Intrathecal NGF administration reduces reactive astrocytosis and changes neurotrophin receptors expression pattern in a rat model of neuropathic pain

Attenuation of p38 MAPK/NF-κB/TRPV1/CGRP is involved in the antinociceptive effect of hesperidin methyl chalcone and taxifolin in paclitaxel-induced peripheral neuropathy

Paclitaxel (PTX)-induced peripheral neuropathy (PIPN) is a disabling side effect of PTX, which adversely affects the life quality of cancer patients. Flavonoids such as hesperidin methyl chalcone (HMC) and taxifolin (TAX) can alleviate neuropathic pain via their anti-inflammatory, antioxidant, neuroprotective, and antinociceptive properties. The current study aimed to assess the efficacy of HMC and TAX in preventing PIPN individually or in combination. Pretreatment with HMC and TAX mitigated PTX-induced mechanical allodynia and hyperalgesia, cold allodynia, and thermal hyperalgesia as well as restore the normal histological architecture. Remarkably, neuropathic pain was relieved by suppression of nerve growth factor (NGF), p38 mitogen-activated protein kinase (p38 MAPK), and transient receptor potential vanilloid type-1 (TRPV1), which ultimately lead to reduced calcitonin gene-related peptide (CGRP). Furthermore, both HMC or TAX enhanced nuclear factor erythroid 2-related factor 2 (Nrf2), leading to elevated glutathione (GSH) and total antioxidant capacity (TAC) along with lowered malondialdehyde (MDA), which in turn, downregulated nuclear factor kappa B P65 (NF-κB P65) and its phosphorylated form and eventually reduced tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β) then lowered the apoptotic indices. Promisingly, the combination of both agents was superior to each drug alone through targeting more diverse signaling pathways and achieving synergistic and comprehensive therapeutic effects. In conclusion, pretreatment with HMC and TAX separately or in combination alleviated PIPN via modulating NGF/p38 MAPK/NF-κB P65/TRPV1/CGRP pathway.

Blood-Derived Eye Drops for the Treatment of Corneal Neuropathic Pain

Blood-derived preparations, including autologous or allogenic serum, umbilical cord serum/plasma, and platelet-rich plasma eye drops, contain various growth factors, cytokines, and immunoglobulins that resemble natural tears. These components play important roles in corneal cell migration, proliferation, and wound healing. Blood-derived eye drops have demonstrated clinical effectiveness across a spectrum of ocular surface conditions, encompassing dry eye disease, Sjögren's syndrome, graft-versus-host disease, and neuropathic corneal pain (NCP). Currently, management of NCP remains challenging. The emergence of blood-derived eye drops represents a promising therapeutic approach. In this review, we discuss the benefits and limitations of different blood-derived eye drops, their mechanisms of action, and treatment efficacy in patients with NCP. Several studies have demonstrated the clinical efficacy of autologous serum eye drops in relieving pain and pain-like symptoms, such as allodynia and photoallodynia. Corneal nerve parameters were also significantly improved, as evidenced by increased nerve fiber density, length, nerve reflectivity, and tortuosity, as well as a decreased occurrence of beading and neuromas after the treatment. The extent of nerve regeneration correlated with improvement in patient-reported photoallodynia. Cord plasma eye drops also show potential for symptom alleviation and corneal nerve regeneration. Future directions for clinical practice and research involve standardizing preparation protocols, establishing treatment guidelines, elucidating underlying mechanisms, conducting long-term clinical trials, and implementing cost-effective measures such as scaling up manufacturing. With ongoing advancements, blood-derived eye drops hold promise as a valuable therapeutic option for patients suffering from NCP.

Nerve Growth Factor/Tyrosine Kinase A Receptor Pathway Enhances Analgesia in an Experimental Mouse Model of Bone Cancer Pain by Increasing Membrane Levels of δ-Opioid Receptors

BACKGROUND

The role of nerve growth factor (NGF)/tyrosine kinase A receptor (TrKA) signaling, which is activated in a variety of pain states, in regulating membrane-associated δ-opioid receptor (mDOR) expression is poorly understood. The hypothesis was that elevated NGF in bone cancer tumors could upregulate mDOR expression in spinal cord neurons and that mDOR agonism might alleviate bone cancer pain.

METHODS

Bone cancer pain (BCP) was induced by inoculating Lewis lung carcinoma cells into the femoral marrow cavity of adult C57BL/6J mice of both sexes. Nociceptive behaviors were evaluated by the von Frey and Hargreaves tests. Protein expression in the spinal dorsal horn of animals was measured by biochemical analyses, and excitatory synaptic transmission was recorded in miniature excitatory synaptic currents.

RESULTS

The authors found that mDOR expression was increased in BCP mice (BCP vs. sham, mean ± SD: 0.18 ± 0.01 g vs. mean ± SD: 0.13 ± 0.01 g, n = 4, P < 0.001) and that administration of the DOR agonist deltorphin 2 (Del2) increased nociceptive thresholds (Del2 vs. vehicle, median [25th, 75th percentiles]: 1.00 [0.60, 1.40] g vs. median [25th, 75th percentiles]: 0.40 [0.16, 0.45] g, n = 10, P = 0.001) and reduced miniature excitatory synaptic current frequency in lamina II outer neurons (Del2 vs. baseline, mean ± SD: 2.21 ± 0.81 Hz vs. mean ± SD: 2.43 ± 0.90 Hz, n = 12, P < 0.001). Additionally, NGF expression was increased in BCP mice (BCP vs. sham, mean ± SD: 0.36 ± 0.03 vs. mean ± SD: 0.16 ± 0.02, n = 4, P < 0.001), and elevated NGF was associated with enhanced mDOR expression via TrKA signaling.

CONCLUSIONS

Activation of mDOR produces analgesia that is dependent on the upregulation of the NGF/TrKA pathway by increasing mDOR levels under conditions of BCP in mice.

EDITOR’S PERSPECTIVE

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

A novel animal model of neuropathic corneal pain-the ciliary nerve constriction model

Introduction

Neuropathic pain arises as a result of peripheral nerve injury or altered pain processing within the central nervous system. When this phenomenon affects the cornea, it is referred to as neuropathic corneal pain (NCP), resulting in pain, hyperalgesia, burning, and photoallodynia, severely affecting patients' quality of life. To date there is no suitable animal model for the study of NCP. Herein, we developed an NCP model by constriction of the long ciliary nerves innervating the eye.

Methods

Mice underwent ciliary nerve constriction (CNC) or sham procedures. Safety was determined by corneal fluorescein staining to assess ocular surface damage, whereas Cochet-Bonnet esthesiometry and confocal microscopy assessed the function and structure of corneal nerves, respectively. Efficacy was assessed by paw wipe responses within 30 seconds of applying hyperosmolar (5M) saline at Days 3, 7, 10, and 14 post-constriction. Additionally, behavior was assessed in an open field test (OFT) at Days 7, 14, and 21.

Results

CNC resulted in significantly increased response to hyperosmolar saline between groups (p < 0.0001), demonstrating hyperalgesia and induction of neuropathic pain. Further, animals that underwent CNC had increased anxiety-like behavior in an open field test compared to controls at the 14- and 21-Day time-points (p < 0.05). In contrast, CNC did not result in increased corneal fluorescein staining or decreased sensation as compared to sham controls (p > 0.05). Additionally, confocal microscopy of corneal whole-mounts revealed that constriction resulted in only a slight reduction in corneal nerve density (p < 0.05), compared to naïve and sham groups.

Discussion

The CNC model induces a pure NCP phenotype and may be a useful model for the study of NCP, recapitulating features of NCP, including hyperalgesia in the absence of ocular surface damage, and anxiety-like behavior.

Effect of recombinant human nerve growth factor treatment on corneal nerve regeneration in patients with neurotrophic keratopathy

Introduction

Neurotrophic Keratopathy (NK) is a neurodegenerative corneal disease that results in diminished corneal sensation. Previous studies have found that Cenegermin 0.002%, a recombinant human nerve growth factor (rhNGF), improves corneal epithelial healing in stage 2 and 3 NK patients. However, rhNGF effect on corneal sensation and nerve regeneration has not been well established. Thus, this study aims to analyze the effect of rhNGF on corneal nerve regeneration using in vivo confocal microscopy (IVCM) and on corneal sensitivity in NK patients.

