ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

Contribution of CD4+ cells in the emotional alterations induced by endometriosis in mice

Endometriosis is a disease defined by the presence of endometrial tissue in extrauterine locations. This chronic condition is frequently associated with pain and emotional disorders and has been related with altered immune function. However, the specific involvement of immune cells in pain and behavioral symptoms of endometriosis has not been yet elucidated. Here, we implement a mouse model of non-surgical endometriosis in which immunocompetent mice develop abdomino-pelvic hypersensitivity, cognitive deficits, anxiety and depressive-like behaviors. This behavioral phenotype correlates with expression of inflammatory markers in the brain, including the immune cell marker CD4. Depletion of CD4 + cells decreases the anxiety-like behavior of mice subjected to the endometriosis model, whereas abdomino-pelvic hypersensitivity, depressive-like behavior and cognitive deficits remain unaltered. The present data reveal the involvement of the immune response characterized by CD4 + white blood cells in the anxiety-like behavior induced by endometriosis in mice. This model, which recapitulates the symptoms of human endometriosis, may be a useful tool to study the immune mechanisms involved in pain and behavioral alterations associated to endometriosis.

Operant Self-medication for Assessment of Spontaneous Pain Relief and Drug Abuse Liability in Mouse Models of Chronic Pain

The search for safe and efficient chronic pain treatments is dampened by the lack of reliable models that faithfully reproduce current pharmacological treatments for chronic spontaneous pain in humans. Preclinical models often assess the antinociceptive efficacy of non-contingent pharmacological treatments evaluated in the short-term. Here, we provide a protocol of contingent operant self-medication in mice, which allows the estimation of spontaneous pain relief and drug abuse liability in models of persistent pain. This paradigm requires preliminary habituation and animal handling, followed by training of mice in operant conditioning boxes, to allow subsequent analgesic drug self-administration. After the initial acquisition of food-maintained operant behavior, a chronic pain sensitization is induced. Posterior intravenous jugular catheterization and coupling of operant conditioning boxes to perfusion pumps allow quantification of operant responding for intravenous drug self-administration. All mice show an initial operant drug self-administration behavior associated with the previous food-maintained operant training. This initial operant responding is extinguished after administration of ineffective treatments, but continues when the compounds have analgesic efficacy or intrinsic reinforcing properties. The identification of a significant drug self-administration selectively expressed in mice exposed to the chronic pain condition is indicative of analgesic drug effects, whereas persistent self-administration in control mice is indicative of abuse liability. The present protocol provides the behavioral and surgical procedures needed to assess spontaneous pain relief and potential for abuse of pharmacological treatments, through contingent analgesic self-medication in mice. Graphic abstract: Experimental design. Animals are subjected to a 5-day food self-administration protocol with a fixed ratio of reinforcement of 1 (FR1, 1 interaction with the active nose-poke causes the release of 1 reinforcer/infusion), to acquire the operant behavior. After this training, mice are subjected to the chronic pain or sham procedure, and four days later an intravenous (i.v.) catheterization is performed, to allow self-administration with the selected compound or its vehicle. Three days after the catheterization, animals start the drug/vehicle self-administration protocol at FR1. The patency of the catheter is evaluated with the thiopental test after the last self-administration session. Adapted from Bura et al. (2018).

In Vitro and In Vivo Pharmacological Characterization of a Novel TRPM8 Inhibitor Chemotype Identified by Small-Scale Preclinical Screening

Transient receptor potential melastatin type 8 (TRPM8) is a target for the treatment of different physio-pathological processes. While TRPM8 antagonists are reported as potential drugs for pain, cancer, and inflammation, to date only a limited number of chemotypes have been investigated and thus a limited number of compounds have reached clinical trials. Hence there is high value in searching for new TRPM8 antagonistic to broaden clues to structure-activity relationships, improve pharmacological properties and explore underlying molecular mechanisms. To address this, the EDASA Scientific in-house molecular library has been screened in silico, leading to identifying twenty-one potentially antagonist compounds of TRPM8. Calcium fluorometric assays were used to validate the in-silico hypothesis and assess compound selectivity. Four compounds were identified as selective TRPM8 antagonists, of which two were dual-acting TRPM8/TRPV1 modulators. The most potent TRPM8 antagonists (BB 0322703 and BB 0322720) underwent molecular modelling studies to highlight key structural features responsible for drug–protein interaction. The two compounds were also investigated by patch-clamp assays, confirming low micromolar potencies. The most potent compound (BB 0322703, IC₅₀ 1.25 ± 0.26 μM) was then profiled in vivo in a cold allodinya model, showing pharmacological efficacy at 30 μM dose. The new chemotypes identified showed remarkable pharmacological properties paving the way to further investigations for drug discovery and pharmacological purposes.

