Peripheral antinociceptive effect of anandamide and drugs that affect the endocannabinoid system on the formalin test in normal and streptozotocin-diabetic rats

Practical Considerations for the Use of Cannabis in Cancer Pain Management—What a Medical Oncologist Should Know

Pain is a highly debilitating emotional and sensory experience that significantly affects quality of life (QoL). Numerous chronic conditions, including cancer, are associated with chronic pain. In the setting of malignancy, pain can be a consequence of the tumor itself or of life-saving interventions, including surgery, chemotherapy, and radiotherapy. Despite significant pharmacological advances and awareness campaigns, pain remains undertreated in one-third of patients. To date, opioids have been the mainstay of cancer pain management. The problematic side effects and unsatisfactory pain relief of opioids have revived patients’ and physicians’ interest in finding new solutions, including cannabis and cannabinoids. The medical use of cannabis has been prohibited for decades, and it remains in Schedule 1 of the Misuse of Drugs Regulations. Currently, the legal context for its usage has become more permissive. Various preclinical and observational studies have aimed to prove that cannabinoids could be effective in cancer pain management. However, their clinical utility must be further supported by high-quality clinical trials.

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.

Bixin attenuates mechanical allodynia, anxious and depressive-like behaviors associated with experimental diabetes counteracting oxidative stress and glycated hemoglobin

Neuropathic pain, depression, and anxiety are common comorbidities in diabetic patients, whose pathophysiology involves hyperglycemia-induced increased oxidative stress. Bixin (BIX), an apocarotenoid extracted from the seeds of Bixa orellana, has been used in traditional medicine to treat diabetes and has been recognized by its antioxidant profile. We aimed to investigate the effect of the BIX over the mechanical allodynia, depressive, and anxious-like behaviors associated with experimental diabetes, along with its involved mechanisms. Streptozotocin-induced diabetic rats were treated for 17 days (starting 14 days after diabetes induction) with the corresponding vehicle, BIX (10, 30 or 90 mg/kg; p.o), or INS (6 IU; s.c.). Mechanical allodynia, depressive, and anxious-like behavior were assessed by electronic Von Frey, forced swimming, and elevated plus-maze tests, respectively. Locomotor activity was assessed by the open field test. Blood glycated hemoglobin (HbA1) and the levels of lipid peroxidation (LPO) and reduced glutathione (GSH) were evaluated on the hippocampus, pre-frontal cortex, lumbar spinal cord, and sciatic nerve. Diabetic animals developed mechanical allodynia, depressive and anxious-like behavior, increased plasma HbA1, increased LPO, and decreased GSH levels in tissues analyzed. Repeated BIX-treatment (at all tested doses) significantly attenuated mechanical allodynia, the depressive (30 and 90 mg/kg) and, anxious-like behaviors (all doses) in diabetic rats, without changing the locomotor performance. BIX (at all tested doses) restored the oxidative parameters in tissues analyzed and reduced the plasma HbA1. Thereby, bixin may represent an alternative for the treatment of comorbidities associated with diabetes, counteracting oxidative stress and plasma HbA1.

The potential anti-inflammatory and anti-nociceptive effects of rat hemopressin (PVNFKFLSH) in experimental arthritis

