The endocannabinoid system as a potential therapeutic target for pain modulation

Daily dosing of cannabidiol (CBD) demonstrates a positive effect on measures of stress in dogs during repeated exposure to car travel

Dogs are routinely exposed to events that may elicit stress and result in negative emotional states which can impact pet dog welfare. One event many dogs living with people are routinely exposed to is car travel, with many dogs displaying behaviors, along with corresponding physiological responses, that are indicative of stress and anxiety. There are a range of management and treatment options that exist from behavior modification, drug therapy, and supplements, often with varying results. The aim of this study was to evaluate whether multiple doses of a tetrahydrocannabinol-free cannabidiol (CBD) distillate over a period of 6 mo could positively influence measures of stress in dogs. In a blinded, parallel design study, dogs (n = 19) underwent a series of short car journeys (test) where a range of physiological and behavioral measures were collected pre, during, and post-test. The car journeys elicited stress in this population of dogs, as indicated by significant changes (P < 0.05) in several stress-related measures (serum cortisol, heart rate, heart rate variability, whining, lip licking, yawning, and qualitative behavioral ratings) observed from baseline to test, which persisted over repeated car travel events. The mitigating effect of CBD treatment varied by measure, with cortisol, whining, lip licking, and qualitative behavioral ratings indicating a significant (P < 0.05) reduction in canine stress compared to the placebo group for at least one time point. Additional research investigating a range of dog populations and stressors is required to fully understand the complex effect of CBD on canine emotional wellbeing.

Serum anandamide and lipids associated with linoleic acid can distinguish interstitial cystitis/bladder pain syndrome from overactive bladder: An exploratory study

OBJECTIVES

Diagnosing interstitial cystitis/bladder pain syndrome presents a major challenge because it relies on subjective symptoms and empirical cystoscopic findings. A practical biomarker should discriminate diseases that cause increased urinary frequency, particularly overactive bladder. Therefore, we aimed to identify blood biomarkers that can discriminate between interstitial cystitis/bladder pain syndrome and overactive bladder.

METHODS

We enrolled patients with Hunner-type interstitial cystitis (n = 20), bladder pain syndrome (n = 20), and overactive bladder (n = 20) and without lower urinary tract symptoms (controls, n = 15) at Ueda Clinic and Nara Medical University Hospital from February 2020 to August 2021. The degree of interstitial cystitis/bladder pain syndrome symptoms was evaluated using the interstitial cystitis symptom and problem indices. Metabolomics analysis was performed on 323 serum metabolites using liquid chromatography time-of-flight mass spectrometry.

RESULTS

In the Hunner-type interstitial cystitis or bladder pain syndrome group, we observed smaller relative areas, including anandamide, acylcarnitine (18:2), linoleoyl ethanolamide, and arachidonic acid, compared to those in the overactive bladder or control group. Notably, the differences in the relative areas of anandamide were statistically significant (median: 3.950e-005 and 4.150e-005 vs. 8.300e-005 and 9.800e-005), with an area under the curve of 0.9321, demonstrating its ability to discriminate interstitial cystitis/bladder pain syndrome.

CONCLUSIONS

Serum anandamide may be a feasible diagnostic biomarker for interstitial cystitis/bladder pain syndrome. Reduced serum anandamide levels may be associated with pain and inflammation initiation, reflecting the pathology of interstitial cystitis/bladder pain syndrome. Furthermore, our findings suggest that abnormal linoleic acid metabolism may be involved in the pathogenesis of interstitial cystitis/bladder pain syndrome.

A single dose of cannabidiol (CBD) positively influences measures of stress in dogs during separation and car travel

Many dogs experience stress when separated from their caregivers, as well as when traveling in vehicles. Pet owners employ various approaches to managing these issues, from training, to giving medications and supplements, often with mixed results. Cannabidiol (CBD) can alleviate stress and anxiety in humans but the effect it has on canine stress is less well-documented. The present study aimed to understand the impact of being left alone and traveling in a car on measures of canine stress, and establish whether a single dose of a tetrahydrocannabinol (THC)-free CBD distillate could positively influence any measures of stress. In a blinded, parallel design study, a population of dogs were either left alone in a familiar room (n = 21) or underwent a short car journey (n = 19). A range of physiological and behavioral measures were collected pre, during and post-test. Significant changes in several stress-related measures (serum cortisol, mean ear temperature, heart rate, heart rate variability, whining and a stressed/anxious behavioral factor) were observed from baseline to test, with the car journey test paradigm eliciting a more pronounced stress response overall. The mitigating effect of CBD treatment varied by measure and test, with some indicating a significant reduction in canine stress compared to the placebo group. Additional research is required to fully understand the complex effect of CBD on canine wellbeing.

Could serum endocannabinoid and N-acylethanolamine levels be important in bipolar disorder?

OBJECTIVES

The endocannabinoid system (ECS) is a critical important neuromodulatory system that interacts with many neurohormonal and neurotransmitter systems in the brain. It plays a pivotal role in emotional responses and mood regulation. The ECS is related with psychotic disorders, depression, anxiety and autism. In this study, we aimed to investigate whether there is any relationship between endocannabinoid and N-acylethanolamine levels with bipolar disorder.

