Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: aberrant innervation of lamina II neurons by Abeta afferents

The effect of progesteron for expression delta (δ) opioid receptor spinal cord through peripheral nerve injury

Background

Neuropathic pain is a major problem to date because of its high prevalence and lack of effective treatment. Neuropathic pain processes can be influenced by many factors and at various levels of the nervous system, including progesterone and the opioid system. The various mechanisms of the effect of progesterone on pain are still controversial, while the effect of progesterone on the activation of the opioid system also needs to be proven. This study aimed to determine the effect of progesterone on pain through the modulation mechanism of the opioid system.

Methods

This research is a completely randomized experimental study using male wistar rats aged around three months at the Experimental Animal Laboratory, Department of Medical Biochemistry, Faculty of Medicine, Airlangga University.

Results

The result was analyzed by using statistical analysis of two independent samples (t-test). The t value was obtained at 6.880, p = 0.000 (p < 0.05).

Conclusion

It was shown that there was a significant difference in the delta (δ) opioid receptor expression between the control group and the progesterone group, which indicated that progesterone causes an increase in the delta (δ) opioid receptor expression in the spinal cord.

•

Experimental study on mice subjects to determine the difference delta (δ) opioid receptors in the spinal cord.

•

Determine the effect of progesterone on pain through the modulation mechanism of the opioid system.

•

The administration of progesterone has a positive effect on the expression of delta (δ) opioid receptors.

Microglia: sculptors of neuropathic pain?

Neuropathic pain presents a huge societal and individual burden. The limited efficacy of current analgesics, diagnostic markers and clinical trial outcome measures arises from an incomplete understanding of the underlying mechanisms. A large and growing body of evidence has established the important role of microglia in the onset and possible maintenance of neuropathic pain, and these cells may represent an important target for future therapy. Microglial research has further revealed their important role in structural remodelling of the nervous system. In this review, we aim to explore the evidence for microglia in sculpting nervous system structure and function, as well as their important role in neuropathic pain, and finally integrate these studies to synthesize a new model for microglia in somatosensory circuit remodelling, composed of six key and inter-related mechanisms. Summarizing the mechanisms through which microglia modulate nervous system structure and function helps to frame a better understanding of neuropathic pain, and provide a clear roadmap for future research.

Role of spinal glial cells in excitability of wide dynamic range neurons and the development of neuropathic pain with the L5 spinal nerve transection in the rats: Behavioral and electrophysiological study

The activation of glial cells affects the neuronal excitability in the spinal cord. Therefore, in this study, we tried to find out the modulatory role of spinal glial cells in the excitability of wide dynamic range (WDR) neurons, induction of the long-term potentiation (LTP) and development of neuropathic pain by L5 spinal nerve transection model in the rats. Forty-eight adult male Wistar rats were used to measure the paw withdrawal threshold to mechanical stimuli and also, to carry out the spinal extracellular single unit recording experiments. In these experiments, spinal nerve ligation (SNL) and a daily injection of propentofylline (1 mg/kg, ip) as a glial cell inhibitor agent, 1 h following nerve ligation during 7-day post-SNL period, were performed. Our findings showed that the mechanical allodynia, and synaptically-evoked firing were caused LTP in the Aδ-fiber, C-fiber and lesser in the Aβ-fiber after high frequency stimulation. Daily injection of propentofylline considerably decreased LTP induction in the Aδ- and C-fibers (P < .001). It was concluded that glial cell activation mediates LTP induction in the spinal cord following peripheral nerve injury. It seems that pain modulatory role of glial cells is partly parallel to changes in neural excitability of the WDR neurons in the dorsal horn of spinal cord.

Looking beyond the intervertebral disc: the need for behavioral assays in models of discogenic pain

Orthopedic research into chronic discogenic back pain has commonly focused on aging- and degeneration-related changes in intervertebral disc structure, biomechanics, and biology. However, the primary spine-related reason for physician office visits is pain. The ambiguous nature of the human condition of discogenic low back pain motivates the use of animal models to better understand the pathophysiology. Discogenic back pain models must consider both emergent behavioral changes following pain induction and changes in the nervous system that mediate such behavior. Looking beyond the intervertebral disc, we describe the different ways to classify pain in human patients and animal models. We describe several behavioral assays that can be used in rodent models to augment disc degeneration measurements and characterize different types of pain. We review rodent models of discogenic pain that employed behavioral pain assays and highlight a need to better integrate neuroscience and orthopedic science methods to extend current understanding of the complex and multifactorial pathophysiology of discogenic back pain.

Perineural capsaicin induces the uptake and transganglionic transport of choleratoxin B subunit by nociceptive C-fiber primary afferent neurons

The distribution of spinal primary afferent terminals labeled transganglionically with the choleratoxin B subunit (CTB) or its conjugates changes profoundly after perineural treatment with capsaicin. Injection of CTB conjugated with horseradish peroxidase (HRP) into an intact nerve labels somatotopically related areas in the ipsilateral dorsal horn with the exceptions of the marginal zone and the substantia gelatinosa, whereas injection of this tracer into a capsaicin-pretreated nerve also results in massive labeling of these most superficial layers of the dorsal horn. The present study was initiated to clarify the role of C-fiber primary afferent neurons in this phenomenon. In L5 dorsal root ganglia, analysis of the size frequency distribution of neurons labeled after injection of CTB-HRP into the ipsilateral sciatic nerve treated previously with capsaicin or resiniferatoxin revealed a significant increase in the proportion of small neurons. In the spinal dorsal horn, capsaicin or resiniferatoxin pretreatment resulted in intense CTB-HRP labeling of the marginal zone and the substantia gelatinosa. Electron microscopic histochemistry disclosed a dramatic, ∼10-fold increase in the proportion of CTB-HRP-labeled unmyelinated dorsal root axons following perineural capsaicin or resiniferatoxin. The present results indicate that CTB-HRP labeling of C-fiber dorsal root ganglion neurons and their central terminals after perineural treatment with vanilloid compounds may be explained by their phenotypic switch rather than a sprouting response of thick myelinated spinal afferents which, in an intact nerve, can be labeled selectively with CTB-HRP. The findings also suggest a role for GM1 ganglioside in the modulation of nociceptor function and pain.

Lack of evidence for ectopic sprouting of genetically labeled Aβ touch afferents in inflammatory and neuropathic trigeminal pain

Background

Mechanical and in particular tactile allodynia is a hallmark of chronic pain in which innocuous touch becomes painful. Previous cholera toxin B (CTB)-based neural tracing experiments and electrophysiology studies had suggested that aberrant axon sprouting from touch sensory afferents into pain-processing laminae after injury is a possible anatomical substrate underlying mechanical allodynia. This hypothesis was later challenged by experiments using intra-axonal labeling of A-fiber neurons, as well as single-neuron labeling of electrophysiologically identified sensory neurons. However, no studies have used genetically labeled neurons to examine this issue, and most studies were performed on spinal but not trigeminal sensory neurons which are the relevant neurons for orofacial pain, where allodynia oftentimes plays a dominant clinical role.

