Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Regulation of Wnt signaling by nociceptive input in animal models

Background

Central sensitization-associated synaptic plasticity in the spinal cord dorsal horn (SCDH) critically contributes to the development of chronic pain, but understanding of the underlying molecular pathways is still incomplete. Emerging evidence suggests that Wnt signaling plays a crucial role in regulation of synaptic plasticity. Little is known about the potential function of the Wnt signaling cascades in chronic pain development.

Results

Fluorescent immunostaining results indicate that β-catenin, an essential protein in the canonical Wnt signaling pathway, is expressed in the superficial layers of the mouse SCDH with enrichment at synapses in lamina II. In addition, Wnt3a, a prototypic Wnt ligand that activates the canonical pathway, is also enriched in the superficial layers. Immunoblotting analysis indicates that both Wnt3a a β-catenin are up-regulated in the SCDH of various mouse pain models created by hind-paw injection of capsaicin, intrathecal (i.t.) injection of HIV-gp120 protein or spinal nerve ligation (SNL). Furthermore, Wnt5a, a prototypic Wnt ligand for non-canonical pathways, and its receptor Ror2 are also up-regulated in the SCDH of these models.

Conclusion

Our results suggest that Wnt signaling pathways are regulated by nociceptive input. The activation of Wnt signaling may regulate the expression of spinal central sensitization during the development of acute and chronic pain.

Randomized control trial of topical clonidine for treatment of painful diabetic neuropathy

A length-dependent neuropathy with pain in the feet is a common complication of diabetes (painful diabetic neuropathy). It was hypothesized that pain may arise from sensitized-hyperactive cutaneous nociceptors, and that this abnormal signaling may be reduced by topical administration of the α(2)-adrenergic agonist, clonidine, to the painful area. This was a randomized, double-blind, placebo-controlled, parallel-group, multicenter trial. Nociceptor function was measured by determining the painfulness of 0.1% topical capsaicin applied to the pretibial area of each subject for 30minutes during screening. Subjects were then randomized to receive 0.1% topical clonidine gel (n=89) or placebo gel (n=90) applied 3 times a day to their feet for 12weeks. The difference in foot pain at week 12 in relation to baseline, rated on a 0-10 numerical pain rating scale (NPRS), was compared between groups. Baseline NPRS was imputed for missing data for subjects who terminated the study early. The subjects treated with clonidine showed a trend toward decreased foot pain compared to the placebo-treated group (the primary endpoint; P=0.07). In subjects who felt any level of pain to capsaicin, clonidine was superior to placebo (P<0.05). In subjects with a capsaicin pain rating ⩾2 (0-10, NPRS), the mean decrease in foot pain was 2.6 for active compared to 1.4 for placebo (P=0.01). Topical clonidine gel significantly reduces the level of foot pain in painful diabetic neuropathy subjects with functional (and possibly sensitized) nociceptors in the affected skin as revealed by testing with topical capsaicin. Screening for cutaneous nociceptor function may help distinguish candidates for topical therapy for neuropathic pain.

TLR3 deficiency impairs spinal cord synaptic transmission, central sensitization, and pruritus in mice

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.

Enhanced hippocampal long-term potentiation and fear memory in Btbd9 mutant mice

Polymorphisms in BTBD9 have recently been associated with higher risk of restless legs syndrome (RLS), a neurological disorder characterized by uncomfortable sensations in the legs at rest that are relieved by movement. The BTBD9 protein contains a BTB/POZ domain and a BACK domain, but its function is unknown. To elucidate its function and potential role in the pathophysiology of RLS, we generated a line of mutant Btbd9 mice derived from a commercial gene-trap embryonic stem cell clone. Btbd9 is the mouse homolog of the human BTBD9. Proteins that contain a BTB/POZ domain have been reported to be associated with synaptic transmission and plasticity. We found that Btbd9 is naturally expressed in the hippocampus of our mutant mice, a region critical for learning and memory. As electrophysiological characteristics of CA3-CA1 synapses of the hippocampus are well characterized, we performed electrophysiological recordings in this region. The mutant mice showed normal input-output relationship, a significant impairment in pre-synaptic activity, and an enhanced long-term potentiation. We further performed an analysis of fear memory and found the mutant mice had an enhanced cued and contextual fear memory. To elucidate a possible molecular basis for these enhancements, we analyzed proteins that have been associated with synaptic plasticity. We found an elevated level of dynamin 1, an enzyme associated with endocytosis, in the mutant mice. These results suggest the first identified function of Btbd9 as being involved in regulating synaptic plasticity and memory. Recent studies have suggested that enhanced synaptic plasticity, analogous to what we have observed, in other regions of the brain could enhance sensory perception similar to what is seen in RLS patients. Further analyses of the mutant mice will help shine light on the function of BTBD9 and its role in RLS.

