Increase of preprotachykinin mRNA and substance P immunoreactivity in spared dorsal root ganglion neurons following partial sciatic nerve injury

Behavioral Animal Models and Neural-Circuit Framework of Depressive Disorder

Depressive disorder is a chronic, recurring, and potentially life-endangering neuropsychiatric disease. According to a report by the World Health Organization, the global population suffering from depression is experiencing a significant annual increase. Despite its prevalence and considerable impact on people, little is known about its pathogenesis. One major reason is the scarcity of reliable animal models due to the absence of consensus on the pathology and etiology of depression. Furthermore, the neural circuit mechanism of depression induced by various factors is particularly complex. Considering the variability in depressive behavior patterns and neurobiological mechanisms among different animal models of depression, a comparison between the neural circuits of depression induced by various factors is essential for its treatment. In this review, we mainly summarize the most widely used behavioral animal models and neural circuits under different triggers of depression, aiming to provide a theoretical basis for depression prevention.

The neuropeptide substance P/neurokinin-1 receptor system and diabetes: From mechanism to therapy

Diabetes is a significant public health issue known as the world's fastest-growing disease condition. It is characterized by persistent hyperglycemia and subsequent chronic complications leading to organ dysfunction and, ultimately, the failure of target organs. Substance P (SP) is an undecapeptide that belongs to the family of tachykinin (TK) peptides. The SP-mediated activation of the neurokinin 1 receptor (NK1R) regulates many pathophysiological processes in the body. There is also a relation between the SP/NK1R system and diabetic processes. Importantly, deregulated expression of SP has been reported in diabetes and diabetes-associated chronic complications. SP can induce both diabetogenic and antidiabetogenic effects and thus affect the pathology of diabetes destructively or protectively. Here, we review the current knowledge of the functional relevance of the SP/NK1R system in diabetes pathogenesis and its exploitation for diabetes therapy. A comprehensive understanding of the role of the SP/NK1R system in diabetes is expected to shed further light on developing new therapeutic possibilities for diabetes and its associated chronic conditions.

Changes in dorsal root ganglion CGRP expression in mouse pinch nerve injury model: Modulation by Somatostatin type-2 receptor

INTRODUCTION

Our previous work has shown that somatostatin effectively inhibits neuropathic pain by activating its type 2 receptor (SSTR2) in the dorsal root ganglion (DRG) and spinal cord of mice. However, the underlying mechanism of this activation has not been elucidated.

METHODS

To explore further mechanisms, we examined pain behavior and the expression of neuropeptides such as calcitonin gene-related peptide (CGRP) in dorsal root ganglion neurons(DRGs) as well as the changes of the number of CGRP-IR DRGs in the mouse model of sciatic pinch nerve injury.

RESULTS

In this model, the number of medium and small DRG neurons in ipsilateral CGRP-IR was slightly increased, but not significantly, compared with sham animals at 3, 7, and 9 days after pinch nerve injury. This correlated with the behavioral readouts of hypersensitivity at the same time points. However, the magnitude of the painful behavior (Autotomy) was observed after application of SSTR2 antagonist (CYN154806, 5 mg/kg) in the injured nerve groups compared to the saline-treated injured group as well as the sham-operated group. Following pinch nerve injury, there was a significant decrease in the number of ipsilateral CGRP-IR small and medium DRG neurons in SSTR2 antagonist (anti-SSTR2)- but not saline-treated mice. These data also correlated with painful behavioral readouts where hypersensitivity was significantly increased by anti-SSTR2 but not saline treatment.

DISCUSSION/CONCLUSION

In all, application of the SSTR2 antagonist to the pinched sciatic nerve suppressed CGRP expression and aggravated painful behavior, suggesting that CGRP expression in DRG neurons can be an important component of the pain mechanism and an indicator of pain behavior.

Evaluation of Associated Genes with Traumatic Pain: A Systematic Review

OBJECTIVES

The knowledge about the molecular pathway of traumatic pain relief is less documented. This systematic review study aimed to identify the genes and molecular pathways associated with various traumatic pains.

METHODS

The online databases such as EMBASE, MEDLINE, PubMed, Cochrane Library, International Clinical Trials Registry Platform, Clinical Trials, Google Scholar, Wiley, ISI Web of Knowledge, and Scopus were searched. Two review authors searched and screened all records' titles and abstracts, and the third expert reviewer author resolved their disagreement. The study's design, various trauma injuries, types of genes, and molecular pathways were recorded. The genes and molecular pathways data were obtained via GeneCards®: The Human Gene Database (https://www.genecards.org).

RESULTS

Studies on a variety of trauma injuries regarding nerve and Spinal Cord Injuries (SCIs) (12 records), Hypertrophic scar with Severe Pain (one record), severe post-traumatic musculoskeletal pain (MSP) (one record), and orthopedic trauma (one record) were included. The main molecular pathways such as the immune system, apoptosis, and death receptor signaling, T-cell antigen receptor (TCR) signaling pathway, oxidative stress, interleukin(s) mediated signaling pathway, biological oxidations, metabolic pathways (especially amino acid metabolism and amino group), focal adhesion, the proliferation of vascular, epithelial, and connective tissue cells, angiogenesis and neural development were identified.

CONCLUSION

The immune system, apoptosis, and metabolic pathways are crucial for understanding the roles of genes in traumatic pain. It is recommended that these identified pathways and related genes be considered therapeutical targets for pain management in patients with trauma injuries. In addition, different forms of trauma injuries require different pathways and related genes to be considered.

The Study of Pain in Rats and Mice

Pain is a clinical syndrome arising from a variety of etiologies in a heterogeneous population, which makes successfully treating the individual patient difficult. Organizations and governments recognize the need for tailored and specific therapies, which drives pain research. This review summarizes the different types of pain assessments currently being used and the various rodent models that have been developed to recapitulate the human pain condition.

Gene Expression Profiling of Cutaneous Injured and Non-Injured Nociceptors in SNI Animal Model of Neuropathic Pain

Nociceptors are a particular subtype of dorsal root ganglion (DRG) neurons that detect noxious stimuli and elicit pain. Although recent efforts have been made to reveal the molecular profile of nociceptors in normal conditions, little is known about how this profile changes in pathological conditions. In this study we exploited laser capture microdissection to specifically collect individual injured and non-injured nociceptive DRG neurons and to define their gene profiling in rat spared nerve injury (SNI) model of neuropathic pain. We found minimal transcriptional changes in non-injured neurons at 7 days after SNI. In contrast, several novel transcripts were altered in injured nociceptors, and the global signature of these LCM-captured neurons differed markedly from that the gene expression patterns found previously using whole DRG tissue following SNI. Pathway analysis of the transcriptomic profile of the injured nociceptors revealed oxidative stress as a key biological process. We validated the increase of caspase-6 (CASP6) in small-sized DRG neurons and its functional role in SNI- and paclitaxel-induced neuropathic pain. Our results demonstrate that the identification of gene regulation in a specific population of DRG neurons (e.g., nociceptors) is an effective strategy to reveal new mechanisms and therapeutic targets for neuropathic pain from different origins.

High-frequency transcutaneous electrical nerve stimulation attenuates postsurgical pain and inhibits excess substance P in rat dorsal root ganglion

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is a common therapeutic modality for pain management, but its effectiveness in skin/muscle incision and retraction (SMIR)-evoked pain is unknown. We aimed to examine the effects of TENS on postoperative pain and the levels of substance P (SP), N-methyl-D-aspartate receptor 1 (NR1), and interleukin 1β (IL-1β) in rat dorsal root ganglion (DRG).