Methods

This is a retrospective, longitudinal, case-control study that included patients with NK, treated with rhNGF for at least 4 weeks, with pre- and post-treatment IVCM images available for analysis. Chart reviews were conducted documenting prior medical and surgical history, clinical signs and symptoms, and corneal sensation using Cochet-Bonnet esthesiometry. Corneal nerve parameters were assessed by IVCM. Sex- and age-matched reference controls were selected from a database of healthy subjects for comparison.

Results

The study included 25 patients, with 22 (88%) stage 1, two (8%) stage 2, and 1 (4%) stage 3 NK patients, with a median age of 64 years (range: 30-93 years). Total, main, and branch nerve densities [median (range) in mm/mm2] were lower in the NK group pre-treatment [2.3 (0.0-21.1); 1.7 (0.0-13.0); 0.5 (0.0-10.2); respectively] vs. controls [22.3 (14.9-29.0); 10.1 (3.2-15.4); and 12.1 (6.2-18.4), (p < 0.0001 for all), respectively]. Post-treatment nerve densities increased compared to pre-treatment to 5.3 (0.0-19.4, p = 0.0083) for total, 3.5 (0.0-13.2, p = 0.0059) for main, and 2.0 (0.0-10.4, p = 0.0251) for branch nerves, but remained lower than controls (p < 0.0001 for all). Corneal sensation increased from 2.3 ± 1.1 cm pre-treatment to 4.1 ± 1.4 cm post-treatment (p = 0.001). Median best corrected visual acuity significantly increased following rhNGF treatment from 0.4 (0.0-1.6) to 0.12 (-0.1 to 1.6) (p = 0.007).

Conclusion

Patients with NK treated with at least 4 weeks of rhNGF, showed a significant increase in corneal nerve densities after treatment. A significant increase in corneal sensation, as well as best corrected visual acuity, was observed following treatment.

Potential role of Schwann cells in neuropathic pain

Neuropathic pain (NPP) is a common syndrome associated with most forms of disease, which poses a serious threat to human health. NPP may persist even after the nociceptive stimulation is eliminated, and treatment is extremely challenging in such cases. Schwann cells (SCs) form the myelin sheaths around neuronal axons and play a crucial role in neural information transmission. SCs can secrete trophic factors to nourish and protect axons, and can further secrete pain-related factors to induce pain. SCs may be activated by peripheral nerve injury, triggering the transformation of myelinated and non-myelinated SCs into cell phenotypes that specifically promote repair. These differentiated SCs provide necessary signals and spatial clues for survival, axonal regeneration, and nerve regeneration of damaged neurons. They can further change the microenvironment around the regions of nerve injury, and relieve the pain by repairing the injured nerve. Herein, we provide a comprehensive overview of the biological characteristics of SCs, discuss the relationship between SCs and nerve injury, and explore the potential mechanism of SCs and the occurrence of NPP. Moreover, we summarize the feasible strategies of SCs in the treatment of NPP, and attempt to elucidate the deficiencies and defects of SCs in the treatment of NPP.

Involvement of nerve growth factor (NGF) in chronic neuropathic pain - a systematic review

Pain is a complex experience, encompassing physiological and psychological components. Amongst the different types of pain, neuropathic pain, resulting from injuries to the peripheral or central nervous system, still constitutes a challenge for researchers and clinicians. Nerve growth factor (NGF) is currently regarded as a key contributor and may serve as a therapeutic target in many types of pain, likely including neuropathic pain. Here, we reviewed the role of NGF in neuropathic pain of peripheral and central origin, also addressing its potential use as a pharmacological target to better help patients dealing with this condition that severely impacts the everyday life. For this, we conducted a search in the databases PubMed and Scopus. Our search resulted in 1103 articles (458 in PubMed and 645 in Scopus). Only articles related to the involvement of NGF in pain or articles that approached its potential use as a target in treatment of pain symptoms were included. Duplicates were eliminated and 274 articles were excluded. After careful analysis, 23 articles were selected for review. Original articles studying the role of NGF in pathology as well as its modulation as a possible therapeutic target were included. We found that NGF is widely regarded as a key player in neuropathic pain and seen as a putative therapeutic target. However, evidence obtained from years of clinical trials highlights the toxic adverse effects of anti-NGF therapeutics, precluding its use in clinical context. Further studies are, thus, needed to improve treatment of chronic neuropathic pain.

Matrix metalloproteinases, purinergic signaling, and epigenetics: hubs in the spinal neuroglial network following peripheral nerve injury

Activation of glial cells (reactive gliosis) and the purinergic pathway, together with metalloproteinase (MMP)-induced remodeling of the neural extracellular matrix (nECM), drive maladaptive changes in the spinal cord following peripheral nerve injury (PNI). We evaluated the effects on spinal maladaptive plasticity through administration of oxidized ATP (oxATP), an antagonist of P2X receptors (P2XR), and/or GM6001, an inhibitor of MMPs, in rats following spared nerve injury (SNI) of the sciatic nerve. With morpho-molecular techniques, we demonstrated a reduction in spinal reactive gliosis and changes in the neuro-glial-nECM crosstalk via expression remodeling of P2XR, nerve growth factor (NGF) receptors (TrkA and p75), and histone deacetylase 2 (HDAC2) after treatments with oxATP/GM6001. Altogether, our data suggest that MMPs and purinergic inhibition have a modulatory impact on key proteins in the neuro-glial-nECM network, acting at different levels from intracellular signaling to epigenetic modifications.

Neural Stem Cells Overexpressing Nerve Growth Factor Improve Functional Recovery in Rats Following Spinal Cord Injury via Modulating Microenvironment and Enhancing Endogenous Neurogenesis

Spinal cord injury (SCI) is a devastating event characterized by severe motor, sensory, and autonomic dysfunction. Currently, there is no effective treatment. Previous studies showed neural growth factor (NGF) administration was a potential treatment for SCI. However, its targeted delivery is still challenging. In this study, neural stem cells (NSCs) were genetically modified to overexpress NGF, and we evaluated its therapeutic value following SCI. Four weeks after transplantation, we observed that NGF-NSCs significantly enhanced the motor function of hindlimbs after SCI and alleviated histopathological damage at the lesion epicenter. Notably, the survival NGF-NSCs at lesion core maintained high levels of NGF. Further immunochemical assays demonstrated the graft of NGF-NSCs modulated the microenvironment around lesion core via reduction of oligodendrocyte loss, attenuation of astrocytosis and demyelination, preservation of neurons, and increasing expression of multiple growth factors. More importantly, NGF-NSCs seemed to crosstalk with and activate resident NSCs, and high levels of NGF activated TrkA, upregulated cAMP-response element binding protein (CREB) and microRNA-132 around the lesion center. Taken together, the transplantation of NGF-NSCs in the subacute stage of traumatic SCI can facilitate functional recovery by modulating the microenvironment and enhancing endogenous neurogenesis in rats. And its neuroprotective effect may be mediated by activating TrkA, up-regulation of CREB, and microRNA-132.

Inhibition of plasminogen/plasmin system retrieves endogenous nerve growth factor and adaptive spinal synaptic plasticity following peripheral nerve injury

Dysfunctions of the neuronal-glial crosstalk and/or impaired signaling of neurotrophic factors represent key features of the maladaptive changes in the central nervous system (CNS) in neuroinflammatory as neurodegenerative disorders. Tissue plasminogen activator (tPA)/plasminogen (PA)/plasmin system has been involved in either process of maturation and degradation of nerve growth factor (NGF), highlighting multiple potential targets for new therapeutic strategies. We here investigated the role of intrathecal (i.t.) delivery of neuroserpin (NS), an endogenous inhibitor of plasminogen activators, on neuropathic behavior and maladaptive synaptic plasticity in the rat spinal cord following spared nerve injury (SNI) of the sciatic nerve. We demonstrated that SNI reduced spinal NGF expression, induced spinal reactive gliosis, altering the expression of glial and neuronal glutamate and GABA transporters, reduced glutathione (GSH) levels and is associated to neuropathic behavior. Beside the increase of NGF expression, i.t. NS administration reduced reactive gliosis, restored synaptic homeostasis, GSH levels and reduced neuropathic behavior. Our results hereby highlight the essential role of tPA/PA system in the synaptic homeostasis and mechanisms of maladaptive plasticity, sustaining the beneficial effects of NGF-based approach in neurological disorders.

