Dendritic spine dysgenesis in neuropathic pain

Tiam1-mediated maladaptive plasticity underlying morphine tolerance and hyperalgesia

Opioid pain medications, such as morphine, remain the mainstay for treating severe and chronic pain. Prolonged morphine use, however, triggers analgesic tolerance and hyperalgesia (OIH), which can last for a long period after morphine withdrawal. How morphine induces these detrimental side effects remains unclear. Here, we show that morphine tolerance and OIH are mediated by Tiam1-coordinated synaptic structural and functional plasticity in the spinal nociceptive network. Tiam1 is a Rac1 GTPase guanine nucleotide exchange factor that promotes excitatory synaptogenesis by modulating actin cytoskeletal dynamics. We found that prolonged morphine treatment activated Tiam1 in the spinal dorsal horn and Tiam1 ablation from spinal neurons eliminated morphine antinociceptive tolerance and OIH. At the same time, the pharmacological blockade of Tiam1-Rac1 signalling prevented the development and reserved the established tolerance and OIH. Prolonged morphine treatment increased dendritic spine density and synaptic NMDA receptor activity in spinal dorsal horn neurons, both of which required Tiam1. Furthermore, co-administration of the Tiam1 signalling inhibitor NSC23766 was sufficient to abrogate morphine tolerance in chronic pain management. These findings identify Tiam1-mediated maladaptive plasticity in the spinal nociceptive network as an underlying cause for the development and maintenance of morphine tolerance and OIH and provide a promising therapeutic target to reduce tolerance and prolong morphine use in chronic pain management.

A novel role for microtubule affinity-regulating kinases in neuropathic pain

BACKGROUND AND PURPOSE

Neuropathic pain affects millions of patients, but there are currently few viable therapeutic options available. Microtubule affinity-regulating kinases (MARKs) regulate the dynamics of microtubules and participate in synaptic remodelling. It is unclear whether these changes are involved in the central sensitization of neuropathic pain. This study examined the role of MARK1 or MARK2 in regulating neurosynaptic plasticity induced by neuropathic pain.

EXPERIMENTAL APPROACH

A rat spinal nerve ligation (SNL) model was established to induce neuropathic pain. The role of MARKs in nociceptive regulation was assessed by genetically knocking down MARK1 or MARK2 in amygdala and systemic administration of PCC0105003, a novel small molecule MARK inhibitor. Cognitive function, anxiety-like behaviours and motor coordination capability were also examined in SNL rats. Synaptic remodelling-associated signalling changes were detected with electrophysiological recording, Golgi-Cox staining, western blotting and qRT-PCR.

KEY RESULTS

MARK1 and MARK2 expression levels in amygdala and spinal dorsal horn were elevated in SNL rats. MARK1 or MARK2 knockdown in amygdala and PCC0105003 treatment partially attenuated pain-like behaviours along with improving cognitive deficit, anxiogenic-like behaviours and motor coordination in SNL rats. Inhibition of MARKs signalling reversed synaptic plasticity at the functional and structural levels by suppressing NR2B/GluR1 and EB3/Drebrin signalling pathways both in amygdala and spinal dorsal horn.

CONCLUSION AND IMPLICATIONS

These results suggest that MARKs-mediated synaptic remodelling plays a key role in the pathogenesis of neuropathic pain and that pharmacological inhibitors of MARKs such as PCC0105003 could represent a novel therapeutic strategy for the management of neuropathic pain.

Dendritic Spines and Pain Memory

Neuropathic pain is a debilitating form of pain arising from injury or disease of the nervous system that affects millions of people worldwide. Despite its prevalence, the underlying mechanisms of neuropathic pain are still not fully understood. Dendritic spines are small protrusions on the surface of neurons that play an important role in synaptic transmission. Recent studies have shown that dendritic spines reorganize in the superficial and deeper laminae of the spinal cord dorsal horn with the development of neuropathic pain in multiple models of disease or injury. Given the importance of dendritic spines in synaptic transmission, it is possible that studying dendritic spines could lead to new therapeutic approaches for managing intractable pain. In this review article, we highlight the emergent role of dendritic spines in neuropathic pain, as well as discuss the potential for studying dendritic spines for the development of new therapeutics.