Delta Opioid Receptor in Astrocytes Contributes to Neuropathic Cold Pain and Analgesic Tolerance in Female Mice

Background: The delta opioid receptor (DOR) contributes to pain control, and a major challenge is the identification of DOR populations that control pain, analgesia, and tolerance. Astrocytes are known as important cells in the pathophysiology of chronic pain, and many studies report an increased prevalence of pain in women. However, the implication of astrocytic DOR in neuropathic pain and analgesia, as well as the influence of sex in this receptor activity, remains unknown. Experimental Approach: We developed a novel conditional knockout (cKO) mouse line wherein DOR is deleted in astrocytes (named GFAP-DOR-KO), and investigated neuropathic mechanical allodynia as well as analgesia and analgesic tolerance in mutant male and female mice. Neuropathic cold allodynia was also characterized in mice of both sexes lacking DOR either in astrocytes or constitutively. Results: Neuropathic mechanical allodynia was similar in GFAP-DOR-KO and floxed DOR control mice, and the DOR agonist SNC80 produced analgesia in mutant mice of both sexes. Interestingly, analgesic tolerance developed in cKO males and was abolished in cKO females. Cold neuropathic allodynia was reduced in mice with decreased DOR in astrocytes. By contrast, cold allodynia was exacerbated in full DOR KO females. Conclusions: These findings show that astrocytic DOR has a prominent role in promoting cold allodynia and analgesic tolerance in females, while overall DOR activity was protective. Altogether this suggests that endogenous- and exogenous-mediated DOR activity in astrocytes worsens neuropathic allodynia while DOR activity in other cells attenuates this form of pain. In conclusion, our results show a sex-specific implication of astrocytic DOR in neuropathic pain and analgesic tolerance. These findings open new avenues for developing tailored DOR-mediated analgesic strategies.

The CB2 cannabinoid receptor as a therapeutic target in the central nervous system

INTRODUCTION

Targeting CB2 cannabinoid receptor (CB2r) represents a promising approach for the treatment of central nervous system disorders. These receptors were identified in peripheral tissues, but also in neurons in the central nervous system. New findings have highlighted the interest to target these central receptors to obtain therapeutic effects devoid of the classical cannabinoid side-effects.

AREAS COVERED

In this review, we searched PubMed (January 1991-May 2021), ClinicalTrials.gov and Cochrane Library databases for articles, reviews and clinical trials. We first introduce the relevance of CB2r as a key component of the endocannabinoid system. We discuss CB2r interest as a possible novel target in the treatment of pain. This receptor has raised interest as a potential target for neurodegenerative disorders treatment, as we then discussed. Finally, we underline studies revealing a novel potential CB2r interest in mental disorders treatment.

EXPERT OPINION

In spite of the interest of targeting CB2r for pain, clinical trials evaluating CB2r agonist analgesic efficacy have currently failed. The preferential involvement of CB2r in preventing the development of chronic pain could influence the failure of clinical trials designed for the treatment of already established pain syndromes. Specific trials should be designed to target the prevention of chronic pain development.

Synergism between oral paracetamol and nefopam in a murine model of postoperative pain

BACKGROUND

The use of paracetamol or nefopam for postoperative pain control is limited by the need of high doses associated with unwanted effects. Previous works suggest positive interactions between both compounds that may be exploited to obtain potentiation of antinociception.

METHODS

Mechanical and heat antinociception induced by oral doses of paracetamol, nefopam or their combination was studied by isobolographic analysis in a murine model of postsurgical pain. The effective doses that produced 50% antinociception (ED₅₀ ) were calculated from the log dose-response curves for each compound. Subsequently, the effects of ED_8.7 s, ED_12.5 s, ED_17.5 s and ED₃₅ s of nefopam and paracetamol combined were assessed.