Astrocyte reactivity in spinal cord and functional impairment after tendon injury in rats

Astrocyte reactivity in the spinal cord may occur after peripheral neural damage. However, there is no data to report such reactivity after Achilles tendon injury. We investigate whether changes occur in the spinal cord, mechanical sensitivity and gait in two phases of repair after Achilles tendon injury. Wistar rats were divided into groups: control (CTRL, without rupture), 2 days post-injury (RUP2) and 21 days post-injury (RUP21). Functional and mechanical sensitivity tests were performed at 2 and 21 days post-injury (dpi). The spinal cords were processed, cryosectioned and activated astrocytes were immunostained by GFAP at 21 dpi. Astrocyte reactivity was observed in the L5 segment of the spinal cord with predominance in the white matter regions and decrease in the mechanical threshold of the ipsilateral paw only in RUP2. However, there was gait impairment in both RUP2 and RUP21. We conclude that during the acute phase of Achilles tendon repairment, there was astrocyte reactivity in the spinal cord and impairment of mechanical sensitivity and gait, whereas in the chronic phase only gait remains compromised.

tDCS and exercise improve anxiety-like behavior and locomotion in chronic pain rats via modulation of neurotrophins and inflammatory mediators

Anxiety disorders cause distress and are commonly found to be comorbid with chronic pain. Both are difficult-to-treat conditions for which alternative treatment options are being pursued. This study aimed to evaluate the effects of transcranial direct current stimulation (tDCS), treadmill exercise, or both, on anxiety-like behavior and associated growth factors and inflammatory markers in the hippocampus and sciatic nerve of rats with neuropathic pain. Male Wistar rats (n = 216) were subjected to sham-surgery or sciatic nerve constriction for pain induction. Fourteen days following neuropathic pain establishment, either bimodal tDCS, treadmill exercise, or a combination of both was used for 20 min a day for 8 consecutive days. The elevated plus-maze test was used to assess anxiety-like behavior and locomotor activity during the early (24 h) or late (7 days) phase after the end of treatment. BDNF, TNF-ɑ, and IL-10 levels in the hippocampus, and BDNF, NGF, and IL-10 levels in the sciatic nerve were assessed 48 h or 7 days after the end of treatment. Rats from the pain groups developed an anxiety-like state. Both tDCS and treadmill exercise provided ethological and neurochemical alterations induced by pain in the early and/or late phase, and a modest synergic effect between tDCS and exercise was observed. These results indicate that non-invasive neuromodulatory approaches can attenuate both anxiety-like status and locomotor activity and alter the biochemical profile in the hippocampus and sciatic nerve of rats with neuropathic pain and that combined interventions may be considered as a treatment option.

Chronic Pain Causes Peripheral and Central Responses in MIA-Induced TMJOA Rats

Chronic pain is the predominant symptom that drives temporomandibular joint osteoarthritis (TMJOA) patients to seek medical care; however, currently used treatment modalities remain less effective. This study aimed to investigate chronic pain and the peripheral and central responses in monoiodoacetate (MIA)-induced TMJOA rats. First, the appropriate dose of MIA was determined based on pain behavior assessment in rats. Alterations of the condylar structure in TMJOA rats were evaluated by histological staining and micro-computed tomography (micro-CT). Second, the period of TMJOA chronic pain was further explored by assessing the numbers of glial fibrillary acidic protein (GFAP)-positive astrocytes and ionized calcium-binding adaptor molecule 1 (IBA-1)-positive microglia in the trigeminal spinal nucleus (TSN) and performing nonsteroidal anti-inflammatory drug (NSAID) efficacy experiments. Finally, the expression of neurofilament 200 (NF200), calcitonin gene-related peptide (CGRP), and isolectin B4 (IB4) in the trigeminal ganglion (TG) and TSN was assessed by immunofluorescence. MIA at 4 mg/kg was considered an appropriate dose. Gradual MIA-induced alterations of the condylar structure were correlated with temporomandibular joint (TMJ) pain. The numbers of GFAP- and IBA-1-positive cells were increased at 2, 3, and 4 weeks after MIA injection. NSAIDs failed to alleviate pain behavior 10 days after MIA injection. CGRP and IB4 levels in the TG and TSN were upregulated at 2 and 4 weeks. These results suggest that TMJOA-related chronic pain emerged 2 weeks after MIA injection. CGRP- and IB4-positive afferents in both the peripheral and central nervous systems may be involved in MIA-induced TMJOA-related chronic pain in rats.

Dietary omega-3 fatty acids prevent neonatal seizure-induced early alterations in the hippocampal glutamatergic system and memory deficits in adulthood

OBJECTIVE

We investigated the influence of dietary omega-3 polyunsaturated fatty acids (n-3 PUFAs) on glutamatergic system modulation after a single episode of neonatal seizures and their possible effects on seizure-induced long-lasting behavioral deficits.

METHODS

Male Wistar rats receiving an omega-3 diet (n-3) or an n-3 deficient diet (D) from the prenatal period were subjected to a kainate-induced seizure model at P7. Glutamate transporter activity and immunocontents (GLT-1 and GLAST) were assessed in the hippocampus at 12, 24, and 48 h after the seizure episode. Fluorescence intensity for glial cells (GFAP) and neurons (NeuN) was assessed 24 h after seizure in the hippocampus. Behavioral analysis (elevated-plus maze and inhibitory avoidance memory task) was performed at 60 days of age.

RESULTS

The D group showed a decrease in glutamate uptake 24 h after seizure. In this group only, the GLT1 content increased at 12 h, followed by a decrease at 24 h. GLAST increased up to 24 h after seizure. GFAP fluorescence was higher, and NeuN fluorescence decreased, in the D group independent of seizures. In adulthood, the D group presented memory deficits independent of seizures, but short-term memory (1.5 h after a training session) was abolished in the D group treated with kainate.

SIGNIFICANCE

N-3 PUFA positively influenced the glutamatergic system during seizure and prevented seizure-related memory deficits in adulthood.

Protective Functions of Reactive Astrocytes Following Central Nervous System Insult

Astrocytes play important roles in numerous central nervous system disorders including autoimmune inflammatory, hypoxic, and degenerative diseases such as Multiple Sclerosis, ischemic stroke, and Alzheimer’s disease. Depending on the spatial and temporal context, activated astrocytes may contribute to the pathogenesis, progression, and recovery of disease. Recent progress in the dissection of transcriptional responses to varying forms of central nervous system insult has shed light on the mechanisms that govern the complexity of reactive astrocyte functions. While a large body of research focuses on the pathogenic effects of reactive astrocytes, little is known about how they limit inflammation and contribute to tissue regeneration. However, these protective astrocyte pathways might be of relevance for the understanding of the underlying pathology in disease and may lead to novel targeted approaches to treat autoimmune inflammatory and degenerative disorders of the central nervous system. In this review article, we have revisited the emerging concept of protective astrocyte functions and discuss their role in the recovery from inflammatory and ischemic disease as well as their role in degenerative disorders. Focusing on soluble astrocyte derived mediators, we aggregate the existing knowledge on astrocyte functions in the maintenance of homeostasis as well as their reparative and tissue-protective function after acute lesions and in neurodegenerative disorders. Finally, we give an outlook of how these mediators may guide future therapeutic strategies to tackle yet untreatable disorders of the central nervous system.

Platelet-rich plasma in umbilical cord blood reduces neuropathic pain in spinal cord injury by altering the expression of ATP receptors

BACKGROUND

Neuropathic pain following injury or dysfunction of the peripheral or CNS is one of the most important medical challenges to treat. Humane platelet-rich plasma (HPRP), which is a rich source of growth factors, may be able to treat and reduce pain caused by spinal cord injury (SCI). In this study, the effect of HPRP on neuropathic pain caused by SCI was investigated.

METHODS

Sixty adult male Wistar rats were randomly divided into 6 groups: control, sham, SCI, vehicle (SCI+platelet-poor plasma), SCI+ PRP2day (injection 48 hrs after SCI) and SCI+PRP14day (injection 14 days after SCI). SCI was induced at the T12-T13 level. Behavioral tests were conducted weekly after injury for six weeks. Allodynia and hyperalgesia were assessed using acetone drops, plantar test and von Frey filament. Cavity size and the number of fibroblasts were determined by H&E stain, and the expression of mTOR, p-mTOR, P₂×₃R and P₂Y₄R were determined using the western blot technique. Data were analyzed using PRISM & SPSS software.

