Accumulating evidence indicates that activation of spinal cord microglia plays an important role in the genesis of neuropathic pain. Resolvin E1 (E1) is derived from omega-3 polyunsaturated fatty acid and exhibits potent anti-inflammatory, pro-resolution, and anti-nociceptive effects. We further examined whether RvE1 could reduce neuropathic pain and modulate spinal cord microglial activation. Intrathecal pre-treatment of RvE1 (100 ng) daily for 3 days partially prevented the development of nerve injury-induced mechanical allodynia and up-regulation of IBA-1 (microglial marker) and TNF-α in the spinal cord dorsal horn. Furthermore, intrathecal post-treatment of RvE1 (100 ng), 3 weeks after nerve injury, transiently reduced mechanical allodynia and heat hyperalgesia. Finally, RvE1 blocked lipopolisaccharide-induced microgliosis and TNF-α release in primary micoglial cultures. Our data suggest that RvE1 may attenuate neuropathic pain via inhibiting microglial signaling. Targeting the anti-inflammatory and pro-resolution lipid mediators may offer new options for preventing and treating neuropathic pain.
Eicosapentaenoic acid (EPA); Tumor necrosis factor-alpha TNF-α; lipopolisacride (LPS); nerve injury; omega-3 poly unsaturated fatty acids; spinal cord
Increasing evidence indicates that the pathogenesis of neuropathic pain is mediated through spinal cord microglia activation. The intracellular protease caspase-6 (CASP6) is known to regulate neuronal apoptosis and axonal degeneration; however, the contribution of microglia and CASP6 in modulating synaptic transmission and pain is unclear. Here, we found that CASP6 is expressed specifically in C-fiber axonal terminals in the superficial spinal cord dorsal horn. Animals exposed to intraplantar formalin or bradykinin injection exhibited CASP6 activation in the dorsal horn. Casp6-null mice had normal baseline pain, but impaired inflammatory pain responses. Furthermore, formalin-induced second-phase pain was suppressed by spinal injection of CASP6 inhibitor or CASP6-neutralizing antibody, as well as perisciatic nerve injection of CASP6 siRNA. Recombinant CASP6 (rCASP6) induced marked TNF-α release in microglial cultures, and most microglia within the spinal cord expressed Tnfa. Spinal injection of rCASP6 elicited TNF-α production and microglia-dependent pain hypersensitivity. Evaluation of excitatory postsynaptic currents (EPSCs) revealed that rCASP6 rapidly increased synaptic transmission in spinal cord slices via TNF-α release. Interestingly, the microglial inhibitor minocycline suppressed rCASP6 but not TNF-α–induced synaptic potentiation. Finally, rCASP6-activated microglial culture medium increased EPSCs in spinal cord slices via TNF-α. Together, these data suggest that CASP6 released from axonal terminals regulates microglial TNF-α secretion, synaptic plasticity, and inflammatory pain.
Epoxyeicosatrienoic acids (EETs) are cytochrome P450-epoxygenase-derived metabolites of arachidonic acid that act as endogenous signaling molecules in multiple biological systems. Here we have investigated the specific contribution of 5,6-EET to transient receptor potential (TRP) channel activation in nociceptor neurons and its consequence for nociceptive processing. We found that, during capsaicin-induced nociception, 5,6-EET levels increased in dorsal root ganglia (DRGs) and the dorsal spinal cord, and 5,6-EET is released from activated sensory neurons in vitro. 5,6-EET potently induced a calcium flux (100 nm) in cultured DRG neurons that was completely abolished when TRPA1 was deleted or inhibited. In spinal cord slices, 5,6-EET dose dependently enhanced the frequency, but not the amplitude, of spontaneous EPSCs (sEPSCs) in lamina II neurons that also responded to mustard oil (allyl isothiocyanate), indicating a presynaptic action. Furthermore, 5,6-EET-induced enhancement of sEPSC frequency was abolished in TRPA1-null mice, suggesting that 5,6-EET presynaptically facilitated spinal cord synaptic transmission by TRPA1. Finally, in vivo intrathecal injection of 5,6-EET caused mechanical allodynia in wild-type but not TRPA1-null mice. We conclude that 5,6-EET is synthesized on the acute activation of nociceptors and can produce mechanical hypersensitivity via TRPA1 at central afferent terminals in the spinal cord.