METHODS

Seventy-nine patients with bipolar disorder diagnosis, who are in the euthymic period, were included in the study. Clinical characteristics, symptoms and serum endocannabinoid and N-acylethanolamine levels were compared. Endocannabinoid and N-acylethanolamine levels were evaluated using liquid chromatography-tandem mass spectrometry.

RESULTS

In total of 79 patients, 44 (55.69%) were females and 35 (44.30%) were males. The mean age of the patients was 42.40 ± 1.10 years. Palmitoylethanolamide (PEA) levels were higher and oleoylethanolamide and 2-arachidonyl glycerol levels were lower in patients who had at least one depressive episode during their life-time illness than in patients who had no depressive episode while arachidonyl ethanolamide levels were unchanged.

CONCLUSIONS

PEA levels were correlated with the history and frequency of depressive episodes and the history of depressive symptoms in patients with bipolar disorder.

The Effects of Cannabinoid Agonist, Heat Shock Protein 90 and Nitric Oxide Synthase Inhibitors on Increasing IL-13 and IL-31 Levels in Chronic Pruritus

BACKGROUND

Heat shock protein 90 (Hsp90) inhibitor and cannabinoid agonists ameliorate dry skin-induced chronic itch. We have recently reported that cannabinoids, hsp90 and nitric oxide (NO) are involved in dry skin-induced itch. Here, we investigated the contribution of the Th2 cell signaling pathway to the antipruritic effect of the hsp90 inhibitor 17-Alilamino-17-demethoxygeldanamycin (17-AAG), nitric oxide synthase (NOS) inhibitor Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME) and cannabinoid agonist WIN 55,212-2 on a dry skin-induced scratch.

METHODS

Dry skin-induced chronic itching was created by topical application of AEW (acetone/diethyl ether/water). WIN 55,212-2 (1 mg/kg, i.p.), L-NAME (1 mg/kg, i.p.) and increasing doses of 17-AAG (1, 3 and 5 mg/kg,i.p.) were administered to Balb/c mice (for each group, n = 6). After these applications, skin tissues were taken from the nape region of all of the mice. Gene and protein expressions of IL-13 and IL-31 were evaluated in skin tissues by RT-PCR and immunohistochemistry, respectively.

RESULTS

IL-13 and IL-31 mRNA expressions and immune positive cell counts were increased in the AEW applied groups. WIN 55,212-2 reduced both of the increased cytokines levels, while L-NAME decreased only the IL-13. 17-AAG dose-dependently reduced the increased cytokine levels. IL-13 and IL-31 levels significantly decreased following the co-administration of these agents.

CONCLUSION

These results show that increased levels of IL-13 and IL-31 are associated with pruritus. Hsp90 inhibition and cannabinoid system activation may induce antipruritic effects through down-regulation of these cytokines.

Spinal cannabinoid CB1 or CB2 receptors activation attenuates mechanical allodynia in streptozotocin-induced diabetic rats

Diabetes is a chronic disease associated with a high number of complications such as peripheral neuropathy, which causes sensorial disturbances and may lead to the development of diabetic neuropathic pain (DNP). The current treatment for DNP is just palliative and the drugs may cause severe adverse effects, leading to discontinuation of treatment. Thus, new therapeutic targets need to be urgently investigated. Studies have shown that cannabinoids have promising effects in the treatment of several pathological conditions, including chronic pain. Thus, we aimed to investigate the acute effect of the intrathecal injection of CB1 or CB2 cannabinoid receptor agonists N-(2-chloroethyl)-5Z, 8Z, 11Z, 14Z-eicosatetraenamide (ACEA) or JWH 133, respectively (10, 30 or 100 μg/rat) on the mechanical allodynia associated with experimental diabetes induced by streptozotocin (60 mg/kg; intraperitoneal) in rats. Cannabinoid receptor antagonists CB1 AM251 or CB2 AM630 (1 mg/kg) were given before treatment with respective agonists to confirm the involvement of cannabinoid CB1 or CB2 receptors. Rats with diabetes exhibited a significant reduction on the paw mechanical threshold 2 weeks after diabetes induction, having the maximum effect observed 4 weeks after the streptozotocin injection. This mechanical allodynia was significantly improved by intrathecal treatment with ACEA or JWH 133 (only at the higher dose of 100 μg). Pre-treatment with AM251 or AM630 significantly reverted the anti-allodynic effect of the ACEA or JWH 133, respectively. Considering the clinical challenge that the treatment of DPN represents, this study showed for the first time, that the intrathecal cannabinoid receptors agonists may represent an alternative for the treatment of DNP.

