Selective inhibition of extracellular signal-regulated kinases 1/2 blocks nerve growth factor to brain-derived neurotrophic factor signaling and suppresses the development of and reverses already established pain behavior in rats

A phenotypic screening platform for chronic pain therapeutics using all-optical electrophysiology

ABSTRACT

Chronic pain associated with osteoarthritis (OA) remains an intractable problem with few effective treatment options. New approaches are needed to model the disease biology and to drive discovery of therapeutics. We present an in vitro model of OA pain, where dorsal root ganglion (DRG) sensory neurons were sensitized by a defined mixture of disease-relevant inflammatory mediators, here called Sensitizing PAin Reagent Composition or SPARC. Osteoarthritis-SPARC components showed synergistic or additive effects when applied in combination and induced pain phenotypes in vivo. To measure the effect of OA-SPARC on neural firing in a scalable format, we used a custom system for high throughput all-optical electrophysiology. This system enabled light-based membrane voltage recordings from hundreds of neurons in parallel with single cell and single action potential resolution and a throughput of up to 500,000 neurons per day. A computational framework was developed to construct a multiparameter OA-SPARC neuronal phenotype and to quantitatively assess phenotype reversal by candidate pharmacology. We screened ∼3000 approved drugs and mechanistically focused compounds, yielding data from over 1.2 million individual neurons with detailed assessment of functional OA-SPARC phenotype rescue and orthogonal "off-target" effects. Analysis of confirmed hits revealed diverse potential analgesic mechanisms including ion channel modulators and other mechanisms including MEK inhibitors and tyrosine kinase modulators. Our results suggest that the Raf-MEK-ERK axis in DRG neurons may integrate the inputs from multiple upstream inflammatory mediators found in osteoarthritis patient joints, and MAPK pathway activation in DRG neurons may contribute to chronic pain in patients with osteoarthritis.

Neuroprotective effect of selumetinib on acrolein-induced neurotoxicity

Abnormal accumulation of acrolein, an α, β unsaturated aldehyde has been reported as one pathological cause of the CNS neurodegenerative diseases. In the present study, the neuroprotective effect of selumetinib (a MEK-ERK inhibitor) on acrolein-induced neurotoxicity was investigated in vitro using primary cultured cortical neurons. Incubation of acrolein consistently increased phosphorylated ERK levels. Co-treatment of selumetinib blocked acrolein-induced ERK phosphorylation. Furthermore, selumetinib reduced acrolein-induced increases in heme oxygenase-1 (a redox-regulated chaperone protein) and its transcriptional factor, Nrf-2 as well as FDP-lysine (acrolein-lysine adducts) and α-synuclein aggregation (a pathological biomarker of neurodegeneration). Morphologically, selumetinib attenuated acrolein-induced damage in neurite outgrowth, including neuritic beading and neurite discontinuation. Moreover, selumetinib prevented acrolein-induced programmed cell death via decreasing active caspase 3 (a hallmark of apoptosis) as well as RIP (receptor-interacting protein) 1 and RIP3 (biomarkers for necroptosis). In conclusion, our study showed that selumetinib inhibited acrolein-activated Nrf-2-HO-1 pathway, acrolein-induced protein conjugation and aggregation as well as damage in neurite outgrowth and cell death, suggesting that selumetinib, a MEK-ERK inhibitor, may be a potential neuroprotective agent against acrolein-induced neurotoxicity in the CNS neurodegenerative diseases.

N-Acetylcysteine causes analgesia in a mouse model of painful diabetic neuropathy

N-Acetylcysteine, one of the most prescribed antioxidant drugs, enhances pain threshold in rodents and humans by activating mGlu2 metabotropic glutamate receptors. Here, we assessed the analgesic activity of N-acetylcysteine in the streptozotocin model of painful diabetic neuropathy and examined the effect of N-acetylcysteine on proteins that are involved in mechanisms of nociceptive sensitization. Mice with blood glucose levels ≥250 mg/dl in response to a single intraperitoneal (i.p.) injection of streptozotocin (200 mg/kg) were used for the assessment of mechanical pain thresholds. Systemic treatment with N-acetylcysteine (100 mg/kg, i.p., either single injection or daily injections for seven days) caused analgesia in diabetic mice. N-acetylcysteine-induced analgesia was abrogated by the Sxc− inhibitors, sulfasalazine (8 mg/kg, i.p.), erastin (30 mg/kg, i.p.), and sorafenib (10 mg/kg, i.p.), or by the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.). Repeated administrations of N-acetylcysteine in diabetic mice reduced ERK1/2 phosphorylation in the dorsal region of the lumbar spinal cord. The analgesic activity of N-acetylcysteine was occluded by the MEK inhibitor, PD0325901 (25 mg/kg, i.p.), the TRPV1 channel blocker, capsazepine (40 mg/kg, i.p.), or by a cocktail of NMDA and mGlu5 metabotropic glutamate receptor antagonists (memantine, 25 mg/kg, plus MTEP, 5 mg/kg, both i.p.). These findings offer the first demonstration that N-acetylcysteine relieves pain associated with diabetic neuropathy and holds promise for the use of N-acetylcysteine as an add-on drug in diabetic patients.

