Methylprednisolone prevents the development of autotomy and neuropathic edema in rats, but has no effect on nociceptive thresholds

Corticosteroids are probably an effective treatment for some types of neuropathic pain and complex regional pain syndromes. This study examined the effects of systemic methylprednisolone (MP) on acute nociception and on pain behavior and hyperalgesia in normal and neuropathic rats. There was no dose-response to intraperitoneal MP (up to 12 mg/kg) for nociceptive thresholds to heat (Peltier) or mechanical (analgesy-meter and von Frey fibers) stimuli in normal rats. Chronic high dose MP (3 mg/kg per day for 21 days) also had no effect on acute nociceptive thresholds in normal rats. After sciatic nerve section in rats a saphenous nerve mediated hyperalgesia to heat and mechanical stimuli gradually developed over 21 days. High dose MP (3 mg/kg per day for 21 days) had no effect on this adjacent neuropathic hyperalgesia. When systemic MP was started immediately after bilateral sciatic and saphenous nerve transection there was a dose-dependent reduction in autotomy behavior. Substance P has been proposed as a mediator of neuropathic pain and edema. Single dose MP (12 mg/kg) slightly reduced the substance P mediated extravasation induced with electrical stimulation of the saphenous nerve. Chronic MP (3.4 mg/kg per day for 28 days) severely reduced the neurogenic extravasation induced with saphenous nerve stimulation. Sciatic sectioned rats developed hindpaw edema between 7 and 14 days after surgery, and this neuropathic edema did not develop in rats chronically treated with MP (3.4 mg/kg per day). These results demonstrate that corticosteroids did not affect nociceptive thresholds in normal or neuropathic hyperalgesic rats. Chronic steroid treatment did prevent the development of autotomy and neuropathic edema, and completely blocked neurogenic extravasation, findings consistent with the hypothesis that primary afferent substance P release mediates autotomy pain behavior and neuropathic edema. This may be a relevant model for examining the effects of corticosteroids on neuropathic pain and complex regional pain syndromes.     

Intact cutaneous C fibre afferent properties in mechanical and cold neuropathic allodynia

Patients with neuropathy, report changes in sensory perception, particularly mechanical and thermal allodynia, and spontaneous pain. Similar sensory changes are seen in experimental neuropathies, in which alteration in primary afferent properties can also be determined. The neural correlate of spontaneous pain is ongoing activity in sensory afferents. Mechanical and heat allodynia are thought to result from lowered activation thresholds in primary afferent and/or central neurones, but the mechanisms underlying cold allodynia are very poorly understood. We investigated nociceptive behaviours and the properties of C and A fibre intact afferents running adjacent to damaged afferents following a partial ligation injury of the saphenous nerve (PSNI). Animals developed mechanical and cold allodynia by 3 days after PSNI. Intact mechanosensitive C fibre afferents developed ongoing activity, and had slower conduction velocities 3 and 7 days following nerve injury, with no change in mechanical threshold. There was a large increase (～46‐fold) in calculated afferent input 3 days after nerve injury, as a result of the ongoing activity in these fibres. Mechano‐cooling‐sensitive C fibre afferents showed both enhanced cooling‐evoked firing, and increased ongoing activity. The afferent barrage associated with mechano‐cooling‐sensitive afferents was increased 26‐fold 7 days after nerve injury. We observed no differences in the properties of intact A fibre mechanosensitive afferents. These studies demonstrate for the first time that the altered nociception seen after PSNI is associated with ongoing activity and enhanced cooling‐evoked activity in intact C fibre afferents in the saphenous nerve, with no concurrent alteration in A fibre afferents.     

