Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

Role of noradrenergic and dopaminergic systems in the antinociceptive effect of N-(3-(phenylselanyl)prop-2-yn-1-yl)benzamide in mice

Pain has a negative impact on public health, reducing quality of life. Unfortunately, current treatments are not fully effective and have adverse effects. Therefore, there is a need to develop new analgesic compounds. Due to promising results regarding the antinociceptive effect of N-(3-(phenylselanyl)prop-2-in-1-yl)benzamide (SePB), this study aimed to evaluate the participation of the dopaminergic and noradrenergic systems in this effect in mice, as well as its toxicity. To this, the antagonists sulpiride (D2/D3 receptor antagonist, 5 mg/kg), SCH-23390 (D1 receptor antagonist, 0.05 mg/kg), prazosin (α1 adrenergic receptor antagonist, 0.15 mg/kg), yohimbine (α2-adrenergic receptors, 0.15 mg/kg) and propranolol (non-selective β-adrenergic antagonist, 10 mg/kg) were administered intraperitoneally to mice 15 min before SePB (10 mg/kg, intragastrically), except for propranolol (20 min). After 26 min of SePB administration, the open field test was performed for 4 min to assess locomotor activity, followed by the tail immersion test to measure the nociceptive response. For the toxicity test, animals received a high dose of 300 mg/kg of SePB. SePB showed an increase in the latency for nociceptive response in the tail immersion test, and this effect was prevented by SCH-23390, yohimbine and propranolol, indicating the involvement of D1, α2 and β-adrenergic receptors in the antinociceptive mechanism of the SePB effect. No changes were observed in the open field test, and the toxicity assessment suggested that SePB has low potential to induce toxicity. These findings contribute to understanding SePB's mechanism of action, with a focus on the development of new alternatives for pain treatment.

Topiramate affords neuroprotection in diabetic neuropathy model via downregulating spinal GFAP/inflammatory burden and improving neurofilament production

The current study aimed to test the neuroprotective action of topiramate in mouse peripheral diabetic neuropathy (DN) and explored some mechanisms underlying this action. Mice were assigned as vehicle group, DN group, DN + topiramate 10-mg/kg and DN + topiramate 30-mg/kg. Mice were tested for allodynia and hyperalgesia and then spinal cord and sciatic nerves specimens were examined microscopically and neurofilament heavy chain (NEFH) immunostaining was performed. Results indicated that DN mice had lower the hotplate latency time (0.46-fold of latency to licking) and lower von-Frey test pain threshold (0.6-fold of filament size) while treatment with topiramate increased these values significantly. Sciatic nerves from DN control mice showed axonal degeneration while spinal cords showed elevated GFAP (5.6-fold) and inflammatory cytokines (∼3- to 4-fold) but lower plasticity as indicated by GAP-43 (0.25-fold). Topiramate produced neuroprotection and suppressed spinal cord GFAP/inflammation but enhanced GAP-43. This study reinforces topiramate as neuroprotection and explained some mechanisms included in alleviating neuropathy.

Neuropathic Pain and Spinal Cord Injury: Management, Phenotypes, and Biomarkers

Chronic neuropathic pain after a spinal cord injury (SCI) continues to be a complex condition that is difficult to manage due to multiple underlying pathophysiological mechanisms and the association with psychosocial factors. Determining the individual contribution of each of these factors is currently not a realistic goal; however, focusing on the primary mechanisms may be more feasible. One approach used to uncover underlying mechanisms includes phenotyping using pain symptoms and somatosensory function. However, this approach does not consider cognitive and psychosocial mechanisms that may also significantly contribute to the pain experience and impact treatment outcomes. Indeed, clinical experience supports that a combination of self-management, non-pharmacological, and pharmacological approaches is needed to optimally manage pain in this population. This article will provide a broad updated summary integrating the clinical aspects of SCI-related neuropathic pain, potential pain mechanisms, evidence-based treatment recommendations, neuropathic pain phenotypes and brain biomarkers, psychosocial factors, and progress regarding how defining neuropathic pain phenotypes and other surrogate measures in the neuropathic pain field may lead to targeted treatments for neuropathic pain after SCI.

Mirabegron is better tolerated than solifenacin in Sjogren's syndrome patients with overactive bladder symptoms-A randomized controlled trial

OBJECTIVES

This study investigates the efficacy and adverse events of beta-3 agonists and antimuscarinic agents for managing overactive bladder syndrome in Sjogren syndrome.

METHODS

Sjogren's syndrome patients with an Overactive Bladder Symptom Score (OABSS) >5 were enrolled and were randomly assigned to mirabegron 50 mg/day or solifenacin 5 mg/day. Patients were evaluated on the recruitment day and reassessed at Week 1, 2, 4, and 12. The study's primary endpoint was to have a significant change in OABSS at Week 12. The secondary endpoint was the adverse event and crossover rate.

RESULTS

A total of 41 patients were included in the final analysis, with 24 in the mirabegron group and 17 in the solifenacin group. The study's primary outcome was a change of the OABSS at Week 12. We found that both mirabegron and solifenacin significantly reduce patients' OABSS after 12 weeks of treatment. The evolution of the OABSS was -3.08 for mirabegron and -3.71 for solifenacin (p = .56). Six out of 17 patients from the solifenacin group crossed over to the mirabegron arm due to severe dry mouth or constipation, while none from the mirabegron arm crossed over to the solifenacin group. Sjogren's syndrome-related pain was also improved in the mirabegron group (4.96-1.67, p = .008) compared to the solifenacin group (4.39-3.4, p = .49).

CONCLUSIONS

Our study showed that mirabegron is equally effective as solifenacin in treating Sjogren's syndrome patients with overactive bladder. Mirabegron is superior to solifenacin in terms of treatment-related adverse events.

Baicalein exerts anxiolytic and antinociceptive effects in a mouse model of posttraumatic stress disorder: Involvement of the serotonergic system and spinal delta-opioid receptors

Post-traumatic stress disorder (PTSD) is a serious mental disease featured by a stress dysfunction that occurs after an individual has faced intense mental stress, often accompanied by anxiety and chronic pain. Currently, the mainstream drug for PTSD is serotonin reuptake inhibitors (SSRIs), however, their pain management for patients is limited. Baicalein, a Chinese traditional herbal medicine, has shown promising results in treating anxiety, depression, and pain. In this study, we found that baicalein may alleviate single prolonged stress (SPS)-induced PTSD-like behaviors in mice without altering baseline nociceptive sensitivity or activity. Meanwhile, baicalein increased the noradrenaline (NE) and serotonin (5-HT) content and decreased the ratio of 5-hydroxyindoleacetic acid (5-HIAA)/5-HT by inhibiting the activity of monoamine oxidase A (MAO-A) in SPS-induce mice. The anxiolytic and antinociceptive effects induced by baicalein were totally abolished by 5-HT depleting agents. Moreover, the anxiolytic effects of baicalein could be abolished by the 5-HT_1A receptor antagonist WAY-100635, and the analgesic effects could be abolished by delta-opioid receptor antagonists in the spinal. Taken together, our study provides compelling evidence that baicalein reversed anxiety-like behaviors and neuropathic pain in PTSD through serotonergic system and spinal delta-opioid receptors.

Precision Medicine in Antidepressants Treatment

Precision medicine uses innovative approaches to improve disease prevention and treatment outcomes by taking into account people's genetic backgrounds, environments, and lifestyles. Treatment of depression is particularly challenging, given that 30-50% of patients do not respond adequately to antidepressants, while those who respond may experience unpleasant adverse drug reactions (ADRs) that decrease their quality of life and compliance. This chapter aims to present the available scientific data that focus on the impact of genetic variants on the efficacy and toxicity of antidepressants. We compiled data from candidate gene and genome-wide association studies that investigated associations between pharmacodynamic and pharmacokinetic genes and response to antidepressants regarding symptom improvement and ADRs. We also summarized the existing pharmacogenetic-based treatment guidelines for antidepressants, used to guide the selection of the right antidepressant and its dose based on the patient's genetic profile, aiming to achieve maximum efficacy and minimum toxicity. Finally, we reviewed the clinical implementation of pharmacogenomics studies focusing on patients on antidepressants. The available data demonstrate that precision medicine can increase the efficacy of antidepressants and reduce the occurrence of ADRs and ultimately improve patients' quality of life.