Inflammatory arthritis, such as rheumatoid arthritis (RA), stands out as one of the main sources of pain and impairment to the quality of life. The use of hemopressin (PVNFKFLSH; Hp), an inverse agonist of type 1 cannabinoid receptor, has proven to be effective in producing analgesia in pain models, but its effect on neuro-inflammatory aspects of RA is limited. In this study, antigen-induced arthritis (AIA) was evoked by the intraarticular (i.art.) injection of methylated bovine serum albumin (mBSA) in male Sprague Dawley rats. Phosphate buffered saline (PBS)-injected ipsilateral knee joints or AIA contralateral were used as control. Nociceptive and inflammatory parameters such as knee joint oedema and leukocyte influx and histopathological changes were carried out in addition to the local measurement of interleukins (IL) IL-6, IL-1β, tumor necrosis factor-α and the immunoreactivity of the neuropeptides substance P (SP) and calcitonin gene related peptide (CGRP) in the spinal cord (lumbar L3-5 segments) of AIA rats. For 4 days, AIA rats were treated daily with a single administration of saline, Hp injected (10 or 20 μg/day, i.art.), Hp given orally (20 μg/Kg, p.o.) or indomethacin (Indo; 5 mg/Kg, i.p.). In comparison to the PBS control group, the induction of AIA produced a significant and progressive mono-arthritis condition. The degree of AIA severity progressively compromised the normal walking pattern and impaired mobility over the next four days in relation to PBS-injected rats or contralateral knee joints. In AIA rats, the reduction of the distance between footprints and disturbances of gait evidenced signs of nociception. This response worsened at day 4, and a loss of footprint from the ipsilateral hind paw was evident. Daily treatment of the animals with Hp either i.art. (10 and 20 μg/knee) or p.o. (20 μg/Kg) as well as Indo (5 mg/Kg, i.p.) ameliorated the impaired mobility in a time-dependent manner (P < 0.05). In parallel, the AIA-injected ipsilateral knee joints reach a peak of swelling 24 h after AIA induction, which persisted over the next four days in relation to PBS-injected rats or contralateral knee joints. There was a significant but not dose-dependent inhibitory effect produced by all dosages and routes of Hp treatments on AIA-induced knee joint swelling (P < 0.05). In addition, the increased synovial levels of MPO activity, total leukocytes number and IL-6, but not IL-1β, were significantly reduced by the lower i.art. dose of Hp. In conclusion, these results successfully demonstrate that Hp may represent a novel therapeutic strategy to treat RA, an effect which is unrelated to the proinflammatory actions of the neuropeptides CGRP and SP.

Investigation of the Involvement of the Endocannabinoid System in TENS-Induced Antinociception

Transcutaneous electrical nerve stimulation (TENS) promotes antinociception by activating the descending pain modulation pathway and consequently releasing endogenous analgesic substances. In addition, recent studies have shown that the endocannabinoid system controls pain. Thus, the present study investigated the involvement of the endocannabinoid system in TENS-induced antinociception of cancer pain using a cancer pain model induced by intraplantar (i.pl.) injections of Ehrlich tumor cells in male Swiss mice. Low- and high-frequency TENS was applied for 20 minutes to the mice's paws, and to investigate the involvement of the endocannabinoid system were used the N-(peperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pitazole-3-carboixamide (AM251), a cannabinoid CB₁ receptor antagonist and (5Z,8Z,11Z,14Z)-5,8,11,14-eicosatetraenyl-methylester phosphonofluoridic acid (MAFP), an inhibitor of the endocannabinoid metabolizing enzyme fatty acid amide hydrolase, injected by via i.pl., intrathecal (i.t.), and intradorsolateral periaqueductal gray matter (i.dl.PAG). Furthermore, liquid chromatography-tandem mass spectrometry, western blot, and immunofluorescence assays were used to evaluate the endocannabinoid anandamide levels, cannabinoid CB₁ receptor protein levels, and cannabinoid CB₁ receptor immunoreactivity, respectively. Low- and high-frequency TENS reduced the mechanical allodynia induced by Ehrlich tumor cells and this effect was reversed by AM251 and potentiated by MAFP at the peripheral and central levels. In addition, TENS increased the endocannabinoid anandamide levels and the cannabinoid CB₁ receptor protein levels and immunoreactivity in the paw, spinal cord, and dorsolateral periaqueductal gray matter. These results suggest that low- and high-frequency TENS is effective in controlling cancer pain, and the endocannabinoid system is involved in this effect at both the peripheral and central levels. PERSPECTIVE: TENS is a nonpharmacological strategy that may be used to control cancer pain. Identification of a new mechanism involved in its analgesic effect could lead to the development of clinical studies as well as an increase in its application, lessening the need for pharmacological treatments.