Depletion of noradrenaline inhibits electrically‐evoked pain in the skin of the human forearm

Guanethidine displaces noradrenaline from sympathetic varicosities, and blocks sympathetic noradrenergic neurotransmission by inhibiting the release of noradrenaline from depleted neural stores. The aim of this study was to determine whether depletion of noradrenaline with guanethidine would oppose thermal hyperalgesia and/or electrically‐evoked pain in mildly‐burnt skin. Guanethidine was transferred by iontophoresis into a small patch of skin on the forearm of 35 healthy human subjects. The heat‐pain threshold to a temperature gradient that increased at 0.5°C/s was then measured at the guanethidine site, a nearby saline‐control iontophoresis site, and in untreated skin. In addition, participants rated pain intensity to a 47°C stimulus that was applied to each site for 7s. Shortly after the iontophoreses, sensitivity to heat was greater at the guanethidine site than the two control sites, suggesting that ejection of noradrenaline from sympathetic varicosities increased sensitivity to heat. One day later, when neural stores of noradrenaline were depleted, sensitivity to heat did not differ between the guanethidine and control sites. The guanethidine pretreatment did not influence thermal hyperalgesia induced by a mild burn, but inhibited pain evoked by electrical stimulation of the skin (0.2mA direct current for 4min). These findings indicate that ongoing sympathetic neural discharge does not normally influence thermal hyperalgesia in inflamed skin, because depleting noradrenergic stores had no effect. However, electrically‐evoked release of noradrenaline may increase nociceptive sensations. Further clarification of this human pain model could provide insights into the mechanism of adrenergic hyperalgesia in certain neuropathic pain syndromes.     

Somatosensory function in patients with and without pain after traumatic peripheral nerve injury

Why traumatic injuries to the peripheral nervous system infrequently result in neuropathic pain is still unknown. The aim of this study was to examine the somatosensory system in patients with traumatic peripheral nerve injury with and without pain to try to unravel possible links to mechanisms underlying development and maintenance of pain. Eighteen patients with spontaneous ongoing pain and 16 patients without pain after unilateral partial peripheral traumatic nerve injury were studied. In the area of partial denervation and in the corresponding contralateral area perception thresholds to warmth, cold, light touch, pressure pain, cold‐ and heat pain were assessed as were pain intensities at suprathreshold heat pain stimulation. Comparing sides patients with pain reported allodynia to cold (p = 0.03) and pressure (p = 0.016) in conjunction with an increase in the perception threshold to non‐painful warmth (p = 0.024) on the injured side. Pain‐free patients reported hypoesthesia to light touch (p = 0.002), cold (p = 0.039) and warmth (p = 0.001) on the injured side. There were no side differences in stimulus–response functions using painful heat stimuli in any of the groups. In addition, no significant difference could be demonstrated in any sensory modality comparing side‐to‐side differences between the two groups. In conclusion, increased pain sensitivity to cold and pressure was found on the injured side in pain patients, pointing to hyperexcitability in the pain system, a finding not verified by a more challenging analysis of side‐to‐side differences between patients with and without pain.     

Pain in primary erythromelalgia--a neuropathic component?

Erythromelalgia is a condition characterized by attacks of red, hot, painful extremities with relief of symptoms by cooling and aggravation by warmth. Although the main emphasis has been on pathophysiological mechanisms related to circulatory changes, recent reports have focused on an involvement of efferent small nerve fibers indicating a neuropathic component. Since the symptoms resemble those described in neuropathic pain, we wanted to investigate the possible affection of afferent nerve fibers.

Twenty-five patients with primary erythromelalgia were examined by neurological testing, neurography and quantitative sensory testing. Thresholds for heat, cold, heat-pain and cold-pain detection were compared with those of a group of 29 healthy controls. The patients had significantly higher median heat (39.5 (36.1–40.8) and cold (29.3 (27.1–30.8)-detection thresholds at the dorsal aspects of their feet compared to the controls (37.0 (35.4–37.7) and 31.2 (30.3–31.5) respectively). These findings show an impaired small fiber function inside or close to the symptomatic area in this group of erythromelalgia patients. Seven patients had brush-evoked allodynia and fourteen had punctate hyperalgesia inside or close to the symptomatic areas in their feet.

