A loose ligature-induced mononeuropathy produces hyperalgesias mediated by both the injured sciatic nerve and the adjacent saphenous nerve

Intrathecal Injection of miR-133b-3p or miR-143-3p Prevents the Development of Persistent Cold and Mechanical Allodynia Following a Peripheral Nerve Injury in Rats

In DRG an increase in miR-133b-3p, miR-143-3p, and miR-1-3p correlates with the lack of development of neuropathic pain following a peripheral nerve injury. Using lentiviral (LV) vectors we found that a single injection of LV-miR-133b-3p or LV-miR-143-3p immediately after a peripheral nerve injury prevented the development of sustained mechanical and cold allodynia. Injection of LV-miR-133b-3p or LV-miR-143-3p by themselves or in combination, on day 3 post-injury produced a partial and transient reduction in mechanical allodynia and a sustained decrease in cold allodynia. Injection of LV-miR-1-3p has no effect. Co-injection of LV-miR-1a with miR-133b-3p or miR-143-3p on day 3 post-injury produced a sustained decrease in mechanical and cold allodynia. In DRG cultures, miR-133b-3p and miR-143-3p but not miR-1-3p, enhanced the depolarization-evoked cytoplasmic calcium increase. Using 3'UTR target clones containing a Gaussian luciferase reporter gene we found that with the 3'UTR-Scn2b, miR-133-3p and miR-143-3p reduced the expression while miR-1-3p enhanced the expression of the reporter gene. With the 3'UTR-TRPM8, miR-133-3p and miR-143-3p reduced the expression and miR-1-3p had no effect. With the 3'UTR-Piezo2, miR-133-3p increased the expression while miR-143-3p and miR-1-3p had no effect. LV-miR133b-3p, LV-miR-143-3p and LV-miR1a-3p reduced Scn2b-mRNA and Piezo2-mRNA. LV-miR133b-3p and LV-miR-143-3p reduced TRPM8-mRNA. LV-miR-133b-3p and LV-miR-143-3p prevent the development of chronic pain when injected immediately after the injury, but are only partially effective when injected at later times. LV-miR-1a-3p had no effect on pain, but complemented the actions of LV-miR-133b-3p or LV-miR-143-3p resulting in a sustained reversal of pain when co-injected 3 days following nerve injury.

NR2B Expression in Rat DRG Is Differentially Regulated Following Peripheral Nerve Injuries That Lead to Transient or Sustained Stimuli-Evoked Hypersensitivity

Following injury, primary sensory neurons undergo changes that drive central sensitization and contribute to the maintenance of persistent hypersensitivity. NR2B expression in the dorsal root ganglia (DRG) has not been previously examined in neuropathic pain models. Here, we investigated if changes in NR2B expression within the DRG are associated with hypersensitivities that result from peripheral nerve injuries. This was done by comparing the NR2B expression in the DRG derived from two modalities of the spared nerve injury (SNI) model, since each variant produces different neuropathic pain phenotypes. Using the electronic von Frey to stimulate the spared and non-spared regions of the hindpaws, we demonstrated that sural-SNI animals develop sustained neuropathic pain in both regions while the tibial-SNI animals recover. NR2B expression was measured at Day 23 and Day 86 post-injury. At Day 23 and 86 post-injury, sural-SNI animals display strong hypersensitivity, whereas tibial-SNI animals display 50 and 100% recovery from post-injury-induced hypersensitivity, respectively. In tibial-SNI at Day 86, but not at Day 23 the perinuclear region of the neuronal somata displayed an increase in NR2B protein. This retention of NR2B protein within the perinuclear region, which will render them non-functional, correlates with the recovery observed in tibial-SNI. In sural-SNI at Day 86, DRG displayed an increase in NR2B mRNA which correlates with the development of sustained hypersensitivity in this model. The increase in NR2B mRNA was not associated with an increase in NR2B protein within the neuronal somata. The latter may result from a decrease in kinesin Kif17, since Kif17 mediates NR2B transport to the soma’s plasma membrane. In both SNIs, microglia/macrophages showed a transient increase in NR2B protein detected at Day 23 but not at Day 86, which correlates with the initial post-injury induced hypersensitivity in both SNIs. In tibial-SNI at Day 86, but not at Day 23, satellite glia cells (SGCs) displayed an increase in NR2B protein. This study is the first to characterize of cell-specific changes in NR2B expression within the DRG following peripheral nerve injury. We discuss how the observed NR2B changes in DRG can contribute to the different neuropathic pain phenotypes displayed by each SNI variant.

An approach to identify microRNAs involved in neuropathic pain following a peripheral nerve injury

Peripheral nerve injury alters the expression of hundreds of proteins in dorsal root ganglia (DRG). Targeting some of these proteins has led to successful treatments for acute pain, but not for sustained post-operative neuropathic pain. The latter may require targeting multiple proteins. Since a single microRNA (miR) can affect the expression of multiple proteins, here, we describe an approach to identify chronic neuropathic pain-relevant miRs. We used two variants of the spared nerve injury (SNI): Sural-SNI and Tibial-SNI and found distinct pain phenotypes between the two. Both models induced strong mechanical allodynia, but only Sural-SNI rats maintained strong mechanical and cold allodynia, as previously reported. In contrast, we found that Tibial-SNI rats recovered from mechanical allodynia and never developed cold allodynia. Since both models involve nerve injury, we increased the probability of identifying differentially regulated miRs that correlated with the quality and magnitude of neuropathic pain and decreased the probability of detecting miRs that are solely involved in neuronal regeneration. We found seven such miRs in L3-L5 DRG. The expression of these miRs increased in Tibial-SNI. These miRs displayed a lower level of expression in Sural-SNI, with four having levels lower than those in sham animals. Bioinformatic analysis of how these miRs could affect the expression of some ion channels supports the view that, following a peripheral nerve injury, the increase of the seven miRs may contribute to the recovery from neuropathic pain while the decrease of four of them may contribute to the development of chronic neuropathic pain. The approach used resulted in the identification of a small number of potentially neuropathic pain relevant miRs. Additional studies are required to investigate whether manipulating the expression of the identified miRs in primary sensory neurons can prevent or ameliorate chronic neuropathic pain following peripheral nerve injuries.