Role of spinal astrocytes through the perisynaptic astrocytic process in pathological pain

Pathological pain is caused by abnormal activity in the neural circuit that transmits nociceptive stimuli. Beyond homeostatic functions, astrocytes actively participate in regulating synaptic transmission as members of tripartite synapses. The perisynaptic astrocytic process (PAP) is the key structure that allows astrocytes to play these roles and not only physically supports synapse formation through cell adhesion molecules (CAMs) but also regulates the efficiency of chemical signaling. Accumulating evidence has revealed that spinal astrocytes are involved in pathological pain by modulating the efficacy of neurotransmitters such as glutamate and GABA through transporters located in the PAP and by directly regulating synaptic transmission through various gliotransmitters. Although various CAMs contribute to pathological pain, insufficient evidence is available as to whether astrocytic CAMs also have this role. Therefore, more in-depth research is needed on how pathological pain is induced and maintained by astrocytes, especially in the PAP surrounding the synapse, and this will subsequently increase our understanding and treatment of pathological pain.

A Novel, Paresthesia-Free Spinal Cord Stimulation Waveform for Chronic Neuropathic Low Back Pain: Six-Month Results of a Prospective, Single-Arm, Dose-Response Study

OBJECTIVES

The aim of this prospective, single-blinded, dose-response study was to evaluate the safety and efficacy of a novel, paresthesia-free (subperception) spinal cord stimulation (SCS) waveform designed to target dorsal horn dendrites for the treatment of chronic neuropathic low back pain (LBP).

MATERIALS AND METHODS

Twenty-seven participants with chronic neuropathic LBP were implanted with a commercial SCS system after a successful trial of SCS therapy. Devices were programmed to deliver the investigative waveform (100 Hz, 1000 μs, T9/T10 bipole) at descending stimulation perception threshold amplitudes (80%, 60%, 40%). Programs were evaluated at six, ten, and 14 weeks, after which participants selected their preferred program, with more follow-up at 26 weeks (primary outcomes). Participants were blinded to the nature of the programming. Pain score (visual analog scale [VAS]), Brief Pain Inventory (BPI), quality of life (EQ-5D-5L), and health status (36-Item Short Form [SF-36]) were measured at baseline and follow-ups. Responder rate, treatment satisfaction, clinician global impression of change, and adverse events (AEs) also were evaluated.

RESULTS

Mean (± SD) baseline VAS was 72.5 ± 11.2 mm. At 26 weeks (n = 26), mean change from baseline in VAS was -51.7 mm (95% CI, -60.7 to -42.7; p < 0.001), with 76.9% of participants reporting ≥50% VAS reduction, and 46.2% reporting ≥80% VAS reduction. BPI, EQ-5D-5L, and SF-36 scores were all statistically significantly improved at 26 weeks (p < 0.001), and 100% of participants were satisfied with their treatment. There were no unanticipated AEs related to the study intervention, device, or procedures.

CONCLUSIONS

This novel, paresthesia-free stimulation waveform may be a safe and effective option for patients with chronic neuropathic LBP eligible for SCS therapy and is deliverable by all current commercial SCS systems.

CLINICAL TRIAL REGISTRATION

This study is registered on anzctr.org.au with identifier ACTRN12618000647235.

Tiam1 coordinates synaptic structural and functional plasticity underpinning the pathophysiology of neuropathic pain

Neuropathic pain is a common, debilitating chronic pain condition caused by damage or a disease affecting the somatosensory nervous system. Understanding the pathophysiological mechanisms underlying neuropathic pain is critical for developing new therapeutic strategies to treat chronic pain effectively. Tiam1 is a Rac1 guanine nucleotide exchange factor (GEF) that promotes dendritic and synaptic growth during hippocampal development by inducing actin cytoskeletal remodeling. Here, using multiple neuropathic pain animal models, we show that Tiam1 coordinates synaptic structural and functional plasticity in the spinal dorsal horn via actin cytoskeleton reorganization and synaptic NMDAR stabilization and that these actions are essential for the initiation, transition, and maintenance of neuropathic pain. Furthermore, an antisense oligonucleotides (ASO) targeting spinal Tiam1 persistently alleviate neuropathic pain sensitivity. Our findings suggest that Tiam1-coordinated synaptic functional and structural plasticity underlies the pathophysiology of neuropathic pain and that intervention of Tiam1-mediated maladaptive synaptic plasticity has long-lasting consequences in neuropathic pain management.