Orchestration of the circadian clock and its association with Alzheimer's disease: Role of endocannabinoid signaling

Circadian rhythms are 24-hour natural rhythms regulated by the suprachiasmatic nucleus, also known as the "master clock". The retino-hypothalamic tract entrains suprachiasmatic nucleus with photic information to synchronise endogenous circadian rhythms with the Earth's light-dark cycle. However, despite the robustness of circadian rhythms, an unhealthy lifestyle and chronic photic disturbances cause circadian rhythm disruption in the suprachiasmatic nucleus's TTFL loops via affecting glutamate and γ-aminobutyric acid-mediated neurotransmission in the suprachiasmatic nucleus. Recently, considerable evidence has been shown correlating CRd with the incidence of Alzheimer's disease. The present review aims to identify the existence and signalling of endocannabinoids in CRd induced Alzheimer's disease through retino-hypothalamic tract- suprachiasmatic nucleus-cortex. Immunohistochemistry has confirmed the expression of cannabinoid receptor 1 in the suprachiasmatic nucleus to modulate the circadian phases of the master clock. Literature also suggests that cannabinoids may alter activity of suprachiasmatic nucleus by influencing the activity of their major neurotransmitter γ-aminobutyric acid or by interacting indirectly with the suprachiasmatic nucleus's two other major inputs i.e., the geniculo-hypothalamic tract-mediated release of neuropeptide Y and serotonergic inputs from the dorsal raphe nuclei. Besides, the expression of cannabinoid receptor 2 ameliorates cognitive deficits via reduction of tauopathy and microglial activation. In conclusion, endocannabinoids may be identified as a putative target for correcting CRd and decelerating Alzheimer's disease.

HPLC-DAD Analysis of Hemp Oil Supplements for Determination of Four Cannabinoids: Cannabidiol, Cannabidiolic Acid, Cannabinol and Delta 9-Tetrahydrocannabinol

Growing consumer interest in hemp oilseed supplements requires quality control. Therefore, appropriate, effective and verified analytical methods are needed for the determination of some bioactive cannabinoids in them. The aim of the study is to present an extended (compared to our previous research) validated high performance liquid chromatography with diode array detection (HPLC-DAD) method for the determination of four cannabinoids (cannabidiol, cannabidiolic acid, cannabinol and delta-9-tetrahydrocannabinol) in an oil matrix, which was used to determine these cannabinoids in seven commercial hemp oil supplements. In our method, the isolation of the target compounds was based on liquid extraction with acetonitrile combined with the freezing (at −41 °C) of the oil phase. The results show that in some cases, the determined concentrations of cannabinoids in the tested supplements differ significantly from those declared by the manufacturers. As for the main medicinal cannabinoid (CBD) in hemp oil supplements, in two cases, the measured concentration was significantly lower (1.45 and 1.81%) than the declared (5 and 5%), and in the other supplements, the obtained results confirm the declared amount of CBD within the error range from 3.29 to 9.2%. Therefore, to ensure the safe and beneficial use of these supplements by consumers, it is necessary to monitor their cannabinoid composition.

The role of endocannabinoid system and TRPV1 receptors in the antidepressant and anxiolytic effects of dipyrone in chronic unpredictable mild stress in mice

Although dipyrone is a widely used analgesic and antipyretic, its mechanism of action is not fully clarified. Recent studies have drawn attention to its central effects and its relationship with the endocannabinoid system. The endocannabinoid system plays important roles in processes such as anxiety, depression, fear, and learning-memory. In this study, we aimed to investigate whether endocannabinoid levels change in the amygdala in chronic unpredictable mild stress model in mice and whether cannabinoid and TRPV1 receptors mediate antidepressant and anxiolytic effects of dipyrone. Mice were submitted to chronic unpredictable mild stress protocol of 6-weeks, then behavioral test were performed. In the first part of the study, dipyrone was injected at doses of 150, 300, and 600 mg/kg (i.p.) during behavioral tests. In the second part, the CB1 antagonist AM 251 (1 mg/kg, i.p.), the CB2 antagonist AM630 (1 mg/kg, i.p.), and the TRPV1 antagonist capsazepine (3 mg/kg, i.p.) were administered alone or in combination with 300 mg/kg dipyrone to observe if these receptors mediate dipyrone effects. Endocannabinoid and N-acylethanolamines levels were measured by LC-MS/MS in amygdala. Our results showed that there were no changes in AEA, 2-AG, PEA, OAE levels in the amygdala in mice exposed to chronic unpredictable mild stress model; dipyrone exerted antidepressant and anxiolytic effects at doses of 300 and 600 mg/kg; its anxiolytic effect appears to be mediated via CB1 receptors, whereas TRPV1 receptors seems to mediate its antidepressant action.

Delivering therapeutic cannabinoids via skin: Current state and future perspectives

Adequate evidence exists in the literature indicating a relatively positive shift with regards to the legal acceptance of cannabis and cannabis-derived products for medicinal purposes in some countries. Concomitantly, scientists are showing renewed interest in cannabis-related research work. Over the years, clinical and preclinical studies have demonstrated the therapeutic significance of cannabinoids for diverse indications. Additionally, efforts are being made to develop cannabis-related products into acceptable prescription products. FDA authorization for the commercial use of four cannabinoid-derived products, available as oral dosage forms is a significant progress already. However, there are certain drawbacks associated with the conventional delivery forms of cannabinoids. These include low oral bioavailability due to hepatic degradation, gastric instability, poor water solubility, and the side effects experienced upon the use of high doses of psychotropic cannabinoids associated with heightened plasma concentrations of the drug. These are however, limitable with the aid of transcutaneous drug delivery. Emerging topical and transdermal strategies could be exploited for the successful development of highly effective delivery systems for cannabinoids. This review discusses the feasibility of delivering therapeutic cannabinoids via skin and provides a comprehensive account of the supporting research studies that have been reported in the literature till date.

