Neuropathic pain: etiology, pathophysiology, mechanisms, and evaluations

New generation capsaicin-diclofenac containing, silicon-based transdermal patch provides prolonged analgesic effect in acute and chronic pain models

OBJECTIVE

Pain is one of the major public health burdens worldwide, however, conventional analgesics are often ineffective. Capsaicin-the active compound of Capsicum species, being responsible for their pungency-has been part of traditional medicine long ago. Capsaicin is a natural agonist of the Transient Receptor Potential Vanilloid 1 receptor-localized on capsaicin-sensitive sensory neurons and strongly involved in pain transmission-, and has been in focus of analgesic drug research for many years. In this study, we aimed to develop a sustained release transdermal patch (transdermal therapeutic system, TTS) combining the advantages of low-concentration capsaicin and diclofenac embedded in an innovative structure, as well as to perform complex preclinical investigations of its analgesic effect.

METHODS

Drug delivery properties of the TTS were investigated with Franz cell and flow-through cell tests. Analgesic effect of the TTS was examined in in vivo models of acute postoperative and inflammatory, chronic neuropathic and osteoarthritic pain.

RESULTS

Modified silicone polymer matrix-based TTS containing low-concentration capsaicin and diclofenac has been developed, releasing both compounds according to zero-order kinetics. Moreover, capsaicin and diclofenac facilitated the liberation of each other. Combined TTS significantly reduced acute postoperative and inflammatory pain, as well as chronic neuropathic and osteoarthritic pain. Interestingly, in acute postoperative and chronic osteoarthritic pain, capsaicin prolonged and potentiated the pain-relieving effect of diclofenac.

CONCLUSIONS

New generation combined low-concentration capsaicin-diclofenac containing TTS can be an effective therapeutic tool in acute and chronic pain states involving neuropathic and inflammatory components.

Ultrasound-Guided Cryoneurolysis for the Treatment of Painful Diabetic Neuropathy of the Foot: A Case Series

Diabetic peripheral neuropathy is one of the most common causes of chronic neuropathic pain. Treatment of peripheral neuropathy has been limited to either treating the underlying cause or using medications, such as tricyclic antidepressants and anticonvulsants, to manage the symptoms. In this case series, we report the use of ultrasound-guided percutaneous cryoneurolysis of the superficial peroneal nerves to treat diabetic neuropathy of the feet. This demonstrates the potential effectiveness and safety of using cryoneurolysis for painful peripheral diabetic neuropathy.

Gut Microbiota Modulation and Its Implications on Neuropathic Pain: A Comprehensive Literature Review

Neuropathic pain (NP) is a chronic pain disorder arising from somatosensory nervous system impairment. Extensive evidence supports the notion that the gut microbiota (GM) is crucial in maintaining human health by performing vital tasks. At the same time, its disruption has been linked to the emergence and advancement of an expanding range of disorders, including NP, in which GM could play a role in its pathophysiology. The crosstalk between the nervous system and GM happens through immune mediators, metabolites, and nervous structures and involves both central and peripheral nervous systems. This literature review aims to thoroughly investigate the function of modulating GM in the treatment of NP. It will achieve this by integrating existing knowledge, identifying underlying mechanisms, and evaluating the possible clinical consequences of exploiting the gut-brain axis. We will cover the main therapeutic applications of the described GM-modulators, such as probiotics, faecal microbiota transplantation, dietary supplements and emotional support, to the main kinds of NP in which any evidence, even if only pre-clinical, has been unravelled in recent years. The explored NP areas include chemotherapy-induced peripheral neuropathy, diabetic neuropathy, trauma-induced neuropathic pain, trigeminal neuralgia, postherpetic neuralgia and low back pain.

Supraorbital Nerve Stimulation for Facial Pain

PURPOSE OF REVIEW

Chronic facial pain is considered one of the conditions that affect quality of daily life of patients significantly and makes them seek medical help. Intractable facial pain with failed trials of medical treatment and other pain management therapies presents a challenge for neurologists, pain specialists, and neurosurgeons. We describe the possibility of proposing peripheral nerve stimulation of the supraorbital nerves to treat patients with medically intractable facial pain. Stimulation of the supraorbital nerves is performed using percutaneously inserted electrodes that are positioned in the epi-fascial plane, traversing the course of the supraorbital nerves. The procedure has two phases starting with a trial by temporary electrodes that are inserted under fluoroscopic guidance and are anchored to the skin. This trial usually lasts for a few days to 2 weeks. If successful, we proceed to the insertion of a permanent electrode that is tunneled under the skin behind the ear toward the infraclavicular region in which we make a pocket for the implantable pulse generator.

RECENT FINDINGS

This procedure has been used in multiple patients with promising results which was published in literature. Literature shows that it provides relief of medically intractable pain, without the need for destructive procedures or more central modulation approaches with a preferable safety profile compared to other invasive procedures. Supraorbital nerve stimulation is now considered a valid modality of treatment for patients with medically intractable facial pain and can be offered as a reliable alternative for the patients while discussing the proper plan of management.