Altered pain sensitivity in 5×familial Alzheimer disease mice is associated with dendritic spine loss in anterior cingulate cortex pyramidal neurons

ABSTRACT

Chronic pain is highly prevalent. Individuals with cognitive disorders such as Alzheimer disease are a susceptible population in which pain is frequently difficult to diagnosis. It is still unclear whether the pathological changes in patients with Alzheimer disease will affect pain processing. Here, we leverage animal behavior, neural activity recording, optogenetics, chemogenetics, and Alzheimer disease modeling to examine the contribution of the anterior cingulate cortex (ACC) neurons to pain response. The 5× familial Alzheimer disease mice show alleviated mechanical allodynia which can be regained by the genetic activation of ACC excitatory neurons. Furthermore, the lower peak neuronal excitation, delayed response initiation, as well as the dendritic spine reduction of ACC pyramidal neurons in 5×familial Alzheimer disease mice can be mimicked by Rac1 or actin polymerization inhibitor in wild-type (WT) mice. These findings indicate that abnormal of pain sensitivity in Alzheimer disease modeling mice is closely related to the variation of neuronal activity and dendritic spine loss in ACC pyramidal neurons, suggesting the crucial role of dendritic spine density in pain processing.

PCC-0105002, a novel small molecule inhibitor of PSD95-nNOS protein-protein interactions, attenuates neuropathic pain and corrects motor disorder associated with neuropathic pain model

In view of postsynaptic density 95kDA (PSD95) tethers neuronal NO synthase (nNOS) to N-methyl-d-aspartate receptor (NMDAR), the PSD95-nNOS complex represents a therapeutic target of neuropathic pain. This study therefore sought to explore the ability of PCC-0105002, a novel PSD95-nNOS small molecule inhibitor, to alter pain sensitivity in rodent neuropathic pain models. Firstly, the IC50 of PCC-0105002 for PSD95 and NOS1 binding activity was determined using an Alpha Screen assay kit. Then, we examined the effects of PCC-0105002 in the mouse formalin test and in the rat spinal nerve ligation (SNL) model, and explored the ability of PCC-0105002 to mediate analgesia and to effect motor coordination in a rota-rod test. Moreover, the mechanisms whereby PCC-0105002 mediates analgesia was explored via western blotting, Golgi staining, and co-immunoprecipitation experiments in dorsal horn. The outcomes indicated that PCC-0105002 exhibited dose-dependent attenuation of phase II pain-associated behaviors in the formalin test. The result indicated that PCC-0105002 disrupted the PSD95-nNOS interaction with IC50 of 1.408 μM. In the SNL model, PCC-0105002 suppressed mechanical allodynia, thermal hyperalgesia, and abnormal dorsal horn wide dynamic range neuron discharge. PCC-0105002 mediated an analgesic effect comparable to that of MK-801, while it was better able to enhance motor coordination as compared with MK-801. Moreover, PCC-0105002 altered signaling downstream of NMDAR and thus functionally and structurally attenuating synaptic plasticity through respective regulation of the NR2B/GluR1/CaMKIIα and Rac1/RhoA pathways. These findings suggest that the novel PSD95-nNOS inhibitor PCC-0105002 is an effective agent for alleviating neuropathic pain, and that it produces fewer motor coordination-associated side effects than do NMDAR antagonists.

5-HT6R null mutatrion induces synaptic and cognitive defects

Serotonin 6 receptor (5-HT6R) is a promising target for a variety of human diseases, such as Alzheimer's disease (AD) and schizophrenia. However, the detailed mechanism underlying 5-HT6R activity in the central nervous system (CNS) is not fully understood. In the present study, 5-HT6R null mutant (5-HT6R^-/- ) mice were found to exhibit cognitive deficiencies and abnormal anxiety levels. 5-HT6R is considered to be specifically localized on the primary cilia. We found that the loss of 5-HT6R affected the Sonic Hedgehog signaling pathway in the primary cilia. 5-HT6R^-/- mice showed remarkable alterations in neuronal morphology, including dendrite complexity and axon initial segment morphology. Neurons lacking 5-HT6R exhibited increased neuronal excitability. Our findings highlight the complexity of 5-HT6R functions in the primary ciliary and neuronal physiology, supporting the theory that this receptor modulates neuronal morphology and transmission, and contributes to cognitive deficits in a variety of human diseases, such as AD, schizophrenia, and ciliopathies.

