Expression of mRNA for brain-derived neurotrophic factor in the dorsal root ganglion following peripheral inflammation

Protein kinase C epsilon activation regulates proliferation, migration, and epithelial to mesenchymal-like transition in rat Schwann cells

Introduction

Protein kinase type C-ε (PKCε) plays an important role in the sensitization of primary afferent nociceptors, promoting mechanical hyperalgesia. In accordance, we showed that PKCε is present in sensory neurons of the peripheral nervous system (PNS), participating in the control of pain onset and chronification. Recently, it was found that PKCε is also implicated in the control of cell proliferation, promoting mitogenesis and metastatic invasion in some types of cancer. However, its role in the main glial cell of the PNS, the Schwann cells (SCs), was still not investigated.

Methods

Rat primary SCs culture were treated with different pharmacologic approaches, including the PKCε agonist dicyclopropyl-linoleic acid (DCP-LA) 500 nM, the human recombinant brain derived neurotrophic factor (BDNF) 1 nM and the TrkB receptor antagonist cyclotraxin B 10 nM. The proliferation (by cell count), the migration (by scratch test and Boyden assay) as well as some markers of SCs differentiation and epithelial-mesenchymal transition (EMT) process (by qRT-PCR and western blot) were analyzed.

Results

Overall, we found that PKCε is constitutively expressed in SCs, where it is likely involved in the switch from the proliferative toward the differentiated state. Indeed, we demonstrated that PKCε activation regulates SCs proliferation, increases their migration, and the expression of some markers (e.g., glycoprotein P0 and the transcription factor Krox20) of SCs differentiation. Through an autocrine mechanism, BDNF activates TrkB receptor, and controls SCs proliferation via PKCε. Importantly, PKCε activation likely promoted a partial EMT process in SCs.

Discussion

PKCε mediates relevant actions in the neuronal and glial compartment of the PNS. In particular, we posit a novel function for PKCε in the transformation of SCs, assuming a role in the mechanisms controlling SCs' fate and plasticity.

Neuronal P2X4 receptor may contribute to peripheral inflammatory pain in rat spinal dorsal horn

Objective

Intense inflammation may result in pain, which manifests as spinal central sensitization. There is growing evidence that purinergic signaling plays a pivotal role in the orchestration of pain processing. Over the last decade the ionotropic P2X purino receptor 4 (P2X4) got into spotlight in neuropathic disorders, however its precise spinal expression was scantily characterized during inflammatory pain. Thus, we intended to analyze the receptor distribution within spinal dorsal horn and lumbar dorsal root ganglia (DRG) of rats suffering in inflammatory pain induced by complete Freund adjuvant (CFA).

Methods

CFA-induced peripheral inflammation was validated by mechanical and thermal behavioral tests. In order to ensure about the putative alteration of spinal P2X4 receptor gene expression qPCR reactions were designed, followed by immunoperoxidase and Western blot experiments to assess changes at a protein level. Colocalization of P2X4 with neuronal and glial markers was investigated by double immunofluorescent labelings, which were subsequently analyzed with IMARIS software. Transmission electronmicroscopy was applied to study the ultrastructural localization of the receptor. Concurrently, in lumbar DRG cells similar methodology has been carried out to complete our observations.

Results

The figures of mechanical and thermal behavioral tests proved the establishment of CFA-induced inflammatory pain. We observed significant enhancement of P2X4 transcript level within the spinal dorsal horn 3 days upon CFA administration. Elevation of P2X4 immunoreactivity within Rexed lamina I-II of the spinal gray matter was synchronous with mRNA expression, and confirmed by protein blotting. According to IMARIS analysis the robust protein increase was mainly detected on primary afferent axonterminals and GFAP-labelled astrocyte membrane compartments, but not on postsynaptic dendrites was also validated ultrastructurally within the spinal dorsal horn. Furthermore, lumbar DRG analysis demonstrated that peptidergic and non-peptidergic nociceptive subsets of ganglia cells were also abundantly positive for P2X4 receptor in CFA model.

Conclusion

Here we provide novel evidence about involvement of neuronal and glial P2X4 receptor in the establishment of inflammatory pain.

Correlation between vitamin D and serum brain derived neurotropic factor levels in type 2 diabetes mellitus patients

Diabetes Mellitus (DM) currently ranks as the most common endocrine disorder worldwide. Current opinion views DM as a group of heterogeneous metabolic diseases characterized by hyperglycemia triggered by defects in the ability of the body to produce or use insulin in type 1 and 2 DM, respectively. Brain-derived neurotrophic factor (BDNF), one of the neurotrophin family of growth factors, has been linked to the pathogenesis of DM and insulin resistance. Moreover, vitamin D has been associated with insulin resistance and DM. Recently, the interactions between vitamin D and BDNF have been investigated in diabetic rats. However, this correlation has never been investigated in humans. Thus, the aim of the present study was to assess the alterations in serum BDNF and vitamin D levels in T2DM patients in Jordan, prior to and following vitamin D supplementation. A combination of non-experimental case-control and experimental designed studies were utilized to assess the relationship between serum BDNF and vitamin D levels in T2DM patients. The levels of BDNF and vitamin D were measured using commercially available ELISA kits, and fasting blood glucose (FBG) and HbA1c levels were measured in medical labs. The results showed that diabetic patients had lower levels of serum vitamin D and higher levels of BDNF compared with the healthy controls. Moreover, linear regression analysis indicated that BDNF levels were inversely correlated with serum vitamin D levels. Furthermore, vitamin D supplementation significantly increased vitamin D serum levels and decreased BDNF serum levels in diabetic patients. Intriguingly, FBG and HbA1c levels were significantly improved post vitamin D supplementation. These data demonstrate a positive effect of vitamin D supplementation in diabetic patients suggesting the implementation of vitamin D as part of future T2DM treatment plans. However, additional studies are needed to investigate the direct link between vitamin D, BDNF, and T2DM.

Brain-derived neurotrophic factor and inflammation in depression: Pathogenic partners in crime?

Major depressive disorder is a debilitating disorder affecting millions of people each year. Brain-derived neurotrophic factor (BDNF) and inflammation are two prominent biologic risk factors in the pathogenesis of depression that have received considerable attention. Many clinical and animal studies have highlighted associations between low levels of BDNF or high levels of inflammatory markers and the development of behavioral symptoms of depression. However, less is known about potential interaction between BDNF and inflammation, particularly within the central nervous system. Emerging evidence suggests that there is bidirectional regulation between these factors with important implications for the development of depressive symptoms and anti-depressant response. Elevated levels of inflammatory mediators have been shown to reduce expression of BDNF, and BDNF may play an important negative regulatory role on inflammation within the brain. Understanding this interaction more fully within the context of neuropsychiatric disease is important for both developing a fuller understanding of biological pathogenesis of depression and for identifying novel therapeutic opportunities. Here we review these two prominent risk factors for depression with a particular focus on pathogenic implications of their interaction.

Case Report: Plasma Biomarkers Reflect Immune Mechanisms of Guillain-Barré Syndrome

This case series reported a group of patients with Guillain-Barré syndrome (GBS) and their plasma cytokine changes before and after immunotherapy. We aimed to understand GBS's pathogenesis and pathophysiology through observing the interval differences of the representative cytokines, which were the thymus and activation regulated chemokine (TARC) for T-cell chemotaxis, CD40 ligand (CD40L) for cosimulation of B and T cells, activated complement component C5/C5a, and brain-derived neurotrophic factor (BDNF) for survival and regenerative responses to nerve injuries. The fluorescence magnetic bead-based multiplexing immunoassay simultaneously quantified the five cytokines in a single sample. From June 2018 to December 2019, we enrolled five GBS patients who had completed before-after blood cytokine measurements. One patient was diagnosed with paraneoplastic GBS and excluded from the following cytokine analysis. The BDNF level decreased consistently in all the patients and made it a potential biomarker for the acute stage of GBS. Interval changes of the other four cytokines were relatively inconsistent and possibly related to interindividual differences in the immune response to GBS triggers, types of GBS variants, and classes of antiganglioside antibodies. In summary, utilizing the multiplexing immunoassay helps in understanding the complex immune mechanisms of GBS and the variation of immune responses in GBS subtypes; this method is feasible for identifying potential biomarkers of GBS.

