Evidence against cholera toxin B subunit as a reliable tracer for sprouting of primary afferents following peripheral nerve injury

Involvement of Oxidative Stress and Nutrition in the Anatomy of Orofacial Pain

Pain is a very important problem of our existence, and the attempt to understand it is one the oldest challenges in the history of medicine. In this review, we summarize what has been known about pain, its pathophysiology, and neuronal transmission. We focus on orofacial pain and its classification and features, knowing that is sometimes purely subjective and not well defined. We consider the physiology of orofacial pain, evaluating the findings on the main neurotransmitters; in particular, we describe the roles of glutamate as approximately 30-80% of total peripheric neurons associated with the trigeminal ganglia are glutamatergic. Moreover, we describe the important role of oxidative stress and its association with inflammation in the etiogenesis and modulation of pain in orofacial regions. We also explore the warning and protective function of orofacial pain and the possible action of antioxidant molecules, such as melatonin, and the potential influence of nutrition and diet on its pathophysiology. Hopefully, this will provide a solid background for future studies that would allow better treatment of noxious stimuli and for opening new avenues in the management of pain.

DARPP-32 cells and neuropil define striosomal system and isolated matrix cells in human striatum

The dorsal striatum forms a central node of the basal ganglia interconnecting the neocortex and thalamus with circuits modulating mood and movement. Striatal projection neurons (SPNs) include relatively intermixed populations expressing D1-type or D2-type dopamine receptors (dSPNs and iSPNs) that give rise to the direct (D1) and indirect (D2) output systems of the basal ganglia. Overlaid on this organization is a compartmental organization, in which a labyrinthine system of striosomes made up of sequestered SPNs is embedded within the larger striatal matrix. Striosomal SPNs also include D1-SPNs and D2-SPNs, but they can be distinguished from matrix SPNs by many neurochemical markers. In the rodent striatum the key signaling molecule, DARPP-32, is a exception to these compartmental expression patterns, thought to befit its functions through opposite actions in both D1- and D2-expressing SPNs. We demonstrate here, however, that in the dorsal human striatum, DARPP-32 is concentrated in the neuropil and SPNs of striosomes, especially in the caudate nucleus and dorsomedial putamen, relative to the matrix neuropil in these regions. The generally DARPP-32-poor matrix contains scattered DARPP-32-positive cells. DARPP-32 cell bodies in both compartments proved negative for conventional intraneuronal markers. These findings raise the potential for specialized DARPP-32 expression in the human striosomal system and in a set of DARPP-32-positive neurons in the matrix. If DARPP-32 immunohistochemical positivity predicts differential functional DARPP-32 activity, then the distributions demonstrated here could render striosomes and dispersed matrix cells susceptible to differential signaling through cAMP and other signaling systems in health and disease. DARPP-32 is highly concentrated in cells and neuropil of striosomes in post-mortem human brain tissue, particularly in the dorsal caudate nucleus. Scattered DARPP-32-positive cells are found in the human striatal matrix. Calbindin and DARPP-32 do not colocalize within every spiny projection neuron in the dorsal human caudate nucleus.

Cutaneous Injection of Resiniferatoxin Completely Alleviates and Prevents Nerve-Injury-Induced Neuropathic Pain

Fifth lumbar (L5) nerve injury in rodent produces neuropathic manifestations in the corresponding hind paw. The aim of this study was to investigate the effect of cutaneous injection of resiniferatoxin (RTX), a TRPV1 receptor agonist, in the rat's hind paw on the neuropathic pain induced by L5 nerve injury. The results showed that intraplantar injection of RTX (0.002%, 100 µL) (1) completely reversed the development of chronic thermal and mechanical hypersensitivity; (2) completely prevented the development of nerve-injury-induced thermal and mechanical hypersensitivity when applied one week earlier; (3) caused downregulation of nociceptive pain markers, including TRPV1, IB4 and CGRP, and upregulation of VIP in the ipsilateral dorsal horn of spinal cord and dorsal root ganglion (DRG) immunohistochemically and a significant reduction in the expression of TRPV1 mRNA and protein in the ipsilateral DRG using Western blot and qRT-PCR techniques; (4) caused downregulation of PGP 9.5- and CGRP-immunoreactivity in the injected skin; (5) produced significant suppression of c-fos expression, as a neuronal activity marker, in the spinal neurons in response to a second intraplantar RTX injection two weeks later. This work identifies the ability of cutaneous injection of RTX to completely alleviate and prevent the development of different types of neuropathic pain in animals and humans.

Regional Targeting of Bladder and Urethra Afferents in the Lumbosacral Spinal Cord of Male and Female Rats: A Multiscale Analysis

Sensorimotor circuits of the lumbosacral spinal cord are required for lower urinary tract (LUT) regulation as well as being engaged in pelvic pain states. To date, no molecular markers have been identified to enable specific visualization of LUT afferents, which are embedded within spinal cord segments that also subserve somatic functions. Moreover, previous studies have not fully investigated the patterning within or across spinal segments, compared afferent innervation of the bladder and urethra, or explored possible structural sex differences in these pathways. We have addressed these questions in adult Sprague Dawley rats, using intramural microinjection of the tract tracer, B subunit of cholera toxin (CTB). Afferent distribution was analyzed within individual sections and 3D reconstructions from sections across four spinal cord segments (L5-S2), and in cleared intact spinal cord viewed with light sheet microscopy. Simultaneous mapping of preganglionic neurons showed their location throughout S1 but restricted to the caudal half of L6. Afferents from both LUT regions extended from L5 to S2, even where preganglionic motor pathways were absent. In L6 and S1, most afferents were associated with the sacral preganglionic nucleus (SPN) and sacral dorsal commissural nucleus (SDCom), with very few in the superficial laminae of the dorsal horn. Spinal innervation patterns by bladder and urethra afferents were remarkably similar, likewise the patterning in male and female rats. In conclusion, microscale to macroscale mapping has identified distinct features of LUT afferent projections to the lumbosacral cord and provided a new anatomic approach for future studies on plasticity, injury responses, and modeling of these pathways.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Co-localization of nociceptive markers in the lumbar dorsal root ganglion and spinal cord of dromedary camel