Cannabinoid Receptors and Ligands: Lessons from CNS Disorders and the Quest for Novel Treatment Venues

The potential use of cannabinoids for therapeutic purposes is at the forefront of cannabinoid research which aims to develop innovative strategies to prevent, manage and treat a broad spectrum of human diseases. This chapter briefly reviews the pivotal role of the endocannabinoid system in modulating the central nervous system and its roles on neurodegenerative diseases and brain disorders. Ligand-induced modulation of cannabinoid 1 and 2 receptors to modulate immune response, decrease neurodegeneration and pain are aspects that are also discussed.

Positron Emission Tomography Imaging of the Endocannabinoid System: Opportunities and Challenges in Radiotracer Development

The endocannabinoid system (ECS) is involved in a wide range of biological functions and comprises cannabinoid receptors and enzymes responsible for endocannabinoid synthesis and degradation. Over the past 2 decades, significant advances toward developing drugs and positron emission tomography (PET) tracers targeting different components of the ECS have been made. Herein, we summarized the recent development of PET tracers for imaging cannabinoid receptors 1 (CB1R) and 2 (CB2R) as well as the key enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), particularly focusing on PET neuroimaging applications. State-of-the-art PET tracers for the ECS will be reviewed including their chemical design, pharmacological properties, radiolabeling, as well as preclinical and human PET imaging. In addition, this review addresses the current challenges for ECS PET biomarker development and highlights the important role of PET ligands to study disease pathophysiology as well as to facilitate drug discovery.

Non-opioid Analgesics and the Endocannabinoid System

Non-steroidal anti-inflammatory drugs produce antinociceptive effects mainly through peripheral cyclooxygenase inhibition. In opposition to the classical non-steroidal anti-inflammatory drugs, paracetamol and dipyrone exert weak anti-inflammatory activity, their antinociceptive effects appearing to be mostly due to mechanisms other than peripheral cyclooxygenase inhibition. In this review, we classify classical non-steroidal anti-inflammatory drugs, paracetamol and dipyrone as “non-opioid analgesics” and discuss the mechanisms mediating participation of the endocannabinoid system in their antinociceptive effects. Non-opioid analgesics and their metabolites may activate cannabinoid receptors, as well as elevate endocannabinoid levels through different mechanisms: reduction of endocannabinoid degradation via fatty acid amide hydrolase and/or cyclooxygenase-2 inhibition, mobilization of arachidonic acid for the biosynthesis of endocannabinoids due to cyclooxygenase inhibition, inhibition of endocannabinoid cellular uptake directly or through the inhibition of nitric oxide synthase production, and induction of endocannabinoid release.

A perspective review on fatty acid amide hydrolase (FAAH) inhibitors as potential therapeutic agents

Fatty acid amide hydrolase (FAAH) is an important enzyme creditworthy of hydrolyzing endocannabinoids and related-amidated signalling lipids, discovery of which has pioneered novel arena of pharmacological canvasses to unwrap its curative potency in various diseased circumstances. It presents contemporary basis for understanding molecules regulating and mediating inflammatory reactions, pain, anxiety, depression, and neurodegeneration. FAAH inhibitors form vital approach for discovery of therapeutic agents that are concerned with local elevation of endocannabinoids under certain stimuli, debarring adverse/unwanted secondary effects from global activation of cannabinoid receptors by exogenous cannabimimetics. During past decades, several molecules with excellent potency developed through tailor-made approaches entered into clinical trials, but none could reach market. Hence, hunt for novel, non-toxic and selective FAAH inhibitors are on horizon. This review summarizes present perception on FAAH in conjunction with its structure, mechanism of catalysis and biological functions. It also foregrounds recent development of molecules belonging to diverse chemical classes as potential FAAH inhibitors bobbing up from in-depth chemical, mechanistic and computational studies published since 2015-November 2019, focusing on their potency. This review will assist readers to obtain rationale on FAAH as potential target for addressing various disease conditions, acquiring significant knowledge on recently established inhibitor scaffolds and their development potentials. New technologies including MD-MM simulations and 3D-QSAR studies allow mechanistic characterization of enzyme. Assessment of in-vitro and in-vivo efficacy of existing FAAH inhibitors will facilitate researchers to design novel ligands utilizing modern drug design methods. The discussions will also impose precaution in decision making process, quashing possibility of late stage failure.

Endocannabinoid and N-acylethanolamide levels in rat brain and spinal cord following systemic dipyrone and paracetamol administration

The cannabinoid system has been suspected to play a role in the mechanisms of action of dipyrone and paracetamol. Our purpose was to measure the local endocannabinoid and N-acylethanolamide levels in the brain and spinal cord of rats following dipyrone and paracetamol administration. Nociception was assessed 1, 5, and 12 h following drug injections in Wistar rats, using tail-flick and hot-plate tests. The antinociceptive effects of dipyrone (150, 300, and 600 mg/kg, i.p.) and paracetamol (30, 100, and 300 mg/kg, i.p.) were observed. After administration of the highest doses of dipyrone and paracetamol, endocannabinoid (N-arachidonoylethanolamide (AEA), 2-arachidonoylglycerol (2-AG)) and N-acylethanolamide (palmitoylethanolamide (PEA), oleoylethanolamide (OEA)) levels were measured in the periaqueductal gray (PAG), rostral ventromedial medulla (RVM), and spinal cords of rats using tandem mass spectrometry with liquid chromatography. Increased 2-AG levels were observed in the PAG and the RVM 12 h after paracetamol injection; dipyrone exerted no action on 2-AG levels. Analgesic administrations led to a reduction in AEA levels in the RVM and spinal cord; similar decreases in PEA and OEA levels were observed in the RVM and the spinal cord. Dipyrone and paracetamol administrations appear to exert complicated effects on endocannabinoid and N-acylethanolamide levels in rats.

