Is sympathetic sprouting in the dorsal root ganglia responsible for the production of neuropathic pain in a rat model?

Sexually Dimorphic Pattern of Pain Mitigation Following Prophylactic Regenerative Peripheral Nerve Interface (RPNI) in a Rat Neuroma Model

BACKGROUND

Treating neuroma pain is a clinical challenge. Identification of sex-specific nociceptive pathways allows a more individualized pain management. The Regenerative Peripheral Nerve Interface (RPNI) consists of a neurotized autologous free muscle using a severed peripheral nerve to provide physiological targets for the regenerating axons.

OBJECTIVE

To evaluate prophylactic RPNI to prevent neuroma pain in male and female rats.

METHODS

F344 rats of each sex were assigned to neuroma, prophylactic RPNI, or sham groups. Neuromas and RPNIs were created in both male and female rats. Weekly pain assessments including neuroma site pain and mechanical, cold, and thermal allodynia were performed for 8 weeks. Immunohistochemistry was used to evaluate macrophage infiltration and microglial expansion in the corresponding dorsal root ganglia and spinal cord segments.

RESULTS

Prophylactic RPNI prevented neuroma pain in both sexes; however, female rats displayed delayed pain attenuation when compared with males. Cold allodynia and thermal allodynia were attenuated exclusively in males. Macrophage infiltration was mitigated in males, whereas females showed a reduced number of spinal cord microglia.

CONCLUSION

Prophylactic RPNI can prevent neuroma site pain in both sexes. However, attenuation of both cold allodynia and thermal allodynia occurred in males exclusively, potentially because of their sexually dimorphic effect on pathological changes of the central nervous system.

Orofacial Pain in Cancer: Part I—Mechanisms

The mechanisms involved, and possible treatment targets, in orofacial pain due to cancer are poorly understood. The aim of the first of this two-part series is to review the involved pathophysiological mechanisms and explore their possible roles in the orofacial region. However, there is a lack of relevant research in the trigeminal region, and we have therefore applied data accumulated from experiments on cancer pain mechanisms in rodent spinal models. In the second part, we review the clinical presentation of cancer-associated orofacial pain at various stages: initial diagnosis, during therapy (chemo-, radiotherapy, surgery), and in the post-therapy period. In the present article, we provide a brief outline of trigeminal functional neuro-anatomy and pain-modulatory pathways. Tissue destruction by invasive tumors (or metastases) induces inflammation and nerve damage, with attendant acute pain. In some cases, chronic pain, involving inflammatory and neuropathic mechanisms, may ensue. Distant, painful effects of tumors include paraneoplastic neuropathic syndromes and effects secondary to the release of factors by the tumor (growth factors, cytokines, and enzymes). Additionally, pain is frequent in cancer management protocols (surgery, chemotherapy, and radiotherapy). Understanding the mechanisms involved in cancer-related orofacial pain will enhance patient management.

Long-term follow-up of peptidergic and nonpeptidergic reinnervation of the epidermis following sciatic nerve reconstruction in rats

OBJECT

Peripheral nerve injuries are a commonly encountered clinical problem and often result in long-term functional deficits. The current gold standard for transected nerves is an end-to-end reconstruction, which results in the intermittent appearance of neuropathic pain.

METHODS

To improve our understanding of the relation between this type of reconstruction and neuropathic pain, the authors transected and immediately end-to-end reconstructed the sciatic nerve in rats. The effect of this procedure on neuropathic pain, as measured by thermal and mechanical hypersensitivity at 4 different time points (5, 10, 20, and 30 weeks), was related to the density of peptidergic and nonpeptidergic fiber innervation in the glabrous skin of rats' hind paws.

RESULTS

Thermal hypersensitivity occurring 20 weeks after reconstruction was accompanied by a significant increase in peptidergic epidermal fibers. However, the lesion-induced reduction in the density of nonpeptidergic epidermal fibers remained decreased at all experimental time points. Moreover, temporal collateral sprouting by undamaged saphenous nerve was visualized using the recently revised Evans blue extravasation technique. Strikingly, as the sciatic nerve repopulated rats' hind paw, the saphenous nerve withdrew to its original territory.

CONCLUSIONS

The authors conclude that the transient thermal hypersensitivity is related to increased density of epidermal peptidergic fibers, which mainly originate from regenerating fibers. Furthermore, a changed composition in the peptidergic and nonpeptidergic epidermal fibers is demonstrated following end-to-end reconstruction of the sciatic nerve.