Novel Therapies for the Treatment of Neuropathic Pain: Potential and Pitfalls

Neuropathic pain affects more than one million people across the globe. The quality of life of people suffering from neuropathic pain has been considerably declining due to the unavailability of appropriate therapeutics. Currently, available treatment options can only treat patients symptomatically, but they are associated with severe adverse side effects and the development of tolerance over prolonged use. In the past decade, researchers were able to gain a better understanding of the mechanisms involved in neuropathic pain; thus, continuous efforts are evident, aiming to develop novel interventions with better efficacy instead of symptomatic treatment. The current review discusses the latest interventional strategies used in the treatment and management of neuropathic pain. This review also provides insights into the present scenario of pain research, particularly various interventional techniques such as spinal cord stimulation, steroid injection, neural blockade, transcranial/epidural stimulation, deep brain stimulation, percutaneous electrical nerve stimulation, neuroablative procedures, opto/chemogenetics, gene therapy, etc. In a nutshell, most of the above techniques are at preclinical stage and facing difficulty in translation to clinical studies due to the non-availability of appropriate methodologies. Therefore, continuing research on these interventional strategies may help in the development of promising novel therapies that can improve the quality of life of patients suffering from neuropathic pain.

Central Neuropathic Pain Syndromes: Current and Emerging Pharmacological Strategies

Central neuropathic pain is caused by a disease or lesion of the brain or spinal cord. It is difficult to predict which patients will develop central pain syndromes after a central nervous system injury, but depending on the etiology, lifetime prevalence may be greater than 50%. The resulting pain is often highly distressing and difficult to treat, with no specific treatment guidelines currently available. This narrative review discusses mechanisms contributing to central neuropathic pain, and focuses on pharmacological approaches for managing common central neuropathic pain conditions such as central post-stroke pain, spinal cord injury-related pain, and multiple sclerosis-related neuropathic pain. Tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors, and gabapentinoids have some evidence for efficacy in central neuropathic pain. Medications from other pharmacologic classes may also provide pain relief, but current evidence is limited. Certain non-pharmacologic approaches, neuromodulation in particular, may be helpful in refractory cases. Emerging data suggest that modulating the primary afferent input may open new horizons for the treatment of central neuropathic pain. For most patients, effective treatment will likely require a multimodal therapy approach.

The κ-Opioid Receptor Agonist U50488H Ameliorates Neuropathic Pain Through the Ca2+/CaMKII/CREB Pathway in Rats

Objective

To observe the ameliorative effect of kappa opioid receptor (KOR) agonist on rats with neuropathic pain (NP) and investigate the mechanism of action of the calcium ion (Ca²⁺)/calcium/calmodulin-dependent protein kinase II (CaMKII)/cyclic AMP response element-binding protein (CREB) pathway.

Methods

A total of 40 Sprague Dawley rats were randomly divided into four groups: sham-operation group (Sham group), NP model group (NP group), NP + KOR agonist U50488H group (NU group) and NP + specific CaMKII antagonist (KN93) + U50488H group (NKU group). The thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) of each group of rats were determined. ELISA was applied to examine the changes in inflammatory factors and oxidative stress factors, and the apoptotic rate in dorsal root ganglia was observed using TUNEL staining. Ca²⁺ concentration, content of oxidative stress index ROS and the release of calcitonin gene-related peptide (CGRP) and N-methyl-D-aspartate receptor (NMDAR) in the dorsal root ganglia were measured by the immunofluorescence assay. Finally, Western blotting was performed to detect expression changes in the Ca²⁺/CaMKII/CREB pathway.

Results

The KOR agonist U50488H could improve the values of TWL and MWT of NP the rats, inhibit inflammatory responses and relieve oxidative stress injury. Its mechanisms of action were associated with U50488H repression of Ca²⁺ influx, reduction of CGRP and NMDAR releases in the dorsal root ganglia and decreases in CaMKII and CREB phosphorylations in NP rats.

Conclusion

The KOR agonist ameliorates NP through suppressing the activity of the Ca²⁺/CaMKII/CREB pathway.

High-frequency spinal cord stimulation produces long-lasting analgesic effects by restoring lysosomal function and autophagic flux in the spinal dorsal horn

High-frequency spinal cord stimulation (HF-SCS) has been established as an effective therapy for neuropathic pain. However, the analgesic mechanisms involved in HF-SCS remain to be clarified. In our study, adult rat neuropathic pain was induced by spinal nerve ligation. Two days after modeling, the rats were subjected to 4 hours of HF-SCS (motor threshold 50%, frequency 10,000 Hz, and pulse width 0.024 ms) in the dorsal horn of the spinal cord. The results revealed that the tactile allodynia of spinal nerve-injured rats was markedly alleviated by HF-SCS, and the effects were sustained for 3 hours after the stimulation had ceased. HF-SCS restored lysosomal function, increased the levels of lysosome-associated membrane protein 2 (LAMP2) and the mature form of cathepsin D (matu-CTSD), and alleviated the abnormally elevated levels of microtubule-associated protein 1A/B-light chain 3 (LC3)-II and sequestosome 1 (P62) in spinal nerve-injured rats. HF-SCS also mostly restored the immunoreactivity of LAMP2, which was localized in neurons in the superficial layers of the spinal dorsal horn in spinal nerve-injured rats. In addition, intraperitoneal administration of 15 mg/kg chloroquine for 60 minutes reversed the expression of the aforementioned proteins and shortened the timing of the analgesic effects of HF-SCS. These findings suggest that HF-SCS may exhibit long-lasting analgesic effects on neuropathic pain in rats through improving lysosomal dysfunction and alleviating autophagic flux. This study was approved by the Laboratory Animal Ethics Committee of China Medical University, Shenyang, China (approval No. 2017PS196K) on March 1, 2017.

