Differences in sympathetic innervation of mouse DRG following proximal or distal nerve lesions

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

Sympathetic-Sensory Coupling as a Potential Mechanism for Acupoints Sensitization

A series of studies have demonstrated acupoint sensitization, in which acupoints can be activated in combination with sensory hypersensitivity and functional plasticity during visceral disorders. However, the mechanisms of acupoint sensitization remain unclear. Neuroanatomy evidence showed nociceptors innervated in acupoints contribute to the mechanism of acupoint sensitization. Increasing studies suggested sympathetic nerve plays a key role in modulating sensory transmission by sprouting or coupling with sensory neuron/nociceptor in the peripheral, forming the functional structure of the sympathetic-sensory coupling. Notably, the sensory inputs of the disease-induced sensitized acupoint contribute to the homeostatic regulation and also involve in delivering therapeutic information under acupuncture, hence, the role of sprouted sympathetic in acupoint function should be given attention. We herein reviewed the current knowledge of sympathetic and its sprouting in pain modulation, then discussed and highlighted the potential value of sympathetic-sensory coupling in acupoint functional plasticity.

Peripheral mechanisms of chronic pain

Acutely, pain serves to protect us from potentially harmful stimuli, however damage to the somatosensory system can cause maladaptive changes in neurons leading to chronic pain. Although acute pain is fairly well controlled, chronic pain remains difficult to treat. Chronic pain is primarily a neuropathic condition, but studies examining the mechanisms underlying chronic pain are now looking beyond afferent nerve lesions and exploring new receptor targets, immune cells, and the role of the autonomic nervous system in contributing chronic pain conditions. The studies outlined in this review reveal how chronic pain is not only confined to alterations in the nervous system and presents findings on new treatment targets and for this debilitating disease.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Profiling sensory neuron microenvironment after peripheral and central axon injury reveals key pathways for neural repair

Sensory neurons with cell bodies in dorsal root ganglia (DRG) represent a useful model to study axon regeneration. Whereas regeneration and functional recovery occurs after peripheral nerve injury, spinal cord injury or dorsal root injury is not followed by regenerative outcomes. Regeneration of sensory axons in peripheral nerves is not entirely cell autonomous. Whether the DRG microenvironment influences the different regenerative capacities after injury to peripheral or central axons remains largely unknown. To answer this question, we performed a single-cell transcriptional profiling of mouse DRG in response to peripheral (sciatic nerve crush) and central axon injuries (dorsal root crush and spinal cord injury). Each cell type responded differently to the three types of injuries. All injuries increased the proportion of a cell type that shares features of both immune cells and glial cells. A distinct subset of satellite glial cells (SGC) appeared specifically in response to peripheral nerve injury. Activation of the PPARα signaling pathway in SGC, which promotes axon regeneration after peripheral nerve injury, failed to occur after central axon injuries. Treatment with the FDA-approved PPARα agonist fenofibrate increased axon regeneration after dorsal root injury. This study provides a map of the distinct DRG microenvironment responses to peripheral and central injuries at the single-cell level and highlights that manipulating non-neuronal cells could lead to avenues to promote functional recovery after CNS injuries or disease.

Cytotoxic Immunity in Peripheral Nerve Injury and Pain

Cytotoxicity and consequent cell death pathways are a critical component of the immune response to infection, disease or injury. While numerous examples of inflammation causing neuronal sensitization and pain have been described, there is a growing appreciation of the role of cytotoxic immunity in response to painful nerve injury. In this review we highlight the functions of cytotoxic immune effector cells, focusing in particular on natural killer (NK) cells, and describe the consequent action of these cells in the injured nerve as well as other chronic pain conditions and peripheral neuropathies. We describe how targeted delivery of cytotoxic factors via the immune synapse operates alongside Wallerian degeneration to allow local axon degeneration in the absence of cell death and is well-placed to support the restoration of homeostasis within the nerve. We also summarize the evidence for the expression of endogenous ligands and receptors on injured nerve targets and infiltrating immune cells that facilitate direct neuro-immune interactions, as well as modulation of the surrounding immune milieu. A number of chronic pain and peripheral neuropathies appear comorbid with a loss of function of cellular cytotoxicity suggesting such mechanisms may actually help to resolve neuropathic pain. Thus while the immune response to peripheral nerve injury is a major driver of maladaptive pain, it is simultaneously capable of directing resolution of injury in part through the pathways of cellular cytotoxicity. Our growing knowledge in tuning immune function away from inflammation toward recovery from nerve injury therefore holds promise for interventions aimed at preventing the transition from acute to chronic pain.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Electroacupuncture promotes the recovery of motor neuron function in the anterior horn of the injured spinal cord

Acupuncture has been shown to lessen the inflammatory reaction after acute spinal cord injury and reduce secondary injury. However, the mechanism of action remains unclear. In this study, a rat model of spinal cord injury was established by compressing the T_8–9 segments using a modified Nystrom method. Twenty-four hours after injury, Zusanli (ST36), Xuanzhong (GB39), Futu (ST32) and Sanyinjiao (SP6) were stimulated with electroacupuncture. Rats with spinal cord injury alone were used as controls. At 2, 4 and 6 weeks after injury, acetylcholinesterase (AChE) activity at the site of injury, the number of medium and large neurons in the spinal cord anterior horn, glial cell line-derived neurotrophic factor (GDNF) mRNA expression, and Basso, Beattie and Bresnahan locomotor rating scale scores were greater in the electroacupuncture group compared with the control group. These results demonstrate that electroacupuncture increases AChE activity, up-regulates GDNF mRNA expression, and promotes the recovery of motor neuron function in the anterior horn after spinal cord injury.