Alpha-Lipoic Acid as an Antioxidant Strategy for Managing Neuropathic Pain

Neuropathic pain (NP) is the most prevalent and debilitating form of chronic pain, caused by injuries or diseases of the somatosensory system. Since current first-line treatments only provide poor symptomatic relief, the search for new therapeutic strategies for managing NP is an active field of investigation. Multiple mechanisms contribute to the genesis and maintenance of NP, including damage caused by oxidative stress. The naturally occurring antioxidant alpha-lipoic acid (ALA) is a promising therapeutic agent for the management of NP. Several pre-clinical in vitro and in vivo studies as well as clinical trials demonstrate the analgesic potential of ALA in the management of NP. The beneficial biological activities of ALA are reflected in the various patents for the development of ALA-based innovative products. This review demonstrates the therapeutic potential of ALA in the management of NP by discussing its analgesic effects by multiple antioxidant mechanisms as well as the use of patented ALA-based products and how technological approaches have been applied to enhance ALA's pharmacological properties.

Kratom alkaloid mitragynine: Inhibition of chemotherapy-induced peripheral neuropathy in mice is dependent on sex and active adrenergic and opioid receptors

Mitragynine (MG) is an alkaloid found in Mitragyna speciosa (kratom) that is used as an herbal remedy for pain relief and opioid withdrawal. MG acts at μ-opioid and α-adrenergic receptors in vitro, but the physiological relevance of this activity in the context of neuropathic pain remains unknown. The purpose of the present study was to characterize the effects of MG in a mouse model of chemotherapy-induced peripheral neuropathy (CIPN), and to investigate the potential impact of sex on MG's therapeutic efficacy. Inhibition of oxaliplatin-induced mechanical hypersensitivity was measured following intraperitoneal administration of MG. Both male and female C57BL/6J mice were used to characterize potential sex-differences in MG's therapeutic efficacy. Pharmacological mechanisms of MG were characterized through pretreatment with the opioid and adrenergic antagonists naltrexone, prazosin, yohimbine, and propranolol (1, 2.5, 5 mg/kg). Oxaliplatin produced significant mechanical allodynia of equal magnitude in both male and females, which was dose-dependently attenuated by repeated MG exposure. MG was more potent in males vs females, and the highest dose of MG (10 mg/kg) exhibited greater anti-allodynic efficacy in males. Mechanistically, activity at µ-opioid, α₁- and α₂-adrenergic receptors, but not β-adrenergic receptors contributed to the effects of MG against oxaliplatin-induced mechanical hypersensitivity. Repeated MG exposure significantly attenuated oxaliplatin-induced mechanical hypersensitivity with greater potency and efficacy in males, which has crucial implications in the context of individualized pain management. The opioid and adrenergic components of MG indicate that it shares pharmacological properties with clinical neuropathic pain treatments.

Peripheral neuropathy: A neglected cause of disability in COPD - A narrative review

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory syndrome with systemic involvement leading to various cardiovascular, metabolic, and neurological comorbidities. It is well known that conditions associated with oxygen deprivation and metabolic disturbance are associated with polyneuropathy, but current data regarding the relationship between COPD and peripheral nervous system pathology is limited. This review summarizes the available data on the association between COPD and polyneuropathy, including possible pathophysiological mechanisms such as the role of hypoxia, proinflammatory state, and smoking in nerve damage; the role of cardiovascular and metabolic comorbidities, as well as the diagnostic methods and screening tools for identifying polyneuropathy. Furthermore, it outlines the available options for managing and preventing polyneuropathy in COPD patients. Overall, current data suggest that optimal screening strategies to diagnose polyneuropathy early should be implemented in COPD patients due to their relatively common association and the additional burden of polyneuropathy on quality of life.

Title: Novel Analgesic Potential of ß2-Agonists for Neuropathic Pain via ß2-Agonist Action

PURPOSE OF REVIEW

Multimodal therapies are often employed to treat chronic pain, and ß2-agonists are a potential drug class that shows promise. The primary aim of this paper is to discuss the role of ß2-agonists as an adjunctive therapy for chronic pain based on the current literature.

RECENT FINDINGS

Recent studies in mouse models have shown that the ß2-adrenergic system plays an essential role in the analgesic properties of antidepressant drugs used to treat neuropathic pain and that the adrenergic relies on an intact endogenous opioid system to be effective. Studies also show that ß2-agonism alone is adequate to exert anti-allodynic effects in a mouse model. This paper summarized the basic physiology and pharmacology of the sympathetic nervous system and specifically the ß2-adrenergic system and summarized current literature in its involvement in the treatment of chronic neuropathic pain.

Antiallodynic action of phosphodiesterase inhibitors in a mouse model of peripheral nerve injury

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory nervous system. It is accompanied by neuronal and non-neuronal alterations, including alterations in intracellular second messenger pathways. Cellular levels of 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) are regulated by phosphodiesterase (PDE) enzymes. Here, we studied the impact of PDE inhibitors (PDEi) in a mouse model of peripheral nerve injury induced by placing a cuff around the main branch of the sciatic nerve. Mechanical hypersensitivity, evaluated using von Frey filaments, was relieved by sustained treatment with the non-selective PDEi theophylline and ibudilast (AV-411), with PDE4i rolipram, etazolate and YM-976, and with PDE5i sildenafil, zaprinast and MY-5445, but not by treatments with PDE1i vinpocetine, PDE2i EHNA or PDE3i milrinone. Using pharmacological and knock-out approaches, we show a preferential implication of delta opioid receptors in the action of the PDE4i rolipram and of both mu and delta opioid receptors in the action of the PDE5i sildenafil. Calcium imaging highlighted a preferential action of rolipram on dorsal root ganglia non-neuronal cells, through PDE4B and PDE4D inhibition. Rolipram had anti-neuroimmune action, as shown by its impact on levels of the pro-inflammatory cytokine tumor necrosis factor-α (TNFα) in the dorsal root ganglia of mice with peripheral nerve injury, as well as in human peripheral blood mononuclear cells (PBMCs) stimulated with lipopolysaccharides. This study suggests that PDEs, especially PDE4 and 5, may be targets of interest in the treatment of neuropathic pain.

β2-adrenoreceptor agonist ameliorates mechanical allodynia in paclitaxel-induced neuropathic pain via induction of mitochondrial biogenesis

Chemotherapy-induced neuropathic pain is a debilitating and common side effect of cancer treatment and so far no effective drug is available for treatment of the serious side effect. Previous studies have demonstrated β2-adrenoreceptor (ADRB2) agonists can attenuate neuropathic pain. However, the role of ADRB2 in paclitaxel -induced neuropathic pain (PINP) remains unclear. In this study, we investigated the effect of formoterol, a long-acting ADRB2 agonist, and related mechanisms in PINP. A rat model of PINP was established by intraperitoneal injection of paclitaxel (2 mg/kg) every other day with a final cumulative dose of 8 mg/kg. Hind paw withdrawal thresholds (PWTs) in response to von Frey filament stimuli were used to evaluate mechanical allodynia. Western blot was used to examine the expression of ADRB2, peroxisome proliferator-activated receptor coactivator-1α (PGC-1α), nuclear respiratory factors 1 (NRF1) and mitochondrial transcription factor A (TFAM) and the immunofluorescence was to detect the cellular localization of ADRB2 and PGC-1α in the spinal cord. Moreover, we measured mitochondrial DNA (mtDNA) copy number by qPCR. In our study, formoterol attenuated established PINP and delayed the onset of PINP. Formoterol restored ADRB2 expression as well as mtDNA copy number and PGC-1α, NRF1, and TFAM protein expression, which are major genes involved in mitochondrial biogenesis, in the spinal cord of PINP rats. Moreover, we found the analgesic effect of formoterol against PINP was partially abolished by PGC-1α inhibitor SR-18292. Collectively, these results demonstrated the activation of ADRB2 with formoterol ameliorates PINP at least partially through induction of mitochondrial biogenesis.

β2-and β3-Adrenergic Receptors Contribute to Cancer-Evoked Pain in a Mouse Model of Osteosarcoma via Modulation of Neural Macrophages

The mechanisms involved in the development and maintenance of cancer pain remain largely unidentified. Recently, it has been reported that β-adrenergic receptors (β-ARs), mainly β2-and β3-ARs, contribute to tumor proliferation and progression and may favor cancer-associated pain and neuroinflammation. However, the mechanism underlying β-ARs in cancer pain is still unknown. Here, we investigated the role of β1-, β2-and β3-ARs in a mouse model of cancer pain generated by the para-tibial injection of K7M2 osteosarcoma cells. Results showed a rapid tumor growth in the soft tissue associated with the development of mechanical allodynia in the hind paw ipsilateral to the injected site. In addition to reduce tumor growth, both propranolol and SR59230A, β1-/β2-and β3-AR antagonists, respectively, attenuated mechanical allodynia, the number of macrophages and an oxidative stress by-product accumulated in the ipsilateral tibial nerve. The selective β1-AR antagonist atenolol was able to slightly reduce the tumor growth but showed no effect in reducing the development of mechanical allodynia. Results suggest that the development of the mechanical allodynia in K7M2 osteosarcoma-bearing mice is mediated by oxidative stress associated with the recruitment of neural macrophages, and that antagonism of β2-and β3-ARs contribute not solely to the reduction of tumor growth, but also in cancer pain. Thus, the targeting of the β2-and β3-ARs signaling may be a promising therapeutic strategy against both tumor progression and the development of cancer-evoke pain in osteosarcoma.