Inhibition of Spinal TRPV1 Reduces NMDA Receptor 2B Phosphorylation and Produces Anti-Nociceptive Effects in Mice with Inflammatory Pain

Transient receptor potential vanilloid 1 (TRPV1) has been implicated in peripheral inflammation and is a mediator of the inflammatory response to various noxious stimuli. However, the interaction between TRPV1 and N-methyl-D-aspartate (NMDA) receptors in the regulation of inflammatory pain remains poorly understood. This study aimed to investigate the analgesic effects of intrathecal administration of capsazepine, a TRPV1 antagonist, on carrageenan-induced inflammatory pain in mice and to identify its interactions with NMDA receptors. Inflammatory pain was induced by intraplantar injection of 2% carrageenan in male ICR mice. To investigate the analgesic effects of capsazepine, pain-related behaviors were evaluated using von Frey filaments and a thermal stimulator placed on the hind paw. TRPV1 expression and NMDA receptor phosphorylation in the spinal cord and glutamate concentration in the spinal cord and serum were measured. Intrathecal treatment with capsazepine significantly attenuated carrageenan-induced mechanical allodynia and thermal hyperalgesia. Moreover, carrageenan-enhanced glutamate and phosphorylation of NMDA receptor subunit 2B in the spinal cord were suppressed by capsazepine administration. These results indicate that TRPV1 and NMDA receptors in the spinal cord are associated with inflammatory pain transmission, and inhibition of TRPV1 may reduce inflammatory pain via NMDA receptors.

Concurrent ocular pain in patients with neurotrophic keratopathy

PURPOSE

To illustrate that ocular pain may occur in patients with neurotrophic keratopathy (NK) that typically are thought to lack symptoms of discomfort, and that aa subset of these patients may also present with neuropathic corneal pain (NCP).

METHOD

Retrospective Case series of 7 stage 1 NK patients who presented with concurrent ocular pain, as confirmed by clinical examination, proparacaine challenge test, and in vivo corneal confocal microscopy (IVCM). Records were assessed for results of ocular surface disease index (OSDI), pain on visual analog scale (VAS), ocular pain assessment survey (OPAS), best-corrected visual acuity (BCVA), corneal fluorescein staining (CFS) score, and IVCM findings. IVCM findings were compared to that of 20 healthy reference controls.

RESULTS

Mean age of patients was 63.7 ± 11.6 (range 44-76) years and 56.9 ± 8.6 (range 42-74) years in reference controls (p = 0.11). At presentation, ocular discomfort was 8.0 ± 1.3 (range 7-10) on VAS and mean OSDI scores were 72.26 ± 6.81 (range 62.50-79.54). Mean BCVA was 20/40, and mean CFS scores were 3.43 ± 0.79 (range 2-4) on the Oxford scale. IVCM analysis showed significant decrease in mean total, main and branch nerve densities in ranges consistent with NK as compared to normal controls (p < 0.001 for all), increased dendritiform cell density in three patients (p < 0.001), and the presence of microneuromas in six of the patients.

CONCLUSION

Patients with NK are thought to present with hypoesthesia. However, nerve damage and inflammation, which play a role in the development of NK may result in the development of chronic ocular pain, such as NCP, resulting in potential underdiagnosis of either disease.

Myelopathy and Reactive Microgliosis and Astrogliosis in Equine Back Pain

Equine chronic back pain (CBP) has been linked to different pathologic processes, which directly or indirectly involve spinal structures. Thus, making diagnosis and management very challenging with most horses with the condition recommended for early retirement from athletic activity. This study described the spinal cord lesions and the development of reactive microgliosis and astrocytosis in the spinal cords of horse with CBP. Thoracolumbar spinal cord segments from three horses euthanized because of unresolved CBP were dissected and grossly and histopathologically examined. The expression of activated microglia and astrocytes were demonstrated immunohistochemically using polyclonal rabbit anti-Iba-1 and anti-glial fibrillary acidic protein antibodies, respectively. All horses had radiological evidence of varying degrees of kissing spine involving six to nine vertebrae with the majority of the lesions graded between 2 and 5. Grossly, there was myelomalacia with intramedullary hemorrhages. The gray matters of the spinal cords were characterized by hemorrhagic malacic lesions with medullary disintegration. Reactive microgliosis and astrocytosis were evident in the spinal dorsal horns. White matter lesions include axonal swollen and/or loss, satellitosis, and varying degrees of dilation of myelin sheaths with some containing macrophages. In conclusion, the presence of reactive microgliosis and astrogliosis in the spinal dorsal horn indicates that they are possible precipitating factors in the development of equine CBP.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Challenges of neuropathic pain: focus on diabetic neuropathy

Neuropathic pain is a frequent condition caused by a lesion or disease of the central or peripheral somatosensory nervous system. A frequent cause of peripheral neuropathic pain is diabetic neuropathy. Its complex pathophysiology is not yet fully elucidated, which contributes to underassessment and undertreatment. A mechanism-based treatment of painful diabetic neuropathy is challenging but phenotype-based stratification might be a way to develop individualized therapeutic concepts. Our goal is to review current knowledge of the pathophysiology of peripheral neuropathic pain, particularly painful diabetic neuropathy. We discuss state-of-the-art clinical assessment, validity of diagnostic and screening tools, and recommendations for the management of diabetic neuropathic pain including approaches towards personalized pain management. We also propose a research agenda for translational research including patient stratification for clinical trials and improved preclinical models in relation to current knowledge of underlying mechanisms.

A Novel Algorithm to Identify Predictors of Treatment Response: Tapentadol Monotherapy or Tapentadol/Pregabalin Combination Therapy in Chronic Low Back Pain?