Sympathetic fibre sprouting in the skin contributes to pain-related behaviour in spared nerve injury and cuff models of neuropathic pain

Background

Cuff and spared nerve injury (SNI) in the sciatic territory are widely used to model neuropathic pain. Because nociceptive information is first detected in skin, it is important to understand how alterations in peripheral innervation contribute to pain in each model. Over 16 weeks in male rats, changes in sensory and autonomic innervation of the skin were described after cuff and SNI using immunohistochemistry to label myelinated (neurofilament 200 positive—NF200+) and peptidergic (calcitonin gene-related peptide positive—CGRP+) primary afferents and sympathetic fibres (dopamine β-hydroxylase positive—DBH+)

Results

Cuff and SNI caused an early loss and later reinnervation of NF200 and CGRP fibres in the plantar hind paw skin. In both models, DBH+ fibres sprouted into the upper dermis of the plantar skin 4 and 6 weeks after injury. Despite these similarities, behavioural pain measures were significantly different in each model. Sympathectomy using guanethidine significantly alleviated mechanical allodynia 6 weeks after cuff, when peak sympathetic sprouting was observed, having no effect at 2 weeks, when fibres were absent. In SNI animals, mechanical allodynia in the lateral paw was significantly improved by guanethidine at 2 and 6 weeks, and the development of cold hyperalgesia, which roughly paralleled the appearance of ectopic sympathetic fibres, was alleviated by guanethidine at 6 weeks. Sympathetic fibres did not sprout into the dorsal root ganglia at 2 or 6 weeks, indicating their unimportance to pain behaviour in these two models.

Conclusions

Alterations in sympathetic innervation in the skin represents an important mechanism that contributes to pain in cuff and SNI models of neuropathic pain.

Progress in Sympathetically Mediated Pathological Pain

Aim of review

Many chronic pain conditions remain difficult to treat, presenting a high burden to society. Conditions such as complex regional pain syndrome may be maintained or exacerbated by sympathetic activity. Understanding the interactions between sympathetic nervous system and sensory system will help to improve the effective management of pathological pain including intractable neuropathic pain and persistent inflammatory pain.

Method

We first described the discovery of abnormal connections between sympathetic and sensory neurons. Subsequently, the functional roles of sympathetic sprouting in altered neuronal excitability and increased pain sensitivity were discussed. The mechanisms of the sympathetic sprouting were focusing on its relationship with neurotrophins, local inflammation, and abnormal spontaneous activity. Finally, we discussed clinical implications and conflicting findings in the laboratory and clinical research with respect to the interaction between sympathetic system and sensory system.

Recent findings

The findings that sprouting of sympathetic fibers into the sensory ganglia (dorsal root ganglion) after peripheral nerve injury, offers a possible explanation of the sympathetic involvement in pain. It is also suggested that releases of adenosine triphosphate (ATP), in addition to norepinephrine, from sympathetic nerve endings play important roles in sympathetic-mediated pain. New evidence indicates the importance of sympathetic innervation in local inflammatory responses.

Summary

Hopefully, this review will reinvigorate the study of sympathetic-sensory interactions in chronic pain conditions, and help to better understand how sympathetic system contributes to this serious clinical problem.

Neuronal degeneration associated with sympathosensory plexuses in the trigeminal ganglia of aged mice that overexpress nerve growth factor

Aberrant sympathetic sprouting is seen in the uninjured trigeminal ganglia of transgenic mice that ectopically express nerve growth factor under the control of the glial fibrillary acidic protein promoter. These sympathetic axons form perineuronal plexuses around a subset of sensory somata in 2- to 3-month-old transgenic mice. Here, we show that aged transgenic mice (i.e., 11-14 and 16-18 months old) have dystrophic sympathetic plexuses (i.e., increased densities of swollen axons), and that satellite glial cells, specifically those in contact with dystrophic plexuses in the aged mice display strong immunostaining for tumor necrosis factor alpha. The colocalization of dystrophic plexuses and reactive satellite glial cells in the aged mice coincides with degenerative features in the enveloped sensory somata. Collectively, these novel results show that, with advancing age, sympathetic plexuses undergo dystrophic changes that heighten satellite glial cell reactivity and that together these cellular events coincide with neuronal degeneration.