Findings

We recently discovered that parvalbumin::Cre (Pv::Cre) labels two types of Aβ touch neurons in trigeminal ganglion. Using a Pv::CreER driver and a Cre-dependent reporter mouse, we specifically labeled these Aβ trigeminal touch afferents by timed taxomifen injection prior to inflammation or infraorbital nerve injury (ION transection). We then examined the peripheral and central projections of labeled axons into the brainstem caudalis nucleus after injuries vs controls. We found no evidence for ectopic sprouting of Pv::CreER labeled trigeminal Aβ axons into the superficial trigeminal noci-receptive laminae. Furthermore, there was also no evidence for peripheral sprouting.

Conclusions

CreER-based labeling prior to injury precluded the issue of phenotypic changes of neurons after injury. Our results suggest that touch allodynia in chronic orofacial pain is unlikely caused by ectopic sprouting of Aβ trigeminal afferents.

Nerve injury induces a new profile of tactile and mechanical nociceptor input from undamaged peripheral afferents

Chronic pain after nerve injury is often accompanied by hypersensitivity to mechanical stimuli, yet whether this reflects altered input, altered processing, or both remains unclear. Spinal nerve ligation or transection results in hypersensitivity to mechanical stimuli in skin innervated by adjacent dorsal root ganglia, but no previous study has quantified the changes in receptive field properties of these neurons in vivo. To address this, we recorded intracellularly from L4 dorsal root ganglion neurons of anesthetized young adult rats, 1 wk after L5 partial spinal nerve ligation (pSNL) or sham surgery. One week after pSNL, hindpaw mechanical withdrawal threshold in awake, freely behaving animals was decreased in the L4 distribution on the nerve-injured side compared with sham controls. Electrophysiology revealed that high-threshold mechanoreceptive cells of A-fiber conduction velocity in L4 were sensitized, with a seven-fold reduction in mechanical threshold, a seven-fold increase in receptive field area, and doubling of maximum instantaneous frequency in response to peripheral stimuli, accompanied by reductions in after-hyperpolarization amplitude and duration. Only a reduction in mechanical threshold (minimum von Frey hair producing neuronal activity) was observed in C-fiber conduction velocity high-threshold mechanoreceptive cells. In contrast, low-threshold mechanoreceptive cells were desensitized, with a 13-fold increase in mechanical threshold, a 60% reduction in receptive field area, and a 40% reduction in instantaneous frequency to stimulation. No spontaneous activity was observed in L4 ganglia, and the likelihood of recording from neurons without a mechanical receptive field was increased after pSNL. These data suggest massively altered input from undamaged sensory afferents innervating areas of hypersensitivity after nerve injury, with reduced tactile and increased nociceptive afferent response. These findings differ importantly from previous preclinical studies, but are consistent with clinical findings in most patients with chronic neuropathic pain.

Conditioned pain modulation: a predictor for development and treatment of neuropathic pain

Psychophysical evaluation of endogenous pain inhibition via conditioned pain modulation (CPM) represents a new generation of laboratory tests for pain assessment. In this review we discuss recent findings on CPM in neuropathic pain and refer to psychophysical, neurophysiological, and methodological aspects of its clinical implications. Typically, chronic neuropathic pain patients express less efficient CPM, to the extent that incidence of acquiring neuropathic pain (e.g. post-surgery) and its intensity can be predicted by a pre-surgery CPM assessment. Moreover, pre-treatment CPM evaluation may assist in the correct choice of serotonin-noradrenalin reuptake inhibitor analgesic agents for individual patients. Evaluation of pain modulation capabilities can serve as a step forward in individualizing pain medicine.

Functional up-regulation of Nav1.8 sodium channel in Aβ afferent fibers subjected to chronic peripheral inflammation

Background

Functional alterations in the properties of Aβ afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Na_v1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na_v1.8 in controlling Aβ-fiber excitability following persistent inflammation.

Methods

Distribution and expression of Na_v1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund’s adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total I_Na and TTX-R Na_v1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na_v1.8-preferring blocker on the electrophysiological properties of Na_v1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.

Results

Our findings revealed that Na_v1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total I_Na and Na_v1.8 peak current densities are enhanced in inflamed large myelinated Aβ-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aβ-fiber neuron excitability by shifting the voltage-dependent activation of Na_v1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na_v1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na_v1.8 regulation in Aβ-fibers contributes to inflammatory pain.

Conclusions

Collectively, these findings support a key role for Na_v1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

Cellular, molecular, and epigenetic mechanisms in non-associative conditioning: implications for pain and memory

Sensitization is a form of non-associative conditioning in which amplification of behavioral responses can occur following presentation of an aversive or noxious stimulus. Understanding the cellular and molecular underpinnings of sensitization has been an overarching theme spanning the field of learning and memory as well as that of pain research. In this review we examine how sensitization, both in the context of learning as well as pain processing, shares evolutionarily conserved behavioral, cellular/synaptic, and epigenetic mechanisms across phyla. First, we characterize the behavioral phenomenon of sensitization both in invertebrates and vertebrates. Particular emphasis is placed on long-term sensitization (LTS) of withdrawal reflexes in Aplysia following aversive stimulation or injury, although additional invertebrate models are also covered. In the context of vertebrates, sensitization of mammalian hyperarousal in a model of post-traumatic stress disorder (PTSD), as well as mammalian models of inflammatory and neuropathic pain is characterized. Second, we investigate the cellular and synaptic mechanisms underlying these behaviors. We focus our discussion on serotonin-mediated long-term facilitation (LTF) and axotomy-mediated long-term hyperexcitability (LTH) in reduced Aplysia systems, as well as mammalian spinal plasticity mechanisms of central sensitization. Third, we explore recent evidence implicating epigenetic mechanisms in learning- and pain-related sensitization. This review illustrates the fundamental and functional overlay of the learning and memory field with the pain field which argues for homologous persistent plasticity mechanisms in response to sensitizing stimuli or injury across phyla.

Properties of spinal lamina III GABAergic neurons in naïve and in neuropathic mice

BACKGROUND

Nerve injury leads to Aβ-fibre-mediated mechanical allodynia that is in part due to an impaired GABAergic inhibition in the spinal cord dorsal horn. The properties and function of GABAergic neurons in spinal cord lamina III, an area where low-threshold mechanosensitive Aβ-fibres terminate are, however, largely unknown.

METHODS

We used transgenic mice, which express enhanced green fluorescent protein (EGFP) under control of the promoter GAD67. The morphology and neurochemical characteristics of GABAergic, EGFP-expressing neurons were characterized. We assessed active and passive membrane properties of spinal lamina III GABAergic neurons in naïve animals and animals with a chronic constriction injury (CCI) of the sciatic nerve.

RESULTS

EGFP-expressing neurons in lamina III were predominantly islet cells (47%), whereas non-EGFP-expressing neurons were largely inverted stalked cells (40%). EGFP-expressing neurons accounted for about 25% of GABAergic neurons in lamina III. Forty-four percent co-expressed glycine, 10% neuronal nitric oxide synthase and 3% co-expressed parvalbumin. We found costaining with protein kinase CβII in 42% of EGFP-expressing neurons but no expression of protein kinase Cγ. Membrane properties and excitability of EGFP-and non-EGFP-expressing neurons from naïve and neuropathic animals were indistinguishable. The most frequent firing pattern was tonic firing (naïve: 35%, neuropathic: 37%) followed by gap firing (naïve: 33%, neuropathic: 25%). Delayed, initial burst and single-spike firing patterns made up the remainder in both groups.