Endocannabinoid modulation of inflammatory hyperalgesia in the IFN-α mouse model of depression

Depression is a well-recognised effect of long-term treatment with interferon-alpha (IFN-α), a widely used treatment for chronic viral hepatitis and malignancy. In addition to the emotional disturbances, high incidences of painful symptoms such as headache and joint pain have also been reported following IFN-α treatment. The endocannabinoid system plays an important role in emotional and nociceptive processing, however it is unknown whether repeated IFN-α administration induces alterations in this system. The present study investigated nociceptive responding in the IFN-α-induced mouse model of depression and associated changes in the endocannabinoid system. Furthermore, the effects of modulating peripheral endocannabinoid tone on inflammatory pain-related behaviour in the IFN-α model was examined. Repeated IFN-α administration (8000 IU/g/day) to male C57/Bl6 mice increased immobility in the forced swim test and reduced sucrose preference, without altering body weight gain or locomotor activity, confirming development of the depressive-like phenotype. There was no effect of repeated IFN-α administration on latency to respond in the hot plate test on day 4 or 7 of treatment, however, formalin-evoked nociceptive behaviour was significantly increased in IFN-α treated mice following 8 days of IFN-α administration. 2-Arachidonoyl glycerol (2-AG) levels in the periaqueductal grey (PAG) and rostroventromedial medulla (RVM), and anandamide (AEA) levels in the RVM, were significantly increased in IFN-α-, but not saline-, treated mice following formalin administration. There was no change in endocannabinoid levels in the prefrontal cortex, spinal cord or paw tissue between saline- or IFNα-treated mice in the presence or absence of formalin. Furthermore, repeated IFN-α and/or formalin administration did not alter mRNA expression of genes encoding the endocannabinoid catabolic enzymes (fatty acid amide hydrolase or monoacylglycerol lipase) or endocannabinoid receptor targets (CB_1, CB₂ or PPARs) in the brain, spinal cord or paw tissue. Intra plantar administration of PF3845 (1 μg/10 μl) or MJN110 (1 μg/10 μl), inhibitors of AEA and 2-AG catabolism respectively, attenuated formalin-evoked hyperalgesia in IFN-α, but not saline-, treated mice. In summary, increasing peripheral endocannabinoid tone attenuates inflammatory hyperalgesia induced following repeated IFN-α administration. These data provide support for the endocannabinoid system in mediating and modulating heightened pain responding associated with IFNα-induced depression.

Novel CB1-ligands maintain homeostasis of the endocannabinoid system in ω3- and ω6-long-chain-PUFA deficiency

Mammalian ω3- and ω6-PUFAs are synthesized from essential fatty acids (EFAs) or supplied by the diet. PUFAs are constitutive elements of membrane architecture and precursors of lipid signaling molecules. EFAs and long-chain (LC)-PUFAs are precursors in the synthesis of endocannabinoid ligands of G_i/o protein-coupled cannabinoid receptor (CB)1 and CB2 in the endocannabinoid system, which critically regulate energy homeostasis as the metabolic signaling system in hypothalamic neuronal circuits and behavioral parameters. We utilized the auxotrophic fatty acid desaturase 2-deficient (fads2^−/−) mouse, deficient in LC-PUFA synthesis, to follow the age-dependent dynamics of the PUFA pattern in the CNS-phospholipidome in unbiased dietary studies of three cohorts on sustained LC-PUFA-free ω6-arachidonic acid- and DHA-supplemented diets and their impact on the precursor pool of CB1 ligands. We discovered the transformation of eicosa-all cis-5,11,14-trienoic acid, uncommon in mammalian lipidomes, into two novel endocannabinoids, 20:3^5,11,14-ethanolamide and 2-20:3^5,11,14-glycerol. Their function as ligands of CB1 has been characterized in HEK293 cells. Labeling experiments excluded Δ8-desaturase activity and proved the position specificity of FADS2. The fads2^−/− mutant might serve as an unbiased model in vivo in the development of novel CB1 agonists and antagonists.