Pacific-Ciguatoxin-2 and Brevetoxin-1 Induce the Sensitization of Sensory Receptors Mediating Pain and Pruritus in Sensory Neurons

Ciguatera fish poisoning (CFP) and neurotoxic shellfish poisoning syndromes are induced by the consumption of seafood contaminated by ciguatoxins and brevetoxins. Both toxins cause sensory symptoms such as paresthesia, cold dysesthesia and painful disorders. An intense pruritus, which may become chronic, occurs also in CFP. No curative treatment is available and the pathophysiology is not fully elucidated. Here we conducted single-cell calcium video-imaging experiments in sensory neurons from newborn rats to study in vitro the ability of Pacific-ciguatoxin-2 (P-CTX-2) and brevetoxin-1 (PbTx-1) to sensitize receptors and ion channels, (i.e., to increase the percentage of responding cells and/or the response amplitude to their pharmacological agonists). In addition, we studied the neurotrophin release in sensory neurons co-cultured with keratinocytes after exposure to P-CTX-2. Our results show that P-CTX-2 induced the sensitization of TRPA1, TRPV4, PAR2, MrgprC, MrgprA and TTX-r NaV channels in sensory neurons. P-CTX-2 increased the release of nerve growth factor and brain-derived neurotrophic factor in the co-culture supernatant, suggesting that those neurotrophins could contribute to the sensitization of the aforementioned receptors and channels. Our results suggest the potential role of sensitization of sensory receptors/ion channels in the induction or persistence of sensory disturbances in CFP syndrome.

Acute and Chronic Exposure of Toluene Induces Genotoxicity in Different Regions of the Brain in Normal and Allergic Mouse Models

Toluene is a widely used industrial organic solvent and is ubiquitous in our environment. The neurobehavioral and neurotoxic effects of toluene are well recognized; however, its genotoxicity is still under discussion. Toluene biotransformation leads to the generation of reactive oxygen species that cause oxidative stress and DNA damages. Individuals with different immunogenetic backgrounds have different sensitivities to toxic chemical exposure. Previous studies have suggested that allergic stimulation may influence the threshold for toluene sensitivity due to the modulation of neurotrophin-related genes. Therefore, we aimed to investigate toluene-induced genotoxicity in different brain regions following acute and chronic exposure in vivo and to further examine whether allergic stimulation may influence the sensitivity to toluene-induced genotoxicity. In this present study, we found that exposure of toluene induced oxidative DNA damages resulting in genotoxicity in different brain regions including cortex, cerebellum, and hippocampus using comet assay. Higher genotoxicity induced by toluene was observed in the hippocampus of control mice compared to OVA-immunized mice. These results provide evidence that toluene-induced genotoxicity may contribute to its neurotoxicity in different immunogenetic individuals.

A Novel Peptide Interfering with proBDNF-Sortilin Interaction Alleviates Chronic Inflammatory Pain

Rationale: Brain-derived neurotrophic factor (BDNF) is a key mediator in the development of chronic pain. Sortilin is known to interact with proBDNF and regulate its activity-dependent secretion in cortical neurons. In a rat model of inflammatory pain with intraplantar injection of complete Freund's adjuvant (CFA), we examined the functional role of proBDNF-sortilin interaction in dorsal root ganglia (DRG).

Methods: Expression and co-localization of BDNF and sortilin were determined by immunofluorescence. ProBDNF-sortilin interaction interface was mapped using co-immunoprecipitation and bimolecular fluorescence complementation assay. The analgesic effect of intrathecal injection of a synthetic peptide interfering with proBDNF-sortilin interaction was measured in the CFA model.

Results: BDNF and sortilin were co-localized and their expression was significantly increased in ipsilateral L4/5 DRG upon hind paw CFA injection. In vivo adeno-associated virus-mediated knockdown of sortilin-1 in L5 DRG alleviated pain-like responses. Mapping by serial deletions in the BDNF prodomain indicated that amino acid residues 71-100 supported the proBDNF-sortilin interaction. A synthetic peptide identical to amino acid residues 89-98 of proBDNF, as compared with scrambled peptide, was found to interfere with proBDNF-sortilin interaction, inhibit activity-dependent release of BDNF in vitro and reduce CFA-induced mechanical allodynia and heat hyperalgesia in vivo. The synthetic peptide also interfered with capsaicin-induced phosphorylation of extracellular signal-regulated kinases in ipsilateral spinal cord of CFA-injected rats.

Conclusions: Sortilin-mediated secretion of BDNF from DRG neurons contributes to CFA-induced inflammatory pain. Interfering with proBDNF-sortilin interaction reduced activity-dependent release of BDNF and might serve as a therapeutic approach for chronic inflammatory pain.

The Role of BDNF in Peripheral Nerve Regeneration: Activity-Dependent Treatments and Val66Met

Despite the ability of peripheral nerves to spontaneously regenerate after injury, recovery is generally very poor. The neurotrophins have emerged as an important modulator of axon regeneration, particularly brain derived neurotrophic factor (BDNF). BDNF regulation and signaling, as well as its role in activity-dependent treatments including electrical stimulation, exercise, and optogenetic stimulation are discussed here. The importance of a single nucleotide polymorphism in the BDNF gene, Val66Met, which is present in 30% of the human population and may hinder the efficacy of these treatments in enhancing regeneration after injury is considered. Preliminary data are presented on the effectiveness of one such activity-dependent treatment, electrical stimulation, in enhancing axon regeneration in mice expressing the met allele of the Val66Met polymorphism.

Diosgenin improves functional recovery from sciatic crushed nerve injury in rats

Peripheral nerve injuries are commonly encountered clinical problem and often result in chronic pain and severe functional deficit. Diosgenin is a plant steroidal saponin and has anti-inflammatory and anticancer effects. In the present study, we investigated the effect of diosgenin on functional recovery following sciatic crushed nerve injury in rats. Walking track analysis for the functional recovery which can be quantified with the sciatic function index (SFI) was conducted. Immunohistochemistry for c-Fos in the ventrolateral periaqueductal gray (vlPAG) and paraventricular nucleus (PVN) and western blot for brain-derived neurotrophic factor (BDNF), tyrosine kinase B (TrkB), cyclooxygenase-2 (COX-2), and inducible nitric oxide synthesis (iNOS) in the sciatic nerve were performed. The right sciatic nerve was crushed for 30 sec using a surgical clip. The animals in the diosgenin-treated groups received orally once a day at the respective doses for 7 consecutive days, starting one day after surgery. Sciatic crushed nerve injury showed characteristic gait changes showing decrease of SFI value. Diosgenin treatment increased the SFI value and suppressed nerve injury-induced c-Fos expression in the vlPAG and PVN. Diosgenin treatment inhibited nerve injury-induced increase of BDNF, TrkB, COX-2, and iNOS expressions. It is possible that diosgenin can be used as the new therapeutic agent for pain control and functional recovery following peripheral nerve injury.