A randomized, controlled trial of a β2-agonist in painful polyneuropathy

ABSTRACT

Experimental data have suggested that in neuropathic pain, tricyclic antidepressants may work solely through a β2-agonist action. The aim of this study was to test if the β2-agonist terbutaline relieves painful polyneuropathy. The study was a randomized, double-blind, placebo-controlled and active-controlled, 3-way, cross-over trial among patients with painful polyneuropathy. The treatment periods were of 5 weeks' duration and were preceded by 1 week for washout and 1 week for baseline observations. The patients received terbutaline (5-15 mg), imipramine (30-150 mg), or placebo in a random order. Drug doses depended on age and metabolizer status. The change in total pain recorded from ratings in diaries (numeric rating scale [NRS] 0-10) was the primary outcome, and the change in rating of specific pain symptoms (NRS 0-10), patient global impression of change, and sleep disturbance were secondary outcomes. Forty-seven patients were randomized. The median score for total pain changed from NRS 6.4 to 6.1 from baseline to week 5 on terbutaline with an average effect during the treatment period as compared with placebo of 0.13 (95% confidence interval -0.12 to 0.38, P = 0.32). The median score for total pain on imipramine changed from NRS 6.6 to 4.8 with an average effect as compared with placebo of -1.17 (95% confidence interval -1.42 to -0.92, P < 0.001). Secondary outcomes were also unaltered by terbutaline but improved by imipramine. The β2-agonist terbutaline has no effect in painful polyneuropathy. β2-agonism seems not to be an important mechanism of action of tricyclic antidepressants in neuropathic pain.

Novel Mechanism for Memantine in Attenuating Diabetic Neuropathic Pain in Mice via Downregulating the Spinal HMGB1/TRL4/NF-kB Inflammatory Axis

Diabetic neuropathic pain (DNP) is a common diabetic complication that currently lacks an efficient therapy. The aim of the current work was to uncover the anti-allodynic and neuroprotective effects of memantine in a model of mouse diabetic neuropathy and its ameliorative effect on the high-mobility group box-1 (HMGB1)/toll-like receptor 4 (TLR4)/nuclear factor-k B (NF-kB) inflammatory axis. Diabetes was prompted by an alloxan injection (180 mg/kg) to albino mice. On the ninth week after diabetes induction, DNP was confirmed. Diabetic mice were randomly allocated to two groups (six mice each); a diabetes mellitus (DM) group and DM+memantine group (10 mg/kg, daily) for five weeks. DNP-related behaviors were assessed in terms of thermal hyperalgesia and mechanical allodynia by hot-plate and von Frey filaments. Enzyme-linked immunosorbent assay (ELISA) kits were used to measure the spinal glutamate, interleukin-1 beta (IL-1β), and tumor necrosis factor-α (TNF-α). The spinal levels of N-methyl-D-aspartate type 1 receptor (NMDAR1), HMGB1, TLR4, and phosphorylated NF-kB were assessed using Western blotting. Histopathological investigation of the spinal cord and sciatic nerves, together with the spinal cord ultrastructure, was employed for assessment of the neuroprotective effect. Memantine alleviated pain indicators in diabetic mice and suppressed excessive NMDAR1 activation, glutamate, and pro-inflammatory cytokine release in the spinal cord. The current study validated the ability of memantine to combat the HMGB1/TLR4/NF-kB axis and modulate overactive glutamate spinal transmission, corroborating memantine as an appealing therapeutic target in DNP.

trans-Resveratrol ameliorates anxiety-like behaviors and neuropathic pain in mouse model of post-traumatic stress disorder

BACKGROUND

trans-Resveratrol has been extensively investigated for its anti-inflammatory, antioxidant, and anti-psychiatric properties. However, whether it could rescue posttraumatic stress disorder-like stress-induced pain abnormality is unknown.

AIM

The present study examined the effects of trans-resveratrol on anxiety-like behavior and neuropathic pain induced by single-prolonged stress, which is a classical animal model for mimicking posttraumatic stress disorder.

METHODS

The single-prolonged stress-induced anxiety-like behavior and pain response were detected by the novelty suppressed feeding, marble burying, locomotor activity, von Frey, and acetone-induced cold allodynia tests in mice. The serum corticosterone levels and glucocorticoid receptor, protein kinase A, phosphorylated cAMP response element binding protein, and brain-derived neurotrophic factor expression were detected by enzyme-linked immunosorbent assay and immunoblot analyses.

RESULTS

trans-Resveratrol reversed single-prolonged stress-induced increased latency to feed and the number of marbles buried in the novelty suppressed feeding and marble burying tests, but did not significantly influence locomotion distance in the locomotor activity test. trans-Resveratrol also reversed single-prolonged stress-induced cold and mechanical allodynia. Moreover, single-prolonged stress induced abnormality in the limbic hypothalamus-pituitary-adrenal axis was reversed by trans-resveratrol, as evidenced by the fact that trans-resveratrol reversed the differential expression of glucocorticoid receptor in the anxiety- and pain-related regions. In addition, trans-resveratrol increased protein kinase A, phosphorylated cAMP response element binding protein, and brain-derived neurotrophic factor levels, which were decreased in mice subjected to single-prolonged stress.

CONCLUSIONS

These results provide compelling evidence that trans-resveratrol protects neurons against posttraumatic stress disorder-like stress insults through regulation of limbic hypothalamus-pituitary-adrenal axis function and activation of downstream neuroprotective molecules such as protein kinase A, phosphorylated cAMP response element binding protein, and brain-derived neurotrophic factor expression.

HDAC6-selective inhibitors decrease nerve-injury and inflammation-associated mechanical hypersensitivity in mice

BACKGROUND

HDAC6 is a class IIB histone deacetylase expressed at many levels of the nociceptive pathway. This study tested the ability of novel and selective HDAC6 inhibitors to alleviate sensory hypersensitivity behaviors in mouse models of peripheral nerve injury and peripheral inflammation.

METHODS

We utilized the murine spared nerve injury (SNI) model for peripheral nerve injury and the Complete Freund's Adjuvant (CFA) model of peripheral inflammation. We applied the Von Frey assay to monitor mechanical allodynia.

RESULTS

Using the SNI model, we demonstrate that daily administration of the brain-penetrant HDAC6 inhibitor, ACY-738, abolishes mechanical allodynia in male and in female mice. Importantly, there is no tolerance to the antiallodynic actions of these compounds as they produce a consistent increase in Von Frey thresholds for several weeks. We observed a similar antiallodynic effect when utilizing the HDAC6 inhibitor, ACY-257, which shows limited brain expression when administered systemically. We also demonstrate that ACY-738 and ACY-257 attenuate mechanical allodynia in the CFA model of peripheral inflammation.

CONCLUSIONS

Overall, our findings suggest that inhibition of HDAC6 provides a promising therapeutic avenue for the alleviation of mechanical allodynia associated with peripheral nerve injury and peripheral inflammation.

Postherpetic Neuralgia: Current Evidence on the Topical Film-Forming Spray with Bupivacaine Hydrochloride and a Review of Available Treatment Strategies

PURPOSE OF REVIEW

This is a comprehensive review of the literature about the use of bupivacaine hydrochloride for the treatment of post-herpetic neuralgia (PHN). It briefly reviews the background, biology, diagnosis and conventional treatment for PHN, and then introduces and compares the recent evidence for the use of topical bupivacaine.