Cannabinoids and agmatine as potential therapeutic alternatives for cisplatin-induced peripheral neuropathy

Cisplatin is a widely used antineoplastic agent in the treatment of various cancers. Peripheral neuropathy is a well-known side effect of cisplatin and has the potential to result in limiting and/or reducing the dose, decreasing the quality of life. Unfortunately, the mechanism for cisplatin-induced neuropathy has not been completely elucidated. Currently, available treatments for neuropathic pain (NP) are mostly symptomatic, insufficient and are often linked with several detrimental side effects; thus, effective treatments are needed. Cannabinoids and agmatine are endogenous modulators that are implicated in painful states. This review explains the cisplatin-induced neuropathy and antinociceptive effects of cannabinoids and agmatine in animal models of NP and their putative therapeutic potential in cisplatin-induced neuropathy.

Characterization of the anterior cingulate cortex in adult tree shrew

The anterior cingulate cortex (ACC) is a key brain region for the perception of pain and emotion. Cellular and molecular mechanisms of the ACC are usually investigated in rodents such as mice and rats. Studies of synaptic mechanisms in primates are limited. To facilitate the translation of basic results from rodents to humans, it is critical to use a primate-like animal model for the investigation of the ACC. The tree shrew presents a great opportunity for this as they have similar genome sequences to primates and are considered to have many similarities to primates. In the present study, by combining anatomy, immunostaining and micro-optical sectioning tomography methods, we examined the morphological properties of the ACC in the tree shrew and compared them with the mouse and rat. We found that the ACC in the tree shrew is significantly larger than those found in the mouse and rat. The sizes of cell bodies of ACC pyramidal cells in tree shrew are also larger than that found in the mouse or rat. Furthermore, there are significantly more apical/basal dendritic branches and apical dendritic spines of ACC pyramidal neurons in tree shrew. These results demonstrate that pyramidal cells of the ACC in tree shrews are more advanced than those found in rodents (mice and rats), indicating that the tree shrew can be used as a useful animal model for studying the cellular mechanism for ACC-related physiological and pathological changes in humans.

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.

An Endocannabinoid Uptake Inhibitor from Black Pepper Exerts Pronounced Anti-Inflammatory Effects in Mice

Guineensine is a dietary N-isobutylamide widely present in black and long pepper (Piper nigrum and Piper longum) previously shown to inhibit cellular endocannabinoid uptake. Given the role of endocannabinoids in inflammation and pain reduction, here we evaluated guineensine in mouse models of acute and inflammatory pain and endotoxemia. Significant dose-dependent anti-inflammatory effects (95.6 ± 3.1% inhibition of inflammatory pain at 2.5 mg/kg ip and 50.0 ± 15.9% inhibition of edema formation at 5 mg/kg ip) and acute analgesia (66.1 ± 28.1% inhibition at 5.0 mg/kg ip) were observed. Moreover, guineensine inhibited proinflammatory cytokine production in endotoxemia. Intriguingly, guineensine and LPS independently induced catalepsy, but in combination this effect was abolished. Both hypothermia and analgesia were blocked by the CB1 receptor inverse agonist rimonabant, but the pronounced hypolocomotion was CB1 receptor-independent. A subsequent screen of 45 CNS-related receptors, ion channels, and transporters revealed apparent interactions of guineensine with the dopamine transporter DAT, 5HT2A, and sigma receptors, uncovering its prospective polypharmacology. The described potent pharmacological effects of guineensine might relate to the reported anti-inflammatory effects of pepper.

Antihyperalgesic Activities of Endocannabinoids in a Mouse Model of Antiretroviral-Induced Neuropathic Pain