Alpha-2 adrenergic stimulation triggers Achilles tenocyte hypercellularity: Comparison between two model systems

The histopathology of tendons with painful tendinopathy is often tendinosis, a fibrosis-like condition of unclear pathogenesis characterized by tissue changes including hypercellularity. The primary tendon cells (tenocytes) have been shown to express adrenoreceptors (mainly alpha-2A) as well as markers of catecholamine production, particularly in tendinosis. It is known that adrenergic stimulation can induce proliferation in other cells. The present study investigated the effects of an exogenously administered alpha-2 adrenergic agonist in an established in vivo Achilles tendinosis model (rabbit) and also in an in vitro human tendon cell culture model. The catecholamine producing enzyme tyrosine hydroxylase and the alpha-2A-adrenoreceptor (α_2A AR) were expressed by tenocytes, and alpha-2 adrenergic stimulation had a proliferative effect on these cells, in both models. The proliferation was inhibited by administration of an α_2A AR antagonist, and the in vitro model further showed that the proliferative alpha-2A effect was mediated via a mitogenic cell signaling pathway involving phosphorylation of extracellular-signal-regulated kinases 1 and 2. The results indicate that catecholamines produced by tenocytes in tendinosis might contribute to the proliferative nature of the pathology through stimulation of the α_2A AR, pointing to a novel target for future therapies. The study furthermore shows that animal models are not necessarily required for all aspects of this research.

Analgesic effect of highly reversible ω-conotoxin FVIA on N type Ca2+ channels

Background

N-type Ca²⁺ channels (Ca_v2.2) play an important role in the transmission of pain signals to the central nervous system. ω-Conotoxin (CTx)-MVIIA, also called ziconotide (Prialt^®), effectively alleviates pain, without causing addiction, by blocking the pores of these channels. Unfortunately, CTx-MVIIA has a narrow therapeutic window and produces serious side effects due to the poor reversibility of its binding to the channel. It would thus be desirable to identify new analgesic blockers with binding characteristics that lead to fewer adverse side effects.

Results

Here we identify a new CTx, FVIA, from the Korean Conus Fulmen and describe its effects on pain responses and blood pressure. The inhibitory effect of CTx-FVIA on N-type Ca²⁺ channel currents was dose-dependent and similar to that of CTx-MVIIA. However, the two conopeptides exhibited markedly different degrees of reversibility after block. CTx-FVIA effectively and dose-dependently reduced nociceptive behavior in the formalin test and in neuropathic pain models, and reduced mechanical and thermal allodynia in the tail nerve injury rat model. CTx-FVIA (10 ng) also showed significant analgesic effects on writhing in mouse neurotransmitter- and cytokine-induced pain models, though it had no effect on acute thermal pain and interferon-γ induced pain. Interestingly, although both CTx-FVIA and CTx-MVIIA depressed arterial blood pressure immediately after administration, pressure recovered faster and to a greater degree after CTx-FVIA administration.

Conclusions

The analgesic potency of CTx-FVIA and its greater reversibility could represent advantages over CTx-MVIIA for the treatment of refractory pain and contribute to the design of an analgesic with high potency and low side effects.

Brain-derived neurotrophic factor mediates non-cell-autonomous regulation of sensory neuron position and identity

During development, neurons migrate considerable distances to reside in locations that enable their individual functional roles. Whereas migration mechanisms have been extensively studied, much less is known about how neurons remain in their ideal locations. We sought to identify factors that maintain the position of postmigratory dorsal root ganglion neurons, neural crest derivatives for which migration and final position play an important developmental role. We found that an early developing population of sensory neurons maintains the position of later born dorsal root ganglia neurons in an activity-dependent manner. Further, inhibiting or increasing the function of brain-derived neurotrophic factor induces or prevents, respectively, migration of dorsal root ganglia neurons out of the ganglion to locations where they acquire a new identity. Overall, the results demonstrate that neurotrophins mediate non-cell-autonomous maintenance of position and thereby the identity of differentiated neurons.