Purpose: High dose monotherapies or drug combinations are used to achieve sufficient analgesia for the treatment of severe chronic low back pain, before invasive therapy options are considered. In order to demonstrate an alternative for an empirical treatment approach, the authors' primary aim was to present an algorithm for the objective identification of treatment predictors. Additionally, the study identified baseline-characteristics in chronic low back pain patients prior to tapentadol PR treatment, as well as scrutinized those patients, either benefitting from a medium/high dose tapentadol PR monotherapy or a combination therapy (medium dose tapentadol PR + pregabalin).

Patients and Methods: The statistical approach included data of a previously published randomized, double blind, phase 3b study which compared the effectiveness and safety of tapentadol PR vs. a combination of tapentadol PR and pregabalin. In total, 46 clinical parameters were included in the statistical prediction models which were applied separately either to 50 patients who already responded well during the titration period (i.e., medium dose tapentadol PR) or to 261 patients with in the comparative treatment period [i.e., monotherapy (high dose tapentadol PR) or combination therapy (medium dose tapentadol PR/pregabalin)].

Results: The first statistical model identified three co-variables (NRS-3, PDQ, SQ) with predictive effects on patients responding well (“optimal responders”) to a medium dose tapentadol PR titration. Those patients presented low baseline pain intensity scores, good sleep quality and high painDETECT scores. The second statistical model identified eight co-variables (PDQ, numbness, SF-12 MCS, SF-12 PCS, VAS, HADS-A, HADS-D, SQ) with predictive effects on patients responding to high dose tapentadol PR monotherapy vs. a combination therapy (tapentadol PR + pregabalin). The high dose tapentadol PR responders indicated high painDETECT scores, little numbness and a good mental health status. Whereas, the combination therapy (tapentadol PR + pregabalin) responders were characterized by severe sleep disturbances and little anxiety.

Conclusion: The statistical analysis characterized chronic low back pain patients and identified factors contributing to a treatment response. Thus, this retrospective statistical algorithm represents an elegant method, which may contribute to future strategies toward a more individualized and improved mechanism based pain therapy.

Dehydrocrenatidine Inhibits Voltage-Gated Sodium Channels and Ameliorates Mechanic Allodia in a Rat Model of Neuropathic Pain

Picrasma quassioides (D. Don) Benn, a medical plant, is used in clinic to treat inflammation, pain, sore throat, and eczema. The alkaloids are the main active components in P. quassioides. In this study, we examined the analgesic effect of dehydrocrenatidine (DHCT), a β-carboline alkaloid abundantly found in P. quassioides in a neuropathic pain rat model of a sciatic nerve chronic constriction injury. DHCT dose-dependently attenuated the mechanic allodynia. In acutely isolated dorsal root ganglion, DHCT completely suppressed the action potential firing. Further electrophysiological characterization demonstrated that DHCT suppressed both tetrodotoxin-resistant (TTX-R) and sensitive (TTX-S) voltage-gated sodium channel (VGSC) currents with IC50 values of 12.36 μM and 4.87 µM, respectively. DHCT shifted half-maximal voltage (V1/2) of inactivation to hyperpolarizing direction by ~16.7 mV in TTX-S VGSCs. In TTX-R VGSCs, DHCT shifted V1/2 of inactivation voltage to hyperpolarizing direction and V1/2 of activation voltage to more depolarizing potential by ~23.9 mV and ~12.2 mV, respectively. DHCT preferred to interact with an inactivated state of VGSCs and prolonged the repriming time in both TTX-S and TTX-R VGSCs, transiting the channels into a slow inactivated state from a fast inactivated state. Considered together, these data demonstrated that the analgesic effect of DHCT was likely though the inhibition of neuronal excitability.

Neuropathic pain and Kv7 voltage-gated potassium channels: The potential role of Kv7 activators in the treatment of neuropathic pain

Neuropathic pain conditions severely and chronically affect the quality of life in a large human population, but the pain conditions are not adequately managed due to poor understanding of their underlying mechanisms. There is a pressing need for further research into this field to help develop effective and nonaddictive medications to treat neuropathic pain. This article first describes general clinical classification of pain, types and symptoms of neuropathic pain, and current practices of clinical management for neuropathic pain. This is followed by a discussion of various cellular and molecular mechanisms responsible for the development and maintenance of neuropathic pain. In this review, we highlight the loss of function of Kv7 voltage-gated potassium as a mechanism of neuropathic pain and the potential use of Kv7 channel activator as subsequent treatment.

Perspectives on the opioid crisis from pain medicine clinicians

Patients experiencing a terminal drug related event reflect a sentinel event. If this pharmacotherapy is a widely used agent, it may be viewed as a catastrophic problem. If patients are dying from illegal drug use when the medical establishment fails them by withdrawing or minimizing their medically prescribed medication, then the burden rests with their health care providers, legislation, and insurance carriers to actively participate in a collegial fashion to achieve parity. Causing a decay in functionality in previously functional patients, may occur with appropriately prescribed opioid medications addressing non-cancer pain when withdrawing or diminishing either with or without patient consent. The members of the medical profession have diminished their prescribing of opioids for their patients out of apparent fear of reprisal, state or federal government sanctions, and other concerned groups. Diminishing former dosages or deleting the opioid medication, preferably in concert with the patient, often results in inequitable patient care. Enforcing sanctioned decreases or ceasing to prescribe from their former required/established opioid medications precipitate patient discord. In absence of opioid misuse, abuse, diversion or addiction based upon medical "guidelines" and with a poor foundation of Evidence Based Medicine the CDC guidelines, it may be masked as a true guideline reflecting a decrement of clinical judgment, wisdom, and compassion. This article also discusses the role of pharmacy chains, insurance carriers, and their pharmacy benefit managers (PBMs) contribution to this multidimensional problem. There may be a potential solution, identified in this paper, if all the associated political, medical and insurance groups work cohesively to improve patient care. This article and the CDC guidelines are not focused at hospice, palliative, end of life care pain management.