Background: Nucleoside reverse transcriptase inhibitors (NRTIs) are the cornerstone of the antiretroviral therapy for human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS). However, their use is sometimes limited by the development of a painful sensory neuropathy, which does not respond well to drugs. Smoked cannabis has been reported in clinical trials to have efficacy in relieving painful HIV-associated sensory neuropathy. Objectives: The aim of this study was to evaluate whether the expression of endocannabinoid system molecules is altered during NRTI-induced painful neuropathy, and also whether endocannabinoids can attenuate NRTI-induced painful neuropathy. Methods: BALB/c mice were treated with 25 mg/kg of 2',3'-dideoxycytidine (ddC, zalcitabine), a NRTI, to induce thermal hyperalgesia. The expression of endocannabinoid system molecules was evaluated by real time polymerase chain reaction in the brain, spinal cord and paw skin at 6 days post ddC administration, a time point when mice had developed thermal hyperalgesia. The effects of the endocannabinoids, N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2-AG), the cannabinoid type 1 (CB1) receptor antagonist AM 251, CB2 receptor antagonist AM 630, and G protein-coupled receptor 55 (GPR55) antagonists ML193 and CID 16020046 on ddC-induced thermal hyperalgesia were evaluated using the hot plate test. Results: ddC treatment resulted in thermal hyperalgesia and increased transcripts of the synthesizing enzyme Plcβ1 and decreased Daglβ in the paw skins, but not Napepld, and Daglα compared to vehicle treatment. Transcripts of the inactivating enzymes Faah and Mgll were downregulated in the brain and/or paw skin but not in the spinal cord of ddC-treated mice. Both AEA and 2-AG had antihyperalgesic effects in mice with ddC-induced thermal hyperalgesia, but had no effect in ddC-naïve mice. The antihyperalgesic activity of AEA was antagonized by AM251 and AM630, whereas the activity of 2-AG was antagonized by AM251, ML193 and CID 16020046, but not by AM630. Conclusion: These data show that ddC induces thermal hyperalgesia, which is associated with dysregulation of the mRNA expression of some endocannabinoid system molecules. The endocannabinoids AEA and 2-AG have antihyperalgesic activity, which is dependent on cannabinoid receptor and GPR55 activation. Thus, agonists of cannabinoid receptors and GPR55 could be useful therapeutic agents for the management of NRTI-induced painful sensory neuropathy.

Antihyperalgesic effect of CB1 receptor activation involves the modulation of P2X3 receptor in the primary afferent neuron

Cannabinoid system is a potential target for pain control. Cannabinoid receptor 1 (CB₁) activation play a role in the analgesic effect of cannabinoids once it is expressed in primary afferent neurons. This study investigates whether the anti-hyperalgesic effect of CB₁ receptor activation involves P2X₃ receptor in primary afferent neurons. Mechanical hyperalgesia was evaluated by electronic von Frey test. Cannabinoid effect was evaluated using anandamide or ACEA, a non-selective or a selective CB₁ receptor agonists, respectively; AM251, a CB₁ receptor antagonist, and antisense ODN for CB₁ receptor. Calcium imaging assay was performed to evaluated α,β-meATP-responsive cultured DRG neurons pretreated with ACEA. Anandamide or ACEA administered in peripheral tissue reduced the carrageenan-induced mechanical hyperalgesia. The reduction in the carrageenan-induced hyperalgesia induced by ACEA was completely reversed by administration of AM251 as well as by the intrathecal treatment with antisense ODN for CB₁ receptor. Also, ACEA reduced the mechanical hyperalgesia induced by bradykinin and by α,β-meATP, a P2X₃ receptor non-selective agonist, but not by tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and chemokine-induced chemoattractant-1 (CINC-1). Finally, CB₁ receptors are co-localized with P2X₃ receptors in DRG small-diameter neurons and the treatment with ACEA reduced the number of α,β-meATP-responsive cultured DRG neurons. Our data suggest that the analgesic effect of CB₁ receptor activation is mediated by a negative modulation of the P2X₃ receptor in the primary afferent neurons.

Peripherally Restricted Cannabinoids for the Treatment of Pain

The use of cannabinoids for the treatment of chronic diseases has increased in the United States, with 23 states having legalized the use of marijuana. Although currently available cannabinoid compounds have shown effectiveness in relieving symptoms associated with numerous diseases, the use of cannabis or cannabinoids is still controversial mostly due to their psychotropic effects (e.g., euphoria, laughter) or central nervous system (CNS)-related undesired effects (e.g., tolerance, dependence). A potential strategy to use cannabinoids for medical conditions without inducing psychotropic or CNS-related undesired effects is to avoid their actions in the CNS. This approach could be beneficial for conditions with prominent peripheral pathophysiologic mechanisms (e.g., painful diabetic neuropathy, chemotherapy-induced neuropathy). In this article, we discuss the scientific evidence to target the peripheral cannabinoid system as an alternative to cannabis use for medical purposes, and we review the available literature to determine the pros and cons of potential strategies that can be used to this end.