In vivo evidence for transdifferentiation of peripheral neurons

It is commonly thought that differentiated neurons do not give rise to new cells, severely limiting the potential for regeneration and repair of the mature nervous system. However, we have identified cells in zebrafish larvae that first differentiate into dorsal root ganglia sensory neurons but later acquire a sympathetic neuron phenotype. These transdifferentiating neurons are present in wild-type zebrafish. However, they are increased in number in larvae that have a mutant voltage-gated sodium channel gene, scn8aa. Sodium channel knock-down promotes migration of differentiated sensory neurons away from the ganglia. Once in a new environment, sensory neurons transdifferentiate regardless of sodium channel expression. These findings reveal an unsuspected plasticity in differentiated neurons that points to new strategies for treatment of nervous system disease.

rTMS for suppressing neuropathic pain: a meta-analysis

This pooled individual data (PID)-based meta-analysis collectively assessed the analgesic effect of repetitive transcranial magnetic stimulation (rTMS) on various neuropathic pain states based on their neuroanatomical hierarchy. Available randomized controlled trials (RCTs) were screened. PID was coded for age, gender, pain neuroanatomical origins, pain duration, and treatment parameters analyses. Coded pain neuroanatomical origins consist of peripheral nerve (PN); nerve root (NR); spinal cord (SC); trigeminal nerve or ganglion (TGN); and post-stroke supraspinal related pain (PSP). Raw data of 149 patients were extracted from 5 (1 parallel, 4 cross-over) selected (from 235 articles) RCTs. A significant (P < .001) overall analgesic effect (mean percent difference in pain visual analog scale (VAS) score reduction with 95% confidence interval) was detected with greater reduction in VAS with rTMS in comparison to sham. Including the parallel study (Khedr et al), the TGN subgroup was found to have the greatest analgesic effect (28.8%), followed by PSP (16.7%), SC (14.7%), NR (10.0%), and PN (1.5%). The results were similar when we excluded the parallel study with the greatest analgesic effect observed in TGN (33.0%), followed by SC (14.7%), PSP (10.5%), NR (10.0%), and PN (1.5%). In addition, multiple (vs single, P = .003) sessions and lower (>1 and < or =10 Hz) treatment frequency range (vs >10 Hz) appears to generate better analgesic outcome. In short, rTMS appears to be more effective in suppressing centrally than peripherally originated neuropathic pain states.

PERSPECTIVE

This is the first PID-based meta-analysis to assess the differential analgesic effect of rTMS on neuropathic pain based on the neuroanatomical origins of the pain pathophysiology and treatment parameters. The derived information serves as a useful resource in regards to treatment parameters and patient population selection for future rTMS-pain studies.

Sympathetic sprouting near sensory neurons after nerve injury occurs preferentially on spontaneously active cells and is reduced by early nerve block

Some chronic pain conditions are maintained or enhanced by sympathetic activity. In animal models of pathological pain, abnormal sprouting of sympathetic fibers around large- and medium-sized sensory neurons is observed in dorsal root ganglia (DRGs). Large- and medium-sized cells are also more likely to be spontaneously active, suggesting that sprouting may be related to neuron activity. We previously showed that sprouting could be reduced by systemic or locally applied lidocaine. In the complete sciatic nerve transection model in rats, spontaneous activity initially originates in the injury site; later, the DRGs become the major source of spontaneous activity. In this study, spontaneous activity reaching the DRG soma was reduced by early nerve blockade (local perfusion of the transected nerve with TTX for the 1st 7 days after injury). This significantly reduced sympathetic sprouting. Conversely, increasing spontaneous activity by local nerve perfusion with K(+) channel blockers increased sprouting. The hyperexcitability and spontaneous activity of DRG neurons observed in this model were also significantly reduced by early nerve blockade. These effects of early nerve blockade on sprouting, excitability, and spontaneous activity were all observed 4-5 wk after the end of early nerve blockade, indicating that the early period of spontaneous activity in the injured nerve is critical for establishing the more long-lasting pathologies observed in the DRG. Individual spontaneously active neurons, labeled with fluorescent dye, were five to six times more likely than quiescent cells to be co-localized with sympathetic fibers, suggesting a highly localized correlation of activity and sprouting.

Efficacy of Stellate Ganglion Blockade for the Management of Type 1 Complex Regional Pain Syndrome

INTRODUCTION

The purpose of this study was to examine the efficacy of stellate ganglion blockade (SGB) in patients with complex regional pain syndromes (CRPS I) of their hands.

METHODS

After IRB approval and patient informed consent, 25 subjects, with a clinical diagnosis of CRPS I of one hand as defined by the International Association for the Study of Pain (IASP) criteria, had three SGB's performed at weekly intervals. Laser Doppler fluxmetric hand perfusion studies were performed on the normal and CRPS I hands pre- and post-SGB therapy. No patient was included in this study if they used tobacco products or any medication or substance that could affect sympathetic function. The appropriate parametric and nonparametric data analyses were performed and a p value <0.05 was used to reject the null hypothesis.