Pain without nociceptors? Nav1.7-independent pain mechanisms

Nav1.7, a peripheral neuron voltage-gated sodium channel, is essential for pain and olfaction in mice and humans. We examined the role of Nav1.7 as well as Nav1.3, Nav1.8, and Nav1.9 in different mouse models of chronic pain. Constriction-injury-dependent neuropathic pain is abolished when Nav1.7 is deleted in sensory neurons, unlike nerve-transection-related pain, which requires the deletion of Nav1.7 in sensory and sympathetic neurons for pain relief. Sympathetic sprouting that develops in parallel with nerve-transection pain depends on the presence of Nav1.7 in sympathetic neurons. Mechanical and cold allodynia required distinct sets of neurons and different repertoires of sodium channels depending on the nerve injury model. Surprisingly, pain induced by the chemotherapeutic agent oxaliplatin and cancer-induced bone pain do not require the presence of Nav1.7 sodium channels or Nav1.8-positive nociceptors. Thus, similar pain phenotypes arise through distinct cellular and molecular mechanisms. Therefore, rational analgesic drug therapy requires patient stratification in terms of mechanisms and not just phenotype.

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Phenotypically identical pain models have different underlying molecular mechanisms

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Nav1.7 expression is required for sympathetic sprouting after neuronal damage

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Oxaliplatin and cancer-induced bone pain are both Nav1.7-independent

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Deleting Nav1.7 in adult mice reverses nerve damage-induced neuropathic pain

Highly localized interactions between sensory neurons and sprouting sympathetic fibers observed in a transgenic tyrosine hydroxylase reporter mouse

Background

Sprouting of sympathetic fibers into sensory ganglia occurs in many preclinical pain models, providing a possible anatomical substrate for sympathetically enhanced pain. However, the functional consequences of this sprouting have been controversial. We used a transgenic mouse in which sympathetic fibers expressed green fluorescent protein, observable in live tissue. Medium and large diameter lumbar sensory neurons with and without nearby sympathetic fibers were recorded in whole ganglion preparations using microelectrodes.

Results

After spinal nerve ligation, sympathetic sprouting was extensive by 3 days. Abnormal spontaneous activity increased to 15% and rheobase was reduced. Spontaneously active cells had Aαβ conduction velocities but were clustered near the medium/large cell boundary. Neurons with sympathetic basket formations had a dramatically higher incidence of spontaneous activity (71%) and had lower rheobase than cells with no sympathetic fibers nearby. Cells with lower density nearby fibers had intermediate phenotypes. Immunohistochemistry of sectioned ganglia showed that cells surrounded by sympathetic fibers were enriched in nociceptive markers TrkA, substance P, or CGRP. Spontaneous activity began before sympathetic sprouting was observed, but blocking sympathetic sprouting on day 3 by cutting the dorsal ramus in addition to the ventral ramus of the spinal nerve greatly reduced abnormal spontaneous activity.

Conclusions

The data suggest that early sympathetic sprouting into the sensory ganglia may have highly localized, excitatory effects. Quantitatively, neurons with sympathetic basket formations may account for more than half of the observed spontaneous activity, despite being relatively rare. Spontaneous activity in sensory neurons and sympathetic sprouting may be mutually re-enforcing.

Increased excitability and spontaneous activity of rat sensory neurons following in vitro stimulation of sympathetic fiber sprouts in the isolated dorsal root ganglion

Many chronic pain conditions including complex regional pain syndrome are exacerbated by sympathetic activity. In animal models, sympathetic fibers sprout into the dorsal root ganglia (DRG) after peripheral nerve injury, forming abnormal connections with sensory neurons. However, functional studies of sympathetic-sensory connections have been limited largely to in vivo studies. This study describes a new method for studying sympathetic-sensory connections in an isolated whole DRG preparation in the rat spinal nerve ligation (SNL) model. Three days after ligation of the ventral ramus of the spinal nerve (SNL), sympathetic fibers sprouting into the DRG were observed to originate largely in the intact dorsal ramus of the spinal nerve, which at the lumbar level is a small branch of the spinal nerve separating from the ventral ramus near the intervertebral foramen. In whole DRG isolated 3 days after SNL, microelectrode recordings of sensory neurons showed that repeated stimulation of the dorsal ramus enhanced spontaneous activity in large and medium diameter neurons and reduced rheobase in large neurons. These effects, which were slow and long lasting, were attributed to stimulation of the sympathetic sprouts because: stimulation had no effect in uninjured DRG; and effects could be reduced or eliminated by a "cocktail" of antagonists of norepinephrine and ATP receptors, by pretreatment with the sympathetic release blocker bretylium, or by pre-cutting the grey ramus through which sympathetic fibers coursed to the ligated DRG. The latter treatment, a relatively minimal form of sympathectomy, was also highly effective in reducing mechanical pain ipsilateral to the SNL.

Chemokine signaling and the management of neuropathic pain

Chemokines and chemokine receptors are widely expressed in the nervous system, where they play roles in the regulation of stem cell migration, axonal path finding, and neurotransmission. Chemokine signaling is also of key importance in the regulation of neuroinflammatory responses. The expression of the chemokine monocyte chemoattractant protein 1 (MCP1) and its receptor (CCR2) is upregulated by dorsal root ganglia neurons in rodent models of neuropathic pain. MCP1 increases the excitability of nociceptive neurons after a peripheral nerve injury, and disruption of MCP1 signaling blocks the development of neuropathic pain. In the spinal cord, microglial cells expressing CCR2 are thought to play an active role in the initiation and maintenance of pain hypersensitivity, and MCP1 may also alter the excitability of spinal neurons in some cases. Other predominant sites of CCR2 activation are found in the peripheral nervous system, thereby explaining, at least in some circumstances, the rapid anti-nociceptive effects of peripherally administered CCR2 antagonists. In this article we discuss the relative roles of CCR2 activation in the peripheral and central nervous systems in relation to the phenomenon of neuropathic pain.

