic target for the treatment of pain. Modulation of nociceptive processing by PPARs is likely to involve both transcription-dependent and transcription-independent mechanisms. This review presents a comprehensive overview of preclinical studies investigating the contribution of PPAR signalling to nociceptive processing in animal models of inflammatory and neuropathic pain. We examine current evidence from anatomical, molecular and pharmacological studies demonstrating a role for PPARs in pain control. We also discuss the limited evidence available from relevant clinical studies and identify areas that warrant further research. LINKED ARTICLES: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

A Pharmacological Rationale to Reduce the Incidence of Opioid Induced Tolerance and Hyperalgesia: A Review

Chronic pain is an important health and social problem. Misuse and abuse of opioids in chronic non-cancer pain management seem to be a huge problem, in some countries. This could probably affect the normal use of such analgesics in patients in need of them. Basic and clinical researches should find the solution to mitigate the potential damage. Dysregulation of mast cell and microglia activation plays an important role in the pathogenesis and management of chronic pain. Persistent mast cell activation sensitizes nociceptors and initiates central nervous system inflammatory processes, involving microglial cell activation and sensitization of spinal somatosensory neurons. Exposure of mast cells and microglia to opioids is well known to provoke activation of these non-neuronal immune cell populations, thereby contributing to an exacerbation of pro-inflammatory and pro-nociceptive processes and promoting, over the long-term, opioid-induced hyperalgesia and tolerance. This review is intended to provide the reader with an overview of the role for these non-neuronal cells in opioid-induced chronic pain and tolerance as a consequence of prolonged exposure to these drugs. In addition, we will examine a potential strategy with the aim to modulate opioid-induced over-activation of glia and mast cells, based on endogenous defense mechanisms and fatty acid amide signaling molecules.

Cannabinoids and Pain: Sites and Mechanisms of Action

The endocannabinoid system, consisting of the cannabinoid₁ receptor (CB₁R) and cannabinoid₂ receptor (CB₂R), endogenous cannabinoid ligands (endocannabinoids), and metabolizing enzymes, is present throughout the pain pathways. Endocannabinoids, phytocannabinoids, and synthetic cannabinoid receptor agonists have antinociceptive effects in animal models of acute, inflammatory, and neuropathic pain. CB₁R and CB₂R located at peripheral, spinal, or supraspinal sites are important targets mediating these antinociceptive effects. The mechanisms underlying the analgesic effects of cannabinoids likely include inhibition of presynaptic neurotransmitter and neuropeptide release, modulation of postsynaptic neuronal excitability, activation of the descending inhibitory pain pathway, and reductions in neuroinflammatory signaling. Strategies to dissociate the psychoactive effects of cannabinoids from their analgesic effects have focused on peripherally restricted CB₁R agonists, CB₂R agonists, inhibitors of endocannabinoid catabolism or uptake, and modulation of other non-CB₁R/non-CB₂R targets of cannabinoids including TRPV1, GPR55, and PPARs. The large body of preclinical evidence in support of cannabinoids as potential analgesic agents is supported by clinical studies demonstrating their efficacy across a variety of pain disorders.

Effect of a new formulation of micronized and ultramicronized N-palmitoylethanolamine in a tibia fracture mouse model of complex regional pain syndrome

Complex regional pain syndrome type 1 (CRPS-I) is a disabling and frequently chronic condition. It involves the extremities and is a frequent consequence of distal tibia and radius fractures. The inflamed appearance of the affected CRPS-I limb suggests that local production of inflammatory mediators may be implicated in the ensuing etiology. A rodent tibia fracture model, characterized by inflammation, chronic unilateral hindlimb warmth, edema, protein extravasation, allodynia and hyperalgesia resembles the clinical features of patients with acute CRPS-I. N-palmitoylethanolamine (PEA), a member of the family of naturally-occurring N-acylethanolamines, is well-known for its ability to modulate inflammatory processes and regulate pain sensitivity. However, the large particle size and lipidic nature of PEA may limit its bioavailability and solubility when given orally. Micronized formulations are frequently used to enhance the dissolution rate of drug and reduce its variability of absorption when orally administered. The aim of this study was to assess the effects of a formulation of micronized and ultramicronized PEA (PEA-MPS), given orally in a mouse model of CRPS-I. CD-1 male mice were subjected to distal tibia fracture and divided into two groups: control and treated with PEA-MPS (PEA micronized 300 mg/kg and ultramicronized 600 mg/kg). Sensibility to pain was monitored in all mice throughout the course of the experiment. Twenty-eight days after tibia fracture induction animals were sacrificed and biochemical parameters evaluated. The PEA-MPS-treated group showed an improved healing process, fracture recovery and fibrosis score. PEA-MPS administration decreased mast cell density, nerve growth factor, matrix metalloproteinase 9 and cytokine expression. This treatment also reduced (poly-ADP)ribose polymerase activation, peroxynitrite formation and apoptosis. Our results suggest that PEA-MPS may be a new therapeutic strategy in the treatment of CRPS-I.

