Motor cortex stimulation in rats with chronic constriction injury

Motor cortex stimulation (MCS) has gained a significant role in treatment of neuropathic pain. In order to evaluate effect of MCS in experimental animals we applied MCS to rats with neuropathic pain, which was evoked by chronic constriction injury (CCI) to the left sciatic nerve. Pain thresholds of both hind limbs were measured before, immediately after MCS, 1 h after MCS and 1 day after MCS. Effect of the stimulation was studied with respect to laterality (contralateral and ipsilateral MCS) and duration (short-term 10-min and long-term 1-h stimulation). It was found out that in control rats MCS did not affect thermal nociceptive thresholds. However, in CCI animals following results were obtained: difference score (difference in paw withdrawal latency between ligated and non-ligated hind limb) significantly decreased after both short- and long-term contralateral MCS; the difference score after the long-term ipsilateral MCS (related to the ligated hind limb) was not significantly different from that of intact animals; the effects of the contralateral short-term and the ipsilateral long-term stimulation faded within 1 h after the end of MCS, while the effect of the contralateral long-term MCS remained 1 h after the end of the MCS and faded within 24 h. It is concluded that MCS in experimental animals exerts similar effects as in human suffering from neuropathic pain and that the effect might be evoked from both cerebral cortices.     

Effects of lowering ambient temperature on pain-related behaviors in a rat model of neuropathic pain

To clarify the mechanism by which changes in chronic pain are induced by cold environments, rats rendered neuropathic by a chronic constriction injury (CCI) to the sciatic nerve were exposed to low ambient temperature (LT; 7 degrees C decrease from 22 degrees C) in a climate-controlled room. LT exposure aggravated pain-related behaviors in CCI rats, i.e., decreased the threshold to von Frey hair and paw pressure stimulation, prolonged the duration of foot withdrawal to pinprick stimulation, and increased the cumulative duration of guarding posture. Lumbar sympathectomy (SYX) did not inhibit LT-induced augmentations of pain-related behaviors in CCI rats. LT exposure decreased the skin temperatures of both hind paws to the same degree in the sham-operated control and SYX rats, while in the CCI and SYX+CCI rats it caused a larger temperature decrease in the injured paw than in the uninjured one. These results indicate that LT exposure augments abnormalities in pain-related behaviors of neuropathic rats, and also suggest that sympathetic nervous activity is not a predominant factor in the augmenting mechanism.     

Botulinum neurotoxin type A counteracts neuropathic pain and facilitates functional recovery after peripheral nerve injury in animal models

A growing interest was recently focused on the use of Botulinum neurotoxin serotype A (BoNT/A) for fighting pain. The aim of this study was to investigate the effects of BoNT/A on neuropathic pain. It was observed that BoNT/A is able to counteract neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve both in mice and in rats. This effect is already present after a single intraplantar (i.pl.) or intrathecal (i.t.) neurotoxin administration that significantly reduces the sciatic nerve ligation-induced mechanical allodynia in mice and rats and thermal hyperalgesia in rats. This effect was evident starting 24 h after the administration of BoNT/A and it was long-lasting, being present 81 or 25 days after i.pl. injection of the higher dose in mice (15 pg/paw) and rats (75 pg/paw), respectively, and 35 days after i.t. injection in rats (75 pg/rat). Moreover, BoNT/A-injected mice showed a quicker recovery of the walking pattern and weight bearing compared to control groups. The behavioral improvement was accompanied by structural modifications, as revealed by the expression of cell division cycle 2 (Cdc2) and growth associated protein 43 (GAP-43) regeneration associated proteins, investigated by immunofluorescence and Western blotting in the sciatic nerve, and by the immunofluorescence expression of S100β and glial fibrillary acidic protein (GFAP) Schwann cells proteins. In conclusion, the present research demonstrate long-lasting anti-allodynic and anti-hyperalgesic effects of BoNT/A in animal models of neuropathic pain together with an acceleration of regenerative processes in the injured nerve, as evidenced by both behavioral and immunohistochemistry/blotting analysis. These results may have important implications in the therapy of neuropathic pain.     