Delayed sympathetic dependence in the spared nerve injury (SNI) model of neuropathic pain

Background

Clinical and experimental studies of neuropathic pain support the hypothesis that a functional coupling between postganglionic sympathetic efferent and sensory afferent fibers contributes to the pain. We investigated whether neuropathic pain-related behavior in the spared nerve injury (SNI) rat model is dependent on the sympathetic nervous system.

Results

Permanent chemical sympathectomy was achieved by daily injection of guanethidine (50 mg/kg s.c.) from age P8 to P21. SNI was performed at adulthood followed by 11 weeks of mechanical and thermal hypersensitivity testing. A significant but limited effect of the sympathectomy on SNI-induced pain sensitivity was observed. The effect was delayed and restricted to cold allodynia-like behavior: SNI-related cold scores were lower in the sympathectomized group compared to the control group at 8 and 11 weeks after the nerve injury but not before. Mechanical hypersensitivity tests (pinprick and von Frey hair threshold tests) showed no difference between groups during the study period. Concomitantly, pericellular tyrosine-hydroxylase immunoreactive basket structures were observed around dorsal root ganglia (DRG) neurons 8 weeks after SNI, but were absent at earlier time points after SNI and in sham operated controls.

Conclusion

These results suggest that the early establishment of neuropathic pain-related behavior after distal nerve injury such as in the SNI model is mechanistically independent of the sympathetic system, whereas the system contributes to the maintenance, albeit after a delay of many weeks, of response to cold-related stimuli.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

NG2 proteoglycan expression in the peripheral nervous system: upregulation following injury and comparison with CNS lesions

The chondroitin sulphate proteoglycan NG2 blocks neurite outgrowth in vitro and thus may be able to inhibit axonal regeneration in the CNS. We have used immunohistochemistry to compare the expression of NG2 in the PNS, where axons regenerate, and the spinal cord, where regeneration fails. NG2 is expressed by satellite cells in dorsal root ganglia (DRG) and in the perineurium and endoneurium of intact sciatic nerves of adult rats. Endoneurial NG2-positive cells were S100-negative. Injury to dorsal roots, ventral rami or sciatic nerves had no effect on NG2 expression in DRG but sciatic nerve section or crush caused an upregulation of NG2 in the damaged nerve. Strongly NG2-positive cells in damaged nerves were S100-negative. The proximal stump of severed nerves was capped by dense NG2, which surrounded bundles of regenerating axons. The distal stump, into which axons regenerated, also contained many NG2-positive/S100-negative cells. Immunoelectron microscopy revealed that most NG2-positive cells in distal stumps had perineurial or fibroblast-like morphologies, with NG2 being concentrated at the poles of the cells in regions exhibiting microvillus-like protrusions or caveolae. Compression and partial transection injuries to the spinal cord also caused an upregulation of NG2, and NG2-positive cells and processes invaded the lesion sites. Transganglionically labelled ascending dorsal column fibres, stimulated to sprout by a conditioning sciatic nerve injury, ended in the borders of lesions among many NG2-positive processes. Thus, NG2 upregulation is a feature of the response to injury in peripheral nerves and in the spinal cord, but it does not appear to limit regeneration in the sciatic nerve.

Parasympathetic nerve fibers invade the upper dermis following sensory denervation of the rat lower lip skin

The sympathetic division of the autonomic nervous system is known to play a role in the genesis of neuropathic pain. In the skin of the rat lower lip (hairy skin), sympathetic and parasympathetic fibers normally innervate the same blood vessels in the lower dermis but do not occur in the upper dermis. However, we have shown that sympathetic fiber migration into the upper dermis occurs following mental nerve lesions (Ruocco et al. [2000] J. Comp. Neurol. 422:287-296). As sensory denervation has a dramatic effect on sympathetic fiber innervation patterns in the rat lower lip skin, we decided to investigate the possible changes in the other autonomic fiber type in the skin-the parasympathetic fiber. Sensory denervation of the rat lower lip was achieved by bilateral transection of the mental nerve, and animals were allowed to recover for 1-8 weeks. Lower lip tissue was processed for double-labeling light microscopic immunocytochemistry (ICC), using antibodies against substance P (SP), which labels a subpopulation of peptidergic sensory fibers, and against the vesicular acetycholine transporter (VAChT), as a marker for parasympathetic fibers. In sham-operated rats, SP-immunoreactive (IR) sensory fibers were found in the epidermis and upper and lower dermal regions, whereas VAChT-IR fibers were confined to the lower dermis. Mental nerve lesions induced the gradual disappearance of SP-IR fibers from all skin layers accompanied by the progressive migration of VAChT-IR fibers into the upper dermis. Cholinergic fiber migration was evident by the second week post surgery, and the ectopic innervation of the upper dermis by these fibers persisted even at the last time point studied (8 weeks) when SP-IR fibers have completely regrown. VAChT-IR fibers were observed in the upper dermis, well above the opening of the sebaceous glands into the hair follicles. These results show that considerable changes occur in the innervation patterns of parasympathetic fibers following mental nerve lesions.