Anti-inflammatory Effect of AZD6244 on Acrolein-Induced Neuroinflammation

Clinically, high levels of acrolein (a highly reactive α, β-unsaturated aldehyde) and acrolein adducts are detected in the brain of patients with CNS neurodegenerative diseases, including Alzheimer's disease and spinal cord injury. Our previous study supports this notion by showing acrolein as a neurotoxin in a Parkinsonian animal model. In the present study, the effect of AZD6244 (an ATP non-competitive MEK1/2 inhibitor) on acrolein-induced neuroinflammation was investigated using BV-2 cells and primary cultured microglia. Our immunostaining study showed that lipopolysaccharide (LPS, an inflammation inducer as a positive control) increased co-localized immunoreactivities of phosphorylated ERK and ED-1 (a biomarker of activated microglia) in the treated BV-2 cells. Similar elevation in co-localized immunoreactivities of phosphorylated ERK and ED-1 was detected in the acrolein-treated BV-2 cells. Furthermore, Western blot assay showed increases in phosphorylated ERK in BV-2 cells subjected to LPS (1 μg/mL) or acrolein (30 μM); these increases were blocked by AZD6244 (10 μM). At the same time, AZD6244 attenuated LPS-induced TNF-α (a pro-inflammatory cytokine) and cyclooxygenase-II (COX II, a pro-inflammatory enzyme). Consistently, AZD6244 reduced acrolein-induced elevations in COX-II mRNA and COX-II protein expression. In addition, AZD6244 inhibited acrolein-induced increases in activated caspase 1 (a biomarker of inflammasome activation) and heme oxygenase-1 (a redox-regulated chaperone protein) in BV-2 cells. Using a transwell migration assay, AZD6244 attenuated acrolein (5 μM)-induced migration of BV-2 cells and primary cultured microglia. In conclusion, our study shows that acrolein is capable of inducing neuroinflammation which involved ERK activation in microglia. Furthermore, AZD6244 is capable of inhibiting acrolein-induced neuroinflammation. Our study suggests that ERK inhibition may be a neuroprotective target against acrolein-induced neuroinflammation in the CNS neurodegenerative diseases.

Rasopathies case report: concurrence of two pathogenic variations de novo in NF1 and KRAS genes in a patient

Background

Rasopathies are a group of genetic malformative syndromes including neurofibromatosis 1, Noonan, LEOPARD, Costello, cardio-facio-cutaneous, Legius, and capillary malformation-arteriovenous malformation syndromes.

Case presentation

We present a female newborn that consulted at the emergency department with refusal to eat and sleepiness. A shortened femur, thickened nucal fold and suspect for agenesis of the corpus callosum were observed in prenatal ultrasound. Her phenotype included hypertelorism, antimongoloid obliquity of the palpebral fissure, prominent forehead, long filtrum, thickened nucal fold, separated nipples, widespread thickened skinfolds and café-au-lait spots. She had a systolic murmur due to pulmonary valve stenosis. The NF1 gene testing found the pathogenic variant p.E2586X (c.7756G > T) in exon 53, not described in any international database or scientific publications yet. Also, a mutation in the Kras gene was detected (p.Val14lle), which is associated with mild Noonan phenotype. Both variations were de novo.

Conclusions

Not all genes and mutations have already been discovered, so it’s important to document new findings, like our patient’s, to enrich and update the international database and broaden all possible knowledge about rasopathies. This is the first case to be described presenting simultaneously two mutations in Kras and NF1 genes, whose possible synergic effect regarding its pathogenicity is unknown, but could be interesting towards therapeutic alternatives.