Crossed-withdrawal reflex in a rat model of neuropathic pain: implications in neural plasticity

Recently we developed a neuropathic rat model employing a distal sciatic nerve branch injury, in which rats show vigorous behavioral signs of neuropathic pain. This study was performed to evaluate the crossed-withdrawal reflex in which any stimuli applied to the uninjured side produces allodynic signs on the injured side in our neuropathic pain model. Rats that received neuropathic surgery developed behavioral signs of neuropathic pain. In addition, these rats developed pain responses of the injured paw to stimuli applied to the contralateral uninjured paw, therefore, demonstrating 'the crossed-withdrawal reflex.' Moreover, electrical stimulation of the uninjured paw developed evoked potentials in the ventral root on the injured side. These results suggest that information processing from input on the uninjured side to output on the injured side, can be facilitated in rats with a nerve injury and that neuroplasticity may contribute to the crossed-withdrawal reflex.     

Both MHC and non-MHC genes regulate development of experimental neuropathic pain in rats

We have previously demonstrated that differences in neuropathic pain-like behaviors after sciatic nerve injury genetically maps to the major histocompatibility complex (MHC) in rats carrying RT1(c) or RT1(av1) haplotypes on the Piebald Virol Glaxo (PVG) background. In order to further explore the genetic contribution to neuropathic pain, we have here examined the MHC-congenic rat strains PVG-RT1(n) and PVG-RT1(av1) and the inbred strains PVG (RT1(c)) and Brown-Norway (BN; RT1(n)). All studied strains developed mechanical hypersensitivity (allodynia-like behavior) of the hind paw after photochemically induced sciatic nerve injury. However, the PVG-RT1(n) and PVG strains displayed significantly more allodynia than PVG-RT1(av1) and BN rats. In addition, the BN strain demonstrated an elevated threshold for the baseline response. The results demonstrate that both MHC and non-MHC genes influence experimental neuropathic pain in rats and also suggest that allelic variation contained in the RT1(av1) haplotype on the PVG background protects against neuropathic pain.     

Minocycline injection in the ventral posterolateral thalamus reverses microglial reactivity and thermal hyperalgesia secondary to sciatic neuropathy

We hypothesized that microglia in the ventral posterolateral (VPL) nucleus of the thalamus are reactive following peripheral nerve injury, and that inhibition of microglia by minocycline injection in the VPL attenuates thermal hyperalgesia. Our results show increased expression of OX-42 co-localized with phosphorylated p38MAPK (P-p38) in the VPL seven days after chronic constriction injury (CCI) of the sciatic nerve. However, astrocytic GFAP expression in the VPL is unchanged 7 and 14 days after CCI. Microinjection of minocycline into the VPL contralateral to CCI reverses thermal hyperalgesia, whereas vehicle injection has no effect on paw withdrawal latency. Minocycline abrogates the increased expression of OX-42 in the VPL after CCI. Therefore, peripheral nerve injury favors a hyperactive microglial phenotype in the VPL, suggesting remote neuroimmune signaling from the damaged nerve to the brain, concomitant with neuropathic behavior that is reversed by local intervention in the VPL to inhibit microglia.     

Antihyperalgesic and Anti-inflammatory Effects of Atorvastatin in Chronic Constriction Injury-Induced Neuropathic Pain in Rats

Atorvastatin is a 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitor used in treatment of hypercholesterolemia and prevention of coronary heart disease. The aim of this study is to investigate the antihyperalgesic and anti-inflammatory effects of atorvastatin (3, 10, and 30 mg/kg by oral gavages for 14 days) in chronic constriction injury (CCI) model of neuropathic pain in rats. CCI caused significant increase in tumor necrosis factor-α, interleukin 1 beta, prostaglandin E2, along with matrix metalloproteases (MMP-2) and nerve growth factor (NGF) levels in sciatic nerve and spinal cord concomitant with mechanical and thermal hyperalgesia, which were significantly reduced by oral administration of atorvastatin for 14 days as compared to CCI rats. Our study demonstrated that atorvastatin attenuates neuropathic pain through inhibition of cytokines, MMP-2, and NGF in sciatic nerve and spinal cord suggesting that atorvastatin could be an additional therapeutic strategy in management of neuropathic pain.     

Anti-allodynic action of the disease-modifying anti-rheumatic drug iguratimod in a rat model of neuropathic pain

Objective

Patients with rheumatoid arthritis experience nociceptive as well as neuropathic pain. The effect of iguratimod (IGU), a disease-modifying anti-rheumatic drug, on neuropathic pain in a rat model of chronic constriction injury (CCI) was examined in this study.