Expression of gp130 and leukaemia inhibitory factor receptor subunits in adult rat sensory neurones: regulation by nerve injury

Members of the interleukin-6 (IL-6) family of cytokines have been implicated as major mediators of the response of the adult nervous system to injury. The basis for an interaction of IL-6 cytokines with adult sensory neurones has been established by analysing the levels and distribution of the two signal-transducing receptor subunits, glycoprotein 130 (gp130) and leukaemia inhibitory factor receptor (LIFR), in the dorsal root ganglion (DRG) of male adult rats before and following nerve injury. All sensory neurones express gp130-immunoreactivity (IR) in the cytoplasm and on the plasma membrane. Levels of gp130 and its intracellular distribution remained unchanged up to 14 days following sciatic nerve axotomy. LIFR-IR was largely absent from the cytoplasm and plasma membrane of sensory neurones, but confined almost exclusively to the nuclear compartment. However, following axotomy, punctate cytoplasmic LIFR-IR was detected which persisted up to 28 days following axotomy. The expression of cytoplasmic LIFR 2 days post-axotomy was proportionally greater in a subset of small diameter sensory neurones which expressed either the sensory neuropeptide CGRP or the cell surface marker isolectin B4. The coexpression of gp130 and LIFR in the same intracellular compartment following axotomy conveys potential responsiveness of injured sensory neurones to members of the IL-6 family of cytokines.

Sympathectomies lead to transient substance P-immunoreactive sensory fibre plasticity in the rat skin

Research using animal models of neuropathic pain has revealed sympathetic sprouting onto dorsal root ganglion cells. More recently, sensory fibre sprouting onto dorsal root ganglion cells has also been observed. Previous work in our laboratory demonstrated persistent sympathetic fibre sprouting in the skin of the rat lower lip following sensory denervation of this region. Therefore, we applied immunocytochemistry to determine the effects of sympathectomies on the terminal fields of sensory fibres. The superior cervical ganglia were removed bilaterally and the effects on the innervation of the skin of the rat lower lip were observed 1, 2, 3, 4, 6 and 8 weeks post-surgery. Substance P and dopamine-beta-hydroxylase immunoreactivities were used to identify a subset of sensory and sympathetic fibres, respectively. We also assessed neurokinin-1 receptor immunoreactivity. Quantitative data was obtained with the aid of an image analysis system. In controls, the epidermis and upper dermis were innervated by substance P-immunoreactive fibres only and upper dermal blood vessels possessed the highest density of neurokinin-1 receptor immunoreactivity. Blood vessels in the lower dermis were innervated by both substance P- and dopamine-beta-hydroxylase-immunoreactive fibres. Following sympathectomies, substance P-immunoreactive fibres in the epidermis and upper dermis were more intensely labelled only 1 and 2 weeks post-surgery when compared to sham controls. The length of substance P-immunoreactive fibres in this region was also increased only on the second week. Neurokinin-1 receptor immunoreactivity in the upper dermis was slightly decreased 1 and 2 weeks post-surgery. In the lower dermis, substance P-immunoreactive fibres associated with blood vessels were more intensely labelled only 1 and 2 weeks post-surgery, and at all post-surgical time points studied, blood vessels in this region were devoid of dopamine-beta-hydroxylase-immunoreactive fibres. The length of substance P-immunoreactive fibres was increased from the first to the third week post-surgery in the lower dermis. These results indicate that sympathectomies lead to transient changes in substance P-immunoreactive fibre innervation and neurokinin-1 receptor expression in rat lower lip skin. The effects are most prominent in the lower dermis probably due to a greater local concentration of nerve growth factor in this region. The plasticity of the interactions between sensory and sympathetic fibres may prove important in the regulation of skin microcirculation and in the generation of painful sensations under normal conditions or following peripheral nerve injuries.

The effect of lumbar sympathectomy in the spinal nerve ligation model of neuropathic pain

To evaluate the sympathetic dependency of pain behaviors in an animal model of neuropathic pain, the effect of surgical sympathectomy on the mechanical sensitivity of the hindpaw was examined in rats with L5 spinal nerve ligation. Mechanical sensitivity was determined by measuring foot withdrawal thresholds to mechanical stimulation with von Frey filaments applied to the base of the third or fourth toe. Tight ligation of the segmental L5 spinal nerve led to the development of mechanical hypersensitivity in the hindpaw. The effects of 2 different procedures of surgical lumbar sympathectomy on mechanical hypersensitivity were compared, limited (resection of sympathetic chain/ganglia L2 to L4 segments) and extensive (resection of L2 to L6 segments) sympathectomies. Mechanical hypersensitivity produced by L5 spinal nerve ligation was partially but significantly reduced by both sympathectomy procedures. In a separate group of rats, the L5 spinal nerve was ligated while irritating the neighboring L4 spinal nerve. This procedure produced a lesser degree of mechanical hypersensitivity, and subsequent sympathectomy had no effect on these animals. These data suggest that sympathectomy is effective in this model only when the animals show severe mechanical hypersensitivity.