CONCLUSION

A change in membrane excitability or discharge pattern of this group of lamina III GABAergic neurons is unlikely the cause for mechanical allodynia in animals with CCI.

Genotype-selective phenotypic switch in primary afferent neurons contributes to neuropathic pain

Pain is normally mediated by nociceptive Aδ and C fibers, while Aβ fibers signal touch. However, after nerve injury, Aβ fibers may signal pain. Using a genetic model, we tested the hypothesis that phenotypic switching in neurotransmitters expressed by Aβ afferents might account for heritable differences in neuropathic pain behavior. The study examined selection-line rats in which one line, high autotomy (HA), shows higher levels of spontaneous pain in the neuroma neuropathy model, and of tactile allodynia in the spinal nerve ligation (SNL) model, than the companion low autotomy (LA) line. Changes in calcitonin gene-related peptide (CGRP) and Substance P expression were evaluated immunohistochemically in L4 and L5 dorsal root ganglia 7 days after SNL surgery. Expression of CGRP was decreased in axotomized small- and medium-diameter neurons in both rat lines. However, in HA but not in LA rats, there was a tenfold increase in CGRP immunoreactivity (CGRP-IR) in large-diameter neurons. Corresponding changes in CGRP-IR in axon terminals in the nucleus gracilis were also seen. Finally, there were indications of enhanced CGRP neurotransmission in deep laminae of the dorsal horn. Substance P immunoreactivity was also upregulated in large-diameter neurons, but this change was similar in the 2 lines. Our findings suggest that phenotypic switching contributes to the heritable difference in pain behavior in HA vs LA rats. Specifically, we propose that in HA rats, but less so in LA rats, injured, spontaneously active Aβ afferents both directly drive CGRP-sensitive central nervous system pain-signaling neurons and also trigger and maintain central sensitization, hence generating spontaneous pain and tactile allodynia.

Maldynia: pathophysiology and management of neuropathic and maladaptive pain--a report of the AMA Council on Science and Public Health

BACKGROUND

Because of disparate taxonomic arrays for classification, the American Academy of Pain Medicine has proposed categorizing pain on a neurobiologic basis as eudynia (nociceptive pain), Greek for "good pain," or maldynia (maladaptive pain), Greek for "bad pain." The latter has been viewed as maladaptive because it may occur in the absence of ongoing noxious stimuli and does not promote healing and repair.

OBJECTIVE

To address recent findings on the pathogenesis of pain following neural injury and consider whether the development of maladaptive pain justifies its classification as a disease and to briefly discuss the scope of pharmacologic and non-pharmacologic approaches employed in patients with such pain.

METHODS

English language reports on studies using human subjects were selected from a PubMed search of the literature from 1995 to August 2010 and from the Cochrane Library. Further information was obtained from Internet sites of medical specialty and other societies devoted to pain management.

RESULTS

Neural damage to either the peripheral or central nervous system provokes multiple processes including peripheral and central sensitization, ectopic activity, neuronal cell death, disinhibition, altered gene expression, and abnormal sprouting and cellular connectivity. A series of neuro-immune interactions underlie many of these mechanisms. Imaging studies have shown that such damage is characterized by functional, structural, and chemical changes in the brain. Such pain is maladaptive in the sense that it occurs in the absence of ongoing noxious stimuli and does not promote healing and repair.

CONCLUSION

As defined, maldynia is a multidimensional process that may warrant consideration as a chronic disease not only affecting sensory and emotional processing but also producing an altered brain state based on both functional imaging and macroscopic measurements. However, the absolute clinical value of this definition is not established.

Stress and tension-type headache mechanisms

Stress is widely demonstrated as a contributing factor in tension-type headache (TTH). The mechanisms underlying this remain unclear at present. Recent research indicates the importance of central pain processes in tension-type headache (TTH) pathophysiology. Concurrently, research with animals and healthy humans has begun to elucidate the relationship between stress and pain processing in the central nervous system, including central pain processes putatively dysfunctional in TTH. Combined, these two fields of research present new insights and hypotheses into possible mechanisms by which stress may contribute to TTH. To date, however, there has been no comprehensive review of this literature. The present paper provides such a review, which may be valuable in facilitating a broader understanding of the central mechanisms by which stress may contribute to TTH.

Preliminary investigation on use of high-resolution optical coherence tomography to monitor injury and repair in the rat sciatic nerve

BACKGROUND AND OBJECTIVE

Optical coherence tomography (OCT) has been used in limited settings to study peripheral nerve injury. The purpose of the study is to determine whether high-resolution OCT can be used to monitor nerve injury and regeneration in the rat sciatic nerve following crush injury, ligation, and transection with microsurgical repair.

STUDY DESIGN/MATERIALS AND METHODS

Forty-five rats were segregated into three groups. The right sciatic nerve was suture ligated (n = 15), cut then microsurgically repaired (n = 15), or crushed (n = 15). The left sciatic nerve served as the control; only surgical exposure and skin closure were performed. Each group was further divided into three subgroups where they were assigned survival durations of 4, 15, or 24 weeks. Following euthanasia, nerves were harvested, fixed in formalin, and imaged at the injury site, as well as proximal and distal ends. The OCT system resolution was approximately 7 microm in tissue with a 1,060 nm central wavelength.

RESULTS

Control (uninjured) nerve tissue showed homogenous signal distribution to a relatively uniform depth; in contrast, damaged nerves showed irregular signal distribution and intensity. Changes in signal distribution were most significant at the injury site and distal regions. Increases in signal irregularity were evident during longer recovery times. Histological analysis determined that OCT imaging was limited to the surrounding perineurium and scar tissue.

CONCLUSION

OCT has the potential to be a valuable tool for monitoring nerve injury and repair, and the changes that accompany wound healing, providing clinicians with a non-invasive tool to treat nerve injuries.

Neuropathic pain: a maladaptive response of the nervous system to damage

Neuropathic pain is triggered by lesions to the somatosensory nervous system that alter its structure and function so that pain occurs spontaneously and responses to noxious and innocuous stimuli are pathologically amplified. The pain is an expression of maladaptive plasticity within the nociceptive system, a series of changes that constitute a neural disease state. Multiple alterations distributed widely across the nervous system contribute to complex pain phenotypes. These alterations include ectopic generation of action potentials, facilitation and disinhibition of synaptic transmission, loss of synaptic connectivity and formation of new synaptic circuits, and neuroimmune interactions. Although neural lesions are necessary, they are not sufficient to generate neuropathic pain; genetic polymorphisms, gender, and age all influence the risk of developing persistent pain. Treatment needs to move from merely suppressing symptoms to a disease-modifying strategy aimed at both preventing maladaptive plasticity and reducing intrinsic risk.