Sensory neuronal P2RX4 receptors controls BDNF signaling in inflammatory pain

Chronic inflammatory and neuropathic pains are major public health concerns. Potential therapeutic targets include the ATP-gated purinergic receptors (P2RX) that contribute to these pathological types of pain in several different cell types. The purinergic receptors P2RX2 and P2RX3 are expressed by a specific subset of dorsal root ganglion neurons and directly shape pain processing by primary afferents. In contrast the P2RX4 and P2RX7 are mostly expressed in myeloid cells, where activation of these receptors triggers the release of various pro-inflammatory molecules. Here, we demonstrate that P2RX4 also controls calcium influx in mouse dorsal root ganglion neurons. P2RX4 is up-regulated in pain-processing neurons during long lasting peripheral inflammation and it co-localizes with Brain-Derived Neurotrophic Factor (BDNF). In the dorsal horn of the spinal cord, BDNF-dependent signaling pathways, phosphorylation of Erk1/2 and of the GluN1 subunit as well as the down regulation of the co-transporter KCC2, which are triggered by peripheral inflammation are impaired in P2RX4-deficient mice. Our results suggest that P2RX4, expressed by sensory neurons, controls neuronal BDNF release that contributes to hyper-excitability during chronic inflammatory pain and establish P2RX4 in sensory neurons as a new potential therapeutic target to treat hyperexcitability during chronic inflammatory pain.

Association Between Neurotrophic Factor Expression and Pain-Related Behavior Induced by Nucleus Pulposus Applied to Rat Nerve Root

STUDY DESIGN

An experimental animal study.

OBJECTIVE

To investigate the relationship between pain-related behavior and the expression of neurotrophic factors in the dorsal root ganglion (DRG) and spinal cord (SC) using a nucleus pulposus (NP) rat model.

SUMMARY OF BACKGROUND DATA

Neurotrophic factors are released from activated glial cells and are associated with pain-related behavior. Nerve growth factor (NGF) is a neurotrophic factor that is induced by inflammation.

METHODS

Rats were divided into an NP group (n = 94) and a sham-operated group (n = 46). NP harvested from the tail was applied to the left L5 DRG. Rats in the NP group were then divided into five subgroups: one non-treatment and four treatment groups. In the treatment groups, a dose of anti-NGF antibody or phosphate-buffered saline was administered into the DRG. Behavioral testing was performed to investigate the mechanical withdrawal threshold of the left hind paw for all groups. Immunohistochemical localization of NGF, phosphorylated p38 (p38), and brain-derived neurotrophic factor (BDNF) in the DRGs and SCs was performed, and the numbers of immunoreactive (IR) cells were counted.

RESULTS

The withdrawal threshold in the nontreatment NP group was significantly decreased for 35 days, and that of the middle- and high-dose treatment rats was significantly higher than the phosphate-buffered saline group values. In the DRG, NGF-IR, p38-IR, and BDNF-IR cells were increased for days 21. In the SC, BDNF-IR, and p38-IR cells were increased from days 7 to 21.

CONCLUSION

In the DRG, NGF expression increased, mechanical thresholds were reduced, and p38 and BDNF expression was increased in the NP group. p38 and BDNF expression was increased in SC neurons during the same period. Inhibition of NGF may be a potential treatment for neuropathic pain due to lumbar disc herniation.

LEVEL OF EVIDENCE

5.

Pain Mechanisms in Peritoneal Diseases Might Be Partially Regulated by Estrogen

To identify factors influencing the differential pain pathogenesis in peritoneal endometriosis (pEM) and peritoneal carcinomatosis in ovarian cancer (pOC), we undertook an experimental study. Tissue samples of 18 patients with pEM, 15 patients with pOC, and 15 unaffected peritoneums as controls were collected during laparoscopy or laparotomy. Immunohistochemical stainings were conducted to identify nerve fibers and neurotrophins in the tissue samples. Additionally, 23 pEM fluids, 25 pOC ascites fluids, and 20 peritoneal fluids of patients with myoma uteri as controls were collected. In these fluids, the expression of neurotrophins was evaluated. The effects of peritoneal fluids and ascites on the neurite outgrowth of chicken sensory ganglia were estimated by using a neuronal growth assay. An electrochemiluminescence immunoassay was carried out to determine the expression of estrogen in the peritoneal fluids and ascites. The total and sensory nerve fiber density was significantly higher in pEM than in pOC ( P < .001 and P < .01). All neurotrophins tested were present in tissue and fluid samples of pEM and pOC. Furthermore, the neurotrophic properties of pEM and pOC fluids were demonstrated, leading to sensory nerve fiber outgrowth. Estrogen concentration in the peritoneal fluids of pEM was significantly higher compared to ascites of pOC ( P < .001). The total and sensory nerve fiber density in the tissue samples as well as the estrogen expression in the peritoneal fluid of pEM was considerably higher than that in pOC, representing the most notable difference found in both diseases. This might explain the differential pain perception in pEM and pOC. Therefore, estrogen might be a key factor in influencing the genesis of pain in endometriosis.

May BDNF Be Implicated in the Exercise-Mediated Regulation of Inflammation? Critical Review and Synthesis of Evidence

Introduction:

Exercise attenuates inflammation and enhances levels of brain-derived neurotrophic factor (BDNF). Exercise also enhances parasympathetic tone, although its role in activating the cholinergic anti-inflammatory pathway is unclear. The physiological pathways of exercise’s effect on inflammation are obscure.

Aims:

To critically review the evidence on the role of BDNF in the anti-inflammatory effects of exercise and its potential involvement in the cholinergic anti-inflammatory pathway.

Methods:

Critical literature review of studies published in MEDLINE, PubMed, CINAHL, Embase, and Cochrane databases.

Results:

BDNF is critically involved in the bidirectional signaling between immune and neurosensory cells and in the regulation of parasympathetic system responses. BDNF is also intricately involved in the inflammatory response: inflammation induces BDNF production, and, in turn, BDNF exerts pro- and/or anti-inflammatory effects. Although exercise modulates BDNF and its receptors in lymphocytes, data on BDNF’s immunoregulatory/anti-inflammatory effects in relation to exercise are scarce. Moreover, BDNF increases cholinergic activity and is modulated by parasympathetic system activation. However, its involvement in the cholinergic anti-inflammatory pathway has not been investigated.

Conclusion:

Converging lines of evidence implicate BDNF in exercise-mediated regulation of inflammation; however, data are insufficient to draw concrete conclusions. We suggest that there is a need to investigate BDNF as a potential modulator/mediator of the anti-inflammatory effects of exercise and of the cholinergic anti-inflammatory pathway during exercise. Such research would have implications for a wide range of inflammatory diseases and for planning targeted exercise protocols.

Up-regulation of brain-derived neurotrophic factor in the dorsal root ganglion of the rat bone cancer pain model

Metastatic bone cancer causes severe pain, but current treatments often provide insufficient pain relief. One of the reasons is that mechanisms underlying bone cancer pain are not solved completely. Our previous studies have shown that brain-derived neurotrophic factor (BDNF), known as a member of the neurotrophic family, is an important molecule in the pathological pain state in some pain models. We hypothesized that expression changes of BDNF may be one of the factors related to bone cancer pain; in this study, we investigated changes of BDNF expression in dorsal root ganglia in a rat bone cancer pain model. As we expected, BDNF mRNA (messenger ribonucleic acid) and protein were significantly increased in L3 dorsal root ganglia after intra-tibial inoculation of MRMT-1 rat breast cancer cells. Among the eleven splice-variants of BDNF mRNA, exon 1–9 variant increased predominantly. Interestingly, the up-regulation of BDNF is localized in small neurons (mostly nociceptive neurons) but not in medium or large neurons (non-nociceptive neurons). Further, expression of nerve growth factor (NGF), which is known as a specific promoter of BDNF exon 1–9 variant, was significantly increased in tibial bone marrow. Our findings suggest that BDNF is a key molecule in bone cancer pain, and NGF-BDNF cascade possibly develops bone cancer pain.