RECENT FINDINGS

PHN is defined by pain lasting 90 days or more after the initial presentation of herpes zoster ("Shingles", HZ) rash and is the most common complication of this disease. A product of re-activation of the Varicella-Zoster virus (VZV), HZ is diagnosed more than 1 million times annually in the United States. Approximately 20% of patients with HZ will experience PHN and will continue to suffer intermittent neuropathic symptoms, including itching and pain, that is sharp, stabbing, throbbing or burning, with the pain localized to the site of their original rash. This long-lasting pain compares with the severity of long-standing rheumatics and osteo-arthritis and is accompanied by severe allodynia causing significant suffering, and a financial burden that is manifested in both healthcare costs and loss of quality-adjusted life years. Prevention of PHN may be achieved with the Zoster vaccine, although there is still a large segment of unvaccinated population. Moreover, the Zoster vaccine is not always effective for prevention. Current treatment includes medical (systemic tricyclic antidepressants, anticonvulsants and opioids, topical lidocaine and capsaicin) and interventional (subcutaneous Botox injections, nerve blocks and nerve stimulation) therapies. These therapies are not always effective, and each carries their own profile of side effects and risks. Moreover, up to 50% of patients with PHN are refractory to management. Recent evidence is emerging to support the use of topical local anesthetics for the treatment of PHN. Two small studies recently found topical lidocaine spray to be effective in treating paroxysmal pain attacks associated with PHN. Bupivacaine is a longer-lasting local anesthetic, and a film-forming formulation allows easy and durable application to the affected skin. Recent studies show that topical film-forming bupivacaine is safe and as effective as lidocaine for the treatment of PHN. PHN is an important though common complication of HZ and can cause long-lasting pain and disability. Current treatment for PNH is limited by efficacy and safety profiles of individual therapies. Recent evidence points to topical local anesthetics as an effective and safe alternative to conventional therapy. Film-forming bupivacaine may offer a durable and safe option for this otherwise difficult to treat syndrome.

Catecholaminergic and opioidergic system mediated effects of reboxetine on diabetic neuropathic pain

RATIONALE

Current data indicate that the noradrenergic system plays a critical role in neuropathic pain treatment. Notably, drugs that directly affect this system may have curative potential in neuropathy-associated pain.

OBJECTIVES

The aim of this study was to evaluate the potential therapeutic efficacy of reboxetine, a potent and selective noradrenaline reuptake inhibitor, on hyperalgesia and allodynia responses in rats with experimental diabetes. Furthermore, mechanistic studies were performed to elucidate the possible mode of actions.

METHODS

Experimental diabetes was induced by a single dose of streptozotocin. Mechanical hyperalgesia, mechanical allodynia, thermal hyperalgesia, and thermal allodynia responses in diabetic rats were evaluated by Randall-Selitto, dynamic plantar, Hargreaves, and warm plate tests, respectively.

RESULTS

Reboxetine treatment (8 and 16 mg/kg for 2 weeks) demonstrated an effect comparable to that of the reference drug, pregabalin, improving the hyperalgesic and allodynic responses secondary to diabetes mellitus. Pretreatment with phentolamine, metoprolol, SR 59230A, and atropine did not alter the abovementioned effects of reboxetine; however, the administration of α-methyl-para-tyrosine methyl ester, propranolol, ICI-118,551, SCH-23390, sulpiride, and naltrindole significantly inhibited these effects. Moreover, reboxetine did not induce a significant difference in the rat plasma glucose levels.

CONCLUSIONS

Our findings indicate that the antihyperalgesic and antiallodynic effects of reboxetine are mediated by the catecholaminergic system; β₂-adrenoceptors; D₁-, D₂/D₃-dopaminergic receptors; and δ-opioid receptors. The results suggest that this analgesic effect of reboxetine, besides its neutral profile on glycemic control, may be advantageous in the pharmacotherapy of diabetic neuropathy-induced pain.

Delta opioid receptors are essential to the antiallodynic action of Β2-mimetics in a model of neuropathic pain

The adrenergic system, because of its reported implication in pain mechanisms, may be a potential target for chronic pain treatment. We previously demonstrated that β₂-adrenoceptors (β₂-ARs) are essential for neuropathic pain treatment by antidepressant drugs, and we showed that agonists of β₂-ARs, that is, β₂-mimetics, had an antiallodynic effect per se following chronic administration. To further explore the downstream mechanism of this action, we studied here the role of the opioid system. We used behavioral, genetic, and pharmacological approaches to test whether opioid receptors were necessary for the antiallodynic action of a short acting (terbutaline) and a long-acting (formoterol) β₂-mimetic. Using the Cuff model of neuropathic pain in mice, we showed that chronic treatments with terbutaline (intraperitoneal) or formoterol (orally) alleviated mechanical hypersensitivity. We observed that these β₂-mimetics remained fully effective in μ-opioid and in κ-opioid receptor deficient mice, but lost their antiallodynic action in δ-opioid receptor deficient mice, either female or male. Accordingly, we showed that the δ-opioid receptor antagonist naltrindole induced an acute relapse of allodynia in mice with neuropathic pain chronically treated with the β₂-mimetics. Such relapse was also observed following administration of the peripheral opioid receptor antagonist naloxone methiodide. These data demonstrate that the antiallodynic effect of long-term β₂-mimetics in a context of neuropathic pain requires the endogenous opioid system, and more specifically peripheral δ-opioid receptors.

Zerumbone Modulates α2A-Adrenergic, TRPV1, and NMDA NR2B Receptors Plasticity in CCI-Induced Neuropathic Pain In Vivo and LPS-Induced SH-SY5Y Neuroblastoma In Vitro Models

Zerumbone has shown great potential in various pathophysiological models of diseases, particularly in neuropathic pain conditions. Further understanding the mechanisms of action is important to develop zerumbone as a potential anti-nociceptive agent. Numerous receptors and pathways function to inhibit and modulate transmission of pain signals. Previously, we demonstrated involvement of the serotonergic system in zerumbone's anti-neuropathic effects. The present study was conducted to determine zerumbone's modulatory potential involving noradrenergic, transient receptor potential vanilloid type 1 (TRPV1) and N-methyl-D-aspartate (NMDA) receptors in chronic constriction injury (CCI)-induced in vitro and lipopolysaccharide (LPS)-induced SH-SY5Y in vitro neuroinflammatory models. von Frey filament and Hargreaves plantar tests were used to assess allodynia and hyperalgesia in the chronic constriction injury-induced neuropathic pain mouse model. Involvement of specific adrenoceptors were investigated using antagonists- prazosin (α₁-adrenoceptor antagonist), idazoxan (α₂-adrenoceptor antagonist), metoprolol (β₁-adrenoceptor antagonist), ICI 118,551 (β₂-adrenoceptor antagonist), and SR 59230 A (β₃-adrenoceptor antagonist), co-administered with zerumbone (10 mg/kg). Involvement of excitatory receptors; TRPV and NMDA were conducted using antagonists capsazepine (TRPV1 antagonist) and memantine (NMDA antagonist). Western blot was conducted to investigate the effect of zerumbone on the expression of α_2A-adrenoceptor, TRPV1 and NMDA NR2B receptors in CCI-induced whole brain samples of mice as well as in LPS-induced SH-SY5Y neuroblastoma cells. Pre-treatment with α₁- and α₂-adrenoceptor antagonists significantly attenuated both anti-allodynic and anti-hyperalgesic effects of zerumbone. For β-adrenoceptors, only β₂-adrenoceptor antagonist significantly reversed the anti-allodynic and anti-hyperalgesic effects of zerumbone. β₁-adrenoceptor antagonist only reversed the anti-allodynic effect of zerumbone. The anti-allodynic and anti-hyperalgesic effects of zerumbone were both absent when TRPV1 and NMDA receptors were antagonized in both nociceptive assays. Zerumbone treatment markedly decreased the expression of α_2A-adrenoceptor, while an up-regulation was observed of NMDA NR2B receptors. Expression of TRPV1 receptors however did not significantly change. The in vitro study, representing a peripheral model, demonstrated the reduction of both NMDA NR2B and TRPV1 receptors while significantly increasing α_2A-adrenoceptor expression in contrast to the brain samples. Our current findings suggest that the α₁-, α₂-, β₁- and β₂-adrenoceptors, TRPV1 and NMDA NR2B are essential for the anti-allodynic and antihyperalgesic effects of zerumbone. Alternatively, we demonstrated the plasticity of these receptors through their response to zerumbone's administration.