Activities of 2-phthalimidethyl nitrate and 2-phthalimidethanol in the models of nociceptive response and edema induced by formaldehyde in mice and preliminary investigation of the underlying mechanisms

The activities of 2-phthalimidethyl nitrate (PTD-NO) and 2-phthalimidethanol (PTD-OH) were recently demonstrated in models of pain and inflammation. We expanded our investigation by evaluating their activities in models of nociceptive and inflammatory pain and inflammatory edema, the preliminary pharmacokinetic parameter for PTD-NO and the role of opioid and cannabinoid pathways in the activity of analogs. Per os (p.o.) administration of PTD-NO or PTD-OH, 1h before intraplantar injection of formaldehyde, inhibited both phases of the nociceptive response (500 and 750 mg/kg) and paw edema (125, 250, 500 and 750 mg/kg). After p.o. administration of PTD-NO, peak plasma concentrations of PTD-NO and PTD-OH were found 0.92 and 1.13 h, respectively. The plasma concentrations of PTD-NO were higher than those of PTD-OH. Intraperitoneal (i.p.) administration of CB1 (AM251) or CB2 (AM630) cannabinoid receptor antagonists (4 or 8 mg/kg, -30 min) or opioid antagonist naltrexone (5 or 10mg/kg, -30 min) did not affect the antinociceptive activities of the analogs. AM251 (8 mg/kg, i.p., -30 min) attenuated the antiedematogenic activity of both analogs, while naltrexone (10mg/kg, i.p., -30 min) only attenuated the antiedematogenic activity of PTD-NO. The antiedematogenic activities of both analogs were not affected by the CB2 cannabinoid antagonist AM630 (4 or 8 mg/kg, i.p., -30 min). Concluding, we expanded the knowledge on the activities of PTD-NO and PTD-OH by showing that these phthalimide analogs also exhibit marked activity in models of nociceptive and inflammatory pain and inflammatory edema. Opioid and cannabinoid mechanisms partially mediate the anti-inflammatory, but not the antinociceptive activity.

Acute administration of a cannabinoid CB1 receptor antagonist impairs stress-induced antinociception in fish

This study evaluated the influence of the pre-treatment with AM251 (a cannabinoid type I receptor (CB1) selective antagonist) on the stress-induced antinociception promoted by restraint in the fish Leporinus macrocephalus. The application of 3 and 5 min of restraint stress promoted an inhibition of the behavioural response to the subcutaneous injection of 3% formaldehyde (increase in locomotor activity), suggesting the activation of an antinociceptive system. The acute intraperitoneal administration of AM251 (3 mg·kg(-1)) impaired this antinociceptive response induced by 3 and 5 min of restraint stress. The fish treated with AM251 before the application of restraint stress presented an increase in locomotor activity after the subcutaneous injection of formaldehyde, similar to fish not exposed to restraint, suggesting that the stress-induced antinociception promoted by restraint in fish is probably mediated by cannabinoid CB1 receptors. The results presented in this paper suggest the participation of the endocannabinoid system in nociception modulation in fish, supporting the hypothesis that an endogenous antinociceptive system activated by restraint stress is present in fish and that the modulation of antinociception by the CB1 receptor is evolutionary well-conserved across vertebrates.