RESULTS

PRP injection showed a higher pain threshold in mechanical allodynia (p<0.0001), cold allodynia (p<0.0001) and thermal hyperalgesia (p<0.0001) than those in the spinal. Animals treated with PRP also reduced cavity size, fibroblast number, p-mTOR/mTOR ratio, and P₂×₃R expression, and increased P₂Y₄R expression. The difference between the two groups was not statistically significant.

CONCLUSIONS

The results showed that PRP reduced SCI-induced allodynia and hyperalgesia by regulating ATP signaling. Using HPRP can open a new window in the treatment of pain caused by damage to the nervous system.

The Delayed-Onset Mechanical Pain Behavior Induced by Infant Peripheral Nerve Injury Is Accompanied by Sympathetic Sprouting in the Dorsal Root Ganglion

Background

Sympathetic sprouting in the dorsal root ganglion (DRG) following nerve injuries had been proved to induce adult neuropathic pain. However, it is unclear whether the abnormal sprouting occurs in infant nerve injury.

Methods

L5 spinal nerve ligation (SNL) or sham surgery was performed on adult rats and 10-day-old pups, and mechanical thresholds and heat hyperalgesia were analyzed on 3, 7, 14, 28, and 56 postoperative days. Tyrosine hydroxylase-labeled sympathetic fibers were observed at each time point, and 2 neurotrophin receptors (p75NTR and TrkA) were identified to explore the mechanisms of sympathetic sprouting.

Results

Adult rats rapidly developed mechanical and heat hyperalgesia from postoperative day 3, with concurrent sympathetic sprouting in DRG. In contrast, the pup rats did not show a significantly lower mechanical threshold until postoperative day 28, at which time the sympathetic sprouting became evident in the DRG. No heat hyperalgesia was presented in pup rats at any time point. There was a late expression of glial p75NTR in DRG of pups from postoperative day 28, which was parallel to the occurrence of sympathetic sprouting. The expression of TrkA did not show such a postoperative syncing change.

Conclusion

The delayed-onset mechanical allodynia in the infant nerve lesion was accompanied with parallel sympathetic sprouting in DRG. The late parallel expression of glial p75NTR injury may play an essential role in this process, which provides novel insight into the treatment of delayed adolescent neuropathic pain.

Dim Light at Night Exposure Induces Cold Hyperalgesia and Mechanical Allodynia in Male Mice

The growing presence of artificial lighting across the globe presents a number of challenges to human and ecological health despite its societal benefits. Exposure to artificial light at night, a seemingly innocuous aspect of modern life, disrupts behavior and physiological functions. Specifically, light at night induces neuroinflammation, which is implicated in neuropathic and nociceptive pain states, including hyperalgesia and allodynia. Because of its influence on neuroinflammation, we investigated the effects of dim light at night exposure on pain responsiveness in male mice. In this study, mice exposed to four days of dim (5 lux) light at night exhibited cold hyperalgesia. Further, after 28 days of exposure, mice exhibited both cold hyperalgesia and mechanical allodynia. No heat/hot hyperalgesia was observed in this experiment. Altered nociception in mice exposed to dim light at night was concurrent with upregulated interleukin-6 and nerve growth factor mRNA expression in the medulla and elevated μ-opioid receptor mRNA expression in the periaqueductal gray region of the brain. The current results support the relationship between disrupted circadian rhythms and altered pain sensitivity. In summary, we observed that dim light at night induces cold hyperalgesia and mechanical allodynia, potentially through elevated neuroinflammation and dysregulation of the endogenous opioid system.

Advances in the diagnosis and treatment of dry eye

The core mechanism of dry eye is the tear film instability. Tear film-oriented diagnosis (TFOD) is a concept to clarify the cause of tear film instability by tear film, and tear film-oriented treatment (TFOT) is a concept to treat dry eye disease by replacing the lacking components of the tear film layer based on the TFOD. In TFOD, the fluorescein breakup pattern of the tear film is important, and the subtype of dry eye can be judged to some extent from the breakup patterns. Current noninvasive devices related to the dynamic analysis of the tear film and visual acuity enabled the diagnosis of dry eye, subtype analysis, and the extent of severity. In Asian countries, secretagogues represent the main treatment in TFOT. Since meibomian gland dysfunction is a factor that greatly affects the tear breakup time, its treatment is also essential in the dry eye treatment strategy. A newly discovered dry eye subtype is the short breakup time-type (BUT) of dry eye. The only abnormal finding in this disease is the short BUT, suggesting a relationship with ocular neuropathic pain and eye strain. Recently, data from many studies have accumulated which show that dry eye is a life-style disease. In addition to the treatment of dry eyes, it is becoming possible to prevent the onset by intervening with the daily habits, diet, exercise and sleep, etc. It has been pointed out that oxidative stress is also involved in the pathology of dry eye, and intervention is being carried out by improving diet and taking supplements. Future research will be needed to link clinical findings to the molecular biological findings in the tear film.

Nerve growth factor gene therapy improves bone marrow sensory innervation and nociceptor-mediated stem cell release in a mouse model of type 1 diabetes with limb ischaemia

AIMS/HYPOTHESIS

Sensory neuropathy is common in people with diabetes; neuropathy can also affect the bone marrow of individuals with type 2 diabetes. However, no information exists on the state of bone marrow sensory innervation in type 1 diabetes. Sensory neurons are trophically dependent on nerve growth factor (NGF) for their survival. The aim of this investigation was twofold: (1) to determine if sensory neuropathy affects the bone marrow in a mouse model of type 1 diabetes, with consequences for stem cell liberation after tissue injury; and (2) to verify if a single systemic injection of the NGF gene exerts long-term beneficial effects on these phenomena.

METHODS

A mouse model of type 1 diabetes was generated in CD1 mice by administration of streptozotocin; vehicle was administered to non-diabetic control animals. Diabetic animals were randomised to receive systemic gene therapy with either human NGF or β-galactosidase. After 13 weeks, limb ischaemia was induced in both groups to study the recovery post injury. When the animals were killed, samples of tissue and peripheral blood were taken to assess stem cell mobilisation and homing, levels of substance P and muscle vascularisation. An in vitro cellular model was adopted to verify signalling downstream to human NGF and related neurotrophic or pro-apoptotic effects. Normally distributed variables were compared between groups using the unpaired Student's t test and non-normally distributed variables were assessed by the Wilcoxon-Mann-Whitney test. The Fisher's exact test was employed for categorical variables.

RESULTS

Immunohistochemistry indicated a 3.3-fold reduction in the number of substance P-positive nociceptive fibres in the bone marrow of type 1 diabetic mice (p < 0.001 vs non-diabetic). Moreover, diabetes abrogated the creation of a neurokinin gradient which, in non-diabetic mice, favoured the mobilisation and homing of bone-marrow-derived stem cells expressing the substance P receptor neurokinin 1 receptor (NK1R). Pre-emptive gene therapy with NGF prevented bone marrow denervation, contrasting with the inhibitory effect of diabetes on the mobilisation of NK1R-expressing stem cells, and restored blood flow recovery from limb ischaemia. In vitro hNGF induced neurite outgrowth and exerted anti-apoptotic actions on rat PC12 cells exposed to high glucose via activation of the canonical neurotrophic tyrosine kinase receptor type 1 (TrkA) signalling pathway.

CONCLUSIONS/INTERPRETATION

This study shows, for the first time, the occurrence of sensory neuropathy in the bone marrow of type 1 diabetic mice, which translates into an altered modulation of substance P and depressed release of substance P-responsive stem cells following ischaemia. NGF therapy improves bone marrow sensory innervation, with benefits for healing on the occurrence of peripheral ischaemia. Nociceptors may represent a new target for the treatment of ischaemic complications in diabetes.