Resolvins, including D and E series resolvins, are endogenous lipid mediators generated during the resolution phase of acute inflammation from the omega-3 polyunsaturated fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Resolvins are known to have potent anti-inflammatory and pro-resolution actions in several animal models of inflammation. Recent findings also demonstrate that resolvin E1 and resolvin D1 can each potently dampen inflammatory and postoperative pain. This review focuses on the mechanisms by which resolvins act on their receptors in immune cells and neurons to normalize exaggerated pain, via regulating inflammatory mediators, transient receptor potential (TRP) ion channels, and spinal cord synaptic transmission. Resolvins may offer novel therapeutic approaches for preventing and treating pain conditions associated with inflammation.
Itch, also known as pruritus, is a common, intractable symptom of several skin diseases, such as atopic dermatitis and xerosis. TLRs mediate innate immunity and regulate neuropathic pain, but their roles in pruritus are elusive. Here, we report that scratching behaviors induced by histamine-dependent and -independent pruritogens are markedly reduced in mice lacking the Tlr3 gene. TLR3 is expressed mainly by small-sized primary sensory neurons in dorsal root ganglions (DRGs) that coexpress the itch signaling pathway components transient receptor potential subtype V1 and gastrin-releasing peptide. Notably, we found that treatment with a TLR3 agonist induces inward currents and action potentials in DRG neurons and elicited scratching in WT mice but not Tlr3–/– mice. Furthermore, excitatory synaptic transmission in spinal cord slices and long-term potentiation in the intact spinal cord were impaired in Tlr3–/– mice but not Tlr7–/– mice. Consequently, central sensitization–driven pain hypersensitivity, but not acute pain, was impaired in Tlr3–/– mice. In addition, TLR3 knockdown in DRGs also attenuated pruritus in WT mice. Finally, chronic itch in a dry skin condition was substantially reduced in Tlr3–/– mice. Our findings demonstrate a critical role of TLR3 in regulating sensory neuronal excitability, spinal cord synaptic transmission, and central sensitization. TLR3 may serve as a new target for developing anti-itch treatment.
Activation of spinal cord glial cells such as microglia and astrocytes has been shown to regulate chronic opioid-induced antinociceptive tolerance and hyperalgesia, due to spinal up-regulation of the proinflammatory cytokines such as interleukin-1 beta (IL-1β). Matrix metalloprotease-9 (MMP-9) has been implicated in IL-1β activation in neuropathic pain. However, it is unclear whether acute opioid treatment can activate glial cells in the peripheral nervous system. We examined acute morphine-induced activation of satellite glial cells (SGCs) and up-regulation of IL-1β in dorsal root ganglia (DRGs), and further investigated the involvement of MMP-9 in these opioid-induced peripheral changes.
Subcutaneous morphine injection (10 mg/kg) induced robust peripheral glial responses, as evidenced by increased GFAP expression in DRGs but not in spinal cords. The acute morphine-induced GFAP expression is transient, peaking at 2 h and declining after 3 h. Acute morphine treatment also increased IL-1β immunoreactivity in SGCs and IL-1β activation in DRGs. MMP-9 and GFAP are expressed in DRG neurons and SGCs, respectively. Confocal analysis revealed a close proximity of MMP-9 and GFAP immunostaining. Importantly, morphine-induced DRG up-regulation of GFAP expression and IL-1β activation was abolished after Mmp9 deletion or naloxone pre-treatment. Finally, intrathecal injections of IL-1β-selective siRNA not only reduced DRG IL-1β expression but also prolonged acute morphine-induced analgesia.
Acute morphine induces opioid receptors- and MMP-9-dependent up-regulation of GFAP expression and IL-1β activation in SGCs of DRGs. MMP-9 could mask and shorten morphine analgesia via peripheral neuron-glial interactions. Targeting peripheral glial activation might prolong acute opioid analgesia.