Brain-derived neurotrophic factor: A steroidogenic regulator of Leydig cells

The brain-derived neurotrophic factor (BDNF) was first recognized for its roles in the peripheral and central nervous systems, and its complex functions on mammalian organs have been extended constantly. However, to date, little is known about its effects on the male reproductive system, including the steroidogenesis of mammals. The purpose of this study was to elucidate the effects of BDNF on testosterone generation of Leydig cells and the underlying mechanisms. We found that BDNF-induced proliferation of TM3 Leydig cells via upregulation of proliferating cell nuclear antigen ( Pcna) and promoted testosterone generation as a result of upregulation of steroidogenic acute regulatory protein ( Star), 3b-hydroxysteroid dehydrogenase ( Hsd3b1), and cytochrome P450 side-chain cleavage enzyme ( Cyp11a1) both in primary Leydig cells and TM3 Leydig cells, which were all attenuated in Bdnf knockdown TM3 Leydig cells. Furthermore, the possible mechanism of testosterone synthesis was explored in TM3 Leydig cells. The results showed that BDNF enhanced extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) phosphorylation, and the effect was disrupted by Bdnf deletion. Moreover, PD98059, a potent selective inhibitor of ERK1/2 activation, compromised BDNF-induced testosterone generation and upregulation of Star, Hsd3b1, and Cyp11a1. The Bdnf knockdown assay, on the other hand, indicated the autocrine effect of BDNF on steroidogenesis in TM3 Leydig cells. On the basis of these results, we concluded that BDNF, acting as an autocrine factor, induced testosterone generation as a result of the upregulation of Star, Hsd3b1, and Cyp11a1 via stimulation of the ERK1/2 pathway.

The role of P2X7R/ERK signaling in dorsal root ganglia satellite glial cells in the development of chronic postsurgical pain induced by skin/muscle incision and retraction (SMIR)

The mechanisms of chronic postsurgical pain remain to be elucidated. We reported here that skin/muscle incision and retraction (SMIR), a rat model of postsurgical pain, phosphorylated the extracellular regulated protein kinases (ERK) signaling components c-Raf, MEK (ERK kinase) and ERK1/2 in lumbar 3 dorsal root ganglion (L3 DRG) in rats. Intrathecal injection of ERK specific inhibitor SCH772984 suppressed the mechanical allodynia induced by SMIR. Furthermore, SMIR upregulated tumor necrosis factor alpha (TNFα) in L3 DRG, which could be inhibited by SCH772984. Intrathecal injection of TNF antagonist Etanercept could also inhibit the mechanical allodynia and the increased ERK phosphorylation in L3 DRG induced by SMIR. In addition, immunofluorescent data showed that P2X7R was located exclusively in GFAP labeled satellite glial cells and was highly colocalized with p-ERK1/2 following SMIR. Pretreatment with P2X7R antagonist Brilliant Blue G (BBG) could also block the mechanical allodynia, inhibited the phosphorylation of c-Raf, MEK, ERK1/2, and decrease the expression of TNF-α. Finally, intrathecal injection of BzATP produced mechanical allodynia and induced ERK phosphorylation in satellite glial cells in L3 DRG. Thus, P2X7R activation in satellite glial cells in L3 DRG, leading to a positive feedback between ERK pathway activation and TNF-α production, is suggested to be involved in the induction of chronic postsurgical pain following SMIR.

Accessibility of axonal G protein coupled mu-opioid receptors requires conceptual changes of axonal membrane targeting for pain modulation

The mechanisms of axonal trafficking and membrane targeting are well established for sodium channels, which are the principle targets for perineurally applied local anaesthetics. However, they have not been thoroughly investigated for G protein coupled receptors such as mu-opioid receptors (MOR). Focusing on these axonal mechanisms, we found that axonal MOR functionality is quite distinct in two different pain states, i.e. hindpaw inflammation and nerve injury. We observed axonal membrane MOR binding and functional G protein coupling exclusively at sites of CCI nerve injury. Moreover at these axonal membrane sites, MOR exhibited extensive co-localization with the membrane proteins SNAP and Na/K-ATPase as well as NGF-dependent enhanced lipid rafts and L1CAM anchoring proteins. Silencing endogenous L1CAM with intrathecal L1CAM specific siRNA, disrupting lipid rafts with the perineurial cholesterol-sequestering agent MβCD, as well as suppressing NGF receptor activation with the perineurial NGF receptor inhibitor K252a abrogated MOR axonal membrane integration, functional coupling, and agonist-elicited antinociception at sites of nerve injury. These findings suggest that local conceptual changes resulting from nerve injury are required for the establishment of functional axonal membrane MOR. Axonal integration and subsequent accessibility of functionally coupled MOR are of great relevance particularly for patients suffering from severe pain due to nerve injury or tumour infiltration.