RESULTS

Oral paracetamol induced dose-dependent relief of postoperative sensitivity and showed higher efficacy reducing mechanical hypersensitivity (ED₅₀ 177.3 ± 15.4 mg/kg) than heat hyperalgesia (ED₅₀ 278.6 ± 43 mg/kg). Oral nefopam induced dose-dependent antinociception with similar efficacy for mechanical and heat hypersensitivity (ED₅₀ s 5.42 ± 0.81 vs. 5.83 ± 0.72). Combinations of increasing isoeffective doses revealed that combined ED_17.5 s (85.76 mg/kg paracetamol and 1.9 mg/kg nefopam) and ED₃₅ s (132.67 mg/kg and 3.73 mg/kg) showed synergistic effects leading to 75% and 90% mechanical antinociception, respectively. These mixtures were defined by interaction indexes of 0.43 and 0.41 and ratios 45:1 and 35:1 paracetamol:nefopam, respectively. The same combinations showed additive effects for the inhibition of incisional thermal hyperalgesia.

CONCLUSIONS AND LIMITATIONS

This work describes a synergistic antinociceptive interaction between low doses of nefopam and paracetamol for the treatment of postoperative hypersensitivity to peripheral stimuli. The promising results obtained on reflexive nociceptive responses of young male mice subjected to plantar surgery highlight the interest of further research evaluating the effects of this mixture on the affective-motivational component of pain and in females and additional age groups. Confirmation of pain-relieving efficacy and safety of this oral combination clinically available in European and Asian countries could provide a useful tool for postsurgical pain management.

SIGNIFICANCE

Early postoperative pain is currently undertreated and has been recognized as a relevant source of chronic postsurgical pain. Oral efficient treatments could facilitate fast-track surgeries and patient recovery at home. Here, we identify in a mouse model of postoperative pain a potent synergistic oral combination consisting of low paracetamol and nefopam doses that provides relief of postsurgical hypersensitivity to mechanical and thermal stimuli. Oral multimodal paracetamol-nefopam mixtures represent a potential clinically available pharmacological strategy for the relief of incisional sensitivity and the promotion of patient recovery.

Role of the endocannabinoid system in a mouse model of Fragile X undergoing neuropathic pain

BACKGROUND

Neuropathic pain is a complex condition characterized by sensory, cognitive and affective symptoms that magnify the perception of pain. The underlying pathogenic mechanisms are largely unknown and there is an urgent need for the development of novel medications. The endocannabinoid system modulates pain perception and drugs targeting the cannabinoid receptor type 2 (CB2) devoid of psychoactive side effects could emerge as novel analgesics. An interesting model to evaluate the mechanisms underlying resistance to pain is the fragile X mental retardation protein knockout mouse (Fmr1KO), a model of fragile X syndrome that exhibits nociceptive deficits and fails to develop neuropathic pain.

METHODS

A partial sciatic nerve ligation was performed to wild-type (WT) and Fmr1KO mice having (HzCB2 and Fmr1KO-HzCB2, respectively) or not (WT and Fmr1KO mice) a partial deletion of CB2 to investigate the participation of the endocannabinoid system on the pain-resistant phenotype of Fmr1KO mice.

RESULTS

Nerve injury induced canonical hypersensitivity in WT and HzCB2 mice, whereas this increased pain sensitivity was absent in Fmr1KO mice. Interestingly, Fmr1KO mice partially lacking CB2 lost this protection against neuropathic pain. Similarly, pain-induced depressive-like behaviour was observed in WT, HzCB2 and Fmr1KO-HzCB2 mice, but not in Fmr1KO littermates. Nerve injury evoked different alterations in WT and Fmr1KO mice at spinal and supra-spinal levels that correlated with these nociceptive and emotional alterations.

CONCLUSIONS

This work shows that CB2 is necessary for the protection against neuropathic pain observed in Fmr1KO mice, raising the interest in targeting this receptor for the treatment of neuropathic pain.

SIGNIFICANCE

Neuropathic pain is a complex chronic pain condition and current treatments are limited by the lack of efficacy and the incidence of important side effects. Our findings show that the pain-resistant phenotype of Fmr1KO mice against nociceptive and emotional manifestations triggered by persistent nerve damage requires the participation of the cannabinoid receptor CB2, raising the interest in targeting this receptor for neuropathic pain treatment. Additional multidisciplinary studies more closely related to human pain experience should be conducted to explore the potential use of cannabinoids as adequate analgesic tools.