Brain-derived neurotrophic factor enhances the excitability of small-diameter trigeminal ganglion neurons projecting to the trigeminal nucleus interpolaris/caudalis transition zone following masseter muscle inflammation

Background

The trigeminal subnuclei interpolaris/caudalis transition zones (Vi/Vc) play an important role in orofacial deep pain, however, the role of primary afferent projections to the Vi/Vc remains to be determined. This study investigated the functional significance of hyperalgesia to the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (trkB) signaling system in trigeminal ganglion (TRG) neurons projecting to the Vi/Vc transition zone following masseter muscle (MM) inflammation.

Results

The escape threshold from mechanical stimulation applied to skin above the inflamed MM was significantly lower than in naïve rats. Fluorogold (FG) labeling was used to identify the TRG neurons innervating the MM, while microbeads (MB) were used to label neurons projecting to the Vi/Vc region. FG/MB-labeled TRG neurons were immunoreactive (IR) for BDNF and trkB. The mean number of BDNF/trkB-IR small/medium-diameter TRG neurons was significantly higher in inflamed rats than in naïve rats. In whole-cell current-clamp experiments, the majority of dissociated small-diameter TRG neurons showed a depolarization response to BDNF that was associated with spike discharge, and the concentration of BDNF that evoked a depolarizing response was significantly lower in the inflamed rats. In addition, the relative number of BDNF-induced spikes during current injection was significantly higher in inflamed rats. The BDNF-induced changes in TRG neuron excitability was abolished by tyrosine kinase inhibitor, K252a.

Conclusion

The present study provided evidence that BDNF enhances the excitability of the small-diameter TRG neurons projecting onto the Vi/Vc following MM inflammation. These findings suggest that ganglionic BDNF-trkB signaling is a therapeutic target for the treatment of trigeminal inflammatory hyperalgesia.

Development of heat hyperalgesia and changes of TRPV1 and NGF expression in rat dorsal root ganglion following joint immobilization

The aim of this study was to examine whether threshold to heat stimuli, and expression of transient receptor potential vanilloid1 (TRPV1) and nerve growth factor (NGF) in dorsal root ganglion (DRG) altered under conditions of long-term limb immobilization. A plastic cast was wrapped around the right limb from the forearm to the forepaw to keep wrist joint at 90° of flexion for 5 weeks. Heat hyperalgesia was tested using the plantar test at 6 h after removing cast. The rats were perfused transcardially with 4 % paraformaldehyde and DRGs were excised at 24 h after removing cast. For size distributions of the TRPV1-IR and NGF-IR neuronal profile, the DRG area measurements over 1000 DRG neurons per animal were measured in each side, on both the immobilized (ipsilateral) and contralateral sides. Ipsilateral withdrawal latency was significantly shorter than contralateral sides. Ipsilateral percentage of immunoreactive neurons in the total DRG neurons was significantly higher than contralateral sides in TRPV1-IR and NGF-IR. Long-term casting induced heat hyperalgesia, and up-regulation and phenotypic change of TRPV1-IR and NGF-IR in DRGs on the immobilized side. These DRG alterations may involve heat hyperalgesia after long-term limb immobilization.

Up-regulation of brain-derived neurotrophic factor is regulated by extracellular signal-regulated protein kinase 5 and by nerve growth factor retrograde signaling in colonic afferent neurons in colitis

Brain-derived neurotrophic factor (BDNF) plays an essential role in sensory neuronal activation in response to visceral inflammation. Here we report that BDNF up-regulation in the primary afferent neurons in the dorsal root ganglia (DRG) in a rat model of colitis is mediated by the activation of endogenous extracellular signal-regulated protein kinase (ERK) 5 and by nerve growth factor (NGF) retrograde signaling. At 7 days of colitis, the expression level of BDNF is increased in conventional neuronal tracing dye Fast Blue labeled primary afferent neurons that project to the distal colon. In these neurons, the phosphorylation (activation) level of ERK5 is also increased. In contrast, the level of phospho-ERK1/2 is not changed in the DRG during colitis. Prevention of the ERK5 activation in vivo with an intrathecal application of the MEK inhibitor PD98059 significantly attenuates the colitis-induced increases in BDNF expression in the DRG. Further studies show that BDNF up-regulation in the DRG is triggered by NGF retrograde signaling which also involves activation of the MEK/ERK pathways. Application of exogenous NGF exclusively to the compartment containing DRG nerve terminals in an ex vivo ganglia-nerve preparation markedly increases the BDNF expression level in the DRG neuronal cell body that is placed in a different compartment; this BDNF elevation is attenuated by U0126, PD98059 and a specific ERK5 inhibitor BIX02188. These results demonstrate the mechanisms and pathways by which BDNF expression is elevated in primary sensory neurons following visceral inflammation that is mediated by increased activity of ERK5 and is likely to be triggered by the elevated NGF level in the inflamed viscera.

TrkB inhibition as a therapeutic target for CNS-related disorders

The interaction of brain-derived neurotrophic factor (BDNF) with its tropomyosin-related kinase receptor B (TrkB) is involved in fundamental cellular processes including neuronal proliferation, differentiation and survival as well as neurotransmitter release and synaptic plasticity. TrkB signaling has been widely associated with beneficial, trophic effects and many commonly used psychotropic drugs aim to increase BDNF levels in the brain. However, it is likely that a prolonged increased TrkB activation is observed in many pathological conditions, which may underlie the development and course of clinical symptoms. Interestingly, genetic and pharmacological studies aiming at decreasing TrkB activation in rodent models mimicking human pathology have demonstrated a promising therapeutic landscape for TrkB inhibitors in the treatment of various diseases, e.g. central nervous system (CNS) disorders and several types of cancer. Up to date, only a few selective and potent TrkB inhibitors have been developed. As such, the use of crystallography and in silico approaches to model BDNF-TrkB interaction and to generate relevant pharmacophores represent powerful tools to develop novel compounds targeting the TrkB receptor.

Evidence for a systemic regulation of neurotrophin synthesis in response to peripheral nerve injury

Up-regulation of neurotrophin synthesis is an important mechanism of peripheral nerve regeneration after injury. Neurotrophin expression is regulated by a complex series of events including cell interactions and multiple molecular stimuli. We have studied neurotrophin synthesis at 2 weeks time-point in a transvertebral model of unilateral or bilateral transection of sciatic nerve in rats. We have found that unilateral sciatic nerve transection results in the elevation of nerve growth factor (NGF) and NT-3, but not glial cell-line derived neurotrophic factor or brain-derived neural factor, in the uninjured nerve on the contralateral side, commonly considered as a control. Bilateral transection further increased NGF but not other neurotrophins in the nerve segment distal to the transection site, as compared to the unilateral injury. To further investigate the distinct role of NGF in regeneration and its potential for peripheral nerve repair, we transduced isogeneic Schwann cells with NGF-encoding lentivirus and transplanted the over-expressing cells into the distal segment of a transected nerve. Axonal regeneration was studied at 2 weeks time-point using pan-neuronal marker NF-200 and found to directly correlate with NGF levels in the regenerating nerve.