Tumor necrosis factor-alpha (TNF-α) is a key proinflammatory cytokine. It is generally believed that TNF-α exerts its effects primarily via TNF receptor subtype-1 (TNFR1). We investigated distinct role of TNFR1 and TNFR2 in spinal cord synaptic transmission and inflammatory pain. Compared to wild-type (WT) mice, TNFR1 and TNFR2 knockout (KO) mice exhibited normal heat sensitivity and unaltered excitatory synaptic transmission in the spinal cord, as revealed by spontaneous excitatory postsynaptic currents (sEPSCs) in lamina II neurons of spinal cord slices. However, heat hyperalgesia after intrathecal TNF-α and the second-phase spontaneous pain in the formalin test were reduced in both TNFR1- and TNFR2-KO mice. In particular, heat hyperalgesia after intraplantar injection of complete Freund's adjuvant (CFA) was decreased in the early phase in TNFR2-KO mice but reduced in both early and later phase in TNFR1-KO mice. Consistently, CFA elicited a transient increase of TNFR2 mRNA levels in the spinal cord on day 1. Notably, TNF-α evoked a drastic increase in sEPSC frequency in lamina II neurons, which was abolished in TNFR1-KO mice and reduced in TNFR2-KO mice. TNF-α also increased NMDA currents in lamina II neurons, and this increase was abolished in TNFR1-KO mice but retained in TNFR2-KO mice. Finally, intrathecal injection of the NMDA receptor antagonist MK-801 prevented heat hyperalgesia elicited by intrathecal TNF-α. Our findings support a central role of TNF-α in regulating synaptic plasticity (central sensitization) and inflammatory pain via both TNFR1 and TNFR2. Our data also uncover a unique role of TNFR2 in mediating early-phase inflammatory pain.
proinflammatory cytokine; central sensitization; TNFR1; TNFR2; formalin; complete Freund's adjuvant
Inflammatory pain such as arthritic pain is typically treated with opioids and COX-2 inhibitors with well-known side effects. Transient receptor potential subtype V1 (TRPV1) and A1 (TRPA1) contribute importantly to the genesis of inflammatory pain via both peripheral mechanisms (peripheral sensitization) and spinal cord mechanisms (central sensitization). Although these TRP channels have been intensively studied, little is known about their endogenous inhibitors. Recent studies have demonstrated that the endogenous lipid mediators resolvins (RvE1 and RvD1), derived from omega-3 unsaturated fatty acids, are potent inhibitors for inflammatory pain, without noticeable side effects. However, the molecular mechanisms underlying resolvins’ distinct analgesic actions in mice are unclear. Resolvin D2 (RvD2) is a novel family member of resolvins. Here we report that RvD2 is a remarkably potent inhibitor of TRPV1 (IC50=0.1 nM) and TRPA1 (IC50= 2 nM) in primary sensory neurons, whereas RvE1 and RvD1 selectively inhibited TRPV1 (IC50=1 nM) and TRPA1 (IC50=9 nM), respectively. Accordingly, RvD2, RvE1, and RvD1 differentially regulated TRPV1 and TRPA1 agonist-elicited acute pain and spinal cord synaptic plasticity (sEPSC frequency increase). RvD2 also abolished inflammation-induced sEPSC increases (frequency and amplitude), without affecting basal synaptic transmission. Intrathecal administration of RvD2 at very low doses (0.01-1 ng) prevented formalin-induced spontaneous pain. Intrathecal RvD2 also reversed adjuvant-induced inflammatory pain without altering baseline pain and motor function. Finally, intrathecal RvD2 reversed C-fiber stimulation-evoked long-term potentiation in the spinal cord. Our findings suggest distinct roles of resolvins in regulating TRP channels and identify RvD2 as a potent endogenous inhibitor for TRPV1/A1 and inflammatory pain.