Effects of Neurotrophic Factors in Glial Cells in the Central Nervous System: Expression and Properties in Neurodegeneration and Injury

Astrocytes, oligodendrocytes, and microglia are abundant cell types found in the central nervous system and have been shown to play crucial roles in regulating both normal and disease states. An increasing amount of evidence points to the critical importance of glia in mediating neurodegeneration in Alzheimer’s and Parkinson’s diseases (AD, PD), and in ischemic stroke, where microglia are involved in initial tissue clearance, and astrocytes in the subsequent formation of a glial scar. The importance of these cells for neuronal survival has previously been studied in co-culture experiments and the search for neurotrophic factors (NTFs) initiated after finding that the addition of conditioned media from astrocyte cultures could support the survival of primary neurons in vitro. This led to the discovery of the potent dopamine neurotrophic factor, glial cell line-derived neurotrophic factor (GDNF). In this review, we focus on the relationship between glia and NTFs including neurotrophins, GDNF-family ligands, CNTF family, and CDNF/MANF-family proteins. We describe their expression in astrocytes, oligodendrocytes and their precursors (NG2-positive cells, OPCs), and microglia during development and in the adult brain. Furthermore, we review existing data on the glial phenotypes of NTF knockout mice and follow NTF expression patterns and their effects on glia in disease models such as AD, PD, stroke, and retinal degeneration.

Efficacy of autologous serum tears for treatment of neuropathic corneal pain

OBJECTIVE

Corneal nerve damage may result in neuropathic corneal pain (NCP). Autologous serum tears (AST) have been shown to results in nerve regeneration and may help alleviate corneal pain. This study aimed to evaluate the efficacy of AST in the treatment of NCP.

METHODS

This was a retrospective case-control study. Sixteen patients suffering from severe NCP and no current ocular surface disease were compared to 12 controls. In vivo confocal microscopy (IVCM) (HRT3/RCM; Heidelberg Engineering GmbH, Germany) of the central corneas was performed bilaterally. Change in pain severity (scale of 0-10), corneal nerve density, tortuosity, reflectivity and presence of beading and micro-neuromas before and after treatment were recorded.

RESULTS

All patients had severe pain, with a mean of 9.1 ± 0.2 (range 8-10). Subbasal nerves were significantly decreased before treatment as compared to controls, including total nerve length (10,935.5 ± 1264.3 vs. 24,714.4 ± 1056.2 μm/mm²; p < 0.0001) and total number of nerves (10.5 ± 1.4 vs. 28.6 ± 2.0; p < 0.0001), respectively. Morphologically, significantly increased reflectivity (2.9 ± 0.2 vs. 1.2 ± 0.1; p = 0.00008) and tortuosity (2.4 ± 0.2 vs. 1.7 ± 0.1; p = 0.001), both graded on a scale of 0-4, were noted. After a mean of 3.8 ± 0.5 months (range 1-8 months) of AST treatment, pain severity decreased to 3.1 ± 0.3 (range 0-4), (p < 0.0001). Further, IVCM demonstrated a significant improvement (p < 0.005) in total nerve length (17,351.3 ± 1395.6  μm/mm²) and number (15.1 ± 1.6), as well as significant decrease in reflectivity (2.4 ± 0.2; p = 0.001) and tortuosity (2.2 ± 0.2; p = 0.001).

CONCLUSION

IVCM demonstrates underlying alterations of the subbasal corneal nerve plexus in patients suffering from debilitating NCP. AST-induced nerve regeneration is seen following treatment with AST, which correlates with improvement in patient symptoms of NCP.

Brain-region specific responses of astrocytes to an in vitro injury and neurotrophins

Astrocytes are a heterogeneous population of glial cells that react to brain insults through a process referred to as astrogliosis. Reactive astrocytes are characterized by an increase in proliferation, size, migration to the injured zone and release of a plethora of chemical mediators such as NGF and BDNF. The aim of this study was to determine whether there are brain region-associated responses of astrocytes to an injury and to the neurotrophins NGF and BDNF. We used the scratch injury model to study the closure of a wound inflicted on a monolayer of astrocytes obtained from cortex, hippocampus or striatum. Our results indicate that the response of astrocytes to a mechanical lesion differ according to brain regions. Astrocytes from the striatum proliferate and repopulate the injury site more rapidly than astrocytes from cortex or hippocampus. We found that the scratch injury induced the upregulation of neurotrophin receptor p75NTR and TrkB.t in astrocytes from all brain regions studied. When astrocytes from all regions were treated with NGF, the neurotrophin induced migration of the astrocytes (assessed in Boyden chambers) and induced wound closure but did not affect proliferation. In contrast, BDNF induced wound closure but only in astrocytes from striatum. Our overall findings show the heterogeneity in astrocyte functions based on their brain region of origin, and how this functional diversity may determine their responses to an injury and to neurotrophins.

Neuropathic Corneal Pain: Approaches for Management

Neuropathic pain is caused by a primary lesion or dysfunction of the nervous system and can occur in the cornea. However, neuropathic corneal pain (NCP) is currently an ill-defined disease. Patients with NCP are extremely challenging to manage, and evidence-based clinical recommendations for the management of patients with NCP are scarce. The objectives of this review are to provide guidelines for diagnosis and treatment of patients with NCP and to summarize current evidence-based literature in this area. We performed a systematic literature search of all relevant publications between 1966 and 2017. Treatment recommendations are, in part, based on methodologically sound randomized controlled trials (RCTs), demonstrating superiority to placebo or relevant control treatments, and on the consistency of evidence, degree of efficacy, and safety. In addition, the recommendations include our own extensive experience in the management of these patients over the past decade. A comprehensive algorithm, based on clinical evaluation and complementary tests, is presented for diagnosis and subcategorization of patients with NCP. Recommended first-line topical treatments include neuroregenerative and anti-inflammatory agents, and first-line systemic pharmacotherapy includes tricyclic antidepressants and an anticonvulsant. Second-line oral treatments recommended include an opioid-antagonist and opiate analgesics. Complementary and alternative treatments, such as cardiovascular exercise, acupuncture, omega-3 fatty acid supplementation, and gluten-free diet, may have additional benefits, as do potential noninvasive and invasive procedures in recalcitrant cases. Medication selection should be tailored on an individual basis, considering side effects, comorbidities, and levels of peripheral and centralized pain. Nevertheless, there is an urgent need for long-term studies and RCTs assessing the efficacy of treatments for NCP.

Can Wound Exudate from Venous Leg Ulcers Measure Wound Pain Status?: A Pilot Study

We investigated the associations between the self-evaluated pain status and two pain biomarker candidates, nerve growth factor and S100A8/A9, in exudate from venous leg ulcer to finally develop an objective pain evaluation method. Patients with venous leg ulcer participated in this cross-sectional observational study conducted between April and October 2014 at two medical facilities. During routine wound care, each participant self-evaluated their pain status at each examination using the 10-point numerical rating scale (present pain intensity) and the short-form McGill Pain Questionnaire 2 (continuous pain, intermittent pain, neuropathic pain, affective descriptors, and total score). Venous leg ulcer exudate sample was collected after wound cleansing. The nerve growth factor and S100A8/A9 concentrations in the venous leg ulcer exudate were measured by enzyme-linked immunosorbent assay and standardized according to the wound area. The association between each pain status and the two standardized protein concentrations was evaluated using Spearman’s correlation coefficient. In 30 sample collected from 13 participants, the standardized nerve growth factor concentration was negatively correlated with continuous pain (ρ = -0.47, P = 0.01), intermittent pain (ρ = -0.48, P = 0.01), neuropathic pain (ρ = -0.51, P = 0.01), and total score (ρ = -0.46, P = 0.01). The standardized S100A8/A9 concentration was positively correlated with present pain intensity (ρ = 0.46, P = 0.03) and continuous pain (ρ = 0.48, P = 0.03). Thus, these two proteins may be useful for objective evaluation of wound pain in venous leg ulcer patients.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

Transcriptional expression of inflammatory mediators in various somatosensory relay centers in the brain of rat models of peripheral mononeuropathy and local inflammation

Contradictory results have been reported regarding the role of inflammatory mediators in the central nervous system in mediating neuropathic pain and inflammatory hyperalgesia following peripheral nerve injury or localized inflammation. The present study aims to correlate between the mRNA expression and protein secretion of proinflammatory cytokines and nerve growth factor (NGF), in the dorsal root ganglia (DRGs), spinal cord, brainstem and thalamus, and pain-related behavior in animal models of peripheral mononeuropathy and localized inflammation. Different groups of rats (n=8, each) were subjected to either lesion of the nerves of their hindpaws to induce mononeuropathy or intraplantar injection of endotoxin (ET) and were sacrificed at various time intervals. TNF-α, IL-1β and NGF mRNA expression and protein levels in the various centers involved in processing nociceptive information were determined, by RT-PCR and ELISA. Control groups were either subjected to sham surgery or to saline injection. Mononeuropathy and ET injection produced significant and sustained increases in the mRNA expression and protein levels of TNF-α, IL-1β and NGF in the ipsilateral and contralateral DRGs, spinal cord, and brainstem. No significant and consistent changes in the mRNA expression of cytokines were noticed in the thalamus, while a downregulation of the NGF-mRNA level was observed. The temporal and spatial patterns of the observed changes in mRNA expression of cytokines and NGF are not closely in phase with the observed allodynia and hyperalgesia in the different models, suggesting that the role of these mediators may not be reduced exclusively to the production and maintenance of pain.