Long-term changes in the distribution of galanin in dorsal root ganglia after sciatic or spinal nerve transection in rats

The neuropeptide galanin is upregulated in primary afferent and sympathetic neurones and might be involved in the development of sympathetic perineuronal baskets ("rings") following nerve injury. Galanin, calcitonin gene-related peptide and tyrosine hydroxylase have been examined immunohistochemically in dorsal root ganglia and associated roots at times up to one year after transection of either sciatic or L5 spinal nerves in adult rats. Small diameter somata containing calcitonin gene-related peptide (with or without galanin) were reduced in number, whereas galanin (and, at later times, calcitonin gene-related peptide) appeared in medium to large diameter cells after both types of lesion. Galanin also appeared in axons in grey rami and somata in lumbar paravertebral ganglia. Within dorsal root ganglia, galanin-positive axons formed perineuronal rings of two types: (i) smooth coiled axons surrounded small (< 30 microm diameter) somata from which they probably arose; these were rare after 12 weeks, particularly after a spinal nerve lesion; and (ii) varicose terminals encircled medium to large galanin-positive somata; some arose from brightly immunofluorescent somata nearby and took nearly a year to disappear. About 30% of varicose galanin-positive rings had associated calcitonin gene-related peptide-positive terminals (partly colocalized) whereas nearly 45% had associated tyrosine hydroxylase-positive terminals (partly colocalized). Synaptophysin was present in swollen axons and in some varicosities of all types. We conclude that, after peripheral nerve lesions, varicose perineuronal rings around large diameter dorsal root ganglion cells may be formed by axotomized primary afferent neurones (some containing calcitonin gene-related peptide) and sympathetic neurones, both of which contain upregulated galanin. Exocytosis from the varicosities may modify the excitability of mechanosensitive somata. Small galanin-positive somata disappear over several months after both lesions as calcitonin gene-related peptide reappears in medium to large neurones.

Sympathetic sprouting in the dorsal root ganglion after spinal nerve ligation: evidence of regenerative collateral sprouting

It is well documented that there is an increase in the number of sympathetic fibers within the dorsal root ganglion (DRG) after a peripheral nerve injury. The present study examined the numbers and distribution of sympathetic fibers in the DRG and their sprouting routes by utilizing various surgical manipulations and retrograde tracing and immunohistochemical staining methods in spinal nerve-ligated neuropathic rats. The appearance of many double immunostained fibers with antibodies to tyrosine hydroxylase (TH) and growth associated protein-43 (GAP-43) in the L5 DRG 1 week after L5 spinal nerve ligation, indicated sprouting of sympathetic fibers. The confined location of early sprouting sympathetic fibers in the distal half of the L5 DRG confirmed that sprouting fibers come primarily from the injured spinal nerve. A second cut proximal to the previously ligated L5 spinal nerve -- a process which would transect the regenerating sympathetic fibers extending from the injury site -- did not change the density of sympathetic fibers in the L5 DRG. When retrograde tracers (fast blue and diamidino yellow) were injected into the L5 spinal nerve and DRG, respectively, the number of double-labeled sympathetic postganglionic neurons was greatly increased after spinal nerve ligation, suggesting the increased number of sympathetic neurons projecting to both the spinal nerve and DRG. All these results indicate that many sympathetic fibers in the DRG are regenerating branches that are sprouting from the proximal part of the injured spinal nerve (regenerative collateral sprouting).

No sympathetic nerve sprouting in rat trigeminal ganglion following painful and non-painful infraorbital nerve neuropathy

Following sciatic nerve injury sympathetic invasion and basket formation is seen in dorsal root ganglia. We examined whether this phenomenon occurs in trigeminal ganglion (TG) following axotomy (IOAx) or chronic constriction injury to the infraorbital nerve (IOCCI). The IOCCI rats developed hyperresponsiveness to pinprick stimulation consistent with this model and the IOAx rats remained hyporesponsive for most of the study period. Immunocytochemistry employing antibodies to tyrosine hydroxylase showed no sympathetic invasion or basket formation 2 and 7 weeks post surgery. This study confirms previous work that found no sympathetic invasion of TG following injury, and shows that this finding is unaffected by the presence or absence of nerve injury induced hyperresponsiveness.

Distinct sprouting responses of sympathetic and peptidergic sensory axons proximal to a sciatic nerve transection in guinea pigs and rats

Sprouting of sympathetic and peptidergic sensory neurones proximal to nerve lesions may reflect upregulation of growth factors around damaged dorsal root ganglion (DRG) cells. Axons containing noradrenaline or calcitonin gene-related peptide were visualized in DRGs and spinal roots of guinea pigs and rats. After sciatic transection in rats, varicose terminals of both types appeared around large DRG somata. These neurones were surrounded by proliferated satellite cells expressing glial fibrillary acidic protein (GFAP) and p75. This did not occur in guinea pigs. Instead, sympathetic axons grew through the DRG and centrally along the dorsal roots (which contained p75-positive glia), avoiding the DRG somata. Thus the glial reaction to peripheral injury differs between species such that, in guinea pigs, the environment in the spinal roots rather than in the DRGs favours sympathetic sprouting.

