Upregulation of spinal cannabinoid-1-receptors following nerve injury enhances the effects of Win 55,212-2 on neuropathic pain behaviors in rats

Allopregnanolone relieves paclitaxel induced mechanical hypersensitivity via inhibiting spinal cord PGE2-EP2 mediated microglia-neuron signaling

Chemotherapy-induced neuropathic pain (CINP) is a serious adverse effect of commonly used chemotherapeutics. Neurosteroid allopregnanolone is suggested to modulate the expression of various receptors or enzymes that involved in pain perception, presenting an analgesic potential. Here, we investigated if allopregnanolone attenuates extracellular signal-regulated kinase (ERK) and its downstream prostaglandin E2 (PGE2) expression in the dorsal spinal cord concomitant with neuropathic pain relief in paclitaxel (PTX)-induced neuropathic pain model rats. The results showed PTX upregulated phosphorylated ERK (p-ERK), PGE2 level, and PGE2 receptor E-prostanoid 2 (EP2) expression in the spinal dorsal horn. Besides, p-ERK inhibitor PD98059 or microglia inhibitor minocycline reduced microglial activation, p-ERK expression, PGE2 release, EP2 expression, and partially alleviated PTX-induced mechanical hypersensitivity. Further, allopregnanolone level in the dorsal spinal cord was observed to decrease in CINP rats, and intragastric administration of exogenous allopregnanolone dose-dependently alleviated PTX-induced mechanical hypersensitivity. Mechanistically, allopregnanolone dose-dependently alleviated PTX-induced microglial activation, p-ERK, PGE2, and EP2 upregulation, as well as cytokines expression in the dorsal spinal cord in CINP rats. Furthermore, subcutaneous injection of allopregnanolone synthesis inhibitor medroxyprogesterone could reduce endogenous allopregnanolone and block all effects of exogenous allopregnanolone in CINP rats. Taken together, these results suggest allopregnanolone presents an analgesic effect for PTX-induced mechanical hypersensitivity, partially via inhibiting the dorsal spinal cord PGE2-EP2 mediated microglia-neuron signaling.

Alcohol and cannabis use for pain management: Translational findings of relative risks, benefits, and interactions

Chronic pain affects over 20% of the global population and contributes to the vast burden of psychiatric illness. While effective treatments for chronic pain remain limited, both alcohol and cannabis have been used for centuries to manage pain and closely associated negative affective symptoms. However, persistent misuse of alcohol and/or cannabis in such a negative reinforcement fashion is hypothesized to increase the risk of severity of substance use disorders (SUDs). The current review describes neurobiological evidence for the analgesic efficacy of alcohol and primary cannabis constituents and how use or co-use of these substances may influence SUD risk.

Elucidating interplay between myrcene and cannabinoid receptor 1 receptors to produce antinociception in mouse models of neuropathic pain

ABSTRACT

The need for nonaddictive and effective treatments for chronic pain are at an all-time high. Historical precedence, and now clinical evidence, supports the use of cannabis for alleviating chronic pain. A plethora of research on delta-9-tetrahydrocannabinol exists, yet cannabis is comprised of a multitude of constituents, some of which possess analgesic potential, that have not been systematically investigated, including the terpene myrcene. Myrcene attenuates pain hypersensitivity in preclinical models and is one of the most abundant terpenes found in cannabis. Despite these findings, it remains unclear how myrcene elicits these effects on nociceptive systems. The present study uses a male and female mouse model of neuropathic pain as well as in vitro experiments with HEK293T cells to explore these questions. We first demonstrate myrcene (1-200 mg/kg i.p.) dose-dependently increases mechanical nociceptive thresholds, where potency was greater in female compared with male pain mice. Testing canonical tetrad outcomes, mice were tested for hypolocomotion and hypothermia after myrcene administration. Myrcene did not alter locomotion or temperature, but female pain mice showed a conditioned place aversion to myrcene. A cannabinoid receptor 1 (CB1) antagonist inhibited myrcene's anti-allodynia. By contrast, in vitro cell culture experiments using a TRUPATH assay revealed myrcene does not directly activate CB1 receptors nor alter CB1 receptor activity elicited by CB1 agonist (CP 55,940) or endocannabinoids (anandamide or 2-arachidonoylglycerol). Understanding engagement of CB1 receptors in pain modulation and myrcene's mechanism of action warrants further study to understand the diversity of cannabis pharmacology and to further the frontier of pain research.

Cannabis constituents for chronic neuropathic pain; reconciling the clinical and animal evidence

Chronic neuropathic pain is a debilitating pain syndrome caused by damage to the nervous system that is poorly served by current medications. Given these problems, clinical studies have pursued extracts of the plant Cannabis sativa as alternative treatments for this condition. The vast majority of these studies have examined cannabinoids which contain the psychoactive constituent delta-9-tetrahydrocannabinol (THC). While there have been some positive findings, meta-analyses of this clinical work indicates that this effectiveness is limited and hampered by side-effects. This review focuses on how recent preclinical studies have predicted the clinical limitations of THC-containing cannabis extracts, and importantly, point to how they might be improved. This work highlights the importance of targeting channels and receptors other than cannabinoid CB1 receptors which mediate many of the side-effects of cannabis.

The endocannabinoid system as a therapeutic target in neuropathic pain: a review

INTRODUCTION

This review highlights the critical role of the endocannabinoid system (ECS) in regulating neuropathic pain and explores the therapeutic potential of cannabinoids. Understanding the mechanisms of the ECS, including its receptors, endogenous ligands, and enzymatic routes, can lead to innovative treatments for chronic pain, offering more effective therapies for neuropathic conditions. This review bridges the gap between preclinical studies and clinical applications by emphasizing ECS modulation for better pain management outcomes.

AREAS COVERED

A review mapped the existing literature on neuropathic pain and the effects of modulating the ECS using natural and synthetic cannabinoids. This analysis examined ECS components and their alterations in neuropathic pain, highlighting the peripheral, spinal, and supraspinal mechanisms. This review aimed to provide a thorough understanding of the therapeutic potential of cannabinoids in the management of neuropathic pain.

EXPERT OPINION

Advances in cannabinoid research have shown significant potential for the management of chronic neuropathic pain. The study emphasizes the need for high-quality clinical trials and collaborative efforts among researchers, clinicians, and regulatory bodies to ensure safe and effective integration of cannabinoids into pain management protocols. Understanding the mechanisms and optimizing cannabinoid formulations and delivery methods are crucial for enhancing therapeutic outcomes.

Cannabis sativa L. Extract Alleviates Neuropathic Pain and Modulates CB1 and CB2 Receptor Expression in Rat

INTRODUCTION

Cannabis sativa L. (CSL) extract has pain-relieving potential due to its cannabinoid content, so the effects of two CSL extracts on alleviating neuropathic pain were investigated in vivo. Methods and groups: Male Wistar rats (n = 130) were divided into groups and received vincristine (0.1 mg/kg) and gabapentin (60 mg/kg) to induce and relieve neuropathic pain or CSL extracts (D and B). The mRNA and protein expression of the cannabinoid receptors type 1 and 2 (CB1R, CB2R) were evaluated in the cerebral cortex, hippocampus, and lymphocytes. Behavioural tests (Tail-Flick and von Frey) were performed on all animals.

RESULTS

VK-induced neuropathic pain was accompanied by decreased CB1R protein level and CB2R mRNA expression in the cortex. Gabapentin relieved pain and increased CB1R protein levels in the hippocampus compared to the vincristine group. Hippocampus CB1R protein expression increased with the administration of extract D (10 mg/kg, 40 mg/kg) and extract B (7.5 mg/kg, 10 mg/kg) compared to VK group. In the cerebral cortex CSL decreased CB1R protein expression (10 mg/kg, 20 mg/kg, 40 mg/kg of extract B) and mRNA level (5 mg/kg, 7.5 mg/kg of extract B; 20 mg/kg of extract D) compared to the VK-group.CB2R protein expression increased in the hippocampus after treatment with extract B (7.5 mg/kg) compared to the VK-group. In the cerebral cortex extract B (10 mg/kg, 20 mg/kg) increased CB2R protein expression compared to VK-group.