Nociceptive markers in mice have been identified in two distinct peptidergic and nonpeptidergic neurons in the dorsal root ganglion (DRG) and distributed in different laminae of the dorsal horn of the spinal cord. Recently, however, a study in humans showed a significant overlapping in these two populations. In this study, we investigated the distribution of various nociceptive markers in the lumbar DRG and spinal cord of the dromedary camel. Immunohistochemical data showed a remarkable percentage of total neurons in the DRG expressed IB4 binding (54.5%), calcitonin gene-related peptide (CGRP; 49.5%), transient receptor potential vanilloid 1 (TRPV1; 48.2%), and nitric oxide synthase (NOS; 30.6%). The co-localization data showed that 89.6% and 74.0% of CGRP- and TRPV1-labeled neurons, respectively, were IB4 positive. In addition, 61.6% and 84.2% of TRPV1- and NOS-immunoreactive neurons, respectively, were also co-localized with CGRP. The distribution of IB4, CGRP, TRPV1, substance P, and NOS immunoreactivities in the spinal cord were observed in lamina I and outer lamina II (IIo). Quantitative data showed that 82.4% of IB4-positive nerve terminals in laminae I and IIo were co-localized with CGRP, and 86.0% of CGRP-labeled terminals were co-localized with IB4. Similarly, 85.1% of NOS-labeled nerve terminals were co-localized with CGRP. No neuropeptide Y (NPY) or cholecystokinin (CCK) immunoreactivities were detected in the DRG, and no co-localization between IB4, NPY, and CCK were observed in the spinal cord. Our results demonstrate marked convergence of nociceptive markers in the primary afferent neurons in camels, which is similar to humans rather than the mouse. The data also emphasizes the importance of interspecies differences when selecting ideal animal models for studying nociception and treating chronic pain.

Neuroimmune System as a Driving Force for Plasticity Following CNS Injury

Following an injury to the central nervous system (CNS), spontaneous plasticity is observed throughout the neuraxis and affects multiple key circuits. Much of this spontaneous plasticity can elicit beneficial and deleterious functional outcomes, depending on the context of plasticity and circuit affected. Injury-induced activation of the neuroimmune system has been proposed to be a major factor in driving this plasticity, as neuroimmune and inflammatory factors have been shown to influence cellular, synaptic, structural, and anatomical plasticity. Here, we will review the mechanisms through which the neuroimmune system mediates plasticity after CNS injury. Understanding the role of specific neuroimmune factors in driving adaptive and maladaptive plasticity may offer valuable therapeutic insight into how to promote adaptive plasticity and/or diminish maladaptive plasticity, respectively.

Perineural application of resiniferatoxin on uninjured L3 and L4 nerves completely alleviates thermal and mechanical hypersensitivity following L5 nerve injury in rats

Fifth lumbar (L5) nerve injury in rats causes neuropathic pain manifested with thermal and mechanical hypersensitivity in the ipsilateral hind paw. This study aimed to determine whether the elimination of unmyelinated primary afferents of the adjacent uninjured nerves (L3 and L4) would alleviate peripheral neuropathic pain. Different concentrations of capsaicin or its analog, resiniferatoxin (RTX), were applied perineurally on either the left L4 or L3 and L4 nerves in Wistar rats whose left L5 nerves were ligated and cut. The application of both capsaicin and RTX on the L4 nerve significantly reduced both thermal and mechanical hypersensitivity. However, only the application of RTX on both L3 and L4 nerves completely alleviated all neuropathic manifestations. Interestingly, responses to thermal and mechanical stimuli were preserved, despite RTX application on uninjured L3, L4, and L5 nerves, which supply the plantar skin in rats. Perineural application of RTX caused downregulation of TRPV1, CGRP, and IB4 binding and upregulation of VIP in the corresponding dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. In comparison, VGLUT1 and NPY immunoreactivities were not altered. RTX application did not cause degenerative or ultrastructural changes in the treated nerves and corresponding DRGs. The results demonstrate that RTX induces neuroplasticity, rather than structural changes in primary afferents, that are responsible for alleviating hypersensitivity and chronic pain. Furthermore, this study suggests that treating uninjured adjacent spinal nerves may be used to manage chronic neuropathic pain following peripheral nerve injury.

Role of Gangliosides in Peripheral Pain Mechanisms

Gangliosides are abundantly occurring sialylated glycosphingolipids serving diverse functions in the nervous system. Membrane-localized gangliosides are important components of lipid microdomains (rafts) which determine the distribution of and the interaction among specific membrane proteins. Different classes of gangliosides are expressed in nociceptive primary sensory neurons involved in the transmission of nerve impulses evoked by noxious mechanical, thermal, and chemical stimuli. Gangliosides, in particular GM1, have been shown to participate in the regulation of the function of ion channels, such as transient receptor potential vanilloid type 1 (TRPV1), a molecular integrator of noxious stimuli of distinct nature. Gangliosides may influence nociceptive functions through their association with lipid rafts participating in the organization of functional assemblies of specific nociceptive ion channels with neurotrophins, membrane receptors, and intracellular signaling pathways. Genetic and experimentally induced alterations in the expression and/or metabolism of distinct ganglioside species are involved in pathologies associated with nerve injuries, neuropathic, and inflammatory pain in both men and animals. Genetic and/or pharmacological manipulation of neuronal ganglioside expression, metabolism, and action may offer a novel approach to understanding and management of pain.