Astrocytes and Microglia-Mediated Immune Response in Maladaptive Plasticity is Differently Modulated by NGF in the Ventral Horn of the Spinal Cord Following Peripheral Nerve Injury

Reactive astrocytes and activated microglia are the key players in several pathophysiologic modifications of the central nervous system. We used the spared nerve injury (SNI) of the sciatic nerve to induce glial maladaptive response in the ventral horn of lumbar spinal cord and examine its role in the remodeling of the tripartite synapse plasticity. Imaging the ventral horn revealed that SNI was associated with both an early microglial and astrocytic activation, assessed, respectively, by analysis of Iba1 and GFAP expression. Microglia, in particular, localized peculiarly surrounding the motor neurons somata. Perineuronal astrocytes, which play a key role in maintaining the homeostasis of neuronal circuitry, underwent a substantial phenotypic change following peripheral axotomy, producing reactive gliosis. The gliosis was associated with the reduction of glial aminoacid transporters (GLT1 and GlyT1) and increase of neuronal glutamate transporter EAAC1. Although the expression of GABAergic neuronal marker GAD65/67 showed no change, glutamate increase, as demonstrated by HPLC analysis, shifted the excitatory/inhibitory balance as showed by the net increase of the glutamate/GABA ratio. Moreover, endogenous NGF levels were altered in SNI animals and not restored by the intrathecal NGF administration. This treatment reverted phenotypic changes associated with reactive astrocytosis, but failed to modify microglia activation. These findings on one hand confirm the correlation between gliopathy and maladaptive plasticity of the spinal synaptic circuitry, on the other hand add new data concerning the complex peculiar behavior of different glial cells in neuronal degenerative processes, defining a special role of microglia in sustaining the inflammatory response.

Changes in the Expressions of Iba1 and Calcitonin Gene-Related Peptide in Adjacent Lumbar Spinal Segments after Lumbar Disc Herniation in a Rat Model

Lumbar disc herniation is commonly encountered in clinical practice and can induce sciatica due to mechanical and/or chemical irritation and the release of proinflammatory cytokines. However, symptoms are not confined to the affected spinal cord segment. The purpose of this study was to determine whether multisegmental molecular changes exist between adjacent lumbar spinal segments using a rat model of lumbar disc herniation. Twenty-nine male Sprague-Dawley rats were randomly assigned to either a sham-operated group (n=10) or a nucleus pulposus (NP)-exposed group (n=19). Rats in the NP-exposed group were further subdivided into a significant pain subgroup (n=12) and a no significant pain subgroup (n=7) using mechanical pain thresholds determined von Frey filaments. Immunohistochemical stainings of microglia (ionized calcium-binding adapter molecule 1; Iba1), astrocytes (glial fibrillary acidic protein; GFAP), calcitonin gene-related peptide (CGRP), and transient receptor potential vanilloid 1 (TRPV1) was performed in spinal dorsal horns and dorsal root ganglions (DRGs) at 10 days after surgery. It was found immunoreactivity for Iba1-positive microglia was higher in the L5 (P=0.004) dorsal horn and in the ipsilateral L4 (P=0.009), L6 (P=0.002), and S1 (P=0.002) dorsal horns in the NP-exposed group than in the sham-operated group. The expression of CGRP was also significantly higher in ipsilateral L3, L4, L6, and S1 segments and in L5 DRGs at 10 days after surgery in the NP-exposed group than in the sham-operated group (P<0.001). Our results indicate that lumbar disc herniation upregulates microglial activity and CGRP expression in many adjacent and ipsilateral lumbar spinal segments.

Neurotrophins and Neuropathic Pain: Role in Pathobiology

Neurotrophins (NTs) belong to a family of trophic factors that regulate the survival, growth and programmed cell death of neurons. In mammals, there are four structurally and functionally related NT proteins, viz. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 and neurotrophin 4. Most research on NTs to date has focussed on the effects of NGF and BDNF signalling via their respective cognate high affinity neurotrophic tyrosine kinase viz TrkA and TrkB receptors. Apart from the key physiologic roles of NGF and BDNF in peripheral and central nervous system function, NGF and BDNF signalling via TrkA and TrkB receptors respectively have been implicated in mechanisms underpinning neuropathic pain. Additionally, NGF and BDNF signalling via the low-affinity pan neurotrophin receptor at 75 kDa (p75NTR) may also contribute to the pathobiology of neuropathic pain. In this review, we critically assess the role of neurotrophins signalling via their cognate high affinity receptors as well as the low affinity p75NTR in the pathophysiology of peripheral neuropathic and central neuropathic pain. We also identify knowledge gaps to guide future research aimed at generating novel insight on how to optimally modulate NT signalling for discovery of novel therapeutics to improve neuropathic pain relief.

Autologous Serum Tears for Treatment of Photoallodynia in Patients with Corneal Neuropathy: Efficacy and Evaluation with In Vivo Confocal Microscopy

OBJECTIVE

Patients suffering from corneal neuropathy may present with photoallodynia; i.e., increased light sensitivity, frequently with a normal slit-lamp examination. This study aimed to evaluate the efficacy of autologous serum tears (AST) for treatment of severe photoallodynia in corneal neuropathy and to correlate clinical findings with corneal subbasal nerve alterations by in vivo confocal microscopy (IVCM).

METHODS

Retrospective case control study with 16 patients with neuropathy-induced severe photoallodynia compared to 16 normal controls. Symptom severity, clinical examination and bilateral corneal IVCM scans were recorded.

RESULTS

All patients suffered from extreme photoallodynia (8.8±1.1) with no concurrent ocular surface disease. Subbasal nerves were significantly decreased at baseline in patients compared to controls; total nerve length (9208±1264 vs 24714±1056 μm/mm(2); P<.0001) and total nerve number (9.6±1.4 vs 28.6±2.0; P<.0001), respectively. Morphologically, significantly increased reflectivity (2.9±0.2 vs 1.8±0.1; P<.0001), beading (in 93.7%), and neuromas (in 62.5%) were seen. AST (3.6±2.1 months) resulted in significantly decreased symptom severity (1.6±1.7; P=.02). IVCM demonstrated significantly improved nerve parameters (P<.005), total nerve length (15451±1595 μm/mm(2)), number (13.9±2.1), and reflectivity (1.9±0.1). Beading and neuromas were seen in only 56.2% and 7.6% of patients.

CONCLUSION

Patients with corneal neuropathy-induced photoallodynia show profound alterations in corneal nerves. AST restores nerve topography through nerve regeneration, and this correlated with improvement in patient-reported photoallodynia. The data support the notion that corneal nerve damage results in alterations in afferent trigeminal pathways to produce photoallodynia.

