Nerve growth factor induces facial heat hyperalgesia and plays a role in trigeminal neuropathic pain in rats

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Differential roles of NMDAR subunits 2A and 2B in mediating peripheral and central sensitization contributing to orofacial neuropathic pain

The spinal N-methyl-d-aspartate receptor (NMDAR), particularly their subtypes NR2A and NR2B, plays pivotal roles in neuropathic and inflammatory pain. However, the roles of NR2A and NR2B in orofacial pain and the exact molecular and cellular mechanisms mediating nervous system sensitization are still poorly understood. Here, we exhaustively assessed the regulatory effect of NMDAR in mediating peripheral and central sensitization in orofacial neuropathic pain. Von-Frey filament tests showed that the inferior alveolar nerve transection (IANX) induced ectopic allodynia behavior in the whisker pad of mice. Interestingly, mechanical allodynia was reversed in mice lacking NR2A and NR2B. IANX also promoted the production of peripheral sensitization-related molecules, such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, brain-derived neurotrophic factor (BDNF), and chemokine upregulation (CC motif) ligand 2 (CCL2), and decreased the inward potassium channel (Kir) 4.1 on glial cells in the trigeminal ganglion, but NR2A conditional knockout (CKO) mice prevented these alterations. In contrast, NR2B CKO only blocked the changes of Kir4.1, IL-1β, and TNF-α and further promoted the production of CCL2. Central sensitization-related c-fos, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adaptor molecule 1 (Iba-1) were promoted and Kir4.1 was reduced in the spinal trigeminal caudate nucleus by IANX. Differential actions of NR2A and NR2B in mediating central sensitization were also observed. Silencing of NR2B was effective in reducing c-fos, GFAP, and Iba-1 but did not affect Kir4.1. In contrast, NR2A CKO only altered Iba-1 and Kir4.1 and further increased c-fos and GFAP. Gain-of-function and loss-of-function approaches provided insight into the differential roles of NR2A and NR2B in mediating peripheral and central nociceptive sensitization induced by IANX, which may be a fundamental basis for advancing knowledge of the neural mechanisms' reaction to nerve injury.

The Role of Anti-Nerve Growth Factor Monoclonal Antibodies in the Control of Chronic Cancer and Non-Cancer Pain

Nerve growth factor (NGF) belongs to the neurotrophin family and plays a fundamental role in the endurance of sensory and sympathetic neurons during embryogenesis. NGF, by interacting with tropomyosin receptor kinase A receptor (TrkA), modulates the pain pathway through the enhancement of the neurotrophic and nociceptor functions. Moreover, it has been demonstrated that NGF is upregulated in patients with chronic pain syndromes, which are difficult to treat. Thus, new non-pharmacological approaches, based on the use of different species-specific monoclonal antibodies (mAbs) targeting the NGF pathway, have been tested for the treatment of chronic pain in preclinical and clinical studies. With regard to preclinical investigations, anti-NGF mAbs have been used for the management of osteoarthritis (OA) and chronic low back pain animal models, with encouraging results. Moreover, anti-NGF mAb therapy is effective in animal models of neuropathic cancer pain. As regards patients with OA, although phase II and phase III clinical trials with tanezumab led to pain reduction, the safety was not observed in all these patients. Here, we review the preclinical and clinical studies on anti-NGF mAb therapy in chronic syndromes, dissect the role of NGF in pain transduction, and highlight the use of anti-NGF mAbs in humans.

Potential Molecular Targets for Treating Neuropathic Orofacial Pain Based on Current Findings in Animal Models

This review highlights potential molecular targets for treating neuropathic orofacial pain based on current findings in animal models. Preclinical research is currently elucidating the pathophysiology of the disease and identifying the molecular targets for better therapies using animal models that mimic this category of orofacial pain, especially post-traumatic trigeminal neuropathic pain (PTNP) and primary trigeminal neuralgia (PTN). Animal models of PTNP and PTN simulate their etiologies, that is, trauma to the trigeminal nerve branch and compression of the trigeminal root entry zone, respectively. Investigations in these animal models have suggested that biological processes, including inflammation, enhanced neuropeptide-mediated pain signal transmission, axonal ectopic discharges, and enhancement of interactions between neurons and glial cells in the trigeminal pathway, are underlying orofacial pain phenotypes. The molecules associated with biological processes, whose expressions are substantially altered following trigeminal nerve damage or compression of the trigeminal nerve root, are potentially involved in the generation and/or exacerbation of neuropathic orofacial pain and can be potential molecular targets for the discovery of better therapies. Application of therapeutic candidates, which act on the molecular targets and modulate biological processes, attenuates pain-associated behaviors in animal models. Such therapeutic candidates including calcitonin gene-related peptide receptor antagonists that have a reasonable mechanism for ameliorating neuropathic orofacial pain and meet the requirements for safe administration to humans seem worth to be evaluated in clinical trials. Such prospective translation of the efficacy of therapeutic candidates from animal models to human patients would help develop better therapies for neuropathic orofacial pain.