Low-level laser therapy alleviates neuropathic pain and promotes function recovery in rats with chronic constriction injury: possible involvements in hypoxia-inducible factor 1α (HIF-1α)

Nerve inflammation plays an important role in the development and progression of neuropathic pain after chronic constrictive injury (CCI). Recent studies have indicated that hypoxia-inducible factor 1α (HIF-1α) is crucial in inflammation. Low-level laser therapy has been used in treating musculoskeletal pain, but rare data directly support its use for neuropathic pain. We investigated the effects of low-level laser on the accumulation of HIF-1α, tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β) in controlling neuropathic pain, as well as on the activation of vascular endothelial growth factor (VEGF) and nerve growth factor (NGF) in promoting functional recovery in a rat CCI model. CCI was induced by placing four loose ligatures around the sciatic nerve of rats. Treatments of low-level laser (660 nm, 9 J/cm(2)) or sham irradiation (0 J/cm(2)) were performed at the CCI sites for 7 consecutive days. The effects of laser in animals with CCI were determined by measuring the mechanical paw withdrawal threshold, as well as the sciatic, tibial, and peroneal function indices. Histopathological and immunoassay analyses were also performed. Low-level laser therapy significantly improved paw withdrawal threshold and the sciatic, tibial, and peroneal functional indices after CCI. The therapy also significantly reduced the overexpressions of HIF-1α, TNF-α, and IL-1β, and increased the amounts of VEGF, NGF, and S100 proteins. In conclusion, a low-level laser could modulate HIF-1α activity. Moreover, it may also be used as a novel and clinically applicable therapeutic approach for the improvement of tissue hypoxia/ischemia and inflammation in nerve entrapment neuropathy, as well as for the promotion of nerve regeneration. These findings might lead to a sufficient morphological and functional recovery of the peripheral nerve.

High-dose remifentanil prevents development of thermal hyperalgesia in a neuropathic pain model

BACKGROUND

Intraoperative nerve lesions can lead to chronic postoperative pain. There are conflicting data as to whether or not anaesthetics administered intraoperatively are beneficial. We investigated if remifentanil administered at the time of nerve injury was able to attenuate neuropathic hypersensitivity.

METHODS

Rats were anaesthetized with isoflurane, endotracheally intubated, and a tail vein catheter was inserted. Rats received an i.v. infusion of either saline or low- or high-dose remifentanil (2 or 20 μg kg(-1) min(-1), respectively) for 20 min. During this time, rats received a spinal nerve L5 transection to induce neuropathic pain or a sham procedure. Behavioural tests to assess mechanical and cold allodynia and heat hyperalgesia were performed on postoperative days 1, 3, 7, 14, 21, and 28.

RESULTS

Sham-operated animals exhibited no hypersensitivity regardless of the intraoperative remifentanil dose. In rats which received spinal nerve L5 transection, mechanical and cold allodynia developed with no significant differences between treatment groups. However, thermal hyperalgesia was reduced in rats given high-dose remifentanil: mean (standard deviation) area under the curve 426 (53) compared with 363 (34) and 342 (24) in saline or low-dose remifentanil treated rats, respectively (P<0.05).

CONCLUSIONS

High-dose remifentanil administered at the time of transection of the spinal nerve at L5 prevents subsequent thermal hyperalgesia.

TNF-alpha and neuropathic pain--a review

Tumor necrosis factor alpha (TNF-α) was discovered more than a century ago, and its known roles have extended from within the immune system to include a neuro-inflammatory domain in the nervous system. Neuropathic pain is a recognized type of pathological pain where nociceptive responses persist beyond the resolution of damage to the nerve or its surrounding tissue. Very often, neuropathic pain is disproportionately enhanced in intensity (hyperalgesia) or altered in modality (hyperpathia or allodynia) in relation to the stimuli. At time of this writing, there is as yet no common consensus about the etiology of neuropathic pain - possible mechanisms can be categorized into peripheral sensitization and central sensitization of the nervous system in response to the nociceptive stimuli. Animal models of neuropathic pain based on various types of nerve injuries (peripheral versus spinal nerve, ligation versus chronic constrictive injury) have persistently implicated a pivotal role for TNF-α at both peripheral and central levels of sensitization. Despite a lack of success in clinical trials of anti-TNF-α therapy in alleviating the sciatic type of neuropathic pain, the intricate link of TNF-α with other neuro-inflammatory signaling systems (e.g., chemokines and p38 MAPK) has indeed inspired a systems approach perspective for future drug development in treating neuropathic pain.

Contralateral high or a combination of high- and low-frequency transcutaneous electrical nerve stimulation reduces mechanical allodynia and alters dorsal horn neurotransmitter content in neuropathic rats

The purpose of the study was to examine the effect of 3 different application strategies for transcutaneous electrical nerve stimulation (TENS) on neuropathy-induced allodynia and dorsal horn neurotransmitter content. Rats were treated with high-frequency, low-frequency, or a combination of high and low-frequency stimulation. TENS was delivered through self-adhesive electrodes daily for 1 hour to rats with a right-sided chronic constriction injury (CCI). Stimulation was delivered to skin or acupuncture points on the left and mechanical and thermal pain thresholds were assessed in the right hind paw. Neurotransmitter content was assessed bilaterally in the dorsal horn of the spinal cord. Daily, high-frequency or a combination of high- and low-frequency TENS reduced mechanical (P < .001), but not thermal allodynia in the right hind paw when compared with untreated CCI rats. Daily high frequency TENS elevated the dorsal horn synaptosomal content of GABA bilaterally (P < .014) and a combination of high- and low-frequency TENS elevated the dorsal horn content of aspartate (P < .001), glutamate (P < .001) and glycine (P < .001) bilaterally over that seen in untreated CCI rats. The present findings support a contralateral approach to the application of TENS and suggest that distinct strategies for TENS application may differentially alter neurotransmission in the central nervous system.

PERSPECTIVE

Because CCI rats are reminiscent of humans with neuropathy, daily high or a combination of high- and low-frequency TENS may reduce mechanical allodynia in humans with neuropathic pain. Because the 2 intervention strategies produce distinctive alterations in spinal cord neurotransmitter content, each may represent a distinctive option for treatment.

