Influence of painful chronic neuropathy on neurogenic inflammation

An Index Combining Lost and Remaining Nerve Fibers Correlates with Pain Hypersensitivity in Mice

Multiple peripheral nerves are known to degenerate after nerve compression injury but the correlation between the extent of nerve alteration and pain severity remains unclear. Here, we used intravital two-photon fluorescence microscopy to longitudinally observe changes in cutaneous fibers in the hind paw of Nav1.8-Cre-tdTomato mice after chronic constriction injury (CCI). Results showed that the CCI led to variable loss of the skin nerve plexus and intraepidermal nerve fibers. The timing of Nav1.8 nerve fiber loss correlated with the development of mechanical hypersensitivity. We compared a scoring approach that assessed whole-paw nerve degeneration with an index that quantified changes in the nerve plexus and terminals in multiple small regions of interest (ROI) from intravital images of the third and fifth toe tips. We found that the number of surviving nerve fibers was not linearly correlated with mechanical hypersensitivity. On the contrary, at 14 days after CCI, the moderately injured mice showed greater mechanical hypersensitivity than the mildly or severely injured mice. This indicates that both surviving and injured nerves are required for evoked neuropathic pain. In addition, these two methods may have the estimative effect as diagnostic and prognostic biomarkers for the assessment of neuropathic pain.

Anti-hyperalgesic and anti-nociceptive potentials of standardized grape seed proanthocyanidin extract against CCI-induced neuropathic pain in rats

BACKGROUND

Neuropathic pain is associated with severe chronic sensory disturbances characterized by spontaneous pain, increased responsiveness to painful stimuli (hyperalgesia) and pain perceived in response to non-noxious stimuli (allodynia). Morphine is effective treatment for neuropathic pain but produces tolerance on chronic use. The present study was designed to explore the anti-nociceptive and anti-hyperalgesic effect of grape seed extract using sciatic nerve ligation-induced neuropathic pain in rats.

METHODS

Chronic constructive injury (CCI) was performed under anesthesia, on one side leg exposed by making a skin incision, and chromic gut ligatures were tied loosely around the sciatic nerve at 1 mm intervals. The treatment with grape seed proanthocyanidin extract (GSPE) (100 and 200 mg/kg, p.o.) was initiated on 7th day post-surgery and continued for next 14 days. Morphine (10 mg/kg, s.c.) alone and morphine in combination with GSPE (100 mg/kg, p.o.) were administered in CCI rats for 5 days starting from 7th day. On 3rd, 7th, 14th and 21st day, behavioral parameters (mechanical allodynia and thermal hyperalgesia) were assessed. Then the animals were killed on 22nd day and biochemical parameters [reduced glutathione (GSH), lipid peroxidation (LPO), catalase, nitrite, superoxide dismutase (SOD)] were assessed.

RESULTS

Ligation of the sciatic nerve significantly induced mechanical allodynia and thermal hyperalgesia and induces oxidative stress (increase in LPO and nitrite) and decline of anti-oxidant enzyme levels (catalase, SOD, GSH) in sciatic nerve homogenate. GSPE (100 and 200 mg/kg, p.o.) attenuated all the behavioural and biochemical parameters. Morphine also significantly reversed the symptoms of neuropathic pain but produced tolerance after 5 days. Further, co-treatment of GSPE (100 mg/kg) with morphine (10 mg/kg, s.c.) in CCI rats significantly reversed the morphine tolerance and enhanced its anti-hyperalgesic effect as compared to the morphine-alone-treated group.

CONCLUSIONS

In the present set of experiments, GSPE showed a significant anti-hyperalgesic and anti-nociceptive effect in rats.

Peripheral NMDA Receptors Mediate Antidromic Nerve Stimulation-Induced Tactile Hypersensitivity in the Rat

