Regular Article
Prostaglandin E2 contributes to the synthesis of brain-derived neurotrophic factor in primary sensory neuron in ganglion explant cultures and in a neuropathic pain model

https://doi.org/10.1016/j.expneurol.2012.01.021Get rights and content

Abstract

Brain-derived neurotrophic factor (BDNF) exists in small to medium size neurons in adult rat dorsal root ganglion (DRG) and serves as a modulator at the first synapse of the pain transmission pathway in the spinal dorsal horn. Peripheral nerve injury increases BDNF expression in DRG neurons, an event involved in the genesis of neuropathic pain. In the present study, we tested the hypothesis that prostaglandin E2 (PGE2) over-produced in injured nerves contributes to the up-regulation of BDNF in DRG neurons. Two weeks after partial sciatic nerve ligation (PSNL), BDNF levels in the ipsilateral L4–L6 DRG of injured rats were significantly increased compared to the contralateral side. Perineural injection of a selective cyclooxygenase (COX2) inhibitor or a PGE2 EP4 receptor antagonist not only dose-dependently relieved PSNL elicited mechanical hypersensitivity, but also suppressed the increased BDNF levels in DRG neurons. PSNL shifted BDNF expression in the ipsilateral DRG from small to medium and larger size injured neurons. BDNF is mainly co-expressed with the EP1 and EP4 while moderately with the EP2 and EP3 receptor subtypes in naïve and PSNL rats. PSNL also shifted the expression of EP1–4 receptors to a larger size population of DRG neurons. In DRG explant cultures, a stabilized PGE2 analog 16,16 dimethyl PGE2 (dmPGE2) or the agonists of EP1 and EP4 receptors significantly increased BDNF levels and the phosphorylated protein kinase A (PKA), extracellular signal-regulated kinase (ERK)/mitogen activated protein kinase (MAPK) and cAMP response element binding protein (CREB). The EP1 and EP4 antagonists, a sequester of nerve growth factor (NGF), the inhibitors of PKA and MEK as well as CREB small interfering RNA suppressed dmPGE2-induced BDNF. Taken together, EP1 and EP4 receptor subtypes, PKA, ERK/MAPK and CREB signaling pathways as well as NGF are involved in PGE2-induced BDNF synthesis in DRG neurons. Injured nerve derived-PGE2 contributes to BDNF up-regulation in DRG neurons following nerve injury. Facilitating the synthesis of BDNF in primary sensory neurons is a novel mechanism underlying the role of PGE2 in the genesis of neuropathic pain.

Highlights

►COX2 inhibitor or EP4 antagonist suppressed nerve injury increased BDNF in DRG neurons. ►BDNF expression is shifted to medium and larger size injured DRG neurons. ►In DRG explant cultures, PGE2 increased BDNF levels. ►EP1 and EP4 receptors, PKA, ERK/MAPK and CREB signaling pathways were involved. ►Injured nerve-derived PGE2 contributes to BDNF up-regulation in DRG neurons following nerve injury.

Introduction

Chronic neuropathic pain is a serious clinical concern which affects more than 6% of the population in the developed countries. This pain condition is usually caused by direct injury or diseases of the nervous systems responsible for pain transmission and perception. Neuropathic pain is generally manifested as spontaneous pain, hyperalgesia (exaggerated response to painful stimulation) and allodynia (painful response to innocuous stimulation). Due to poorly understood underlying mechanisms, the treatment of neuropathic pain is too often unsatisfactory. Thus vigorous research has focused on its central and peripheral mechanisms to uncover novel therapeutic targets. As a peripheral mechanism, the role of the inflammatory responses occurring in injured nerves has drawn great attention. We and others have previously shown that cyclooxygenase 2 (COX2), the rate limiting enzyme in the synthesis of the well-known pain mediator prostaglandin E2 (PGE2), was dramatically increased in invading macrophages (Ma and Eisenach, 2002, Ma and Eisenach, 2003a) and Schwann cells (Muja and Devries, 2004, Takahashi et al., 2004) of injured nerves. Levels of PGE2 in injured nerves are consequently elevated (Ma and Quirion, 2005, Muja and Devries, 2004, Schafers et al., 2004). Four PGE2 EP receptors, particularly EP1 and EP4, are up-regulated in injured nerves (Ma and Eisenach, 2003b) and DRG neurons (Ma et al., 2010a).