METHODS

High-frequency (100 Hz) TENS was administered daily beginning on postoperative day 1 (POD1) and continued until animal subjects were killed for tissues. Mechanical sensitivity to von Frey stimuli (6g and 15g) and the levels of NR1, SP, and IL-1β in DRG were assessed in the sham-operated, SMIR-operated, TENS after SMIR surgery, and placebo-TENS after SMIR surgery groups.

RESULTS

Skin/muscle incision and retraction rats exhibited a significant hypersensitivity to von Frey stimuli on POD3. In contrast with SMIR rats, SMIR-operated rats receiving TENS therapy demonstrated a rapid recovery of mechanical hypersensitivity. The SMIR-operated rats showed an up-regulation of NR1, SP, and IL-1β in DRG on PODs 14 and 28, whereas the SMIR-operated rats after TENS administration reversed this up-regulation. By contrast, the placebo-TENS after SMIR operation did not alter postsurgical pain nor the levels of NR1, SP, and IL-1β.

CONCLUSIONS

Our data demonstrated that TENS intervention reduced persistent postoperative pain caused by SMIR operation. Up-regulation of NR1, SP, and IL-1β in DRG, activated after SMIR surgery, is important in the development of prolonged postincisional pain. The TENS pain relief may be related to the suppression of NR1, SP, and IL-1β in DRG of SMIR rats.

Therapeutic ultrasound suppresses neuropathic pain and upregulation of substance P and neurokinin-1 receptor in rats after peripheral nerve injury

We studied the mechanisms and impact of therapeutic ultrasound (TU) for pain caused by nerve injury. TU began on post-operative day 5 (POD5) and then continued daily for the next 22 d. Sensitivity to thermal and mechanical stimuli and levels of neurokinin-1 receptor, substance P, tumor necrosis factor-α and interleukin-6 in the sciatic nerve were examined. On POD7, chronic constriction injury rats undergoing TU at an intensity of 1 W/cm(2), but not 0.25 or 0.5 W/cm(2), had increases in both the mechanical withdrawal threshold and the thermal withdrawal latency compared with the chronic constriction injury group. Moreover, chronic constriction injury rats exhibited upregulation of neurokinin-1 receptor, substance P, tumor necrosis factor-α and interleukin-6 in the sciatic nerve on PODs 14 and 28, whereas TU inhibited their increased expression. We suggest that the efficacy of TU is dependent on its ability to limit the upregulation of neurokinin-1 receptor, substance P, tumor necrosis factor-α and interleukin-6 around the injured sciatic nerve.

Nerve stretch injury induced pain pattern and changes in sensory ganglia in a clinically relevant model of limb-lengthening in rabbits

We used a model of tibial lengthening in rabbits to study the postoperative pain pattern during limb-lengthening and morphological changes in the dorsal root ganglia (DRG), including alteration of substance P (SP) expression. Four groups of animals (naive; OG: osteotomized only group; SDG/FDG: slow/fast distraction groups, with 1 mm/3 mm lengthening a day, respectively) were used. Signs of increasing postoperative pain were detected until the 10(th) postoperative day in OG/SDG/FDG, then they decreased in OG but remained higher in SDG/FDG until the distraction finished, suggesting that the pain response is based mainly on surgical trauma until the 10(th) day, while the lengthening extended its duration and increased its intensity. The only morphological change observed in the DRGs was the presence of large vacuoles in some large neurons of OG/SDG/FDG. Cell size analysis of the S1 DRGs showed no cell loss in any of the three groups; a significant increase in the number of SP-positive large DRG cells in the OG; and a significant decrease in the number of SP-immunoreactive small DRG neurons in the SDG/FDG. Faster and larger distraction resulted in more severe signs of pain sensation, and further reduced the number of SP-positive small cells, compared to slow distraction.

Forced treadmill running suppresses postincisional pain and inhibits upregulation of substance P and cytokines in rat dorsal root ganglion

Exercise causes a variety of psychophysical effects (eg, alterations in pain sensation). Tissue injury induces mediator releases in the spinal cord resulting in pain hypersensitivity; however, the contribution of the dorsal root ganglion (DRG) is poorly understood. In this study, we tested if forced treadmill running can attenuate postoperative pain and alter substance P (SP) or proinflammatory cytokine level in the DRG by using a rat model of skin/muscle incision and retraction (SMIR). We evaluated mechanical sensitivity to von Frey stimuli (6 and 15 g) and expression of SP, interleukin-1β, and interleukin-6 in the DRG of sham-operated sedentary rats, SMIR sedentary rats, sham-operated rats with forced treadmill running, and SMIR rats with forced treadmill running. At postoperative day 8, trained rats ran for 5 days per week for 4 weeks on a treadmill 70 minutes/d with an intensity of 18 m/min. On postoperative day 6, SMIR sedentary rats displayed a significant mechanical hypersensitivity that persisted until postoperative day 35. By comparison, SMIR-operated rats, which received forced treadmill running, exhibited a quick recovery from mechanical hypersensitivity. SMIR sedentary rats showed an upregulation of SP, interleukin-1β, and interleukin-6 in the DRG at postoperative days 14 and 28, whereas SMIR-operated rats receiving forced treadmill running reversed this upregulation at postoperative day 28. We concluded that forced treadmill running alleviated persistent postincisional pain caused by SMIR surgery. This appears to be protective against postoperative pain, which probably relates to the downturn in excess SP, interleukin-1β, and interleukin-6 in the DRG.

PERSPECTIVE

Controlling the expression of SP, interleukin-6, and interleukin-1β in the DRG can help manage postoperative pain. This finding could potentially help clinicians and physical therapists who seek to examine how exercise may attenuate postsurgical pain and its mechanism.

De novo expression of neurokinin-1 receptors by spinoparabrachial lamina I pyramidal neurons following a peripheral nerve lesion

Lamina I of the spinal dorsal horn is a major site of integration and transmission to higher centers of nociceptive information from the periphery. One important primary afferent population that transmits such information to the spinal cord expresses substance P (SP). These fibers terminate in contact with lamina I projection neurons that express the SP receptor, also known as the neurokinin-1 receptor (NK-1r). Three types of lamina I projection neurons have been described: multipolar, fusiform, and pyramidal. Most neurons of the first two types are thought to be nociceptive and express the NK-1r, whereas most pyramidal neurons are nonnociceptive and do not express the NK-1r. In this immunocytochemical and behavioral study, we induced a neuropathic pain-like condition in the rat by means of a polyethylene cuff placed around in the sciatic nerve. We document that this lesion led to a de novo expression of NK-1r on pyramidal neurons as well as a significant increase in SP-immunoreactive innervation onto these neurons. These phenotypic changes were evident at the time of onset of neuropathic pain-related behavior. Additionally, we show that, after a noxious stimulus (intradermal capsaicin injection), these NK-1r on pyramidal neurons were internalized, providing evidence that these neurons become responsive to peripheral noxious stimulation. We suggest that the changes following nerve lesion in the phenotype and innervation pattern of pyramidal neurons are of significance for neuropathic pain and/or limb temperature regulation.

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

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.

Role of prostaglandin E2 in the synthesis of the pro-inflammatory cytokine interleukin-6 in primary sensory neurons: an in vivo and in vitro study

Following various types of nerve injury, cyclooxygenase 2 and prostaglandin E2 (PGE2) are universally and chronically up-regulated in injured nerves and contribute to the genesis of neuropathic pain. Persistent high levels of PGE2 likely exert chronic effects on nociceptive dorsal root ganglion (DRG) neurons. In the present study, we tested the hypothesis that injured nerve-derived PGE2 contributes to the up-regulation of the pro-inflammatory cytokine interleukin-6 (IL-6) in DRG neurons following partial sciatic nerve ligation. In naive adult rats, IL-6 was expressed in only a few small size DRG neurons which all co-expressed EP4 receptors. Partial sciatic nerve ligation increased and shifted IL-6 expression from small to medium and large size damaged DRG neurons. Perineural injection of a selective cyclooxygenase 2 inhibitor or a selective EP4 receptor antagonist significantly suppressed the up-regulation of IL-6 in DRG, suggesting that injured nerve derived PGE2 contributes to the de novo synthesis of IL-6 in DRG neurons through EP4 receptors. In cultured sensory ganglion explants, a stabilized PGE2 analog increased IL-6 mRNA and protein levels through the activation of EP4, protein kinase A, protein kinase C, extracellular regulated protein kinase/MAPK, cAMP response element binding protein and NFκB signalling pathways. Taken together, these data indicate that facilitating the de novo synthesis of pain-related cytokines in injured medium and large size DRG neurons is a novel mechanism underlying the role of injured nerve derived PGE2 in the genesis of neuropathic pain.