CONCLUSION

Alterations in cannabinoid receptor expression do not fully account for the observed behavioural changes in rats. Therefore, additional signalling pathways may contribute to the initiation and transmission of neuropathic pain. The Cannabis extracts tested demonstrated antinociceptive effects comparable to gabapentin, highlighting the antinociceptive properties of Cannabis extracts for human use.

Additive antinociceptive action of intrathecal anandamide reuptake inhibitor and morphine in the management of post-incisional pain in rats

Spinal opioids have mixed efficacy and their adverse effects force treatment cessation of postoperative pain. Consequently, there is an ongoing search for new therapeutic strategies. Here, we evaluated the analgesic efficacy of intrathecal UCM707, an anandamide reuptake inhibitor, and morphine combination. Firstly, we assessed the effects of morphine (1, 5 and 10 μg), UCM707 (75 μg) and its combination in the hot plate. Then, morphine + UCM707 at sub-effective doses was evaluated in a rat post-incisional pain model. In addition, μ-, CB1r-, CB2r- and TRPV1-antagonists were pre-administered before the combination. Activation of μ-opioid and CB1r, and Cnr1, Cnr2, Oprm1 and TRPV1 expressions were evaluated in the lumbar sacra and periaqueductal grey by [35 S]-GTPγS binding autoradiography and qPCR studies. In the hot plate, morphine (1 μg) and UCM707 (75 μg) induced a more robust analgesic effect than each drug alone. Morphine plus UCM707 did not modify μ-opioid nor CB1 receptor function in the PAG or LS. Cnr1 and TRPV1 expression increased in the lumbar sacra (LS). Morphine plus UCM707 significantly reduced post-incisional pain at 1 and 4 days after surgery. Cnr1, Cnr2 and TRPV1 expressions increased in the LS. Blockade of μ-opioid receptor reduced combination effects on days 1 and 4. CB1r- and CB2r-antagonism reduced morphine + UCM707 effects on days 1 and 4, respectively. CB1r and TRPV1-antagonism improved their antinociceptive effects on day 4. These results revealed a synergistic/additive analgesic effect of UCM707 and morphine combination controlling postincisional pain. CB1r, CB2r and TRPV1 contribute differently as central sensitization occurs.

Navigating Preclinical Models and Medications for Peripheral Neuropathy: A Review

Peripheral neuropathy (PN) is a multifaceted disorder characterised by peripheral nerve damage, manifesting in symptoms like pain, weakness, and autonomic dysfunction. This review assesses preclinical models in PN research, evaluating their relevance to human disease and their role in therapeutic development. The Streptozotocin (STZ)-induced diabetic rat model is widely used to simulate diabetic neuropathy but has limitations in faithfully replicating disease onset and progression. Cisplatin-induced PN models are suitable for studying chemotherapy-induced peripheral neuropathy (CIPN) and closely resemble human pathology. However, they may not fully replicate the spectrum of sensory and motor deficits. Paclitaxel-induced models also contribute to understanding CIPN mechanisms and testing neuroprotective agents. Surgical or trauma-induced models offer insights into nerve regeneration and repair strategies. Medications such as gabapentin, pregabalin, duloxetine, and fluoxetine have demonstrated promise in these models, enhancing our understanding of their therapeutic efficacy. Despite progress, developing models that accurately mirror human PN remains imperative due to its complex nature. Continuous refinement and innovative approaches are critical for effective drug discovery. This review underscores the strengths and limitations of current models and advocates for an integrated approach to address the complexities of PN better and optimise treatment outcomes.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

URB937 Prevents the Development of Mechanical Allodynia in Male Rats with Trigeminal Neuralgia

Cannabinoids are proposed for alleviating neuropathic pain, but their use is limited by cannabimimetic side effects. The inhibition of the fatty acid amide hydrolase (FAAH), the degrading enzyme of the endocannabinoid anandamide, has received attention as an alternative to cannabinoids in the treatment of neuropathic pain. Here, we investigated the effect of URB937, a blood-brain barrier impermeant FAAH inhibitor, on experimentally induced mechanical allodynia in an animal model of trigeminal neuralgia. Male Sprague-Dawley rats were subjected to chronic constriction injury of the infraorbital nerve (IoN-CCI); operated animals were treated sub-chronically with URB937 (1 mg/kg, i.p.) or vehicle before or after trigeminal mechanical allodynia establishment. We also assayed mRNA expression levels of the pain neuropeptide calcitonin gene-related peptide (CGRP) and cytokines in the medulla, cervical spinal cord, and trigeminal ganglion ipsilateral to IoN-CCI using rt-PCR. URB937 treatment prevented the development of mechanical allodynia and IoN-CCI-induced changes in mRNA expression levels of CGRP and cytokines in the evaluated areas. When administered after allodynia development, URB937 prevented IoN-CCI-induced changes in CGRP and cytokine gene expression; this was not associated with a significant abrogation of the mechanical allodynia. These findings suggest that URB937 may counteract, but not reverse, the development of allodynia in trigeminal neuralgia. Further research is needed to elucidate the underlying mechanisms.

A type II cannabis extract and a 1:1 blend of Δ(9)-tetrahydrocannabinol and cannabidiol display distinct antinociceptive profiles and engage different endocannabinoid targets when administered into the subarachnoid space

Introduction: Cannabis extracts are being increasingly used to mitigate chronic pain. Current guidelines for their prescription rely on Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) content as well as the ratio of these major cannabinoids present in the blend. Here we assessed whether these descriptors were representative of product effectiveness to produce a desired outcome such as analgesia. Methods: In this study, we used a rat model of diabetic neuropathy and assessed the reduction in mechanical allodynia following intrathecal injection of pure THC, pure CBD, a 1:1 mix of these compounds and a "balanced" chemotype II cannabis extract. Engagement of endocannabinoid targets by different treatments was investigated using CB1 (AM251) and CB2 (AM630) receptor antagonists as well as a TRPV1 channel blocker (capsazepine). Results: Antinociceptive responses induced by an equivalent amount of THC administered in its pure form, as a THC:CBD mix or as a "balanced" extract were distinct. Furthermore, the 1:1 THC:CBD mix and the balanced extract had not only different response profiles but their relative engagement of CB1, CB2 receptors and TRPV1 channels was distinct. Discussion: These findings indicate that antinociceptive responses and targets engaged by blended cannabinoids are composition-specific, and cannot be simply inferred from THC and CBD contents. This information may have implications in relation to the way medicinal cannabis products are prescribed.

Novel Topical Anandamide Formulation for Alleviating Peripheral Neuropathic Pain

Peripheral neuropathy (PN) is a condition of peripheral nerve damage leading to severe pain. The first line therapies are associated with adverse psychotropic effects (PSE) and second line therapies are not efficient enough to relieve pain. There is an unmet drug need for relieving pain effectively without PSE in PN. Anandamide, an endocannabinoid activates cannabinoid receptors to relieve the pain due to peripheral neuropathy (PN). Anandamide has a very short biological half-life as they are extensively metabolized by fatty acid amide hydrolase enzyme (FAAH). Regional delivery of safe FAAH inhibitor (FI) with anandamide would be beneficial for PN without psychotropic effects. The objective of the study is to identify a safe FAAH inhibitor and deliver the anandamide in combination with the FAAH inhibitor topically for the management of PN. The FAAH inhibition potential of silymarin constituents was evaluated by molecular docking and in vitro studies. The topical gel formulation was developed to deliver anandamide and FI. The formulation was assessed in chemotherapeutic agent-induced peripheral neuropathy rat models to relieve mechanical-allodynia and thermal-hyperalgesia. The molecular docking studies demonstrated that the Prime MM-GBSA free energy of silymarin constituents were in the order of silybin > isosilybin > silychristin > taxifolin > silydianin. In in vitro studies, silybin 20 µM inhibited >61.8% of FAAH activity and increased the half-life of anandamide. The developed formulation increased permeation of anandamide and silybin across the porcine skin. Furthermore, on the application of anandamide and anandamide-silybin gel to rat paws, there was a significant increase in the pain threshold for allodynic and hyperalgesic stimulus up to 1 h and 4 h, respectively. The topical anandamide with silybin delivery approach could serve to alleviate peripheral neuropathy efficiently and thus could minimize unwanted CNS side effects of synthetic or natural cannabinoids in patients.