Expression of CHL1 and L1 by neurons and glia following sciatic nerve and dorsal root injury

Cell adhesion molecules (CAMs), particularly L1, are important for axonal growth on Schwann cells in vitro. We have used in situ hybridization to study the expression of mRNAs for L1 and its close homologue CHL1, by neurons regenerating their axons in vivo, and have compared CAM expression with that of GAP-43. Adult rat sciatic nerves were crushed (allowing functional regeneration), or cut and ligated to maintain axonal sprouting but prevent reconnection with targets. In other animals lumbar dorsal roots were transected to produce slow regeneration of the central axons of sensory neurons. In unoperated animals L1 and CHL1 mRNAs were expressed at moderate levels by small- to medium-sized sensory neurons and L1 mRNA was expressed at moderate levels by motor neurons. Many large sensory neurons expressed neither L1 nor CHL1 mRNAs and motor neurons expressed little or no CHL1 mRNA. Neither motor nor sensory neurons showed any obvious upregulation of L1 mRNA after axotomy. Increased CHL1 mRNA was found in motor neurons and small- to medium-sized sensory neurons 3 days to 2 weeks following sciatic nerve crush, declining toward control levels by 5 weeks when regeneration was complete. Cut and ligation injuries caused a prolonged upregulation of CHL1 mRNA (and GAP-43 mRNA), indicating that reconnection with target tissues may be required to signal the return to control levels. Large sensory neurons did not upregulate CHL1 mRNA after axotomy and thus regenerated within the sciatic nerve without producing CHL1 or L1. Dorsal root injuries caused a modest, slow upregulation of CHL1 mRNA by some sensory neurons. CHL1 mRNA was also upregulated by many presumptive Schwann cells in injured nerves and by some satellite cells around large sensory neurons after sciatic nerve injuries and was transiently upregulated by some astrocytes in the degenerating dorsal columns after dorsal rhizotomy.

Peripheral nerve injury leads to the establishment of a novel pattern of sympathetic fibre innervation in the rat skin

Peripheral nerve injury has been shown to result in sympathetic fibre sprouting around dorsal root ganglia (DRG) neurons. It has been suggested that this anomalous sympathetic fibre innervation of the DRG plays a role in neuropathic pain. Other studies have suggested an interaction between sympathetic and sensory fibres more peripherally. To date, no anatomical study of these possible interactions in the terminal fields of sensory and sympathetic fibres has been performed; therefore, the authors set out to study them in the rat lower lip after bilateral lesions of a sensory nerve, the mental nerve (MN). Immunocytochemistry for both substance P (SP) and dopamine-beta-hydroxylase (DbetaH) was performed. Within the first week post-MN lesions, the SP-immunoreactive (IR) fibres had degenerated almost completely, whereas DbetaH-IR fibres were found in the upper dermis, an area from which they normally are absent. These DbetaH-IR fibres were present in the upper dermis at all postsurgery times studied (1, 2, 3, 4, 6, and 8 weeks). It is noteworthy that, although, by week 6 post-MN lesions, SP-IR fibre reinnervation of the lower lip was occurring, the DbetaH-IR fibres still were present in the upper dermis. Quantification revealed that the migration and branching of the DbetaH-IR fibres into the upper dermis occurred gradually and was most significant at 4 weeks post-MN lesions, as demonstrated by the fact that the DbetaH-IR fibres were found 169.6 +/- 91.4 microm away from the surface of the skin compared with 407.1 +/- 78.4 microm away in sham-operated animals. These findings suggest that the ectopic innervation of the upper dermis by sympathetic fibres may be important in the genesis of neuropathic pain through the interactions of sympathetic and SP-containing sensory fibres.

Partial sciatic nerve transection induced tyrosine hydroxidase immunoreactive axon sprouting around both injured and spared dorsal root ganglion neurons which project to the gracile nucleus in middle-aged rats

After partial sciatic nerve transection (PSNT), sympathetic axons sprout into the lumbar dorsal root ganglia (DRG), a phenomenon implicated in neuropathic pain. We asked whether sympathetic sprouting is directed to injured or spared DRG neurons and whether these neurons project to the gracile nucleus. Using combined fluorescent dye tracing and tyrosine hydroxylase (TH) immunohistochemistry, we found that 4 weeks after PSNT in rats 8-10 months old, 51% of the neurons surrounded by TH-immunoreactive (IR) axons were spared, while 43% were injured. Seventy-nine percent projected to the gracile nucleus. Sympathetic sprouting induced by PSNT is not directed preferentially to injured or spared DRG neurons, but does show a preference for DRG neurons projecting to the gracile nucleus.