Changes in synaptic effectiveness of myelinated joint afferents during capsaicin-induced inflammation of the footpad in the anesthetized cat

The present series of experiments was designed to examine, in the anesthetized cat, the extent to which the synaptic efficacy of knee joint afferents is modified during the state of central sensitization produced by the injection of capsaicin into the hindlimb plantar cushion. We found that the intradermic injection of capsaicin increased the N2 and N3 components of the focal potentials produced by stimulation of intermediate and high threshold myelinated fibers in the posterior articular nerve (PAN), respectively. This facilitation lasted several hours, had about the same time course as the paw inflammation and was more evident for the N2 and N3 potentials recorded within the intermediate zone in the L6 than in the L7 spinal segments. The capsaicin-induced facilitation of the N2 focal potentials, which are assumed to be generated by activation of fibers signaling joint position, suggests that nociception may affect the processing of proprioceptive and somato-sensory information and, probably also, movement. In addition, the increased effectiveness of these afferents could activate, besides neurons in the intermediate region, neurons located in the more superficial layers of the dorsal horn. As a consequence, normal joint movements could produce pain representing a secondary hyperalgesia. The capsaicin-induced increased efficacy of the PAN afferents producing the N3 focal potentials, together with the reduced post-activation depression that follows high frequency autogenetic stimulation of these afferents, could further contribute to the pain sensation from non-inflamed joints during skin inflammation in humans. The persistence, after capsaicin, of the inhibitory effects produced by stimulation of cutaneous nerves innervating non-inflamed skin regions may account for the reported reduction of the articular pain sensations produced by trans-cutaneous stimulation.

Local Nogo-66 administration reduces neuropathic pain after sciatic nerve transection in rat

Neuropathic pain after periphery nerve injury is frequently accompanied by the regeneration of the injured nerve fibers. We tested in this study whether local administration of Nogo-66, a well-studied axon growth inhibiting peptide in the central nerve system, could reduce the pain related behavior after sciatic nerve transection in rat. Nogo-66 peptide was purified as a GST fusion protein. Its inhibitory function was testified by neurite outgrowth assay of primary cultured neurons, and then it was given directly at the lesion site by a minipump for 2 weeks. Mechanical nociceptive withdrawal responses and heat hyperalgesia responses were assessed during a 4-week period, and autotomy was evaluated during a 6-week period. The results showed that the mechanical allodynia and heat hyperalgesia scores of the rats treated with GST-Nogo-66 were significantly higher than the controls between 7 and 14 days after sciatic nerve transection. The autotomy scores in the GST-Nogo-66 group were significantly lower than the controls from 28 days after surgery. Taken together, the results of our present study suggest that Nogo-66 may be utilized to decrease the neuropathic pain after periphery nerve injury.

Refinement of Symptoms of Neuropathic Pain Measurements After Various Transections of the Nerve Endings of the Sciatic and Femoral Nerve in Rats: An Exploratory Behavioral Analysis

BACKGROUND

Many animal models can be used to study the underlying pathophysiological mechanisms of neuropathic pain. Most of these models rely on a partial denervation of the limb of the animal by ligating a selected nerve. In this study, we performed nerve lesions on three peripheral nerves supplying the plantar side of the rat hindpaw by differentially transecting the saphenous, the tibial, and the sural nerves alone or in paired combinations.

METHODS

The development of neuropathic pain symptoms at three different anatomical areas (medial, central, and lateral) of the glabrous skin of the hindpaw was evaluated by sensory testing over a 12-wk period. Mechanical hyperalgesia (pinprick), cold allodynia (acetone), and abnormalities of hindpaw posture were continuously present in animals with tibial and tibial and saphenous nerve transection.

RESULTS

Transection of the tibial and sural nerves induced cold allodynia and moderate mechanical hyperalgesia. Transection of the sural, the saphenous, or both nerves simultaneously induced no signs of specific neuropathic pain behavior and no abnormalities in posture of the affected hindpaw were noted after adequate stimulation.

CONCLUSIONS

The overlapping innervation of nerve distribution can complicate the interpretation of nerve ligation studies of peripheral neuropathies.

Transcutaneous Electrical Nerve Stimulation for the Management of Neuropathic Pain: The Effects of Frequency and Electrode Position on Prevention of Allodynia in a Rat Model of Complex Regional Pain Syndrome Type II

Background and Purpose. Complex regional pain syndrome type II (CPSII) is a painful condition that develops following a nerve injury. Although transcutaneous electrical nerve stimulation (TENS) relieves the pain of CPSII, the stimulation parameters that would best prevent the development of the condition are not known. The purpose of this study was to compare the ability of several different stimulation strategies to reduce the development of allodynia. Subjects. Sprague-Dawley rats were used in the study. Methods. A chronic constriction injury (CCI) to the right sciatic nerve was used to induce allodynia. Two groups of CCI rats received high-frequency TENS to the lumbar paravertebral region with electrodes positioned on the skin overlying either the right or left paraspinal musculature. Two additional groups of CCI rats received low-frequency TENS to acupuncture points in the right or left hind limbs. A fifth group of CCI rats received no TENS intervention. Thermal and mechanical pain thresholds were assessed in the right hind paw before and 12 days after the CCI surgery. The TENS was delivered 1 hour per day beginning on the day of surgery. Results. Daily high-frequency TENS reduced the development of mechanical allodynia in CCI rats, and low-frequency TENS reduced the development of thermal allodynia, but only when TENS was delivered on the left side. Discussion and Conclusion. The results indicate that TENS delivered contralateral to a nerve injury best reduces allodynia development. Comprehensive reduction of allodynia development would require a combination of high- and low-frequency TENS intervention.

Ameroid rings for gradual chronic constriction of the sciatic nerve in rats: contribution of different nerves to neuropathic pain

Mononeuropathy was induced by placing an ameroid ring around the sciatic nerve and was compared with chronic constriction injury (CCI) of the sciatic nerve [Pain 33 (1988) 87] in rats. Mechanical allodynia was assessed and the role of sciatic and saphenous afferents (Adelta and C) in thermal hyperalgesia investigated. A shorter duration of mechanical allodynia in ameroid rats as compared to CCI rats was observed. Thermal hyperalgesia was observed in the saphenous innervated skin of the hindpaw for Adelta and C nociceptors in ameroid and for Adelta nociceptors only in CCI rats, respectively. The sciatic innervated skin showed a thermal hypoalgesia with a fast onset for Adelta afferents and a slower onset for C afferents in CCI and ameroid rats. The duration of both thermal hypo- and hyperalgesia was longer in ameroid rats. We conclude that ameroid rings are a useful tool for the investigation of long-duration hyperalgesic effects of nerve injury, as the effects were more stable and seen for a longer time (>8 weeks) as compared to the CCI model. The uninjured saphenous afferents, in particular C fibers, mediate thermal hyperalgesia after chronic constriction of the sciatic nerve using an ameroid ring.