Cannabidiol, cannabinol and their combinations act as peripheral analgesics in a rat model of myofascial pain

OBJECTIVE

This study investigated whether local intramuscular injection of non-psychoactive cannabinoids, cannabidiol (CBD), cannabinol (CBN), cannabichromene (CBC) and their combinations can decrease nerve growth factor (NGF)-induced masticatory muscle sensitization in female rats.

DESIGN

In awake rats, changes in mechanical sensitivity induced by intramuscular injection of NGF and cannabinoids were measured by applying an electronic von Frey hair over the masseter muscle to measure the withdrawal response. The effect of CBD (5 mg/ml) and CBN (1 mg/ml) or their combinations CBD/CBN (1:1 mg/ml or 5:1 mg/ml) were assessed. To confirm a peripheral action, electrophysiological experiments were undertaken in anesthetized rats to examine whether intramuscular injections of CBD (5 mg/ml) and CBN (1 mg/ml) altered the mechanical threshold of masticatory muscle mechanoreceptors.

RESULTS

In behavioral experiments, CBD (5 mg/ml) or CBN (1 mg/ml) decreased NGF-induced mechanical sensitization. Combinations of CBD/CBN induced a longer-lasting reduction of mechanical sensitization than either compound alone. No significant change in mechanical withdrawal threshold was observed in the contralateral masseter muscles and no impairment of motor function was found with the inverted screen test after any of the treatments. Consistent with behavioral results, CBD (5 mg/ml), CBN (1 mg/ml) and the combination of CBD/CBN (1:1 mg/ml) increased the mechanical threshold of masseter muscle mechanoreceptors. However, combining CBD/CBN (5:1 mg/ml) at a higher ratio reduced the duration of this effect. This may indicate an inhibitory effect of higher concentrations of CBD on CBN.

CONCLUSIONS

These results suggest that peripheral application of these non-psychoactive cannabinoids may provide analgesic relief for chronic muscle pain disorders such as temporomandibular disorders and fibromyalgia without central side effects.

Brain activity of anandamide: a rewarding bliss?

Anandamide is a lipid mediator that acts as an endogenous ligand of CB1 receptors. These receptors are also the primary molecular target responsible for the pharmacological effects of Δ9-tetrahydrocannabinol, the psychoactive ingredient in Cannabis sativa. Several studies demonstrate that anandamide exerts an overall modulatory effect on the brain reward circuitry. Several reports suggest its involvement in the addiction-producing actions of other abused drugs, and it can also act as a behavioral reinforcer in animal models of drug abuse. Importantly, all these effects of anandamide appear to be potentiated by pharmacological inhibition of its metabolic degradation. Enhanced brain levels of anandamide after treatment with inhibitors of fatty acid amide hydrolase, the main enzyme responsible for its degradation, seem to affect the rewarding and reinforcing actions of many drugs of abuse. In this review, we will provide an overview from a preclinical perspective of the current state of knowledge regarding the behavioral pharmacology of anandamide, with a particular emphasis on its motivational/reinforcing properties. We will also discuss how modulation of anandamide levels through inhibition of enzymatic metabolic pathways could provide a basis for developing new pharmaco-therapeutic tools for the treatment of substance use disorders.

Contribution of spinal 5-HT5A receptors to the antinociceptive effects of systemically administered cannabinoid agonist WIN 55,212-2 and morphine

The antinociceptive effects of cannabinoids and opioids have been known for centuries. Serotonin and its receptors are also known to play important roles in nociception. However, the contribution of spinal 5-HT_5A receptors in antinociceptive effects of cannabinoids and opioids has not been studied. We conducted this study to clarify spinal mechanisms of the actions of the antinociceptive effects of cannabinoids and opioids. Hot plate and tail flick tests were used to assess the antinociceptive activity in Balb/c mice. WIN 55,212-2, a nonselective CB1 and CB2 agonist, and morphine exerted significant antinociceptive effects at 1, 3, and 10 mg/kg doses administered intraperitoneally in both hot plate and tail flick tests. The selective 5-HT_5A receptor antagonist SB-699551 (10 nmol/mouse) was administered intrathecally 10 min before the agonists. SB-699551 significantly reduced the antinociceptive effect of both WIN 55,212-2 and morphine. In the rotarod test, WIN 55,212-2 disrupted the motor coordination at a dose of 10 mg/kg, while morphine did not affect this function at any dose. Our findings show that spinal 5-HT_5A receptors are involved in the antinociceptive effects of WIN 55,212-2 and morphine.