Spinophilin modulates pain through suppressing dendritic spine morphogenesis via negative control of Rac1-ERK signaling in rat spinal dorsal horn

Both spinophilin (SPN, also known as neurabin 2) and Rac1 (a member of Rho GTPase family) are believed to play key roles in dendritic spine (DS) remodeling and spinal nociception. However, how SPN interacts with Rac1 in the above process is unknown. Here, we first demonstrated natural existence of SPN-protein phosphatase 1-Rac1 complex in the spinal dorsal horn (DH) neurons by both double immunofluorescent labeling and co-immunoprecipitation, then the effects of SPN over-expression and down-regulation on mechanical and thermal pain sensitivity, GTP-bound Rac1-ERK signaling activity, and spinal DS density were studied. Over-expression of SPN in spinal neurons by intra-DH pAAV-CMV-SPN-3FLAG could block both mechanical and thermal pain hypersensitivity induced by intraplantar bee venom injection, however it had no effect on the basal pain sensitivity. Over-expression of SPN also resulted in a significant decrease in GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-MCS). In sharp contrast, knockdown of SPN in spinal neurons by intra-DH pAAV-CAG-eGFP-U6-shRNA[SPN] produced both pain hypersensitivity and dramatic elevation of GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-shRNA [NC]). Moreover, knockdown of SPN resulted in increase in DS density while over-expression of it had no such effect. Collectively, SPN is likely to serve as a regulator of Rac1 signaling to suppress DS morphogenesis via negative control of GTP-bound Rac1-ERK activities at postsynaptic component in rat DH neurons wherein both mechanical and thermal pain sensitivity are controlled.

Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

WITHDRAWN: PCC-0105002, a novel small molecule inhibitor of PSD95-nNOS protein-protein interactions, attenuates neuropathic pain and corrects motor coordination-associated side effects in neuropathic pain model

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Dendritic Spines in the Spinal Cord: Live Action Pain

Dendritic spines are microscopic protrusions on neurons that house the postsynaptic machinery necessary for neurotransmission between neurons. As such, dendritic spine structure is intimately linked with synaptic function. In pathology, dendritic spine behavior and its contribution to disease are not firmly understood. It is well known that dendritic spines are highly dynamic in vivo. In our recent publication, we used an intravital imaging approach, which permitted us to repeatedly visualize the same neurons located in lamina II, a nociceptive processing region of the spinal cord. Using this imaging platform, we analyzed the intravital dynamics of dendritic spine structure before and after nerve injury-induced pain. This effort revealed a time-dependent relationship between the progressive increase in pain outcome, and a switch in the steady-state fluctuations of dendritic spine structure. Collectively, our in vivo study demonstrates how injury that leads to abnormal pain may also contribute to synapse-associated structural remodeling in nociceptive regions of the spinal cord dorsal horn. By combining our live-imaging approach with measures of neuronal activity, such as with the use of calcium or other voltage-sensitive dyes, we expect to gain a more complete picture of the relationship between dendritic spine structure and nociceptive physiology.

Spinal Wnt5a Plays a Key Role in Spinal Dendritic Spine Remodeling in Neuropathic and Inflammatory Pain Models and in the Proalgesic Effects of Peripheral Wnt3a

Wnt signaling represents a highly versatile signaling system, which plays critical roles in developmental morphogenesis as well as synaptic physiology in adult life and is implicated in a variety of neural disorders. Recently, we demonstrated that Wnt3a is able to recruit multiple noncanonical signaling pathways to alter peripheral sensory neuron function in a nociceptive modality-specific manner. Furthermore, several studies recently reported an important role for Wnt5a acting via canonical and noncanonical signaling in spinal processing of nociception in a number of pathologic pain disorders. Here, using diverse molecular, genetic, and behavioral approaches in mouse models of pain in vivo, we report a novel role for Wnt5a signaling in nociceptive modulation at the structural level. In models of chronic pain, using male and female mice, we found that Wnt5a is released spinally from peripheral sensory neurons, where it recruits the tyrosine kinase receptors Ror2 and Ryk to modulate dendritic spine rearrangement. Blocking the Wnt5a-Ryk/Ror2 axis in spinal dorsal horn neurons prevented activity-dependent dendritic spine remodeling and significantly reduced mechanical hypersensitivity induced by peripheral injury as well as inflammation. Moreover, we observed that peripheral Wnt3a signaling triggers the release of Wnt5a in the spinal cord, and inhibition of spinal Wnt5a signaling attenuates the functional impact of peripheral Wnt3a on nociceptive sensitivity. In conclusion, this study reports a novel role for the Wnt signaling axis in coordinating peripheral and spinal sensitization and shows that targeting Wnt5a-Ryk/ROR2 signaling alleviates both structural and functional mechanisms of nociceptive hypersensitivity in models of chronic pain in vivo SIGNIFICANCE STATEMENT There is a major need to elucidate molecular mechanisms underlying chronic pain disorders to develop novel therapeutic approaches. Wnt signaling represents a highly versatile signaling system, which plays critical roles during development and adult physiology, and it was implicated in several diseases, including chronic pain conditions. Using mouse models, our study identifies a novel role for Wnt5a signaling in nociceptive modulation at the spinal cord level. We observed that Wnt5a recruits Ror2 and Ryk receptors to enhance dendritic spine density, leading to nociceptive sensitization. Blocking the Wnt5a-Ryk/Ror2 interaction in the spinal dorsal horn prevented spine remodeling and significantly reduced inflammatory and neuropathic hypersensitivity. These findings provide proof-of-concept for targeting spinal Wnt signaling for alleviating nociceptive hypersensitivity in vivo.