Purinergic Modulation of Spinal Neuroglial Maladaptive Plasticity Following Peripheral Nerve Injury

Modulation of spinal reactive gliosis following peripheral nerve injury (PNI) is a promising strategy to restore synaptic homeostasis. Oxidized ATP (OxATP), a nonselective antagonist of purinergic P2X receptors, was found to recover a neuropathic behavior following PNI. We investigated the role of intraperitoneal (i.p.) OxATP treatment in restoring the expression of neuronal and glial markers in the mouse spinal cord after sciatic spared nerve injury (SNI). Using in vivo two-photon microscopy, we imaged Ca(2+) transients in neurons and astrocytes of the dorsal horn of spinal cord at rest and upon right hind paw electrical stimulation in sham, SNI, and OxATP-treated mice. Neuropathic behavior was investigated by von Frey and thermal plantar test. Glial [glial fibrillary acidic protein (GFAP), ionized calcium-binding adaptor molecule 1 (Iba1)] and GABAergic [vesicular GABA transporter (vGAT) and glutamic acid decarboxylase 65/76 (GAD65/67)] markers and glial [glutamate transporter (GLT1) and GLAST] and neuronal amino acid [EAAC1, vesicular glutamate transporter 1 (vGLUT1)] transporters have been evaluated. In SNI mice, we found (i) increased glial response, (ii) decreased glial amino acid transporters, and (iii) increased levels of neuronal amino acid transporters, and (iv) in vivo analysis of spinal neurons and astrocytes showed a persistent increase of Ca(2+) levels. OxATP administration reduced glial activation, modulated the expression of glial and neuronal glutamate/GABA transporters, restored neuronal and astrocytic Ca(2+) levels, and prevented neuropathic behavior. In vitro studies validated that OxATP (i) reduced levels of reactive oxygen species (ROS), (ii) reduced astrocytic proliferation, (iii) increase vGLUT expression. All together, these data support the correlation between reactive gliosis and perturbation of the spinal synaptic homeostasis and the role played by the purinergic system in modulating spinal plasticity following PNI.

Fulranumab for treatment of diabetic peripheral neuropathic pain: A randomized controlled trial

OBJECTIVE

To assess efficacy and safety of fulranumab, a fully human monoclonal antibody against nerve growth factor, in patients with diabetic peripheral neuropathic pain (DPNP).

METHODS

In this phase II, double-blind, placebo-controlled trial, patients with moderate to severe DPNP were randomized to treatments with fulranumab (1, 3, or 10 mg) or placebo administered subcutaneously every 4 weeks.

RESULTS

Because of early study termination (clinical hold) by the US Food and Drug Administration, 77 (intent-to-treat) of the planned 200 patients were enrolled. The primary endpoint, the mean reduction of average daily pain at week 12 compared with baseline, showed a positive dose-response relationship (p = 0.014, 1-sided); the pair-wise comparison between the 10-mg group and placebo was significant (unadjusted p = 0.040, 2-sided). An exploratory responder analysis revealed that a greater proportion of patients in the 10-mg group reported ≥30% reduction in the average DPNP intensity compared with placebo at week 12 (p = 0.006). Although not statistically significant, several secondary endpoints showed directionally similar results to the primary efficacy dose-response relationship. During the combined efficacy and safety extension phases, the top 3 treatment-emergent adverse events in the combined fulranumab group were arthralgia (11%), edema peripheral (11%), and diarrhea (9%). No cases of joint replacement or death were reported.

CONCLUSION

Despite early study termination, fulranumab treatment resulted in dose-dependent efficacy and was generally well tolerated.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that in patients with DPNP, fulranumab 10 mg reduces pain by 1.2 points on an 11-point scale compared with placebo.

Induction of a reactive state in perineuronal satellite glial cells akin to that produced by nerve injury is linked to the level of p75NTR expression in adult sensory neurons

Satellite glial cells (SGCs) surrounding primary sensory neurons are similar to astrocytes of the central nervous system in that they buffer the extracellular environment via potassium and calcium channels and express the intermediate filament glial fibrillary acidic protein (GFAP). Peripheral nerve injury induces a reactive state in SGCs that includes SGC proliferation, increased SGC/SGC coupling via gap junctions, decreased inward rectifying potassium channel 4.1 (Kir 4.1) expression and increased expression of GFAP and the common neurotrophin receptor, p75NTR. In contrast, neuronal p75NTR expression, normally detected in ∼80% of adult rat sensory neurons, decreases in response to peripheral axotomy. Given the differential regulation of p75NTR expression in neurons versus SGCs with injury, we hypothesized that reduced signaling via neuronal p75NTR contributes to the induction of a reactive state in SGCs. We found that reducing neuronal p75NTR protein expression in uninjured sensory neurons by intrathecal subarachnoid infusion of p75NTR-selective anti-sense oligodeoxynucleotides for one week was sufficient to induce a "reactive-like" state in the perineuronal SGCs akin to that normally observed following peripheral nerve injury. This reactive state included significantly increased SGC p75NTR, GFAP and gap junction protein connexin-43 protein expression, increased numbers of SGCs surrounding individual sensory neurons and decreased SGC Kir 4.1 channel expression. Collectively, this supports the tenet that reductions in target-derived trophic support leading to, or as a consequence of, reduced neuronal p75NTR expression plays a critical role in switching the SGC to a reactive state.

Astrocyte-neuron interplay in maladaptive plasticity

The complexity of neuronal networks cannot only be explained by neuronal activity so neurobiological research in the last decade has focused on different components of the central nervous system: the glia. Glial cells are fundamental elements for development and maintenance of physiological brain work. New data confirm that glia significantly influences neuronal communication through specific molecules, named "gliotransmitters", and their related receptors. This new approach to the traditional model of the way synapses work is also supported by changes occurring in pathological conditions, such as neurodegenerative diseases or toxic/traumatic injury to nervous system. Experimental models have revealed that glial cells are the starting point of damage progression that subsequently involves neurons. The "bedside to bench" approach has demonstrated that clinical phenotypes are strictly related to neuronal death, however it is conceivable that the disease begins earlier, years before clinical onset. This temporal gap is necessary to determine complex changes in the neuro-glial network organization and produce a "maladaptive plasticity". We review the function of glial cells in health and disease, pointing the putative mechanisms of maladaptive plasticity, suggesting that glial cells may represent a fascinating therapeutic target to prevent irreversible neuronal cell death.

Role of nerve growth factor-tyrosine kinase receptor A signaling in paclitaxel-induced peripheral neuropathy in rats

The mechanisms underlying paclitaxel-induced peripheral neuropathy remain unknown. Nerve growth factor (NGF) is a representative neurotrophic factor that maintains neuronal function, promotes survival, and mediates neuropathic pain. We investigated expression levels of NGF and its receptors in the dorsal root ganglia (DRG) and spinal dorsal horn (DH) following paclitaxel treatment. Intraperitoneal (I.P.) administration of paclitaxel induced significant mechanical hypersensitivity and cold allodynia in rats, significantly increased the expression of NGF and its receptor tyrosine kinase receptor A (trkA) in the DRG, and increased NGF expression in the DH. In contrast, paclitaxel treatment did not alter the mRNA levels of NGF or its receptors in the DRG, DH, sciatic nerve, or hindpaw skin. Moreover, expression of NEDD4-2, a negative regulator of trkA, was significantly increased in the DRG of paclitaxel-treated rats. Intrathecal (I.T.) administration of the tyrosine kinase receptor inhibitor k252a significantly alleviated mechanical hypersensitivity in paclitaxel-treated rats. Our results suggest that NGF-trkA signaling is involved in mechanical allodynia in paclitaxel-induced neuropathy.

Satellite glial cell proliferation in the trigeminal ganglia after chronic constriction injury of the infraorbital nerve

We have examined satellite glial cell (SGC) proliferation in trigeminal ganglia following chronic constriction injury of the infraorbital nerve. Using BrdU labeling combined with immunohistochemistry for SGC specific proteins we positively confirmed proliferating cells to be SGCs. Proliferation peaks at approximately 4 days after injury and dividing SGCs are preferentially located around neurons that are immunopositive for ATF-3, a marker of nerve injury. After nerve injury there is an increase GFAP expression in SGCs associated with both ATF-3 immunopositive and immunonegative neurons throughout the ganglia. SGCs also express the non-glial proteins, CD45 and CD163, which label resident macrophages and circulating leukocytes, respectively. In addition to SGCs, we found some Schwann cells, endothelial cells, resident macrophages, and circulating leukocytes were BrdU immunopositive.