The Role of Nuclear Hormone Receptors in Cannabinoid Function

Since the early 2000s, evidence has been accumulating that most cannabinoid compounds interact with the nuclear hormone family peroxisome proliferator-activated receptors (PPARs). This can be through direct binding of these compounds to PPARs, metabolism of cannabinoid to other PPAR-activating chemicals, or indirect activation of PPAR through cell signaling pathways. Delivery of cannabinoids to the nucleus may be facilitated by fatty acid-binding proteins and carrier proteins. All PPAR isoforms appear to be activated by cannabinoids, but the majority of evidence is for PPARα and γ. To date, little is known about the potential interaction of cannabinoids with other nuclear hormones. At least some (but not all) of the well-known biological actions of cannabinoids including neuroprotection, antiinflammatory action, and analgesic effects are partly mediated by PPAR-activation, often in combination with activation of the more traditional target sites of action. This has been best investigated for the endocannabinoid-like compounds palmitoylethanolamide and oleoylethanolamine acting at PPARα, and for phytocannabinoids or their derivatives activation acting at PPARγ. However, there are still many aspects of cannabinoid activation of PPAR and the role it plays in the biological and therapeutic effects of cannabinoids that remain to be investigated.

Microstructural Abnormalities Were Found in Brain Gray Matter from Patients with Chronic Myofascial Pain

Myofascial pain, presented as myofascial trigger points (MTrPs)-related pain, is a common, chronic disease involving skeletal muscle, but its underlying mechanisms have been poorly understood. Previous studies have revealed that chronic pain can induce microstructural abnormalities in the cerebral gray matter. However, it remains unclear whether the brain gray matters of patients with chronic MTrPs-related pain undergo alteration. In this study, we employed the Diffusion Kurtosis Imaging (DKI) technique, which is particularly sensitive to brain microstructural perturbation, to monitor the MTrPs-related microstructural alterations in brain gray matter of patients with chronic pain. Our results revealed that, in comparison with the healthy controls, patients with chronic myofascial pain exhibited microstructural abnormalities in the cerebral gray matter and these lesions were mainly distributed in the limbic system and the brain areas involved in the pain matrix. In addition, we showed that microstructural abnormalities in the right anterior cingulate cortex (ACC) and medial prefrontal cortex (mPFC) had a significant negative correlation with the course of disease and pain intensity. The results of this study demonstrated for the first time that there are microstructural abnormalities in the brain gray matter of patients with MTrPs-related chronic pain. Our findings may provide new insights into the future development of appropriate therapeutic strategies to this disease.

Antinociception induced by galanin in anterior cingulate cortex in rats with acute inflammation

The present study was performed to explore the role of galanin in nociceptive modulation in anterior cingulate cortex (ACC) of rats with acute inflammation, and the changes in galanin and galanin receptor 2 (Gal R2) expressions in rats with acute inflammation. Intra-ACC injection of galanin induced antinociception in rats with acute inflammation, the antinociceptive effects induced by galanin were attenuated significantly by intra-ACC injection of the Gal R2 antagonist M871, indicating an involvement of Gal R2 in nociceptive modulation in ACC in rats with acute inflammation. Furthermore, we found that both the galanin mRNA expression and galanin content increased significantly in ACC in rats with acute inflammation than that in normal rats. Moreover, both the mRNA levels of Gal R2 and the content of Gal R2 in ACC increased significantly in rats with acute inflammation than that in normal rats. These results demonstrated that galanin induced antinociception in ACC in rats with acute inflammation. And there were changes in the expression of galanin and Gal R2 in rats with acute inflammation.