We investigated the role of peripheral NMDA receptors (NMDARs) in antidromic nerve stimulation-induced tactile hypersensitivity outside the skin area innervated by stimulated nerve. Tetanic electrical stimulation (ES) of the decentralized L5 spinal nerve, which induced enlargement of plasma extravasation, resulted in tactile hypersensitivity in the L4 plantar dermatome of the hind-paw. When intraplantar (i.pl.) injection was administered into the L4 dermatome before ES, NMDAR and group-I metabotropic Glu receptor (mGluR) antagonists and group-II mGluR agonist but not AMPA/kainate receptor antagonist prevented ES-induced hypersensitivity. I.pl. injection of PKA or PKC inhibitors also prevented ES-induced hypersensitivity. When the same injections were administered after establishment of ES-induced hypersensitivity, hypersensitivity was partially reduced by NMDAR antagonist only. In naïve animals, i.pl. Glu injection into the L4 dermatome induced tactile hypersensitivity, which was blocked by NMDAR antagonist and PKA and PKC inhibitors. These results suggest that the peripheral release of Glu, induced by antidromic nerve stimulation, leads to the expansion of tactile hypersensitive skin probably via nociceptor sensitization spread due to the diffusion of Glu into the skin near the release site. In addition, intracellular PKA- and PKC-dependent mechanisms mediated mainly by NMDAR activation are involved in Glu-induced nociceptor sensitization and subsequent hypersensitivity.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Effects of removal of dietary polyunsaturated fatty acids on plasma extravasation and mechanical allodynia in a trigeminal neuropathic pain model

Background

Neuropathic pain (NP) is partially mediated by neuroinflammatory mechanisms, and also modulates local neurogenic inflammation. Dietary lipids, in particular the total amount and relative proportions of polyunsaturated fatty acids (PUFAs) of the ω-3 and ω-6 families, have been reported to modify the threshold for thermal and mechanical allodynia in the partial sciatic nerve ligation model of NP in rats. The effects of dietary lipids on other popular NP models, such as the chronic constriction injury (CCI), have not yet been examined. It is also unknown whether dietary PUFAs exert any effect on the capsaicin (CAP)-induced neurogenic inflammation under control or NP conditions. In this study we investigated these interrelated phenomena in the trigeminal territory, which has been much less explored, and for which not all data derived from limb nerves can be directly applied.

Results

We studied the effects of a CCI of the infraorbital nerve (IoN) on the development of mechanical allodynia and CAP-induced plasma extravasation in rats fed either a regular diet (RD), or a modified diet (MD) with much lower total content and ω-3:ω-6 ratio of PUFAs. In rats kept on MD, mechanical allodynia following CCI-IoN was more pronounced and developed earlier. Extravasation was substantially increased in naive rats fed MD, and displayed differential diet-depending changes one and four weeks after CCI-IoN. When compared with basal levels (in naive and/or sham cases), the net effect of CCI-IoN on ipsilateral extravasation was a reduction in the MD group, but an increase in the RD group, effectively neutralizing the original intergroup differences.

Conclusion

In summary, PUFA intake reduces CAP-induced neurogenic plasma extravasation in the trigeminal territory, and their removal significantly alters the mechanical allodynia and the plasma extravasation that result from a unilateral CCI-IoN. It is likely that this "protective" effect of dietary lipids is temporary. Also, the presence of contralateral effects of CCI-IoN precludes using the contralateral side as control.

AM404, an anandamide transport inhibitor, reduces plasma extravasation in a model of neuropathic pain in rat: role for cannabinoid receptors

Neuropathic pain consequent to peripheral nerve injury has been associated with local inflammation. Following noxious stimulation afferent fibres release substance P (SP) and calcitonin-gene related peptide (CGRP), which are closely related to oedema formation and plasma leakage. The effect of the anandamide transport blocker AM404 has been studied on plasma extravasation after chronic constriction injury (CCI) which consists in a unilateral loose ligation of the rat sciatic nerve (Bennett and Xie, 1988). AM404 (1-3-10 mg kg(-1)) reduced plasma extravasation in the legated paw, measured as mug of Evans Blue per gram of fresh tissue. A strong effect on vascular permeability was also produced by the synthetic cannabinoid agonist WIN 55,212-2 (0.1-0.3-1 mg kg(-1)). Using specific antagonists or enzyme inhibitors, we demonstrate that cannabinoids act at several levels: data on the 3rd day suggest a strong involvement of substance P (SP) and calcitonin gene-related peptide (CGRP) in the control of vascular tone, whereas at the 7th and 14th days the major role seems to be played by prostaglandins (PGs) and nitric oxide (NO). Capsaicin injection in ligated paws of AM404- or WIN 55,212-2-treated rats resulted in an increase of Evans Blue extravasation, suggesting the involvement of the cannabinergic system in the protective effect of C fibres of ligated paws. Taken together, these data demonstrate the efficacy of cannabinoids in controlling pain behaviour through the modulation of several pain mediators and markers of vascular reactivity, such as SP, CGRP, PGs and NO.