Influence of vagotomy on monosynaptic transmission at second-order nucleus tractus solitarius synapses

Manipulations of vagal activity are used to treat medical pathologies, but the underlying CNS changes caused by these treatments are not well understood. Furthermore, heart and lung transplant as well as treatments for many gastrointestinal disorders result in section of the vagus nerve (vagotomy). Following unilateral vagotomy under isoflurane anesthesia of Sprague-Dawley rats, electrophysiological properties were recorded with whole cell patch techniques in horizontal brain stem slices. Vagotomy significantly reduced the median amplitude of evoked excitatory postsynaptic currents (evEPSCs; -121; n = 43) in the nucleus tractus solitarius (NTS) when compared with controls (-157 pA; n = 66; P < 0.05) but had no significant effect on the passive properties or on the average amplitude or frequency of miniature EPSCs. The degree of synaptic failure exhibited during a 50-Hz train of stimuli was used to define two separate classes of synapses: "low failure" and "high failure" (HF); failure rates <5 and > or =5%, respectively. HF synapses had significantly smaller median evEPSCs (-88 vs. -184 pA; P < 0.05). After vagotomy, the percentage of HF synapses nearly doubled to 56% (n = 24/43) when compared with controls (30%; n = 20/66). Additionally, the overall percentage of failures after the second to fifth stimuli significantly increased by at least twofold. These results suggest that vagotomy causes a decrease in synaptic efficacy by both increasing the overall percentage of synaptic failures and shifting the population of NTS synapses toward more HF transmission. In addition, the alterations due to vagotomy are likely to be presynaptic in nature.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Ifenprodil induced antinociception and decreased the expression of NR2B subunits in the dorsal horn after chronic dorsal root ganglia compression in rats

BACKGROUND

Spinal N-methyl D-aspartate receptors play an important role in the pathogenesis of neuropathic pain, and administration of N-methyl D-aspartate receptor antagonists can attenuate this hyperpathia. Ifenprodil is an antagonist selective for N-methyl D-aspartate receptor 2B (NR2B) subunits. Several researches have reported effective analgesia of ifenprodil in animal models of neuropathic pain. We extended this work to include chronic compression of the dorsal root ganglia (CCD).

METHODS

The paw withdrawal mechanical threshold and paw withdrawal thermal latency tests were used to assess mechanical allodynia and thermal hyperalgesia after a CCD operation and intrathecal injection of ifenprodil. We used immunohistochemistry and immunoblotting to investigate the effect of ifenprodil on NR2B subunits expression in CCD rats.

RESULTS

The data revealed increased expression of NR2B subunits in the superficial dorsal horn in CCD rats. We found that, in addition to a marked suppression of thermal hyperalgesia and mechanical allodynia, intrathecal injection ifenprodil treatment causes a decreased expression of NR2B in the spinal cord.

CONCLUSIONS

These data suggest that ifenprodil induced antinociception in CCD rats and provided further evidence for the important role of NR2B subunits in the development of neuropathic pain.

Targeted mutation of EphB1 receptor prevents development of neuropathic hyperalgesia and physical dependence on morphine in mice

EphB receptor tyrosine kinases, which play important roles in synaptic connection and plasticity during development and in matured nervous system, have recently been implicated in processing of pain after nerve injury and morphine dependence. Subtypes of the EphB receptors that may contribute to the neuropathic pain and morphine dependence have not been identified. Here we demonstrate that the subtype EphB1 receptor is necessary for development of neuropathic pain and physical dependence on morphine. The results showed that peripheral nerve injury produced thermal hyperalgesia in wild-type (EphB1+/+) control littermate mice, but not in EphB1 receptor homozygous knockout (EphB1-/-) and heterozygous knockdown (EphB1+/-) mice. Hyperalgesia in the wild-type mice was inhibited by intrathecal administration of an EphB receptor blocking reagent EphB2-Fc (2 microg). Intrathecal administration of an EphB receptor activator ephrinB1-Fc (1 microg) evoked thermal hyperalgesia in EphB1+/+, but not EphB1-/- and EphB1+/- mice. Cellularly, nerve injury-induced hyperexcitability of the medium-sized dorsal root ganglion neurons was prevented in EphB1-/- and EphB1+/- mice. In chronically morphine-treated mice, most of the behavioral signs and the overall score of naloxone-precipitated withdrawal were largely diminished in EphB1-/- mice compared to those in the wild-type. These findings indicate that the EphB1 receptor is necessary for development of neuropathic pain and physical dependence on morphine and suggest that the EphB1 receptor is a potential target for preventing, minimizing, or reversing the development of neuropathic pain and opiate dependence.

Identity of myelinated cutaneous sensory neurons projecting to nocireceptive laminae following nerve injury in adult mice

It is widely thought that, after peripheral injury, some low-threshold mechanoreceptive (LTMR) afferents "sprout" into pain-specific laminae (I-II) of the dorsal horn and are responsible for chronic pain states such as mechanical allodynia. Although recent studies have questioned this hypothesis, they fail to account for a series of compelling results from single-fiber analyses showing extensive projections from large-diameter myelinated afferents into nocireceptive layers after nerve injury. Here we show that, in the thoracic spinal cord of naïve adult mouse, all myelinated nociceptors gave rise to terminal projections throughout the superficial dorsal horn laminae (I-II). Most (70%) of these fibers had large-diameter axons with recurving flame-shaped central arbors that projected throughout the dorsal horn laminae I-V. This morphology was reminiscent of that attributed to sprouted LTMRs described in previous studies. After peripheral nerve axotomy, we found that LTMR afferents with narrow, uninflected somal action potentials did not sprout into superficial laminae of the dorsal horn. Only myelinated noiceptive afferents with broad, inflected somal action potentials were found to give rise to recurving collaterals and project into superficial "pain-specific" laminae after axotomy. We conclude that the previously undocumented central morphology of large, myelinated cutaneous nociceptors may very well account for the morphological findings previously thought to require sprouting of LTMRs.

Locomotor dysfunction and pain: the scylla and charybdis of fiber sprouting after spinal cord injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.

Nociceptin-receptor deficiency prevents postherpetic pain without effects on acute herpetic pain in mice

Using nociceptin-receptor-deficient mice, we studied the participation of nociceptin in herpetic and postherpetic allodynia in mice. Although nociceptin-receptor deficiency did not affect the development of skin lesions and herpetic allodynia, it prevented postherpetic allodynia. Messenger ribonucleic acid (mRNA) of pronociceptin increased in the dorsal horn of lumbar enlargement on day 6, but not on day 40, after inoculation. No changes were observed in the mRNA of the nociceptin receptor. Inhibition of herpetic allodynia by repeated oral administration of gabapentin (100 mg/kg) alleviated the overexpression of mRNA of pronociceptin, as well as the severity of postherpetic allodynia. These results suggest that the spinal nociceptin system is involved in the transitional process from herpetic allodynia to postherpetic allodynia.

Upregulation and redistribution of ephrinB and EphB receptor in dorsal root ganglion and spinal dorsal horn neurons after peripheral nerve injury and dorsal rhizotomy

EphrinB-EphB receptor signaling plays diverse roles during development, but recently has been implicated in synaptic plasticity in the matured nervous system and in pain processes. The present study investigated the correlation between expression of ephrinB and EphB receptor proteins and chronic constriction injury (CCI) of the sciatic nerve and dorsal rhizotomy (DR) in dorsal root ganglion (DRG) and spinal cord (SC); and interaction of CCI and DR on expression of these signals. Adult, male Sprague-Dawley rats were employed and thermal sensitivity was determined in the sham operated CCI and DR rats. Western blot and immunobiochemistry analysis and immunofluorescence staining techniques were used to detect the expression and location of the ephrinB-EphB receptor proteins in DRG and SC. The results showed that expression of ephrinB1 and EphB1 receptor proteins was significantly upregulated in DRG and SC in a time-dependent manner corresponding to the development of thermal hyperalgesia after CCI. The increased expression is predominately located in the medium- and small-sized DRG neurons, the superficial layers of spinal dorsal horn (DH) neurons, and the IB4 positive nociceptive terminals. DR increases ephrinB1 in DRG, not SC and EphB receptor in SC, not DRG. DR suppressed CCI-induced upregulation of ephrinB1 in SC and EphB1 receptor in DRG and SC. These findings indicate that ephrinB-EphB receptor activation and redistribution in DRG and DH neurons after nerve injury could contribute to neuropathic pain. This study may also provide a new mechanism underlying DR-induced analgesia in clinic.