docosahexaenoic acid (DHA); eicosapentaenoic acid (EPA); long-term potentiation; omega-3 polyunsaturated fatty acid; primary sensory neurons; spontaneous EPSCs
Mechanisms of inflammatory pain are not fully understood. We investigated the role of TRPV1 and TNF-α, two critical mediators for inflammatory pain, in regulating spinal cord synaptic transmission. We found in mice lacking Trpv1 the frequency but not the amplitude of spontaneous EPSCs (sEPSCs) in lamina II neurons of spinal cord slices is reduced. Further, C-fiber-induced spinal long-term potentiation (LTP) in vivo is abolished in Trpv1 knockout mice. TNF-α also increases sEPSC frequency but not amplitude in spinal lamina IIo neurons, and this increase is abolished in Trpv1 knockout mice. Single-cell PCR analysis revealed that TNF-α-responding neurons in lamina IIo are exclusively excitatory (vGluT2+) neurons. Notably, neuroprotectin-1 (NPD1), an anti-inflammatory lipid mediator derived from omega-3 polyunsaturated fatty acid (docosahexaenoic acid) blocks TNF-α- and capsaicin-evoked sEPSC frequency increases but has no effect on basal synaptic transmission. Strikingly, NPD1 potently inhibits capsaicin-induced TRPV1 current (IC50=0.4 nM) in dissociated dorsal root ganglion neurons, and this IC50 is ≈ 500 times lower than that of AMG9810, a commonly used TRPV1 antagonist. NPD1 inhibition of TRPV1 is mediated by GPCRs, since the effects were blocked by pertussis toxin. In contrast, NPD1 had not effect on mustard oil-induced TRPA1 currents. Spinal injection of NPD1, at very low doses (0.1–10 ng), blocks spinal LTP and reduces TRPV1-dependent inflammatory pain, without affecting baseline pain. NPD1 also reduces TRPV1-independent but TNF-α-dependent pain hypersensitivity. Our findings demonstrate a novel role of NPD1 in regulating TRPV1/TNF-α-mediated spinal synaptic plasticity and identify NPD1 as a novel analgesic for treating inflammatory pain.
central sensitization; docosahexaenoic acid; long-term potentiation; omega-3 polyunsaturated fatty acid; primary sensory neurons; single-cell PCR; spontaneous EPSCs; TRPA1
Toll-like receptors (TLRs) are typically expressed in immune cells to regulate innate immunity. Here we report that functional TLR7 is expressed in C-fiber primary sensory neurons and important for inducing itch (pruritis) but not necessary for eliciting mechanical, thermal, inflammatory and neuropathic pain in mice. Thus, we have uncovered TLR7 as a novel itch mediator and a potential therapeutic target for anti-itch treatment in skin disease conditions.
Inflammatory pain, such as arthritis pain, is a growing health problem1. Inflammatory pain is generally treated with opioids and cyclooxygenase (COX) inhibitors, but both are limited by side effects. Recently, resolvins, a novel family of lipid mediators including RvE1 and RvD1 derived from omega-3 polyunsaturated fatty acid, show remarkable potency in treating disease conditions associated with inflammation2, 3. Here we report that peripheral (intraplantar) or spinal (intrathecal) administration of RvE1 or RvD1 (0.3–20 ng) potently reduces inflammatory pain behaviors in mice induced by intraplantar injection of formalin, carrageenan or complete Freund’s adjuvant, without affecting basal pain perception. Intrathecal RvE1 also inhibits spontaneous pain and heat and mechanical hypersensitivity evoked by intrathecal capsaicin and TNF-α. RvE1 plays anti-inflammatory roles via reducing neutrophil infiltration, paw edema, and proinflammatory cytokine expression. RvE1 also abolishes TRPV1- and TNF-α-induced excitatory postsynaptic current increase and TNF-α-evoked NMDA receptor hyperactivity in spinal dorsal horn neurons, via inhibition of ERK signaling pathway. Thus, we demonstrate a novel role of resolvins in normalizing spinal synaptic plasticity that has been implicated in generating pain hypersensitivity. Given the remarkable potency of resolvins and well known side effects of opioids and COX inhibitors, resolvins may represent novel analgesics for treating inflammatory pain.