Contribution of PKMζ-dependent and independent amplification to components of experimental neuropathic pain

Injuries can induce adaptations in pain processing that result in amplification of signaling. One mechanism may be analogous to long-term potentiation and involve the atypical protein kinase C, PKMζ. The possible contribution of PKMζ-dependent and independent amplification mechanisms to experimental neuropathic pain was explored in rats with spinal nerve ligation (SNL) injury. SNL increased p-PKMζ in the rostral anterior cingulate cortex (rACC), a site that mediates, in part, the unpleasant aspects of pain. Inhibition of PKMζ within the rACC by a single administration of ζ-pseudosubstrate inhibitory peptide (ZIP) reversed SNL-induced aversiveness within 24 hours, whereas N-methyl-d-aspartate receptor blockade with MK-801 had no effects. The SNL-induced aversive state (reflecting "spontaneous" pain), was re-established in a time-dependent manner, with full recovery observed 7 days post-ZIP administration. Neither rACC ZIP nor MK-801 altered evoked responses. In contrast, spinal ZIP or MK-801, but not scrambled peptide, transiently reversed evoked hypersensitivity, but had no effect on nerve injury-induced spontaneous pain. PKMζ phosphorylation was not altered by SNL in the spinal dorsal horn. These data suggest that amplification mechanisms contribute to different aspects of neuropathic pain at different levels of the neuraxis. Thus, PKMζ-dependent amplification contributes to nerve injury-induced aversiveness within the rACC. Moreover, unlike mechanisms maintaining memory, the consequences of PKMζ inhibition within the rACC are not permanent in neuropathic pain, possibly reflecting the re-establishment of amplification mechanisms by ongoing activity of injured nerves. In the spinal cord, however, both PKMζ-dependent and independent mechanisms contribute to amplification of evoked responses, but apparently not spontaneous pain.

Acute morphine induces matrix metalloproteinase-9 up-regulation in primary sensory neurons to mask opioid-induced analgesia in mice

Background

Despite decades of intense research efforts, actions of acute opioids are not fully understood. Increasing evidence suggests that in addition to well-documented antinociceptive effects opioids also produce paradoxical hyperalgesic and excitatory effects on neurons. However, most studies focus on the pronociceptive actions of chronic opioid exposure. Matrix metalloproteinase 9 (MMP-9) plays an important role in neuroinflammation and neuropathic pain development. We examined MMP-9 expression and localization in dorsal root ganglia (DRGs) after acute morphine treatment and, furthermore, the role of MMP-9 in modulating acute morphine-induced analgesia and hyperalgesia in mice.

Results

Subcutaneous morphine induced a marked up-regulation of MMP-9 protein in DRGs but not spinal cords. Morphine also increased MMP-9 activity and mRNA expression in DRGs. MMP-9 up-regulation peaked at 2 h but returned to the baseline after 24 h. In DRG tissue sections, MMP-9 is expressed in small and medium-sized neurons that co-express mu opioid receptors (MOR). In DRG cultures, MOR agonists morphine, DAMGO, and remifentanil each increased MMP-9 expression in neurons, whereas the opioid receptor antagonist naloxone and the MOR-selective antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) suppressed morphine-induced MMP-9 expression. Notably, subcutaneous morphine-induced analgesia was enhanced and prolonged in Mmp9 knockout mice and also potentiated in wild-type mice receiving intrathecal injection of MMP-9 inhibitors. Consistently, intrathecal injection of specific siRNA targeting MMP-9 reduced MMP-9 expression in DRGs and enhanced and prolonged morphine analgesia. Subcutaneous morphine also produced heat hyperalgesia at 24 h, but this opioid-induced hyperalgesia was not enhanced after MMP-9 deletion or inhibition.

Conclusions

Transient MMP-9 up-regulation in DRG neurons can mask opioid analgesia, without modulating opioid-induced hyperalgesia. Distinct molecular mechanisms (MMP-9 dependent and independent) control acute opioid-induced pronociceptive actions (anti-analgesia in the first several hours and hyperalgesia after 24 h). Targeting MMP-9 may improve acute opioid analgesia.

Deconstructing the neuropathic pain phenotype to reveal neural mechanisms

After nerve injury maladaptive changes can occur in injured sensory neurons and along the entire nociceptive pathway within the CNS, which may lead to spontaneous pain or pain hypersensitivity. The resulting neuropathic pain syndromes present as a complex combination of negative and positive symptoms, which vary enormously from individual to individual. This variation depends on a diversity of underlying pathophysiological changes resulting from the convergence of etiological, genotypic, and environmental factors. The pain phenotype can serve therefore, as a window on underlying pathophysiological neural mechanisms and as a guide for developing personalized pain medicine.