Diagnostic accuracy of an identification tool for localized neuropathic pain based on the IASP criteria

OBJECTIVE

Based on the clear neuroanatomical delineation of many neuropathic pain (NP) symptoms, a simple tool for performing a short structured clinical encounter based on the IASP diagnostic criteria was developed to identify NP. This study evaluated its accuracy and usefulness.

METHODS

A case-control study was performed in 19 pain clinics within Spain. A pain clinician used the experimental screening tool (the index test, IT) to assign the descriptions of non-neuropathic (nNP), non-localized neuropathic (nLNP), and localized neuropathic (LNP) to the patients' pain conditions. The reference standard was a formal clinical diagnosis provided by another pain clinician. The accuracy of the IT was compared with that of the Douleur Neuropathique en 4 questions (DN4) and the Leeds Assessment of Neuropathic Signs and Symptoms (LANSS).

RESULTS

Six-hundred and sixty-six patients were analyzed. There was a good agreement between the IT and the reference standard (kappa =0.722). The IT was accurate in distinguishing between LNP and nLNP (83.2% sensitivity, 88.2% specificity), between LNP and the other pain categories (nLNP + nNP) (80.0% sensitivity, 90.7% specificity), and between NP and nNP (95.5% sensitivity, 89.1% specificity). The accuracy in distinguishing between NP and nNP was comparable with that of the DN4 and the LANSS. The IT took a median of 10 min to complete.

CONCLUSIONS

A novel instrument based on an operationalization of the IASP criteria can not only discern between LNP and nLNP, but also provide a high level of diagnostic certainty about the presence of NP after a short clinical encounter.

An Inflammation-Centric View of Neurological Disease: Beyond the Neuron

Inflammation is a complex biological response fundamental to how the body deals with injury and infection to eliminate the initial cause of cell injury and effect repair. Unlike a normally beneficial acute inflammatory response, chronic inflammation can lead to tissue damage and ultimately its destruction, and often results from an inappropriate immune response. Inflammation in the nervous system (“neuroinflammation”), especially when prolonged, can be particularly injurious. While inflammation per se may not cause disease, it contributes importantly to disease pathogenesis across both the peripheral (neuropathic pain, fibromyalgia) and central [e.g., Alzheimer disease, Parkinson disease, multiple sclerosis, motor neuron disease, ischemia and traumatic brain injury, depression, and autism spectrum disorder] nervous systems. The existence of extensive lines of communication between the nervous system and immune system represents a fundamental principle underlying neuroinflammation. Immune cell-derived inflammatory molecules are critical for regulation of host responses to inflammation. Although these mediators can originate from various non-neuronal cells, important sources in the above neuropathologies appear to be microglia and mast cells, together with astrocytes and possibly also oligodendrocytes. Understanding neuroinflammation also requires an appreciation that non-neuronal cell—cell interactions, between both glia and mast cells and glia themselves, are an integral part of the inflammation process. Within this context the mast cell occupies a key niche in orchestrating the inflammatory process, from initiation to prolongation. This review will describe the current state of knowledge concerning the biology of neuroinflammation, emphasizing mast cell-glia and glia-glia interactions, then conclude with a consideration of how a cell's endogenous mechanisms might be leveraged to provide a therapeutic strategy to target neuroinflammation.

Capsaicin patch for persistent postoperative pain after thoracoscopic surgery, report of two cases

Effective postoperative pain control after thoracic surgery is a significant clinical issue because it reduces pulmonary complications and accelerates the pace of recovery. Persistent postoperative pain syndrome is a recognized and frequent complication after thoracoscopic surgery. The capsaicin 8% patch contains a high concentration of synthetic capsaicin approved for treatment of peripheral neuropathic pain in adults. Little clinical data exist on the use of capsaicin patch in thoracic persistent postoperative pain syndrome. This report included two patients who were evaluated after receiving capsaicin for thoracic surgery. Satisfactory pain relief was achieved in both cases without side effects.

Antinociception induced by a novel α2A adrenergic receptor agonist in rodents acute and chronic pain models

The mechanisms and antinociceptive effects of a novel α_2A adrenoceptor agonist, 3-(2-chloro-6-fluorobenzil)-imidazolinide-2,4-dione (PT-31) were investigated using animal models of acute and chronic pain. The effects of PT-31 on pain responses were examined using hot plate and formalin tests in mice and spinal nerve ligation (SNL)-induced hyperalgesia in rats. The effects of antagonists acting on α adrenoceptor were assessed to investigate the interaction of these pathways upon PT-31 induced antinociception. PT-31 effects on motor activity/skills and on hemodynamic parameters were also evaluated. PT-31 had dose-dependent antinociception effects on hot-plate and formalin-injection induced pain responses. Thermal hyperalgesia and mechanical allodynia were reduced following a 7 d treatment with PT-31 (1, 5, and 10mg/kg/d, p.o.), and those effects were attenuated by yohimbine (5mg/kg), atropine (2mg/kg), L-nitro arginine methyl ester (L-NAME; 30mg/kg), or naloxone (2mg/kg). In contrast to clonidine, PT-31 did not have locomotor or hemodynamic effects in rats. The present results suggest that PT-31 represents a candidate for pain treatment with advantages over clonidine, namely no locomotor or hemodynamic impairments.