Activation of CB1R alleviates central sensitization by regulating HCN2-pNR2B signaling in a chronic migraine rat model

BACKGROUND

Central sensitization has been widely accepted as an underlying pathophysiological mechanism of chronic migraine (CM), activation of cannabinoid type-1 receptor (CB1R) exerts antinociceptive effects by relieving central sensitization in many pain models. However, the role of CB1R in the central sensitization of CM is still unclear.

METHODS

A CM model was established by infusing inflammatory soup (IS) into the dura of male Wistar rats for 7 days, and hyperalgesia was assessed by the mechanical and thermal thresholds. In the periaqueductal gray (PAG), the mRNA and protein levels of CB1R and hyperpolarization-activated cyclic nucleotide-gated cation channel 2 (HCN2) were measured by qRT-PCR and western blotting. After intraventricular injection of Noladin ether (NE) (a CB1R agonist), ZD 7288 (an HCN2 blocker), and AM 251 (a CB1R antagonist), the expression of tyrosine phosphorylation of N-methyl-D-aspartate receptor subtype 2B (pNR2B), calcium-calmodulin-dependent kinase II (CaMKII), and phosphorylated cAMP-responsive element binding protein (pCREB) was detected, and central sensitization was evaluated by the expression of calcitonin gene-related peptide (CGRP), c-Fos, and substance P (SP). Synaptic-associated protein (postsynaptic density protein 95 (PSD95) and synaptophysin (Syp)) and synaptic ultrastructure were detected to explore synaptic plasticity in central sensitization.

RESULTS

We observed that the mRNA and protein levels of CB1R and HCN2 were both significantly increased in the PAG of CM rats. The application of NE or ZD 7288 ameliorated IS-induced hyperalgesia; repressed the pNR2B/CaMKII/pCREB pathway; reduced CGRP, c-Fos, SP, PSD95, and Syp expression; and inhibited synaptic transmission. Strikingly, the application of ZD 7288 relieved AM 251-evoked elevation of pNR2B, CGRP, and c-Fos expression.

CONCLUSIONS

These data reveal that activation of CB1R alleviates central sensitization by regulating HCN2-pNR2B signaling in CM rats. The activation of CB1R might have a positive influence on the prevention of CM by mitigating central sensitization.

Combined non-psychoactive Cannabis components cannabidiol and β-caryophyllene reduce chronic pain via CB1 interaction in a rat spinal cord injury model

The most frequently reported use of medical marijuana is for pain relief. However, its psychoactive component Δ9-tetrahydrocannabinol (THC) causes significant side effects. Cannabidiol (CBD) and β-caryophyllene (BCP), two other cannabis constituents, possess more benign side effect profiles and are also reported to reduce neuropathic and inflammatory pain. We evaluated the analgesic potential of CBD and BCP individually and in combination in a rat spinal cord injury (SCI) clip compression chronic pain model. Individually, both phytocannabinoids produced dose-dependent reduction in tactile and cold hypersensitivity in male and female rats with SCI. When co-administered at fixed ratios based on individual A50s, CBD and BCP produced enhanced dose-dependent reduction in allodynic responses with synergistic effects observed for cold hypersensitivity in both sexes and additive effects for tactile hypersensitivity in males. Antinociceptive effects of both individual and combined treatment were generally less robust in females than males. CBD:BCP co-administration also partially reduced morphine-seeking behavior in a conditioned place preference (CPP) test. Minimal cannabinoidergic side effects were observed with high doses of the combination. The antinociceptive effects of the CBD:BCP co-administration were not altered by either CB2 or μ-opioid receptor antagonist pretreatment but, were nearly completely blocked by CB1 antagonist AM251. Since neither CBD or BCP are thought to mediate antinociception via CB1 activity, these findings suggest a novel CB1 interactive mechanism between these two phytocannabinoids in the SCI pain state. Together, these findings suggest that CBD:BCP co-administration may provide a safe and effective treatment option for the management of chronic SCI pain.

Changes in the expression of endocannabinoid system components in an experimental model of chemotherapy-induced peripheral neuropathic pain: Evaluation of sex-related differences

Chemotherapy-induced neuropathic pain is a serious clinical problem and one of the major side effects in cancer treatment. The endocannabinoid system (ECS) plays a crucial role in regulating pain neurotransmission, and changes in the expression of different components of the ECS have been reported in experimental models of persistent pain. In addition, sex differences have been observed in ECS regulation and function. The aim of our study was to evaluate whether administration of oxaliplatin, a neurotoxic antineoplastic agent, induced changes in the expression of ECS components in peripheral and central stations of the pain pathway, and if those changes exhibited sexual dimorphism. Adult male and female rats were injected with oxaliplatin or saline, and mechanical and cold hypersensitivity and allodynia were evaluated using Von Frey and Choi Tests. The mRNA levels corresponding to cannabinoid receptors (CB1, CB2), cannabinoid-related receptors (GPR55, 5HT1A, TRPV1) and to the main enzymes involved in the synthesis (DAGL, DAGL, NAPE-PLD) and degradation (MGL, FAAH) of endocannabinoids were assessed in lumbar dorsal root ganglia (DRGs) and spinal cord by using real time RT-PCR. In addition, the levels of the main endocannabinoids, 2-arachidonoylglycerol (2-AG) and anandamide (AEA), were evaluated using commercial ELISA kits. Oxaliplatin administration induced the development of mechanical and cold hypersensitivity and allodynia in male and female animals. Oxaliplatin also induced early and robust changes in the expression of several components of the ECS in DRGs. A marked upregulation of CB1, CB2, 5HT1A and TRPV1 was detected in both sexes. Interestingly, while DAGL mRNA levels remained unchanged, DAGL was downregulated in male and upregulated in female rats. Finally, MGL and NAPE-PLD showed increased levels only in male animals, while FAAH resulted upregulated in both sexes. In parallel, reduced 2-AG and AEA levels were detected in DRGs from male or female rats, respectively. In the lumbar spinal cord, only TRPV1 mRNA levels were found to be upregulated in both sexes. Our results reveal previously unreported changes in the expression of cannabinoid receptors, ligands and enzymes occurring mainly in the peripheral nervous system and displaying certain sexual dimorphism. These changes may contribute to the physiopathology of oxaliplatin-induced neuropathic pain in male and female rats. A better understanding of these dynamic changes will facilitate the development of mechanism- and sex-specific approaches to optimize the use of cannabinoid-based medicines for the treatment of chemotherapy-induced pain.

Expression and Kinetics of Endogenous Cannabinoids in the Brain and Spinal Cord of a Spare Nerve Injury (SNI) Model of Neuropathic Pain

The role of endogenous cannabinoids in neuropathic pain has been actively studied, among which 2-arachidonoyl glycerol (2-AG) has received the most attention. However, owing to its chemical properties, direct detection of 2-AG distribution in tissues is difficult. Moreover, although desorption electrospray ionization mass spectrometry imaging (DESI-MSI) has enabled the detection of 2-AG, its distribution in the brain and spinal cord of neuropathic pain models has not been reported. In this study, the expression and distribution of 2-AG in the brain and spinal cord of a spare nerve injury (SNI) mice model of neuropathic pain was examined using DESI-MSI. The brain and lumbar spinal cord were collected and analyzed on days 3, 7, and 21 after treatment. On days 3 and 7 after treatment, 2-AG expression in the SNI model was decreased in the hypothalamus, midbrain, and especially in the periaqueductal gray (PAG) region but increased in the lumbar spinal cord. On day 21, the SNI model showed decreased 2-AG expression in the hypothalamus, but the difference from the control was not significant. Furthermore, there were no differences in 2-AG expression between the lumbar spinal cord, midbrain, or PAG. These data suggest that 2-AG might be involved in pain control.