P2X3 and P2X2/3 receptors activation induces articular hyperalgesia by an indirect sensitization of the primary afferent nociceptor in the rats' knee joint

We have previously shown that endogenous adenosine 5'-triphosphate (ATP), via P2X3 and P2X2/3 receptors, plays an essential role in carrageenan-induced articular hyperalgesia model in rats' knee joint. In the present study, we used the rat knee joint incapacitation test, Enzyme-Linked Immunosorbent Assay (ELISA), and myeloperoxidase enzyme activity assay, to test the hypothesis that the activation of P2X3 and P2X2/3 receptors by their agonist induces articular hyperalgesia mediated by the inflammatory mediators bradykinin, prostaglandin, sympathomimetic amines, pro-inflammatory cytokines and by neutrophil migration. We also tested the hypothesis that the activation of P2X3 and P2X2/3 receptors contributes to the articular hyperalgesia induced by the inflammatory mediators belonging to carrageenan inflammatory cascade. The non-selective P2X3 and P2X2/3 receptors agonist αβ-meATP induced a dose-dependent articular hyperalgesia, which was significantly reduced by the selective antagonists for P2X3 and P2X2/3 receptors (A-317491), bradykinin B₁- (DALBK) or B₂-receptors (bradyzide), β₁-(atenolol) or β₂-adrenoceptors (ICI-118,551), by the pre-treatment with cyclooxygenase inhibitor (indomethacin) or with the nonspecific selectin inhibitor (Fucoidan). αβ-meATP induced the release of pro-inflammatory cytokines TNFα, IL-1β, IL-6, and CINC-1, as well as the neutrophil migration. Moreover, the co-administration of A-317491 significantly reduced the articular hyperalgesia induced by bradykinin, prostaglandin E₂ (PGE₂), and dopamine. These findings suggest that peripheral P2X3 and P2X2/3 receptors activation induces articular hyperalgesia by an indirect sensitization of the primary afferent nociceptor of rats' knee joint through the release of inflammatory mediators. Further, they also indicate that the activation of these purinergic receptors by endogenous ATP mediates the bradykinin-, PGE₂-, and dopamine-induced articular hyperalgesia.

Relative prevalence of 10 types of pharmacodynamic interactions in psychiatric treatment

OBJECTIVE

To assess the relative prevalence and factors affecting the prescription of medication combinations with a theoretical efficacy limiting pharmacodynamic interaction, defined as two medications with opposing indications and side effects or antagonistic action at the primary receptor of mechanism of action.

METHOD

One hundred sixteen combinations were identified for 10 types of pharmacodynamic interactions. PubMed was searched for each combination to assess the quality of evidence either supporting clinical use or verifying reduced efficacy. Micromedex was searched to determine the presence of warnings to prescribers of reduced efficacy. The prevalence in clinical practice was determined by computer review of the Genoa Healthcare database for all prescribers at 10 participating community mental health centers. The expected prevalence was calculated as the product of the probability of each medication prescribed alone and was compared with the actual prevalence of the combination using the test of proportions.

RESULTS

The frequency of prescription of eight combinations met the Bonferroni corrected level of significance of p < 0.001. Four were combinations of amphetamine and D2 antagonists and each were prescribed less often than chance, p = 0.0001 consistent with epidemiological studies and multiple animal studies verifying an efficacy limiting interaction. Despite epidemiological studies indicating increased risk of accidents, alprazolam and amphetamine were prescribed more often than chance, p = 0.0001. Micromedex generated warnings for efficacy limiting interactions for five other combinations, but with no subsequent change in prescription frequency.

CONCLUSIONS

Neither presence of medical evidence nor warnings from Micromedex consistently affect the prescription of combinations with pharmacodynamic efficacy limiting interactions.

Peripheral Delta Opioid Receptors Mediate Formoterol Anti-allodynic Effect in a Mouse Model of Neuropathic Pain

Neuropathic pain is a challenging condition for which current therapies often remain unsatisfactory. Chronic administration of β2 adrenergic agonists, including formoterol currently used to treat asthma and chronic obstructive pulmonary disease, alleviates mechanical allodynia in the sciatic nerve cuff model of neuropathic pain. The limited clinical data currently available also suggest that formoterol would be a suitable candidate for drug repurposing. The antiallodynic action of β2 adrenergic agonists is known to require activation of the delta-opioid (DOP) receptor but better knowledge of the molecular mechanisms involved is necessary. Using a mouse line in which DOP receptors were selectively ablated in neurons expressing Nav1.8 sodium channels (DOP cKO), we showed that these DOP peripheral receptors were necessary for the antiallodynic action of the β2 adrenergic agonist formoterol in the cuff model. Using a knock-in mouse line expressing a fluorescent version of the DOP receptor fused with the enhanced green fluorescent protein (DOPeGFP), we established in a previous study, that mechanical allodynia is associated with a smaller percentage of DOPeGFP positive small peptidergic sensory neurons in dorsal root ganglia (DRG), with a reduced density of DOPeGFP positive free nerve endings in the skin and with increased DOPeGFP expression at the cell surface. Here, we showed that the density of DOPeGFP positive free nerve endings in the skin is partially restored and no increase in DOPeGFP translocation to the plasma membrane is observed in mice in which mechanical pain is alleviated upon chronic oral administration of formoterol. This study, therefore, extends our previous results by confirming that changes in the mechanical threshold are associated with changes in peripheral DOP profile. It also highlights the common impact on DOP receptors between serotonin noradrenaline reuptake inhibitors such as duloxetine and the β2 mimetic formoterol.

Synaptic Communication upon Gentle Touch

Gentle touch sensation in mammals depends on synaptic transmission from primary sensory cells (Merkel cells) to secondary sensory neurons. Hoffman et al. (2018) identify norepinephrine and β₂-adrendergic receptors as the neurotransmitter-receptor pair responsible for sustained touch responses. The findings may deepen understanding of how drugs affect touch and pain sensation.

[Update in the pharmacological treatment of neuropathic pain]

We provide an updated review of the pharmacological treatment of neuropathic pain, with emphasis on the latest evidence-based recommendations. Drugs proposed as first line include tricyclic antidepressants (particularly amitriptyline), serotonin-noradrenaline reuptake inhibitors (particularly duloxetine), pregabalin and gabapentin. Second-line treatments include 5% lidocaine medicated plasters and capsaicin 8% patches, only for peripheral neuropathic pain and tramadol; whereas potent opioids and botulinum toxin A (for peripheral neuropathic pain) are considered third-line treatments. Future perspectives include the development of new drugs and a more personalised therapeutic approach, which is made possible by recent progress in the assessment and understanding of neuropathic pain.

A novel tissue-selective β2-adrenoceptor agonist with minimized cardiovascular effects, 5-HOB, attenuates neuropathic pain in mice

Objective

5-HOB is a novel tissue selective, 5-hydroxybenzothiazolone-derived β2 adrenoceptor agonist with minimized cardiovascular effects while retaining efficacy on skeletal muscle in preclinical experiments unlike conventional β2 adrenoceptor agonists, however its effect on the nervous system has not been evaluated yet. Therefore, 5-HOB was evaluated in a mouse model of neuropathic pain.

Results

5-HOB alleviated neuropathic allodynia in a dose dependent manner and reversed the changes in hind paw withdrawal thresholds to the sham control levels. The dose attenuating neuropathic allodynia was slightly lower than the dose inducing skeletal muscle hypertrophy. In conclusion, as reported with known β2 adrenoceptor agonists, 5-HOB was also effective in attenuating neuropathic pain in mice in addition to its effect on skeletal muscle.

Salvia officinalis, Rosmarinic and Caffeic Acids Attenuate Neuropathic Pain and Improve Function Recovery after Sciatic Nerve Chronic Constriction in Mice

The leaves of Salvia officinalis L. have a traditional reputation for the management of pain in Morocco. This study was conducted to investigate the curative effects of Salvia officinalis (SO) and its major constituents Rosmarinic (ROS) and Caffeic acids (CAF) on peripheral neuropathic pain in mice. Chronic constriction injury (CCI) was induced in mice, and neuropathic pain behaviors tests were evaluated by mechanical, chemical, thermal sensation tests and functional recovery of the sciatic nerve at different time intervals, i.e., (day 0, 1, 7, 14, and 21). Ethanolic extract of SO (100 and 200 mg/kg, p.o.), ROS (10 and 20 mg/kg, i.p.), CAF (30 and 40 mg/kg, i.p.), and CLOM (5 mg/kg, i.p., a positive control) was given for 21 days after surgery. Hematological and biochemical parameters were also measured as well as histopathological analysis. CCI produced significant development in mechanical and thermal hyperalgesia, cold allodynia, and rise in the sciatic functional index in mice. Chronic treatments with SO extract, ROS, CAF, and CLOM for 3 weeks significantly increased mechanical sensibility, cold, and thermal withdrawal latency and enhanced functional recovery of the injured nerve. The same treatments remarkably ameliorated hematological parameters and did not alter biochemical levels. The histopathological findings had revealed the protective effect of SO, ROS, and CAF against the CCI-induced damage. Our data support the use of SO in folk medicine to alleviate pain. Their main phenolic constituents could be promising antineuropathic compounds, which may be attributed to their biological activities including anti-inflammatory, antioxidant, and neuroprotective effects. SO leaves may be a good candidate to treat neuropathic pain.