Spread of excitation across modality borders in spinal dorsal horn of neuropathic rats

Under physiological conditions, nociceptive information is mainly processed in superficial laminae of the spinal dorsal horn, whereas non-nociceptive information is processed in deeper laminae. Neuropathic pain patients often suffer from touch-evoked pain (allodynia), suggesting that modality borders are disrupted in their nervous system. We studied whether excitation evoked in deep dorsal horn neurons either via stimulation of primary afferent Abeta-fibres, by direct electrical stimulation or via glutamate microinjection leads to activation of neurons in the superficial dorsal horn. We used Ca(2+)-imaging in transversal spinal cord slices of neuropathic and control animals to monitor spread of excitation from the deep to the superficial spinal dorsal horn. In neuropathic but not control animals, a spread of excitation occurred from the deep to the superficial dorsal horn. The spread of excitation was synaptically mediated as it was blocked by the AMPA receptor antagonist CNQX. In contrast, block of NMDA receptors was ineffective. In control animals, the violation of modality borders could be reproduced by bath application of GABA(A) and glycine receptor antagonists. Furthermore, we could show that neuropathic animals were more prone to synchronous network activity than control animals. Thus, following peripheral nerve injury, excitation generated in dorsal horn areas which process non-nociceptive information can invade superficial dorsal horn areas which normally receive nociceptive input. This may be a spinal mechanism of touch-evoked pain.

Intrathecal neuropeptide Y reduces behavioral and molecular markers of inflammatory or neuropathic pain

Our previous work indicates that the intrathecal administration of neuropeptide Y (NPY) acts at its cognate receptors to reduce behavioral signs of nociception in several models of inflammatory pain, including the formalin test. The present study extends these findings to a rat model of peripheral neuropathic pain, and then evaluates the hypothesis that NPY inhibits inflammation- and nerve injury-induced activation of spinal nociceptive transmission. Here we show that NPY dose-dependently reduced behavioral signs of mechanical and cold hypersensitivity in the spared nerve injury (SNI) model. Intrathecal administration of either a Y1 (BIBO3304) or a Y2 (BIIE0246) receptor antagonist dose-dependently reversed the anti-allodynic actions of NPY. To monitor the effects of NPY on the stimulus-induced activation of spinal nociresponsive neurons, we quantified protein expression of the immediate-early gene c-fos in lamina I-VI of the L4-L5 dorsal horn, with special attention to the mediolateral pattern of Fos immunohistochemical staining after SNI. Either tactile stimulation of the hindpaw ipsilateral to nerve injury, or intraplantar injection of noxious formalin, increased the number of Fos-like immunoreactive profiles. Tactile stimulation evoked a mediolateral pattern of Fos expression corresponding to the innervation territory of the uninjured (sural) nerve. We found that intrathecal NPY reduced both formalin- and SNI-induced Fos expression. NPY inhibition of SNI-induced Fos expression was localized to the sural (uninjured) innervation territory, and could be blocked by intrathecal BIBO3304 and BIIE0246. We conclude that NPY acts at spinal Y1 and Y2 receptors to reduce spinal neuron activity and behavioral signs of inflammatory or neuropathic pain.

Capsaicin, transient receptor potential (TRP) protein subfamilies and the particular relationship between capsaicin receptors and small primary sensory neurons

A number of subfamilies of the capsaicin receptor, collectively called TRP, have been reported since the discovery of vanilloid receptor 1 (VR1). The term 'TRP' is derived from 'transient receptor potential', which means the transient and rapid defect of reaction following long stimulation with light in the photoreceptor cells of mutant Drosophila. The common features of TRP family members are the centrally situated six transmembrane domain, in which an ion channel is located, three to four ankirin repeats at the N-terminus and a TRP domain comprising 25 amino acids at the C-terminus. The TRP family members are present in animals, including invertebrates and vertebrates, and in the cells in various tissues in individual animals. During evolution, the original TRP seems to have acquired a wide variety of functions related to sensing the inner or outer environment (e.g. to sensing light (Drosophila), osmolarity, protons, temperature, ligands and mechanical force). In mammals, the TRPV subfamily is exclusively expressed in small- to medium-sized primary sensory neurons that also co-express some chemical markers (i.e. isolectin B4 (IB4), fluoride-resistant acid phosphatase (FRAP), the P2X3 purinoceptor (a receptor provoked by ATP-induced nociception) and Ret, a glial cell line-derived neurotrophic factor receptor). There is a paradox in that regardless of the marked or complete loss of noxious, small sensory neurons (polymodal nociceptors) in mice treated with capsaicin during the neonatal period, as well as in VR1 (TRPV1)-deficient knock-out mice, the responses to noxious heat are normal. Regarding the paradox in mice treated with capsaicin as neonates, our explanation is that although capsaicin probably reduces the number of a subgroup of small neurons (IB4-, VR1+), the remaining IB4+ (VR1-) neurons can sense noxious heat normally. One working hypothesis is that mice lacking TRPV1/2 can sense noxious heat under normal conditions, presumably via another still unknown pathway, and TRPV1 has been suggested to be involved in noxious heat transduction under pathological conditions, such as inflammation and tissue injury. Further studies will be required to clarify these complexities. Mice treated with capsaicin as neonates would provide a model to investigate the above paradoxes, as would TRPV1-knock-out mice, although different mechanisms may be operating in the two models.

Capsaicin receptor (TRPV1) and non-erosive reflux disease

BACKGROUND/AIM

Non-erosive reflux disease (NERD) is a common and heterogeneous disorder. We hypothesized that changes in peripheral innervation may lead to hyperalgesia and contribute to the development of the disorder.

METHODS

Patients referred for evaluation of reflux symptoms with wireless pH monitoring were asked to provide demographic and clinical data and complete a survey related to severity of reflux symptoms. Endoscopies were performed to rule out macroscopic abnormalities of the esophageal mucosa. Biopsies obtained 2 cm above the gastroesophageal junction were stained for protein gene product 9.5 (PGP 9.5; general neuronal marker) and TRPV1 (capsaicin receptor) immunoreactivity. The density of immunoreactive fibers in the esophageal mucosa was determined morphometrically.

RESULTS

A total of 39 patients without evidence of Barrett's metaplasia, erosive or ulcerative esophagitis were enrolled. Most patients had daily symptoms. The total esophageal acid exposure time was 5.6+/-0.6%, with 16 patients (41%) having increased acid reflux. Immunoreactivity for PGP 9.5 or TRPV1 was detected in papillary structures as well as within the epithelium (free intra-epithelial endings). Total acid-exposure time, but not symptom score or duration correlated significantly with density of PGP 9.5 immunoreactivity and TRPV1 positive fibers.