The N-methyl-D-aspartate-evoked cytoplasmic calcium increase in adult rat dorsal root ganglion neuronal somata was potentiated by substance P pretreatment in a protein kinase C-dependent manner

The involvement of substance P (SP) in neuronal sensitization through the activation of the neurokinin-1-receptor (NK1r) in postsynaptic dorsal horn neurons has been well established. In contrast, the role of SP and NK1r in primary sensory dorsal root ganglion (DRG) neurons, in particular in the soma, is not well understood. In this study, we evaluated whether SP modulated the NMDA-evoked transient increase in cytoplasmic Ca2+ ([Ca2+]cyt) in the soma of dissociated adult DRG neurons. Cultures were treated with nerve growth factor (NGF), prostaglandin E2 (PGE2) or both NGF+PGE2. Treatment with NGF+PGE2 increased the percentage of N-methyl-D-aspartate (NMDA) responsive neurons. There was no correlation between the percentage of NMDA responsive neurons and the level of expression of the NR1 and NR2B subunits of the NMDA receptor or of the NK1r. Pretreatment with SP did not alter the percentage of NMDA responsive neurons; while it potentiated the NMDA-evoked [Ca2+]cyt transient by increasing its magnitude and by prolonging the period during which small- and some medium-sized neurons remained NMDA responsive. The SP-mediated potentiation was blocked by the SP-antagonist ([D-Pro4, D-Trp7,9]-SP (4-11)) and by the protein kinase C (PKC) blocker bisindolylmaleimide I (BIM); and correlated with the phosphorylation of PKCε. The Nk1r agonist [Sar9, Met(O2)11]-SP (SarMet-SP) also potentiated the NMDA-evoked [Ca2+]cyt transient. Exposure to SP or SarMet-SP produced a rapid increase in the labeling of phosphorylated-PKCε. In none of the conditions we detected phosphorylation of the NR2B subunit at Ser-1303. Phosphorylation of the NR2B subunit at Tyr1472 was enhanced to a similar extent in cells exposed to NMDA, SP or NMDA+SP, and that enhancement was blocked by BIM. Our findings suggest that NGF and PGE2 may contribute to the injury-evoked sensitization of DRG neurons in part by enhancing their NMDA-evoked [Ca2+]cyt transient in all sized DRG neurons; and that SP may further contribute to the DRG sensitization by enhancing and prolonging the NMDA-evoked increase in [Ca2+]cyt in small- and medium-sized DRG neurons.

Endogenous neurotrophins and plasticity following spinal deafferentation

Neurons intrinsic to the spinal cord dorsal horn receive input from various classes of long-distance projection systems. Two of the best known of these are primary afferent and descending monoaminergic axons. Together with intrinsic interneurons, activity in these axonal populations shapes the early part of the sensory experience before it is transmitted to supraspinal structures via ascending projection axons. Injury to dorsal roots, which contain the centrally projecting branches of primary afferent axons, results in their permanent disconnection from the spinal cord, as well as sensory dysfunction such as pain. In animals, experimental dorsal root injuries affecting a small number of roots produce dynamic behavioural changes, providing evidence for the now familiar concept that sensory processing at the level of the spinal cord is not hard-wired. Changes in behaviour following rhizotomy suggest changes in spinal sensory circuitry, and we and others have shown that the density of spinal serotonergic axons as well as processes of inhibitory interneurons increases following rhizotomy. Intact primary afferent axons are less apt to sprout into denervated territory. Recent work from our group has asked (1) what is the stimulus that induces sprouting of serotonergic (and other) axons and (2) what prevents spared primary afferent axons from occupying the territory of those lost to injury. This article will review the evidence that a single factor upregulated by dorsal root injury, brain-derived neurotrophic factor (BDNF), underpins both serotonergic sprouting and a lack of primary afferent plasticity. BDNF also differentially modulates some of the behavioural consequences of dorsal root injury: antagonizing endogenous BDNF improves spontaneous mechanosensory recovery but prevents recovery from rhizotomy-induced hypersensitivity to cold. These findings reinforce the notion that in disease states as complex and variable as spinal cord injury, single pharmacological interventions are unlikely to produce meaningful results. However, understanding the differences in capacity for plasticity among different systems, as well as their triggers, should allow for more patient-tailored therapies.

Chronic prostaglandin E2 treatment induces the synthesis of the pain-related peptide substance P and calcitonin gene-related peptide in cultured sensory ganglion explants

Prostaglandin E2 (PGE2) is a well known pain and pro-inflammatory mediator abundantly produced in inflamed tissue. It causes pain by directly exciting nociceptive primary sensory neurons (nociceptors) and indirectly stimulating the release of pain-related peptide substance P (SP) and calcitonin gene-related peptide (CGRP). In an ex vivo culture of sensory ganglion explants, we tested the hypothesis that PGE2 could induce the synthesis of SP and CGRP in nociceptors. A stabilized PGE2 analog, 16,16-dimethyl PGE2, in a concentration- and time-dependent manner, significantly increased mRNA and peptide levels of SP and CGRP. The agonists of EP1 and EP4 receptors also significantly increased SP and CGRP levels. Moreover, 16,16-dimethyl PGE2-induced SP and CGRP were blocked by EP1 and EP4 antagonists as well as the inhibitors of both protein kinase A and protein kinase C. Nerve growth factor was partially involved in PGE2-induced SP and CGRP synthesis. Taken together, these results indicate that PGE2 contributes to the synthesis of SP and CGRP in nociceptors, an event mediated by EP1 and EP4 receptors, nerve growth factor and protein kinase A and protein kinase C signalling pathways. We thus conclude that facilitating the synthesis of pain-related peptides in nociceptors is a novel mechanism underlying the role of PGE2 in nociception and chronic pain states.

A novel method to quantify histochemical changes throughout the mediolateral axis of the substantia gelatinosa after spared nerve injury: characterization with TRPV1 and substance P

Nerve injury dramatically increases or decreases protein expression in the spinal cord dorsal horn. Whether the spatial distribution of these changes is restricted to the central innervation territories of injured nerves or could spread to adjacent territories in the dorsal horn is not understood. To address this question, we developed a simple computer software-assisted method to precisely distinguish and efficiently quantify immunohistochemical staining patterns across the mediolateral axis of the dorsal horn 2 weeks after transection of either the tibial and common peroneal nerves (thus sparing the sural branch, spared nerve injury, [SNI]), the tibial nerve, or the common peroneal and sural nerves. Using thiamine monophosphatase (TMP) histochemistry, we determined that central terminals of the tibial, common peroneal, sural, and posterior cutaneous nerves occupy the medial 35%, medial-central 20%, central-lateral 20%, and lateral 25% of the substantia gelatinosa, respectively. We then used these calculations to show that SNI reduced the expression of SP and TRPV1 immunoreactivity within the tibial and peroneal innervation territories in the L4 dorsal horn, without changing expression in the uninjured, sural sector. We conclude that SNI-induced loss of SP and TRPV1 in central terminals of dorsal horn is restricted to injured fibers. Our new method enables direct comparison of injured and uninjured terminals in the dorsal horn so as to better understand their relative contributions to mechanisms of chronic pain.