Increased PGE2 level in injured nerves is a long-lasting event in rats and human with nerve injury (Durrenberger et al., 2006, Ma et al., 2010a) and thus likely exerts long-term effects on EP receptor bearing cells, e.g. invading macrophages through autocrine or paracrine pathways, and on nociceptive DRG neurons (nociceptors) by stimulating en passant injured or spared axons, a hypothesis that we previously formulated (Ma and Quirion, 2008). Of these chronic effects, facilitating the production of pain-related mediators such as neuropeptides, growth factors, ion channels, cytokines and chemokines in injured nerves as well as in DRG neurons is highly possible. In fact, the maintenance of neuropathic pain mainly depends on the long-term up-regulation of these pain-related molecules (Scholz and Woolf, 2007). Therefore, the persistent facilitation of the synthesis of pain-related mediators in injured nerves and DRG might be one of the mechanisms underlying the role of injured nerve-derived COX2 and PGE2 in the initiation and maintenance of neuropathic pain. Indeed, we have previously shown that injured nerve derived-COX2 and PGE2 facilitate the up-regulation of pro-inflammatory cytokine interleukin-6 (IL-6) (Ma and Quirion, 2005) and pain peptide calcitonin gene-related peptide (CGRP) (Ma and Quirion, 2006, Ma et al., 2010b) in invading macrophages following partial sciatic nerve ligation (PSNL). We recently showed that injured nerve-derived COX2 and PGE2 are involved in the up-regulation of IL-6 (St-Jacques and Ma, 2011) and in the production of pain-related peptide substance P (SP) and CGRP as well as transient receptor potential vanilloid-1 (TRPV1) in DRG neurons (Ma, 2010, Ma et al., 2010a) following PSNL. In the current study, we explored the contributing role of injured nerve derived COX2 and PGE2 in the up-regulation of the pain-related neurotrophin brain-derived neurotrophic factor (BDNF) in DRG neurons of rats experiencing neuropathic pain elicited by PSNL.

The role of BDNF in adulthood to serve as a pain modulator at spinal dorsal horn level has been well documented (Merighi et al., 2008, Pezet et al., 2002, Thompson et al., 1999). BDNF exists in small to medium size DRG neurons of naïve adult rats (Zhou et al., 1999). It can be anterogradely transported to the superficial spinal dorsal horn and released as a neuromodulator to act on its high-affinity tyrosine kinase receptor, trkB, present in nociceptive neurons, thus modulating the synaptic efficacy of the first synapse in the nociception pathway (Merighi et al., 2008, Pezet et al., 2002, Thompson et al., 1999). BDNF is up-regulated in DRG neurons following peripheral inflammation and nerve injury (Obata and Noguchi, 2006). A growing body of evidence has shown that the increased levels of BDNF in DRG neurons and spinal dorsal horn contribute to the genesis of neuropathic pain through BDNF/trkB signaling (Deng et al., 2000, Fukuoka et al., 2001, Quintao et al., 2008, Wang et al., 2009, Yajima et al., 2002, Yajima et al., 2005, Zhou et al., 2000)

Although it is generally agreed that BDNF is up-regulated in DRG neurons following nerve injury, the type of DRG neurons up-regulating BDNF levels remains controversial. After either complete or partial nerve injury, BDNF was shown to be down-regulated in small and medium neurons and de novo synthesized in medium and large neurons (Zhou et al., 1999), over-expressed in uninjured small neurons (Fukuoka et al., 2001), or increased in small, medium and large size DRG neurons (Ha et al., 2001). It is necessary to use tract tracing method to revisit this issue in the PSNL model in which injured and spared DRG neurons are intermingled. Most importantly, it is necessary to determine the factors which are responsible for the up-regulation of BDNF in DRG neurons following nerve injury. NGF has been well known to induce BDNF in trkA containing DRG neurons after nerve injury (Pezet and Mcmahon, 2006). PGE2 could be another candidate factor for the induction of BDNF as it was shown to induce NGF and BDNF in brain neurons (Rage et al., 2006) and astrocytes (Toyomoto et al., 2004). Thus, it is highly possible that injured nerve derived-COX2 and PGE2 contributes, directly or indirectly through NGF, to the up-regulation of BDNF in DRG neurons resulting from nerve injury.