Inhibitory effect of valproate sodium on pain behavior in diabetic mice involves suppression of spinal histone deacetylase 1 and inflammatory mediators

Anti-epileptic medications are included in the international guidelines for managing neuropathic pain. Valproate sodium (VPS) was recently described as "the forgotten analgesic" and has been reported to relief pain in various models of neuropathic pain. Some studies reported anti-inflammatory and histone deacetylase 1 (HDA1) inhibitory properties for sodium valproate. The aim of the current study was to investigate the modulatory effect of VPS on pain behavior and inflammatory reactions in alloxan-induced diabetic neuropathy focusing on HDA1 inhibition and glia reactivity. 28 Male Swiss albino mice were allocated into four groups, (1) vehicle group, (2) alloxan-diabetic group, (3 & 4) alloxan+VPS (25 or 50 mg/kg) groups. VPS was given daily for 5 weeks by oral gavage. Pain behavior demonstrated increased allodynia (von-Frey filaments) and hyperalgesia (hot-plate test) in alloxan-diabetic mice that was reduced significantly by at least one of VPS doses. Sciatic nerves in diabetic mice showed increased histopathology score, increased silver staining for the nerves-indicating myelopathy- and a decrease in immunostaining for nerve growth factor. Spinal cord of diabetic mice showed greater histopathologic score, increased CD11b and glia fibrillary acidic protein (GFAP) immunostaining than vehicle treated mice. Molecular investigations highlighted greater content of spinal histone deacetylases, tumor necrosis factor-α (TNF-α) and interlukin-1β (IL1β) that were favorably modified by VPS. Overall, the current data confirmed that the pain killing and anti-inflammatory activity of VPS is at least partly mediated through inhibition of spinal HDA1 and glia reactivity. These findings support the view of inviting antiepileptics for treating neuropathies.

Donepezil improves neuropathy through activation of AMPK signalling pathway in streptozotocin-induced diabetic mice

Diabetic neuropathy (DN) is a common complication of diabetes mellitus and is associated with structural changes in the nerves. However, the molecular basis for DN is poorly understood. Adenosine monophosphate activated protein kinase (AMPK) has been shown to regulate the activity of some kinases including protein kinase B (AKT), mitogen-activated protein kinases (MAPK) and mammalian target of rapamycin complex 1 (mTORC1) that represent important signalling pathways modulating the function of peripheral nociceptive neuron. Donepezil can activate AMPK and exerts neuroprotective effects. In this study, streptozotocin (45 mg/kg for 5 Day, i.p.) was used to induce experimental DN. After confirmation of development of neuropathy, mice were randomly distributed into five groups: Group 1; negative control group received saline (0.9%NaCl), Group 2; diabetic mice received saline, Group (3-5); diabetic mice received daily donepezil (1, 2 or 4 mg/kg, p.o.) respectively for 20 days. Mice were then sacrificed under anesthesia then their sciatic nerve and spinal cord were dissected out and processed for biochemical and histopathological studies. Diabetic mice revealed severe histological abnormalities including degenerated neurons in the spinal cord and swollen myelin sheath with inflammatory edema observed in sciatic nerves. In addition, diabetic mice showed reduced expression of p-AMPK in sciatic nerves with consequent activation of AKT/MAPK/4EBP1. A significant upregulation of the N-Methyl-d-aspartate (NMDA) receptors in both cervical and lumbar regions of spinal cord of diabetic mice was also demonstrated. Donepezil, an AMPK activator, blocked the phosphorylation of AKT/MAPK/4EBP1, down regulate the expression of NMDA receptors and reversed hyperalgesia developed in diabetic mice. Therefore, Donepezil could be a potential pharmacological agent for management of DN.

A Dual Noradrenergic Mechanism for the Relief of Neuropathic Allodynia by the Antidepressant Drugs Duloxetine and Amitriptyline

In addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. Despite the widespread use of these drugs, the mechanism underlying their therapeutic action in this pain context remains partly elusive. The present study combined data collected in male and female mice from a model of neuropathic pain and data from the clinical setting to understand how antidepressant drugs act. We show two distinct mechanisms by which the selective inhibitor of serotonin and noradrenaline reuptake duloxetine and the tricyclic antidepressant amitriptyline relieve neuropathic allodynia. One of these mechanisms is acute, central, and requires descending noradrenergic inhibitory controls and α_2A adrenoceptors, as well as the mu and delta opioid receptors. The second mechanism is delayed, peripheral, and requires noradrenaline from peripheral sympathetic endings and β₂ adrenoceptors, as well as the delta opioid receptors. We then conducted a transcriptomic analysis in dorsal root ganglia, which suggested that the peripheral component of duloxetine action involves the inhibition of neuroimmune mechanisms accompanying nerve injury, including the downregulation of the TNF-α-NF-κB signaling pathway. Accordingly, immunotherapies against either TNF-α or Toll-like receptor 2 (TLR2) provided allodynia relief. We also compared duloxetine plasma levels in the animal model and in patients and we observed that patients' drug concentrations were compatible with those measured in animals under chronic treatment involving the peripheral mechanism. Our study highlights a peripheral neuroimmune component of antidepressant drugs that is relevant to their delayed therapeutic action against neuropathic pain.SIGNIFICANCE STATEMENT In addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. However, the mechanism by which antidepressant drugs can relieve neuropathic pain remained in part elusive. Indeed, preclinical studies led to contradictions concerning the anatomical and molecular substrates of this action. In the present work, we overcame these apparent contradictions by highlighting the existence of two independent mechanisms. One is rapid and centrally mediated by descending controls from the brain to the spinal cord and the other is delayed, peripheral, and relies on the anti-neuroimmune action of chronic antidepressant treatment.

Antihyperalgesic Activity of Atomoxetine on Diabetes-Induced Neuropathic Pain: Contribution of Noradrenergic and Dopaminergic Systems

Atomoxetine is a selective noradrenaline reuptake inhibitor drug. Based on the knowledge that agents increasing monoamine levels in the central nervous system have therapeutic potential for neuropathic pain, it is planned to investigate the possible efficacy of atomoxetine on diabetes-induced hyperalgesia, in this study. Randall-Selitto (mechanical noxious stimuli) and Hargreaves (thermal noxious stimuli) tests were used to evaluate nociceptive perception of rats. Obtained data indicated that streptozotocin-induced diabetes causes significant decreases in the paw withdrawal threshold and paw withdrawal latency values of the animals, respectively. However, atomoxetine administered at 3 mg/kg/day for 7 and 14 days improved these diabetes-induced hyperalgesia responses. Furthermore, antihyperalgesic activity was antagonized with α-methyl-para-tyrosine methyl ester, phentolamine, propranolol, and sulpiride pre-treatments. The same effect was not reversed, however, by SCH 23390. These findings demonstrated, for the first time, that atomoxetine possesses significant antihyperalgesic activity on diabetes-induced neuropathic pain and this effect seems to be mediated by α- and β-adrenergic and D₂/D₃ dopaminergic receptors. Results of this present study seem to offer a new indication for an old drug; atomoxetine, but these preclinical data should first be confirmed with further well-designed clinical trials.

Effect of autophagy on allodynia, hyperalgesia and astrocyte activation in a rat model of neuropathic pain

Primary damage or dysfunction of the nervous system may cause or initiate neuropathic pain. However, it has been difficult to establish an effective treatment for neuropathic pain, as the mechanisms responsible for its pathology remain largely unknown. Autophagy is closely associated with the pathological process of neurodegenerative diseases, neuropathic injury and cancer, among others. The aim of the present study was to examine the changes in the autophagy-lysosomal pathway and discuss the effects of autophagy on allodynia, hyperalgesia and astrocyte activation in neuropathic pain. A neuropathic pain model was induced by chronic constriction injury (CCI) in rats. Inducers and inhibitors of autophagy and lysosomes were used to assess autophagy, allodynia, hyperalgesia and astrocyte activity. Neuropathic pain was found to induce an increase in the levels of the autophagy-related proteins, LC3II and Beclin 1 and, and in those of the lysosomal proteins, lysosomal-associated membrane protein type 2 (LAMP2) and Ras-related protein Rab-7a (RAB7), whereas p62 levels were found to decrease from day 1 to 14 following CCI. The autophagy inducer, rapamycin, further increased the LC3II, Beclin 1, lysosomal-associated membrane protein 2 (LAMP2) and Ras-related protein Rab-7a (RAB7) expression levels, and decreased the p62 expression levels, which were accompanied by alleviation of allodynia, hyperalgesia and astrocyte activation in the rats subjected to CCI; the autophagy inhibitor, 3-methyladenine, reversed these effects. The use of the lysosomal inhibitors, bafilomycin and chloroquine, resulted in the accumulation of LC3II and Beclin 1, a decrease in the levels of LAMP2 and RAB7, and the exacerbation of allodynia, hyperalgesia and astrocyte activation in rats with neuropathic pain. On the whole, the findings of this study indicate that neuropathic pain activates autophagy, which alleviates mechanical and thermal hyperalgesia and suppresses astrocyte activity. Therefore, neuropathic pain induced by CCI in rats appears to be mediated via the autophagy-lysosomal pathway.