Assessment of Growth Factors, Cytokines, and Cellular Markers in Saliva of Patients with Trigeminal Neuralgia

We proposed to perform a comparative analysis of growth factors, cytokines, and chemokine receptors on the salivary cells in the saliva obtained from trigeminal neuralgia (TN) and normal subjects. Saliva was collected from TN and healthy subjects. Salivary cells were isolated by centrifugation. The expression of the cell surface marker was analyzed by flow cytometry. A cytometric bead array was done to measure the levels of cytokines and growth factors on the flow cytometer. Saliva from TN subjects showed lower growth factor levels of Angiopoietin-2, bFGF, HGF, SCF, TGF-α, and VEGF and higher cytokine levels of IL-1β, TNF-α, CCL2, IL-17A, IL-6, and CXCL8, as well as higher expression levels of chemokine receptors CCR1 (CD191), CR3 (CD11b), CCR2 (CD192), CXCR5 (CD185), and CCR5 (CD196) in the cells from TN saliva. A certain set of cytokines and growth factors in the saliva, as well as chemokine receptors on salivary cells, could be a useful tool in the diagnostics and prognostics of trigeminal neuralgia. Trigeminal neuralgia is one of the significant pathological conditions in the class of chronic diseases around the world. Many targeted approaches are being tried by various research groups to utilize the information of the inflammatory microenvironment to resolve the pathology of chronic TN.

Carvedilol Exerts Neuroprotective Effect on Rat Model of Diabetic Neuropathy

Diabetic neuropathy (DN) commonly occurs in diabetics, affecting approximately 50% of both type 1 and 2 diabetic patients. It is a leading cause of non-traumatic amputations. Oxidative stress could play a key role in the pathophysiology of DN. This study aimed to investigate the potential neuroprotective effect of carvedilol on STZ-induced DN in rats. Thirty male Sprague Dawley rats (weighing 200–250 g) were randomly divided into five groups (six/group), where group 1 (negative control) received only the vehicle (0.5% of carboxymethyl cellulose orally 1 ml/kg). DN was induced by a single injection of remaining rats with streptozotocin (STZ; 50 mg/kg, i.p.). After diabetes induction, group 2 served as the diabetic untreated animals; while groups 3 and 4 were treated with carvedilol (1 and 10 mg/kg/d, orally, respectively). Group 5 received a-lipoic acid as a reference neuroprotective (100 mg/kg/d, orally). All treatments were continued for 45 days after diabetes induction, followed by behavioural tests. After sacrificing the animals, dorsal root ganglia, and sciatic nerves were collected for histopathological examination and biochemical assessments. Briefly, STZ administration caused cold allodynia, induced oxidative stress, and increased nerve growth factor (NGF) concentration. Nevertheless, carvedilol improved the behavioural tests, ameliorated the oxidative imbalance as manifested by reducing malondialdehyde, restoring glutathione content, and superoxide dismutase activity. Carvedilol also decreased NGF concentration in DRG homogenate. In conclusion, this study demonstrates the neuroprotective effect of carvedilol in an experimentally induced DN rat model through–at least partly–its antioxidant effect and reduced NGF concentration in DRG.

The opposing contribution of neurotrophin-3 and nerve growth factor to orofacial heat hyperalgesia in rats