The relationship between dorsal horn neurotransmitter content and allodynia in neuropathic rats treated with high-frequency transcutaneous electric nerve stimulation

OBJECTIVE

To examine the relation between axon terminal neurotransmitter content in the dorsal horn and allodynia in neuropathic rats treated with high-frequency transcutaneous electric nerve stimulation (TENS).

DESIGN

A completely randomized experimental design. Two groups of rats received a chronic constriction injury to the right sciatic nerve, and 2 groups did not. The rats were either treated or not treated with TENS.

SETTING

Research laboratory.

ANIMALS

Adult male Sprague-Dawley rats (150-165g).

INTERVENTIONS

TENS was delivered daily for 1 hour to the chronic constriction injury rats or to the uninjured rats through self-adhesive electrodes applied to the skin innervated by the right dorsal rami of lumbar spinal nerves 1 to 6.

MAIN OUTCOME MEASURES

Thermal and mechanical pain thresholds were assessed bilaterally in the hind paws of all rats twice before the chronic constriction injury surgery (baseline) and then 12 days after the surgery. An analogous time frame of assessment was used for rats that did not have chronic constriction injury surgery. Thermal and mechanical allodynia were expressed as difference scores between the pain thresholds of the right and left hind paws. These values were normalized to differences that existed between the 2 paws at baseline. The amino acid content of dorsal horn axon terminals was assessed bilaterally with high-pressure liquid chromatography, and values were normalized to wet weight.

RESULTS

The mean level of thermal and mechanical allodynia did not differ between the TENS-treated and untreated rats with chronic constriction injury. However, there was a significant relation between the dorsal horn, axon terminal content of glutamate (adjusted R(2)=.45, P<.01) and glycine (adjusted R(2)=.51, P<.005) and the magnitude of mechanical allodynia present in TENS-treated chronic constriction injury rats, but not in any other group. As axon terminal glutamate and glycine decreased in the right dorsal horn and increased in the left, mechanical allodynia was reduced or absent. When this trend was reversed, mechanical allodynia was more severe. Daily TENS also reduced the mean axon terminal content of aspartate, glutamate, and glycine bilaterally in the chronic constriction injury rats from the level observed in untreated neuropathic rats (P<.05).

CONCLUSION

The variability in responsiveness of mechanical allodynia to daily TENS treatment in neuropathic rats is related to the axon terminal content of glutamate and glycine in the dorsal horn. These findings may help explain a similar variability in humans when TENS is used to treat neuropathic pain.

Reflex sympathetic dystrophy syndrome and neuromediators

Concepts related to the pathophysiology of reflex sympathetic dystrophy syndrome (RSDS) are changing. Although sympathetic influences are still viewed as the most likely mechanism underlying the development and/or perpetuation of RSDS, these influences are no longer ascribed to an increase in sympathetic tone. Rather, the most likely mechanism may be increased sensitivity to catecholamines due to sympathetic denervation with an increase in the number and/or sensitivity of peripheral axonal adrenoceptors. Several other pathophysiological mechanisms have been suggested, including neurogenic inflammation with the release of neuropeptides by primary nociceptive afferents and sympathetic efferents. These neuromediators, particularly substance P, calcitonin gene-related peptide, and neuropeptide Y (NPY), may play a pivotal role in the genesis of pain in RSDS. They induce an inflammatory response (cutaneous erythema and edema) and lower the pain threshold. Neurogenic inflammation at the site of the lesion with neuromediator accumulation or depletion probably contributes to the pathophysiology of RSDS. However, no single neuromediator has been proved responsible, and other hypotheses continue to arouse interest.

Activation of diffuse noxious inhibitory controls (DNIC) in rats with an experimental peripheral mononeuropathy

Diffuse noxious inhibitory controls (DNIC), which involve supraspinal structures and modulate the transmission of nociceptive signals, were investigated in rats with chronic constriction injury of the sciatic nerve. Nerve-injured rats with increased sensitivity to mechanical and thermal stimulation on the operated side were anesthetized and recordings were made from trigeminal convergent neurons. Inhibitions of C-fiber-evoked neuronal responses during and after the application of nociceptive conditioning stimuli to the hindpaw, were measured to evaluate DNIC. The conditioning stimuli consisted of graded natural (pressure and heat) and electrical stimuli and were applied alternately to non-operated and operated hindpaws. Compared with the non-operated paw, inhibitions elicited by pressure on the operated hindpaw were increased significantly at all stimulus intensities. Albeit to a lesser extent, inhibitions elicited by thermal stimulation of the operated paw were also increased in the nerve-injured animals. Such exacerbation of DNIC-induced inhibitions produced by mechanical and thermal stimulation of the operated paw can be explained by an increase in the afferent input to the spinal cord. In contrast to the results obtained with natural stimulations, inhibitions evoked from the operated and non-operated paws were similar when graded electrical stimulation was used as the conditioning stimulus. This was true regardless of the intensity and frequency of stimulation and regardless of whether the stimuli were applied transcutaneously or directly to the sciatic nerve. The clear-cut difference between the results obtained with natural and electrical conditioning stimuli suggests that the nociceptive neurons involved in the triggering of DNIC may not be sensitized at the central level. Peripheral mechanisms such as the sensitization of nerve injured fibers and/or sprouting of nerve terminals may thus be the main causes of DNIC increase in this model of neuropathic pain.