Signaling pathways and gene co-expression modules associated with cytoskeleton and axon morphology in breast cancer survivors with chronic paclitaxel-induced peripheral neuropathy

Background

The major dose-limiting toxicity of paclitaxel, one of the most commonly used drugs to treat breast cancer, is peripheral neuropathy (paclitaxel-induced peripheral neuropathy). Paclitaxel-induced peripheral neuropathy, which persists into survivorship, has a negative impact on patient’s mood, functional status, and quality of life. Currently, no interventions are available to treat paclitaxel-induced peripheral neuropathy. A critical barrier to the development of efficacious interventions is the lack of understanding of the mechanisms that underlie paclitaxel-induced peripheral neuropathy. While data from preclinical studies suggest that disrupting cytoskeleton- and axon morphology-related processes are a potential mechanism for paclitaxel-induced peripheral neuropathy, clinical evidence is limited. The purpose of this study in breast cancer survivors was to evaluate whether differential gene expression and co-expression patterns in these pathways are associated with paclitaxel-induced peripheral neuropathy.

Methods

Signaling pathways and gene co-expression modules associated with cytoskeleton and axon morphology were identified between survivors who received paclitaxel and did (n = 25) or did not (n = 25) develop paclitaxel-induced peripheral neuropathy.

Results

Pathway impact analysis identified four significantly perturbed cytoskeleton- and axon morphology-related signaling pathways. Weighted gene co-expression network analysis identified three co-expression modules. One module was associated with paclitaxel-induced peripheral neuropathy group membership. Functional analysis found that this module was associated with four signaling pathways and two ontology annotations related to cytoskeleton and axon morphology.

Conclusions

This study, which is the first to apply systems biology approaches using circulating whole blood RNA-seq data in a sample of breast cancer survivors with and without chronic paclitaxel-induced peripheral neuropathy, provides molecular evidence that cytoskeleton- and axon morphology-related mechanisms identified in preclinical models of various types of neuropathic pain including chemotherapy-induced peripheral neuropathy are found in breast cancer survivors and suggests pathways and a module of genes for validation and as potential therapeutic targets.

Dendritic Spine Dynamics after Peripheral Nerve Injury: An Intravital Structural Study

Neuropathic pain is an intractable medical condition with few or no options for effective treatment. Emerging evidence shows a strong structure-function relationship between dendritic spine dysgenesis and the presence of neuropathic pain. Postmortem tissue analyses can only imply dynamic structural changes associated with injury-induced pain. Here, we profiled the in vivo dynamics of dendritic spines over time on the same superficial dorsal horn (lamina II) neurons before and after peripheral nerve injury-induced pain. We used a two-photon, whole-animal imaging paradigm that permitted repeat imaging of the same dendritic branches of these neurons in C57/Bl6 Thy1-YFP male mice. Our study demonstrates, for the first time, the ongoing, steady-state changes in dendritic spine dynamics in the dorsal horn associated with peripheral nerve injury and pain. Ultimately, the relationship between altered dendritic spine dynamics and neuropathic pain may serve as a structure-based opportunity to investigate mechanisms of pain following injury and disease.SIGNIFICANCE STATEMENT This work is important because it demonstrates for the first time: (1) the powerful utility of intravital study of dendritic spine dynamics in the superficial dorsal horn; (2) that nerve injury-induced pain triggers changes in dendritic spine steady-state behavior in the spinal cord dorsal horn; and (3) this work opens the door to further investigations in vivo of spinal cord dendritic spine dynamics in the context of injury and disease.