N-acylethanolamide metabolizing enzymes are upregulated in human neural progenitor-derived neurons exposed to sub-lethal oxidative stress

N-acyl amides (NAAs) are a class of lipids that consist of an acyl group N-linked to an amino acid, neurotransmitter, taurine or ethanolamide group (N-acylethanolamines or NAEs) and include some endocannabinoids (eCB) such as anandamide. These lipids are synthesized in a wide variety of organisms and in multiple cell types, including neurons. NAEs are involved in numerous cellular and physiological processes and their concentrations are elevated in response to ischemia and physical trauma to play a role in neuroprotection. The neuroprotective properties of eCB NAEs make the protein targets of these compounds attractive targets for clinical intervention for a variety of conditions. The most promising of these targets include cannabinoid receptor type 1 (CB1), cannabinoid receptor type 2 (CB2), fatty acid amide hydrolase (FAAH), N-acylethanolamine acid amidase (NAAA), and N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD). Further characterization of these targets in a more contemporary model system of neurodegeneration and neuroprotection will allow us to fully describe their role and mechanism of action in neuroprotection against oxidative stress leading to better utilization in the clinical setting. Human stem cell-derived or human neural progenitor cell-derived cells, such as ReN cells, have become more utilized for the study of human neuronal development and neurodegenerative diseases. ReN cells can be easily differentiated thereby circumventing the need for using transformed cell lines and primary neurons as cell model systems. In this study, we determined whether ReN cells, a superior cell model system for studying neurodevelopment, differentiation, and neuroprotection, express proteins involved in canonical eCB NAE signaling and whether oxidative stress can induce their expression. We determined that sublethal oxidative stress upregulates the expression of all eCB proteins tested. In addition, we determined that oxidative stress increases the nuclear localization of FAAH, and to a lesser extent, NAAA and NAPE-PLD. This study is a first step toward determining how oxidative stress affects CB1, CB2, FAAH, NAAA, and NAPE-PLD expression and their potential defense against oxidative stress. As such, our data is important for further determining the role of eCB metabolizing proteins and eCB receptors against oxidative stress.

Intrathecal Actions of the Cannabis Constituents Δ(9)-Tetrahydrocannabinol and Cannabidiol in a Mouse Neuropathic Pain Model

(1) Background: The psychoactive and non-psychoactive constituents of cannabis, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), synergistically reduce allodynia in various animal models of neuropathic pain. Unfortunately, THC-containing drugs also produce substantial side-effects when administered systemically. We examined the effectiveness of targeted spinal delivery of these cannabis constituents, alone and in combination. (2) Methods: The effect of acute intrathecal drug delivery on allodynia and common cannabinoid-like side-effects was examined in a mouse chronic constriction injury (CCI) model of neuropathic pain. (3) Results: intrathecal THC and CBD produced dose-dependent reductions in mechanical and cold allodynia. In a 1:1 combination, they synergistically reduced mechanical and cold allodynia, with a two-fold increase in potency compared to their predicted additive effect. Neither THC, CBD nor combination THC:CBD produced any cannabis-like side-effects at equivalent doses. The anti-allodynic effects of THC were abolished and partly reduced by cannabinoid CB1 and CB2 receptor antagonists AM281 and AM630, respectively. The anti-allodynic effects of CBD were partly reduced by AM630. (4) Conclusions: these findings indicate that intrathecal THC and CBD, individually and in combination, could provide a safe and effective treatment for nerve injury induced neuropathic pain.

Systematic reviews of randomized controlled trials of cannabinoid products in chronic pain conditions and for symptoms associated with multiple sclerosis: what do they tell us?

INTRODUCTION

: To investigate whether published systematic reviews of randomized controlled trials provide sufficient clarity to inform prescribing of cannabinoid products aimed for medicinal use, we examined their features and findings in two well-researched areas: chronic cancer/noncancer pain and multiple sclerosis (MS)-related symptoms.

AREAS COVERED

: Structured searches from January 2011 to 2 February 2021 identified 31 systematic reviews (with/without meta-analyses) that met the inclusion criteria. Support for the efficacy of cannabinoids was minimal in cancer pain, and somewhat stronger in noncancer (especially neuropathic) pain and MS spasticity. All systematic reviews and most meta-analyses grouped cannabinoid products together without appropriate consideration of their differential attributes (active constituent(s), concentration/strength, dosage forms, administration route), dosing regimens or treatment durations. Patient populations and efficacy outcome measures were inhomogeneous, particularly for studies in noncancer pain and MS. Separate results for specific cannabinoid formulations were rarely provided.

EXPERT OPINION

: The therapeutic effect of cannabinoids, as already demonstrated for some products, is not reflected clearly in the current range of systematic reviews and meta-analyses in chronic pain and MS. To truly inform evidence-based practice, future publications should aim to present results by individual product from well-conducted clinical trials using appropriate and homogeneous outcome measures in well-defined patient populations.

Cannabinoid Use for Pain Reduction in Spinal Cord Injuries: A Meta-Analysis of Randomized Controlled Trials

Background: Spinal cord injury (SCI) often involves multimodal pain control. This study aims to evaluate the efficacy and safety of cannabinoid use for the reduction of pain in SCI patients. Methods and Findings: This study followed the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement. We searched PubMed, EMBASE, Scopus, Cochrane, Web of Science, and ClinicalTrials.gov for relevant randomized controlled trials (RCTs) reporting the efficacy (e.g., pain relief) or safety (e.g., adverse events) of cannabinoids in patients with SCI, from inception to 25 December 2021. The study quality and the quality of evidence were evaluated by Cochrane ROB 2.0 and the Grading of Recommendations, Assessment, Development, and Evaluations system (GRADE), respectively. We used the random-effects model to perform the meta-analysis. From a total of 9,500 records, we included five RCTs with 417 SCI patients in the systematic review and meta-analysis. We judged all five of the included RCTs as being at high risk of bias. This meta-analysis indicated no significant difference in pain relief between the cannabinoids and placebo in SCI patients (mean difference of mean differences of pain scores: -5.68; 95% CI: -13.09, 1.73; p = 0.13; quality of evidence: very low), but higher odds of adverse events were found in SCI patients receiving cannabinoids (odds ratio: 3.76; 95% CI: 1.98, 7.13; p < 0.0001; quality of evidence: moderate). Conclusion: The current best evidence suggests that cannabinoids may not be beneficial for pain relief in SCI patients, but they do increase the risks of adverse events, including dizziness, somnolence, and dysgeusia, compared to the placebo. Cannabinoids should not be regularly suggested for pain reduction in SCI patients. Updating the systematic reviews and meta-analyses by integrating future RCTs is necessary to confirm these findings.

Cannabis and pain: a scoping review

For centuries, cannabis has been used with many different purposes, including medicinal use, usually bypassing any formal approval process. However, during the last decade, interest in cannabis in medicine has been increasing, and several countries, including the United States and Canada, have produced their own legislation about marihuana and cannabis-based medicines. Because of this, interest in research has been increasing and evidence about its medical effects is becoming necessary. We conducted a review examining the evidence of cannabis in pain. Cannabis had been shown to be useful in acute and chronic pain, however recently, these results have been controverted. Within the different types of chronic pain, it has a weak evidence for neuropathic, rheumatic pain, and headache, modest evidence for multiple sclerosis related pain, and as adjuvant therapy in cancer pain. There is no strong evidence to recommend cannabis in order to decrease opioids in patients with chronic use. Even though cannabis-based medications appear to be mostly safe, mild adverse effects are common; somnolence, sedation, amnesia, euphoric mood, hyperhidrosis, paranoia, and confusion may limit the use of cannabis in clinical practice. Risks have not been systematically analyzed. Special concern arises on how adverse effect might affect vulnerable population such as elderly patients. More research is needed in order to evaluate benefits and risks, as well as the ideal administration route and dosages. As cannabis use increases in several countries, answers to these questions might be coming soon.