Resolvin D2 is a potent endogenous inhibitor for transient receptor potential subtype V1/A1, inflammatory pain, and spinal cord synaptic plasticity in mice: distinct roles of resolvin D1, D2, and E1

Inflammatory pain such as arthritic pain is typically treated with opioids and cyclo-oxygenase-2 inhibitors with well known side effects. Transient receptor potential subtype vanilloid 1 (TRPV1) and TRP ankyryn 1 (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 ω-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. RvD2 is a novel family member of resolvins. Here we report that RvD2 is a remarkably potent inhibitor of TRPV1 (IC(50) = 0.1 nm) and TRPA1 (IC(50) = 2 nm) in primary sensory neurons, whereas RvE1 and RvD1 selectively inhibited TRPV1 (IC(50) = 1 nm) and TRPA1 (IC(50) = 9 nm), respectively. Accordingly, RvD2, RvE1, and RvD1 differentially regulated TRPV1 and TRPA1 agonist-elicited acute pain and spinal cord synaptic plasticity [spontaneous EPSC (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/TRPA1 and inflammatory pain.

Targeting neuronal adenylyl cyclase for the treatment of chronic pain

Pain research is currently undergoing dramatic changes. In the area of basic pain research, new discoveries have been made towards the understanding of pain transmission, modulation and plasticity. However, many of these basic discoveries have not yet led to the development of new drugs for the treatment of chronic pain. One major reason for this disconnection is the lack of translational research and drug discovery based directly on the novel pain mechanism. In this review, I focus on activity-dependent potentiation in pain-related cortical areas and recent translational research on adenylyl cyclase subtype 1 (AC1) as a novel target for treating chronic pain. In particular, I discuss the AC1 inhibitor, NB001, which produces powerful analgesic effects in animal models of chronic pain by inhibiting chronic pain-related cortical potentiation.

Resolving TRPV1- and TNF-α-mediated spinal cord synaptic plasticity and inflammatory pain with neuroprotectin D1

Mechanisms of inflammatory pain are not fully understood. We investigated the role of TRPV1 (transient receptor potential subtype V1) 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 knock-out mice. TNF-α also increases sEPSC frequency but not amplitude in spinal outer lamina II (lamina IIo) neurons, and this increase is abolished in Trpv1 knock-out 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 ω-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 (IC(50) = 0.4 nm) in dissociated dorsal root ganglion neurons, and this IC(50) 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 no 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.

How can animal models inform on the transition to chronic symptoms in whiplash?

STUDY DESIGN

A nonsystematic review of the literature.

OBJECTIVE

The objective was to present general schema for mechanisms of whiplash pain and review the role of animal models in understanding the development of chronic pain from whiplash injury.

SUMMARY OF BACKGROUND DATA

Extensive biomechanical and clinical studies of whiplash have been performed to understand the injury mechanisms and symptoms of whiplash injury. However, only recently have animal models of this painful disorder been developed based on other pain models in the literature.

METHODS

A nonsystematic review was performed and findings were integrated to formulate a generalized picture of mechanisms by which chronic whiplash pain develops from mechanical tissue injuries.

RESULTS

The development of chronic pain from tissue injuries in the neck due to whiplash involves complex interactions between the injured tissue and spinal neuroimmune circuits. A variety of animal models are beginning to define these mechanisms.

CONCLUSION

Continued work is needed in developing appropriate animal models to investigate chronic pain from whiplash injuries and care must be taken to determine whether such models aim to model the injury event or the pain symptoms.

Hyperalgesia by synaptic long-term potentiation (LTP): an update

Long-term potentiation of synaptic strength (LTP) in nociceptive pathways shares principle features with hyperalgesia including induction protocols, pharmacological profile, neuronal and glial cell types involved and means for prevention. LTP at synapses of nociceptive nerve fibres constitutes a contemporary cellular model for pain amplification following trauma, inflammation, nerve injury or withdrawal from opioids. It provides a novel target for pain therapy. This review summarizes recent progress which has been made in unravelling the properties and functions of LTP in the nociceptive system and in identifying means for its prevention and reversal.

DNA methylation of SPARC and chronic low back pain

Background

The extracellular matrix protein SPARC (Secreted Protein, Acidic, Rich in Cysteine) has been linked to degeneration of the intervertebral discs and chronic low back pain (LBP). In humans, SPARC protein expression is decreased as a function of age and disc degeneration. In mice, inactivation of the SPARC gene results in the development of accelerated age-dependent disc degeneration concurrent with age-dependent behavioral signs of chronic LBP.

DNA methylation is the covalent modification of DNA by addition of methyl moieties to cytosines in DNA. DNA methylation plays an important role in programming of gene expression, including in the dynamic regulation of changes in gene expression in response to aging and environmental signals.

We tested the hypothesis that DNA methylation down-regulates SPARC expression in chronic LBP in pre-clinical models and in patients with chronic LBP.

Results

Our data shows that aging mice develop anatomical and behavioral signs of disc degeneration and back pain, decreased SPARC expression and increased methylation of the SPARC promoter. In parallel, we show that human subjects with back pain exhibit signs of disc degeneration and increased methylation of the SPARC promoter. Methylation of either the human or mouse SPARC promoter silences its activity in transient transfection assays.

Conclusions

This study provides the first evidence that DNA methylation of a single gene plays a role in chronic pain in humans and animal models. This has important implications for understanding the mechanisms involved in chronic pain and for pain therapy.