PERSPECTIVE

A simple computer software-assisted algorithm was developed to precisely distinguish and efficiently quantify immunohistochemical staining patterns across the mediolateral axis of the dorsal horn after distal sciatic-branch transection. This method will facilitate a better understanding of the relative contribution of injured and uninjured terminals to mechanisms of chronic pain.

Nerve growth factor mediates mechanical allodynia in a mouse model of type 2 diabetes

C57BLKS db/db (db/db) mice develop a neuropathy with features of human type 2 diabetic neuropathy. Here, we demonstrate that these mice develop transient mechanical allodynia at the early stage of diabetes. We hypothesized that nerve growth factor (NGF), which enhances the expression of key mediators of nociception (i.e. substance P [SP] and calcitonin gene-related peptide), contributes to the development of mechanical allodynia in these mice. We found that NGF, SP, and calcitonin gene-related peptide gene expression is upregulated in the dorsal root ganglion (DRG) of db/db mice before or during the period that they develop mechanical allodynia. There were more small- to medium-sized NGF-immunopositive DRG neurons in db/db mice than in control db+ mice; these neurons also expressed SP, consistent with its role in nociception. Nerve growth factor expression in the hind paw skin was also increased in a variety of dermal cell types and nerve fibers, suggesting the contribution of a peripheral source of NGF to mechanical allodynia. The upregulation of NGF coincided with enhanced tropomyosin-related kinase A receptor phosphorylation in the DRG. Finally, an antibody against NGF inhibited mechanical allodynia and decreased the numbers of SP-positive DRG neurons in db/db mice. These results suggest that inhibition of NGF action is a potential strategy for treating painful diabetic neuropathy.

Joint distraction magnitude is associated with different behavioral outcomes and substance P levels for cervical facet joint loading in the rat

The facet joint is a common source of pain in both the neck and low back, and can be injured by abnormal loading of the spinal joints. Whereas a host of nociceptive changes including neuronal activation, neuropeptide expression, and inflammatory mediator responses has been reported for rat models of joint pain, no such responses have been explicitly investigated or quantified for painful mechanical injury to the facet joint. Two magnitudes of joint loading were separately imposed in a rat model of cervical facet joint distraction: Painful and nonpainful distractions. Behavioral outcomes were defined by assessing mechanical hyperalgesia in the shoulders and forepaws. Substance P (SP) mRNA and protein levels were quantified in the dorsal root ganglion (DRG) and spinal cord at days 1 and 7 following distraction. Painful distraction produced mechanical hyperalgesia that was significantly greater (P < .010) than that for a nonpainful distraction. Painful distraction significantly increased spinal SP mRNA (P = .048) and SP protein expression in the DRG (P = .013) at day 7 compared to nonpainful distraction. However, spinal SP protein for painful distraction was significantly less (P = .024) than that for nonpainful distraction at day 1. Joint distractions producing different behavioral outcomes modulate SP mRNA and protein in the DRG and spinal cord, suggesting that SP responses may be involved with different temporal responses in painful joint loading.

PERSPECTIVE

SP mRNA and protein in the DRG and spinal cord are quantified at 2 time points after cervical facet joint distractions that separately do or do not produce mechanical hyperalgesia. Studies describe a role for SP to contribute to pain produced by mechanical joint loading.

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

Neuropathic pain (NeP) is a debilitating disease afflicting mostly the aged population. Inflammatory responses in injured nerves play a pivotal role in the pathogenesis of NeP. Injured nerve derived cyclooxygenase 2/prostaglandin E2 (COX2/PGE2) contributes to the genesis of NeP at the early stage in young rats. Here we show that COX2/PGE2 is involved in the maintenance of NeP at a chronic stage in aged rats. Eighteen months after partial sciatic nerve ligation (PSNL), NeP remained prominent in aged rats. COX2 expressing macrophages and PGE2 levels were increased in injured nerves. PGE2 receptors (EP1 and EP4) and pain-related ion channel transient receptor potential vanilloid-1 (TRPV1) were increased in the ipsilateral dorsal root ganglion (DRG) neurons of aged PSNL rats. Perineural injection of a selective COX2 inhibitor NS-398 relieved NeP, reversed PSNL increased expression of EP1, EP4 and TRPV1 and suppressed the levels of pain-related peptide substance P and calcitonin gene-related peptide in DRG neurons. These data suggest that injured nerve-derived PGE2 contributes to the maintenance of NeP at the chronic stage in aged rats. Chronically facilitating the synthesis of pain-related molecules in nociceptive DRG neurons is a novel mechanism underpinning the contribution of PGE2.

Immediate electrical stimulation enhances regeneration and reinnervation and modulates spinal plastic changes after sciatic nerve injury and repair

We have studied whether electrical stimulation immediately after nerve injury may enhance axonal regeneration and modulate plastic changes at the spinal cord level underlying the appearance of hyperreflexia. Two groups of adult rats were subjected to sciatic nerve section followed by suture repair. One group (ES) received electrical stimulation (3 V, 0.1 ms at 20 Hz) for 1 h after injury. A second group served as control (C). Nerve conduction, H reflex, motor evoked potentials, and algesimetry tests were performed at 1, 3, 5, 7 and 9 weeks after surgery, to assess muscle reinnervation and changes in excitability of spinal cord circuitry. The electrophysiological results showed higher levels of reinnervation, and histological results a significantly higher number of regenerated myelinated fibers in the distal tibial nerve in group ES in comparison with group C. The monosynaptic H reflex was facilitated in the injured limb, to a higher degree in group C than in group ES. The amplitudes of motor evoked potentials were similar in both groups, although the MEP/M ratio was increased in group C compared to group ES, indicating mild central motor hyperexcitability. Immunohistochemical labeling of sensory afferents in the spinal cord dorsal horn showed prevention of the reduction in expression of substance P at one month postlesion in group ES. In conclusion, brief electrical stimulation applied after sciatic nerve injury promotes axonal regeneration over a long distance and reduces facilitation of spinal motor responses.

Nitric oxide-NGF mediated PPTA/SP, ADNP, and VIP expression in the peripheral nervous system

Nerve growth factor (NGF)-deprivation or axotomy of dorsal root ganglion (DRG) neurons causes stress, which they cope by triggering various mechanisms. Among several molecular changes, in the present study, we demonstrate preprotachykinin-A-substance P (PPTA-SP) and activity-dependent neuroprotective protein-vasoactive intestinal peptide (ADNP-VIP) expression pattern using DRG neurons-Schwann cells coculture and axotomy model. In the presence of NGF, DRG cultures showed high levels of PPTA and ADNP mRNA expression, which were significantly suppressed in the absence of NGF and/or nitric oxide synthase (NOS) inhibition by NG-nitro-L-arginine methyl ester (L-NAME), suggesting that both NGF and nitric oxide (NO) can regulate PPTA and ADNP expression. However, treating coculture with NO donor, diethylenetriamine nitric oxide (DETA-NO) did not increase PPTA and ADNP expression in the presence or absence of NGF, although there was a marginal increase in ADNP expression in the absence of NGF. NGF-deprivation increases endogenous NO; thus, DETA-NO had no further effect on PPTA and ADNP expression. Alternatively, NGF produced from NO-stimulated Schwann cells influence gene expression. In addition, interestingly, DETA-NO treatment of Schwann cells alone suppresses both PPTA and ADNP, suggesting differential response of DRG neurons-Schwann cells coculture to DETA-NO. SP and ADNP immunostaining of axotomized DRGs revealed significant reduction in SP and ADNP compared to intact DRG, which was partially recovered in neuronal NOS blocker, 7-nitroindazole (7-NI)-treated DRGs, particularly intense ADNP staining in satellite glia. As ADNP is VIP-responsive gene, we further explored VIP expression in DRGs. Axotomy increased VIP in DRG neurons, but 7-NI treatment caused intense VIP staining in satellite glia. These observations suggest a complex interaction of NO-NGF with PPTA/SP and ADNP-VIP in neuron-glial communication when neurons are stressed.