Novel analgesic effects of melanin-concentrating hormone on persistent neuropathic and inflammatory pain in mice

The melanin-concentrating hormone (MCH) is a peptidergic neuromodulator synthesized by neurons in the lateral hypothalamus and zona incerta. MCHergic neurons project throughout the central nervous system, indicating the involvements of many physiological functions, but the role in pain has yet to be determined. In this study, we found that pMCH^-/- mice showed lower baseline pain thresholds to mechanical and thermal stimuli than did pMCH^+/+ mice, and the time to reach the maximum hyperalgesic response was also significantly earlier in both inflammatory and neuropathic pain. To examine its pharmacological properties, MCH was administered intranasally into mice, and results indicated that MCH treatment significantly increased mechanical and thermal pain thresholds in both pain models. Antagonist challenges with naltrexone (opioid receptor antagonist) and AM251 (cannabinoid 1 receptor antagonist) reversed the analgesic effects of MCH in both pain models, suggesting the involvement of opioid and cannabinoid systems. MCH treatment also increased the expression and activation of CB1R in the medial prefrontal cortex and dorsolateral- and ventrolateral periaqueductal grey. The MCH1R antagonist abolished the effects induced by MCH. This is the first study to suggest novel analgesic actions of MCH, which holds great promise for the application of MCH in the therapy of pain-related diseases.

CB1 Receptors Mediated Inhibition of ATP-Induced [Ca2+]i Increase in Cultured Rat Spinal Dorsal Horn Neurons

Spinal cannabinoid receptor 1 (CB₁R) and purinergic P2X receptors (P2XR) play a critical role in the process of pathological pain. Both CB₁R and P2XR are expressed in spinal dorsal horn (DH) neurons. It is not clear whether CB₁ receptor activation modulates the function of P2X receptor channels within dorsal horn. For this reason, we observed the effect of CP55940 (cannabinoid receptor agonist) on ATP-induced Ca²⁺ mobilization in cultured rat DH neurons. The changes of intracellular calcium concentration ([Ca²⁺]i) were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescent indicator. 100 μM ATP caused [Ca²⁺]i increase in cultured DH neurons. ATP-evoked [Ca²⁺]i increase in DH neurons was blocked by chelating extracellular Ca²⁺ and P2 purinoceptor antagonist PPADS. At the same time, ATP-γ-S (a non-hydrolyzable ATP analogue) mimicked the ATP action, while P2Y receptor agonist ADP failed to evoke [Ca²⁺]i increase in cultured DH neurons. These data suggest that ATP-induced [Ca²⁺]i elevation in cultured DH neurons is mediated by P2X receptor. Subsequently, we noticed that, in cultured rat DH neurons, ATP-induced Ca²⁺ mobilization was inhibited after pretreated with CP55940 with a concentration-dependent manner, which implies that the opening of P2X receptor channels are down-regulated by activation of cannabinoid receptor. The inhibitory effect of CP55940 on ATP-induced Ca²⁺ response was mimicked by ACEA (CB₁R agonist), but was not influenced by AM1241 (CB₂R agonist). Moreover, the inhibitory effect of CP55940 on ATP-induced Ca²⁺ mobilization was blocked by AM251 (CB₁ receptor antagonist), but was not influenced by AM630 (CB₂ receptor antagonist). In addition, we also observed that forskolin (an activator of adenylate cyclase) and 8-Br-cAMP (a cell-permeable cAMP analog) reversed the inhibitory effect of CP55940, respectively. In a summary, our observations raise a possibility that CB₁R rather than CB₂R can downregulate the opening of P2X receptor channels in DH neurons. The reduction of cAMP/PKA signaling is a key element in the inhibitory effect of CB₁R on P2X-channel-induced Ca²⁺ mobilization.

Blockade of Cannabinoid CB1 Receptors in the Dorsal Periaqueductal Gray Unmasks the Antinociceptive Effect of Local Injections of Anandamide in Mice

Divergent results in pain management account for the growing number of studies aiming at elucidating the pharmacology of the endocannabinoid/endovanilloid anandamide (AEA) within several pain-related brain structures. For instance, the stimulation of both Transient Receptor Potential Vanilloid type 1 (TRPV1) and Cannabinoid type 1 (CB1) receptors led to paradoxical effects on nociception. Here, we attempted to propose a clear and reproducible methodology to achieve the antinociceptive effect of exogenous AEA within the dorsal periaqueductal gray (dPAG) of mice exposed to the tail-flick test. Accordingly, male Swiss mice received intra-dPAG injection of AEA (CB1/TRPV1 agonist), capsaicin (TRPV1 agonist), WIN (CB1 agonist), AM251 (CB1 antagonist), and 6-iodonordihydrocapsaicin (6-IODO) (TRPV1 selective antagonist) and their nociceptive response was assessed with the tail-flick test. In order to assess AEA effects on nociception specifically at vanilloid or cannabinoid (CB) substrates into the dPAG, mice underwent an intrinsically inactive dose of AM251 or 6-IODO followed by local AEA injections and were subjected to the same test. While intra-dPAG AEA did not change acute pain, local injections of capsaicin or WIN induced a marked TRPV1- and CB1-dependent antinociceptive effect, respectively. Regarding the role of AEA specifically at CB/vanilloid substrates, while the blockade of TRPV1 did not change the lack of effects of intra-dPAG AEA on nociception, local pre-treatment of AM251, a CB1 antagonist, led to a clear AEA-induced antinociception. It seems that the exogenous AEA-induced antinociception is unmasked when it selectively binds to vanilloid substrates, which might be useful to address acute pain in basic and perhaps clinical trials.