It has been proposed that neurotrophin-3 acts in a manner that is opposed to nerve growth factor, especially in the modulation of heat hyperalgesia. Injury to the constriction of the infraorbital nerve (CION) is a well-established model of trigeminal neuropathic pain that leads to robust heat, cold, and mechanical hyperalgesia. Here, we assessed the effect of local neurotrophin-3 treatment on CION-induced hyperalgesia, and we examined some mechanisms related to the effect of neurotrophin-3. Neurotrophin-3 (1 µg/50 µl) injected into the upper lip of CION rats caused a significant and long-lasting reduction of CION-induced heat hyperalgesia, but failed to affect cold and mechanical hyperalgesia. Increased levels of neurotrophin-3 were detected in the injured nerve at the time point that represents the peak of heat hyperalgesia. The anti-hyperalgesic effect of neurotrophin-3 was markedly reduced in the presence of an antagonist of TrkA receptors (K-252a, 1 μg/50 μl). Moreover, association of lower doses of neurotrophin-3 with an antibody anti-nerve growth factor resulted in a synergistic anti-hyperalgesic effect in CION rats. Local injection of nerve growth factor (3 µg/50 µl) or the TRPV1 agonist capsaicin (1 μg/50 μl), but not neurotrophin-3 injection (1 µg/50 µl), resulted in long-lasting facial heat hyperalgesia, which was both significantly reduced by previous neurotrophin-3 local treatment. In conclusion, we suggest that neurotrophin-3 is a potent modulator of facial heat hyperalgesia, which may exert an inhibitory influence on the trkA pathway. Neurotrophin-3 treatment may represent a promising approach, especially in pain conditions associated with increased levels of nerve growth factor.

Role of Nerve Growth Factor in Orofacial Pain

Some chronic pain conditions in the orofacial region are common and the mechanisms underlying orofacial pain are unresolved. Nerve growth factor (NGF) is a member of a family of neurotrophins and regulates the growth, maintenance and development of neurons. Increasing evidence suggests that NGF plays a crucial role in the generation of pain and hyperalgesia in different pain states. This review investigates the role of NGF in orofacial pain and their underlying cellular mechanisms, which may provide essential guidance to drug-discovery programmes. A systemic literature search was conducted in Pubmed focusing on NGF and orofacial pain. Articles were reviewed, and those discussing in vitro studies, animal evidence, clinical course, and possible mechanisms were summarized. We found a hyperalgesic effect of NGF in peripheral sensitization in orofacial pain models. We also summarize the current knowledge regarding NGF-dependent pain mechanism, which is initiated by retrograde transport of the ligand-receptor complex, ensuing transcriptional regulation of many important nociceptor genes involved in nociceptive processing. Phase III trials suggest that anti-NGF drug is endorsed with anti-inflammatory and pain-relieving effects with good tolerance in a variety of pain conditions, including pain associated with osteoarthritis and chronic lower back pain. Based on the data reviewed herein, NGF is believed to be an important hyperalgesic mediator in orofacial pain. The identification of underlying mechanisms and pathways of orofacial pain opens new frontiers for pain management.

Analgesic Effects of Directed Repetitive Transcranial Magnetic Stimulation in Acute Neuropathic Pain After Spinal Cord Injury

Objectives

Central neuropathic pain (CNP) often appears following spinal cord injury (SCI), but current treatments are not always successful. In this study, we evaluated the analgesic effects of repetitive transcranial magnetic stimulation (rTMS) applied over the hand area of the motor cortex in patients with acute CNP after SCI.

Methods

A total of 48 patients with complete or incomplete SCI and acute CNP participated in this study and were randomized to receive either rTMS (10 Hz, 1,500 stimuli; N = 24) or a sham intervention (N = 24) for three weeks. The numeric rating scale (NRS) and Short-Form McGill Pain Questionnaire-2 (Chinese Edition; SF-MPQ-2-CN) were analyzed to assess the degree of pain. Brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) were collected to explore expression influenced by rTMS. Motor-evoked potential (MEP) latency and maximal amplitude were measured to determine neurophysiological changes. The assessments were carried out at baseline (T0), three days (T1), one week (T2), two weeks (T3), and three weeks (T4) after onset of treatment.

Results

The analysis showed significant treatment–time interactions for the quality and intensity of pain, as measured by NRS (P < 0.001, η2 = 0.441) and SF-MPQ-2 (P < 0.001, η2 = 0.590). Compared with the sham group, the NRS and SF-MPQ2-CN scores were significantly lower on the third day (P < 0.001, Cohen’s d = 1.135; P = 0.006, Cohen’s d = 0.616) and after one week (P < 0.001, Cohen’s d = 0.846; P = 0.012, Cohen’s d = 0.557) of treatment. In addition, the serum levels of BDNF and NGF were significantly higher in the treated group after three weeks (P = 0.015, Cohen’s d = 0.539; P = 0.009, Cohen’s d = 0.580), and the MEP amplitude increased by 109.59% (P = 0.033, Cohen’s d = 0.464).

Conclusions

These findings indicate that 10 Hz rTMS over the hand area of the motor cortex could alleviate acute CNP in the early phase of SCI and could enhance MEP parameters and modulate BDNF and NGF secretion.