Cyclotraxin-B, a new TrkB antagonist, and glial blockade by propentofylline, equally prevent and reverse cold allodynia induced by BDNF or partial infraorbital nerve constriction in mice

Several lines of evidence indicate that brain-derived neurotrophic factor (BDNF) plays a key role as a central pronociceptive modulator of pain, acting through postsynaptic TrkB receptors that trigger intracellular signaling cascades leading to central sensitization. The overall aim of this study was to investigate to what extent BDNF could participate in the generation and maintenance of trigeminal neuropathic pain. The results showed that acute intracisternal administration of nanogram doses of BDNF in naïve mice elicited long-lasting, dose-related, cold allodynic responses to topical application of acetone onto vibrissal pad skin. The systemic administration of cyclotraxin-B (CTX-B), a new TrkB receptor antagonist, or propentofylline, an inhibitor of glial activation, was able to either prevent or reverse the effects of intracisternal BDNF on cold nociception. In addition, the blockade of TrkB receptor by CTX-B inhibited the mechanisms that either initiate or maintain cold allodynia in the ipsilateral vibrissal pad skin after unilateral constriction of the infraorbital nerve. These observations raise the possibility that BDNF is capable on its own of conveying many features of the signaling mechanisms that underlie central sensitization caused by nerve constriction.

PERSPECTIVE

Although further studies are necessary to examine in detail the mechanisms underlying the strong anti-allodynic action of CTX-B, this compound may represent an interesting lead for the development of novel therapeutic strategies aimed at preventing and/or suppressing central sensitization associated with neuropathic pain.

Early regenerative effects of NGF-transduced Schwann cells in peripheral nerve repair

Peripheral nerve injury leads to a rapid and robust increase in the synthesis of neurotrophins which guide and support regenerating axons. To further optimize neurotrophin supply at the earliest stages of regeneration, we over-expressed NGF in Schwann cells (SCs) by transducing these cells with a lentiviral vector encoding NGF (NGF-SCs). Transplantation of NGF-SCs in a rat sciatic nerve transection/repair model led to significant increase of NGF levels 2weeks after injury and correspondingly to substantial improvement in axonal regeneration. Numbers of NF200, ChAT and CGRP-positive axon profiles, as well as the gastrocnemius muscle weights, were significantly higher in the NGF-Schwann cell group compared to the animals that received control SCs transduced with a lentiviral vector encoding GFP (GFP-SCs). Comparison with other models of NGF application signifies the important role of this neurotrophin during the early stages of regeneration, and supports the importance of developing combined gene and cell therapy for peripheral nerve repair.

Up-regulation of dorsal root ganglia BDNF and trkB receptor in inflammatory pain: an in vivo and in vitro study

Background

During inflammation, immune cells accumulate in damaged areas and release pro-inflammatory cytokines and neurotrophins. Brain-derived neurotrophic factor (BDNF) plays a neuromodulatory role in spinal cord dorsal horn via the post-synaptic tyrosine protein kinase B (trkB) receptor to facilitate pain transmission. However, the precise role of BDNF and trkB receptor in the primary sensory neurons of dorsal root ganglia (DRG) during inflammation remains to be clarified. The aim of this study was to investigate whether and how BDNF-trkB signaling in the DRG is involved in the process of inflammatory pain.

Methods

We used complete Freund's adjuvant- (CFA-) induced and tumor necrosis factor-α- (TNF-α-) induced inflammation in rat hindpaw as animal models of inflammatory pain. Quantification of protein and/or mRNA levels of pain mediators was performed in separate lumbar L3-L5 DRGs. The cellular mechanism of TNF-α-induced BDNF and/or trkB receptor expression was examined in primary DRG cultures collected from pooled L1-L6 DRGs. Calcitonin gene-related peptide (CGRP), BDNF and substance P release were also evaluated by enzyme immunoassay.

Results

CFA injection into rat hindpaw resulted in mechanical hyperalgesia and significant increases in levels of TNF-α in the inflamed tissues, along with enhancement of BDNF and trkB receptor as well as the pain mediators CGRP and transient receptor potential vanilloid receptor subtype 1 (TRPV1) in DRG. Direct injection of TNF-α into rat hindpaw resulted in similar effects with retrograde transport of TNF-α along the saphenous nerve to DRG during CFA-induced inflammation. Primary DRG cultures chronically treated with TNF-α showed significant enhancement of mRNA and protein levels of BDNF and trkB receptor, BDNF release and trkB-induced phospho-ERK1/2 signal. Moreover, CGRP and substance P release were enhanced in DRG cultures after chronic TNF-α treatment or acute BDNF stimulation. In addition, we found that BDNF up-regulated trkB expression in DRG cultures.

Conclusions

Based on our current experimental results, we conclude that inflammation and TNF-α up-regulate the BDNF-trkB system in DRG. This phenomenon suggests that up-regulation of BDNF in DRG may, in addition to its post-synaptic effect in spinal dorsal horn, act as an autocrine and/or paracrine signal to activate the pre-synaptic trkB receptor and regulate synaptic excitability in pain transmission, thereby contributing to the development of hyperalgesia.

Decoy strategy targeting the brain-derived neurotrophic factor exon I to attenuate tactile allodynia in the neuropathic pain model of rats

The mechanism underlying neuropathic pain is still largely unclear. Recently, much attention has been focused on the role of brain-derived neurotrophic factor (BDNF) as a neuromodulator in the spinal cord. We previously reported that the expression of Bdnf exon I mRNA was remarkably up-regulated in the dorsal root ganglion (DRG) neurons with the rat L5 spinal nerve ligation (SNL) model. In the present study, we investigated whether neuropathic pain response would be reduced by the inhibition of the Bdnf exon I in the rat SNL model. We identified the promoter region of exon I and synthesized the decoy ODNs targeting the region. Reverse transcription-polymerase chain reaction analysis confirmed that the decoy ODN treatment reduced SNL-induced Bdnf exon I mRNA up-regulation in ipsilateral L4 and L5 DRGs. Furthermore, post-treatment with the decoy ODNs significantly attenuated SNL-induced tactile allodynia. This study suggested that decoy ODNs targeting the Bdnf exon I might provide a novel analgesic strategy for the treatment of neuropathic pain.

Tumor necrosis factor-α increases brain-derived neurotrophic factor expression in trigeminal ganglion neurons in an activity-dependent manner

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1β and tumor necrosis factor-α (TNFα) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNFα on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNFα increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNFα increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNFα-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNFα on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNFα-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNFα stimulates de novo synthesis of BDNF in these neurons. Thus, TNFα should be considered in mechanisms of BDNF-dependent neuronal plasticity.

Alternation of gene expression in trigeminal ganglion neurons following complete Freund's adjuvant or capsaicin injection into the rat face

The hyperexcitability of trigeminal ganglion (TG) neurons following inflammation or C-fiber stimulation is known to be involved in a variety of changes in gene expression in TG neurons, resulting in pain abnormalities in orofacial regions. We analyzed nocifensive behavior following complete Freund's adjuvant (CFA) or capsaicin injection into the maxillary whisker pad, and gene expression in the TG neurons using microarray analysis. The head-withdrawal latency to capsaicin injection or the head-withdrawal threshold to mechanical stimulation of the whisker pad skin in CFA-treated rats was significantly decreased compared to vehicle-treated rats. Many up-regulated and down-regulated genes in the TG neurons of each model were reported. Genes which have not been linked to peripheral inflammation or C-fiber activation were detected. Moreover, microarray chip containing a number of non-coding sequences was also up-regulated by C-fiber activation. These findings suggest that the diverse gene expressions in TG neurons are differentially involved in the inflammatory chronic pain and the acute pain induced by C-fiber activation, and the hyperexcitation of C-fibers are associated with the activation of certain non-coding RNAs.