Endocannabinoid activation of CB1 receptors contributes to long-lasting reversal of neuropathic pain by repetitive spinal cord stimulation

BACKGROUND

Spinal cord stimulation (SCS) has been shown to be effective in the management of certain neuropathic pain conditions, however, the underlying mechanisms are incompletely understood. In this study, we investigated repetitive SCS in a rodent neuropathic pain model, revealing long-lasting and incremental attenuation of hyperalgesia and a mechanism of action involving endocannabinoids.

METHOD

Animals were implanted with monopolar electrodes at the time of partial sciatic nerve injury. Dorsal columns at spinal segments T12/13 were stimulated 3 days later (early SCS), and again at day 7 (late SCS) using low-frequency parameters. Hypersensitivity to cutaneous mechanical stimuli was assessed using von Frey filaments. Pharmacological agents, selected to identify endocannabinoid and opioid involvement, were administered intraperitoneally, 10 min before SCS.

RESULTS

Early SCS caused partial reversal of mechanical hypersensitivity with corresponding changes in the biomarker of central sensitization, [phospho-Tyr¹⁴⁷² ]-GluN2B. The partial reversal of hyperalgesia by early SCS was amplified by co-administration of LY 2183240, an inhibitor of endocannabinoid reuptake/breakdown. This amplification was inhibited by a CB₁ R antagonist, AM251, but not by a CB₂ R antagonist, AM630. Early SCS-induced reversal of hyperalgesia was attenuated by naloxone, indicating a role for opioids. Late SCS resulted in an incremental level of reversal of hyperalgesia, which was inhibited by AM251, but not by CB₂ or opioid receptor antagonists.

CONCLUSION

The endocannabinoid system, and in particular the CB₁ R, plays a pivotal role in the long-lasting and incremental reversal of hyperalgesia induced by repetitive SCS in a neuropathic pain model.

SIGNIFICANCE

Alternative parameters for repetitive spinal cord stimulation (SCS) at 25/10 Hz elicit particularly long-lasting and incremental reversal of hyperalgesia in a neuropathic pain model through a mechanism involving endocannabinoids.

Descending serotonergic and noradrenergic systems do not regulate the antipruritic effects of cannabinoids

BACKGROUND

For centuries, cannabinoids have been known to be effective in pain states. Itch and pain are two sensations sharing a lot in common.

OBJECTIVE

The goal of this research was to observe whether the cannabinoid agonist WIN 55,212-2 reduces serotonin-induced scratching behaviour and whether neurotoxic destruction of descending serotonergic and noradrenergic pathways mediate the antipruritic effect of WIN 55,212-2. Material and methods Scratching behaviour was induced by intradermal injection of serotonin (50 µg/50 µl/mouse) to Balb/c mice. The neurotoxins 5,7-dihydroxytryptamine (5,7-DHT, 50 μg/mouse) and 6-hydroxydopamine (6-OHDA, 20 μg/mouse) are applied intrathecally to deplete serotonin and noradrenaline in the spinal cord. WIN 55,212-2 (1, 3, 10 mg/kg, i.p.) dose-dependently attenuated serotonin-induced scratches. Neurotoxic destruction of neither the serotonergic nor the noradrenergic systems by 5,7-DHT and 6-OHDA, respectively, had any effect on the antipruritic action of WIN 55,212-2.

CONCLUSION

Our findings indicate that cannabinoids dose-dependently reduce serotonin-induced scratching behaviour and neurotoxic destruction of descending inhibitory pathways does not mediate this antipruritic effect.

Changes in nociceptin/orphanin FQ levels in rat brain regions after acute and chronic cannabinoid treatment in conjunction with the development of antinociceptive tolerance

It has been indicated that acute and chronic morphine administrations enhance nociceptin/orphanin FQ (N/OFQ) levels in the brain, which might play role in the development of tolerance to the antinociceptive effect of morphine. Accordingly, N/OFQ receptor (NOP) antagonists have been shown to prevent the development of antinociceptive tolerance to morphine. Our aim is to observe whether cannabinoids, similarly to opioids, enhance N/OFQ levels in pain-related brain regions and whether antagonism of NOP receptors attenuates the development of tolerance to the antinociceptive effect of cannabinoids. Hot plate and Tail flick tests are used to assess the antinociceptive response in Sprague-Dawley rats. N/OFQ levels are measured in cortex, amygdala, hypothalamus, periaqueductal gray, nucleus raphe magnus and locus coeruleus of rat brains using Western blotting and immunohistochemistry. Within 9 days, animals became completely tolerant to the antinociceptive effect of the cannabinoid agonist WIN 55,212-2 (2, 4, 6 mg/kg, i.p.). Chronic administration of JTC-801, a NOP receptor antagonist, at a dose that exerted no effect on its own (1 mg/kg, i.p.), attenuated development of tolerance to the antinociceptive effect of WIN 55,212-2 (4 mg/kg, i.p.). Western blotting and immunohistochemistry results showed that N/OFQ levels significantly increased in amygdala, periaqueductal gray, nucleus raphe magnus and locus coeruleus of rat brains when WIN 55,212-2 was combined with JTC-801. We hypothesize that, similar to opioids, chronic cannabinoid + NOP antagonist administration may enhance N/OFQ levels and NOP receptor antagonism prevents development of tolerance to cannabinoid antinociception.