Continuous intra-arterial application of substance P induces signs and symptoms of experimental complex regional pain syndrome (CRPS) such as edema, inflammation and mechanical pain but no thermal pain

Substance P is involved in nociception in both the peripheral nervous system and the CNS and has been documented to play a crucial role in the complex regional pain syndrome (CRPS). So far, however, most experimental animal models are restricted to the effect of neurokinin-1 receptor blockers to inhibit substance P and do not directly evaluate its action. Thus, this study was conducted to test the hypothesis that local application of substance P causes signs and symptoms of CRPS. For this purpose rats received a continuous infusion of either substance P or saline over 24 h delivered by a mini-osmotic pump connected to an intrafemoral catheter. Animals were analyzed at either day 1 (n=6, each group) or day 4 (n=5, each group) after start of infusion. Substance P application caused a significant and long-lasting decrease in paw withdrawal thresholds upon mechanical stimulation, while animals did not present with thermal allodynia at days 1 and 4 after onset of infusion. In addition, severe s.c. edema was observed in all animals receiving substance P. In vivo fluorescence microscopy of the extensor digitorum longus muscle of the affected hind paw revealed enhanced leukocyte-endothelial cell interaction with a significant rise in the number of leukocytes both rolling along and firmly adhering to the wall of postcapillary venules, while saline-exposed animals were free of this local inflammatory response. Muscle cell apoptosis, as assessed by in vivo bisbenzimide staining, terminal deoxynucleotidyl transferase nick end labeling analysis and caspase 3-cleavage, could not be observed in either of the animals. In summary, the present study indicates that substance P is responsible for neurogenic inflammation, including local cell response, edema formation and mechanical pain, while it seems not to contribute to the generation of thermal allodynia.

Alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain

Allodynia or hyperalgesia induced by peripheral nerve injury may be involved in changes in the sensitivity of neurotransmitters at the spinal cord level. In order to clarify the functional role of neurotransmitters in peripheral nerve injury, we used rats with nerve injury induced by chronic constriction of the sciatic nerve (CCI rat model) and estimated the effects of the intrathecal injection of drugs known to affect glutamate and tachykinin receptors. In sham-operated rats, the NMDA receptor agonist NMDA and AMPA-kinate receptor agonist RS-(5)-bromowillardin reduced withdrawal latency. The non-competitive NMDA receptor antagonist MK-801, competitive NMDA receptor antagonist AP-5 and AMPA-kinate receptor antagonist NBQX increased withdrawal latency. Substance P (SP) increased the withdrawal latency but only transitorily. The NK1 receptor antagonist RP67580 increased withdrawal latency, but the NK2 receptor antagonist SR48968 did not show an effect. In CCI rats, RS-(5)-bromowillardin reduced withdrawal latency, but NMDA did not show an effect. NBQX increased withdrawal latency, while MK-801 and AP-5 showed little or no effect. SP reduced withdrawal latency, and both RP67580 and SR48968 increased it. These results indicate that the alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in the spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain.

Neuropathy-induced osteopenia in rats is not due to a reduction in weight born on the affected limb

Changes in bone mineral density (BMD) are associated with clinical neuropathies. Following nerve injury in the rat, there is a loss of BMD, which may be related to nerve injury or reduced mechanical loading. The purpose of this study was to investigate if altered mechanical loading is solely responsible for the observed loss of BMD in neuropathic pain models. In addition, we sought to study the action of chronic bisphosphonate treatment on both neuropathy-induced osteopenia and pain. We therefore had two hypotheses: firstly, that nerve injuries can have variable effects on hind limb bone loss in rats which are not attributable to differences in the extent of hind limb disuse and, secondly, that bisphosphonate treatment can reverse bone loss in a rat mononeuropathy model, and this is not attributable to bisphosphonate effects on nociception or hind paw unweighting. Male Sprague-Dawley rats were subject to chronic constriction injury (CCI), partial sciatic nerve ligation (PSN) or L5 + L6 spinal nerve ligation (SNL). Loss of BMD, defined as a numerically lower BMD as compared to control animals, was extreme following CCI (maximum ipsilateral/contralateral difference of 0.023 +/- 0.011); BMD loss following either PSN or SNL in the rat was subtle (0.010 +/- 0.002 and 0.013 +/- 0.012 g/cm2, respectively), significant only at early time points and had resolved by 7 weeks post-surgery. Chronic bisphosphonate treatment significantly inhibited CCI-induced osteopenia in the rat without inhibiting the reduction in weight-bearing tactile allodynia or mechanical hyperalgesia. Loss of BMD is observed in rats in a variety of neuropathic pain models. Lack of correlation between neuropathy-induced bone loss and weight bearing demonstrates that the bone loss is not simply a function of reduced mechanical loading and suggests that altered bone-nerve signaling is involved. Furthermore, chronic bisphosphonate treatment inhibits neuropathy-induced osteopenia without affecting behavioral measurements of neuropathic pain. This indicates that osteopenia is not directly related to neuropathic pain behaviors.