PIEZOs mediate neuronal sensing of blood pressure and the baroreceptor reflex

Activation of stretch-sensitive baroreceptor neurons exerts acute control over heart rate and blood pressure. Although this homeostatic baroreflex has been described for more than 80 years, the molecular identity of baroreceptor mechanosensitivity remains unknown. We discovered that mechanically activated ion channels PIEZO1 and PIEZO2 are together required for baroreception. Genetic ablation of both Piezo1 and Piezo2 in the nodose and petrosal sensory ganglia of mice abolished drug-induced baroreflex and aortic depressor nerve activity. Awake, behaving animals that lack Piezos had labile hypertension and increased blood pressure variability, consistent with phenotypes in baroreceptor-denervated animals and humans with baroreflex failure. Optogenetic activation of Piezo2-positive sensory afferents was sufficient to initiate baroreflex in mice. These findings suggest that PIEZO1 and PIEZO2 are the long-sought baroreceptor mechanosensors critical for acute blood pressure control.

A single GABA neuron receives contacts from myelinated primary afferents of two adjacent peripheral nerves. A possible role in neuropathic pain

GAD67-EGFP mice were used in a series of experiments to provide anatomical evidence for the role of the reduction in myelinated primary afferent input to GABA spinal neurons in the production of neuropathic pain following peripheral L5 nerve injury. First, we confirmed that L5 injury in these mice produced mechanical and thermal hyperalgesia in the ipsilateral foot. Second, we injected a mixture of cholera toxin subunit-B (CTb) and isolectin B4 (IB4) in the sciatic nerve to selectively label its myelinated and unmyelinated primary afferents. Results showed that primary afferents of sciatic nerve extend from L2-L6 spinal segments. Third, we determined the central terminations of myelinated primary afferents of L4 and L5 spinal nerves following CTb injection in either nerve. The myelinated primary afferents of both nerves terminated in the corresponding and two to three rostral spinal segments with some fibers descending to terminate in the segment caudal to the level at which they entered indicating an intermingling of their terminals at the dorsal horn of the spinal cord. Fourthly, we injected CTb in L5 nerve and CTb HRP-conjugate in L4 nerve. Confocal microscopy and subsequent image analyses showed that individual EGFP-labeled neurons in L4 segment receive myelinated primary afferent contacts from both L4 and L5 nerves. Eliminating inputs from L5 nerve following its injury would result in less involvement of spinal GABA neurons which would very likely initiate neuronal sensitization in L4 segment. This could lead to the development of hyperalgesia in response to the stimulation of the adjacent uninjured L4 nerve.

Changes in the axon terminals of primary afferents from a single vibrissa in the rat trigeminal nuclei after active touch deprivation or exposure to an enriched environment

Lasting modifications of sensory input induce structural and functional changes in the brain, but the involvement of primary sensory neurons in this plasticity has been practically ignored. Here, we examine qualitatively and quantitatively the central axonal terminations of a population of trigeminal ganglion neurons, whose peripheral axons innervate a single mystacial vibrissa. Vibrissa follicles are heavily innervated by myelinated and unmyelinated fibers that exit the follicle mainly through a single deep vibrissal nerve. We made intraneural injections of a mixture of cholera-toxin B (CTB) and isolectin B4, tracers for myelinated and unmyelinated fibers, respectively, in three groups of young adult rats: controls, animals subjected to chronic haptic touch deprivation by unilateral whisker trimming, and rats exposed for 2 months to environmental enrichment. The regional and laminar pattern of terminal arborizations in the trigeminal nuclei of the brain stem did not show gross changes after sensory input modification. However, there were significant and widespread increases in the number and size of CTB-labeled varicosities in the enriched condition, and a prominent expansion in both parameters in laminae III-IV of the caudal division of the spinal nucleus in the whisker trimming condition. No obvious changes were detected in IB4-labeled terminals in laminae I-II. These results show that a prolonged exposure to changes in sensory input without any neural damage is capable of inducing structural changes in terminals of primary afferents in mature animals, and highlight the importance of peripheral structures as the presumed earliest players in sensory experience-dependent plasticity.

Histological identification of phrenic afferent projections to the spinal cord

Limited data are available regarding the spinal projections of afferent fibers in the phrenic nerve. We describe a method that robustly labels phrenic afferent spinal projections in adult rats. The proximal end of the cut phrenic nerve was secured in a microtube filled with a transganglionic tracer (cholera toxin β-subunit, CT-β, or Cascade Blue) and tissues harvested 96-h later. Robust CT-β labeling occurred in C3-C5 dorsal root ganglia cell bodies and phrenic afferent projections were identified in the mid-cervical dorsal horn (laminae I-III), intermediate grey matter (laminae IV, VII) and near the central canal (laminae X). Afferent fiber labeling was reduced or absent when CT-β was delivered to the intrapleural space or directly to the hemidiaphragm. Soaking the phrenic nerve with Cascade Blue also produced robust labeling of mid-cervical dorsal root ganglia cells bodies, and primary afferent fibers were observed in spinal grey matter and dorsal white matter. Our results show that the 'nerve soak' method effectively labels both phrenic motoneurons and phrenic afferent projections, and show that primary afferents project throughout the ipsilateral mid-cervical gray matter.