Fatty acid amide hydrolase inhibitor URB597 prevented tolerance and cognitive deficits induced by chronic morphine administration in rats

Inhibitors of the endocannabinoid metabolic enzyme fatty acid amide hydrolase exert therapeutic effects, but might also be associated with some of the adverse effects of cannabis. However, at least one fatty acid amide hydrolase inhibitor, URB597, has beneficial effects without signs of abuse or dependence. Although previous investigations have evaluated URB597-morphine interactions, the effects of URB597 on morphine tolerance and cognition deficits have not been studied previously. Rats were rendered tolerant to or dependent on morphine by an injection of morphine (10 mg/kg, subcutaneous) twice daily, respectively, for 7 or 10 days. URB597 (1 mg/kg, intraperitoneal) was administered before morphine. The tail-flick and passive avoidance learning tests were used to evaluate tolerance and cognition. Chronic morphine injection led to significant tolerance to the antinociceptive effect on days 5 and 7. URB597 completely prevented the development of morphine tolerance. URB597 also enhanced memory acquisition in the passive avoidance learning test, and although morphine impaired memory, URB597 alleviated this effect. These data show that URB597 protects against tolerance and memory deficits in chronic usage of morphine and suggests URB597 as a promising candidate for the treatment of adverse effects of opioids.

Endocannabinoid system as a regulator of tumor cell malignancy - biological pathways and clinical significance

The endocannabinoid system (ECS) comprises cannabinoid receptors (CBs), endogenous cannabinoids, and enzymes responsible for their synthesis, transport, and degradation of (endo)cannabinoids. To date, two CBs, CB1 and CB2, have been characterized; however, orphan G-protein-coupled receptor GPR55 has been suggested to be the third putative CB. Several different types of cancer present abnormal expression of CBs, as well as other components of ECS, and this has been shown to correlate with the clinical outcome. Although most effects of (endo)cannabinoids are mediated through stimulation of classical CBs, they also interact with several molecules, either prosurvival or proapoptotic molecules. It should be noted that the mode of action of exogenous cannabinoids differs significantly from that of endocannabinoid and results from the studies on their activity both in vivo and in vitro could not be easily compared. This review highlights the main signaling pathways involved in the antitumor activity of cannabinoids and the influence of their activation on cancer cell biology. We also discuss changes in the expression pattern of the ECS in various cancer types that have an impact on disease progression and patient survival. A growing amount of experimental data imply possible exploitation of cannabinoids in cancer therapy.

High Times for Painful Blues: The Endocannabinoid System in Pain-Depression Comorbidity

Depression and pain are two of the most debilitating disorders worldwide and have an estimated cooccurrence of up to 80%. Comorbidity of these disorders is more difficult to treat, associated with significant disability and impaired health-related quality of life than either condition alone, resulting in enormous social and economic cost. Several neural substrates have been identified as potential mediators in the association between depression and pain, including neuroanatomical reorganization, monoamine and neurotrophin depletion, dysregulation of the hypothalamo-pituitary-adrenal axis, and neuroinflammation. However, the past decade has seen mounting evidence supporting a role for the endogenous cannabinoid (endocannabinoid) system in affective and nociceptive processing, and thus, alterations in this system may play a key role in reciprocal interactions between depression and pain. This review will provide an overview of the preclinical evidence supporting an interaction between depression and pain and the evidence supporting a role for the endocannabinoid system in this interaction.

Contribution of nociceptin/orphanin FQ receptors to the anti-nociceptive and hypothermic effects of dipyrone

BACKGROUND

Dipyrone is one of the most commonly used non-opioid analgesic and antipyretic drug. Its anti-nociceptive and hypothermic effects have long been suspected to be centrally mediated. The involvement of the most recently discovered opioid peptide, nociceptin/orphanin FQ (N/OFQ), and its receptor (NOP) in pain transmission is controversial. It appears to be pro-nociceptive when administered supra-spinally, but exerts anti-nociceptive effects when injected spinally or systemically.

OBJECTIVE

Investigation of the role of the N/OFQ system in paracetamol-induced anti-nociception and hypothermia led us to determine its role in the anti-nociceptive and hypothermic effects of dipyrone. Material and Methods Hot-plate and tail-flick tests were used to assess nociception, and a rectal thermometer was used to measure rectal temperature in mice.

RESULTS

Mice injected with dipyrone (150, 300, 600 mg/kg, i.p.) displayed dose-related anti-nociception and hypothermia. The NOP receptor antagonist JTC-801 (3 mg/kg, i.p.), at a dose that exerted no effect when used alone, alleviated dipyrone-induced anti-nociception but did not reverse dipyrone-induced hypothermia.

CONCLUSION

We conclude that NOP receptors participate in the anti-nociceptive, but not in the hypothermic, effects of dipyrone.