Involvement of substance P and calcitonin gene-related peptide in development and maintenance of neuropathic pain from spinal nerve injury model of rat

Recently, it has been suggested that uninjured primary sensory neurons contribute to neuropathic pain induced by peripheral nerve injury. However, there is lack of evidences of roles of normal pain transmitting substances such as substance P and calcitonin gene-related peptide (CGRP) in neuropathic pain. Whether substance P and CGRP have a role in spinal nerve-injured neuropathic pain model was tested. Male rats were subjected to L5 and L6 spinal nerve transection (SNT), and mechanical hyperalgesia was evaluated by measuring paw withdrawal threshold (PWT). SNT induced a persistent PWT decrease, a sign of neuropathic pain. Lidocaine was soaked on spinal nerves or intrathecally injected 10 min before SNT to block neuronal discharges caused by the injury, and L703,606 (NK1 receptor antagonist) and CGRP8-37 (CGRP receptor antagonist) were intrathecally injected into the rats to block actions of substance P and CGRP released from central nerve terminals in the spinal cord by injury discharges. The treatments with lidocaine, L703,606 and CGRP8-37 delayed the onset of neuropathic pain by 1-4 days, compared with the saline-treated rats. After neuropathic pain was established, intrathecal injections of L703,606 and CGRP8-37 significantly mitigated mechanical hyperalgesia for 20 min. These results suggest that substance P and CGRP are involved in the development and maintenance of neuropathic pain and that these peptides from the central terminals of intact sensory neurons contribute to the maintenance of peripheral nerve injury-induced neuropathic pain.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Differential expression of the capsaicin receptor TRPV1 and related novel receptors TRPV3, TRPV4 and TRPM8 in normal human tissues and changes in traumatic and diabetic neuropathy

BACKGROUND

Transient receptor potential (TRP) receptors expressed by primary sensory neurons mediate thermosensitivity, and may play a role in sensory pathophysiology. We previously reported that human dorsal root ganglion (DRG) sensory neurons co-expressed TRPV1 and TRPV3, and that these were increased in injured human DRG. Related receptors TRPV4, activated by warmth and eicosanoids, and TRPM8, activated by cool and menthol, have been characterised in pre-clinical models. However, the role of TRPs in common clinical sensory neuropathies needs to be established.

METHODS

We have studied TRPV1, TRPV3, TRPV4, and TRPM8 in nerves (n = 14) and skin from patients with nerve injury, avulsed dorsal root ganglia (DRG) (n = 11), injured spinal nerve roots (n = 9), diabetic neuropathy skin (n = 8), non-diabetic neuropathic nerve biopsies (n = 6), their respective control tissues, and human post mortem spinal cord, using immunohistological methods.

RESULTS

TRPV1 and TRPV3 were significantly increased in injured brachial plexus nerves, and TRPV1 in hypersensitive skin after nerve repair, whilst TRPV4 was unchanged. TRPM8 was detected in a few medium diameter DRG neurons, and was unchanged in DRG after avulsion injury, but was reduced in axons and myelin in injured nerves. In diabetic neuropathy skin, TRPV1 expressing sub- and intra-epidermal fibres were decreased, as was expression in surviving fibres. TRPV1 was also decreased in non-diabetic neuropathic nerves. Immunoreactivity for TRPV3 was detected in basal keratinocytes, with a significant decrease of TRPV3 in diabetic skin. TRPV1-immunoreactive nerves were present in injured dorsal spinal roots and dorsal horn of control spinal cord, but not in ventral roots, while TRPV3 and TRPV4 were detected in spinal cord motor neurons.

CONCLUSION

The accumulation of TRPV1 and TRPV3 in peripheral nerves after injury, in spared axons, matches our previously reported changes in avulsed DRG. Reduction of TRPV1 levels in nerve fibres in diabetic neuropathy skin may result from the known decrease of nerve growth factor (NGF) levels. The role of TRPs in keratinocytes is unknown, but a relationship to changes in NGF levels, which is produced by keratinocytes, deserves investigation. TRPV1 represents a more selective therapeutic target than other TRPs for pain and hypersensitivity, particularly in post-traumatic neuropathy.

Modification of classical neurochemical markers in identified primary afferent neurons with Aβ-, Aδ-, and C-fibers after chronic constriction injury in mice

It is functionally important to differentiate between primary afferent neurons with A-fibers, which are nociceptive or nonnociceptive, and C-fibers, which are mainly nociceptive. Neurochemical markers such as neurofilament 200 (NF200), substance P (SP), and isolectin B4 (IB4) have been useful to distinguish between A- and C-fiber neurons. However, the expression patterns of these markers change after peripheral nerve injury, so that it is not clear whether they still distinguish between fiber types in models of neuropathic pain. We identified neurons with Abeta-, Adelta-, and C-fibers by their conduction velocity (corrected for utilization time) in dorsal root ganglia taken from mice after a chronic constriction injury (CCI) of the sciatic nerve and control mice, and later stained them for IB4, SP, calcitonin gene-related peptide (CGRP), NF200, and neuropeptide Y (NPY). NF200 remained a good marker for A-fiber neurons, and IB4 and SP remained good markers for C-fiber neurons after CCI. NPY was absent in controls but was expressed in A-fiber neurons after CCI. After CCI, a group of C-fiber neurons emerged that expressed none of the tested markers. The size distribution of the markers was investigated in larger samples of unidentified dorsal root ganglion neurons and, together with the results from the identified neurons, provided only limited evidence for the expression of SP in Abeta-fiber neurons after CCI. The extent of up-regulation of NPY showed a strong inverse correlation with the degree of heat hyperalgesia.

Differential RIP antigen (CNPase) expression in peripheral ensheathing glia

The RIP monoclonal antibody is commonly used to identify oligodendrocytes. Recently, the RIP antigen was identified as 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a known non-compact myelin protein [Watanabe, M., Sakurai, Y., Ichinose, T., Aikawa, Y., Kotani, M., Itoh, K., 2006. Monoclonal antibody Rip specifically recognizes 2',3'-cyclic nucleotide 3'-phosphodiesterase in oligodendrocytes. J. Neurosci. Res. 84, 525-533]. In the present study we characterize normal and axotomy-induced changes in RIP immunoreactivity in peripheral glia. In myelinating Schwann cells, RIP demarcated paranodal regions of myelinated axons and clearly defined Schmidt-Lantermann incisures. Surprisingly, RIP immunoreactivity was not confined to myelinating glia. Robust RIP immunoreactivity was present in Remak bundles in mixed nerves and in sympathetic ganglia and grey rami. Following peripheral nerve injury, RIP immunoreactivity was redistributed diffusely throughout de-differentiating Schwann cell cytoplasm. In uninjured rats, low levels of RIP immunoreactivity were detectable in satellite cells surrounding dorsal root ganglion (DRG) neurons and in terminal Schwann cells at neuromuscular junctions. This pattern suggested a correlation between RIP immunoreactivity and the amount of axon-glial contact. We therefore injured the L5 spinal nerve to induce sympathetic sprouting and pericellular basket formation in the DRG, and asked whether relatively RIP-negative satellite glia, which normally contact only neuronal somata, would upregulate the RIP antigen upon contact with sprouting sympathetic axons. All perineuronal sympathetic sprouts infiltrated heavily RIP-immunoreactive satellite cell sheaths. RIP immunoreactivity was absent from placode-derived olfactory ensheathing glia, indicating that the relationship between axon-glial contact and RIP-immunoreactivity is restricted to peripheral ensheathing glia of the neural crest-derived Schwann cell lineage.