Cyclic urea derivatives as potent NK1 selective antagonists

A series of novel five- and six-membered ring urea derivatives have been described as potent and selective NK1 receptor antagonists. Several compounds in this series exhibited good oral activity and brain penetration. Syntheses of these compounds are also described herein.

In vivo evidence for apoptosis, but not inflammation in the hindlimb muscle of neuropathic rats

Loose ligation of the rat sciatic nerve (chronic constriction injury (CCI) model) provokes signs and symptoms like those observed in complex regional pain syndrome (CRPS) patients. Neurogenic inflammation is a purported cause of neuropathic pain despite inconsistent evidence to support this hypothesis. To clarify this issue, we examined effects of CCI on microcirculation, inflammatory cell-cell interaction and cell integrity in muscle tissue using intravital fluorescence microscopic, molecular and immunohistochemical techniques. CCI-rats, but not sham-operated animals developed symptoms of neuropathic pain and oedema on the ipsilateral hindpaw. Despite signs of neuropathic pain, high resolution in vivo multifluorescence microscopy revealed physiological values for functional capillary density, leukocyte-endothelial cell interaction and microvascular permeability in muscle tissue of CCI-animals, similarly as found in controls, indicating absence of perfusion failure and inflammatory cell reaction. However, CCI-animals represented with marked apoptosis of bisbenzimide-stained muscle tissue cells, as given by in vivo fluorescence microscopic assessment of cell death-associated condensation, fragmentation and/or crescent-shaped formation of their nuclear chromatin. Apoptosis was further confirmed by increased caspase 3 protein levels and positive terminal deoxyuridine nick end labeling histochemistry. The present study demonstrates that sciatic nerve ligation-induced neuropathy causes cell apoptosis without concomitant inflammation-associated microcirculatory dysfunction in muscle tissue. Beside the well-known pattern of neuropathic pain, the CCI-model has now additionally been shown to reflect the response of muscle tissue to impaired innervation, i.e. prompting muscle cells to undergo non-inflammatory apoptotic cell death. This finding deserves further investigation in that apoptosis may contribute to neuropathic pain conditions like CRPS.

Increased substance P and tumor necrosis factor-alpha level in the paws following formalin injection in rat tail

We previously described a rat model where the injection of formalin in the tail induced a facilitation of the hindpaw withdrawal reflexes (hyperalgesia). In the present work, after injecting formalin in the tail, we measured the levels of pro-nociceptive mediators tumor necrosis factor-alpha (TNF) and substance P (SP) in the rat paws. A significant increase of SP levels was evident in the hindpaw, whereas no changes in SP were observed in the forepaw. Both in the hindpaw and in the forepaw the TNF levels were higher than normal at each stage of measurement. Our results indicate that a prolonged neuronal activation induced by formalin injection is associated with a change in nociceptive and inflammatory mediators in distal sites of the body. The fact that SP levels are changed in the hindpaw but not in the forepaw might point to the activation of a mechanism of retrograde signaling from central synapses to paw afferent nerves.