When comparing the individual results, there is a tendency to clustering of patients in two separate groups; reduced small fiber input/no hyperalgesia and normal thermal thresholds/hyperalgesia.

Our results showing an affection of afferent small nerve fibers together with the nature of the symptoms, suggest that the pain experienced by erythromelalgia patients could have a neuropathic component.     

Divergent peripheral effects of pituitary adenylate cyclase-activating polypeptide-38 on nociception in rats and mice

Pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38) and its receptors have been shown in the spinal dorsal horn, on capsaicin-sensitive sensory neurons and inflammatory cells. The role of PACAP in central pain transmission is controversial, and no data are available on its function in peripheral nociception. Therefore, the aim of the present study was to analyze the effects of locally or systemically administered PACAP-38 on nocifensive behaviors, inflammatory/neuropathic hyperalgesia and afferent firing. Intraplantar PACAP-38 (0.2 nmol) injection inhibited carrageenan-evoked inflammatory mechanical allodynia, mild heat injury-induced thermal hyperalgesia, as well as nocifensive behaviors in the early and late phases of the formalin test in rats. However, the above dose did not alter basal mechanical or heat thresholds. In mice, PACAP-38 (0.2 nmol/kg s.c.) significantly diminished acetic acid-induced abdominal contractions, but exerted no effect on sciatic nerve ligation-induced neuropathic mechanical hyperalgesia. In contrast, local PACAP-38 injection markedly increased rotation-induced afferent firing in the inflamed rat knee joint clearly demonstrating a peripheral sensitization in this organ. These actions were blocked by VPAC1/VPAC2 receptor antagonist pretreatment, but were not altered by PAC1 receptor antagonism. This paper presents the first data for the peripheral actions of PACAP-38 on nociceptive transmission mediated by VPAC receptors. These effects seem to be divergent depending on the mechanisms of nociceptor activation and the targets of PACAP actions. In acute somatic and visceral inflammatory pain models, PACAP exerts anti-nociceptive, anti-hyperalgesic and anti-allodynic effects. It has no significant peripheral role in traumatic mononeuropathy, but induces mechanical sensitization of knee joint primary afferents.     

Supraspinal cholecystokinin may drive tonic descending facilitation mechanisms to maintain neuropathic pain in the rat

Complete or partial spinal section at T(8) has been shown to block tactile allodynia but not thermal hyperalgesia following L(5)/L(6) spinal nerve ligation (SNL), suggesting the supraspinal integration of allodynia in neuropathic pain. In the present study, the possibility of mediation of nerve injury-associated pain through tonic activity of descending nociceptive facilitation arising from the rostroventromedial medulla (RVM) was investigated. Specifically, the actions of brainstem cholecystokinin and the possible importance of sustained afferent input from injured nerve fibers were determined using pharmacological and physiological approaches in rats with SNL. Lidocaine given bilaterally into the RVM blocked tactile allodynia and thermal hyperalgesia in SNL rats and was inactive in sham-operated rats. Bilateral injection of L365,260 (CCK(B) receptor antagonist) into the RVM also reversed both tactile allodynia and thermal hyperalgesia. Microinjection of CCK-8 (s) into the RVM of naive rats produced a robust tactile allodynic effect and a more modest hyperalgesia. CCK immunoreactivity was not significantly different between SNL and sham-operated rats. The anti-nociceptive effect of morphine given into the ventrolateral periaqueductal gray region (PAG) was substantially reduced by SNL. The injection of L365,260 into the RVM or of bupivacaine at the site of nerve injury restored the potency and efficacy of PAG morphine in SNL rats. These results suggest that changes in supraspinal processing are likely to contribute to the observed poor efficacy of opioids in clinical states of neuropathic pain. These data also indicate that the activation of descending nociceptive facilitatory pathways is important in the maintenance of neuropathic pain, appears to be dependent on CCK release, and may be driven from sustained afferent input from injured nerves to brainstem sites. Collectively, these data support the hypothesis that abnormal tonic activity of descending facilitation mechanisms may underlie chronic pain from peripheral nerve injury.     