Efficacy of cannabis-based medications compared to placebo for the treatment of chronic neuropathic pain: a systematic review with meta-analysis

Background

Chronic neuropathic pain (NP) presents therapeutic challenges. Interest in the use of cannabis-based medications has outpaced the knowledge of its efficacy and safety in treating NP. The objective of this review was to evaluate the effectiveness of cannabis-based medications in individuals with chronic NP.

Methods

Randomized placebo-controlled trials using tetrahydrocannabinol (THC), cannabidiol (CBD), cannabidivarin (CBDV), or synthetic cannabinoids for NP treatment were included. The MEDLINE, Cochrane Library, EMBASE, and Web of Science databases were examined. The primary outcome was the NP intensity. The risk of bias analysis was based on the Cochrane handbook.

Results

The search of databases up to 2/1/2021 yielded 379 records with 17 RCTs included (861 patients with NP). Meta-analysis showed that there was a significant reduction in pain intensity for THC/CBD by -6.624 units (P < .001), THC by -8.681 units (P < .001), and dronabinol by -6.0 units (P = .008) compared to placebo on a 0-100 scale. CBD, CBDV, and CT-3 showed no significant differences. Patients taking THC/CBD were 1.756 times more likely to achieve a 30% reduction in pain (P = .008) and 1.422 times more likely to achieve a 50% reduction (P = .37) than placebo. Patients receiving THC had a 21% higher improvement in pain intensity (P = .005) and were 1.855 times more likely to achieve a 30% reduction in pain than placebo (P < .001).

Conclusion

Although THC and THC/CBD interventions provided a significant improvement in pain intensity and were more likely to provide a 30% reduction in pain, the evidence was of moderate-to-low quality. Further research is needed for CBD, dronabinol, CT-3, and CBDV.

No antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol in male and female rats with persistent inflammatory pain

Studies have demonstrated antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol (THC) in animals, but whether such synergy occurs against all types of pain and in humans is unclear. Because a majority of chronic pain patients are women, and sex differences in morphine and THC potencies have been observed in rodents, the present study examined sex-specific effects of morphine and THC given alone and in combination, in rats with persistent inflammatory pain. On day 1, baseline mechanical and thermal response thresholds, hindpaw weight-bearing, locomotor activity, and hindpaw thickness were determined. Inflammation was then induced via hindpaw injection of complete Freund's adjuvant (CFA). Three days later, morphine (s.c.), THC (i.p) or a morphine-THC combination (1:1, 3:1 and 1:3 dose ratios) was administered, and behavioral testing was conducted at 30-240 min postinjection. Morphine alone was antiallodynic and antihyperalgesic, with no sex differences, but at some doses increased weight-bearing on the CFA-treated paw more in males than females. THC alone reduced mechanical allodynia with similar potency in both sexes, but reduced thermal hyperalgesia and locomotor activity with greater potency in females than males. All morphine-THC combinations reduced allodynia and hyperalgesia, but isobolographic analysis of mechanical allodynia data showed no significant morphine-THC synergy in either sex. Additionally, whereas morphine alone was antinociceptive at doses that did not suppress locomotion, morphine-THC combinations suppressed locomotion and did not increase weight-bearing on the inflamed paw. These results suggest that THC is unlikely to be a beneficial adjuvant when given in combination with morphine for reducing established inflammatory pain.

Untapped endocannabinoid pharmacological targets: Pipe dream or pipeline?

It has been established that the endogenous cannabinoid (endocannabinoid) system plays key modulatory roles in a wide variety of pathological conditions. The endocannabinoid system comprises both cannabinoid receptors, their endogenous ligands including 2-arachidonoylglycerol (2-AG), N-arachidonylethanolamine (anandamide, AEA), and enzymes that regulate the synthesis and degradation of endogenous ligands which include diacylglycerol lipase alpha (DAGL-α), diacylglycerol lipase beta (DAGL-β), fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL), α/β hydrolase domain 6 (ABHD6). As the endocannabinoid system exerts considerable involvement in the regulation of homeostasis and disease, much effort has been made towards understanding endocannabinoid-related mechanisms of action at cellular, physiological, and pathological levels as well as harnessing the various components of the endocannabinoid system to produce novel therapeutics. However, drug discovery efforts within the cannabinoid field have been slower than anticipated to reach satisfactory clinical endpoints and raises an important question into the validity of developing novel ligands that therapeutically target the endocannabinoid system. To answer this, we will first examine evidence that supports the existence of an endocannabinoid system role within inflammatory diseases, neurodegeneration, pain, substance use disorders, mood disorders, as well as metabolic diseases. Next, this review will discuss recent clinical studies, within the last 5 years, of cannabinoid compounds in context to these diseases. We will also address some of the challenges and considerations within the cannabinoid field that may be important in the advancement of therapeutics into the clinic.

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.

Cannabinoids in the Treatment of Back Pain

Marijuana is increasingly utilized for the treatment of multiple medical problems, including back pain, in the United States. Although there is strong preclinical evidence supporting the promise of cannabinoids in the treatment of back pain, there is a paucity of clinical data supporting their use in clinical practice. Opioids are an important medication for the treatment of acute and chronic back pain, but utilization of opioid-based regimens have likely contributed to the growing opioid epidemic. The significant risk of morbidity, mortality, and dependence secondary to opioid medications have increased the interest in nonopioid medications, including cannabinoid-based pain regimens, in treating back pain. This review will provide an overview on the pharmacology, drug delivery methods, clinical evidence, and safety considerations critical to understanding the potential role of cannabinoids in the treatment of back pain.

The influence of rat strain on the development of neuropathic pain and comorbid anxio-depressive behaviour after nerve injury

Back-translating the clinical manifestations of human disease burden into animal models is increasingly recognized as an important facet of preclinical drug discovery. We hypothesized that inbred rat strains possessing stress hyper-reactive-, depressive- or anxiety-like phenotypes may possess more translational value than common outbred strains for modeling neuropathic pain. Rats (inbred: LEW, WKY, F344/ICO and F344/DU, outbred: Crl:SD) were exposed to Spared Nerve Injury (SNI) and evaluated routinely for 6 months on behaviours related to pain (von Frey stimulation and CatWalk-gait analysis), anxiety (elevated plus maze, EPM) and depression (sucrose preference test, SPT). Markers of stress reactivity together with spinal/brain opioid receptor expression were also measured. All strains variously developed mechanical allodynia after SNI with the exception of stress-hyporesponsive LEW rats, despite all strains displaying similar functional gait-deficits after injury. However, affective changes reflective of anxiety- and depressive-like behaviour were only observed for F344/DU in the EPM, and for Crl:SD in SPT. Although differences in stress reactivity and opioid receptor expression occurred, overall they were relatively unaffected by SNI. Thus, anxio-depressive behaviours did not develop in all strains after nerve injury, and correlated only modestly with degree of pain sensitivity or with genetic predisposition to stress and/or affective disturbances.

Advances in the Treatment of Chronic Pain by Targeting GPCRs

Pain is an essential protective mechanism that the body uses to alert or prevent further damage. Pain sensation is a complex event involving perception, transmission, processing, and response. Neurons at different levels (peripheral, spinal cord, and brain) are responsible for these pro- or antinociceptive activities to ensure an appropriate response to external stimuli. The terminals of these neurons, both in the peripheral endings and in the synapses, are equipped with G protein-coupled receptors (GPCRs), voltage- and ligand-gated ion channels that sense structurally diverse stimuli and inhibitors of neuronal activity. This review will focus on the largest class of sensory proteins, the GPCRs, as they are distributed throughout ascending and descending neurons and regulate activity at each step during pain transmission. GPCR activation also directly or indirectly controls the function of co-localized ion channels. The levels and types of some GPCRs are significantly altered in different pain models, especially chronic pain states, emphasizing that these molecules could be new targets for therapeutic intervention in chronic pain.