Tooth pulp inflammation increases brain-derived neurotrophic factor expression in rodent trigeminal ganglion neurons

Nociceptive pathways with first-order neurons located in the trigeminal ganglion (TG) provide sensory innervation to the head, and are responsible for a number of common chronic pain conditions, including migraines, temporomandibular disorders and trigeminal neuralgias. Many of those conditions are associated with inflammation. Yet, the mechanisms of chronic inflammatory pain remain poorly understood. Our previous studies show that the neurotrophin brain-derived neurotrophic factor (BDNF) is expressed by adult rat TG neurons, and released from cultured newborn rat TG neurons by electrical stimulation and calcitonin gene-related peptide (CGRP), a well-established mediator of trigeminal inflammatory pain. These data suggest that BDNF plays a role in activity-dependent plasticity at first-order trigeminal synapses, including functional changes that take place in trigeminal nociceptive pathways during chronic inflammation. The present study was designed to determine the effects of peripheral inflammation, using tooth pulp inflammation as a model, on regulation of BDNF expression in TG neurons of juvenile rats and mice. Cavities were prepared in right-side maxillary first and second molars of 4-week-old animals, and left open to oral microflora. BDNF expression in right TG was compared with contralateral TG of the same animal, and with right TG of sham-operated controls, 7 and 28 days after cavity preparation. Our ELISA data indicate that exposing the tooth pulp for 28 days, with confirmed inflammation, leads to a significant upregulation of BDNF in the TG ipsilateral to the affected teeth. Double-immunohistochemistry with antibodies against BDNF combined with one of nociceptor markers, CGRP or transient receptor potential vanilloid type 1 (TRPV1), revealed that BDNF is significantly upregulated in TRPV1-immunoreactive (IR) neurons in both rats and mice, and CGRP-IR neurons in mice, but not rats. Overall, the inflammation-induced upregulation of BDNF is stronger in mice compared to rats. Thus, mouse TG provides a suitable model to study molecular mechanisms of inflammation-dependent regulation of BDNF expression in vivo.

Role of neuropathic pain mechanisms in the pathogenesis of pain in chronic pancreatitis

Abdominal pain is a major clinical symptom in patients with chronic pancreatitis. Alleviation of abdominal pain is important in the treatment of chronic pancreatitis. More and more studies show that the pain in chronic pancreatitis is similar to neuropathic pain. In this article, we will review the role of neuropathic pain mechanisms in the pathogenesis of pain in chronic pancreatitis by describing the clinical features of the pain, peripheral sensitization, central sensitization and changes in electroencephalogram. Understanding of the mechanisms responsible for pain in chronic pancreatitis may have important implications for the treatment of the disease.

Reg-2 expression in dorsal root ganglion neurons after adjuvant-induced monoarthritis

Reg-2 is a secreted protein that is expressed de novo in motoneurons, sympathetic neurons, and dorsal root ganglion (DRG) neurons after nerve injury and which can act as a Schwann cell mitogen. We now show that Reg-2 is also upregulated by DRG neurons in inflammation with a very unusual expression pattern. In a rat model of monoarthritis, Reg-2 immunoreactivity was detected in DRG neurons at 1 day, peaked at 3 days (in 11.6% of DRG neurons), and was still present at 10 days (in 5%). Expression was almost exclusively in the population of DRG neurons that expresses the purinoceptor P2X(3) and binding sites for the lectin Griffonia simplicifolia IB4, and which is known to respond to glial cell line-derived neurotrophic factor (GDNF). Immunoreactivity was present in DRG cell bodies and central terminals in the dorsal horn of the spinal cord. In contrast, very little expression was seen in the nerve growth factor (NGF) responsive and substance P expressing population. However intrathecal delivery of GDNF did not induce Reg-2 expression, but leukemia inhibitory factor (LIF) had a dramatic effect, inducing Reg-2 immunoreactivity in 39% of DRG neurons and 62% of P2X(3) cells. Changes in inflammation have previously been observed predominantly in the neuropeptide expressing, NGF responsive, DRG neurons. Our results show that changes also take place in the IB4 population, possibly driven by members of the LIF family of neuropoietic cytokines. In addition, the presence of Reg-2 in central axon terminals implicates Reg-2 as a possible modulator of second order dorsal horn cells.

Pain facilitation and activity-dependent plasticity in pain modulatory circuitry: role of BDNF-TrkB signaling and NMDA receptors

Pain modulatory circuitry in the brainstem exhibits considerable synaptic plasticity. The increased peripheral neuronal barrage after injury activates spinal projection neurons that then activate multiple chemical mediators including glutamatergic neurons at the brainstem level, leading to an increased synaptic strength and facilitatory output. It is not surprising that a well-established regulator of synaptic plasticity, brain-derived neurotrophic factor (BDNF), contributes to the mechanisms of descending pain facilitation. After tissue injury, BDNF and TrkB signaling in the brainstem circuitry is rapidly activated. Through the intracellular signaling cascade that involves phospholipase C, inositol trisphosphate, protein kinase C, and nonreceptor protein tyrosine kinases; N-methyl-D-aspartate (NMDA) receptors are phosphorylated, descending facilitatory drive is initiated, and behavioral hyperalgesia follows. The synaptic plasticity observed in the pain pathways shares much similarity with more extensively studied forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which typically express NMDA receptor dependency and regulation by trophic factors. However, LTP and LTD are experimental phenomena whose relationship to functional states of learning and memory has been difficult to prove. Although mechanisms of synaptic plasticity in pain pathways have typically not been related to LTP and LTD, pain pathways have an advantage as a model system for synaptic modifications as there are many well-established models of persistent pain with clear measures of the behavioral phenotype. Further studies will elucidate cellular and molecular mechanisms of pain sensitization and further our understanding of principles of central nervous system plasticity and responsiveness to environmental challenge.

Expression changes of multiple brain-derived neurotrophic factor transcripts in selective spinal nerve ligation model and complete Freund's adjuvant model

Brain-derived neurotrophic factor (BDNF) expression changes in the dorsal root ganglion (DRG) and spinal cord in some pain models. Recently, rat BDNF transcripts containing novel 5' untranslated exons were identified and characterized, and a new numbering system for rat BDNF exons was introduced. We examined the expression profiles of these novel BDNF transcripts in bilateral L4/5 DRGs in an L5-selective spinal nerve ligation (SSNL) model and bilateral L5 DRGs in a complete Freund's adjuvant (CFA) model of rats. L5SSNL increased significantly (P<0.05) the expression of total BDNF mRNA and exon I, IIA, IIB, IIC, III, IV, VI, and IXA transcripts in ipsilateral L4 DRG. Although expression of total BDNF mRNA remained unchanged in ipsilateral L5 DRG in the L5SSNL model, expression of exon I transcript increased significantly (P<0.05) and that of exon IV transcript decreased significantly (P<0.05). The expression profiles of the variant exons in ipsilateral L4 DRG of the L5SSNL model were quite similar to those in ipsilateral DRG of the CFA model, and exon I transcript was the most common BDNF mRNA in these DRGs. Although L5SSNL increased significantly (P<0.05) the expression of total BDNF mRNA and exon IIC and IXA transcripts in the contralateral L4/5 DRGs, CFA treatment did not alter the expression of total BDNF mRNA or specific transcripts in the contralateral DRGs. These findings suggest that exon I plays an important role in the increase in BDNF expression in ipsilateral DRGs regardless of condition.