Lumbar Sympathectomy Attenuates Cold Allodynia But Not Mechanical Allodynia and Hyperalgesia in Rats With Spared Nerve Injury

In certain patients with neuropathic pain, the pain is dependent on activity in the sympathetic nervous system. To investigate whether the spared nerve injury model (SNI) produced by injury to the tibial and common peroneal nerves and leaving the sural nerve intact is a model for sympathetically maintained pain, we measured the effects of surgical sympathectomy on the resulting mechanical allodynia, mechanical hyperalgesia, and cold allodynia. Decreases of paw withdrawal thresholds to von Frey filament stimuli and increases in duration of paw withdrawal to pinprick or acetone stimuli were observed in the ipsilateral paw after SNI, compared with their pre-SNI baselines. Compared with sham surgery, surgical lumbar sympathectomy had no effect on the mechanical allodynia and mechanical hyperalgesia induced by SNI. However, the sympathectomy significantly attenuated the cold allodynia induced by SNI. These results suggest that the allodynia and hyperalgesia to mechanical stimuli in the SNI model is not sympathetically maintained. However, the sympathetic nervous system may be involved, in part, in the mechanisms of cold allodynia in the SNI model. PERSPECTIVE: The results of our study suggest that the SNI model is not an appropriate model of sympathetically maintained mechanical allodynia and hyperalgesia but may be useful to study the mechanisms of cold allodynia associated with sympathetically maintained pain states.     

Mechanisms of adrenosensitivity in capsaicin induced hyperalgesia.

It is well known that iontophoresis of norepinephrine in capsaicin treated skin is followed by an increase in thermal hyperalgesia. It is unclear if this action on nocicepitive afferents involves the release of prostaglandins. The aim of the present study was to determine: (1) the effect of norepinephrine iontophoresis on spontaneous and evoked pain in the human skin after topical application of capsaicin; (2) the effect of cyclooxygenase (COX) inhibition on changes in pain perception induced by norepinephrine application. METHODS: Ten volunteers were included in the study. Iontophoresis of norepinephrine or saline was performed in a randomized cross over design on the volar aspect of the forearm after topical application of capsaicin. In the second part of the study single iv. injections of saline or acetylsalicylic acid were performed in a randomized double blind cross over design. After the injection norepinephrine iontophoresis was performed on the skin treated with topical capsaicin. Spontaneous pain, mechanical hyperalgesia as well as warm and heat pain thresholds were measured before and after each iontophoresis. RESULTS: Norepinephrine did enhance spontaneous pain and mechanical and thermal hyperalgesia in capsaicin treated skin. Inhibition of COX I and II had no effect on the norepinephrine induced changes in pain perception. CONCLUSION: The results do not support the assumption that in human skin sensitized by topical capsaicin application of norepinephrine acts on nociceptive afferents via the release of prostaglandins. Thus, a direct action of norepinephrine on adrenergic receptors in the membrane of the afferent fibers is most likely.     

Increased efficacy of early spinal cord stimulation in an animal model of neuropathic pain

Although spinal cord stimulation (SCS) is an established treatment for chronic neuropathic pain, pain relief is still not successful in a large group of patients. We suggest that the success of SCS may be related to the timing of SCS during the development of chronic neuropathic pain. We therefore compared the effect of SCS applied after 24 h of neuropathic pain (early SCS) and after 16 days of neuropathic pain (late SCS). For early SCS, male Sprague—Dawley rats (n =13) were implanted with an SCS device, followed by a partial ligation of the sciatic nerve. Using von Frey monofilaments, tactile allodynia was assessed 24 h after ligation. Animals with tactile allodynia received 30 min of SCS. Withdrawal thresholds were assessed just before SCS, during SCS and until the return to pre‐stimulation withdrawal threshold. Results were compared with the data from late SCS (n =29). Out of the 13 allodynic animals that received early SCS, 10 (77%) responded to SCS with significantly increased withdrawal thresholds, compared to 38% in the late SCS group. The increase of the withdrawal threshold in the early SCS group could still be noticed 90 min after termination of SCS. In more than half of these animals, pre‐stimulation withdrawal thresholds were reached only the next day. Early SCS resulted in an increased number of responders to SCS and furthermore an increased duration of the effect of SCS as compared to late SCS. Early SCS treatment of neuropathic rats is more effective as compared to the late SCS treatment.     