Cannabinoid Receptors: An Update on Cell Signaling, Pathophysiological Roles and Therapeutic Opportunities in Neurological, Cardiovascular, and Inflammatory Diseases

The identification of the human cannabinoid receptors and their roles in health and disease, has been one of the most significant biochemical and pharmacological advancements to have occurred in the past few decades. In spite of the major strides made in furthering endocannabinoid research, therapeutic exploitation of the endocannabinoid system has often been a challenging task. An impaired endocannabinoid tone often manifests as changes in expression and/or functions of type 1 and/or type 2 cannabinoid receptors. It becomes important to understand how alterations in cannabinoid receptor cellular signaling can lead to disruptions in major physiological and biological functions, as they are often associated with the pathogenesis of several neurological, cardiovascular, metabolic, and inflammatory diseases. This review focusses mostly on the pathophysiological roles of type 1 and type 2 cannabinoid receptors, and it attempts to integrate both cellular and physiological functions of the cannabinoid receptors. Apart from an updated review of pre-clinical and clinical studies, the adequacy/inadequacy of cannabinoid-based therapeutics in various pathological conditions is also highlighted. Finally, alternative strategies to modulate endocannabinoid tone, and future directions are also emphasized.

Incerta-thalamic Circuit Controls Nocifensive Behavior via Cannabinoid Type 1 Receptors

Pain is a source of substantial discomfort. Abnormal activity in both the zona incerta (ZI) and posterior complex of the thalamus (Po) are implicated in neuropathic pain, but their exact roles remain unclear. In particular, the precise cell types and molecular mechanisms of the ZI-Po circuit that regulate nociception are largely uncharacterized. Here, we found that parvalbumin (PV)-positive neuronal projections from the ventral ZI (ZIv) to the Po (ZIv-Po) are critical for promoting nocifensive behaviors, whereas selectively inhibiting ZIv-Po activity reduces nocifensive withdrawal responses. Furthermore, cannabinoid type 1 receptors (CB₁Rs) are expressed specifically at ZIv-Po axon terminals in this circuit, and cannabinoids attenuate nocifensive responses through presynaptic inhibition. Selective inhibition of the ZIv-Po circuit or administration of cannabinoids into the Po are sufficient to ameliorate pathological pain. These findings identify the critical role of the ZIv-Po circuit and its modulation by endocannabinoids in controlling nocifensive behaviors.

Disability-specific genes GRIN1, GRIN2 and CNR1 show injury-dependent protein expression in the lumbar spinal cord of CCI rats

The sensory changes triggered by peripheral nerve injury result from functional changes in both neurons and glia in the dorsal horn of the spinal cord. Whether the disrupted affective-motivational states often comorbid with injury-evoked changes in sensation are driven directly by these functional changes is a question only recently investigated. Using a combination of GeneChip microarrays and RT-PCR techniques we identified differences in mRNA expression unique to rats with sustained changes to their social behaviour following sciatic nerve chronic constriction injury (CCI). Amongst these changes were the mRNAs encoding several of the NMDA subunits and the CB1 receptor. However, as protein translation is not a necessary consequence of the upregulation or downregulation of genes we decided to evaluate the functional significance of our initial observations using immunohistochemical detection of their translated protein products to determine their location and abundance in the lumbar spinal cord. Spinal cord tissue from rats with ('Affected'), and without ('Unaffected') changes in social behaviour after CCI was compared with tissue from uninjured controls. The expression of NMDA-1 (NR1) subunit, NMDA-2D subunit, Cannabinoid Receptor 1 (CB1), Glucocorticoid Receptor (GR) and Glial Fibrillary Acidic Protein (GFAP) immunoreactivities was quantified for these rats and revealed that nerve injury increased the expression of NMDA-2D, CB1 and GFAP immunoreactivity compared to uninjured controls. However, these changes were not specific to rats whose social behaviours were 'Affected' or 'Unaffected' by the nerve injury. Our data thus suggest that the development and expression of changes in social behaviour seen in a proportion of rats following CCI are unlikely to be directly related to the spinal changes in NMDA-2D, CB1 and GFAP expression induced by the nerve injury.

Pain in neuropsychiatry: Insights from animal models

Pain is the most common symptom reported in clinical practice, meaning that it is associated with many pathologies as either the origin or a consequence of other illnesses. Furthermore, pain is a complex emotional and sensorial experience, as the correspondence between pain and body damage varies considerably. While these issues are widely acknowledged in clinical pain research, until recently they have not been extensively considered when exploring animal models, important tools for understanding pain pathophysiology. Interestingly, chronic pain is currently considered a risk factor to suffer psychiatric disorders, mainly stress-related disorders like anxiety and depression. Conversely, pain appears to be altered in many psychiatric disorders, such as depression, anxiety and schizophrenia. Thus, pain and psychiatric disorders have been linked in epidemiological and clinical terms, although the neurobiological mechanisms involved in this pathological bidirectional relationship remain unclear. Here we review the evidence obtained from animal models about the co-morbidity of pain and psychiatric disorders, placing special emphasis on the different dimensions of pain.

Targeting Peripherally Restricted Cannabinoid Receptor 1, Cannabinoid Receptor 2, and Endocannabinoid-Degrading Enzymes for the Treatment of Neuropathic Pain Including Neuropathic Orofacial Pain

Neuropathic pain conditions including neuropathic orofacial pain (NOP) are difficult to treat. Contemporary therapeutic agents for neuropathic pain are often ineffective in relieving pain and are associated with various adverse effects. Finding new options for treating neuropathic pain is a major priority in pain-related research. Cannabinoid-based therapeutic strategies have emerged as promising new options. Cannabinoids mainly act on cannabinoid 1 (CB1) and 2 (CB2) receptors, and the former is widely distributed in the brain. The therapeutic significance of cannabinoids is masked by their adverse effects including sedation, motor impairment, addiction and cognitive impairment, which are thought to be mediated by CB1 receptors in the brain. Alternative approaches have been developed to overcome this problem by selectively targeting CB2 receptors, peripherally restricted CB1 receptors and endocannabinoids that may be locally synthesized on demand at sites where their actions are pertinent. Many preclinical studies have reported that these strategies are effective for treating neuropathic pain and produce no or minimal side effects. Recently, we observed that inhibition of degradation of a major endocannabinoid, 2-arachydonoylglycerol, can attenuate NOP following trigeminal nerve injury in mice. This review will discuss the above-mentioned alternative approaches that show potential for treating neuropathic pain including NOP.

FAAH inhibition as a preventive treatment for migraine: A pre-clinical study

BACKGROUND

Fatty-acid amide hydrolase (FAAH) is an intracellular serine hydrolase that catalyzes the cleavage of endogenous fatty-acid amides, including the endocannabinoid anandamide (AEA). We previously reported that the peripherally restricted FAAH inhibitor URB937, which selectively increases AEA levels outside the central nervous system, reduces hyperalgesia and c-Fos expression in the trigeminal nucleus caudalis (TNC) and the locus coeruleus in an animal model of migraine based on nitroglycerin (NTG) administration.

AIM

To further investigate the relevance of FAAH inhibition in the NTG animal model of migraine by testing the effects of the globally active FAAH inhibitor URB597.

METHODS

Our experimental approach involved mapping neuronal c-Fos protein expression, measurement of AEA levels in brain areas and in trigeminal ganglia, evaluation of pain-related behavior and quantification of molecular mediators in rats that received URB597 (2 mg/kg i.p.) either before or after NTG administration (10 mg/kg, i.p.).

RESULTS

Pre-treatment with URB597 significantly reduced c-Fos immunoreactivity in the TNC and inhibited NTG-induced hyperalgesia in the orofacial formalin test. This behavioral response was associated with a decrease in neuronal nitric oxide synthase, calcitonin gene-related peptide and cytokine gene expression levels in central and peripheral structures. Administration of URB597 after NTG had no such effect.

CONCLUSIONS

The findings suggest that global FAAH inhibition may offer a therapeutic approach to the prevention, but not the abortive treatment, of migraine attacks. Further studies are needed to elucidate the exact cellular and molecular mechanisms underlying the protective effects of FAAH inhibition.