Cholera Toxin B Subunit Shows Transneuronal Tracing after Injection in an Injured Sciatic Nerve

Cholera toxin B subunit (CTB) has been extensively used in the past for monosynaptic mapping. For decades, it was thought to lack the ability of transneuronal tracing. In order to investigate whether biotin conjugates of CTB (b-CTB) would pass through transneurons in the rat spinal cord, it was injected into the crushed left sciatic nerve. For experimental control, the first order afferent neuronal projections were defined by retrograde transport of fluorogold (FG, a non-transneuronal labeling marker as an experimental control) injected into the crushed right sciatic nerve in the same rat. Neurons containing b-CTB or FG were observed in the dorsal root ganglia (DRG) at the L4-L6 levels ipsilateral to the tracer injection. In the spinal cord, b-CTB labeled neurons were distributed in all laminae ipsilaterally between C7 and S1 segments, but labeling of neurons at the cervical segment was abolished when the T10 segment was transected completely. The interneurons, distributed in the intermediate gray matter and identified as gamma-aminobutyric acid-ergic (GABAergic), were labeled by b-CTB. In contrast, FG labeling was confined to the ventral horn neurons at L4-L6 spinal segments ipsilateral to the injection. b-CTB immunoreactivity remained to be restricted to the soma of neurons and often appeared as irregular patches detected by light and electron microscopy. Detection of monosialoganglioside (GM1) in b-CTB labeled neurons suggests that GM1 ganglioside may specifically enhance the uptake and transneuronal passage of b-CTB, thus supporting the notion that it may be used as a novel transneuronal tracer.

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.

Selective innervation of NK1 receptor-lacking lamina I spinoparabrachial neurons by presumed nonpeptidergic Aδ nociceptors in the rat

Fine myelinated (Aδ) nociceptors are responsible for fast, well-localised pain, but relatively little is known about their postsynaptic targets in the spinal cord, and therefore about their roles in the neuronal circuits that process nociceptive information. Here we show that transganglionically transported cholera toxin B subunit (CTb) labels a distinct set of afferents in lamina I that are likely to correspond to Aδ nociceptors, and that most of these lack neuropeptides. The vast majority of lamina I projection neurons can be retrogradely labelled from the lateral parabrachial area, and these can be divided into 2 major groups based on expression of the neurokinin 1 receptor (NK1r). We show that CTb-labelled afferents form contacts on 43% of the spinoparabrachial lamina I neurons that lack the NK1r, but on a significantly smaller proportion (26%) of those that express the receptor. We also confirm with electron microscopy that these contacts are associated with synapses. Among the spinoparabrachial neurons that received contacts from CTb-labelled axons, contact density was considerably higher on NK1r-lacking cells than on those with the NK1r. By comparing the density of CTb contacts with those from other types of glutamatergic bouton, we estimate that nonpeptidergic Aδ nociceptors may provide over half of the excitatory synapses on some NK1r-lacking spinoparabrachial cells. These results provide further evidence that synaptic inputs to dorsal horn projection neurons are organised in a specific way. Taken together with previous studies, they suggest that both NK1r(+) and NK1r-lacking lamina I projection neurons are directly innervated by Aδ nociceptive afferents.

A putative relay circuit providing low-threshold mechanoreceptive input to lamina I projection neurons via vertical cells in lamina II of the rat dorsal horn

Background

Lamina I projection neurons respond to painful stimuli, and some are also activated by touch or hair movement. Neuropathic pain resulting from peripheral nerve damage is often associated with tactile allodynia (touch-evoked pain), and this may result from increased responsiveness of lamina I projection neurons to non-noxious mechanical stimuli. It is thought that polysynaptic pathways involving excitatory interneurons can transmit tactile inputs to lamina I projection neurons, but that these are normally suppressed by inhibitory interneurons. Vertical cells in lamina II provide a potential route through which tactile stimuli can activate lamina I projection neurons, since their dendrites extend into the region where tactile afferents terminate, while their axons can innervate the projection cells. The aim of this study was to determine whether vertical cell dendrites were contacted by the central terminals of low-threshold mechanoreceptive primary afferents.

Results

We initially demonstrated contacts between dendritic spines of vertical cells that had been recorded in spinal cord slices and axonal boutons containing the vesicular glutamate transporter 1 (VGLUT1), which is expressed by myelinated low-threshold mechanoreceptive afferents. To confirm that the VGLUT1 boutons included primary afferents, we then examined vertical cells recorded in rats that had received injections of cholera toxin B subunit (CTb) into the sciatic nerve. We found that over half of the VGLUT1 boutons contacting the vertical cells were CTb-immunoreactive, indicating that they were of primary afferent origin.

Conclusions

These results show that vertical cell dendritic spines are frequently contacted by the central terminals of myelinated low-threshold mechanoreceptive afferents. Since dendritic spines are associated with excitatory synapses, it is likely that most of these contacts were synaptic. Vertical cells in lamina II are therefore a potential route through which tactile afferents can activate lamina I projection neurons, and this pathway could play a role in tactile allodynia.

Fifth lumbar spinal nerve injury causes neurochemical changes in corresponding as well as adjacent spinal segments: a possible mechanism underlying neuropathic pain