α2- and β2-Adrenoreceptor-Mediated Efficacy of the Atypical Antidepressant Agomelatine Combined With Gabapentin to Suppress Allodynia in Neuropathic Rats With Ligated Infraorbital or Sciatic Nerve

Previous data showed that neuropathic pain induced by mechanical lesion of peripheral nerves has specific characteristics and responds differently to alleviating drugs at cephalic versus extracephalic level. This is especially true for tricyclic antidepressants currently used for alleviating neuropathic pain in humans which are less effective against cephalic neuropathic pain. Whether this also applies to the antidepressant agomelatine, with its unique pharmacological properties as MT₁/MT₂ melatonin receptor agonist and 5-HT_2B/5-HT_2C serotonin receptor antagonist, has been investigated in two rat models of neuropathic pain. Acute treatments were performed 2 weeks after unilateral chronic constriction (ligation) injury to the sciatic nerve (CCI-SN) or the infraorbital nerve (CCI-ION), when maximal mechanical allodynia had developed in ipsilateral hindpaw or vibrissal pad, respectively, in Sprague–Dawley male rats. Although agomelatine (45 mg/kg i.p.) alone was inactive, co-treatment with gabapentin, at an essentially ineffective dose (50 mg/kg i.p.) on its own, produced marked anti-allodynic effects, especially in CCI-ION rats. In both CCI-SN and CCI-ION models, suppression of mechanical allodynia by ‘agomelatine + gabapentin’ could be partially mimicked by the combination of 5-HT_2C antagonist (SB 242084) + gabapentin, but not by melatonin or 5-HT_2B antagonist (RS 127445, LY 266097), alone or combined with gabapentin. In contrast, pretreatment by idazoxan, propranolol or the β₂ antagonist ICI 118551 markedly inhibited the anti-allodynic effect of ‘agomelatine + gabapentin’ in both CCI-SN and CCI-ION rats, whereas pretreatment by the MT₁/MT₂ receptor antagonist S22153 was inactive. Altogether these data indicate that ‘agomelatine + gabapentin’ is a potent anti-allodynic combination at both cephalic and extra-cephalic levels, whose action implicates α₂- and β₂-adrenoreceptor-mediated noradrenergic neurotransmission.

The effects of propranolol on heart rate variability and quantitative, mechanistic, pain profiling: a randomized placebo-controlled crossover study

Background and aims

The autonomic nervous system (ANS) is capable of modulating pain. Aberrations in heart rate variability (HRV), reflective of ANS activity, are associated with experimental pain sensitivity, chronic pain, and more recently, pain modulatory mechanisms but the underlying mechanisms are still unclear. HRV is lowered during experimental pain as well as in chronic pain conditions and HRV can be increased by propranolol, which is a non-selective β-blocker. Sensitization of central pain pathways have been observed in several chronic pain conditions and human mechanistic pain biomarkers for these central pain pathways include temporal summation of pain (TSP) and conditioned pain modulation (CPM). The current study aimed to investigate the effect of the β-blocker propranolol, and subsequently assessing the response to standardized, quantitative, mechanistic pain biomarkers.

Methods

In this placebo-controlled, double-blinded, randomized crossover study, 25 healthy male volunteers (mean age 25.6 years) were randomized to receive 40 mg propranolol and 40 mg placebo. Heart rate, blood pressure, and HRV were assessed before and during experimental pain tests. Cuff pressure pain stimulation was used for assessment of pain detection (cPDTs) and pain tolerance (cPTTs) thresholds, TSP, and CPM. Offset analgesia (OA) was assessed using heat stimulation.

Results

Propranolol significantly reduced heart rate (p<0.001), blood pressure (p<0.02) and increased HRV (p<0.01) compared with placebo. No significant differences were found comparing cPDT (p>0.70), cPTT (p>0.93), TSP (p>0.70), OA-effect (p>0.87) or CPM (p>0.65) between propranolol and placebo.

Conclusions

The current study demonstrated that propranolol increased HRV, but did not affect pressure pain sensitivity or any pain facilitatory or modulatory outcomes.

Implications

Analgesic effects of propranolol have been reported in clinical pain populations and the results from the current study could indicate that increased HRV from propranolol is not associated with peripheral and central pain pathways in healthy male subjects.

Neuroprotective effect of duloxetine in a mouse model of diabetic neuropathy: Role of glia suppressing mechanisms

AIMS

Painful diabetic neuropathy (PDN) is one of the most frequent complications of diabetes and the current therapies have limited efficacy. This study aimed to study the neuroprotective effect of duloxetine, a serotonin noradrenaline reuptake inhibitor (SNRI), in a mouse model of diabetic neuropathy.

MAIN METHODS

Nine weeks after developing of PDN, mice were treated with either saline or duloxetine (15 or 30 mg/kg) for four weeks. The effect of duloxetine was assessed in terms of pain responses, histopathology of sciatic nerve and spinal cord, sciatic nerve growth factor (NGF) gene expression and on the spinal expression of astrocytes (glial fibrillary acidic protein, GFAP) and microglia (CD₁₁b).

KEY FINDINGS

The present results highlighted that duloxetine (30 mg/kg) increased the withdrawal threshold in von-Frey test. In addition, both doses of duloxetine prolonged the licking time and latency to jump in the hot-plate test. Moreover, duloxetine administration downregulated the spinal expression of both CD₁₁b and GFAP associated with enhancement in sciatic mRNA expression of NGF.

SIGNIFICANCE

The current results highlighted that duloxetine provided peripheral and central neuroprotective effects in neuropathic pain is, at least in part, related to its downregulation in spinal astrocytes and microglia. Further, this neuroprotective effect was accompanied by upregulation of sciatic expression of NGF.

The antinociceptive effects of ferulic acid on neuropathic pain: involvement of descending monoaminergic system and opioid receptors

Neuropathic pain can be considered as a form of chronic stress that may share common neuropathological mechanism between pain and stress-related depression and respond to similar treatment. Ferulic acid (FA) is a major active component of angelica sinensis and has been reported to exert antidepressant-like effects; however, it remains unknown whether FA ameliorate chronic constriction injury (CCI)-induced neuropathic pain and the involvement of descending monoaminergic system and opioid receptors. Chronic treatment with FA (20, 40 and 80 mg/kg) ameliorated mechanical allodynia and thermal hyperalgesia in von Frey hair and hot plate tasks, accompanied by increasing spinal noradrenaline (NA) and serotonin (5-HT) levels. Subsequent study suggested that treatment of CCI animals with 40 and 80 mg/kg FA also inhibited spinal MAO-A levels. FA's effects on mechanical allodynia or thermal hyperalgesiawas blocked by 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) via pharmacological depletion of spinal noradrenaline or serotonin. Moreover, the anti-allodynic action of FA on mechanical stimuli was prevented by pre-treatment with beta2-adrenoceptor antagonist ICI 118,551, or by the delta-opioid receptor antagonist naltrindole. While the anti-hyperalgesia on thermal stimuli induced by FA was blocked by pre-treatment with 5-HT1A receptor antagonist WAY-100635, or with the irreversible mu-opioid receptor antagonist beta-funaltrexamine. These results suggest that the effect of FA on neuropathic pain is potentially mediated via amelioration of the descending monoaminergic system that coupled with spinal beta2- and 5-HT1A receptors and the downstream delta- and mu-opioid receptors differentially.

Experimental Drugs for Neuropathic Pain

BACKGROUND

Neuropathic pain (NP) is an important public health problem and despite recent progress in the understanding, diagnosis, pathophysiological mechanisms and the treatment of NP, many patients remain refractory to pharmacotherapy.

OBJECTIVE

Currently used drugs have limited efficacy and dose-limiting adverse effects, and thus there is a substantial need for further development of novel medications for its treatment. Alternatively, drugs approved for use in diseases other than NP can be applied as experimental for NP conditions. This paper covers advances in the field of NP treatment.

RESULTS

The prime focus of this paper is on drugs with well-established pharmacological activity whose current therapeutic applications are distinct from NP. These drugs could be a potential novel treatment of NP. Data from preclinical studies and clinical trials on these experimental drugs are presented. The development of advanced methods of genomics enabled to propose new targets for drugs which could be effective in the NP treatment.