Methods

CCI was induced by making four ligations on the left sciatic nerve. Rats with stable signs of static allodynia were selected 2 weeks after the surgery and drug treatments were started (day 0). The test drugs were orally administered once daily for 15 days. The threshold of mechanical pain response in the hind paw was evaluated by the von Frey hair test in a blinded manner. To observe histological changes in the spinal cord, the L4 region was subjected to immunohistochemical analysis for the detection of microglial cells.

Results

IGU showed an anti-allodynic effect on CCI-induced neuropathic pain at days 6 and 14, but not at 90 min after the first administration of IGU. This effect of IGU was observed until day 21. Furthermore, IGU decreased the number of Iba-1-positive cells, which had been increased at the ipsilateral side of the dorsal horn by CCI.

Conclusions

These results suggest that IGU suppresses neuropathic pain via a different mechanism from that of current therapeutics.

     

Analgesic effect of intrathecally γ-aminobutyric acid transporter-1 inhibitor NO-711 administrating on neuropathic pain in rats

To investigate the analgesic effect of intrathecally administered γ-aminobutyric acid (GABA) transporter-1 inhibitor NO-711 on the sciatic nerve chronic constriction injury (CCI) rats. 5 days after intrathecal catheter placement, neuropathic pain model was established by CCI of sciatic nerve on rats. Withdrawal thresholds for mechanical allodynia and latency for thermal hyperalgesia were measured in all animals. All rats operated upon for CCI displayed decreased withdrawal thresholds for mechanical allodynia and latency for thermal hyperalgesia, which has significant difference compared with sham groups. After intrathecal NO-711 administration, withdrawal thresholds and latency were significantly increased on CCI rats compared with control group after 1 day. The results show that GABA transporter-1 inhibitor could effectively develop analgesic effect in sciatic nerve CCI rats’ model.     

Effects of 17β-estradiol on glucose transporter 1 expression and endothelial cell survival following focal ischemia in the rats

To characterize various animal models of neuropathic pain, we compared three previously developed rat models using the same behavioral testing methods. These models involve: (1) chronic constriction injury by loose ligation of the sciatic nerve (CCI); (2) tight ligation of the partial sciatic nerve (PSL); and (3) tight ligation of spinal nerves (SNL). Comparisons were made for the time course of behavioral signs representing various components of neuropathic pain as well as for the effects of surgical sympathectomy. In general, all three methods of peripheral nerve injury produced behavioral signs of both ongoing and evoked pain with similar time courses. However, there was a considerable difference in the magnitude of each pain component between models. Signs of mechanical allodynia were largest in the SNL injury and smallest in the CCI model. On the other hand, behavioral signs representing ongoing pain were much more prominent in the CCI model than in the other two. Although the behavioral signs of neuropathic pain tended to decrease after sympathectomy in all three models, the change was most evident in the SNL model. The results of the present study suggest that the three rat models tested have contrasting features, yet all are useful neuropathic pain models, possibly representing different populations of human neuropathic pain patients.     

Neuropathic pain in aged rats: behavioral responses and astrocytic activation

We used the Bennett and Xie (1988) model of chronic neuropathic pain to study the effect of age on thermal and tactile sensitivity and on astrocytic activation in the dorsal horn of the spinal cord after nerve injury. Fischer 344 FBNF1 hybrid rats in three age groups, 4-6, 14-16, and 24-26 months, were studied. Rats were either unligated (day 0, control) or the left sciatic nerve was loosely ligated to cause a chronic constriction injury (CCI). CCI causes a neuropathic pain condition characterized by tactile allodynia and thermal hyperalgesia. Rats were behaviorally assessed for tactile and thermal sensitivity of their ligated and unligated hind paws up to 35 days postligation. Rats were sacrificed before or at various days postligation, and activated astrocytes were identified at the L4-L5 levels of their spinal cords by use of an antibody to glial fibrillary acid protein (GFAP). The number of GFAP-ir astrocytes in the dorsal horn of the spinal cord in the control, uninjured condition decreased with age (P < or = 0.001) but increased after CCI in all three age groups. After CCI, astrocytic activation in the cord was less robust in aged rats than in younger ones (P < or = 0.01). Not all the CCI rats displayed hyperalgesia to touch and to heat. Rats with an increased sensitivity to heat had increased levels of GFAP-ir in their cords; however, rats with decreased thermal sensitivity also displayed increased GFAP-ir. Thus the presence of activated astrocytes was not correlated with a single behavioral manifestation of neuropathic pain.     