Previous investigations of the anatomical basis of the neuropathic-like manifestations in the spinal nerve ligation animal model have shown that the central terminations of the unmyelinated primary afferents of L5 spinal nerve are not restricted to the corresponding L5 spinal segment, and rather extend to two spinal segments rostrally and one segment caudally where they intermingle with primary afferents of the adjacent L4 spinal nerve. The aim of the present study was to investigate the neurochemical changes in the dorsal horn of the spinal cord and DRGs after L5 nerve injury in rats. In the first experiment, the right L5 nerve was ligated and sectioned for 14 days, and isolectin B4 (IB4, a tracer for unmyelinated primary afferents) was injected into the left L5 nerve. The results showed that the vasoactive intestinal peptide (VIP) was up-regulated in laminae I-II of L3-L6 spinal segments on the right side in exactly the same areas where IB4 labelled terminals were revealed on the left side. In the second experiment, L5 was ligated and sectioned and the spinal cord and DRGs were stained immunocytochemically with antibodies raised against various peptides known to be involved in pain transmission and hyperalgesia. The results showed that L5 nerve lesion caused down-regulation of substance P, calcitonin-gene related peptide and IB4 binding and up-regulation of neuropeptide Y and neurokinin-1 receptor in the dorsal horn of L4 and L5 spinal segments. Similar neurochemical changes were observed only in the corresponding L5 DRG with minimal effects observed in L3, L4 and L6 DRGs. Although, L5 nerve injury caused an up-regulation in NPY, no change in SP and CGRP immunoreactivity was observed in ipsilateral garcile nucleus. These neuroplastic changes in the dorsal horn of the spinal cord, in the adjacent uninjured territories of the central terminations of the adjacent uninjured nerves, might explain the mechanism of hyperalgesia after peripheral nerve injury.

Artemin induced functional recovery and reinnervation after partial nerve injury

Systemic artemin promotes regeneration of dorsal roots to the spinal cord after crush injury. However, it is unclear whether systemic artemin can also promote peripheral nerve regeneration, and functional recovery after partial lesions distal to the dorsal root ganglion (DRG) remains unknown. In the present investigation, male Sprague Dawley rats received axotomy, ligation, or crush of the L5 spinal nerve or sham surgery. Starting the day of injury, animals received intermittent subcutaneous artemin or vehicle across 2weeks. Sensory thresholds to tactile or thermal stimuli were monitored for 6weeks after injury. Immunohistochemical analyses of the DRG and nerve regeneration were performed at the 6-week time point. Artemin transiently reversed tactile and thermal hypersensitivity after axotomy, ligation, or crush injury. Thermal and tactile hypersensitivity reemerged within 1week of treatment termination. However, artemin-treated rats with nerve crush, but not axotomy or ligation, subsequently showed gradual return of sensory thresholds to preinjury baseline levels by 6weeks after injury. Artemin normalized labeling for NF200, IB4, and CGRP in nerve fibers distal to the crush injury, suggesting persistent normalization of nerve crush-induced neurochemical changes. Sciatic and intradermal administration of dextran or cholera toxin B distal to the crush injury site resulted in labeling of neuronal profiles in the L5 DRG, suggesting regeneration functional restoration of nonmyelinated and myelinated fibers across the injury site into cutaneous tissue. Artemin also diminished ATF3 and caspase 3 expression in the L5 DRG, suggesting persistent neuroprotective actions. A limited period of artemin treatment elicits disease modification by promoting sensory reinnervation of distal territories and restoring preinjury sensory thresholds.

Differential joint-specific corticospinal tract projections within the cervical enlargement

The motor cortex represents muscle and joint control and projects to spinal cord interneurons and-in many primates, including humans-motoneurons, via the corticospinal tract (CST). To examine these spinal CST anatomical mechanisms, we determined if motor cortex sites controlling individual forelimb joints project differentially to distinct cervical spinal cord territories, defined regionally and by the locations of putative last-order interneurons that were transneuronally labeled by intramuscular injection of pseudorabies virus. Motor cortex joint-specific sites were identified using intracortical-microstimulation. CST segmental termination fields from joint-specific sites, determined using anterograde tracers, comprised a high density core of terminations that was consistent between animals and a surrounding lower density projection that was more variable. Core terminations from shoulder, elbow, and wrist control sites overlapped in the medial dorsal horn and intermediate zone at C5/C6 but were separated at C7/C8. Shoulder sites preferentially terminated dorsally, in the dorsal horn; wrist/digit sites, more ventrally in the intermediate zone; and elbow sites, medially in the dorsal horn and intermediate zone. Pseudorabies virus injected in shoulder, elbow, or wrist muscles labeled overlapping populations of predominantly muscle-specific putative premotor interneurons, at a survival time for disynaptic transfer from muscle. At C5/C6, CST core projections from all joint zones were located medial to regions of densely labeled last-order interneurons, irrespective of injected muscle. At C7/C8 wrist CST core projections overlapped the densest interneuron territory, which was located in the lateral intermediate zone. In contrast, elbow CST core projections were located medial to the densest interneuron territories, and shoulder CST core projections were located dorsally and only partially overlapped the densest interneuron territory. Our findings show a surprising fractionation of CST terminations in the caudal cervical enlargement that may be organized to engage different spinal premotor circuits for distal and proximal joint control.

Co-development of proprioceptive afferents and the corticospinal tract within the cervical spinal cord

In maturity, skilled movements depend on coordination of control signals by descending pathways, such as the corticospinal tract (CST), and proprioceptive afferents (PAs). An important locus for this coordination is the spinal cord intermediate zone. Convergence of CST and PA terminations onto common regions leads to interactions that may underlie afferent gating and modulation of descending control signals during movements. We determined establishment of CST and PA terminations within common spinal cord regions and development of synaptic interactions in 4-week-old cats, which is before major spinal motor circuit refinement, and two ages after refinement (weeks 8, 11). We examined the influence of one or the other system on monosynaptic responses, on the spinal cord surface and locally in the intermediate zone, evoked by either CST or deep radial nerve (DRN) stimulation. DRN stimulation suppressed CST monosynaptic responses at 4 weeks, but this converted to facilitation by 8 weeks. This may reflect a strategy to limit CST movement control when it has aberrant immature connections, and could produce errant movements. CST stimulation showed delayed development of mixed suppression and facilitation of DRN responses. We found development of age-dependent overlap of PA and CST terminations where interactions were recorded in the intermediate zone. Our findings reveal a novel co-development of different inputs onto common spinal circuits and suggest a logic to CST-PA interactions at an age before the CST has established connectional specificity with spinal circuits.