Primary Headache Disorders- Part 2: Tension-type headache and medication overuse headache

In Part 2 of Primary Headache disorders, we discuss the fourth Primary Headache Disorder, Tension-Type Headache (TTHA). We are again using the ICHD-III (Beta) definitions of such headaches, taking into consideration episodic and chronic TTHA, as well as the presence or absence of pericranial muscle tenderness. We discuss the pathophysiology and pharmacotherapeutic treatment of TTHA, and the aspects of the Myofascial Pain Syndrome that enhance and help the development of TTHA. We then discuss Medication Overuse Headache (MOH), itself a Secondary headache disorder, but one that is extremely important as it assists with the chronification of both migraine and TTHA. Finally we discuss how to manage and treat those patients with MOH. Chronic migraine, which is TTHA, Migraine as well as, in many patients, MOH, is discussed along with the treatment of this multifaceted disorder.

Spinal dopaminergic involvement in the antihyperalgesic effect of antidepressants in a rat model of neuropathic pain

Antidepressants such as tricyclic antidepressants, and serotonin noradrenaline reuptake inhibitors are a first-line treatment for neuropathic pain. Here, we aimed to determine the involvement of the spinal dopaminergic system in the antihyperalgesic effects of antidepressants in a rat model of neuropathic pain induced by spinal nerve ligation (SNL). The right L5 spinal nerve of male Sprague-Dawley rats was ligated under inhalation anesthesia to induce hyperalgesia. Behavioral testing was performed by measuring ipsilateral hindpaw withdrawal thresholds after intraperitoneal injection of amitriptyline, duloxetine, milnacipran, and fluoxetine. D2-like receptors were blocked by intrathecal administration of sulpiride. We also determined the concentrations of dopamine in the spinal cord using microdialysis after injection of antidepressants. The dopamine contents in the spinal dorsal horn were also measured in normal and SNL rats at 2, 3, 4, and 8 weeks after SNL surgery. Intraperitoneal injection of amitriptyline, duloxetine, milnacipran, and fluoxetine (3-30mg/kg) produced antihyperalgesic effects, and prevented by intrathecal pre-injection of sulpiride (30μg). Microdialysis revealed the dopamine levels in the spinal cord were increased after intraperitoneal injection of each antidepressant (10mg/kg). Furthermore, the dopamine content in homogenized spinal cord tissue were increased at 2 weeks after SNL and then subsequently declined. Our results suggest that the effect of antidepressants against neuropathic pain is related to modulation of not only noradrenalin and serotonin but also dopamine levels in the spinal cord.

Topical phenytoin for the treatment of neuropathic pain

We developed and tested a new putative analgesic cream, based on the anticonvulsant phenytoin in patients suffering from treatment refractory neuropathic pain. The use of commercial topical analgesics is not widespread due to the facts that capsaicin creams or patches can give rise to side effects, such as burning, and analgesic patches (e.g., lidocaine 5% patches) have complex handling, especially for geriatric patients. Only in a few countries, compounded creams based on tricyclic antidepressants or other (co-)analgesics are available. Such topical analgesic creams, however, are easy to administer and have a low propensity for inducing side effects. We, therefore, developed a new topical cream based on 5% and 10% phenytoin and described three successfully treated patients suffering from neuropathic pain. All patients were refractory to a number of other analgesics. In all patients, phenytoin cream was effective in reducing pain completely, without any side effects, and the tolerability was excellent. The onset of action of the phenytoin creams was within 30 minutes. Phenytoin cream might become a new treatment modality of the treatment of neuropathic pain.

Rethinking chronic pain in a primary care setting

Chronic pain substantially impacts patient function and quality of life and is a burden to society at large in terms of increased health care utilization and loss of productivity. As a result, there is an increasing recognition of chronic pain as a public health crisis. However, there remains wide variability in clinical practices related to the prevention, assessment, and treatment of chronic pain. Certain fundamental aspects of chronic pain are often neglected including the contribution of the psychological, social, and contextual factors associated with chronic pain. Also commonly overlooked is the importance of understanding the likely neurobiological mechanism(s) of the presenting pain and how they can guide treatment selection. Finally, physicians may not recognize the value of using electronic medical records to systematically capture data on pain and its impact on mood, function, and sleep. Such data can be used to monitor onset and maintenance of treatments effects at the patient level and evaluate costs at the systems level. In this review we explain how these factors play a critical role in the development of a coordinated, evidence-based treatment approach tailored to meet specific needs of the patient. We also discuss some practical approaches and techniques that can be implemented by clinicians in order to enhance the assessment and management of individuals with chronic pain in primary care settings.

Neuroinflammation, Mast Cells, and Glia: Dangerous Liaisons

The perspective of neuroinflammation as an epiphenomenon following neuron damage is being replaced by the awareness of glia and their importance in neural functions and disorders. Systemic inflammation generates signals that communicate with the brain and leads to changes in metabolism and behavior, with microglia assuming a pro-inflammatory phenotype. Identification of potential peripheral-to-central cellular links is thus a critical step in designing effective therapeutics. Mast cells may fulfill such a role. These resident immune cells are found close to and within peripheral nerves and in brain parenchyma/meninges, where they exercise a key role in orchestrating the inflammatory process from initiation through chronic activation. Mast cells and glia engage in crosstalk that contributes to accelerate disease progression; such interactions become exaggerated with aging and increased cell sensitivity to stress. Emerging evidence for oligodendrocytes, independent of myelin and support of axonal integrity, points to their having strong immune functions, innate immune receptor expression, and production/response to chemokines and cytokines that modulate immune responses in the central nervous system while engaging in crosstalk with microglia and astrocytes. In this review, we summarize the findings related to our understanding of the biology and cellular signaling mechanisms of neuroinflammation, with emphasis on mast cell-glia interactions.