Spinal cord motor neuron plasticity accompanies second-degree burn injury and chronic pain

Burn injuries and associated complications present a major public health challenge. Many burn patients develop clinically intractable complications, including pain and other sensory disorders. Recent evidence has shown that dendritic spine neuropathology in spinal cord sensory and motor neurons accompanies central nervous system (CNS) or peripheral nervous system (PNS) trauma and disease. However, no research has investigated similar dendritic spine neuropathologies following a cutaneous thermal burn injury. In this retrospective investigation, we analyzed dendritic spine morphology and localization in alpha-motor neurons innervating a burn-injured area of the body (hind paw). To identify a molecular regulator of these dendritic spine changes, we further profiled motor neuron dendritic spines in adult mice treated with romidepsin, a clinically approved Pak1-inhibitor, or vehicle control at two postburn time points: Day 6 immediately after treatment, or Day 10 following drug withdrawal. In control treated mice, we observed an overall increase in dendritic spine density, including structurally mature spines with mushroom-shaped morphology. Pak1-inhibitor treatment reduced injury-induced changes to similar levels observed in animals without burn injury. The effectiveness of the Pak1-inhibitor was durable, since normalized dendritic spine profiles remained as long as 4 days despite drug withdrawal. This study is the first report of evidence demonstrating that a second-degree burn injury significantly affects motor neuron structure within the spinal cord. Furthermore, our results support the opportunity to study dendritic spine dysgenesis as a novel avenue to clarify the complexities of neurological disease following traumatic injury.

When pain gets stuck: the evolution of pain chronification and treatment resistance

It is well-recognized that, despite similar pain characteristics, some people with chronic pain recover, whereas others do not. In this review, we discuss possible contributions and interactions of biological, social, and psychological perturbations that underlie the evolution of treatment-resistant chronic pain. Behavior and brain are intimately implicated in the production and maintenance of perception. Our understandings of potential mechanisms that produce or exacerbate persistent pain remain relatively unclear. We provide an overview of these interactions and how differences in relative contribution of dimensions such as stress, age, genetics, environment, and immune responsivity may produce different risk profiles for disease development, pain severity, and chronicity. We propose the concept of "stickiness" as a soubriquet for capturing the multiple influences on the persistence of pain and pain behavior, and their stubborn resistance to therapeutic intervention. We then focus on the neurobiology of reward and aversion to address how alterations in synaptic complexity, neural networks, and systems (eg, opioidergic and dopaminergic) may contribute to pain stickiness. Finally, we propose an integration of the neurobiological with what is known about environmental and social demands on pain behavior and explore treatment approaches based on the nature of the individual's vulnerability to or protection from allostatic load.

Dendritic spine dysgenesis in superficial dorsal horn sensory neurons after spinal cord injury

Neuropathic pain is a major complication of spinal cord injury, and despite aggressive efforts, this type of pain is refractory to available clinical treatment. Our previous work has demonstrated a structure–function link between dendritic spine dysgenesis on nociceptive sensory neurons in the intermediate zone, laminae IV/V, and chronic pain in central nervous system and peripheral nervous system injury models of neuropathic pain. To extend these findings, we performed a follow-up structural analysis to assess whether dendritic spine remodeling occurs on superficial dorsal horn neurons located in lamina II after spinal cord injury. Lamina II neurons are responsible for relaying deep, delocalized, often thermally associated pain commonly experienced in spinal cord injury pathologies. We analyzed dendritic spine morphometry and localization in tissue obtained from adult rats exhibiting neuropathic pain one-month following spinal cord injury. Although the total density of dendritic spines on lamina II neurons did not change after spinal cord injury, we observed an inverse relationship between the densities of thin- and mushroom-shaped spines: thin-spine density decreased while mushroom-spine density increased. These structural changes were specifically noted along dendritic branches within 150 µm from the soma, suggesting a possible adverse contribution to nociceptive circuit function. Intrathecal treatment with NSC23766, a Rac1-GTPase inhibitor, significantly reduced spinal cord injury-induced changes in both thin- and mushroom-shaped dendritic spines. Overall, these observations demonstrate that dendritic spine remodeling occurs in lamina II, regulated in part by the Rac1-signaling pathway, and suggests that structural abnormalities in this spinal cord region may also contribute to abnormal nociception after spinal cord injury.