Electroacupuncture alleviates morphine‑induced hyperalgesia by regulating spinal CB1 receptors and ERK1/2 activity

Electroacupuncture (EA), a traditional Chinese therapeutic technique, is considered an effective method for treating certain painful neuropathies induced by various neuropathological damage. The current study investigated the effect of EA on intrathecal (IT) morphine-induced hyperalgesia (MIH) and examined the hypothesis that activation of cannabinoid receptor 1 (CB1) could enhance the antinociceptive effect of EA on MIH via regulation of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway. Using a rat model of IT MIH, mechanical and thermal hyperalgesia were evaluated by an electronic von Frey filament and hotplate at baseline (1 day before IT administration) and at days 1, 3, 5 and 7 after IT administration. Rats received IT normal saline, IT morphine or IT morphine + EA at ST36-GB34. The protein levels of ERK1/2, phosphorylated (p)-ERK1/2 and CB1 in the spinal cord were assayed by western blotting. Furthermore, the effect of IT injection of the CB1 agonist WIN 55,212-2 and the CB1 antagonist SR141716 on the antinociceptive effect of EA in rats with MIH was investigated. Nociceptive behavior and ERK1/2, phosphorylated (p)-ERK1/2 and CB1 protein levels were evaluated as mentioned above. The results revealed that chronic IT injections of morphine induced a significant decrease in mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) accompanied with remarkable upregulation of p-ERK1/2 in the spinal cord, which could be attenuated by EA at the ST36-GB34 acupoints. In the rat model of MIH, IT injection of WIN 55,212-2 combined with EA induced a significant increase in MWT and TWL accompanied with a significant decrease in p-ERK1/2 and a significant increase in CB1 protein level compared with EA alone, while SR141716 induced the opposite results. The present study suggests that EA alleviates hyperalgesia induced by IT injection of morphine partially through the inhibition of ERK1/2 activation. Activation of the CB1 receptor enhances the antinociceptive effect of EA in rats with MIH partly through the regulation of the spinal CB1-ERK1/2 signaling pathway.

Selective modulation of the cannabinoid type 1 (CB1) receptor as an emerging platform for the treatment of neuropathic pain

Neuropathic pain is caused by a lesion or dysfunction in the nervous system, and it may arise from illness, be drug-induced or caused by toxin exposure. Since the discovery of two G-protein-coupled cannabinoid receptors (CB1 and CB2) nearly three decades ago, there has been a rapid expansion in our understanding of cannabinoid pharmacology. This is currently one of the most active fields of neuropharmacology, and interest has emerged in developing cannabinoids and other small molecule modulators of CB1 and CB2 as therapeutics for neuropathic pain. This short review article provides an overview of the chemotypes currently under investigation for the development of novel neuropathic pain treatments targeting CB1 receptors.

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.

Cannabinoids and Pain: New Insights From Old Molecules

Cannabis has been used for medicinal purposes for thousands of years. The prohibition of cannabis in the middle of the 20th century has arrested cannabis research. In recent years there is a growing debate about the use of cannabis for medical purposes. The term ‘medical cannabis’ refers to physician-recommended use of the cannabis plant and its components, called cannabinoids, to treat disease or improve symptoms. Chronic pain is the most commonly cited reason for using medical cannabis. Cannabinoids act via cannabinoid receptors, but they also affect the activities of many other receptors, ion channels and enzymes. Preclinical studies in animals using both pharmacological and genetic approaches have increased our understanding of the mechanisms of cannabinoid-induced analgesia and provided therapeutical strategies for treating pain in humans. The mechanisms of the analgesic effect of cannabinoids include inhibition of the release of neurotransmitters and neuropeptides from presynaptic nerve endings, modulation of postsynaptic neuron excitability, activation of descending inhibitory pain pathways, and reduction of neural inflammation. Recent meta-analyses of clinical trials that have examined the use of medical cannabis in chronic pain present a moderate amount of evidence that cannabis/cannabinoids exhibit analgesic activity, especially in neuropathic pain. The main limitations of these studies are short treatment duration, small numbers of patients, heterogeneous patient populations, examination of different cannabinoids, different doses, the use of different efficacy endpoints, as well as modest observable effects. Adverse effects in the short-term medical use of cannabis are generally mild to moderate, well tolerated and transient. However, there are scant data regarding the long-term safety of medical cannabis use. Larger well-designed studies of longer duration are mandatory to determine the long-term efficacy and long-term safety of cannabis/cannabinoids and to provide definitive answers to physicians and patients regarding the risk and benefits of its use in the treatment of pain. In conclusion, the evidence from current research supports the use of medical cannabis in the treatment of chronic pain in adults. Careful follow-up and monitoring of patients using cannabis/cannabinoids are mandatory.

Synthetic peripherally-restricted cannabinoid suppresses chemotherapy-induced peripheral neuropathy pain symptoms by CB1 receptor activation

Chemotherapy-induced peripheral neuropathy (CIPN) is a severe and dose-limiting side effect of cancer treatment that affects millions of cancer survivors throughout the world and current treatment options are extremely limited by their side effects. Cannabinoids are highly effective in suppressing pain symptoms of chemotherapy-induced and other peripheral neuropathies but their widespread use is limited by central nervous system (CNS)-mediated side effects. Here, we tested one compound from a series of recently developed synthetic peripherally restricted cannabinoids (PRCBs) in a rat model of cisplatin-induced peripheral neuropathy. Results show that local or systemic administration of 4-{2-[-(1E)-1[(4-propylnaphthalen-1-yl)methylidene]-1H-inden-3-yl]ethyl}morpholine (PrNMI) dose-dependently suppressed CIPN mechanical and cold allodynia. Orally administered PrNMI also dose-dependently suppressed CIPN allodynia symptoms in both male and female rats without any CNS side effects. Co-administration with selective cannabinoid receptor subtype blockers revealed that PrNMI's anti-allodynic effects are mediated by CB1 receptor (CB1R) activation. Expression of CB2Rs was reduced in dorsal root ganglia from CIPN rats, whereas expression of CB1Rs and various endocannabinoid synthesizing and metabolizing enzymes was unaffected. Daily PrNMI treatment of CIPN rats for two weeks showed a lack of appreciable tolerance to PrNMI's anti-allodynic effects. In an operant task which reflects cerebral processing of pain, PrNMI also dose-dependently suppressed CIPN pain behaviors. Our results demonstrate that PRCBs exemplified by PrNMI may represent a viable option for the treatment of CIPN pain symptoms.

Contribution of Endocannabinoid Gene Expression and Genotype on Low Back Pain Susceptibility and Chronicity

BACKGROUND

A major research emphasis has been focused on defining the molecular changes that occur from acute to chronic pain to identify potential therapeutic targets for chronic pain. As the endocannabinoid system is dynamically involved in pain signaling, a plausible mechanism that may contribute to chronic pain vulnerability involves alterations in the amount of circulating endocannabinoids. Therefore, this study sought to examine cannabinoid type 1 (CNR1), type 2 (CNR2) receptors, fatty acid amide hydrolase (FAAH), and the vanilloid receptor (transient receptor potential cation channel subfamily V member 1 [TRPV1]) gene expression profiles among individuals with acute and chronic low back pain (cLBP) at their baseline visit. We also assessed associations among selected single nucleotide polymorphisms (SNPs) of FAAH and CNR2 and measures of somatosensory function and self-report pain measures.Using a previously established quantitative sensory testing protocol, we comprehensively assessed somatosensory parameters among 42 acute LBP, 42 cLBP, and 20 pain-free participants. Samples of whole blood were drawn to examine mRNA expression and isolate genomic DNA for genotyping.CNR2 mRNA was significantly upregulated in all LBP patients compared with controls. However, FAAH mRNA and TRPV1 mRNA were significantly upregulated in cLBP compared with controls. A significant association was observed between FAAH SNP genotype and self-report pain measures, mechanical and cold pain sensitivity among LBP participants. cLBP participants showed increased FAAH and TRPV1 mRNA expression compared with acute LBP patients and controls.Further research to characterize pain-associated somatosensory changes in the context of altered mRNA expression levels and SNP associations may provide insight on the molecular underpinnings of maladaptive chronic pain.