Postnatal refinement of proprioceptive afferents in the cat cervical spinal cord

Proprioceptive afferent (PA) information is integrated with signals from descending pathways, including the corticospinal tract (CST), by spinal interneurons in the dorsal horn and intermediate zone for controlling movements. PA spinal projections, and the reflexes that they evoke, develop prenatally. The CST projects to the spinal cord postnatally, and its connections are subsequently refined. Consequently, the tract becomes effective in transmitting control signals from motor cortex to muscle. This suggests sequential development of PAs and the CST rather than co-development. In this study we determined if there was also late postnatal refinement of PA spinal connections, which would support PA-CST co-development. We examined changes in PA spinal connections at 4 weeks of age, when CST terminations are immature, at 8 weeks, after CST refinement, and at 11 weeks, when CST terminations are mature. We electrically stimulated PA afferents in the deep radial nerve. Evoked PA responses were small and not localized at 4 weeks. By 8 and 11 weeks, responses were substantially larger and maximal in laminae VI and dorsal VII. We used intramuscular injection of cholera toxin β subunit to determine the distribution of PAs from the extensor carpii radialis muscle in the cervical enlargement at the same ages as in the electrophysiological studies. We found a reduction of the distribution of PAs with age that paralleled the physiological changes. This age-related sharpening of PA spinal connections also paralleled CST development, suggesting coordinated PA-CST co-development rather than sequential development. This is likely to be important for the development of adaptive motor control.

Simultaneous identification of unmyelinated and myelinated primary somatic afferents by co-injection of isolectin B4 and Cholera toxin subunit B into the sciatic nerve of the rat

Several studies have used the transganglionic tracers cholera toxin subunit B (CTb) and either Bandeiraea simplicifolia isolectin B4 (IB4) or wheat-germ agglutinin (WGA) to label myelinated and unmyelinated afferent fibres respectively. In this study, we aim to determine whether co-injection of CTb and either IB4 or WGA into the sciatic nerve of rat will selectively label myelinated and unmyelinated simultaneously. A double immunofluorescence approach was used to detect these tracers in dorsal root ganglia (DRGs) and afferent fibre terminals in the spinal cord. CTb- and IB4-labelled neurons were seen mainly in L4 and L5 DRGs, with CTb labelling detected primarily in large sized neurons and IB4 staining seen mainly in smaller cells. Only a minority of CTb labelled DRG neuron profiles (5.1%) were also labelled with IB4. In the spinal cord, IB4-labelling was largely confined to lamina II of spinal segments L3-L5, whereas CTb-labelled terminals were seen in all laminae but sparse in lamina II. Confocal microscopy showed no evidence for colocalisation of CTb and IB4 labelling in any terminals in laminae I-III. Although the central distribution of CTb labelling in laminae I and II inner-IV had the same rostro-caudal and medio-lateral coverage as IB4 labelling in spinal segments L3-L5, CTb labelling in ventral laminae (of putative proprioceptor afferents) extended between T12 and S1. Similar patterns of central labelling were found when CTb and WGA were injected together. We therefore concluded that this co-injection approach provides a reliable method to identify both myelinated and unmyelinated somatic primary afferents simultaneously.

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region.

Functional roles of intrinsic neurotrophin-3 in spinal neuroplasticity of cats following partial ganglionectomy

This study detected the effects of endogenous neurotrophin-3 (NT-3) on the collateral sprouting derived from the L6 dorsal root ganglion (DRG) after unilateral removal of adjacent DRGs (L1–L5 and L7) in cats. Cholera toxin B tracing revealed significant neurite growth from the spared L6 DRG and axonal sprouting in the dorsal column. There was a significant increase in the number of NT-3 and trkC immunopositive neurons as well as in NT-3 protein level in the spared DRG by immunohistochemistry and enzyme-linked immunoadsorbent assay. NT-3 and its mRNA and trkC were located mainly in large- and medium-sized DRG neurons. NT-3 antibody neutralization in vivo and in vitro results in marked reduction in sprouted fibers. These findings point to an important role of NT-3 in neural plasticity at dorsal column axons.

Acute and chronic sectioning of fifth lumbar spinal nerve has equivalent effects on the primary afferents of sciatic nerve in rat spinal cord

The mechanism of neuropathic pain may be associated with sprouting of uninjured primary afferents of peripheral nerves into regions of the spinal cord denervated through peripheral injury. However, this remains controversial. Therefore, the purpose of the present investigation was, first, to determine in detail the central distributions of the unmyelinated primary afferents of each of the L4, L5, and L6 components of sciatic nerve, then to assess the distribution of afferent sciatic terminals following acute and chronic injury to (L5) nerve. First, we injected isolectin B4 (IB4), into the sciatic nerves in three groups of rats, each of which had two of the three L4, L5, or L6 components ligated and cut, and the one remaining, uninjured. Although the terminal labelling found in the L5 segment of the spinal cord originated from the L5 component, some terminal labelling remained in cases when either the L4 or L6 component was intact. Second, tracers transported in predominantly unmyelinated (IB4 and WGA-HRP) or myelinated (cholera toxin subunit B) nerves were injected into the sciatic nerve following acute or chronic (21-day) injury restricted to the L5 component. In each case, the central distribution of nerve terminals in the spinal dorsal horn was equivalent following either acute or chronic injury to the L5 component. Consequently, these data provide no support for the suggestion that neuropathic pain in spinal ligation model results from uninjured L4 and L6 components sprouting to occupy sites vacated by the injured L5 component of the sciatic nerve.