Therefore, in the present study we attempted to address four specific aims to test this hypothesis. Our first aim was to identify the DRG neuron type(s) in which BDNF is up-regulated following PSNL. The second aim was to determine if blocking COX2 and PGE2 signaling suppresses the up-regulated levels of BDNF in DRG neurons. The third aim was to determine if exogenous PGE2 directly increases BDNF protein levels in cultured DRG neurons. The fourth aim was to determine if NGF is involved in PGE2-induced BDNF synthesis. Finally, we attempted to identify the EP receptor subtype(s) and signaling transduction pathways underlying PGE2-induced BDNF. Part of data in this study has been reported in abstract form (Cruz Duarte et al., 2010).

Section snippets

Partial sciatic nerve ligation

Sprague–Dawley rats (male, 250–350 g in body weight, Charles River, St-Constant, Québec, Canada) were used in this study. Animal care and maintenance were in accordance with the protocols and guidelines approved by McGill University Animal Care Committee and the Canadian Council for Animal Care. Adequate measures have been taken to minimize pain and discomfort of animals. Anesthetic inhalation of isoflurane (5% for induction and 2% for maintenance, Animal Source Center, McGill University,

Partial sciatic nerve ligation up-regulates BDNF levels in the ipsilateral L4–6 DRG neurons

In DRG of naive rats (not shown) or in the uninjured contralateral DRG of PSNL rats (Fig. 1G), only a small proportion of DRG neurons expressed BDNF (about 20%) and the majority of BDNF-IR neurons were of small to medium size. Two weeks after PSNL, the percentage of BDNF-IR neurons was significantly increased in the ipsilateral DRG (Fig. 1G, p < 0.001). A size frequency analysis revealed that the majority of BDNF-IR neurons in the ipsilateral DRG of PSNL rats were of small (< 500 μm2 in soma area)

Injured nerve derived-COX2 and PGE2 contribute to the up-regulation of BDNF in the ipsilateral DRG neurons

The role of BDNF in nociception, inflammatory pain and neuropathic pain has been explored extensively (Merighi et al., 2008, Obata and Noguchi, 2006, Shu and Mendell, 1999). It is generally agreed that BDNF released from primary sensory afferents serves as a neuromodulator at the first synapse of nociception pathway at the spinal dorsal horn level. The modulating effects of BDNF on nociception are mediated through its trkB receptor which is enriched in both DRG neurons and dorsal horn neurons (

Acknowledgments

This study was supported by the operating grants from Canadian Institutes of Health Research to Weiya Ma (grant nos. 87372 and 89892).

References (64)

  • W. Ma et al.

    Medium and large injured dorsal root ganglion cells increase TRPV-1, accompanied by increased alpha2C-adrenoceptor co-expression and functional inhibition by clonidine

    Pain

    (2005)
  • W. Ma et al.

    Injured nerve-derived COX2/PGE2 contributes to the maintenance of neuropathic pain in aged rats

    Neurobiol. Aging

    (2010)
  • A. Merighi et al.

    Neurotrophins in spinal cord nociceptive pathways

    Prog. Brain Res.

    (2004)
  • A. Merighi et al.

    BDNF as a pain modulator

    Prog. Neurobiol.

    (2008)
  • K. Miki et al.

    Calcitonin gene-related peptide increase in the rat spinal dorsal horn and dorsal column nucleus following peirpheral nerve injury: up-regulation in a subpopulation of primary afferent sensory neurons

    Neuroscience

    (1997)
  • K. Obata et al.

    BDNF in sensory neurons and chronic pain

    Neurosci. Res.

    (2006)
  • S. Pezet et al.