Surgical Induction of Endometriosis in Female Mice

Endometriosis is a common gynecological disease characterized by the presence of endometrial tissue outside the uterine cavity. It is frequently associated with pain, infertility and a reduced quality of life, and it lacks adequate treatment. Several rodent models of endometriosis have been developed through heterologous and homologous transplantation of endometrial tissue into the abdominal compartment. Here we describe a surgical procedure to generate a syngeneic model of endometriosis in immunocompetent mice with intact uterine and ovarian tissues. In this model, four uterine fragments from a donor mouse at diestrus are sutured to the abdominal wall of a recipient mouse. One month after surgeries, endometrial implants develop into cysts with glandular epithelium and stroma, mimicking the endometriotic lesions observed in women with endometriosis. Therefore, this mouse model provides a valuable tool to study the pathophysiology of endometriosis and the efficacy of potential treatments.

The endocannabinoid system in modulating fear, anxiety, and stress

The endocannabinoid system is widely expressed in the limbic system, prefrontal cortical areas, and brain structures regulating neuroendocrine stress responses, which explains the key role of this system in the control of emotions. In this review, we update recent advances on the function of the endocannabinoid system in determining the value of fear-evoking stimuli and promoting appropriate behavioral responses for stress resilience. We also review the alterations in the activity of the endocannabinoid system during fear, stress, and anxiety, and the pathophysiological role of each component of this system in the control of these protective emotional responses that also trigger pathological emotional disorders. In spite of all the evidence, we have not yet taken advantage of the therapeutic implications of this important role of the endocannabinoid system, and possible future strategies to improve the treatment of these emotional disorders are discussed.


Protective role of neuronal and lymphoid cannabinoid CB2 receptors in neuropathic pain

Cannabinoid CB₂ receptor (CB₂) agonists are potential analgesics void of psychotropic effects. Peripheral immune cells, neurons and glia express CB₂; however, the involvement of CB₂ from these cells in neuropathic pain remains unresolved. We explored spontaneous neuropathic pain through on-demand self-administration of the selective CB₂ agonist JWH133 in wild-type and knockout mice lacking CB₂ in neurons, monocytes or constitutively. Operant self-administration reflected drug-taking to alleviate spontaneous pain, nociceptive and affective manifestations. While constitutive deletion of CB₂ disrupted JWH133-taking behavior, this behavior was not modified in monocyte-specific CB₂ knockouts and was increased in mice defective in neuronal CB₂ knockouts suggestive of increased spontaneous pain. Interestingly, CB₂-positive lymphocytes infiltrated the injured nerve and possible CB₂transfer from immune cells to neurons was found. Lymphocyte CB₂depletion also exacerbated JWH133 self-administration and inhibited antinociception. This work identifies a simultaneous activity of neuronal and lymphoid CB₂that protects against spontaneous and evoked neuropathic pain.

Mu and delta opioid receptors play opposite nociceptive and behavioural roles on nerve-injured mice

BACKGROUND AND PURPOSE

Mu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations.

EXPERIMENTAL APPROACH

We generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC).

KEY RESULTS

Constitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL.

CONCLUSIONS AND IMPLICATIONS

MOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.

Disease-modifying effects of natural Δ9-tetrahydrocannabinol in endometriosis-associated pain

Endometriosis is a chronic painful disease highly prevalent in women that is defined by growth of endometrial tissue outside the uterine cavity and lacks adequate treatment. Medical use of cannabis derivatives is a current hot topic and it is unknown whether phytocannabinoids may modify endometriosis symptoms and development. Here we evaluate the effects of repeated exposure to Δ9-tetrahydrocannabinol (THC) in a mouse model of surgically-induced endometriosis. In this model, female mice develop mechanical hypersensitivity in the caudal abdomen, mild anxiety-like behavior and substantial memory deficits associated with the presence of extrauterine endometrial cysts. Interestingly, daily treatments with THC (2 mg/kg) alleviate mechanical hypersensitivity and pain unpleasantness, modify uterine innervation and restore cognitive function without altering the anxiogenic phenotype. Strikingly, THC also inhibits the development of endometrial cysts. These data highlight the interest of scheduled clinical trials designed to investigate possible benefits of THC for women with endometriosis.