CONCLUSION

Even in the absence of macroscopic injury, esophageal acid exposure is associated with changes in mucosal innervation of the esophagus, thus potentially further enhancing symptoms in patients with gastroesophageal reflux.

NMDA receptor antagonist treatment at the time of nerve injury prevents injury-induced changes in spinal NR1 and NR2B subunit expression and increases the sensitivity of residual pain behaviours to subsequently administered NMDA receptor antagonists

Spinal NMDA receptors (NMDA R) are important in neuropathic sensitisation and acute administration of antagonists can provide temporary attenuation of sensitisation. If establishment of the chronic pain state could be prevented by brief administration of such agents at or around the time of nerve injury (pre-emptive analgesia) it might be possible to avoid many of the unacceptable side effects associated with repeated administration of these or other antagonists. Several reports describe aspects of effective pre-emptive analgesia from NMDA R antagonists in animal models of neuropathic pain. The first aim of the present study was to make a direct comparison of changes in mechanical allodynia, cold allodynia and thermal hyperalgesia following nerve injury, demonstrating their increasing degree of susceptibility to pre-emptive NMDA R antagonist treatment. Secondly, we used immunoblotting and immunohistochemistry to investigate the effects of nerve injury on NMDA receptor subunit expression, revealing increased expression of NR2B, but not NR2A and reduced NR1 in the superficial dorsal horn. These changes were attenuated following NMDA receptor antagonist pre-treatment. Thirdly, we investigated the pharmacological properties of residual mechanical allodynia and cold allodynia that remained after pre-emptive treatment and revealed a greater sensitivity to NMDA R antagonists. These findings indicate that in addition to a marked suppression of thermal hyperalgesia and cold allodynia, pre-emptive treatment with NMDA R antagonist causes a lasting change in spinal NMDA R complexes such that remaining mechanical allodynia should be more effectively targeted by NMDA R antagonists.

Altered expression and function of sodium channels in large DRG neurons and myelinated A-fibers in early diabetic neuropathy in the rat

Differential alterations of sodium channels in small nociceptive C-fiber DRG neurons have been implicated in diabetic neuropathy. In this study, we investigated sodium currents and the expression of sodium channels in large A-fiber DRG neurons in diabetic rats. Compared with controls, large neurons from diabetic rats showed significant increases in both total and TTX-S sodium currents and approximately -15mV shifts in their voltage-dependent activation kinetics. TTX-R Na(v)1.9 sodium current was also significantly increased, whereas no alteration of TTX-R Na(v)1.8 current was observed in neurons from diabetic rats. Sodium current induced by fast- or slow-voltage ramps increased markedly in the diabetic neurons as well. Immunofluorescence studies showed significant increases in the levels and number of large DRG neurons from diabetic rats expressing Na(v)1.2, Na(v)1.3, Na(v)1.7, and Na(v)1.9 whereas Na(v)1.8 decreased. We also observed a decrease in the number of nodes of Ranvier expressing Na(v)1.8 and in staining intensity of Na(v)1.6 and Na(v)1.8 at nodes. Our results suggest that alterations of sodium channels occur in large DRG neurons and A-fibers, and may play an important role in diabetic sensory neuropathy.

Inhibition of Protein Tyrosine Kinases Attenuated Aβ-Fiber-Evoked Synaptic Transmission in Spinal Dorsal Horn of Rats With Sciatic Nerve Transection

Peripheral nerve injury leads to the establishment of a novel synaptic connection between afferent Abeta-fiber and lamina II neurons in spinal dorsal horn, which is hypothesized to underlie mechanical allodynia. However, how the novel synapses transmit nociceptive information is poorly understood. In the present study, the role of protein tyrosine kinases (PTKs) in Abeta-fiber-evoked excitatory postsynaptic currents (EPSCs) recorded in lamina II neurons in transverse spinal cord slices of rats was investigated using the whole-cell patch-clamp recording technique. In the slices from sciatic nerve transection (SNT) rats, genistein (50 microM), a broad-spectrum PTKs inhibitor, or PP2 (20 microM), a selective Src family tyrosine kinase inhibitor, significantly reduced the amplitude of Abeta-fiber EPSCs. In sham-operated rats, however, Abeta-fiber EPSCs were insensitive to genistein and PP2. The N-methyl-D-aspartate (NMDA) receptor antagonist AP-V (50 microM) suppressed Abeta-fiber EPSCs in slices from SNT rats but not from sham-operated rats. Following nerve injury, the slow inward currents elicited by bath application of NMDA (100 muM) significantly increased at -70 mV. In SNT rats, genistein and PP2 reduced Abeta-fiber-evoked EPSCs mediated by NMDA receptor; however, genistein produced little effect on Abeta-fiber EPSCs mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor. These data suggested that PTKs, especially Src family members, participated in Abeta-fiber-evoked synaptic transmission following sciatic nerve injury via potentiation of NMDA receptor function.

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.

Thiamine, pyridoxine, cyanocobalamin and their combination inhibit thermal, but not mechanical hyperalgesia in rats with primary sensory neuron injury

Neuropathic pain after nerve injury is severe and intractable, and current drugs and nondrug therapies offer substantial pain relief to no more than half of affected patients. The present study investigated the analgesic roles of the B vitamins thiamine (B1), pyridoxine (B6) and cyanocobalamin (B12) in rats with neuropathic pain caused by spinal ganglia compression (CCD) or loose ligation of the sciatic nerve (CCI). Thermal hyperalgesia was determined by a significantly shortened latency of foot withdrawal to radiant heat, and mechanical hyperalgesia was determined by a significantly decreased threshold of foot withdrawal to von Frey filaments stimulation of the plantar surface of hindpaw. Results showed that (1) intraperitoneal injection of B1 (5, 10, 33 and 100 mg/kg), B6 (33 and 100 mg/kg) or B12 (0.5 and 2 mg/kg) significantly reduced thermal hyperalgesia; (2) the combination of B1, B6 and B12 synergistically inhibited thermal hyperalgesia; (3) repetitive administration of vitamin B complex (containing B1/B6/B12 33/33/0.5 mg/kg, for 1 and 2 wk) produced long-term inhibition of thermal hyperalgesia; and (4) B vitamins did not affect mechanical hyperalgesia or normal pain sensation, and exhibited similar effects on CCD and CCI induced-hyperalgesia. The present studies demonstrate effects of B vitamins on pain and hyperalgesia following primary sensory neurons injury, and suggest the possible clinical utility of B vitamins in the treatment of neuropathic painful conditions following injury, inflammation, degeneration or other disorders in the nervous systems in human beings.

Selective C-fiber deafferentation of the spinal dorsal horn prevents lesion-induced transganglionic transport of choleragenoid to the substantia gelatinosa in the rat

The effect of neonatal capsaicin treatment, producing selective elimination of almost all unmyelinated C-fiber sensory axons, was studied on lesion-induced transganglionic labelling of the substantia gelatinosa of the spinal cord by choleragenoid. In both control and capsaicin-pretreated rats, the injection of choleragenoid-horseradish peroxidase conjugate into the intact sciatic nerves resulted in intense labelling only of the deeper layers of the spinal dorsal horn. In the control but not the capsaicin-pretreated rats, the injection of the tracer into sciatic nerves transected 2 weeks previously produced an intense homogeneous labelling of the substantia gelatinosa. It is concluded that the uptake and axonal transport of choleragenoid by capsaicin-sensitive C-fiber afferents may be accounted for by the lesion-induced transganglionic labelling of the substantia gelatinosa, rather than by A-fiber sprouting.