Interleukin-17 levels in rat models of nerve damage and neuropathic pain

In the present study, we assessed IL-17 levels at 3 and 8 days following various forms of injuries to the sciatic nerve and related the cytokine levels to the pain behaviors associated with the injuries. The four experimental models employed were chronic constriction injury (CCI), partial sciatic ligation (PSL), complete sciatic transection (CST) and perineural inflammation (Neuritis). Behavior withdrawal thresholds for mechanical stimulus and withdrawal latency for thermal stimulation were used to measure mechanical allodynia and thermal hyperalgesia. IL-17 levels of the affected, contralateral and naïve rats’ sciatic nerve were assessed employing enzyme-linked immunosorbent assay (ELISA). Rats exposed to CCI and Neuritis displayed significant mechanical allodynia and thermal hyperalgesia 3, 5 and 8 days following the procedure, rats exposed to PSL displayed significant mechanical allodynia 5 and 8 days following the procedure and rats exposed to CST developed significant hypoesthesia. Three days following the procedure, IL-17 levels increased significantly compared to naïve rats only in the PSL model. Eight days following the procedure, IL-17 levels in nerves exposed to CCI, CST, PSL and Neuritis were significantly elevated compare to intact nerve levels. It is likely that IL-17 has a limited role in the acute phase of nerve injury and the associated acute pain, but may have a role in later phases of the processes of the development of neuropathic pain.     

Comparison of three models of neuropathic pain in mice using a new method to assess cold allodynia: The double plate technique

The recent identification of receptors sensitive to cold stimuli increased the significance of using mice to study cold allodynia, one of the important features of neuropathic pain. However, commonly used techniques (simple cold plate and acetone technique) may be inappropriate to study cold allodynia in mice because of problems of interpretation. We have developed a new method for assessing aversion to a cold non-noxious stimulus. It consists of calculating the time that mice spend on a non-noxious cold plate during their explorative behavior versus a thermoneutral one. We used three different models of neuropathic pain: chronic constriction injury of the sciatic nerve (CCI), partial sciatic nerve ligation (PSL) and chronic constriction of the saphenous nerve (CCS) with their respective sham groups and naive animals to assess the double plate in comparison to the acetone drop technique. All operated mice displayed cold allodynia with both methods. The response to acetone and the time spent on the cold plate were correlated (r = −0.93) and we also showed that the CCI mice were more sensitive to cold. Pharmacological validation of this technique showed that CCI induced cold allodynia was alleviated by gabapentin. In conclusion, the double plate technique provides a new, relevant method for assessing cold allodynia in mice. The advantages and drawbacks with the other techniques are discussed.     

Activation of hippocampal MEK1 contributes to the cumulative antinociceptive effect of electroacupuncture in neuropathic pain rats

Background

Electroacupuncture (EA) intervention can relieve a variety of pain; however, optimal EA protocols have not been clearly determined. In addition, although central mitogen-activated protein kinase kinase (MEK) signaling has been shown to be involved in the antinociceptive effect of acupuncture stimulation, its characteristics at different time-points of EA intervention have not been fully elucidated. Therefore, the present study investigated the relationship between the effects of different numbers of EA intervention sessions and the activation of MEK1 in the hippocampus and hypothalamus in a rat model of neuropathic pain.

Methods

After ligation of the left sciatic nerve, which induces chronic constriction injury (CCI), the acupoints Zusanli (ST36) and Yanglingquan (GB34) were applied. The thermal withdrawal latency of the hind paw was used to evaluate the effect of EA on pain thresholds. Intra-hippocampus microinjection of PD98059, a MEK inhibitor, was performed to validate the involvement of MEK in EA analgesia. The hippocampus and hypothalamus were harvested to examine the phosphorylation levels of MEK (pMEK) by western blotting.

Results

In CCI rats, the thermal pain threshold of the affected hind paw decreased significantly relative to the control. Following subsequent daily EA interventions, CCI-induced ipsilateral hyperalgesia was markedly improved from day 4 and the analgesic effect of EA lasted 3 days after cessation of EA. Four sessions of EA markedly suppressed CCI-induced decrease of hippocampal pMEK1 (normalized to the total MEK level). In contrast, successive sessions of EA intervention gradually down-regulated the CCI-induced up-regulation of hypothalamic pMEK1 along with the increase numbers of EA intervention. However, EA did not exert the same analgesic effect after microinjection of PD98059 into the contralateral hippocampus during the first 3 days of EA intervention.