The alpha(2A) adrenoceptor and the sympathetic postganglionic neuron contribute to the development of neuropathic heat hyperalgesia in mice

We have addressed the role of the sympathetic nervous system in the development and maintenance of neuropathic pain. Using a new neuropathic mouse model, we examined the development of hyperalgesia in transgenic mice lacking functional alpha(2A) adrenoceptors and in sympathectomized wild-type mice, to determine if sympathetic-sensory coupling generates hyperalgesia. The development of neuropathic heat hyperalgesia required the presence of both the alpha(2A) adrenoceptor and the sympathetic postganglionic neuron (SPGN), but the development of mechanical hyperalgesia did not require either the alpha(2A) adrenoceptor or the SPGN, indicating different mechanisms of sensitization. These results suggest that the development of neuropathic heat hyperalgesia, but not mechanical hyperalgesia, requires sympathetic-sensory coupling in the peripheral nervous system. Nerve injury enhanced the analgesic efficacy of the alpha(2) adrenoceptor agonist dexmedetomidine, and paradoxically also induced an analgesic response to alpha(2) adrenoceptor antagonists. The alpha(2) agonist-evoked analgesia to mechanical stimuli was mediated by activating central alpha(2A) adrenoceptors, possibly at the spinal level. The peripherally restricted alpha(2) antagonist L659,066 evoked analgesia for heat, but not for mechanical stimuli, findings which support the hypothesis that the peripheral alpha(2) adrenoceptor plays a role in both the development and the maintenance of neuropathic heat hyperalgesia. The alpha(2) antagonist-evoked analgesia for heat stimuli was mediated by blocking peripheral and probably central alpha(2) adrenoceptors, while the analgesia for mechanical stimuli was mediated by blocking central alpha(2A) adrenoceptors. Intradermal injections with an alpha(2) agonist or antagonist had no effect on nociceptive thresholds, indicating that sympathetic-sensory coupling at the level of the cutaneous nociceptor did not contribute to the maintenance of neuropathic hyperalgesia.

Spontaneous activity of rat dorsal horn cells in spinal segments of sciatic projection following transection of sciatic nerve or of corresponding dorsal roots

Natural forms of stimulation were used to compare the spontaneous and evoked activity of dorsal horn neurons in three groups of rats: controls with no surgical lesion, rats with transection of the sciatic nerve and rats with transection of the dorsal roots at the same segmental level. In control rats, cells encountered in the dorsal horn were classified according to their peripheral field as tactile specific, convergent tactile and nociceptive, nociceptive, or movement driven. In 20 control animals, only 20% of the 140 cells with a peripheral field were spontaneously active. After sciatic nerve transection made on the side of recording a few days previously (18 rats), all of the 141 cells studied showed spontaneous activity, only 69 of them having a peripheral field. After dorsal root transections a few days previously (nine rats), 25 spontaneously active cells were found in the dorsal horn ipsilateral to the section, none with a peripheral field. Spontaneous activities of cells without a peripheral field were separated into three types as a function of bursting pattern, which were similar following both types of transection. The spontaneous activity shown by dorsal horn cells without peripheral fields following dorsal root transection precludes attribution of spontaneous spiking in such cells to abnormal input from the periphery, and shows that abnormal activity can develop in deafferented dorsal horn cells themselves. A possible role played by this spontaneous activity in deafferentation pain is considered.

Reflex sympathetic dystrophy: facts and hypotheses

Reflex sympathetic dystrophy (RSD) syndrome has been recognized clinically for many years. It is most often initiated by trauma to a nerve, neural plexus, or soft tissue. Diagnostic criteria are the presence of regional pain and other sensory changes following a noxious event. The pain is associated with changes in skin colour, skin temperature, abnormal sweating, oedema, and sometimes motor abnormalities. The clinical course is commonly divided into three stages: first (acute or hyperaemic), second (dystrophic or ischaemic), and third (atrophic) stage. The diagnosis is primarily clinical, but roentgenography, scintigraphy, thermography, electromyography and assessment of nerve conduction velocity can help to confirm the diagnosis. Although a wide variety of treatments have been recommended, the only therapies found to be effective in large studies aim at interfering with the activity of the sympathetic nervous system. To this end, efferent sympathetic nerve activity can be interrupted surgically or chemically. Alternatively, adrenoceptor blockers may be used to relieve pain. Numerous theories have been proposed to explain the pathophysiology. Sympathetic dysfunction, which often has been purported to play a pivotal role in RSD, has been suggested to consist of an increased rate of efferent sympathetic nerve impulses towards the involved extremity induced by increased afferent activity. However, the results of several experimental studies suggest that sympathetic dysfunction consists of supersensitivity to catecholamines induced by (partial) autonomic denervation. Besides, it has been suggested that excitation of sensory nerve fibres at axonal level causes release of neuropeptides at the peripheral endings of these fibres. These neuropeptides may induce vasodilation, increase vascular permeability, and excite surrounding sensory nerve fibres -- a phenomenon referred to as neurogenic inflammation. At the level of the central nervous system, it has been suggested that the increased input from peripheral nociceptors alters the central processing mechanisms.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

High-frequency transcutaneous electrical nerve stimulation alters thermal but not mechanical allodynia following chronic constriction injury of the rat sciatic nerve

OBJECTIVE

To determine if daily transcutaneous electrical nerve stimulation (TENS) can alter the thermal and mechanical allodynia that develops after chronic constriction injury (CCI) to the right sciatic nerve of rats.

DESIGN

A completely randomized experimental design was used. Four groups of rats underwent CCI surgery to the right sciatic nerve and either were not treated with TENS or received TENS starting at different times after the CCI surgery.

INTERVENTIONS

TENS was delivered daily for 1 hour to CCI rats through self-adhesive electrodes applied to skin innervated by the right dorsal rami of lumbar spinal nerves L1-6. Rats of different groups received daily TENS starting immediately, 20 to 30 hours, or 3 days after the CCI surgery.

MAIN OUTCOME MEASURE

Thermal and mechanical pain thresholds of hind paws were assessed bilaterally in all rats twice before the CCI surgery (baseline) and then 2, 7, 12, and 14 days after surgery. Thermal and mechanical allodynia were expressed as difference scores between the pain thresholds of right and left hind paws. These values were normalized to differences that existed between the two paws at baseline.

RESULTS

Daily TENS beginning immediately after CCI surgery prevented the development of thermal allodynia at all assessment times (p < .05). Daily TENS starting 1 day after surgery reduced thermal allodynia, but only on days 2 and 14 (p < .05). Daily TENS beginning 3 days after surgery had no effect on the development of thermal allodynia. Regardless of when it was started, daily TENS did not consistently alter mechanical allodynia in CCI rats.

CONCLUSION

It appears that daily TENS can prevent thermal but not mechanical allodynia in this model. However, early intervention with the treatment is critical if it is to be effective at all.