Peripheral inflammation induces persistent central sensitization characterized by mechanical allodynia and heat hyperalgesia that are mediated by distinct mechanisms. Compared to well-demonstrated mechanisms of heat hyperalgesia, mechanisms underlying the development of mechanical allodynia and contralateral pain are incompletely known. In this study, we investigated the distinct role of spinal JNK in heat hyperalgesia, mechanical allodynia, and contralateral pain in an inflammatory pain model. Intraplantar injection of complete Freund’s adjuvant (CFA) induced bilateral mechanical allodynia but unilateral heat hyperalgesia. CFA also induced a bilateral activation (phosphorylation) of JNK in the spinal cord, and the phospho JNK1 (pJNK1) levels were much higher than that of pJNK2. Notably, both pJNK and JNK1 were expressed in GFAP-positive astrocytes. Intrathecal infusion of a selective peptide inhibitor of JNK, D-JNKI-1, starting before inflammation via an osmotic pump, reduced CFA-induced mechanical allodynia in the maintenance phase but had no effect on CFA-induced heat hyperalgesia. A bolus intrathecal injection of D-JNKI-1 or SP600126, a small molecule inhibitor of JNK also reversed mechanical allodynia bilaterally. In contrast, peripheral (intraplantar) administration of D-JNKI-1 reduced the induction of CFA-induced heat hyperalgesia but did not change mechanical allodynia. Finally, CFA-induced bilateral mechanical allodynia was attenuated in mice lacking JNK1 but not JNK2. Taken together, our data suggest that spinal JNK, in particular JNK1 plays an important role in the maintenance of persistent inflammatory pain. Our findings also reveal a unique role of JNK1 and astrocyte network in regulating tactile allodynia and contralateral pain.
c-Jun-N-terminal kinase; MAP kinase; Astrocytes; Spinal cord; Contralateral pain; Inflammation; Tactile allodynia
Cancer pain significantly affects the quality of cancer patients, and current treatments for this pain are limited. C-Jun N-terminal kinase (JNK) has been implicated in tumor growth and neuropathic pain sensitization. We investigated the role of JNK in cancer pain and tumor growth in a skin cancer pain model. Injection of luciferase-transfected B16-Fluc melanoma cells into a hindpaw of mouse induced robust tumor growth, as indicated by increase in paw volume and fluorescence intensity. Pain hypersensitivity in this model developed rapidly (<5 days) and reached a peak in 2 weeks, and was characterized by mechanical allodynia and heat hyperalgesia. Tumor growth was associated with JNK activation in tumor mass, dorsal root ganglion (DRG), and spinal cord and a peripheral neuropathy, such as loss of nerve fibers in the hindpaw skin and induction of ATF-3 expression in DRG neurons. Repeated systemic injections of D-JNKI-1 (6 mg/kg, i.p.), a selective and cell permeable peptide inhibitor of JNK, produced an accumulative inhibition of mechanical allodynia and heat hyperalgesia. A bolus spinal injection of D-JNKI-1 also inhibited mechanical allodynia. Further, JNK inhibition suppressed tumor growth in vivo and melanoma cell proliferation in vitro. In contrast, repeated injections of morphine (5 mg/kg), a commonly used analgesic for terminal cancer, produced analgesic tolerance after 1 day and did not inhibit tumor growth. Our data reveal a marked peripheral neuropathy in this skin cancer model and important roles of the JNK pathway in cancer pain development and tumor growth. JNK inhibitors such as D-JNKI-1 may be used to treat cancer pain.
Bulleyaconitine A (BLA) is an analgesic and antiinflammatory drug isolated from Aconitum plants. BLA has several potential targets, including voltage-gated Na+ channels. We tested whether BLA elicited long-lasting cutaneous analgesia, when co-injected with lidocaine and epinephrine, as a model for prolonged infiltration anesthesia.
The local anesthetic properties of BLA were assessed by the patch-clamp technique in HEK293t cells expressing Nav1.7 and Nav1.8 neuronal Na+ channels, both crucial for nociception. Drug solutions (0.6 mL) were injected subcutaneously via rat shaved dorsal skin. Inhibition of the cutaneous trunci muscle reflex was evaluated by pinpricks. Skin cross-sections were stained with hematoxylin and eosin or with antibodies against PGP9.5.
BLA at 10 µM interacted minimally with resting or inactivated Nav1.7 and Nav1.8 Na+ channels when infrequently stimulated to +50 mV for 3 ms. However, when stimulated at 2 Hz for 1000 pulses, their peak Na+ currents were >90% reduced by BLA. This use-dependent inhibition was not significantly reversed after 15-min washing. Complete nociceptive blockade after injection of lidocaine (0.5%)/epinephrine (1:200,000) lasted for approximately 1 h in rats; full recovery occurred after approximately 6 h. Co-injection of 0.125 mM BLA with lidocaine/epinephrine increased the duration of complete nociceptive blockade to 24 h. Full recovery occurred after approximately 6 days. Skin histology including peripheral nerve fibers appeared unaffected by BLA.