    BDNF: a neuromodulator in nociceptive pathways?

    Brain Res. Rev.

    (2002)
  • N.L. Quintao et al.

    The role of neurotrophic factors in genesis and maintenance of mechanical hypernociception after brachial plexus avulsion in mice

    Pain

    (2008)
  • F. Rage et al.

    IL-1beta regulation of BDNF expression in rat cultured hypothalamic neurons depends on the presence of glial cells

    Neurochem. Int.

    (2006)
  • M. Schafers et al.

    Cyclooxygenase inhibition in nerve-injury- and TNF-induced hyperalgesia in the rat

    Exp. Neurol.

    (2004)
  • Z. Seltzer et al.

    A novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury

    Pain

    (1990)
  • H. Sun et al.

    Nerve injury-induced tactile allodynia is mediated via ascending spinal dorsal column projections

    Pain

    (2001)
  • J.P. Syriatowicz et al.

    Hyperalgesia due to nerve injury: role of prostaglandins

    Neuroscience

    (1999)
  • M. Takahashi et al.

    Cyclooxygenase-2 expression in Schwann cells and macrophages in the sciatic nerve after single spinal nerve injury in rats

    Neurosci. Lett.

    (2004)
  • X. Tao et al.

    Ca2 + influx regulates BDNF transcription by a CREB family transcription factor-dependent mechanism

    Neuron

    (1998)
  • M. Toyomoto et al.

    Prostaglandins are powerful inducers of NGF and BDNF production in mouse astrocyte cultures

    FEBS Lett.

    (2004)
  • H. Vanegas et al.

    Prostaglandins and cycloxygenases in the spinal cord

    Prog. Neurobiol.

    (2001)
  • Y. Yajima et al.

    Involvement of a spinal brain-derived neurotrophic factor/full-length TrkB pathway in the development of nerve injury-induced thermal hyperalgesia in mice

    Brain Res.

    (2002)
  • J. Zhao et al.

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

    Mol. Cell. Neurosci.

    (2006)
  • X.F. Zhou et al.

    Injured primary sensory neurons switch phenotype for brain-derived neurotrophic factor in the rat

    Neuroscience

    (1999)
  • L.J. Zhou et al.

    Limited BDNF contributes to the failure of injury to skin afferents to produce a neuropathic pain condition

    Pain

    (2010)
  • P. Cruz Duarte et al.

    Contribution of injured nerve-derived COX2/PGE2 to the up-regulation of brain derived neurotrophic factor in dorsal root ganglion neurons of neuropathic rats

    Soc. Neurosci.

    (2010)
  • Cited by (50)

    • The omega-3 lipid 17,18-EEQ sensitizes TRPV1 and TRPA1 in sensory neurons through the prostacyclin receptor (IP)

      2020, Neuropharmacology
      Citation Excerpt :

      The involvement of PKA in 17,18-EEQ-mediated TRPV1 sensitization implies activation of a Gs-coupled receptor of the lipid in sensory neurons. The most abundantly expressed lipid receptors in the DRGs that are known to be promiscuous in their ligand binding are the prostanoid receptors, DP1, EP1-4, FP, TP and IP (Cruz Duarte et al., 2012; Narumiya et al., 1999; Zinn et al., 2017). From these receptors only the DP1, EP2, EP4 and IP-receptor is Gs-coupled and activates PKA (Hirata and Narumiya, 2011).

    • Erythrocyte membrane-encapsulated celecoxib improves the cognitive decline of Alzheimer's disease by concurrently inducing neurogenesis and reducing apoptosis in APP/PS1 transgenic mice

      2017, Biomaterials
      Citation Excerpt :

      As a downstream target of COX-2, prostaglandin E2 (PGE2) has exhibited positive effects on neurogenesis in the dentate gyrus of adult rats [4]. Indeed, PGE2 contributed to the synthesis of brain-derived neurotrophic factor (BDNF) in adult rat dorsal root ganglion (DRG) neurons, which potentially influence neurogenesis [5]. In addition, EP2 has been found to be responsible for the production of BDNF in human glial cells [6].

    View all citing articles on Scopus
    View full text