Research progress of the role and mechanism of extracellular signal-regulated protein kinase 5 (ERK5) pathway in pathological pain

Extracellular signal-regulated protein kinase 5 (ERK5), also known as big mitogen-activated protein kinase 1 (MAPK1), is an important member of ERK family, which is a subfamily of the large MAPK family. ERK5 is expressed in many tissues, including the dorsal root ganglion (DRG) neurons and the spinal cord. In this review, we focus on elaborating ERK5-associated pathway in pathological pain, in which the ERK5/CREB (cyclic adenosine monophosphate (cAMP)-response element-binding protein) pathway plays a crucial role in the transduction of pain signal and contributes to pain hypersensitivity. ERK5 activation in the spinal dorsal horn occurs mainly in microglia. The activation of ERK5 can be mediated by N-methyl-D-aspartate (NMDA) receptors. We also elaborate the relationship between ERK5 activation and nerve growth factor-tyrosine kinase A (NGF-TrkA), and the connection between ERK5 activation and brain-derived neurotrophic factor (BDNF) in pathological pain in detail.

The Dorsal Root Ganglion as a Therapeutic Target for Chronic Pain

Chronic neuropathic pain is a widespread problem with negative personal and societal consequences. Despite considerable clinical neuroscience research, the goal of developing effective, reliable, and durable treatments has remained elusive. The critical role played by the dorsal root ganglion (DRG) in the induction and maintenance of chronic pain has been largely overlooked in these efforts, however. It may be that, by targeting this site, robust new options for pain management will be revealed. This review summarizes recent advances in the knowledge base for DRG-targeted treatments for neuropathic pain:• Pharmacological options including the chemical targeting of voltage-dependent calcium channels, transient receptor potential channels, neurotrophin production, potentiation of opioid transduction pathways, and excitatory glutamate receptors.• Ablation or modulation of the DRG via continuous thermal radiofrequency and pulsed radiofrequency treatments.• Implanted electrical neurostimulator technologies.• Interventions involving the modification of DRG cellular function at the genetic level by using viral vectors and gene silencing methods.

RAS and downstream RAF-MEK and PI3K-AKT signaling in neuronal development, function and dysfunction

In postmitotic neurons, the activation of RAS family small GTPases regulates survival, growth and differentiation. Dysregulation of RAS or its major effector pathway, the cascade of RAF-, mitogen-activated and extracellular-signal regulated kinase kinases (MEK), and extracellular-signal regulated kinases (ERK) causes the RASopathies, a group of neurodevelopmental disorders whose pathogenic mechanisms are the subject of intense research. I here summarize the functions of RAS-RAF-MEK-ERK signaling in neurons in vivo, and discuss perspectives for harnessing this pathway to enable novel treatments for nervous system injury, the RASopathies, and possibly other neurological conditions.

Plant Natural Product Puerarin Ameliorates Depressive Behaviors and Chronic Pain in Mice with Spared Nerve Injury (SNI)

Simultaneous relief of the pain from body and brain remains an ongoing challenge. The aim of the present study was to clarify whether plant-derived isoflavone puerarin could ameliorate comorbid depression and pain. We investigated the effects of puerarin on depressive-like behaviors and neuropathic pain in C57BL/6 N mice with spared nerve injury (SNI). After SNI surgery, mice were allowed to recover spontaneously for 7 days and subsequently treated with puerarin, anti-depressant citalopram, and analgesic ibuprofen, alone or in combination, for 8 or 14 days. Forced swim test and tail suspension test were used to assess depressive-like behaviors, whereas von Frey filament test was used to estimate the sensitivity to the mechanical stimulation. Our results suggested that puerarin effectively ameliorated depression and pain in SNI mice although citalopram exhibited anti-depressant activity. In contrast, ibuprofen showed lesser activities against SNI-induced depression and pain. Further mechanistic studies revealed the uniqueness of puerarin as follows: (1) puerarin did not recover SNI-induced depletion of reduced glutathione and loss of superoxide dismutase (SOD), whereas citalopram and ibuprofen showed somewhat antioxidant activities; (2) puerarin markedly promoted the activation of CREB pathway although puerarin and citalopram activated ERK pathway to the same extent; (3) puerarin rapidly and persistently induced brain-derived neurotrophic factor (BDNF) expression whereas citalopram only induced BDNF expression after a prolonged stimulation. Collectively, these results suggest that puerarin may ameliorate the SNI-induced depression and pain via activating ERK, CREB, and BDNF pathways. Puerarin may serve as new lead compound for the development of novel therapeutics for depression and pain comorbidity.