Sympathetic nerve sprouting fails to occur in the trigeminal ganglion after peripheral nerve injury in the rat

Peripheral nerve injury induces sprouting of sympathetic nerve fibers in dorsal root ganglia after spinal nerve injury. In the present study, we sought to determine the extent of intraganglionic noradrenergic sprouting in the trigeminal system. The inferior alveolar nerve, a major branch of the mandibular division, or the infraorbital nerve of the maxillary division was either ligated or chronically constricted in Sprague-Dawley rats and recovery permitted for either 2-3 or 6-9 weeks. In some animals both nerves were injured. Using immunohistochemistry with tyrosine hydroxylase antibodies, we found no signs of sympathetic nerve fiber sprouting in the trigeminal ganglion after injury. In contrast, sciatic nerve injury in rat littermates induced a widespread autonomic nerve outgrowth in affected DRGs. Thus, sensory ganglion sympathetic nerve sprouting does not seem to be a general outcome of PNS injury, but is restricted to certain specific locations. Sympathetic nerve fiber networks that surround primary sensory neurons have been suggested to form a structural basis for interactions between the sympathetic and sensory nervous systems after PNS injury. Such interactions, sometimes resulting in paraesthesia or dysaesthesia in patients, appear to be less common in territories innervated by the trigeminal nerve than in spinal nerve regions. The lack of injury-induced intraganglionic sympathetic sprouting in the trigeminal ganglion may help to explain this observation.

Causes and consequences of sympathetic basket formation in dorsal root ganglia

Injury to peripheral nerves can result in severe and intractable neuropathic pain, and in some cases the symptoms are sympathetically maintained. In recent years much effort has been put into elucidating the anatomical nature of nerve injury-induced sympathetic-sensory coupling. The demonstration of sympathetic sprouting into dorsal root ganglia (DRG) of nerve-injured rats has led to the suggestion that this phenomenon might underlie sympathetically-maintained pain. As a result, several studies have been undertaken to determine what factor or factors are responsible for the sprouting, and for the formation of abnormal sympathetic terminal arborizations or 'baskets' around some DRG neurons. In this review we examine in particular the roles of nerve growth factor (NGF) and the cytokines leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), as these have all been shown to contribute to sympathetic sprouting. We also stress the role of satellite cells within axotomized DRG, as these have been shown to express not only neurotrophin mRNA, but also the low-affinity neurotrophin receptor p75. We propose a mechanism for sympathetic sprouting in the DRG involving; (i) the activation of satellite cells on the DRG by a factor such as LIF or IL-6, and (ii) the generation of a sympathetic axon-guiding gradient by p75-bound neurotrophins on the activated satellite cells. We also highlight the possibility that a sympathetic sprouting signal may be derived from the periphery, as NGF, LIF and IL-6 are all produced as a result of Wallerian degeneration, and can be retrograde transported to the DRG. The possible relevance of sympathetic sprouting in the DRG to neuropathic pain is also discussed.

Adrenergic innervation of rat sensory ganglia following proximal or distal painful sciatic neuropathy: distinct mechanisms revealed by anti-NGF treatment

Sympathetic axons invade dorsal root ganglia (DRG) following nerve injury, and activity in the resulting pericellular axonal 'baskets' may underlie painful sympathetic-sensory coupling. Sympathetic sprouting into the DRG may be stimulated by nerve growth factor (NGF). To test this hypothesis, we investigated the effect of daily anti-NGF administration on pain and on sprouting in the DRG induced by chronic sciatic constriction injury (CCI) or L5 spinal nerve ligation (SNL). These models have been shown to differ subtly in the onset of pain behaviours and adrenergic sprouting, and we now demonstrate a fundamental difference in the way sympathetic axons invade the DRG: after CCI, perivascular noradrenergic collaterals sprouted into the DRG in a manner dependent upon peripherally derived NGF. In contrast, after SNL, regenerating sympathetic axons were diverted towards the DRG from the spinal nerve by the obstructing ligature, and this effect was only moderately impeded by anti-NGF. The differential dependence on anti-NGF suggests that adrenergic innervation of the DRG after SNL and CCI may reflect regenerative and collateral sprouting, respectively. Pain behaviour was similarly affected: anti-NGF completely prevented CCI-induced thermal hyperalgesia and mechanoallodynia, but the same treatment only partly relieved these symptoms following SNL. These differences emphasize that although CCI and SNL may result in similar behavioural abnormalities, the underlying mechanisms may be governed by distinct processes, differentially dependent on peripheral NGF. These mechanistic differences will have to be considered in the development of appropriate treatment strategies for neuropathic pain produced by different types of pathology.

Glial overexpression of NGF enhances neuropathic pain and adrenergic sprouting into DRG following chronic sciatic constriction in mice

Adrenergic sprouts within axotomized dorsal root ganglia (DRG) may contribute to neuropathic pain, and may arise under the influence of nerve growth factor (NGF). We investigated effects of chronic constriction injury (CCI) on behavior and sprouting in mice in which NGF overexpression is driven by a glial protein (GFAP) promotor. GFAP-NGF mice were naturally hyperresponsive to radiant heat, and had enhanced ipsilateral responses to thermal and mechanical stimulation following CCI compared to wild-type mice. Sympathetic axons were already present in intact DRG of GFAP-NGF mice. Following CCI, sprouting in ipsilateral and to a lesser extent contralateral DRG occurred in both genotypes, but the sprout density 2 weeks post-lesion was much greater in GFAP-NGF mice. These results demonstrate a connection between the endogenous ectopic overexpression of NGF and (1) neuropathic pain behaviour and (2) sympathetic sprouting in the DRG.