Endometriosis is a common disease in women caused by tissue that lines the uterus growing outside the uterine cavity on to other organs in the pelvis. This can cause a variety of symptoms including chronic pelvic pain, infertility, and pain during menstruation or sexual intercourse. These symptoms may contribute to anxiety, depression, loss of working ability and a reduced quality of life.

Currently available treatments for endometriosis, including hormonal therapy and surgery, have a limited effect and can produce unwanted side effects. For example, women who undergo surgery to remove the growths may experience post-surgical pain or a recurrence. As a result, women with endometriosis often rely on self-management strategies like dietary changes or exercise. Although cannabis consumption has a large number of potential side effects and can lead to substance abuse, it has been shown to provide pain relief in some conditions. But it is unknown whether it could be useful for treating endometriosis.

Now, Escudero-Lara et al. have created a mouse model that mimics some of the conditions of human endometriosis: pelvic pain, anxiety and memory impairments. The mice were treated with moderate doses of Δ9-tetrahydrocannabinol (THC), which is the main pain-relieving component of cannabis. The THC reduced pelvic pain and cognitive impairments in the mice with the endometriosis-like condition, but it had no effect on their anxious behavior. Escudero-Lara et al. also noticed that endometrial growths were also smaller in the treated mice indicating that THC may also inhibit endometriosis development.

These experiments suggest that THC may be a useful treatment for patients with endometriosis. Clinical trials are already ongoing to test whether these findings translate to patients with the condition. Although THC and cannabis are readily available in some areas, Escudero-Lara et al. discourage using unregulated cannabis products due to the potential risks.

Disease-modifying effects of natural Δ9-tetrahydrocannabinol in endometriosis-associated pain

Endometriosis is a chronic painful disease highly prevalent in women that is defined by growth of endometrial tissue outside the uterine cavity and lacks adequate treatment. Medical use of cannabis derivatives is a current hot topic and it is unknown whether phytocannabinoids may modify endometriosis symptoms and development. Here we evaluate the effects of repeated exposure to Δ9-tetrahydrocannabinol (THC) in a mouse model of surgically-induced endometriosis. In this model, female mice develop mechanical hypersensitivity in the caudal abdomen, mild anxiety-like behavior and substantial memory deficits associated with the presence of extrauterine endometrial cysts. Interestingly, daily treatments with THC (2 mg/kg) alleviate mechanical hypersensitivity and pain unpleasantness, modify uterine innervation and restore cognitive function without altering the anxiogenic phenotype. Strikingly, THC also inhibits the development of endometrial cysts. These data highlight the interest of scheduled clinical trials designed to investigate possible benefits of THC for women with endometriosis.

Sigma-1 receptor modulates neuroinflammation associated with mechanical hypersensitivity and opioid tolerance in a mouse model of osteoarthritis pain

BACKGROUND AND PURPOSE

Osteoarthritic pain is a chronic disabling condition lacking effective treatment. Continuous use of opioid drugs during osteoarthritic pain induces tolerance and may result in dose escalation and abuse. Sigma-1 (σ1) receptors, a chaperone expressed in key areas for pain control, modulates μ-opioid receptor activity and represents a promising target to tackle these problems. The present study investigates the efficacy of the σ1 receptor antagonist E-52862 to inhibit pain sensitization, morphine tolerance, and associated electrophysiological and molecular changes in a murine model of osteoarthritic pain.

EXPERIMENTAL APPROACH

Mice received an intra-knee injection of monoiodoacetate followed by 14-day treatment with E-52862, morphine, or vehicle, and mechanical sensitivity was assessed before and after the daily doses.

KEY RESULTS

Monoiodoacetate-injected mice developed persistent mechanical hypersensitivity, which was dose-dependently inhibited by E-52862. Mechanical thresholds assessed before the daily E-52862 dose showed gradual recovery, reaching complete restoration by the end of the treatment. When repeated treatment started 15 days after knee injury, E-52862 produced enhanced short-term analgesia, but recovery to baseline threshold was slower. Both a σ1 receptor agonist and a μ receptor antagonist blocked the analgesic effects of E-52862. An acute, sub-effective dose of E-52862 restored morphine analgesia in opioid-tolerant mice. Moreover, E-52862 abolished spinal sensitization in osteoarthritic mice and inhibited pain-related molecular changes.