Involvement of leptin signaling in the development of cannabinoid CB2 receptor-dependent mirror image pain

Neuropathic pain typically appears in a region innervated by an injured or diseased nerve and, in some instances, also on the contralateral side. This so-called mirror image pain is often observed in mice lacking CB2 receptors after sciatic nerve injury, but the underlying mechanisms for this phenotype largely remain unclear. Here we focused on peripheral leptin signaling, which modulates neuropathic pain development and interacts with the endocannabinoid system. Leptin production is induced at the site of nerve injury in CB2-deficient mice (CB2-KO) mice and wild type controls (WT). However, induction of leptin receptor expression was only observed in the injured nerve of CB2-KO mice. This was paralleled by a stimulation of the leptin receptor-downstream STAT3 signaling and an infiltration of F4/80-positive macrophages. Interestingly, an upregulation of leptin receptor expression STAT3 activity and macrophage infiltration was also observed on the non-injured nerve of CB2-KO mice thus reflecting the mirror image pain in CB2-KO animals. Importantly, perineurally-administered leptin-neutralizing antibodies reduced mechanical hyperalgesia, blocked mirror image pain and inhibited the recruitment of F4/80-positive macrophages. These results identify peripheral leptin signaling as an important modulator of CB2 signaling in neuropathic pain.

Cannabinoid WIN-55,212-2 mesylate inhibits tumor necrosis factor-α-induced expression of nitric oxide synthase in dorsal root ganglion neurons

Tumor necrosis factor-α (TNF-α) is an established pain modulator in the peripheral nervous system. Elevated levels of TNF-α in dorsal root ganglion (DRG) neurons reportedly is critical for neuropathic pain processing. It has been shown that the production of nitric oxide, a key player in the development and maintenance of nociception, depends on the expression of nitric oxide synthases (NOSs) and their activities. Accumulating evidence also supports an important role of cannabinoids in modulating neuropathic pain. In this study, we explored the effects and the underlying mechanisms of crosstalk between TNF-α and cannabinoid on the expression/activity of NOS in DRG neurons. With or without knockdown of p38 mitogen-activated protein kinase (MAPK), DRG neurons were treated with TNF-α in the presence or absence of synthetic cannabinoid WIN-55,212-2 mesylate (WIN-55) and selective cannabinoid receptor (CB) antagonists. TNF-α significantly increased the NOS activity as well as the mRNA stability and expression of neuronal NOS (nNOS) in DRG neurons; this was abolished by inhibiting p38 MAPK signaling. WIN-55 inhibited TNF-α-induced p38 MAPK activity as well as TNF-α-induced increase of mRNA stability and expression/activity of nNOS; the inhibitory effect of WIN-55 was blocked by a selective CB2 antagonist. Our findings suggest that TNF-α induces the expression/activity of nNOS in DRG neurons by increasing its mRNA stability by a p38 MAPK-dependent mechanism; WIN-55 inhibits this effect of TNF-α by inhibiting p38 MAPK via CB2. By linking the functions of TNF-α, NOS and cannabinoid in DRG neurons, this study adds new insights into the molecular mechanisms underlying the pharmacologic effects of cannabinoids on neuropathic pain as well as into the pathophysiology of neuropathic pain.

Molecular Understanding of the Activation of CB1 and Blockade of TRPV1 Receptors: Implications for Novel Treatment Strategies in Osteoarthritis

Osteoarthritis (OA) is a joint disease in which cartilage degenerates as a result of mechanical and biochemical changes. The main OA symptom is chronic pain involving both peripheral and central mechanisms of nociceptive processing. Our previous studies have implicated the benefits of dual- over single-acting compounds interacting with the endocannabinoid system (ECS) in OA treatment. In the present study, we focused on the specific molecular alterations associated with pharmacological treatment. OA was induced in Wistar rats by intra-articular injection of 3 mg of monoiodoacetate (MIA). Single target compounds (URB597, an FAAH inhibitor, and SB366791, a TRPV1 antagonist) and a dual-acting compound OMDM198 (FAAH inhibitor/TRPV1 antagonist) were used in the present study. At day 21 post-MIA injection, rats were sacrificed 1 h after i.p. treatment, and changes in mRNA expression were evaluated in the lumbar spinal cord by RT-qPCR. Following MIA administration, we observed 2-4-fold increase in mRNA expression of targeted receptors (Cnr1, Cnr2, and Trpv1), endocannabinoid degradation enzymes (Faah, Ptgs2, and Alox12), and TRPV1 sensitizing kinases (Mapk3, Mapk14, Prkcg, and Prkaca). OMDM198 treatment reversed some of the MIA effects on the spinal cord towards intact levels (Alox12, Mapk14, and Prkcg). Apparent regulation of ECS and TRPV1 in response to pharmacological intervention is a strong justification for novel ECS-based multi-target drug treatment in OA.

The Endogenous Cannabinoid System: A Budding Source of Targets for Treating Inflammatory and Neuropathic Pain

A great need exists for the development of new medications to treat pain resulting from various disease states and types of injury. Given that the endogenous cannabinoid (that is, endocannabinoid) system modulates neuronal and immune cell function, both of which play key roles in pain, therapeutics targeting this system hold promise as novel analgesics. Potential therapeutic targets include the cannabinoid receptors, type 1 and 2, as well as biosynthetic and catabolic enzymes of the endocannabinoids N-arachidonoylethanolamine and 2-arachidonoylglycerol. Notably, cannabinoid receptor agonists as well as inhibitors of endocannabinoid-regulating enzymes fatty acid amide hydrolase and monoacylglycerol lipase produce reliable antinociceptive effects, and offer opioid-sparing antinociceptive effects in myriad preclinical inflammatory and neuropathic pain models. Emerging clinical studies show that 'medicinal' cannabis or cannabinoid-based medications relieve pain in human diseases such as cancer, multiple sclerosis, and fibromyalgia. However, clinical data have yet to demonstrate the analgesic efficacy of inhibitors of endocannabinoid-regulating enzymes. Likewise, the question of whether pharmacotherapies aimed at the endocannabinoid system promote opioid-sparing effects in the treatment of pain reflects an important area of research. Here we examine the preclinical and clinical evidence of various endocannabinoid system targets as potential therapeutic strategies for inflammatory and neuropathic pain conditions.

Endocannabinoids: Effectors of glucocorticoid signaling

For decades, there has been speculation regarding the interaction of cannabinoids with glucocorticoid systems. Given the functional redundancy between many of the physiological effects of glucocorticoids and cannabinoids, it was originally speculated that the biological mechanisms of cannabinoids were mediated by direct interactions with glucocorticoid systems. With the discovery of the endocannabinoid system, additional research demonstrated that it was actually the opposite; glucocorticoids recruit endocannabinoid signaling, and that the engagement of endocannabinoid signaling mediated many of the neurobiological and physiological effects of glucocorticoids. With the development of advances in pharmacology and genetics, significant advances in this area have been made, and it is now clear that functional interactions between these systems are critical for a wide array of physiological processes. The current review acts a comprehensive summary of the contemporary state of knowledge regarding the biological interactions between glucocorticoids and endocannabinoids, and their potential role in health and disease.

Recent Advances in the Realm of Allosteric Modulators for Opioid Receptors for Future Therapeutics

Opioids, and more specifically μ-opioid receptor (MOR) agonists such as morphine, have long been clinically used as therapeutics for severe pain states but often come with serious side effects such as addiction and tolerance. Many studies have focused on bringing about analgesia from the MOR with attenuated side effects, but its underlying mechanism is not fully understood. Recently, focus has been geared toward the design and elucidation of the orthosteric site with ligands of various biological profiles and mixed subtype opioid activities and selectivities, but targeting the allosteric site is an area of increasing interest. It has been shown that allosteric modulators play key roles in influencing receptor function such as its tolerance to a ligand and affect downstream pathways. There has been a high variance of chemical structures that provide allosteric modulation at a given receptor, but recent studies and reviews tend to focus on the altered cellular mechanisms instead of providing a more rigorous description of the allosteric ligand's structure-function relationship. In this review, we aim to explore recent developments in the structural motifs that potentiate orthosteric binding and their influences on cellular pathways in an effort to present novel approaches to opioid therapeutic design.

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.