The analgesic potency of dexmedetomidine is enhanced after nerve injury: a possible role for peripheral alpha2-adrenoceptors

This study investigated the analgesic potency and site of action of systemic dexmedetomidine, a selective alpha2-adrenoceptor (alpha2AR) agonist, in normal and neuropathic rats. Ligation of the L5-6 spinal nerves produced a chronic mechanical and thermal neuropathic hyperalgesia in rats. von Frey fibers and a thermoelectric Peltier device were used to measure mechanical and heat withdrawal thresholds over the hindpaw. Systemic dexmedetomidine dose-dependently increased the mechanical and thermal thresholds in the control animals (50% effective dose [ED50] 144 and 180 microg/kg intraperitoneally [i.p.], respectively). Neuropathic animals responded to much smaller doses of dexmedetomidine with mechanical and thermal ED50 values of 52 and 29 microg/kg i.p., respectively. There was no difference between the control and neuropathic animals with respect to dexmedetomidine-evoked sedation, as determined by decreased grid crossings in an open-field activity chamber (ED50 12 and 9 microg/kg i.p., respectively). Atipamezole, a selective alpha2AR antagonist, blocked the analgesic and sedative actions of dexmedetomidine inboth the neuropathic and control animals. However, L-659,066, a peripherally restricted alpha2AR antagonist, could only block the analgesic actions of dexmedetomidine in the neuropathic rats, with no effect in control animals. In conclusion, nerve injury enhanced the analgesic but not the sedative potency of systemic dexmedetomidine and may have shifted the site of alpha2 analgesic action to outside the blood-brain barrier.

IMPLICATIONS

We tested the analgesic efficacy of the alpha2 agonist dexmedetomidine in normal and nerve-injured rats. The analgesic potency of dexmedetomidine was enhanced after nerve injury with a site of action outside the central nervous system. Peripherally restricted alpha2 agonists may be useful in the management of neuropathic pain.

Motor denervation induces altered muscle fibre type densities and atrophy in a rat model of neuropathic pain

Loose ligation of a sciatic nerve in rats (chronic constriction injury; CCI) provokes sensory, autonomic, and motor disturbances like those observed in humans with partial peripheral nerve injury. So far, it is unknown whether these motor disturbances result from (mechanical) allodynia or from damage to the motor neuron. These considerations prompted us to assess, in CCI rats, the density of motor axons in both the ligated sciatic nerve and the ipsilateral femoral nerve. To this end, we determined the number of cholinesterase positive fibres. It has been demonstrated previously that muscle fibre type density may be used as a measure of motor denervation and/or hypokinesia. Therefore, the myofibrillar ATPase reaction was employed to assess fibre type density in biopsies obtained from the lateral gastrocnemius muscle (innervated by sciatic nerve) and rectus femoris muscle (innervated by femoral nerve). We observed axonal degeneration of motor fibres within the loosely ligated sciatic nerve, both at an intermediate (day 21) and at a late stage (day 90) after nerve injury. The reduction in the number of motor nerve fibres was more pronounced distal to the site of the ligatures than proximal. A (less pronounced) reduction of motor fibres was observed in the ipsilateral (non-ligated) femoral nerve. In line with these findings, we observed altered fibre type densities in muscle tissue innervated by the ligated sciatic nerve as well as the non-ligated femoral nerve indicative of motor denervation rather than hypokinesia. The findings of this study suggest that the motor disorder induced by partial nerve injury involves degeneration of motor nerve fibres not only within the primarily affected nerve but also within adjacent large peripheral nerves. This spread outside the territory of the primarily affected nerve suggests degeneration of motor neurons at the level of the central nervous system.

Somatotopic redistribution of c-fos expressing neurons in the superficial dorsal horn after peripheral nerve injury

The functional somatotopic reorganization of the lumbar spinal cord dorsal horn after nerve injury was studied in the rat by mapping the stimulus-evoked distribution of neurons expressing proto-oncogene c-fos. In three different nerve injury paradigms, the saphenous nerve was electrically stimulated at C-fibre strength at survival times ranging from 40 h to more than six months: 1) Saphenous nerve stimulation from three weeks onwards after ipsilateral sciatic nerve transection resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous territory in laminae I-II, and an expansion of the saphenous territory into the denervated sciatic territory until 14 weeks postinjury. 2) Saphenous nerve stimulation from five days onwards after ipsilateral sciatic nerve section combined with saphenous nerve crush resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous nerve territory, and an expansion of the saphenous nerve territory into the denervated sciatic nerve territory. 3) Stimulation of the crushed nerve (without previous adjacent nerve section) at five days, but not at eight months resulted in a temporary increase in the number of Fos-immunoreactive neurons within the territory of the injured nerve, and no change in area at either survival time. The results indicate that nerve injury results in an increased capacity of afferents in an adjacent uninjured, or regenerating nerve, to excite neurons both in its own and in the territory of the permanently injured nerve in the dorsal horn. The onset and duration of the increased postsynaptic excitability and expansion depends on the types of nerve injuries involved. These findings indicate the complexity of the central changes that follows in nerve injuries that contain a mixture of uninjured, regenerating and permanently destroyed afferents.

Colchicine treatment of the sciatic nerve reduces neurogenic extravasation, but does not affect nociceptive thresholds or collateral sprouting in neuropathic or normal rats

The effect of topical colchicine treatment of the sciatic nerve on sciatic and saphenous nociceptive thresholds and neurogenic extra-vasation was investigated in normal and neuropathic rats. After a pilot investigation using several different concentrations of colchicine it was determined that treating the sciatic nerve with 5 mM colchicine did not usually affect the heat nociceptive threshold over the sciatic innervated plantar surface of the hindpaw. Mechanical nociception and motor function were also unchanged. Electrical stimulation of the sciatic nerve after intravenous injection of Evans blue dye causes extravasation of the dye in the cutaneous distribution of the nerve. The area and quantity of sciatic extravasation were measured 3 weeks after treating the sciatic nerve with colchicine. This treatment results in a marked loss of neurogenic extravasation, but there were no changes in the sciatic and saphenous mediated heat and mechanical nociceptive thresholds. The area of saphenous nociceptive innervation was mapped using pinch responses and saphenous neurogenic extravasation acutely after sciatic section. There was no change in the cutaneous distribution of saphenous nociceptive fibers when measured 3 weeks after the sciatic colchicine treatment. Some rats had their sciatic nerves transected immediately after colchicine treatment (5 and 50 mM) and the saphenous nociceptive thresholds and autotomy scores were followed postoperatively. Colchicine pretreatment of the sciatic nerve has no effect on the development of hyperalgesia or autotomy. Colchicine blocks axonal transport in peripheral nerve, including the orthograde transport of tachykinins, which probably explains its ability to induce prolonged reductions in sciatic neurogenic extravasation at concentrations that spare C-fiber nociceptor function. Sciatic nerve colchicine treatment does not trigger nociceptive fiber collateral sprouting from the adjacent saphenous nerve, nor does it influence the development of hyperalgesia and autotomy behavior after sciatic transection.