Nerve injury-induced tactile allodynia is present in the absence of FOS labeling in retrogradely labeled post-synaptic dorsal column neurons

The dorsal column pathway consists of direct projections from primary afferents and of ascending fibers of the post-synaptic dorsal column (PSDC) cells. This pathway mediates touch but may also mediate allodynia after nerve injury. The role of PSDC neurons in nerve injury-induced mechanical allodynia is unknown. Repetitive gentle, tactile stimulus or noxious pinch was applied to the ipsilateral hindpaw of rats with spinal nerve ligation (SNL) or sham surgery that had previously received tetramethylrhodamine dextran in the ipsilateral n. gracilis. Both touch and noxious stimuli produced marked increases in FOS expression in other cells throughout all laminae of the ipsilateral dorsal horn after nerve injury. However, virtually none of the identified PSDC cells expressed FOS immunofluorescence in response to repetitive touch or pinch in either the nerve-injured or sham groups. In contrast, labeled PSDC cells expressed FOS in response to ureter ligation and labeled spinothalamic tract (STT) cells expressed FOS in response to noxious pinch. Identified PSDC neurons from either sham-operated or SNL rats did not express immunoreactivity to substance P, CGRP, NPY, PKCY, MOR, the NK1 and the NPY-Y1 receptor. Retrogradely labeled DRG cells of nerve injured rats were large diameter neurons, which expressed NPY, but no detectable CGRP or substance P. Spinal nerve injury sensitizes neurons in the spinal dorsal horn to repetitive light touch but PSDC neurons apparently do not participate in touch-evoked allodynia. Sensitization of these non-PSDC neurons may result in activation of projections integral to the spinal/supraspinal processing of enhanced pain states and of descending facilitation, thus priming the central nervous system to interpret tactile stimuli as being aversive.

Activity-dependent slowing of conduction velocity in uninjured L4 C fibers increases after an L5 spinal nerve injury in the rat

Growing evidence suggests that uninjured afferents may play an important role in neuropathic pain following nerve injury. The excitability of nociceptive neurons in the L4 spinal nerve appears to be enhanced following an injury to the adjacent L5 spinal nerve. In this study, we investigated whether the action-potential conduction properties of unlesioned, unmyelinated fibers are also altered. A teased-fiber technique was used to record from single C fibers from the L4 spinal nerve of the rat in vitro. Repeated electrical stimulation of the tibial nerve was used to investigate activity-dependent slowing of conduction velocity. Twin pulse stimulation at a 50 ms interpulse interval allowed investigation of supranormal conduction velocity. Blinded experiments were performed 8-10 days after sham surgery and after an L5 spinal nerve ligation (L5 SNL). Activity-dependent slowing revealed two populations of C fibers, a "nociceptor" population with a large degree of activity-dependent slowing and a "non-nociceptor" population with a smaller degree of activity-dependent slowing. Both populations showed enhanced activity-dependent slowing of conduction velocity and enhanced supranormal conduction velocities in lesioned animals compared to sham animals. Activity-dependent slowing was also enhanced after an L5 SNL in the mouse. These alterations in conduction velocity may reflect changes in expression of ion channels responsible for the membrane excitability. These data provide additional evidence that a nerve injury leads to persistent alterations in the properties of adjacent uninjured, unmyelinated fibers.

Upregulation of brain-derived neurotrophic factor in the sensory pathway by selective motor nerve injury in adult rats

Selective motor nerve injury by lumbar 5 ventral root transection (L5 VRT) induces neuropathic pain, but the underlying mechanisms remain unknown. Previously, increased expression and secretion of brain-derived neurotrophic factor (BDNF) had been implicated in injury-induced neuropathic pain in the sensory system. In this study, as a step to examine potential roles of BDNF in L5 VRT-induced neuropathic pain, we investigated BDNF gene and protein expression in adult rats with L5 VRT. L5 VRT induced a dramatic upregulation of BDNF mRNA in intact sensory neurons in the ipsilateral L5 dorsal root ganglia (DRG), in non-neuronal cells in the ipsilateral sciatic nerve, and in motoneurons in the ipsilateral spinal cord. L5 VRT also induced de novo synthesis of BDNF mRNA in spinal dorsal horn neurons and in glial cells in the white matter of the ipsilateral spinal cord. Consistent with the mRNA expression pattern, BDNF protein was also mainly upregulated in all populations of sensory neurons in the ipsilateral L5 DRG and in spinal neurons and glia. Quantitative analysis by ELISA showed that the BDNF content in the DRG and sciatic nerve peaked on day 1 and remained elevated 14 days after L5 VRT. These results suggest that increased BDNF expression in intact primary sensory neurons and spinal cord may be an important factor in the induction of neuropathic pain without axotomy of sensory neurons.

Time course of substance P expression in dorsal root ganglia following complete spinal nerve transection

Recent evidence suggests that substance P (SP) is up-regulated in primary sensory neurons following axotomy and that this change occurs in larger neurons that do not usually produce SP. If this is so, then the up-regulation may allow normally neighboring, uninjured, and nonnociceptive dorsal root ganglion (DRG) neurons to become effective in activating pain pathways. By using immunohistochemistry, we performed a unilateral L5 spinal nerve transection on male Wistar rats and measured SP expression in ipsilateral L4 and L5 DRGs and contralateral L5 DRGs at 1-14 days postoperatively (dpo) and in control and sham-operated rats. In normal and sham-operated DRGs, SP was detectable almost exclusively in small neurons (< or =800 microm2). After surgery, the mean size of SP-positive neurons from the axotomized L5 ganglia was greater at 2, 4, 7, and 14 dpo. Among large neurons (>800 microm2) from the axotomized L5, the percentage of SP-positive neurons increased at 2, 4, 7, and 14 dpo. Among small neurons from the axotomized L5, the percentage of SP-positive neurons was increased at 1 and 3 dpo but was decreased at 7 and 14 dpo. Thus, SP expression is affected by axonal damage, and the time course of the expression is different between large and small DRG neurons. These data support a role for SP-producing, large DRG neurons in persistent sensory changes resulting from nerve injury.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

Antisense knock down of TRPA1, but not TRPM8, alleviates cold hyperalgesia after spinal nerve ligation in rats

Patients with neuropathic pain frequently experience hypersensitivity to cold stimulation. However, the underlying mechanisms of this enhanced sensitivity to cold are not well understood. After partial nerve injury, the transient receptor potential ion channel TRPV1 increases in the intact small dorsal root ganglion (DRG) neurons in several neuropathic pain models. In the present study, we precisely examined the incidence of cold hyperalgesia and the changes of TRPA1 and TRPM8 expression in the L4 and L5 DRG following L5 spinal nerve ligation (SNL), because it is likely that the activation of two distinct populations of TRPA1- and TRPM8-expressing small neurons underlie the sensation of cold. We first confirmed that L5 SNL rats developed cold hyperalgesia for more than 14 days after surgery. In the nearby uninjured L4 DRG, TRPA1 mRNA expression increased in trkA-expressing small-to-medium diameter neurons from the 1st to 14th day after the L5 SNL. This upregulation corresponded well with the development and maintenance of nerve injury-induced cold hyperalgesia of the hind paw. In contrast, there was no change in the expression of the TRPM8 mRNA/protein in the L4 DRG throughout the 2-week time course of the experiment. In the injured L5 DRG, on the other hand, both TRPA1 and TRPM8 expression decreased over 2 weeks after ligation. Furthermore, intrathecal administration of TRPA1, but not TRPM8, antisense oligodeoxynucleotide suppressed the L5 SNL-induced cold hyperalgesia. Our data suggest that increased TRPA1 in uninjured primary afferent neurons may contribute to the exaggerated response to cold observed in the neuropathic pain model.