Tizanidine exerts anti-nociceptive effects in spared nerve injury model of neuropathic pain through inhibition of TLR4/NF-κB pathway

Recently, α2-adrenoceptors (α2-AR) agonists have been shown to have anti-nociceptive effects and thus may become a promising therapeutic strategy for neuropathic pain. tizanidine is a highly selective α2-AR agonist, but the effect mechanism of tizanidine in neuropathic pain remains largely unknown. The present study investigated whether tizanidine has anti-nociceptive effects in spared nerve injury (SNI) model of neuropathic pain in rats, as well as explored the underlying molecular mechanism. We found that the rats in SNI group showed significantly higher mechanical and thermal hyperalgesia, accompanied with increased production of proinflammatory cytokines including interleukin-1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α), as well as the activation of Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling. PDTC, an inhibitor of TLR4/NF-κB signaling, could significantly attenuate the SNI-induced mechanical and thermal hyperalgesia and the production of proinflammatory cytokines. Moreover, treatment with tizanidine also attenuated the SNI-induced mechanical and thermal hyperalgesia, suppressed production of the proinflammatory cytokines, and inhibited the activation of TLR4/NF-κB pathway, which could be reversed by pretreatment with BRL44408, a selective α2-AR antagonist. Taken these findings together, we demonstrated that tizanidine has anti-nociceptive effects on neuropathic pain via inhibiting the production of proinflammatory cytokines through suppressing the activation of TLR4/NF-κB p65 signaling pathway.

Nerve growth factor: a neuroimmune crosstalk mediator for all seasons

Neurotrophic factors comprise a broad family of biomolecules - most of which are peptides or small proteins - that support the growth, survival and differentiation of both developing and mature neurons. The prototypical example and best-characterized neurotrophic factor is nerve growth factor (NGF), which is widely recognized as a target-derived factor responsible for the survival and maintenance of the phenotype of specific subsets of peripheral neurons and basal forebrain cholinergic nuclei during development and maturation. In addition to being active in a wide array of non-nervous system cells, NGF is also synthesized by a range of cell types not considered as classical targets for innervation by NGF-dependent neurons; these include cells of the immune-haematopoietic lineage and populations in the brain involved in neuroendocrine functions. NGF concentrations are elevated in numerous inflammatory and autoimmune states such as multiple sclerosis, chronic arthritis, systemic lupus erythematosus and mastocytosis, in conjunction with increased accumulation of mast cells. Intriguingly, NGF seems to be linked also with diabetic pathology and insulin homeostasis. Mast cells and NGF appear involved in neuroimmune interactions and tissue inflammation. As mast cells are capable of producing and responding to NGF, this suggests that alterations in mast cell behaviour could provoke maladaptive neuroimmune tissue responses, including those of an autoimmune nature. Moreover, NGF exerts a modulatory role on sensory nociceptive nerve physiology in the adult, which appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. NGF can therefore be viewed as a multifactorial modulator of neuro-immune-endocrine functions.

Neurotrophin receptors in the pathogenesis, diagnosis and therapy of neurodegenerative diseases

In the last few years, exciting properties have emerged regarding the activation, signaling, mechanisms of action, and therapeutic targeting of the two types of neurotrophin receptors: the p75^NTR with its intracellular and extracellular peptides, the Trks, their precursors and their complexes. This review summarizes these new developments, with particular focus on neurodegenerative diseases. Based on the evolving knowledge, innovative concepts have been formulated regarding the pathogenesis of these diseases, especially the Alzheimer's and two other, the Parkinson's and Huntington's diseases. The medical progresses include original procedures of diagnosis, started from studies in mice and now investigated for human application, based on innovative classes of receptor agonists and blockers. In parallel, comprehensive studies have been and are being carried out for the development of drugs. The relevance of these studies is based on the limitations of the therapies employed until recently, especially for the treatment of Alzheimer's patients. Starting from well known drugs, previously employed for non-neurodegenerative diseases, the ongoing progress has lead to the development of small molecules that cross rapidly the blood-brain barrier. Among these molecules the most promising are specific blockers of the p75^NTR receptor. Additional drugs, that activate Trk receptors, were shown effective against synaptic loss and memory deficits. In the near future such approaches, coordinated with treatments with monoclonal antibodies and with developments in the microRNA field, are expected to improve the therapy of neurodegenerative diseases, and may be relevant also for other human disease conditions.