Blockade of Nogo receptor ligands promotes functional regeneration of sensory axons after dorsal root crush

A major impediment for regeneration of axons within the CNS is the presence of multiple inhibitory factors associated with myelin. Three of these factors bind to the Nogo receptor, NgR, which is expressed on axons. Administration of exogenous blockers of NgR or NgR ligands promotes the regeneration of descending axonal projections after spinal cord hemisection. A more detailed analysis of CNS regeneration can be made by examining the growth of specific classes of sensory axons into the spinal cord after dorsal root crush injury. In this study, we assessed whether administration of a soluble peptide fragment of the NgR (sNgR) that binds to and blocks all three NgR ligands can promote regeneration after brachial dorsal root crush in adult rats. Intraventricular infusion of sNgR for 1 month results in extensive regrowth of myelinated sensory axons into the white and gray matter of the dorsal spinal cord, but unmyelinated sensory afferents do not regenerate. In concert with the anatomical growth of sensory axons into the cord, there is a gradual restoration of synaptic function in the denervated region, as revealed by extracellular microelectrode recordings from the spinal gray matter in response to stimulation of peripheral nerves. These positive synaptic responses are correlated with substantial improvements in use of the forelimb, as assessed by paw preference, paw withdrawal to tactile stimuli and the ability to grasp. These results suggest that sNgR may be a potential therapy for restoring sensory function after injuries to sensory roots.

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.

Chapter 27: Neural plasticity after nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory, and autonomic functions in the denervated segments of the body due to the interruption of axons, degeneration of distal nerve fibers, and eventual death of axotomized neurons. Functional deficits caused by nerve injuries can be compensated by reinnervation of denervated targets by regenerating injured axons or by collateral branching of undamaged axons, and remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of adequate target reinnervation; however, plasticity has limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain and hyperreflexia. After axotomy, neurons shift from a transmitter to a regenerative phenotype, activating molecular pathways that promote neuronal survival and axonal regeneration. Peripheral nerve injuries also induce a cascade of events, at the molecular, cellular, and system levels, initiated by the injury and progressing throughout plastic changes at the spinal cord, brainstem nuclei, thalamus, and brain cortex. Mechanisms involved in these changes include neurochemical changes, functional alterations of excitatory and inhibitory synaptic connections, sprouting of new connections, and reorganization of sensory and motor central maps. An important direction for research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, and are also able to modulate central nervous system reorganization, amplifying positive adaptive changes that improve functional recovery and also reducing undesirable effects.

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.

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.

Electrical stimulation of intact peripheral sensory axons in rats promotes outgrowth of their central projections

A lesion of a peripheral nerve before a second injury (conditioning lesion, CL), enhances peripheral and central regeneration of dorsal root ganglion (DRG) axons. This effect is mediated by elevated neuronal cAMP. Here we wanted to investigate whether electrical stimulation (ES) of an intact nerve, which has been shown to accelerate peripheral axon outgrowth, is also effective in promoting axon regeneration of injured DRG axons in vitro and of the central DRG axons in vivo and, whether this effect is mediated by elevation of cAMP. For the in vitro assay, the intact sciatic nerve of adult rats was stimulated at 20 Hz for 1 h, 7 days before harvest and primary culture of DRG neurons on a growth permissive substrate. In the in vivo study, the central axons of the lumbosacral DRGs were cut in the Th8 dorsal column, and the sciatic nerve was either cut or left intact, and subjected to 1 h ES at 20 Hz or 200 Hz. In vitro, ES increased neurite outgrowth 4-fold as compared to non-stimulated DRG neurons. In vivo, ES at 20 Hz significantly increased axon outgrowth into the central lesion site as compared to the Sham control. The 20 Hz ES was as effective as the CL in increasing axon outgrowth into the lesion site but not in promoting axonal elongation even though 20 Hz ES increased intracellular cAMP levels in DRG neurons as effectively as the CL. Thus elevation of cAMP may account for the central axonal outgrowth after ES and a CL.

Sensory and autonomic nerve changes in the monosodium glutamate-treated rat: a model of type II diabetes

Rats that had been injected with monosodium glutamate (MSG) neonatally were studied for up to 70 weeks and compared with age-matched control rats to study changes in glucose tolerance and in sympathetic and sensory nerves. At 61 and 65 weeks of age, there were significant differences in glucose tolerance between the MSG and control groups, and the MSG group had raised fasting blood glucose. These changes were not associated with changes in the number of beta-cells in the islets of Langerhans. In addition, the diabetic MSG-treated rats had central obesity and cataracts. Hypoalgesia to thermal stimuli was present in MSG-treated rats as early as 6 weeks and persisted at 70 weeks. However, no differences were observed in the distribution of substance P, the neurokinin-1 receptor or calcitonin gene-related peptide in the dorsal horn of L3-L5 at this age (70 weeks). Diabetic MSG-treated animals at 65 and 70 weeks of age had significantly reduced noradrenaline concentrations in the heart, tail artery and ileum, while concentrations in the adrenal gland and corpus cavernosum were significantly increased. There was also a significant increase in adrenal adrenaline, dopamine and serotonin, largely attributable to changes in weight of the adrenal gland in the MSG-treated animals. The results indicate that MSG-treated animals develop a form of type II diabetes by about 60 weeks of age, and that there are significant changes in amine levels in various tissues associated with these developments.

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.