Spinal neuropeptide responses in persistent and transient pain following cervical nerve root injury

STUDY DESIGN

Behavioral and immunohistochemical analysis in rat models of persistent and transient allodynia.

OBJECTIVES

To examine separate cervical nerve root injuries (compression, transection) for producing behavioral hypersensitivity and investigate spinal neuropeptides to understand relationships to pain symptoms.

SUMMARY OF BACKGROUND DATA

Mechanical cervical nerve root injury can be a source of neck pain. Painful lumbar radiculopathy models show that different nerve root ligation intensities produce differential allodynia responses. Spinal neuropeptides can mediate pain responses. Yet, little is known about their contributions to pain in the cervical spine.

METHODS

Rats underwent separate procedures on the right C7 nerve roots: transection (n = 12), 10-gf compression for 15 minutes (n = 11), or sham (n = 5). Ipsilateral forepaw mechanical allodynia was measured after surgery for 7 days. C7 spinal cord tissue was analyzed by immunohistochemistry for substance P and calcitonin gene-related peptide (CGRP) expression on days 1 and 7 for each injury; densitometry quantified immunoreactivity in lamina I of the ipsilateral dorsal horn.

RESULTS

Both injuries immediately produced significant increases in allodynia. Sensitivity was sustained following root compression, and at day 7, was not different from day 1. By day 7 after transection, allodynia had returned to baseline and sham levels, significantly decreasing from day 1 (P = 0.0012). Spinal substance P and CGRP were increased over normal at day 1 for both injuries and decreased with time for CGRP after transection, which paralleled behaviors. For individual rats, substance P was significantly (P < 0.001) correlated with CGRP expression for both injuries.

CONCLUSIONS

Compression and transection of the cervical nerve root produce different forepaw allodynia responses, with persistent and transient sensitivity, respectively. Spinal neuropeptide expression in these models parallels this sensitivity, suggesting their potential role in pain symptoms.

A proximal E-box modulates NGF effects on rat PPT-A promoter activity in cultured dorsal root ganglia neurones

The rat preprotachykinin A (rtPPTA) promoter fragment spanning -865+92, relative to the major transcriptional start, has previously been demonstrated to be nerve growth factor (NGF) responsive in primary cultures of rat dorsal root ganglion (DRG) neurones [Harrison, P.T., Dalziel, R.G., Ditchfield, N.A., Quinn, J.P., 1999. Neuronal-specific and nerve growth factor-inducible expression directed by the preprotachykinin-A promoter delivered by an adeno-associated virus vector. Neuroscience 94, 997-1003]. In this communication, we demonstrate that an E box element at -60, in part, regulates the activity of this rtPPT-A promoter fragment in DRG neurones in response to NGF. Differential regulation of the promoter is observed in the presence or absence of NGF when the E Box site is present. Under basal conditions binding of proteins to this -60 element may antagonise promoter activity. Hence, in the absence of NGF, mutation of the -60 E box increased reporter gene expression. Further, comparison of levels of reporter gene expression supported by both WT and mutated promoter indicate that in the presence of NGF the -60 E box element also plays a role as an activator domain. This represents a novel mechanism for NGF regulation of rtPPT-A. Similarly, an important role for this signalling pathway was observed in neonate rat DRG neuronal cultures, which require NGF for their survival, namely mutation of the -60 element resulted in higher levels of reporter gene expression.

Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation

To investigate whether activation of mitogen-activated protein kinase (MAPK) in damaged and/or undamaged primary afferents participates in neuropathic pain after partial nerve injury, we examined the phosphorylation of extracellular signal-regulated protein kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK) in the L4 and L5 dorsal root ganglion (DRG) in the L5 spinal nerve ligation (SNL) model. We first confirmed, using activating transcription factor 3 and neuropeptide Y immunoreactivity, that virtually all L4 DRG neurons are spared from axotomy in this model. In the injured L5 DRG, the L5 SNL induced the activation of ERK, p38, and JNK in different populations of DRG neurons. In contrast, in the uninjured L4 DRG, the L5 SNL induced only p38 activation in tyrosine kinase A-expressing small- to medium-diameter neurons. Intrathecal ERK, p38, and JNK inhibitor infusions reversed SNL-induced mechanical allodynia, whereas only p38 inhibitor application attenuated SNL-induced thermal hyperalgesia. Furthermore, the L5 dorsal rhizotomy did not prevent SNL-induced thermal hyperalgesia. We therefore hypothesized that p38 activation in the uninjured L4 DRG might be involved in the development of heat hypersensitivity in the L5 SNL model. In fact, the treatment of the p38 inhibitor and also anti-nerve growth factor reduced SNL-induced upregulation of brain-derived neurotrophic factor and transient receptor potential vanilloid type 1 expression in the L4 DRG. Together, our results demonstrate that the L5 SNL induces differential activation of MAPK in injured and uninjured DRG neurons and, furthermore, that MAPK activation in the primary afferents may participate in generating pain hypersensitivity after partial nerve injury.

A??-afferents activate neurokinin-1 receptor in dorsal horn neurons after nerve injury

We provide new evidence demonstrating that peripheral nerve injury produces profound alterations in synaptic input to dorsal horn neurons mediated by non-nociceptive sensory neurons, and activation of neurokinin-1 receptor may be involved in the enhanced synaptic response and thus contribute to the tactile allodynia. Our results show that Abeta-fiber-evoked field potential significantly increased in the first postoperative week and decreased thereafter while maximal mechanical allodynia was exhibited. The neurokinin-1 receptor antagonist L703,606 significantly reduced Abeta-fiber-evoked field potential in nerve-injured but not in sham-operated animals. The non-N-methyl-D-aspartate receptor antagonist CNQX inhibited Abeta-fiber-evoked field potential in both nerve-injured and sham-operated rats, while the N-methyl-D-aspartate receptor antagonist MK-801 did not affect Abeta-fiber-evoked field potential in either CCI or sham-operated animals.

Tumor-induced injury of primary afferent sensory nerve fibers in bone cancer pain

Bone is the most common site of chronic pain in patients with metastatic cancer. What remains unclear are the mechanisms that generate this pain and why bone cancer pain can be so severe and refractory to treatment with opioids. Here we show that following injection and confinement of NCTC 2472 osteolytic tumor cells within the mouse femur, tumor cells sensitize and injure the unmyelinated and myelinated sensory fibers that innervate the marrow and mineralized bone. This tumor-induced injury of sensory nerve fibers is accompanied by an increase in ongoing and movement-evoked pain behaviors, an upregulation of activating transcription factor 3 (ATF3) and galanin by sensory neurons that innervate the tumor-bearing femur, upregulation of glial fibrillary acidic protein (GFAP) and hypertrophy of satellite cells surrounding sensory neuron cell bodies within the ipsilateral dorsal root ganglia (DRG), and macrophage infiltration of the DRG ipsilateral to the tumor-bearing femur. Similar neurochemical changes have been described following peripheral nerve injury and in other non-cancerous neuropathic pain states. Chronic treatment with gabapentin did not influence tumor growth, tumor-induced bone destruction or the tumor-induced neurochemical reorganization that occurs in sensory neurons or the spinal cord, but it did attenuate both ongoing and movement-evoked bone cancer-related pain behaviors. These results suggest that even when the tumor is confined within the bone, a component of bone cancer pain is due to tumor-induced injury to primary afferent nerve fibers that innervate the tumor-bearing bone. Tumor-derived, inflammatory, and neuropathic mechanisms may therefore be simultaneously driving this chronic pain state.