Conclusions

EA intervention can induce time-dependent cumulative analgesia in neuropathic pain rats after 4 successive sessions of daily EA intervention, which is at least in part related to the activation of hippocampal MEK1.

     

Transient early expression of TNF-alpha in sciatic nerve and dorsal root ganglia in a mouse model of painful peripheral neuropathy

The proinflammatory cytokine tumor necrosis factor-alpha (TNF) is an important mediator in neuropathic pain. We investigated the temporal pattern of TNF mRNA expression in the sciatic nerve, in dorsal root ganglia (DRG) and spinal cord in the mouse chronic constriction injury model of neuropathy with quantitative real-time polymerase chain reaction. Neuropathic pain-like behaviour was monitored by evaluating thermal hyperalgesia and mechanical allodynia. Pain-related behaviour and TNF expression were evaluated 6 h, 1, 3, 7 and 14 days after injury. Naive animals and sham-operated mice were used as controls. We found an early upregulation of sciatic nerve TNF mRNA levels in chronic constriction injury (CCI) and sham-operated animals 6 h after surgery: 1 day later TNF overexpression was present in CCI mice only and disappeared 3 days after injury. The mRNA cytokine levels were elevated in DRG 1 and 3 days after surgery in CCI animals only, while the cytokine was not modulated in the spinal cord. A significant hyperalgesia was present in CCI and sham-operated mice at 6 h and 1 day, while at later time point only CCI mice presented lower thresholds. Mechanical allodynia was already present only in CCI animals 6 h from surgery and remained constant up to the 14 th day. The results indicate that a transient early TNF upregulation takes place in peripheral nervous system after CCI that can activate a cascade of proinflammatory/pronociceptive mediators.     

Effects of repeated administered ghrelin on chronic constriction injury of the sciatic nerve in rats

Chronic constriction injury (CCI) is a peripheral mononeuropathic pain model that is caused by an injury to the peripheral nervous system and refractory to available conventional treatment. Mechanisms involved in neuropathic pain are still unclear. Previous studies reveal that proinflammatory cytokines contribute to CCI-induced peripheral nerve pathology. Ghrelin, a novel identified gastric peptide, has been shown to have antinociceptive activity and also anti-inflammatory properties by decreasing proinflammatory cytokines. Therefore, the aim of the present study was to investigate the effects of ghrelin on the CCI and its relationship with proinflammatory cytokines in rats. Wistar rats underwent sciatic nerve ligation to induce CCI fallowed by repeated ghrelin administrations (50 and 100 μg/kg i.p., once daily) for a period of 14 days. Mechanical hyperalgesia was assessed before surgery and at day 14 after CCI. TNF-α, IL-1β and IL-6 were measured in blood and spinal cord. The changes of sciatic nerve was assessed histologically by both light and electron microscopy. Ghrelin attenuated mechanical hyperalgesia, reduced spinal TNF-α and IL-1β levels and enhanced sciatic nerve injury with correlated morphometric recovery. These results indicate that the protective effect by ghrelin in the spinal cord is mediated through the suppression of TNF-α and IL-1β. Thus ghrelin may be a promising peptide in the management of neuropathic pain.     

Comparison of sympathetic sprouting in sensory ganglia in three animal models of neuropathic pain

Sympathetic postganglionic fibers sprout in the dorsal root ganglion (DRG) after peripheral nerve injury. Therefore, one possible contributing factor of sympathetic dependency of neuropathic pain is the extent of sympathetic sprouting in the DRG after peripheral nerve injury. The present study compared the extent of sympathetic sprouting in the DRG as well as in the injured peripheral nerve in three rat neuropathic pain models: (1) the chronic constriction injury model (CCI); (2) the partial sciatic nerve ligation injury model (PSI); and (3) the segmental spinal nerve ligation injury model (SSI). All three methods of peripheral nerve injury produced behavioral signs of ongoing and evoked pain with some differences in the magnitude of each pain component. The density of sympathetic fibers in the DRG was significantly higher at all examined postoperative times than controls in the SSI model, while it was somewhat higher than controls only at the last examined postoperative time (20 weeks) in the CCI and PSI models. Therefore, data suggest that, although sympathetic changes in the DRG may contribute to neuropathic pain syndromes in the SSI model, other mechanisms seem to be more important in the CCI and PSI models at early times following peripheral nerve injury.     