Absence of mechanical allodynia and Abeta-fiber sprouting after sciatic nerve injury in mice lacking membrane-type 5 matrix metalloproteinase

Matrix metalloproteinases (MMPs) are a family of endopeptidases that degrade extracellular matrix components. Membrane-type 5 MMP (MT5-MMP/MMP-24) was identified as neuron-specific, and is believed to contribute to neuronal circuit formation and plasticity. To elucidate its function in vivo, we have generated mice lacking MT5-MMP by gene targeting. MT5-MMP-deficient mice were born without obvious morphological abnormalities. No apparent histological defects were observed in the nervous system either. However, MT5-MMP-deficient mice did not develop neuropathic pain with mechanical allodynia after sciatic nerve injury, though responses to acute noxious stimuli were normal. Neuropathic pain induced by peripheral nerve lesions is known to accompany structural reorganization of the nervous system. Intraneural injection of cholera toxin B subunit, a transganglionic tracer, into the injured sciatic nerve of wild-type mice revealed that the myelinated Abeta-fiber primary afferents sprouted from laminae III-VI of the dorsal horn of the spinal cord and invaded lamina II. However, no such sprouting and invasion of Abeta-fibers were observed in MT5-MMP-deficient mice. These findings suggest that MT5-MMP is essential for the development of mechanical allodynia and plays an important role in neuronal plasticity in this mouse model.

Alterations in primary afferent input to substantia gelatinosa of adult rat spinal cord after neonatal capsaicin treatment

Primary afferent fibers are divided into three main subgroups: Abeta-, Adelta-, and C-fibers. Morphological studies have demonstrated that neonatal capsaicin treatment (NCT) depletes C-fiber inputs in the spinal dorsal horn; the electrophysiological features of the NCT-induced changes, however, remain unclear. This issue was addressed by performing whole-cell voltage-clamp recordings from substantia gelatinosa (SG) neurons in dorsal root-attached spinal cord slices. When estimated from excitatory postsynaptic currents (EPSCs) evoked by stimulating primary afferent fibers, 24 (49%) of 49 neurons examined exhibited C-fiber EPSCs that were either monosynaptic (n = 15) or polysynaptic (n = 9) in origin; only two of the neurons had Abeta-fiber responses. In NCT rats, however, SG neurons exhibiting C-fiber-mediated EPSCs decreased to 7% (3 of 41 neurons tested), whereas Abeta-fiber EPSCs were observed in 21 (51%) of the neurons, and 14 (67%) of them exhibited monosynaptic ones. There was no change in the cell proportion having Adelta-fiber innervation after NCT. Our electrophysiological data suggest that NCT diminishes primary afferent C-fiber inputs while enhancing Abeta-fiber direct innervation in the SG in adulthood.

Lack of evidence for sprouting of Abeta afferents into the superficial laminas of the spinal cord dorsal horn after nerve section

The central arborizations of large myelinated cutaneous afferents normally extend as far dorsally as the ventral part of lamina II in rat spinal cord. Woolf et al. (1992) reported that after nerve injury some of these afferents sprouted into lamina I and the dorsal part of lamina II, and it has been suggested that this could contribute to allodynia associated with neuropathic pain. Part of the evidence for sprouting was on the basis of the use of cholera toxin B subunit as a selective tracer for A-fibers, and the validity of this approach has recently been questioned; however, sprouting was also reported in experiments involving intra-axonal labeling of chronically axotomized afferents. We have used intra-axonal labeling in the rat to examine central terminals of 58 intact sciatic afferents of presumed cutaneous origin and 38 such afferents axotomized 7-10 weeks previously. Both normal and axotomized populations included axons with hair follicle afferent-like morphology and arbors that entered the ventral half of lamina II; however, none of these extended farther dorsally. We also performed bulk labeling of myelinated afferents by injecting biotinylated dextran into the lumbar dorsal columns bilaterally 8-11 weeks after unilateral sciatic nerve section. We observed that both ipsilateral and contralateral to the sectioned nerve, arbors of axons with hair follicle afferent-like morphology in the sciatic territory extended only as far as the ventral half of lamina II. Therefore these results do not support the hypothesis that Abeta afferents sprout into the superficial laminas after nerve section.

Effects and consequences of nerve injury on the electrical properties of sensory neurons

Nociceptive pain alerts the body to potential or actual tissue damage. By contrast, neuropathic or "noninflammatory" pain, which results from injury to the nervous system, serves no useful purpose. It typically continues for years after the original injury has healed. Sciatic nerve lesions can invoke chronic neuropathic pain that is accompanied by persistent, spontaneous activity in primary afferent fibers. This activity, which reflects changes in the properties and functional expression of Na+, K+, and Ca2+ channels, initiates a further increase in the excitability of second-order sensory neurons in the dorsal horn. This change persists for many weeks. The source of origin of the pain thus moves from the peripheral to the central nervous system. We hypothesize that this centralization of pain involves the inappropriate release of peptidergic neuromodulators from primary afferent fibers. Peptides such as substance P, neuropeptide Y (NPY), calcitonin-gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) may promote enduring changes in excitability as a consequence of neurotrophic actions on ion channel expression in the dorsal horn. Findings that form the basis of this hypothesis are reviewed. Study of the neurotrophic control of ion channel expression by spinal peptides may thus provide new insights into the etiology of neuropathic pain.

Transganglionic transport of choleragenoid by capsaicin-sensitive C-fibre afferents to the substantia gelatinosa of the spinal dorsal horn after peripheral nerve section

Choleratoxin B subunit-binding thick myelinated, A-fibre and unmyelinated, capsaicin-sensitive nociceptive C-fibre primary afferent fibres terminate in a strict topographic and somatotopic manner in the spinal cord dorsal horn. Injection of choleratoxin B subunit-horseradish peroxidase conjugate into injured but not intact peripheral nerves produced transganglionic labelling of primary afferents not only in the deeper layers (Rexed's laminae III-IV), but also in the substantia gelatinosa (Rexed's laminae II) of the spinal dorsal horn. This was interpreted in terms of a sprouting response of the Abeta-myelinated afferents and suggested a contribution to the pathogenesis of neuropathic pain [Nature 355 (1992) 75; J Comp Neurol 360 (1995) 121]. By utilising the selective neurotoxic effect of capsaicin, we examined the role of C-fibre sensory ganglion neurons in the mechanism of this phenomenon. Elimination of these particular, capsaicin-sensitive C-fibre afferents by prior intrathecal or systemic capsaicin treatment inhibited transganglionic labelling by the choleratoxin B subunit-horseradish peroxidase conjugate of the substantia gelatinosa evoked by chronic sciatic nerve section. More importantly, prior perineural capsaicin treatment of the transected nerve proximal to the anticipated site of injection of choleragenoid 12 hours later prevented the labelling of the substantia gelatinosa, but not that of the deeper layers. Electron microscopic examination of the dorsal roots revealed no significant difference in the proportion of labelled myelinated fibres relating to the intact (54.4+/-5.5%) and the transected (62.4+/-5.4%) sciatic nerves. In contrast, the proportion of labelled unmyelinated dorsal root axons relating to the transected, but not the intact nerves showed a significant, six-fold increase after sciatic nerve transection (intact: 4.9+/-1.3%; transected: 35+/-6.7%). These observations indicate that peripheral nerve lesion-induced transganglionic labelling of the substantia gelatinosa by choleratoxin B subunit-horseradish peroxidase may be primarily accounted for by the uptake and transganglionic transport of choleragenoid by injured capsaicin-sensitive C-fibre afferents rather than a sprouting response of A-fibre afferents. The present findings suggest an essential role of capsaicin-sensitive primary sensory neurons in lesion-induced spinal neuroplastic changes and provide further support for C-fibre nociceptor neurons being promising targets for the development of new strategies in pain management.