Submodality-selective hyperalgesia adjacent to partially injured sciatic nerve in the rat is dependent on capsaicin-sensitive afferent fibers and independent of collateral sprouting or a dorsal root reflex

We studied submodality dependence of sensory changes produced by unilateral ligation of the sciatic or the saphenous nerve in the rat. We focused especially on sensory changes in the skin area adjacent to the innervation area of the injured nerve. Moreover, we examined the roles of capsaicin-sensitive nociceptive fibers, collateral sprouting and a dorsal root reflex in sensory changes observed behaviorally. Assessment of sensory changes was performed by a pattern of behavioral tests: hot-plate test and hindlimb withdrawal responses induced by radiant heat, hot-water bath, innocuous mechanical stimuli, and noxious mechanical stimuli. In one group, the saphenous nerve ipsilateral to the sciatic ligation was topically treated with capsaicin (1%) at the time of the surgery. A proximal stump of a saphenous nerve strand was orthodromically stimulated to induce a dorsal root reflex (an antidromic volley) in nociceptive fibers of the saphenous nerve trunk. For visualization of plasma extravasation induced by a dorsal root reflex, a dye-labeling (Evans blue) technique was used. A collateral sprouting of nociceptive fibers of the uninjured saphenous nerve was evaluated by determining the plasma extravasation response induced by antidromic stimulation of the saphenous nerve. Three and 10 days following the sciatic constriction injury, the hindlimb withdrawal threshold evoked by noxious mechanical stimulation of the medial side of the paw (the innervation are of the intact saphenous nerve) was significantly decreased. There was no corresponding thermal hyperalgesia adjacent to the injured sciatic nerve. Chronic constriction of the saphenous nerve did not produce any significant hyper- or hypoalgesia to mechanical or thermal stimulation of the uninjured sciatic nerve area. Topical treatment of the ipsilateral (intact) saphenous nerve at the time of the sciatic nerve ligation completely prevented the development of mechanical hyperalgesia in the medial side of the paw (the innervation area of the saphenous nerve). No dorsal root reflex in nociceptive fibers mediating the adjacent hyperalgesia could be evoked. No collateral sprouting of the uninjured nociceptive fibers of the saphenous nerve was observed. The results indicate that the constriction injury of the sciatic nerve produced a selective hyperalgesia to mechanical stimulation in the innervation area of the neighboring saphenous nerve. At the peripheral level, the mechanical hyperalgesia adjacent to the innervation area of the injured nerve was mediated by capsaicin-sensitive nociceptive fibers. Collateral sprouting of nociceptive fibers from the uninjured to the injured innervation area did not contribute to the present sensory findings. The sciatic nerve injury did not induce a dorsal root reflex in nociceptive fibers innervating the hyperalgesic saphenous nerve area.

Role of input from saphenous afferents in altered spinal processing of noxious signal that follows sciatic nerve constriction in rats

An investigation was made into a possible contribution of saphenous nerve to altered central processing of noxious information following sciatic nerve chronic constriction (CCI) in rats. Spinal sciatic neuron spontaneous and noxious evoked activities were recorded in CCI rats, CCI rats with saphenous nerve sectioned just before the sciatic nerve constriction and sham operated rats. The results show (1) high values of spontaneous and noxious evoked activities with prolonged afterdischarges in CCI rats with intact saphenous, (2) comparable high values of spontaneous activity, but significantly reduced noxious evoked activity and afterdischarges in CCI rats with sectioned saphenous, (3) values in the normal range in the sham rats. Potential mechanisms underlying these results are discussed.

Distal tibial mononeuropathy in diabetic and nondiabetic rats reared on wire cages: an experimental entrapment neuropathy

Using electrophysiological recordings, we studied a distal tibial mononeuropathy that involves the hind foot of rats reared in cages with wire grid flooring. In an initial set of experiments, serial sciatic-tibial motor conduction recordings were made in smaller or larger rats reared in cages with wire grid or sawdust flooring. Electrophysiological features of the neuropathy were loss in the amplitude of the distal tibial nerve M potential recorded over hind limb foot muscles, temporal dispersion of the potential, often into multiple peaks, and a prolonged distal latency of the response. The changes in M amplitude were more apparent in larger rats with a greater body weight. In a second series of experiments we studied sciatic-tibial conduction over 16 weeks in nondiabetic rats and rats rendered diabetic with streptozotocin raised and wire grid or plastic flooring. Tibial mononeuropathy developed in both wire grid-reared groups, but there was evidence that it appeared earlier in diabetic rats. Electrophysiological changes of distal mononeuropathy also obscured the expected slowing of sciatic-tibial motor conduction velocity from diabetics. Tibial mononeuropathy in rats reared on wire grid flooring may be a useful animal model of human entrapment neuropathy but its presence can confound studies of experimental neuropathy. Rats used in studies of experimental neuropathy should be housed in plastic cages with sawdust or shavings flooring.

Spinal cord stimulation in animal models of mononeuropathy: effects on the withdrawal response and the flexor reflex

Spinal cord stimulation (SCS) is efficacious for pain due to injury of peripheral nerves, and therefore models of mononeuropathy appear to be particularly suitable for an experimental approach to the study of mechanisms underlying the clinical effect of this mode of treatment in chronic neuropathic pain. Virtually all previous experimental studies on SCS have utilized acute and nociceptive types of peripheral pain stimuli to explore the attenuating effects of SCS. In the present study we made use of the two models of supposedly painful neuropathy developed by Bennett and Xie (1988) and Seltzer et al. (1990) to explore the effect of SCS applied with stimulus parameters similar to those used in clinical practice. In rats subjected to ligatures of the sciatic nerve according to these two methods, SCS was applied via chronically implanted electrodes, or acutely via a laminectomy in the lower thoracic region. In awake, freely moving animals SCS produced a marked increase of the withdrawal thresholds to innocuous mechanical stimuli in the form of von Frey filaments. This threshold elevation lasted for up to 40 min after 10 min of SCS. In about one-half of the animals there was also a moderate, but short-lasting increase in the intact leg. The degree and duration of the withdrawal threshold elevation was clearly related to the intensity of SCS which was kept below the level of which a response in the thoracic or leg musculature was produced. In a second series of experiments the effect of SCS, applied acutely via a laminectomy, on the early component (latency: 8-12 msec) of the flexor reflex was studied. As a result of nerve ligation with either of the methods used, the thresholds for evoking the early as well as the late component in the nerve-ligated leg were significantly lower than in the intact one. SCS resulted in a marked and long-lasting increase of the threshold of the early component in the nerve-ligated leg. On the intact side only a slight and short-lasting increase was observed. The late, C fibre-mediated component was not influenced by SCS. The first component of the flexor reflex is conceivably mediated by A beta-fibre activation and it presumably corresponds to the withdrawal response induced by innocuous mechanical stimuli. The lack of effect of SCS on the late reflex component indicates that it selectively influences transmission of A-fibre activity. (ABSTRACT TRUNCATED AT 400 WORDS)