The Emerging Role of HMGB1 in Neuropathic Pain: A Potential Therapeutic Target for Neuroinflammation

Neuropathic pain (NPP) is intolerable, persistent, and specific type of long-term pain. It is considered to be a direct consequence of pathological changes affecting the somatosensory system and can be debilitating for affected patients. Despite recent progress and growing interest in understanding the pathogenesis of the disease, NPP still presents a major diagnostic and therapeutic challenge. High mobility group box 1 (HMGB1) mediates inflammatory and immune reactions in nervous system and emerging evidence reveals that HMGB1 plays an essential role in neuroinflammation through receptors such as Toll-like receptors (TLR), receptor for advanced glycation end products (RAGE), C-X-X motif chemokines receptor 4 (CXCR4), and N-methyl-D-aspartate (NMDA) receptor. In this review, we present evidence from studies that address the role of HMGB1 in NPP. First, we review studies aimed at determining the role of HMGB1 in NPP and discuss the possible mechanisms underlying HMGB1-mediated NPP progression where receptors for HMGB1 are involved. Then we review studies that address HMGB1 as a potential therapeutic target for NPP.

Retrospective Study on the Analgesic Activity of a Topical (TT-CTAC) Cream in Patients With Diabetic Neuropathy and Other Chronic Pain Conditions

Treatment of chronic pain in diabetic neuropathy or neuropathic pain of other origins is challenging. Compounded topical formulations have evolved as potential treatment options. The objective of this retrospective study was to evaluate the efficacy of a compounded topical cream (Transdermal Therapeutics). Two versions of TT-CTAC cream were evaluated: cream 6B and cream 7B. Both creams contain ketamine (10%), baclofen (2%), gabapentin (6%), amitriptyline (4%), bupivacaine (2%), and clonidine (0.2%). Additionally, one cream (7B) contains nifedipine (2%). The primary efficacy outcome was the change in numeric pain intensity score from pretreatment to posttreatment. Secondary outcomes were qualitative grading (excellent, good, poor, or no effect), reduction in oral medication, and avoiding referral to a pain specialist. Information on 283 patients was evaluated, 205 received the 7B and 78 received 6B creams. The pain score decreased by 2.4 ± 2.4 (35%) with the 6B cream (from 7.8 ± 1.6 to 5.4 ± 2.0, P < 0.001) and by 3.0 ± 2.4 (40%) with the 7B cream (from 7.5 ± 1.7 to 4.5 ± 2.2, P < 0.001). Excellent or good effects were reported in 82% of the patients in the 6B and in 70% in the 7B groups. Reduction in oral pain medication was seen in 35% of the patients in the 7B and in 20% in the 6B groups. In the opinion of the treating physicians, the cream therapy caused the avoidance of a pain specialist referral in 53% of the patients in the 6B and in 39% in the 7B groups. The creams were equally effective in diabetic neuropathy, neuropathic pain, or other chronic pain states. We conclude that both creams provided excellent pain relief in the majority of the patients studied and may be a useful modality for pain therapy.

Efficacy and Safety of Topiramate for Essential Tremor: A Meta-Analysis of Randomized Controlled Trials

Essential tremor (ET) is the most common movement disorder that is frequently treated by propranolol or primidone. However, 30% of patients with ET do not respond to either propranolol or primidone. The objective of this study was to assess the efficacy and safety of topiramate for ET.We searched the MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials for relevant randomized controlled trials on the effects of topiramate for ET. A meta-analysis technique was applied to estimate the efficacy and safety of topiramate. The primary outcome was the change in the Fahn-Tolosa-Marin tremor rating scale (TRS). The secondary outcomes included the respective change in the location, motor tasks/function and function disability scores, and adverse events.We included 3 randomized controlled trials with a total of 294 participants. Topiramate was significantly better than placebo in reducing TRS of patients with ET (mean difference [MD] -8.58, 95% confidence interval [CI] -15.46 to -1.70). Changes from the scales of upper limb tremor severity (MD -5.12, 95% CI -7.79 to -2.45), motor tasks/function (MD -5.07, 95% CI -7.12 to -3.03), and functional disability (MD -4.72, 95% CI -6.77 to -2.67) were significantly greater with topiramate than with placebo. More participants taking topiramate experienced adverse events leading to withdrawal than those taking placebo (risk difference 19%, 95% CI 11%-27%).There is consistent evidence supporting the efficacy of topiramate in treating ET; however, a significant proportion of participants withdrew due to its adverse effects.

Analgesic effect of electroacupuncture on paclitaxel-induced neuropathic pain via spinal opioidergic and adrenergic mechanisms in mice

This study was designed to determine the antinociceptive effect and related neuronal mechanism of electroacupuncture (EA) on paclitaxel (PTX)-induced neuropathic pain in mice. PTX (4 mg/kg, i.p.) was administered once a day for 5 consecutive days to induce neuropathic pain. EA stimulation (2 mA, 2 Hz, 30 min) was applied at the ST36 acupoint bilaterally once in every 2 days. Repeated EA stimulation significantly attenuated PTX-induced mechanical allodynia and thermal hyperalgesia. In a separate set of experiment, the antinociceptive effect of a single EA stimulation 8 days after PTX treatment was reduced by intrathecal pretreatment with naloxone (opioid receptor antagonist), idazoxan (alpha2-adrenoceptor antagonist) or propranolol (beta-adrenoceptor antagonist), but not prazosin (alpha1-adrenoceptor antagonist). Moreover, EA remarkably suppressed the PTX-enhanced phosphorylation of the NMDA receptor NR2B subunit in the spinal dorsal horn, and intrathecal pretreatment of naloxone, idazoxan (IDA) or propranolol blocked the effect of EA. In conclusion, EA stimulation at the ST36 acupoint significantly diminished PTX-induced neuropathic pain in mice via the mediation of spinal opioid receptor, alpha2- and beta-adrenoceptors.