Differential activation of MAPK in injured and uninjured DRG neurons following chronic constriction injury of the sciatic nerve in rats

To investigate the intracellular signal transduction pathways involved in the pathophysiological mechanisms of neuropathic pain after partial nerve injury, we examined the activation of extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) in the chronic constriction injury (CCI) model. The CCI induced an increase in the phosphorylation of ERK in predominantly injured medium-sized and large-sized DRG neurons and in satellite glial cells. Treatment with the MAPK kinase 1/2 inhibitor, U0126, suppressed CCI-induced mechanical allodynia and partially reversed the increase in neuropeptide Y (NPY) expression in damaged DRG neurons. In contrast, the CCI induced the activation of p38, mainly in uninjured small-to-medium-diameter DRG neurons and in satellite glial cells. The p38 inhibitor, SB203580, reversed the CCI-induced heat hyperalgesia and also the increase in brain-derived neurotrophic factor (BDNF) expression in intact DRG neurons. On the other hand, the nerve growth factor (NGF)-induced increase in BDNF expression in small-to-medium-diameter neurons was reversed by SB203580, whereas the anti-NGF-induced increase in NPY in medium-sized and large-sized neurons was partially blocked by U0126. Taken together, our results demonstrate that the activation of ERK and p38 and also the changes in NPY and BDNF expression may occur in different populations of DRG neurons after CCI, partially through alterations in the target-derived NGF. These changes in injured and intact primary afferents are likely to have a substantial role in pathological states, and MAPK pathways in nociceptors may be potential targets for the development of novel analgesics.

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Partial sciatic nerve transection causes redistribution of pain-related peptides and lowers withdrawal threshold

Complete nerve transection results in loss of sensation and paralysis of the involved extremity. Such injury drastically reduces content of the nociceptive peptides, substance P, and somatostatin in the dorsal horn of the spinal cord and dorsal root ganglia innervating the limb. Partial nerve injuries occur more commonly in clinical practice, however, and frequently result in the development of chronic neuropathic pain. To investigate mechanisms underlying this pathologic pain syndrome, rats were subjected to partial sciatic nerve transection. Withdrawal thresholds determined with Von Frey hairs dropped dramatically in the operated limb. On postoperative Day 4, thresholds had decreased from 15 g to less than 5 g on the operated side, whereas those on the contralateral (unoperated) side or those from sham-operated rats did not change. Sciatic hemisection had no effect on total content of either substance P or somatostatin in the dorsal spinal cord and lumbar dorsal root ganglia as measured by radioimmunoassay on postoperative Days 4, 7, or 14. However, when examined immunohistochemically, there was a marked redistribution of both peptides associated with partial transection. On the contralateral side or in sham-operated rats, both substance P and somatostatin were confined to the superficial laminae of the dorsal horn. By contrast, on the operated side, content of both peptides was reduced by more than half in the superficial laminae. There was a compensatory increase in content in the deeper laminae where nociceptive peptides are not usually found. Redistribution of substance P and somatostatin may be due to axonal sprouting, increased peptide expression by interneurons, or aberrant expression of nociceptive peptides by neurons normally mediating mechanical sensation. The presence of increased levels of nociceptive peptides in regions of the spinal cord that mediate innocuous sensation may underlie development of allodynia.

Single-cell laser-capture microdissection and RNA amplification

Generating gene-expression profiles from laser-captured cells requires the successful combination of laser-capture microdissection, RNA extraction, RNA amplification, and microarray analysis. To permit single-cell gene-expression profiling, the RNA amplification method has to be sufficiently powerful to bridge the gap between the amount of RNA available from a single cell to what is required by the microarray, a gap that spans 5 to 6 orders of magnitude. This chapter focuses on the amplification of RNA using a two-round T7 RNA amplification method. The protocols described are adapted for laser-captured material and have been used to generate gene expression profiles from single laser-captured cells.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Intraplantar injection of a cyclooxygenase inhibitor ketorolac reduces immunoreactivities of substance P, calcitonin gene-related peptide, and dynorphin in the dorsal horn of rats with nerve injury or inflammation

We previously reported that partial sciatic nerve ligation (PSNL) dramatically up-regulates cyclooxygenase 2 (COX2) in injured sciatic nerve, and local injection of the COX inhibitor, ketorolac, reverses tactile allodynia and suppresses increased phosphorylation of the transcription factor cAMP responsive element binding protein [Eur J Neurosci 15 (2002) 1037]. These findings suggest that peripheral prostaglandins (PGs) are over-produced and contribute to the central plasticity and the maintenance of neuropathic pain after nerve injury. PGs, particularly PGE2, are well known to facilitate the release of the pro-nociceptive neuropeptide substance P (SP) and calcitonin gene-related peptide (CGRP) from primary sensory afferents. Thus, suppressing peripheral PG over-production may inhibit the release of these two neuropeptides from primary afferents and thereby increase the content of these neuropeptides remaining in afferent terminals in the dorsal horn. In this study we tested this hypothesis by examining the immunoreactivities of SP and CGRP in the dorsal horn of PSNL rats intraplantarly injected with saline and ketorolac. Four weeks after PSNL, SP- and CGRP-immunoreactivities (IR) in the ipsilateral dorsal horn were not significantly different from the contralateral side. Five days following intraplantar injection of ketorolac, CGRP- and SP-IR in the ipsilateral and contralateral dorsal horn were dramatically reduced compared with saline-injected PSNL rats. Local ketorolac also suppressed PSNL-induced increase in dynorphin-IR in dorsal horn neurons. Since abundant production of PGs during inflammation is well documented, we further examined the effect of intraplantar ketorolac on neuropeptide expression in the dorsal horn following carrageenan inflammation. We observed that co-administration of ketorolac with carrageenan in the hindpaw also reduced SP- and dynorphin-IR in the ipsilateral and contralateral dorsal horn. These findings are in contrast to our hypothesis, suggesting that peripherally over-produced PGs following nerve injury and inflammation possibly contribute to the production of SP and CGRP in primary sensory neurons, to the up-regulation of dynorphin in the dorsal horn neurons, and finally to the mechanisms of neuropathic and inflammation pain.

Effects of spinal nerve ligation on immunohistochemically identified neurons in the L4 and L5 dorsal root ganglia of the rat

This study examined the effect of spinal nerve ligation on different populations of immunohistochemically identified neurons in the dorsal root ganglia (DRG) of the rat. The optical fractionator method was used to count neurons in the ipsilateral L4 and L5 DRG 1-20 weeks after ligation of the L5 and L6 spinal nerves, sham surgery, or no surgery. One week after ligation, neurons in the L5 DRG that were labeled by IB4, a marker of unmyelinated primary afferent neurons, were largely absent. The numbers of IB4-labeled neurons then progressively increased to reach control values by 20 weeks. A smaller, sustained decrease occurred in the number of small-, medium- and large-sized neurons immunoreactive for calcitonin gene-related peptide (CGRP), a marker for peptidergic primary afferents, in the L5 DRG. There was a proportionately greater decrease in the numbers of medium- to large-sized CGRP-like immunoreactive neurons. The number of myelinated afferents in the L5 DRG, identified by their staining for neurofilament protein (N52), did not change after ligation. However, closer examination revealed a significant decrease in the numbers of large-sized neurons, coupled with an increase in the numbers of small- to medium-sized neurons, and the appearance of a novel population of very small-sized neurons labeled by N52. The numbers and cell size distributions of IB4-labeled, CGRP-like immunoreactive, and N52-labeled neurons were unchanged in the adjacent L4 DRG. Unlike the L5 DRG, injury-induced changes in the expression of various receptors, neurotransmitters and neurotrophic factors in the L4 DRG are not confounded by a change in the immunohistochemical phenotype of primary afferent neurons.