Sympathetic modulation of activity in Adelta- and C-primary nociceptive afferents after intradermal injection of capsaicin in rats

Neuropathic and inflammatory pain can be modulated by the sympathetic nervous system. In some pain models, sympathetic postganglionic efferents are involved in the modulation of nociceptive transmission in the periphery. The purpose of this study is to examine the sensitization of Adelta- and C-primary afferent nociceptors induced by intradermal injection of capsaicin (CAP) to see whether the presence of sympathetic efferents is essential for the sensitization. Single primary afferent discharges were recorded from the tibial nerve after the fiber types were identified by conduction velocity in anesthetized rats. An enhanced response of some Adelta- and most C-primary afferent fibers to mechanical stimuli was seen in sham-sympathectomized rats after CAP (1%, 15 mul) injection, but the enhanced responses of both Adelta- and C-fibers were reduced after sympathetic postganglionic efferents were removed. Peripheral pretreatment with norepinephrine by intraarterial injection could restore and prolong the CAP-induced enhancement of responses under sympathectomized conditions. In sympathetically intact rats, pretreatment with an alpha(1)-adrenergic receptor antagonist (terazosin) blocked completely the enhanced responses of C-fibers after CAP injection in sympathetically intact rats without significantly affecting the enhanced responses of Adelta-fibers. In contrast, a blockade of alpha(2)-adrenergic receptors by yohimbine only slightly reduced the CAP-evoked enhancement of responses. We conclude that the presence of sympathetic efferents is essential for the CAP-induced sensitization of Adelta- and C-primary afferent fibers to mechanical stimuli and that alpha(1)-adrenergic receptors play a major role in the sympathetic modulation of C-nociceptor sensitivity in the periphery.

A tale of two neurons in the upper airways: pain versus itch

Pain and itch sensations are induced by depolarization of distinct populations of unmyelinated type C, and possibly other, neurons. Both sets of neurons and sensations serve critical protective mechanisms that maintain the integrity and patency of the upper airways. When noxious or pruritic stimuli are applied on the afferent nerve ending, pain and itch are appreciated at the thalamic and parietal cortex. In the mucosa, this neuronal depolarization spreads via the peripheral efferent axon response mechanism. Neuropeptides such as substance P and calcitonin gene-related peptide are released from neurosecretory varicosities on the nociceptive C fibers. The exact functions of axon responses differ between humans and rodents, and in health and disease. Separate itch- and pain-specific peripheral type C fibers, secondary relay interneurons in the spinal cord dorsal horn, anatomical locations in the lateral spinothalamic tract, and thalamic nuclei demonstrate that all nociceptive nerves are not the same. Other types of irritant-sensitive trigeminal neurons might be discovered that could mediate other unique sensations, specific axon responses, or central nervous system functions.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Inhibition of pain responses by activation of CB(2) cannabinoid receptors

Cannabinoid receptor agonists diminish responses to painful stimuli. Extensive evidence demonstrates that CB(1) cannabinoid receptor activation inhibits pain responses. Recently, the synthesis of CB(2) cannabinoid receptor-selective agonists has allowed testing whether CB(2) receptor activation inhibits pain. CB(2) receptor activation is sufficient to inhibit acute nociception, inflammatory hyperalgesia, and the allodynia and hyperalgesia produced in a neuropathic pain model. Studies using site-specific administration of agonist and antagonist have suggested that CB(2) receptor agonists inhibit pain responses by acting at peripheral sites. CB(2) receptor activation also inhibits edema and plasma extravasation produced by inflammation. CB(2) receptor-selective agonists do not produce central nervous system (CNS) effects typical of cannabinoids retaining agonist activity at the CB(1) receptor. Peripheral antinociception without CNS effects is consistent with the peripheral distribution of CB(2) receptors. CB(2) receptor agonists may have promise for the treatment of pain and inflammation without CNS side effects.

Enhanced release of adenosine in rat hind paw following spinal nerve ligation: involvement of capsaicin-sensitive sensory afferents

Modulation of endogenous adenosine levels by inhibition of adenosine metabolism produces a peripheral antinociceptive effect in a neuropathic pain model. The present study used microdialysis to investigate the neuronal mechanisms modulating extracellular adenosine levels in the rat hind paw following tight ligation of the L5 and L6 spinal nerves. Subcutaneous injection of 50 microl saline into the nerve-injured paw induced a rapid and short-lasting increase in extracellular adenosine levels in the subcutaneous tissues of the rat hind paw ipsilateral to the nerve injury. Saline injection did not increase adenosine levels in sham-operated rats or non-treated rats. The adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine and the adenosine deaminase inhibitor 2'-deoxycoformycin, at doses producing a peripheral antinociceptive effect, did not further enhance subcutaneous adenosine levels in the nerve-injured paw. Systemic pretreatment with capsaicin, a neurotoxin selective for small-diameter sensory afferents, markedly reduced the saline-evoked release of adenosine in rat hind paw following spinal nerve ligation. Systemic pretreatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic afferent nerves, did not affect release. These results suggest that following nerve injury, peripheral capsaicin-sensitive primary sensory afferent nerve terminals are hypersensitive, and are able to release adenosine following a stimulus that does not normally evoke release in sham-operated or intact rats. Sympathetic postganglionic afferents do not appear to be involved in such release. The lack of effect on such release by the inhibitors of adenosine metabolism suggests an altered peripheral adenosine system following spinal nerve ligation.