BLA inhibits Nav1.7 and Nav1.8 Na+ currents in a use-dependent manner. Co-injection of BLA at ≤0.125 mM with lidocaine and epinephrine elicits complete cutaneous analgesia that lasts for up to 24 h without adverse effects.
Neuropathic pain affects millions of people globally and could be a disease on its own right. Current treatments focus on blocking neurotransmission and have resulted in limited success. Recent progress points to an important role of neuroinflammation in the pathogenesis of neuropathic pain. Matrix metalloproteases (MMPs) comprise a large family of zinc endopeptidases that have been implicated in the generation of neuroinflammation via cleavage of extracellular matrix proteins and activation of proinflammatory cytokines and chemokines. However, little is known about the role of MMPs in chronic pain regulation. Our recent study has shown that neuropathic pain development in the early and late phase requires MMP-9 and MMP-2, respectively. Inhibition of MMP-9 or MMP-2 may provide a new strategy for the prevention and treatment of neuropathic pain.
Our previous study showed that activation of c-jun-N-terminal kinase (JNK) in spinal astrocytes plays an important role in neuropathic pain sensitization. We further investigated how JNK regulates neuropathic pain. In cultured astrocytes, TNF-α transiently activated JNK via TNF receptor-1. Cytokine array indicated that the chemokine CCL2/MCP-1 (monocyte chemoattractant protein-1) was strongly induced by the TNF-α/JNK pathway. MCP-1 upregulation by TNF-α was dose-dependently inhibited by the JNK inhibitors SP600125 and D-JNKI-1. Spinal injection of TNF-α produced JNK-dependent pain hypersensitivity and MCP-1 upregulation in the spinal cord. Further, spinal nerve ligation (SNL) induced persistent neuropathic pain and MCP-1 upregulation in the spinal cord, and both were suppressed by D-JNKI-1. Remarkably, MCP-1 was primarily induced in spinal cord astrocytes after SNL. Spinal administration of MCP-1 neutralizing antibody attenuated neuropathic pain. Conversely, spinal application of MCP-1 induced heat hyperalgesia and phosphorylation of extracellular signal-regulated kinase (ERK) in superficial spinal cord dorsal horn neurons, indicative of central sensitization (hyperactivity of dorsal horn neurons). Patch clamp recordings in lamina II neurons of isolated spinal cord slices showed that MCP-1 not only enhanced spontaneous excitatory synaptic currents (sEPSCs) but also potentiated NMDA- and AMPA-induced currents. Finally, the MCP-1 receptor CCR2 was expressed in neurons and some non-neuronal cells in the spinal cord. Taken together, we have revealed a previously unknown mechanism of MCP-1 induction and action. MCP-1 induction in astrocytes following JNK activation contributes to central sensitization and neuropathic pain facilitation by enhancing excitatory synaptic transmission. Inhibition of the JNK/MCP-1 pathway may provide a new therapy for neuropathic pain management.
MAP kinase; chemokine; CCL2; CCR2; TNF-α; nerve injury; glia; neural-glial interaction
Treatment of neuropathic pain, triggered by multiple insults to the nervous system, is a clinical challenge because the underlying mechanisms of neuropathic pain development remain poorly understood 1-4. Most treatments do not differentiate between different phases of neuropathic pain pathophysiology and simply focus on blocking neurotransmission, producing transient pain relief. Here, we report that early and late phase neuropathic pain development after nerve injury require different matrix metalloproteinases (MMPs). After spinal nerve ligation, MMP-9 shows a rapid and transient upregulation in injured DRG primary sensory neurons consistent with an early phase of neuropathic pain, whereas MMP-2 shows a delayed response in DRG satellite cells and spinal astrocytes consistent with a late phase of neuropathic pain. Local inhibition of MMP-9 via an intrathecal route inhibits the early phase of neuropathic pain, whereas inhibition of MMP-2 suppresses late phase of neuropathic pain. Further, intrathecal administration of MMP-9 or MMP-2 is sufficient to produce neuropathic pain symptoms. Following nerve injury, MMP-9 induces neuropathic pain through interleukin-1β cleavage and microglia activation at early times, whereas MMP-2 maintains neuropathic pain through interleukin-1β cleavage and astrocyte activation at later times. Inhibition of MMP may provide a novel therapeutic approach for the treatment of neuropathic pain at different phases.