Hyperalgesia and increased neuropathic pain-like response in mice lacking galanin receptor 1 receptors

The neuropeptide galanin may have a role in modulation of nociception, particularly after peripheral nerve injury. The effect of galanin is mediated by at least three subtypes of receptors. In the present study, we assessed the nociceptive sensitivity in mice lacking the galanin receptor 1 gene (Galr1) and the development of neuropathic pain-like behaviours after photochemically induced partial sciatic nerve ischaemic injury. Under basal condition, Galr1 knock-out (Galr1(-/-)) mice had shortened response latency on the hot plate, but not tail flick and paw radiant heat, tests. The mechanical sensitivity was not different between Galr1(-/-) and wild type (Galr1(+/+)) mice, whereas the cold response was moderately enhanced in Galr1(-/-) mice. Both Galr1(-/-) mice and Galr1(+/+) controls developed mechanical and heat hypersensitivity after partial sciatic nerve injury. The duration of such pain-like behaviours was significantly increased in Galr1(-/-). The Galr1(-/-) mice and Galr1(+/+) mice did not differ in their recovery from deficits in toe-spread after sciatic nerve crush.The results provide some evidence for an inhibitory function for the neuropeptide galanin acting on galanin receptor 1 (GALR1) in nociception and neuropathic pain after peripheral nerve injury in mice.     

The anterior cingulate cortex projection to the dorsomedial striatum modulates hyperalgesia in a chronic constriction injury mouse model

IntroductionThe aim of the study was to study the role of the anterior cingulate cortex (ACC)-dorsal midbrain striatum (DMS) in neuropathic pain in mice.Material and methodsOptogenetics has been increasingly used in neuroscience research to selectively and precisely control the activity of a defined group of central neurons to determine their roles in behavioral functions in animals. The most important opsins are blue-sensitive ChR2 and yellow-sensitive NpHR. Calcium-calmodulin dependent protein kinase Iiα (CaMKIIα) is mostly expressed in the pyramidal excitatory neurons. Mice were injected with AAV2/9-CamKII-ChR2-mCherry, AAV2/9-CamKII-eNpHR3.0-GFP or AAV2/9-CamKII-mCherry virus in the ACC region, and the optical fiber implantation was performed in the ACC or DMS region. Mice were then followed up for 2 to 8 weeks and behavioral tests were carried out in the presence or absence of the blue/yellow light (473 nm/589 nm). Pain behavioral tests with or without the blue/yellow light at the same time were performed on the third and the seventh day after the chronic constriction injury of sciatic nerve model (CCI) was established. The pain thresholds of left and right hind limbs of mice in all groups were measured.ResultsNo matter whether activating the neurons in ACC or DMS, compared with normal mice in the ChR2-off-right group, and the mCherry-on-right group, the thermal pain threshold and mechanical pain threshold of the normal mice in the ChR2-on-right group were significantly lower. When inhibiting the neurons in the ACC or DMS, on day 3 and day 7 after CCI operation, the thermal pain threshold and mechanical pain threshold of the CCI mice of the NpHR-on-right group were significantly higher compared with the NpHR-off-right and mCherry-on-right groups.ConclusionsThe anterior cingulate cortex-dorsal midbrain striatum may be involved in the regulation of neuropathic pain in mice.     

Local expression of inducible nitric oxide synthase in an animal model of neuropathic pain

Peripheral nerve injury is associated with local inflammation and neuropathic pain. In this study we investigated the local expression of the inducible isoform of nitric oxide synthase (iNOS) following a chronic constriction injury (CCI) to the sciatic nerve, a rat model of neuropathic pain. Western blot analysis and immunohistochemical co-localization methods were used to identify temporal and spatial expression of iNOS and its cells of origin. Changes in mRNA were analyzed by RT-PCR and iNOS specific primers. We report that CCI injury induced local iNOS expression in both macrophages and Schwann cells within and distal to the injury site. The local increase in iNOS mRNA expression paralleled both the temporal and spatial protein expression. This study supports the hypothesis that CCI is associated with a local inflammatory reaction mediated at least in part by iNOS. Local activation of the iNOS-NO system may play an important role in the pathogenesis of peripheral nerve injury and neuropathic pain.