Behavioral and molecular processing of visceral pain in the brain of mice: impact of colitis and psychological stress

Gastrointestinal disorders with abdominal pain are associated with central sensitization and psychopathologies that are often exacerbated by stress. Here we investigated the impact of colitis induced by dextran sulfate sodium (DSS) and repeated water avoidance stress (WAS) on spontaneous and nociception-related behavior and molecular signaling in the mouse brain. DSS increased the mechanical pain sensitivity of the abdominal skin while both WAS and DSS enhanced the mechanical and thermal pain sensitivity of the plantar skin. These manifestations of central sensitization were associated with augmented c-Fos expression in spinal cord, thalamus, hypothalamus, amygdala and prefrontal cortex. While WAS stimulated phosphorylation of mitogen-activated protein kinase (MAPK) p42/44, DSS activated another signaling pathway, both of which converged on c-Fos. The DSS- and WAS-induced hyperalgesia in the abdominal and plantar skin and c-Fos expression in the brain disappeared when the mice were subjected to WAS+DSS treatment. Intrarectal allyl isothiocyanate (AITC) evoked aversive behavior (freezing, reduction of locomotion and exploration) in association with p42/44 MAPK and c-Fos activation in spinal cord and brain. These effects were inhibited by morphine, which attests to their relationship with nociception. DSS and WAS exerted opposite effects on AITC-evoked p42/44 MAPK and c-Fos activation, which indicates that these transduction pathways subserve different aspects of visceral pain processing in the brain. In summary, behavioral perturbations caused by colitis and psychological stress are associated with distinct alterations in cerebral signaling. These findings provide novel perspectives on central sensitization and the sensory and emotional processing of visceral pain stimuli in the brain.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

Neurotrophins and Neuropathic Pain: Role in Pathobiology

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

Increasing progranulin levels and blockade of the ERK1/2 pathway: upstream and downstream strategies for the treatment of progranulin deficient frontotemporal dementia

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disorder marked by mild-life onset and progressive changes in behavior, social cognition, and language. Loss-of-function progranulin gene (GRN) mutations are the major cause of FTLD with TDP-43 protein inclusions (FTLD-TDP). Disease-modifying treatments for FTLD-TDP are not available yet. Mounting evidence indicates that cell cycle dysfunction may play a pathogenic role in neurodegenerative disorders including FTLD. Since cell cycle re-entry of posmitotic neurons seems to precede neuronal death, it was hypothesized that strategies aimed at preventing cell cycle progression would have neuroprotective effects. Recent research in our laboratory revealed cell cycle alterations in lymphoblasts from FTLD-TDP patients carrying a null GRN mutation, and in PGRN deficient SH-SY5Y neuroblastoma cells, involving overactivation of the ERK1/2 signaling pathway. In this work, we have investigated the effects of PGRN enhancers drugs and ERK1/2 inhibitors, in these cellular models of PGRN-deficient FTLD. We report here that both restoring the PGRN content, by suberoylanilide hydroxamic acid (SAHA) or chloroquine (CQ), as blocking ERK1/2 activation by selumetinib (AZD6244) or MEK162 (ARRY-162), normalized the CDK6/pRb pathway and the proliferative activity of PGRN deficient cells. Moreover, we found that SAHA and selumetinib prevented the cytosolic TDP-43 accumulation in PGRN-deficient lymphoblasts. Considering that these drugs are able to cross the blood-brain barrier, and assuming that the alterations in cell cycle and signaling observed in lymphoblasts from FTLD patients could be peripheral signs of the disease, our results suggest that these treatments may serve as novel therapeutic drugs for FTLD associated to GRN mutations.