Upregulation of substance P in low-threshold myelinated afferents is not required for tactile allodynia in the chronic constriction injury and spinal nerve ligation models

It has been proposed that substance P and calcitonin gene-related peptide (CGRP) are upregulated in low-threshold myelinated primary afferents after certain types of nerve injury, and that release of substance P from these afferents contributes to the resulting tactile allodynia. To test this hypothesis, we looked for neuropeptides in Abeta primary afferent terminals in the ipsilateral gracile nucleus and spinal dorsal horn in three nerve injury models: sciatic nerve transection (SNT), spinal nerve ligation (SNL), and chronic constriction injury (CCI). We also looked for evidence of neurokinin 1 (NK1) receptor internalization in the dorsal horn after electrical stimulation of Abeta afferents. We found no evidence of either substance P or CGRP expression in injured Abeta terminals in the spinal cord in any of the models. Although substance P was not detected in terminals of injured afferents in the gracile nucleus, CGRP was expressed in between 32 and 68% of these terminals, with a significantly higher proportion in the SNL and CCI models, compared with SNT. In addition, we did not detect any Abeta-evoked NK1 receptor internalization in neurons from laminas I, III, or IV of the dorsal horn in the CCI or SNL models. These results do not support the proposal that substance P is present at significant levels in the terminals of injured Abeta primary afferents in neuropathic models. They also suggest that any release of substance P from injured Abeta afferents is unlikely to activate NK1 receptors in the dorsal horn or contribute to neuropathic pain.

Peripheral nerve injury induces reorganization of galanin-containing afferents in the superficial dorsal horn of monkey spinal cord

Peripheral nerve injury-induced structural and chemical modifications of the sensory circuits in the dorsal horn of the spinal cord contribute to the mechanism of neuropathic pain. In contrast to the topographic projection of primary afferents in laminae I-IV in the rat spinal cord, the primary afferents of Macaca mulatta monkeys almost exclusively project into laminae I-II of the spinal cord. After peripheral nerve injury, up-regulation of galanin has been found in sensory neurons in both monkey and rat dorsal root ganglia. However, the nerve injury-induced ultrastructural modification of galanin-containing afferents in the monkey spinal cord remains unknown. Using immunoelectron microscopy, we found that 3 weeks after unilateral sciatic nerve transection, the number of galanin-containing afferents was increased in ipsilateral lamina II of monkey spinal cord. Branching of these galanin-positive afferents was often observed. The afferent terminals contained a large number of synaptic vesicles, peptidergic vesicles and mitochondria, whereas the number of synapses was markedly reduced. Some of the afferents-enriched microtubules were often packed into bundles. Moreover, galanin-labeling could be associated with endosomal structures in many dendrites and axonal terminals of dorsal horn neurons. These results suggest that peripheral nerve injury induces an expansion of the central projection of galanin-containing afferents in lamina II of the monkey spinal cord, not only by increasing galanin levels in primary afferents but also by triggering afferent branching.

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.

Synaptic plasticity in the adult spinal dorsal horn: The appearance of new functional connections following peripheral nerve regeneration

Peripherally regenerated fibers were impaled in the dorsal columns. Each impaled fiber's adequate stimulus was determined and the fiber was activated by passing brief (200 ms) current pulses through the microelectrode. Cord dorsum potentials (CDPs) elicited by fiber stimulation were recorded at 8 sites, and then the fiber was injected with Neurobiotin (NB). In the same preparations, dorsal horn cells were impaled and their receptive fields (RFs) mapped; areas of skin from which the most vigorous responses were elicited were noted. Needle electrodes inserted into these cutaneous "hot spots" were used to electrically activate minimal numbers of peripherally regenerated fibers while simultaneously recording the resulting CDPs and any intracellular EPSPs. This allowed determination of connectivity between regenerated fibers and dorsal horn cells with overlapping RFs. In agreement with findings in intact animals, NB revealed long-ranging collaterals which were not seen using intraaxonally injected horseradish peroxidase (HRP). Although there was no qualitative difference in their morphology compared to those seen in controls, the correlation between spatial distribution of boutons and amplitudes of the monosynaptic CDPs of peripherally regenerated fibers revealed significant shifts in the functional efficacy of many central connections. Transcutaneous electrical stimulation revealed a significantly higher incidence of connectivity between regenerated fibers and cells with overlapping RFs at 9-12 months (86%) than at 5-6 months (34%). Although there was no obvious anatomical reorganization of afferent projections in the dorsal horn, the observed functional changes with time following transection show the formation of new functional central connections.

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.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Dose-dependent beneficial and detrimental effects of ROCK inhibitor Y27632 on axonal sprouting and functional recovery after rat spinal cord injury

Axonal regeneration within the injured central nervous system (CNS) is hampered by multiple inhibitory molecules in the glial scar and the surrounding disrupted myelin. Many of these inhibitors stimulate, either directly or indirectly, the Rho intracellular signaling pathway, providing a strong rationale to target it following spinal cord injuries. In this study, we infused either control (PBS) or a ROCK inhibitor, Y27632 (2 mM or 20 mM, 12 microl/day for 14 days) into the intrathecal space of adult rats starting immediately after a cervical 4/5 dorsal column transection. Histological analysis revealed that high dose-treated animals displayed significantly more axon sprouts in the grey matter distal to injury compared to low dose-treated rats. Only the high dose regimen stimulated sprouting of the dorsal ascending axons along the walls of the lesion cavity. Footprint analysis revealed that the increased base of support normalized significantly faster in control and high dose-treated animals compared to low dose animals. Forepaw rotation angle, and the number of footslips on a horizontal ladder improved significantly more by 6 weeks in high dose animals compared to the other two groups. In a food pellet reaching test, high dose animals performed significantly better than low dose animals, which failed to recover. There was no evidence of mechanical allodynia in any treatment group; however, the slightly shortened heat withdrawal times normalized only with the high dose treatment. Collectively, our data support beneficial effects of high dose Y27632 treatment but indicate that low doses might be detrimental.