P2X receptors: New players in cancer pain

Pain is unfortunately a quite common symptom for cancer patients. Normally pain starts as an episodic experience at early cancer phases to become chronic in later stages. In order to improve the quality of life of oncological patients, anti-cancer treatments are often accompanied by analgesic therapies. The P2X receptor are adenosine triphosphate (ATP) gated ion channels expressed by several cells including neurons, cancer and immune cells. Purinergic signaling through P2X receptors recently emerged as possible common pathway for cancer onset/growth and pain sensitivity. Indeed, tumor microenvironment is rich in extracellular ATP, which has a role in both tumor development and pain sensation. The study of the different mechanisms by which P2X receptors favor cancer progression and relative pain, represents an interesting challenge to design integrated therapeutic strategies for oncological patients. This review summarizes recent findings linking P2X receptors and ATP to cancer growth, progression and related pain. Special attention has been paid to the role of P2X2, P2X3, P2X4 and P2X7 in the genesis of cancer pain and to the function of P2X7 in tumor growth and metastasis. Therapeutic implications of the administration of different P2X receptor blockers to alleviate cancer-associated pain sensations contemporarily reducing tumor progression are also discussed.

Contribution of α,β-Amyrenone to the Anti-Inflammatory and Antihypersensitivity Effects of Aleurites moluccana (L.) Willd

The aim of the study was to analyze the constituents of the dichloromethane fraction obtained from A. moluccana and also to evaluate the anti-inflammatory and antinociceptive properties of α,β-amyrenone isolated from A. moluccana in mice. The dichloromethane fraction was evaluated by gas chromatography and submitted to purification. The mixture of α,β-amyrenone was isolated and then evaluated using the carrageenan-induced paw-oedema or pleurisy and CFA-induced arthritis models in mice. Five triterpenes, α,β-amyrenone, glutinol, and α,β-amyrin were isolated from dichloromethane fraction of A. moluccana leaf extract. The mixture of α,β-amyrenone, dosed orally, was able to reduce mechanical hypersensitivity and paw-oedema induced by carrageenan, interfering with neutrophil migration. Similar results were observed in the carrageenan-induced pleurisy model. Repeated administration of the compounds was also effective in reducing the mechanical sensitization and oedema developed in the arthritis model induced by CFA. In conclusion, the results demonstrate that α,β-amyrenone interferes in both acute and chronic inflammatory processes. We can infer that these effects involve, at least in part, a reduction in the neutrophil migration. Therefore, it seems reasonable to suggest that α,β-amyrenone could represent a new therapeutic tool for the management of painful and inflammatory diseases, especially those presenting a chronic profile.

Inflammation triggers production of dimethylsphingosine from oligodendrocytes

Neuropathic pain is a chronic, refractory condition that arises after damage to the nervous system. We previously showed that an increased level of the endogenous metabolite N,N-dimethylsphingosine (DMS) in the central nervous system (CNS) is sufficient to induce neuropathic pain-like behavior in rats. However, several important questions remain. First, it has not yet been demonstrated that DMS is produced in humans and its value as a therapeutic target is therefore unknown. Second, the cell types within the CNS that produce DMS are currently unidentified. Here we provide evidence that DMS is present in human CNS tissue. We show that DMS levels increase in demyelinating lesions isolated from patients with multiple sclerosis, an autoimmune disease in which the majority of patients experience chronic pain. On the basis of these results, we hypothesized that oligodendrocytes may be a cellular source of DMS. We show that human oligodendrocytes produce DMS in culture and that the levels of DMS increase when oligodendrocytes are challenged with agents that damage white matter. These results suggest that damage to oligodendrocytes leads to increased DMS production which in turn drives inflammatory astrocyte responses involved in sensory neuron sensitization. Interruption of this pathway in patients may provide analgesia without the debilitating side effects that are commonly observed with other chronic pain therapies.

Clinical characteristics, patient-reported outcomes, and previous therapeutic management of patients with uncontrolled neuropathic pain referred to pain clinics

Background. The aim of this report was to evaluate the clinical profile and previous management of patients with uncontrolled neuropathic pain who were referred to pain clinics. Methods. We included adult patients with uncontrolled pain who had a score of ≥4 in the DN4 questionnaire. In addition to sociodemographic and clinical data, we evaluated pain levels using a visual analog scale as well as anxiety, depression, sleep, disability, and treatment satisfaction employing validated tools. Results. A total of 755 patients were included in the study. The patients were predominantly referred to pain clinics by traumatologists (34.3%) and primary care physicians (16.7%). The most common diagnoses were radiculopathy (43%) and pain of oncological origin (14.3%). The major cause for uncontrolled pain was suboptimal treatment (88%). Fifty-three percent of the patients were depressed, 43% had clinical anxiety, 50% rated their overall health as bad or very bad, and 45% noted that their disease was severely or extremely interfering with their daily activities. Conclusions. Our results showed that uncontrolled neuropathic pain is a common phenomenon among the specialties that address these clinical entities and, regardless of its etiology, uncontrolled pain is associated with a dramatic impact on patient well-being.