Pulsed radiofrequency: a review of the basic science as applied to the pathophysiology of radicular pain: a call for clinical translation

Radicular pain is an important health care problem, with only limited evidence-based treatments available. Treatment selection should ideally target documented pathophysiological pathways. In herniated discs, a sequence in the inflammatory cascade can be observed that initiates and maintains increased nociceptive signal input. Inflammatory mediators including tumor necrosis factor α are released from the nucleus pulposus and the degenerating peripheral nerve, which, in turn, induces production of neurotrophins like nerve growth factor and brain-derived neurotrophic factor. Neurotrophins interfere not only with the generation of ectopic firing of nociceptive neurons in the dorsal root ganglion but also with the excitability and sensitization of neuronal transmission in the dorsal spinal horn. Radicular pain is further characterized by the electrophysiological spreading of the afferent nociceptive input over different spinal nerve roots. Both the complex pathophysiological pathways involved and the spreading of the nociceptive signal make radicular pain difficult to treat. Pulsed radiofrequency (PRF) is considered an option in treatment of radicular pain. To understand and increase the efficiency of PRF interventional treatments in radicular pain, both in vitro and in vivo studies aiming at elucidating part of the mechanism of action of PRF are described. Potential factors that may improve the efficacy of PRF treatment in radicular pain are discussed.

Changes in compressed neurons from dogs with acute and severe cauda equina constrictions following intrathecal injection of brain-derived neurotrophic factor-conjugated polymer nanoparticles

This study established a dog model of acute multiple cauda equina constriction by experimental constriction injury (48 hours) of the lumbosacral central processes in dorsal root ganglia neurons. The repair effect of intrathecal injection of brain-derived neurotrophic factor with 15 mg encapsulated biodegradable poly(lactide-co-glycolide) nanoparticles on this injury was then analyzed. Dorsal root ganglion cells (L₇) of all experimental dogs were analyzed using hematoxylin-eosin staining and immunohistochemistry at 1, 2 and 4 weeks following model induction. Intrathecal injection of brain-derived neurotrophic factor can relieve degeneration and inflammation, and elevate the expression of brain-derived neurotrophic factor in sensory neurons of compressed dorsal root ganglion. Simultaneously, intrathecal injection of brain-derived neurotrophic factor obviously improved neurological function in the dog model of acute multiple cauda equina constriction. Results verified that sustained intraspinal delivery of brain-derived neurotrophic factor encapsulated in biodegradable nanoparticles promoted the repair of histomorphology and function of neurons within the dorsal root ganglia in dogs with acute and severe cauda equina syndrome.

Spinal mitogen-activated protein kinase phosphatase-3 (MKP-3) is necessary for the normal resolution of mechanical allodynia in a mouse model of acute postoperative pain

The mechanisms that drive the normal resolution of acute postoperative pain are not completely understood. We hypothesize a pivotal role of a major spinal mitogen-activated protein kinase (MAPKs) regulator, MAPK phosphatase (MKP)-3, in the resolution of postoperative pain. We used wild-type and MKP-3 knock-out (KO) mice, a paw incision model of acute postoperative pain, and behavioral and molecular biology experiments. We observed persistent mechanical allodynia in mice lacking MKP-3 (postoperative day 21), concurrently with persistent phosphorylation of spinal p38 and extracellular signal-regulated kinases (ERK)-1/2 on postoperative day 12, while both MAPK phosphorylation and allodynia resolved on postoperative day 7 in wild-type mice. Spinal p-ERK was expressed mainly in neurons and microglia, while spinal p-p38 was expressed mostly in microglia in MKP-3 KO mice, and their selective pharmacological inhibition reduced the persistent allodynia observed in these mice. Our findings strongly suggest that dysregulation of MKP-3 prevents spontaneous resolution of acute postoperative pain and drives its transition to persistent pain via persistent neuronal and microglial MAPK phosphorylation in the spinal cord.

Neonatal bee venom exposure induces sensory modality-specific enhancement of nociceptive response in adult rats

OBJECTIVE

Previous studies have shown that inflammatory pain at the neonatal stage can produce long-term structural and functional changes in nociceptive pathways, resulting in altered pain perception in adulthood. However, the exact pattern of altered nociceptive response and associated neurochemical changes in the spinal cord in this process is unclear.

METHOD

In this study, we used an experimental paradigm in which each rat first received intraplantar bee venom (BV) or saline injection on postnatal day 1, 4, 7, 14, 21, or 28. This was followed 2 months later by a second intraplantar bee venom injection in the same rats to examine the difference in nociceptive responses.