Neurotrophins, endocannabinoids and thermo-transient receptor potential: a threesome in pain signalling

Because of the social and economic costs of chronic pain, there is a growing interest in unveiling the cellular and molecular mechanisms underlying it with the aim of developing more effective medications. Pain signalling is a multicomponent process that involves the peripheral and central nervous systems. At the periphery, nociceptor sensitisation by pro-inflammatory mediators is a primary step in pain transduction. Although pain is multifactorial at cellular and molecular levels, it is widely accepted that neurotrophin (TrkA, p75NTR, Ret and GFRs), cannabinoid (CB1 and CB2), and thermo-transient receptor potential (TRPs; TRPV1, TRPA1 and TRPM8) receptors play a pivotal role. They form a threesome for which endocannabinoids appear to be a first line of defence against pain, while neurotrophins and thermoTRPs are the major generators of painful signals. However, endocannabinoids may exhibit nociceptive activity while some neurotrophins may display anti-nociception. Accordingly, a clear-cut knowledge of the modulation and context-dependent function of these signalling cascades, along with the molecular and dynamic details of their crosstalk, is critical for understanding and controlling pain transduction. Here, the recent progress in this fascinating topic, as well as the tantalizing questions that remain unanswered, will be discussed. Furthermore, we will underline the need for using a systems biology approach (referred to as systems pain) to uncover the dynamics and interplay of these intricate signalling cascades, taking into consideration the molecular complexity and cellular heterogeneity of nociceptor populations. Nonetheless, the available information confirms that pharmacological modulation of this signalling triad is a highly valuable therapeutic strategy for effectively treating pain syndromes.

Influence of nitric oxide synthase or cyclooxygenase inhibitors on cannabinoids activity in streptozotocin-induced neuropathy

BACKGROUND

Influence of a relatively specific inhibitor cyclooxygenase (COX)-2, celecoxib, a relatively specific inhibitor of neuronal nitric oxide synthase (NOS), 7-Ni, and a relatively selective inhibitor of inducible NOS, L-NIL, on the action of a preferentially selective CB1 cannabinoid receptor agonist, Met-F-AEA and a selective CB2 cannabinoid receptor agonist, AM 1241 was investigated, in a streptozotocin (STZ)-induced neuropathy.

METHODS

Studies were performed on male Wistar rats. Changes in nociceptive thresholds were determined using mechanical stimuli - the modification of the classic paw withdrawal test described by Randall-Selitto. Diabetes was induced by a single administration of STZ.

RESULTS

In a diabetic neuropathic pain model, pretreatment with celecoxib, L-NIL and 7-Ni, significantly increased the antihyperalgesic activity of both Met-F-AEA and AM 1241.

CONCLUSIONS

The results of this study seemed to indicate that the interaction between cannabinoid, COX-2 and NOS(s) systems might exist. Concomitant administration of small doses of CB1 and/or CB2 receptor agonists and COX-2 or NOS inhibitors can be effective in the alleviation of diabetic neuropathic pain.

Improving the translation of analgesic drugs to the clinic: animal models of neuropathic pain

Neuropathic pain remains an area of considerable unmet clinical need. Research based on preclinical animal models has failed to deliver truly novel treatment options, questioning the predictive value of these models. This review addresses the shortcomings of rodent in vivo models commonly used in the field and highlights approaches which could increase their predictivity, including more clinically relevant assays, outcome measures and animal characteristics. The methodological quality of animal studies also needs to be improved. Low internal validity and incomplete reporting lead to a waste of valuable research resources and animal lives, and ultimately prevent an objective assessment of the true predictivity of in vivo models.

Activation of spinal cannabinoid CB2 receptors inhibits neuropathic pain in streptozotocin-induced diabetic mice