CONCLUSION

Experimental drugs for NP can be a treatment option which should be tailor-made for each individual on the basis of pain features, previous therapies, associated clinical conditions, recurrence of pain, adverse effects, contraindications and patients' preferences. At present, there are only some agents which may have potential as novel treatments. Increasing knowledge about mechanisms underlying NP, mechanisms of drug action, as well as available data from preclinical and clinical studies make botulinum toxin A, minocycline, ambroxol, statins and PPAR agonists (ATx086001) promising potential future treatment options.

A cyclic pathway of P2 × 7, bradykinin, and dopamine receptor activation induces a sustained articular hyperalgesia in the knee joint of rats

OBJECTIVE

We investigated whether: (1) P2 × 7 receptor activation by its agonist (BzATP) induces articular hyperalgesia in the rat's knee joint via inflammatory mechanisms and (2) activation of P2 × 7 receptors by endogenous ATP contributes to the articular hyperalgesia induced by bradykinin, TNF-α, IL-1β, CINC-1, PGE_2, and dopamine.

METHODS

The articular hyperalgesia was quantified using the rat knee joint incapacitation test. The knee joint inflammation, characterized by the concentration of pro-inflammatory cytokines and by neutrophil migration, was quantified in the synovial lavage fluid by ELISA and myeloperoxidase enzyme activity assay, respectively.

RESULTS

BzATP induced a dose-dependent articular hyperalgesia in the rat's knee joint that was significantly reduced by the selective antagonists for P2 × 7, bradykinin B₁ or B₂ receptors, β₁ or β₂ adrenoceptors, and by pre-treatment with Indomethacin. BzATP induced a local increase of TNF-α, IL-1β, IL-6, and CINC-1 concentration and neutrophil migration into the knee joint. The co-administration of the selective P2 × 7 receptor antagonist A-740003 significantly reduced the articular hyperalgesia induced by bradykinin and dopamine, but not by TNF-α, IL-1β, CINC-1, and PGE₂.

CONCLUSIONS

P2 × 7 receptor activation induces articular hyperalgesia mediated by the previous inflammatory mediator release. P2 × 7 receptor-induced articular hyperalgesia is sustained by the involvement of this purinergic receptor in bradykinin and dopamine-induced hyperalgesia in the knee joint.

Monoaminergic descending pathways contribute to modulation of neuropathic pain by increasing-intensity treadmill exercise after peripheral nerve injury

This study characterizes the impact of increasing-intensity treadmill exercise (iTR) on noradrenergic (NE) and serotonergic (5HT) modulation of neuropathic pain. Following sciatic nerve transection and repair (SNTR) rats developed significant mechanical and thermal hyperalgesia that was partially prevented by iTR performed during the first 2weeks after injury. Marked decrease in the expression of 5HT_2A and α_1A and β-, but not α_2A adrenergic receptors in the spinal cord dorsal horn was associated to SNTR and recovered by iTR, particularly in lamina II. iTR significantly increased 5HT_2A in periaqueductal grey (PAG), raphe magnus (RM) and dorsal raphe nucleus (DRN), with a pattern suggesting reorganization of serotonergic excitatory interconnections between PAG and DRN. iTR also increased the expression of α_1A in locus coeruleus (LC) and DRN, and β₂ in LC, indicating that exercise enhanced activity of NE neurons, likely by activating autologous projections from DRN and PAG. iTR hypoalgesia was antagonized by blockade of β₂ and 5HT_2A receptors with administration of butoxamine and ketanserin. The neurotoxin DSP4 was injected to induce depletion of NE projections from LC before starting iTR. DSP4 treatment worsened mechanical hyperalgesia, but iTR hypoalgesia was similarly produced. Moreover, 5HT_2A expression in LC further increased after DSP4 injection, all these results suggesting an intrinsic regulation of 5HT and NE activity between PAG, DRN and LC neurons activated by iTR. Finally, iTR significantly reduced microglial reactivity in LC and increased non-microglial BDNF expression, an effect that was reverted by butoxamine, implicating BDNF regulation in central 5HT/NE actions on neuropathic pain.

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3β (GSK3β). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.

Neuropathic Pain and Spinal Cord Injury: Phenotypes and Pharmacological Management

Chronic neuropathic pain is a complicated condition after a spinal cord injury (SCI) that often has a lifelong and significant negative impact on life after the injury; therefore, improved pain management is considered a significant and unmet need. Neuropathic pain mechanisms are heterogeneous and the difficulty in determining their individual contribution to specific pain types may contribute to poor treatment outcomes in this population. Thus, identifying human neuropathic pain phenotypes based on pain symptoms, somatosensory changes, or cognitive and psychosocial factors that reflect specific spinal cord or brain mechanisms of neuropathic pain is an important goal. Once a pain phenotype can be reliably replicated, its relationship with biomarkers and clinical treatment outcomes can be analyzed, and thereby facilitate translational research and further the mechanistic understanding of individual differences in the pain experience and in clinical trial outcomes. The present article will discuss clinical aspects of SCI-related neuropathic pain, neuropathic pain phenotypes, pain mechanisms, potential biomarkers and pharmacological interventions, and progress regarding how defining neuropathic pain phenotypes may lead to more targeted treatments for these difficult pain conditions.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Inhibition of the NMDA and AMPA receptor channels by antidepressants and antipsychotics

It is known that some antidepressants and antipsychotics directly inhibit NMDA-type ionotropic glutamate receptors. In this study we systematically studied action of seven drugs (Fluoxetine, Citalopram, Desipramine, Amitriptyline, Atomoxetine, Chlorpromazine, and Clozapine) on NMDA receptors and Ca²⁺-permeable and -impermeable AMPA receptors in rat brain neurons by whole-cell patch-clamp technique. Except for weak effect of fluoxetine, all drugs were virtually inactive against Ca²⁺-impermeable AMPA receptors. Fluoxetine and desipramine significantly inhibited Ca²⁺-permeable AMPA receptors (IC₅₀=43±7 and 105±12µM, respectively). Desipramine, atomoxetine and chlorpromazine inhibited NMDA receptors in clinically relevant low micromolar concentrations, while citalopram had only weak effect. All tested medicines have been clustered into two groups by their action on NMDA receptors: desipramine, amitriptyline, chlorpromazine, and atomoxetine display voltage- and magnesium-dependent open channel blocking mechanism. Action of fluoxetine and clozapine was found to be voltage- and magnesium-independent. All voltage-dependent compounds could be trapped in closed NMDA receptor channels. Possible contribution of NMDA receptor inhibition by certain antidepressants and antipsychotics to their analgesic effects in neuropathic pain is discussed.

Agomelatine: a new opportunity to reduce neuropathic pain-preclinical evidence

Antidepressants are first-line treatments of neuropathic pain but not all these drugs are really effective. Agomelatine is an antidepressant with a novel mode of action, acting as an MT1/MT2 melatonergic receptor agonist and a 5-HT2C receptor antagonist that involves indirect norepinephrine release. Melatonin, serotonin, and norepinephrine have been involved in the pathophysiology of neuropathic pain. Yet, no study has been conducted to determine agomelatine effects on neuropathic pain in animal models. Using 3 rat models of neuropathic pain of toxic (oxaliplatin/OXA), metabolic (streptozocin/STZ), and traumatic (sciatic nerve ligation/CCI [chronic constriction nerve injury]) etiologies, we investigated the antihypersensitivity effect of acute and repeated agomelatine administration. We then determined the influence of melatonergic, 5-HT2C, α-2 and β-1/2 adrenergic receptor antagonists in the antihypersensitivity effect of agomelatine. The effect of the combination of agomelatine + gabapentin was evaluated using an isobolographic approach. In STZ and CCI models, single doses of agomelatine significantly and dose dependently reduced mechanical hypersensitivity. After daily administrations for 2 weeks, this effect was confirmed in the CCI model and agomelatine also displayed a marked antihypersensitivity effect in the OXA model. The antihypersensitivity effect of agomelatine involved melatonergic, 5-HT2C, and α-2 adrenergic receptors but not beta adrenoceptors. The isobolographic analysis demonstrated that the combination of agomelatine + gabapentin had additive effects. Agomelatine exerts a clear-cut antihypersensitivity effect in 3 different neuropathic pain models. Its effect is mediated by melatonergic and 5-HT2C receptors and, although agomelatine has no affinity, also by α-2 adrenergic receptors. Finally, agomelatine combined with gabapentin produces an additive antihypersensitivity effect.