RESULTS

Symptom onset of CRPS I until the initiation of SGB therapy ranged between 3 to 34 weeks. Following the SGB series, patient pain relief was as follows: group I, 10/25 (40%) had complete symptom relief; group II, 9/25 (36%) had partial relief and group III, 6/25 (24%) had no relief. The duration of symptoms until SGB therapy was: group I, 4.6 +/- 1.8 weeks, group II, 11.9 +/- 1.6 weeks and group III, 35.8 +/- 27 weeks. Compared with the normal control hand, the skin perfusion in the CRPS I affected hand was greater in group I and decreased in groups II and III.

DISCUSSION

The results of our study demonstrate that an inverse relationship exists between hand perfusion and the duration of symptoms of CRPS I. On the other hand, a positive correlation exists between SGB efficacy and how soon SGB therapy is initiated. A duration of symptoms greater than 16 weeks before the initial SGB and/or a decrease in skin perfusion of 22% between the normal and affected hands adversely affects the efficacy of SGB therapy.

Individual differences in the sensitivity of cold allodynia to phentolamine in neuropathic rats

In neuropathic rats sensitive to phentolamine (alpha-adrenoreceptor antagonist, 2 mg/kg, i.p.), prazosin (alpha1-adrenoreceptor antagonist, 0.5 mg/kg, i.p.) significantly attenuated cold allodynia whereas yohimbine (alpha2-adrenoreceptor antagonist, 0.5 mg/kg, i.p.) had no significant effect. In neuropathic rats insensitive to phentolamine, yohimbine significantly exacerbated cold allodynia whereas prazosin had no significant effect. These results suggest that the individual differences in the sensitivity of cold allodynia to phentolamine may be due to the difference in the alpha-adrenoreceptor subtype predominantly involved in cold allodynia.

Electrophysiologic characterization of the antinociceptive actions of S18616, a novel and potent alpha2-adrenoceptor agonist, after acute and persistent pain states

alpha (2)-Adrenoceptor (AR) agonists are active in behavioral models of persistent pain involving tissue and nerve damage. We evaluated the spinal effect of a novel, potent, and selective alpha (2)-AR agonist, [7,8](2-chlorobenzo)-2-amino-1-aza-3-oxa[4,5]spirodeca-1,7-diene (S18616), on the responses of dorsal horn neurons in halothane-anesthetized rats. Intrathecal administration of S18616 (0.1 to 3.0 microg) dose-dependently suppressed C- and A delta-fiber evoked responses but not the A beta-fiber evoked response. Drug effects were reversed by the alpha (2)-AR antagonists, atipamezole and idazoxan (100 microg). In rats with unilateral spinal nerve (L5-L6) ligation performed 2 weeks before study, S18616 (0.1 to 3.0 microg) dose-dependently suppressed the C- and A delta-fiber evoked responses and blocked "wind-up" in these neurons. The potency was comparable between nerve-injured and sham-operated rats, and S18616 was equally effective against responses to thermal and high-intensity mechanical stimuli. Interestingly, the effectiveness of S18616 on the low-intensity mechanical evoked response was significantly enhanced after nerve injury. Finally, S18616 (0.3 and 3.0 microg) reduced the neuronal responses produced by intraplantar injection of formalin. In conclusion, S18616 dose-dependently and potently inhibits the responses of dorsal horn neurons to peripheral stimulation in normal, inflamed, and neuropathic rats. These data support the use of spinal S18616 and other alpha (2)-AR agonists in the management of clinical pain.

Sympathetic sprouting in sensory ganglia depends on the number of injured neurons

We examined whether the extent of sympathetic sprouting in the dorsal root ganglion was a function of the number of injured nerve fibers. We compared two groups of rats. One group was subjected to unilateral superior and inferior caudal trunk transections at the level between the S1 and S2 spinal nerves (S-I group) and the other group was subjected to unilateral superior caudal trunk transection at the same level (S group). Immunohistochemical staining with tyrosine hydroxylase (TH) antibody of the S1 DRG revealed that the degree of TH-immunoreactive fibers was more extensive in the S-I group than in the S group. However, there was no difference in the severity of neuropathic pain behaviors between the two groups. These results suggest that the extent of sympathetic sprouting in the DRG following peripheral nerve injury is proportionally related to the amount of injured nerve fibers, but not related to the degree of neuropathic pain behaviors.

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.