Peripheral hyperalgesia in experimental neuropathy: mediation by alpha 2-adrenoreceptors on post-ganglionic sympathetic terminals

Rats in which the sciatic nerve is partially transected develop hyperalgesia which is relieved by sympathectomy. We carried out experiments using this model of experimental peripheral neuropathy to examine the peripheral mechanisms underlying sympathetically maintained pain. Subcutaneous injection of noradrenaline (NA) into the affected paw exacerbated the hyperalgesia but had no effect in control animals. Injection of the non-specific alpha-adrenergic blocker phentolamine and the alpha 2-adrenergic blocker yohimbine significantly relieved the hyperalgesia, while injection of the alpha 1-adrenergic blocker prazosin had no effect. Peripheral injection of the alpha 2-adrenergic agonist clonidine had no significant effect, while injection of the alpha 1-adrenergic agonist phenylephrine produced slight exacerbation of mechanical hyperalgesia. Hyperalgesia was eliminated by peripheral injection of indomethacin into the affected paw. Following a chemical sympathectomy, hyperalgesia was eliminated and injection of NA into the hyperalgesic paw had no effect on pain thresholds. We concluded that NA exacerbates hyperalgesia in this experimental model by acting on alpha 2-adrenoreceptors which are located on post-ganglionic sympathetic terminals. Our results are consistent with the proposal (Levine et al. 1986) that activation of these adrenoreceptors brings about an increased release of prostaglandins which sensitises nociceptors.

The resolution of neuropathic hyperalgesia following motor and sensory functional recovery in sciatic axonotmetic mononeuropathies

Nerve lesions producing extensive axonal loss can induce painful hyperalgesic states in man. The affect of axonal regeneration and end-organ reinnervation on hyperalgesia and pain is controversial. This study used two axonotmetic models, the sciatic crush injury (CI) and the sciatic chronic constrictive injury (CCI), to investigate the affects of nerve regeneration and reinnervation on hyperalgesia and presumed painful behavior in rats. The sciatic CI resulted in a transient loss of both sciatic motor function and the withdrawal response to pinch and heat in the sciatic distribution. Extensive recovery of motor function, pinch and heat response occurred over days 23-38 post-crush injury. This temporally corresponded with a plateau in the hindpaw autotomy score and a resolution of the saphenous-mediated pressure and heat hyperalgesia (adjacent neuropathic hyperalgesia; ANH) which developed over the medial dorsum of the hindpaw following the sciatic CI. In contrast, with sciatic transection and distal stump excision, no motor recovery occurs, large areas of the hindpaw remain unresponsive to heat and pinch, and the saphenous mediated ANH fails to resolve over a period of 3 months. When sciatic CI was compared to contralateral sciatic transection within the same rat, the bilateral saphenous-mediated pressure and heat thresholds were initially identical, but by 23-27 days post-crush, the crush side thresholds became hypoalgesic relative to the section side. This demonstrates an attenuation of the crush-induced ANH which temporally corresponds to the recovery of motor and sensory function. When the sciatic nerve was proximally crushed and distally transected (3 cm below the crush site), the saphenous-mediated pressure and heat threshold changes were identical (over 6 weeks of serial testing) to those produced by a contralateral sciatic transection within the same rat. This indicates that the microenvironments surrounding the regenerating axon tips did not differentially affect the development of ANH following sciatic CI or transection. The sciatic CCI resulted in a transient loss of hindpaw motor function without the loss of pinch or heat withdrawal responses in the sciatic distribution. Motor function recovery occurred primarily over days 23-59 post-ligature. During this prolonged period of motor function recovery there was a resolution of the sciatic-mediated plantar surface heat hyperalgesia and the saphenous-mediated heat ANH. The above data support the hypothesis that the successful regeneration of distal axons after axonotmetic lesions can initiate the resolution of neuropathic hyperalgesia.

Extra-territorial pain in rats with a peripheral mononeuropathy: mechano-hyperalgesia and mechano-allodynia in the territory of an uninjured nerve

The abnormal pain sensations that accompany peripheral neuropathies are sometimes found in a distribution that does not coincide with the territories of nerves or posterior roots. This 'extra-territorial' pain is one of the lines of evidence that has been advanced to support the proposal that these patients suffer from a psychogenic disorder. In the present experiments, rats were prepared with a unilateral chronic constriction injury (CCI) to the sciatic nerve. Beginning on the first postoperative day and continuing for at least 18 days, exaggerated withdrawal reflexes to pinprick stimulation, indicative of mechano-hyperalgesia, were seen on the side of nerve injury in the hindpaw territories of both the injured sciatic nerve and the uninjured saphenous nerve. Beginning on postoperative day 4 and continuing for at least the next 3 weeks, the withdrawal responses to von Frey hair stimulation on the nerve-injured side occurred at a significantly lower threshold, indicating the presence of mechano-allodynia. The severity and time course of the mechano-allodynia were similar in both nerve territories. When tested 18 days after the CCI, mechano-allodynia in the saphenous territory was abolished by an acute saphenous transection, but unaffected by sciatic transection. Conversely, mechano-allodynia evoked from the mid-plantar sciatic territory was abolished by acute sciatic transection, but unaffected by saphenous transection. These results show that rats with an experimental painful peripheral mononeuropathy have extra-territorial pain like that seen in man. Extra-territorial pain may be partly or entirely due to a peripheral nerve injury-evoked dysfunction of pain processing neurons in the central nervous system.