A novel rat forelimb model of neuropathic pain produced by partial injury of the median and ulnar nerves

The vast majority of human peripheral nerve injuries occur in the upper limb, whereas the most animal studies have been conducted using the hindlimb models of neuropathic pain, involving damages of the sciatic or lumbar spinal nerve(s). We attempted to develop a rat forelimb model of peripheral neuropathy by partial injury of the median and ulnar nerves. The halves of each nerve were transected by microscissors at about 5 mm proximal from the elbow joint and behavioral signs of neuropathic pain, such as mechanical and cold allodynia, and heat hyperalgesia, were monitored up to 126 days following nerve injury. Mechanical allodynia was assessed by measuring the forepaw withdrawal threshold to von Frey filaments, and cold allodynia was evaluated by measuring the time spent in lifting or licking the forepaw after applying acetone to it. Heat hyperalgesia was also monitored by investigating the forepaw withdrawal latencies using the Hargreaves' test. After the nerve injury, the experimental animals exhibited long‐lasting clear neuropathic pain‐like behaviors, such as reduced forepaw withdrawal threshold to von Frey filaments, the increased response duration of the forepaw to acetone application, and the decreased withdrawal latency to radiant heat stimulation. These behaviors were significantly alleviated by administration of gabapentin (5 or 50 mg/kg, i.p.) in a dose‐dependent manner. Therefore, these abnormal sensitivities are interpreted as the signs of neuropathic pain following injury of the median and ulnar nerves. Our rat forelimb model of neuropathic pain may be useful for studying human neuropathic pain and screening for valuable drug candidates.     

Nerve injury-induced tactile allodynia is mediated via ascending spinal dorsal column projections

Peripheral nerve injury produces signs of neuropathic pain including tactile allodynia and thermal hyperalgesia, sensory modalities which may be associated with different neuronal pathways. Studies of spinally-transected, nerve-injured rats have led to suggestions that thermal hyperalgesia may be mediated predominately through local spinal circuitry whereas ascending input to supraspinal sites is critical to the manifestation of tactile allodynia. Here, the nature of ascending spinal input mediating tactile allodynia was explored using selective spinal lesions. Male Sprague-Dawley rats received L(5)/L(6) spinal nerve ligation (SNL) and ipsilateral or contralateral (relative to the SNL side) lesions including spinal hemisections and bilateral and unilateral dorsal column lesions. The rats were maintained in a sling and monitored for tactile allodynia by measuring withdrawal thresholds to probing with von Frey filaments 24 h after the hemisection. Rats receiving dorsal column lesions demonstrated no motor deficits while rats receiving spinal hemisection showed paralysis of the paw which nevertheless responded to strong noxious stimulation. Spinal hemisection ipsilateral, but not contralateral, to SNL completely abolished tactile allodynia while maintaining spinal nocifensive reflexes to noxious pinch. Bilateral and ipsilateral dorsal column lesions blocked tactile allodynia while contralateral dorsal column lesions did not. Administration of lidocaine into the nucleus gracilis ipsilateral to SNL also blocked tactile allodynia, but did not alter thermal hyperalgesia in SNL rats or increase thermal nociceptive responses in sham-operated rats. Lidocaine microinjected into the contralateral nucleus gracilis produced no changes in responses to tactile or thermal stimuli in either group. These results indicate that tactile allodynia after peripheral nerve injury is dependent upon inputs to supraspinal sites. Furthermore, it is apparent that afferent signals interpreted as tactile allodynia course through the ipsilateral dorsal columns and are relayed through the nucleus gracilis. This neuronal pathway is consistent with the interpretation that tactile allodynia pursuant to peripheral nerve injury is transmitted to the central nervous system by means of large diameter, myelinated fibers.     