Neuropathic pain is associated with alterations of nitric oxide synthase immunoreactivity and catalytic activity in dorsal root ganglia and spinal dorsal horn

Previous experiments have suggested that nitric oxide may play an important role in nociceptive transmission in the spinal cord. To assess the possible roles of neuronal nitric oxide synthase (nNOS) in spinal sensitization after nerve injury, we examined the distribution of nNOS immunoreactivity in dorsal root ganglia (DRGs) and dorsal horn of the corresponding spinal segments. NOS catalytic activity was also determined by monitoring the conversion of [3H]arginine to [3H]citrulline in the lumbar (L4-L6) spinal cord segments and DRGs in rats 21 days after unilateral loose ligation of the sciatic nerve. Behavioral signs of tactile and cold allodynia developed in the nerve-ligated rats within 1 week after surgery and lasted up to 21 days. Immunocytochemical staining revealed a significant increase (approximately 6.7-fold) of nNOS-immunoreactive neurons and fibers in the DRGs L4-L6. No significant changes were detected in the number of nNOS-positive neurons in laminae I-II of the spinal segments L4-L6 ipsilateral to nerve ligation. However, an increased number of large stellate or elongated somata in deep laminae III-V of the L5 segment expressed high nNOS immunoreactivity. The alterations of NOS catalytic activity in the spinal segments L4-L6 and corresponding DRGs closely correlated with nNOS distribution detected by immunocytochemistry. No such changes were detected in the contralateral DRGs or spinal cord of sham-operated rats. The results indicate that marked alterations of nNOS in the DRG cells and in the spinal cord may contribute to spinal sensory processing as well as to the development of neuronal plasticity phenomena in the dorsal horn.

Effect of lumbar 5 ventral root transection on pain behaviors: a novel rat model for neuropathic pain without axotomy of primary sensory neurons

A peripheral nerve injury often causes neuropathic pain but the underlying mechanisms remain obscure. Several established animal models of peripheral neuropathic pain have greatly advanced our understanding of the diverse mechanisms of neuropathic pain. A common feature of these models is primary sensory neuron injury and the commingle of intact axons with degenerating axons in the sciatic nerve. Here we investigated whether neuropathic pain could be induced without sensory neuron injury following exposure of their peripheral axons to the milieu of Wallerian degeneration. We developed a unilateral lumbar 5 ventral root transection (L5 VRT) model in adult rats, in which L5 ventral root fibers entering the sciatic nerve were sectioned in the spinal canal. This model differs from previous ones in that DRG neurons and their afferents are kept uninjured and intact afferents expose to products of degenerating efferent ventral root fibers in the sciatic nerve and the denervated muscles. We found that the L5 VRT produced rapid (24 h after transection), robust and prolonged (56 days) bilateral mechanical allodynia, to a similar extent to that in rats with L5 spinal nerve transection (L5 SNT), cold allodynia and short-term thermal hyperalgesia (14 days). Furthermore, L5 VRT led to significant inflammation as demonstrated by infiltration of ED-1-positive monocytes/macrophages in the DRG, sciatic nerve and muscle fibers. These findings demonstrated that L5 VRT produced behavioral signs of neuropathic pain with high mechanical sensitivity and thermal responsiveness, and suggested that neuropathic pain can be induced without damage to sensory neurons. We propose that neuropathic pain in this model may be mediated by primed intact sensory neurons, which run through the milieu of Wallerian degeneration and inflammation after nerve injury. The L5 VRT model manifests the complex regional pain syndrome in some human patients, and it may provide an additional dimension to dissect out the mechanisms underlying neuropathic pain.