Comparison of the pain suppressive effects of clinical and experimental painful conditioning stimuli

Studies in healthy volunteers suggested that the classical counterirritation phenomenon (i.e. pain inhibits pain effect) might depend on diffuse noxious inhibitory controls (DNIC), which modulate the spinal transmission of nociceptive signals. In the present study, we sought to determine whether similar mechanisms were at play in patients with different subtypes of neuropathic pain. Ten patients presenting with a traumatic peripheral nerve injury associated with dynamic mechano-allodynia (i.e. pain triggered by brushing) or static mechano-allodynia (i.e. pain triggered by light pressure stimuli) were included in this study. To investigate counterirritation mechanisms in these patients, we analysed the RIII nociceptive flexion reflex and concomitant painful sensation elicited by electrical stimulation of the sural nerve. We compared the effects of heterotopic 'clinical' conditioning stimuli (i.e. pain evoked by brushing or pressure within the allodynic area located in the upper limb or chest) to those of experimental heterotopic noxious stimuli (HNCS) consisting of a cold pressor test or tourniquet test applied to the normal upper limb. Static mechano-allodynia induced inhibitions of both the RIII reflex and the concomitant painful sensation. These effects were similar to those induced by HNCS and were probably due to an increased activation of DNIC. In contrast, in patients with dynamic allodynia, brushing within the allodynic area reduced the pain sensation at the foot, but did not inhibit the electrophysiological responses, suggesting that in this case the counterirritation effect may take place at the supraspinal level. Thus, the mechanisms of counterirritation are not univocal, but depend on the pathophysiological mechanisms of clinical pain.

Resiniferatoxin induces paradoxical changes in thermal and mechanical sensitivities in rats: mechanism of action

Resiniferatoxin (RTX), an ultrapotent analog of capsaicin, has been used as a tool to study the role of capsaicin-sensitive C fibers in pain. Recently, we found that RTX diminished the thermal sensitivity but unexpectedly increased the sensitivity to tactile stimulation in adult rats. In this study, we explored the potential mechanisms involved in RTX-induced changes in somatosensory function. An intraperitoneal injection of 200 microg/kg RTX, but not its vehicle, rapidly produced an increase in the paw withdrawal latency to a heat stimulus. Also, profound tactile allodynia developed in all the RTX-treated rats in 3 weeks. This paradoxical change in thermal and mechanical sensitivities lasted for at least 6 weeks. Electron microscopic examination of the sciatic nerve revealed a loss of unmyelinated fibers and extensive ultrastructural damage of myelinated fibers in RTX-treated rats. Immunofluorescence labeling showed a diminished vanilloid receptor 1 immunoreactivity in dorsal root ganglia neurons and the spinal dorsal horn of RTX-treated rats. Furthermore, two transganglionic tracers, horseradish peroxidase conjugates of cholera toxin B subunit (CTB) and isolectin-B(4) of Bandeiraea simplicifolia (IB(4)), were injected into the opposite sides of the sciatic nerve to trace myelinated and unmyelinated afferent terminations, respectively, in the spinal dorsal horn. In RTX-treated rats, IB(4)-labeled terminals in the dorsal horn were significantly reduced, and CTB-labeled terminals appeared to sprout into lamina II of the spinal dorsal horn. Thus, this study demonstrates that systemic RTX diminishes the thermal pain sensitivity by depletion of unmyelinated afferent neurons. The delayed tactile allodynia induced by RTX is likely attributable to damage to myelinated afferent fibers and their abnormal sprouting in lamina II of the spinal dorsal horn. These data provide new insights into the potential mechanisms of postherpetic neuralgia.

The role of the dorsal columns in neuropathic behavior: evidence for plasticity and non-specificity

Despite conflicting clinical and experimental evidence, textbook description of somatic sensations continues to follow a rigid dichotomy based on the concept that pain sensation is transmitted cephalad primarily through anterolateral pathways, while touch is mediated through the dorsal column pathway. This study provides an example of the dynamic rerouting in the transmission of the nociceptive signals following injuries to the peripheral and central processes of sensory neurons. In two rat models for mononeuropathy, the chronic constriction injury model [Bennett, G.J., Xie, Y.K., Pain 33 (1988) 87-107] and the spared nerve injury model [Decosterd, I., Woolf, C.J., Pain 87 (2000) 149-158], we demonstrate that selective dorsal columns lesion produced significant decrease of tactile and cold allodynias and thermal hyperalgesia which were assessed by the Von Frey hair filaments, the acetone drop test and the heat-induced paw withdrawal, respectively. These manifestations, however, can reappear 2 weeks after bilateral dorsal column lesion in rats subjected to spared nerve injury mononeuropathy and appear also in animals sustaining chronic bilateral dorsal column lesion followed by either model of mononeuropathy. Lesion of the dorsal column on the side opposite to the neuropathic leg did not alter the neuropathic manifestations in both animal models. Changes in the sequence of timing of the dorsal column lesion and induction of mononeuropathy, suggest that the effects of the former last for 1 to 2 weeks. The results of this study show that the dorsal columns are involved in neuropathic manifestations and at the same time are not necessary for their full development and persistence. Furthermore, these results shade doubts on the validity of the concept of segregation of pathways involved in the transmission of neuropathic manifestations. Therefore, principles governing acute pain transmission are not necessarily applicable to chronic pain situations. The latter conditions seem to engage other available pathways to reestablish the pain signaling system.

Changes of sodium channel expression in experimental painful diabetic neuropathy

Although pain is experienced by many patients with diabetic neuropathy, the pathophysiology of painful diabetic neuropathy is not understood. Substantial evidence indicates that dysregulated sodium channel gene transcription contributes to hyperexcitability of dorsal root ganglion neurons, which may produce neuropathic pain after axonal transection. In this study, we examined sodium channel mRNA and protein expression in dorsal root ganglion neurons in rats with streptozotocin-induced diabetes and tactile allodynia, using in situ hybridization and immunocytochemistry for sodium channels Na(v)1.1, Na(v)1.3, Na(v)1.6, Na(v)1.7, Na(v)1.8, and Na(v)1.9. Our results show that, in rats with experimental diabetes, there is a significant upregulation of mRNA for the Na(v)1.3, Na(v)1.6, and Na(v)1.9 sodium channels and a downregulation of Na(v)1.8 mRNA 1 and 8 weeks after onset of allodynia. Channel protein levels display parallel changes. Our results demonstrate dysregulated expression of the genes for sodium channels Na(v)1.3, Na(v)1.6, Na(v)1.8, and Na(v)1.9 in dorsal root ganglion neurons in experimental diabetes and suggest that misexpression of sodium channels contributes to neuropathic pain associated with diabetic neuropathy.