Neurokinin-1 (NK-1) receptor and brain-derived neurotrophic factor (BDNF) gene expression is differentially modulated in the rat spinal dorsal horn and hippocampus during inflammatory pain

Persistent pain produces complex alterations in sensory pathways of the central nervous system (CNS) through activation of various nociceptive mechanisms. However, the effects of pain on higher brain centers, particularly the influence of the stressful component of pain on the limbic system, are poorly understood. Neurokinin-1 (NK-1) receptors and brain-derived neurotrophic factor (BDNF), known neuromediators of hyperalgesia and spinal central sensitization, have also been implicated in the plasticity and neurodegeneration occurring in the hippocampal formation during exposures to various stressors. Results of this study showed that injections of complete Freund's adjuvant (CFA) into the hind paw increased NK-1 receptor and BDNF mRNA levels in the ipsilateral dorsal horn, supporting an important role for these nociceptive mediators in the amplification of ascending pain signaling. An opposite effect was observed in the hippocampus, where CFA down-regulated NK-1 receptor and BDNF gene expression, phenomena previously observed in immobilization models of stress and depression. Western blot analyses demonstrated that in the spinal cord, CFA also increased levels of phosphorylated cAMP response element-binding protein (CREB), while in the hippocampus the activation of this transcription factor was significantly reduced, further suggesting that tissue specific transcription of either NK-1 or BDNF genes may be partially regulated by common intracellular transduction mechanisms mediated through activation of CREB. These findings suggest that persistent nociception induces differential regional regulation of NK-1 receptor and BDNF gene expression and CREB activation in the CNS, potentially reflecting varied roles of these neuromodulators in the spinal cord during persistent sensory activation vs. modulation of the higher brain structures such as the hippocampus.

NT-3 Expression in Spared DRG and the Associated Spinal Laminae as well as Its Anterograde Transport in Sensory Neurons Following Removal of Adjacent DRG in Cats

Neurotrophin-3 plays an important role in survival and differentiation of sensory and sympathetic neurons, sprouting of neurites, synaptic reorganization, and axonal growth. The present study evaluated changes in expression of NT-3 in the spinal cord and L6 dorsal root ganglion (DRG), after ganglionectomy of adjacent dorsal roots in cats. NT-3 immunoreactivity increased at 3 days post-operation (dpo), but decreased at 10 dpo in spinal lamina II after ganglionectomy of L1-L5 and L7-S2 (leaving L6 DRG intact). Conversely, NT-3 immunoreactivity decreased on 3 dpo, but increased on 10 dpo in the nucleus dorsalis. Very little NT-3 mRNA signal was detected in the spinal cord, despite the changes in NT-3 expression. The above changes may be related to changes in NT-3 expression in the DRG. Ganglionectomy of L1-L5 and L7-S2 resulted in increase in NT-3 immunoreactivity and mRNA in small and medium-sized neurons, but decreased expression in large neurons of L6 DRG at 3 dpo. It is possible that increased NT-3 in spinal lamina II is derived from anterograde transport from small- and medium-sized neurons of L6 DRG, whereas decreased NT-3 immunoreactivity in the nucleus dorsalis is due to decreased transport of NT-3 from large neurons in the DRG at this time. This notion is supported by observations on NT-3 distribution in the dorsal root of L6 after ligation of the nerve root. The above results indicate that DRG may be a source of neurotrophic factors such as NT-3 to the spinal cord, and may contribute to plasticity in the spinal cord after injury.

Expression profiles of BDNF splice variants in cultured DRG neurons stimulated with NGF

Expression of brain-derived neurotrophic factor (BDNF) mRNA is increased in the dorsal root ganglion (DRG) in response to peripheral inflammation. Nerve growth factor (NGF) from inflammatory tissue is thought to induce expression of BDNF. Recently, it was reported that the BDNF gene has eight non-coding exons that are transcribed independently into several splice variants. Expression of these splice variants in DRG neurons stimulated with NGF has not been studied. We examined changes in expression of BDNF splice variants in a rat model of peripheral inflammation and in cultured DRG neurons exposed to NGF. Total BDNF mRNA was increased by inflammation in vivo and by NGF in vitro. Among all splice variants, exon 1-9 showed the greatest increase in expression in both experiments. Our results indicate that exon 1-9 contributes to changes in total BDNF levels and may play an important role in the acute response of DRG to NGF.

Neurotrophin activation of NFAT-dependent transcription contributes to the regulation of pro-nociceptive genes

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) play key roles in the development of inflammation-induced hyperalgesia by triggering the expression of pro-nociceptive genes within primary afferent and spinal neurons. However, the mechanisms by which neurotrophins elicit gene expression remain largely unknown. Recently, neurotrophins have been shown to activate members of the calcineurin (CaN)-regulated, nuclear factor of activated T-cells (NFATc) family of transcription factors within brain. Thus, we hypothesized that NFATc transcription factors couple neurotrophin signaling to gene expression within primary afferent and spinal neurons. In situ hybridization revealed NFATc4 mRNA within the dorsal root ganglion and spinal cord. In cultured dorsal root ganglion cells, NGF triggered NFAT-dependent transcription in a CaN-sensitive manner. Further, increased BDNF expression following NGF treatment relied on CaN, thereby suggesting that NGF regulates BDNF transcription via activation of NFATc4. Within cultured spinal cells, BDNF also activated CaN-dependent, NFAT-regulated gene expression. Interestingly, BDNF stimulation increased the expression of the pro-nociceptive genes cyclooxygenase-2, neurokinin-1 receptor, inositol trisphosphates receptor type 1, and BDNF itself, through both NFAT-dependent and NFAT-independent transcriptional mechanisms. Our results suggest that regulation of pro-nociceptive genes through activation of NFAT-dependent transcription is one mechanism by which NGF and BDNF signaling contributes to the development of persistent pain states.

Up-regulation of calcitonin gene-related peptide and receptor tyrosine kinase TrkB in rat bladder afferent neurons following TNBS colitis

Colonic inflammation has profound effects on the urinary bladder physiology and produces hypersensitivity of bladder afferent neurons and neurogenic bladder overactivity. Calcitonin gene-related peptide (CGRP) expressed in dorsal root ganglia (DRG) plays an important role in mediating sensory perception following visceral inflammation. In the present study, we determined that the expression of CGRP was increased in bladder afferent neurons in lumbosacral DRG following tri-nitrobenzene sulfonic acid (TNBS)-induced colitis in rat. After colitis, the percentage of bladder afferent neurons expressing CGRP was increased in L1 (61.2+/-2.9% in colitis vs. 37.7+/-5.1% in controls; p<0.05) and S1 DRG (26.3+/-2.3% in colitis vs. 15.5+/-1.9% in controls; p<0.01). We also demonstrated that the expression of tyrosine kinase receptor TrkB was increased in L1 (39.7+/-2.9% in colitis vs. 25.2+/-4.3% in controls; p<0.05) and S1 DRG (45.6+/-3.8% in colitis vs. 38.3+/-3.6% in controls; p<0.01) following colitis. CGRP and TrkB were co-stored in a subpopulation of DRG neurons in control and colitic animals and the number of DRG cells co-expressing CGRP and TrkB was significantly increased in L1 (2.7-fold, p<0.01) and S1 DRG (2.4-fold, p<0.01) following colitis. In cultured DRG, exogenous BDNF application significantly increased CGRP expression, which was blocked by TrkB selective inhibitor K252a. These results suggest that up-regulation of CGRP and TrkB in bladder afferent neurons may play a role in colon-to-bladder cross-sensitization following colitis.