Synaptosomal-associated protein 25 may be an intervention target for improving sensory and locomotor functions after spinal cord contusion

Synaptosomal-associated protein 25 kDa (SNAP-25) is localized on the synapse and participates in exocytosis and neurotransmitter release. Decreased expression of SNAP-25 is associated with Alzheimer's disease and attention deficit/hyperactivity disorder. However, the expression of SNAP-25 in spinal cord contusion injury is still unclear. We hypothesized that SNAP-25 is associated with sensory and locomotor functions after spinal cord injury. We established rat models of spinal cord contusion injury to detect gene changes with a gene array. A decreased level of SNAP-25 was detected by quantitative real time-polymerase chain reaction and western blot assay at 1, 3, 7, 14 and 28 days post injury. SNAP-25 was localized in the cytoplasm of neurons of the anterior and posterior horns, which are involved in locomotor and sensory functions. Our data suggest that reduced levels of SNAP-25 are associated with sensory and locomotor functions in rats with spinal cord contusion injury.

Dendritic spine remodeling following early and late Rac1 inhibition after spinal cord injury: evidence for a pain biomarker

Neuropathic pain is a significant complication following spinal cord injury (SCI) with few effective treatments. Drug development for neuropathic pain often fails because preclinical studies do not always translate well to clinical conditions. Identification of biological characteristics predictive of disease state or drug responsiveness could facilitate more effective clinical translation. Emerging evidence indicates a strong correlation between dendritic spine dysgenesis and neuropathic pain. Because dendritic spines are located on dorsal horn neurons within the spinal cord nociceptive system, dendritic spine remodeling provides a unique opportunity to understand sensory dysfunction after SCI. In this study, we provide support for the postulate that dendritic spine profiles can serve as biomarkers for neuropathic pain. We show that dendritic spine profiles after SCI change to a dysgenic state that is characteristic of neuropathic pain in a Rac1-dependent manner. Suppression of the dysgenic state through inhibition of Rac1 activity is accompanied by attenuation of neuropathic pain. Both dendritic spine dysgenesis and neuropathic pain return when inhibition of Rac1 activity is lifted. These findings suggest the utility of dendritic spines as structural biomarkers for neuropathic pain.

Involvement of Rac1 signalling pathway in the development and maintenance of acute inflammatory pain induced by bee venom injection

BACKGROUND AND PURPOSE

The Rho GTPase, Rac1, is involved in the pathogenesis of neuropathic pain induced by malformation of dendritic spines in the spinal dorsal horn (sDH) neurons. In the present study, the contribution of spinal Rac1 to peripheral inflammatory pain was studied.

EXPERIMENTAL APPROACH

Effects of s.c. bee venom (BV) injection on cellular localization of Rac1 in the rat sDH was determined with double labelling immunofluorescence. Activation of Rac1 and its downstream effector p21-activated kinase (PAK), ERKs and p38 MAPK in inflammatory pain states was evaluated with a pull-down assay and Western blotting. The preventive and therapeutic analgesic effects of intrathecal administration of NSC23766, a selective inhibitor of Rac1, on BV-induced spontaneous nociception and pain hypersensitivity were investigated.

KEY RESULTS

Rac1 labelling was mainly localized within neurons in both the superficial and deep layers of the sDH in rats of naïve, vehicle-treated and inflamed (BV injected) groups. GTP-Rac1-PAK and ERKs/p38 were activated following s.c. BV injection. Post-treatment with intrathecal NSC23766 significantly inhibited GTP-Rac1 activity and phosphorylation of Rac1-PAK, ERKs and p38 MAPK in the sDH. Both pre-treatment and post-treatment with intrathecal NSC23766 dose-dependently attenuated the paw flinches, primary thermal and mechanical hyperalgesia and the mirror-image thermal hyperalgesia induced by BV injection, but without affecting the baseline pain sensitivity and motor coordination.

CONCLUSIONS AND IMPLICATIONS

The spinal GTP-Rac1-PAK-ERK/p38MAPK signalling pathway is involved in both the development and maintenance of peripheral inflammatory pain and can be used as a potential molecular target for developing a novel therapeutic strategy for clinical pain.