CONCLUSION AND IMPLICATIONS

These findings show dual effects of σ1 receptor antagonism alleviating both short- and long-lasting antinociception during chronic osteoarthritis pain. They identify E-52862 as a promising pharmacological agent to treat chronic pain and avoid opioid tolerance.

Blockade of the Sigma-1 Receptor Relieves Cognitive and Emotional Impairments Associated to Chronic Osteoarthritis Pain

Osteoarthritis is the most common musculoskeletal disease worldwide, often characterized by degradation of the articular cartilage, chronic joint pain and disability. Cognitive dysfunction, anxiety and depression are common comorbidities that impact the quality of life of these patients. In this study, we evaluated the involvement of sigma-1 receptor (σ1R) on the nociceptive, cognitive and emotional alterations associated with chronic osteoarthritis pain. Monosodium iodoacetate (MIA) was injected into the knee of Swiss-albino CD1 mice to induce osteoarthritis pain, which then received a repeated treatment with the σ1R antagonist E-52862 or its vehicle. Nociceptive responses and motor performance were assessed with the von Frey and the Catwalk gait tests. Cognitive alterations were evaluated using the novel object recognition task, anxiety-like behavior with the elevated plus maze and the zero-maze tests, whereas depressive-like responses were determined using the forced swimming test. We also studied the local effect of the σ1R antagonist on cartilage degradation, and its central effects on microglial reactivity in the medial prefrontal cortex. MIA induced mechanical allodynia and gait abnormalities that were prevented by the chronic treatment with the σ1R antagonist. E-52862 also reduced the memory impairment and the depressive-like behavior associated to osteoarthritis pain. Interestingly, the effect of E-52862 on depressive-like behavior was not accompanied by a modification of anxiety-like behavior. The pain-relieving effects of the σ1R antagonist were not due to a local effect on the articular cartilage, since E-52862 treatment did not modify the histological alterations of the knee joints. However, E-52862 induced central effects revealed by a reduction of the cortical microgliosis observed in mice with osteoarthritis pain. These findings show that σ1R antagonism inhibits mechanical hypersensitivity, cognitive deficits and depressive-like states associated with osteoarthritis pain in mice. These effects are associated with central modulation of glial activity but are unrelated to changes in cartilage degradation. Therefore, targeting the σ1R with E-52862 represents a promising pharmacological approach with effects on multiple aspects of chronic osteoarthritis pain that may go beyond the strict inhibition of nociception.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Opioid-induced hyperalgesia in a murine model of postoperative pain: role of nitric oxide generated from the inducible nitric oxide synthase

BACKGROUND

Opioid-induced delayed hyperalgesia and allodynia have been reported in human and animal models. The authors evaluated the influence of different opioids used during clinical anesthesia on nociceptive sensitivity and incisional pain in mice. The role of the inducible nitric oxide synthase on surgical pain and opioid-induced pronociception also was investigated.

METHODS

CD1 mice were used to study the efficacy of opioids inducing pronociception and enhancing incisional pain. The implication of nitric oxide generated from the inducible nitric oxide synthase was investigated using knockout mice (C57/BL6) for its gene. Mice underwent right hind paw surgery under sevoflurane anesthesia combined with subcutaneous administration of saline or the opioids fentanyl (0.05 mg/kg), alfentanil (1 mg/kg), and remifentanil (0.04 mg/kg). Nociception was evaluated daily for 7 days using paw-pressure, plantar, and von Frey tests.

RESULTS

The antinociceptive effect of opioids was followed by long-lasting thermal hyperalgesia and mechanical allodynia (each lasting between 2 and 7 days), but not mechanical hyperalgesia. Intraoperative infusion of opioids significantly enhanced incisional pain in all tests. The most prominent effects were observed with remifentanil. The inducible nitric oxide synthase gene deletion attenuated both remifentanil- and incision-induced pronociceptive effects. In mutant mice for the inducible nitric oxide synthase gene, remifentanil was still efficient in enhancing incisional pain, but the global pronociceptive effect was attenuated significantly as compared with wild-type mice.

CONCLUSIONS

The authors' study demonstrates that the intraoperative administration of fentanyl or remifentanil enhances the extent and duration of postoperative pain. The results suggest a role of the nitric oxide systems in the cause of acute postoperative pain and opioid-induced pronociception.