Clinical relevance of the neurotrophins and their receptors

The neurotrophins are growth factors required by discrete neuronal cell types for survival and maintenance, with a broad range of activities in the central and peripheral nervous system in the developing and adult mammal. This review examines their role in diverse disease states, including Alzheimer's disease, depression, pain and asthma. In addition, the role of BDNF (brain-derived neurotrophic factor) in synaptic plasticity and memory formation is discussed. Unlike the other neurotrophins, BDNF is secreted in an activity-dependent manner that allows the highly controlled release required for synaptic regulation. Evidence is discussed which shows that sequestration of NGF (nerve growth factor) is able to reverse symptoms of inflammatory pain and asthma in animal models. Both pain and asthma show an underlying pathophysiology linked to increases in endogenous NGF and subsequent NGF-dependent increase in BDNF. Conversely, in Alzheimer's disease, there is a role for NGF in the treatment of the disease and a recent clinical trial has shown benefit from its exogenous application. In addition, reductions in BDNF, and changes in the processing and usage of NGF, are evident and it is possible that both NGF and BDNF play a part in the aetiology of the disease process. This highly selective choice of functions and disease states related to neurotrophin function, although in no way comprehensive, illustrates the importance of the neurotrophins in the brain, the peripheral nervous system and in non-neuronal tissues. Ways in which the neurotrophins, their receptors or agonists/antagonists may act therapeutically are discussed.

Neurotrophic factors in peripheral neuropathies: therapeutic implications

Neurotrophic factors are proteins which promote the survival of specific neuronal populations. Many have other physiological effects on neurons such as inducing morphological differentiation, enhancing nerve regeneration, stimulating neurotransmitter expression, and otherwise altering the physiological characteristics of neurons. These properties suggest that neurotrophic factors are highly promising as potential therapeutic agents for neurological disease. Neurotrophic factors will most likely be applied to the peripheral nervous system initially, since there are fewer problems for large proteins to gain access to peripheral neurons. Many of the most intensively studied factors are active in the peripheral nervous system. These include the neurotrophins (nerve growth factor, brain derived neurotrophic factor, neurotrophin-3, neurotrophin-4/5), the insulin like growth factors, ciliary neurotrophic factor, and glial cell derived neurotrophic factor and its related proteins. The biology of these factors and their receptors in the peripheral nervous system is reviewed here. We also review data suggesting that abnormal availability of some factors may contribute towards the pathogenesis of certain types of peripheral neuropathy. Finally, the pre-clinical data suggesting that individual factors might be effective in treating neuropathy is reviewed, along with data relating to possible side effects of neurotrophic factor therapy. Several factors have already entered clinical trials with variable success. The data from these trials is reviewed as well.

Distinct expression of TRPM8, TRPA1, and TRPV1 mRNAs in rat primary afferent neurons with adelta/c-fibers and colocalization with trk receptors

The transient receptor potential (TRP) superfamily of cation channels contains four temperature-sensitive channels, named TRPV1-4, that are activated by heat stimuli from warm to that in the noxious range. Recently, two other members of this superfamily, TRPA1 and TRPM8, have been cloned and characterized as possible candidates for cold transducers in primary afferent neurons. Using in situ hybridization histochemistry and immunohistochemistry, we characterized the precise distribution of TRPA1, TRPM8, and TRPV1 mRNAs in the rat dorsal root ganglion (DRG) and trigeminal ganglion (TG) neurons. In the DRG, TRPM8 mRNA was not expressed in the TRPV1-expressing neuronal population, whereas TRPA1 mRNA was only seen in some neurons in this population. Both A-fiber and C-fiber neurons expressed TRPM8, whereas TRPV1 was almost exclusively seen in C-fiber neurons. All TRPM8-expressing neurons also expressed TrkA, whereas the expression of TRPV1 and TRPA1 was independent of TrkA expression. None of these three TRP channels were coexpressed with TrkB or TrkC. The TRPM8-expressing neurons were more abundant in the TG compared with the DRG, especially in the mandibular nerve region innervating the tongue. Our data suggest heterogeneity of TRPM8 and TRPA1 expression by subpopulations of primary afferent neurons, which may result in the difference of cold-sensitive primary afferent neurons in sensitivity to chemicals such as menthol and capsaicin and nerve growth factor.

BDNF in sensory neurons and chronic pain

Neurotrophic factors, which support neuronal survival and growth during development of the nervous system, have been shown to play significant roles in the transmission of physiologic and pathologic pain. Brain-derived neurotrophic factor (BDNF), synthesized in the primary sensory neurons, is anterogradely transported to the central terminals of the primary afferents in the spinal dorsal horn, where it is involved in the modulation of painful stimuli. In models of inflammatory and neuropathic pain, BDNF synthesis is greatly increased in different populations of dorsal root ganglion (DRG) neurons. Furthermore, it is now known that the activation of mitogen-activated protein kinases occurs in these sensory neurons and contributes to persistent inflammatory and neuropathic pain by regulating BDNF expression. The recent discovery that BDNF upregulation in the DRG and spinal cord contributes to chronic pain hypersensitivity indicates that blocking BDNF in sensory neurons could provide a fruitful strategy for the development of novel analgesics.

Nociceptor-derived brain-derived neurotrophic factor regulates acute and inflammatory but not neuropathic pain

Conditional mouse knock-outs provide an informative approach to drug target validation where no pharmacological blockers exist or global knock-outs are lethal. Here, we used the Cre-loxP system to delete BDNF in most nociceptive sensory neurons. Conditional null animals were healthy with no sensory neuron loss. However, pain-related behavior was substantially altered. Baseline thermal thresholds were reduced. Carrageenan-induced thermal hyperalgesia was inhibited. Formalin-induced pain behavior was attenuated in the second phase, and this correlated with abolition of NMDA receptor NR1 Ser896/897 phosphorylation and ERK1 and ERK2 activation in the dorsal horn; AMPA receptor phosphorylation (GluR1/Ser831) was unaffected. NGF-induced thermal hyperalgesia was halved, and mechanical secondary hyperalgesia caused by intramuscular NGF was abolished. By contrast, neuropathic pain behavior developed normally. Nociceptor-derived BDNF thus plays an important role in regulating inflammatory pain thresholds and secondary hyperalgesia, but BDNF released only from nociceptors plays no role in the development of neuropathic pain.

The effect of site and type of nerve injury on the expression of brain-derived neurotrophic factor in the dorsal root ganglion and on neuropathic pain behavior

A number of rat neuropathy models have been developed to simulate human neuropathic pain conditions, such as spontaneous pain, hyperalgesia, and allodynia. In the present study, to determine the relative importance of injury site (proximal or distal to the primary afferent neurons) and injury type (motor or sensory), we examined pain-related behaviors and changes of brain-derived neurotrophic factor expression in the dorsal root ganglion in sham-operated rats, and in the L5 dorsal rhizotomy, L5 ventral rhizotomy, L5 dorsal rhizotomy+ventral rhizotomy, and L5 spinal nerve transection models. L5 ventral rhizotomy and spinal nerve transection produced not only mechanical and heat hypersensitivity, but also an increase in brain-derived neurotrophic factor mRNA/protein in the L5 dorsal root ganglion at 7 days after surgery. In contrast, rats in the L5 dorsal rhizotomy and dorsal rhizotomy+ventral rhizotomy groups did not show both pain behaviors at 7 days after surgery, despite brain-derived neurotrophic factor upregulation in medium- and large-size neurons in the L5 dorsal root ganglion. On the other hand, L5 spinal nerve transection, but not dorsal rhizotomy, dorsal rhizotomy+ventral rhizotomy or ventral rhizotomy, increased the expression of brain-derived neurotrophic factor in the L4 dorsal root ganglion at 7 days after surgery. Taken together, these findings suggest that the upregulation of brain-derived neurotrophic factor expression in the L4 and L5 dorsal root ganglion neurons may be, at least in part, involved in the pathophysiological mechanisms of neuropathic pain and that the selective nerve root injury models may be useful for studying the underlying mechanisms of chronic pain after nerve injury.