RESULTS

We found that neonatal inflammatory pain induced by the first BV injection significantly reduced baseline paw withdrawal mechanical threshold, but not baseline paw withdrawal thermal latency, when rats were examined 2 months from the first BV injection. Neonatal inflammatory pain also exacerbated mechanical, but not thermal, hyperalgesia in response to the second BV injection in these same rats. Rats exposed to neonatal inflammation also showed up-regulation of spinal NGF, TrkA receptor, BDNF, TrkB receptor, IL-1β, and COX-2 expression following the second BV injection, especially with prior BV exposure on postnatal day 21 or 28.

CONCLUSION

These results indicate that neonatal inflammation produces sensory modality-specific changes in nociceptive behavior and alters neurochemistry in the spinal cord of adult rats. These results also suggest that a prior history of inflammatory pain during the developmental period might have an impact on clinical pain in highly susceptible adult patients.

Epigenetic regulation of BDNF expression in the primary sensory neurons after peripheral nerve injury: implications in the development of neuropathic pain

Brain-derived neurotrophic factor (BDNF) is known to be up-regulated in the dorsal root ganglion (DRG) after peripheral nerve injury, and to contribute to neuropathic pain. Here, we found that thermal hyperalgesia and mechanical allodynia at day 7 post-injury were inhibited only when anti-BDNF antibody was intrathecally administrated at day 2 post-injury. Consistent with behavioral results, Western blot analysis showed that the expression levels of BDNF protein in the spinal dorsal horn were markedly induced during early stage post-injury. Moreover, the maximal increase in BDNF mRNA expression in the DRG was observed at day 1 post-injury, and significantly elevated levels were sustained for at least 14 days. Four of five BDNF mRNA transcripts were up-regulated after nerve injury, and the most inducible transcript was exon I. Using a chromatin immunoprecipitation (ChIP) assay, we found that nerve injury promotes histone H3 and H4 acetylation, transcriptionally active modifications, at BDNF promoter I at day 1 post-injury, and the levels of histone acetylation remain elevated for at least 7 days. Taken together, our findings suggest that an initial increase in BDNF exon I expression controlled by epigenetic mechanisms might have a crucial role in the development of neuropathic pain.

Spinal nerve injury causes upregulation of ErbB2 and ErbB3 receptors in rat dorsal root ganglia

It is generally known that peripheral nerve injury causes changes in expression of some growth factors in the dorsal root ganglion. Altered expression of ErbB receptors, a well-known growth factor in somatic cells, reportedly follows peripheral nerve injury in the spinal dorsal horn; however, it remains unknown whether the expression of these receptors is altered in the dorsal root ganglion after nerve injury. Therefore, this study examined the gene expression profiles of ErbB receptors in bilateral lumbar (L)4/L5 dorsal root ganglia, using L5-selective spinal nerve ligation in model rats as a peripheral nerve injury model. The expression of ErbB2 and ErbB3 was observed in the dorsal root ganglia of the mature rat, despite ErbB1 and ErbB4 showing only subtle expression. We also demonstrated that peripheral nerve injury induced significant increases in ErbB2 and ErbB3 in the ipsilateral dorsal root ganglion as compared with uninjured nerve. Expression changes in ErbB receptors appear to play important roles in nerve injury and subsequent nerve regeneration.

Pleiotrophin promotes microglia proliferation and secretion of neurotrophic factors by activating extracellular signal-regulated kinase 1/2 pathway

Pleiotrophin (PTN) is an effective neuroprotective factor and its expression is strikingly increased in microglia after ischemia/reperfusion injury. However, whether PTN could provide neurotrophic support to neurons by regulating microglia function is not clear. In this study, we demonstrated that the expression of PTN was induced in microglia after oxygen-glucose deprivation/reperfusion. PTN promoted the proliferation of microglia by enhancing the G1 to S phase transition. PTN also stimulated the secretion of brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and nerve growth factor (NGF) in microglia, but did not upregulate the expression of proinflammatory factors such as TNF-α, IL-1β and iNOS. Mechanistically, we found that PTN increased the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 in microglia in both concentration-dependent and time-dependent manners. In addition, ERK1/2 inhibitor U0126 abolished the proliferation and G1 to S phase transition of microglia stimulated by PTN, and inhibited the production of BDNF, CNTF and NGF induced by PTN. In conclusion, our results demonstrated that PTN-ERK1/2 pathway plays important role in regulating microglia growth and secretion of neurotrophic factors. These findings provide new insight into the neuroprotective role of PTN and suggest that PTN is a new target for therapeutic intervention of stroke.