Efficacy, safety, and tolerability of fulranumab, an anti-nerve growth factor antibody, in the treatment of patients with moderate to severe osteoarthritis pain

Nerve growth factor (NGF) is increased in chronic pain conditions. This study examined analgesic efficacy and safety of fulranumab, a fully human monoclonal anti-NGF antibody, in adults with chronic osteoarthritis pain. Patients (n=466, intent-to-treat) were randomized to receive, in addition to their current pain therapy, subcutaneous injections in 1 of 6 parallel treatment groups: placebo (n=78), fulranumab 1 mg (n=77) or 3 mg (n=79) every 4 weeks (Q4wk), 3 mg (n=76), 6 mg (n=78), or 10 mg (n=78) every 8 weeks (Q8wk). Primary efficacy results showed that fulranumab significantly reduced the average pain intensity score (P < or = 0.030) from baseline to week 12 compared with placebo in the 3mgQ4wk, 6mgQ8wk, and 10mgQ8wk groups. Secondary efficacy outcomes indicated that significant improvement occurred compared with placebo at week 12 on the Western Ontario and McMaster Universities Osteoarthritis Index subscales of pain, stiffness, and physical function (P < 0.040) across all fulranumab groups except 1mgQ4wk, on the Brief Pain Inventory-Short Form subscales of pain intensity (P < or = 0.016) and pain interference (P < or = 0.030) in the 3mgQ4wk and 10mgQ8wk groups, and on the Patient Global Assessment score (P < or = 0.040) in the 3mgQ4wk, 6mgQ8wk, and 10mgQ8wk groups. The most common (> or = 5% of patients) treatment-emergent adverse events in overall fulranumab groups during the first 12weeks included paresthesia (7%), headache (5%), and nasopharyngitis (5%). Most neurologic-related treatment-emergent adverse events were mild or moderate and resolved at the end of week 12. Serious adverse events occurred in 3 patients, but they were not neurologically related and resolved before study completion. Fulranumab treatment resulted in statistically significant efficacy in pain measures and physical function versus placebo and was generally well tolerated.

Swim therapy reduces mechanical allodynia and thermal hyperalgesia induced by chronic constriction nerve injury in rats

OBJECTIVE

Neuropathic pain is common and often difficult to treat because it generally does not respond well to the currently available pain medications or nerve blocks. Recent studies in both humans and animals have suggested that exercise may induce a transient analgesia and reduce acute pain in normal healthy individuals. We examined whether swim therapy could alleviate neuropathic pain in rats.

DESIGN

Rats were trained to swim over a 2-week period in warm water. After the rats were trained, neuropathic pain was induced by constricting the right sciatic nerve, and regular swimming was resumed. The sensitivity of each hind paw was monitored using the Hargreaves test and von Frey test to evaluate the withdrawal response thresholds to heat and touch.

RESULTS

The paw ipsilateral to the nerve ligation expressed pain-like behaviors including thermal hyperalgesia and mechanical allodynia. Regular swim therapy sessions significantly reduced the mechanical allodynia and thermal hyperalgesia. Swim therapy had little effect on the withdrawal thresholds for the contralateral paw. In addition, swim therapy alone did not alter the thermal or mechanical thresholds of normal rats.

CONCLUSIONS

The results suggest that regular exercise, including swim therapy, may be an effective treatment for neuropathic pain caused by nerve injuries. This study, showing that swim therapy reduces neuropathic pain behavior in rats, provides a scientific rationale for clinicians to test the efficacy of exercise in the management of neuropathic pain. It may prove to be a safe and cost-effective therapy in a variety of neuropathic pain states.

Double gene therapy with granulocyte colony-stimulating factor and vascular endothelial growth factor acts synergistically to improve nerve regeneration and functional outcome after sciatic nerve injury in mice

Peripheral-nerve injuries are a common clinical problem and often result in long-term functional deficits. Reconstruction of peripheral-nerve defects is currently undertaken with nerve autografts. However, there is a limited availability of nerves that can be sacrificed and the functional recovery is never 100% satisfactory. We have previously shown that gene therapy with vascular endothelial growth factor (VEGF) significantly improved nerve regeneration, neuronal survival, and muscle activity. Our hypothesis is that granulocyte colony-stimulating factor (G-CSF) synergizes with VEGF to improve the functional outcome after sciatic nerve transection. The left sciatic nerves and the adjacent muscle groups of adult mice were exposed, and 50 or 100 μg (in 50 μl PBS) of VEGF and/or G-CSF genes was injected locally, just below the sciatic nerve, and transferred by electroporation. The sciatic nerves were transected and placed in an empty polycaprolactone (PCL) nerve guide, leaving a 3-mm gap to challenge nerve regeneration. After 6 weeks, the mice were perfused and the sciatic nerve, the dorsal root ganglion (DRG), the spinal cord and the gastrocnemius muscle were processed for light and transmission electron microscopy. Treated animals showed significant improvement in functional and histological analyses compared with the control group. However, the best results were obtained with the G-CSF+VEGF-treated animals: quantitative analysis of regenerated nerves showed a significant increase in the number of myelinated fibers and blood vessels, and the number of neurons in the DRG and motoneurons in the spinal cord was significantly higher. Motor function also showed that functional recovery occurred earlier in animals receiving G-CSF+VEGF-treatment. The gastrocnemius muscle showed an increase in weight and in the levels of creatine phosphokinase, suggesting an improvement of reinnervation and muscle activity. These results suggest that these two factors acted synergistically and optimized the nerve repair potential, improving regeneration after a transection lesion.

Calcium imaging of living astrocytes in the mouse spinal cord following sensory stimulation

Astrocytic Ca²⁺ dynamics have been extensively studied in ex vivo models; however, the recent development of two-photon microscopy and astrocyte-specific labeling has allowed the study of Ca²⁺ signaling in living central nervous system. Ca²⁺ waves in astrocytes have been described in cultured cells and slice preparations, but evidence for astrocytic activation during sensory activity is lacking. There are currently few methods to image living spinal cord: breathing and heart-beating artifacts have impeded the widespread application of this technique. We here imaged the living spinal cord by two-photon microscopy in C57BL6/J mice. Through pressurized injection, we specifically loaded spinal astrocytes using the red fluorescent dye sulforhodamine 101 (SR101) and imaged astrocytic Ca²⁺ levels with Oregon-Green BAPTA-1 (OGB). Then, we studied astrocytic Ca²⁺ levels at rest and after right electrical hind paw stimulation. Sensory stimulation significantly increased astrocytic Ca²⁺ levels within the superficial dorsal horn of the spinal cord compared to rest. In conclusion, in vivo morphofunctional imaging of living astrocytes in spinal cord revealed that astrocytes actively participate to sensory stimulation.

Nerve growth factor induces cell cycle arrest of astrocytes

Neurotrophins can influence multiple cellular functions depending on the cellular context and the specific receptors they interact with. These neurotrophic factors have been extensively studied for their ability to support neuronal survival via Trk receptors and to induce apoptosis via the p75(NTR). However, the p75(NTR) is also detected on cell populations that do not undergo apoptosis in response to neurotrophins. In particular, the authors have detected p75(NTR) expression on astrocytes during development and after seizure-induced injury. In this study, the authors investigated the role of Nerve growth factor (NGF) in regulating astrocyte proliferation and in influencing specific aspects of the cell cycle. The authors have demonstrated that NGF prevents the induction of cyclins and their association with specific cyclin-dependent kinases, and thereby prevents progression through the G1 phase of the cell cycle. Since the authors have previously shown that p75(NTR) but not TrkA, is expressed in astrocytes, these data suggest that activation of p75(NTR) promotes withdrawal of astrocytes from the cell cycle, which may have important consequences during development and after injury.

Role of neurotrophins in neuropathic pain

Neurotrophins (NTs) belong to a family of structurally and functionally related proteins, they are the subsets of neurotrophic factors. Neurotrophins are responsible for diverse actions in the developing peripheral and central nervous systems. They are important regulators of neuronal function, affecting neuronal survival and growth. They are able to regulate cell death and survival in development as well as in pathophysiologic states. NTs and their receptors are expressed in areas of the brain that undergo plasticity, indicating that they are able to modulate synaptic plasticity.

Recently, neurotrophins have been shown to play significant roles in the development and transmission of neuropathic pain. Neuropathic pain is initiated by a primary lesion or dysfunction in the nervous system. It has a huge impact on the quality of life. It is debilitating and often has an associated degree of depression that contributes to decreasing human well being. Neuropathic pain ranks at the first place for sanitary costs.

Neuropathic pain treatment is extremely difficult. Several molecular pathways are involved, making it a very complex disease. Excitatory or inhibitory pathways controlling neuropathic pain development show altered gene expression, caused by peripheral nerve injury. At present there are no valid treatments over time and neuropathic pain can be classified as an incurable disease.

Nowadays, pain research is directing towards new molecular methods. By targeting neurotrophin molecules it may be possible to provide better pain control than currently available.