The role of spinal cannabinoid systems in neuropathic pain of streptozotocin (STZ)-induced diabetic mice was studied. In normal mice, injection of the cannabinoid receptor agonist WIN-55,212-2 (1 and 3μg, i.t.) dose-dependently prolonged the tail-flick latency, whereas there were no changes with the injection of either cannabinoid CB1 (AM 251, 1 μg, i.t.) or CB2 (AM 630, 4 μg, i.t.) receptor antagonists. AM 251 (1 μg, i.t.), but not AM 630 (4 μg, i.t.), significantly inhibited the prolongation of the tail-flick latency induced by WIN-55,212-2 (3 μg, i.t.). In STZ-induced diabetic mice, the tail-flick latency was significantly shorter than that in normal mice. A low dose of WIN-55,212-2 (1 μg, i.t.) significantly recovered the tail-flick latency in STZ-induced diabetic mice. The effect of WIN-55,212-2 (1 μg, i.t.) in STZ-induced diabetic mice was significantly inhibited by AM 630 (4 μg, i.t.), but not AM 251 (1 μg). The selective cannabinoid CB2 receptor agonist L-759,656 (19 and 38 μg, i.t.) also dose-dependently recovered the tail-flick latency in STZ-induced diabetic mice, and this recovery was inhibited by AM 630 (4 μg, i.t.). The protein levels of cannabinoid CB1 receptors, CB2 receptors and diacylglycerol lipase α (DGL-α), the enzyme that synthesizes endocannabinoid 2-arachidonoylglycerol, in the spinal cord were examined using Western blotting. The protein levels of both cannabinoid CB1 and CB2 receptors were increased in STZ-induced diabetic mice, whereas the protein level of DGL-α was significantly decreased. These results indicate that spinal cannabinoid systems are changed in diabetic mice and suggest that cannabinoid CB2 receptor agonists might have an ability to recover diabetic neuropathic pain.

Diabetic neuropathy and oxidative stress: therapeutic perspectives

Diabetic neuropathy (DN) is a widespread disabling disorder comprising peripheral nerves' damage. DN develops on a background of hyperglycemia and an entangled metabolic imbalance, mainly oxidative stress. The majority of related pathways like polyol, advanced glycation end products, poly-ADP-ribose polymerase, hexosamine, and protein kinase c all originated from initial oxidative stress. To date, no absolute cure for DN has been defined; although some drugs are conventionally used, much more can be found if all pathophysiological links with oxidative stress would be taken into account. In this paper, although current therapies for DN have been reviewed, we have mainly focused on the links between DN and oxidative stress and therapies on the horizon, such as inhibitors of protein kinase C, aldose reductase, and advanced glycation. With reference to oxidative stress and the related pathways, the following new drugs are under study such as taurine, acetyl-L-carnitine, alpha lipoic acid, protein kinase C inhibitor (ruboxistaurin), aldose reductase inhibitors (fidarestat, epalrestat, ranirestat), advanced glycation end product inhibitors (benfotiamine, aspirin, aminoguanidine), the hexosamine pathway inhibitor (benfotiamine), inhibitor of poly ADP-ribose polymerase (nicotinamide), and angiotensin-converting enzyme inhibitor (trandolapril). The development of modern drugs to treat DN is a real challenge and needs intensive long-term comparative trials.

Orofacial sensory changes after streptozotocin-induced diabetes in rats

Peripheral neuropathy is a common complication of diabetes and is often accompanied by episodes of pain. There is evidence that diabetic neuropathy may affect the trigeminal nerve, altering the transmission of orofacial sensory information. Structural changes in the trigeminal ganglia may be involved in the development of these sensory alterations. Herein, we evaluate the development of orofacial sensory changes after streptozotocin-induced diabetes in rats, and their sensitivity to pregabalin and morphine treatments. Furthermore, stereological analysis of the trigeminal ganglia was performed. Diabetic rats showed similar responses to 1% formalin applied into the upper lip compared to normoglycemic rats on weeks 1, 2 and 4 after streptozotocin. Additionally, there was no difference in the facial mechanical threshold of normoglycemic and diabetic rats, on weeks 1 up to 5 after streptozotocin, while the paw mechanical threshold of diabetic rats was significantly reduced. In contrast, diabetic rats developed long-lasting orofacial heat and cold hyperalgesia. Moreover, stereological analyses revealed significant neuronal loss in the trigeminal ganglia of diabetic compared to normoglycemic rats. Pregabalin treatment (30mg/kg, p.o.) of diabetic rats resulted in marked and prolonged (up to 6h) reduction of heat and cold orofacial hyperalgesia. Likewise, morphine treatment (2.5mg/kg, s.c.) abolished orofacial heat and cold hyperalgesia, but its effect was significant only up to 1h after the administration. In conclusion, the results of the present study demonstrated that streptozotocin-treated rats developed long-lasting orofacial heat and cold hyperalgesia, which is more amenable to reduction by pregabalin than morphine.