Decrease in spinal CGRP and substance P is not related to neuropathic pain in a rat model

We tested the hypothesis that the decrease in spinal levels of SP and CGRP after peripheral nerve injury is related to neuropathic pain. We compared two groups of rats, both of which were subjected to unilateral transection of the inferior and superior caudal trunks between the S1 and S2 spinal nerves. One group exhibited well-developed neuropathic signs after the nerve injury, whereas the other group showed poorly developed signs despite the same nerve injury. The decrease in immunoreactivity of CGRP and SP in the S1 dorsal horn (injured segment) was not significantly different between the two groups. These results suggest that the decrease in spinal levels of CGRP and SP after peripheral nerve injury is not related to neuropathic pain.

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.

Novel drugs for neuropathic pain

Neuropathic pain is often inadequately controlled by conventional analgesics. Because the aetiology of neuropathic pain is only partially understood, specific treatment approaches have not been defined. A variety of pharmacological treatments have been proposed. However, for only a small minority of drugs used in neuropathic pain has the scientific evidence been evaluated in a satisfactory manner. The present review of the recent literature reveals the potential of certain novel drugs in treatment of neuropathic pain.

Sympathetic-sensory coupling after L5 spinal nerve lesion in the rat and its relation to changes in dorsal root ganglion blood flow

Transection of the L5 spinal nerve in rats results in allodynia- and hyperalgesia-like behavior to mechanical stimulation which are thought to be mediated by ectopic activity arising in lesioned afferent neurons mainly in the dorsal root ganglion (DRG). It has been suggested that the neuropathic pain behavior is dependent on the sympathetic nervous system. In rats 3-56 days after L5 spinal nerve lesion, we tested responses of axotomized afferent fibers recorded in the dorsal root of the lesioned segment to norepinephrine (NE, 0.5 microg/kg) injected intravenously and to selective electrical stimulation of the lumbar sympathetic trunk (LST). In some experiments we measured blood flow in the DRG by laser Doppler flowmetry. The majority of lesioned afferent fibers with spontaneous activity responded to neither LST stimulation (82.4%) nor NE (71.4%). In those which did react to LST stimulation, responses occurred only at high stimulation frequencies (likely to be above the physiological range), and they could be mimicked by non-adrenergic vasoconstrictor drugs (angiotensin II, vasopressin). Excitatory responses to LST stimulation were closely correlated with the stimulation-induced phasic vasoconstrictions in the DRG. We therefore hypothesized that the activation of lesioned afferents might be brought about indirectly by an impaired blood supply to the DRG. To test this hypothesis we induced a strong and sustained baseline vasoconstriction in the DRG by blocking endothelial nitric oxide synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME) applied systemically. L-NAME enhanced baseline vascular resistance in the DRG about threefold and also increased stimulation-induced vasoconstrictions. After L-NAME, the majority of axotomized neurons with spontaneous activity were activated by LST stimulation (76%) or NE (75%). Again, activations closely followed stimulation-induced phasic vasoconstrictions in the DRG provided that a critical level of vasoconstriction was exceeded. In the present study, inhibitory responses to LST stimulation were generally rare and could be reversed to activation by prolonged stimulation or after L-NAME. These results show that sympathetic-sensory coupling occurs only in a minority of axotomized afferents after L5 spinal nerve injury. Like previous studies, they cast doubt on the notion that the L5 spinal nerve lesion is a good model for sympathetically maintained pain. Since responses of lesioned afferent neurons to LST stimulation and NE could be provoked with high reliability after inducing vasoconstriction in the DRG, and since they mirrored stimulation-induced vasoconstrictions in the DRG, it appears that in this model the association of sympathetic activity with afferent discharge occurs mainly when perfusion of the DRG is impaired.

Role of signals from the dorsal root ganglion in neuropathic pain in a rat model

We examined whether signals from the neuroma or the dorsal root ganglion of the injured segment are critical for the generation of neuropathic pain. To this aim, we used a rat model of peripheral neuropathy made by transecting the inferior and superior caudal trunks at the level between the S1 and S2 spinal nerves under enflurane anesthesia. These animals displayed tail-withdrawal responses to normally innocuous mechanical stimulation applied to the tail with a von Frey hair (2 g). Also, these animals, compared to pre-surgical value, displayed shorter tail-withdrawal latencies following immersion of the tail to warm (40 degrees C) or cold (4 degrees C) water. Transection of the S1 spinal nerve between the dorsal root ganglion and neuroma did not change the behavioral signs of neuropathic pain. In contrast, S1 dorsal rhizotomy significantly reduced the behavioral signs. The data suggest that signals arising from the dorsal root ganglion cells of the injured segment, but not from the neuroma, are critical for the generation of neuropathic pain in this model.