Basic Science (30)

Sympathetic‐sensory coupling after L5 spinal nerve lesion in the rat and its relation to changes in dorsal root ganglion blood flow. (Christian‐Albrechts‐Universität, Kiel, Germany) Pain 2000;87:335–345. In rats 3‐56 days after L5 spinal nerve lesion, the authors of this study tested the responses of axotomized afferent fibers recorded in the dorsal root of the lesioned segment to norepinephrine (NE, 0.5 μg/kg) injected intravenously and to selective electrical stimulation of the lumbar sympathetic trunk (LST). In some experiments, blood flow was measured in the dorsal root ganglion (DRG) by laser Doppler flowmetry. The majority of lesioned afferent fibers with spontaneous activity responded to neither LST stimulation (82.4%) nor NE (71.4%). In those that did react to LST stimulation, responses occurred only at high stimulation frequencies and they could be mimicked by nonadrenergic vasoconstrictor drugs (angiotensin II, vasopressin). Excitatory responses to LST stimulation were closely correlated with the stimulation‐induced phasic vasoconstrictions in the DRG. Therefore, the activation of lesioned afferents might be brought about indirectly by an impaired blood supply to the DRG. To test this hypothesis, a strong and sustained baseline vasoconstriction in the DRG was induced by blocking endothelial nitric oxide synthesis with N^G‐nitro‐L‐arginine methyl ester (L‐NAME) applied systemically. L‐NAME enhanced baseline vascular resistance in the DRG about threefold and also increased stimulation‐induced vasoconstrictions. After L‐NAME, the majority of axotomized neurons with spontaneous activity were activated by LST stimulation (76%) or NE (75%). Again, activations closely followed stimulation‐induced phasic vasoconstrictions in the DRG provided that a critical level of vasoconstriction was exceeded. Inhibitory responses to LST stimulation were generally rare and could be reversed to activation by prolonged stimulation or after L‐NAME. These results show that sympathetic‐sensory coupling occurs only in a minority of axotomized afferents after L5 spinal nerve injury. Like previous studies, they cast doubt on the notion that L5 spinal nerve lesion is a good model for sympathetically maintained pain. Since responses of lesioned afferent neurons to LST stimulation and NE could be provoked with high reliability after inducing vasoconstriction in the DRG, and since they mirrored stimulation‐induced vasoconstrictions in the DRG, it appears that in this model the association of sympathetic activity with afferent discharge occurs mainly when perfusion of the DRG is impaired. Comments by Marshall Devor, PhD. Animal research has revealed that massive ectopic afferent discharge is generated in the dorsal root ganglion (DRG) following nerve injury. This source of ectopic firing is in addition to activity that may be generated at the nerve injury site (Tinel sign). The combined impulse barrage is almost certainly an important cause of neuropathic pain. It has been discovered recently that the intensity of the ectopic DRG barrage is modulated by sympathetic efferent activity. This is a potential link between sympathetic activity and symptomatology in sympathetic related pain conditions such as complex regional pain syndrome/reflex sympathetic dystrophy. But what is the mechanism of the sympathetic‐sensory coupling? Considerable evidence indicates that axotomized DRG neurons become abnormally adrenosensitivity. Habler et al now point out an additional, indirect, coupling mechanism … modulation of intrinsic DRG blood flow. That is, sympathetic activity may reduce DRG blood flow inducing partial ischemia, with the ischemia causing neural excitation. This finding may also have broader implications as spinal pathology can also compromise DRG blood flow. Might low back pain and sciatica, for example, derive from this same process? It is surprising that ectopic firing in the DRG has played so little part in clinical thinking and practice in patients with neuropathic pain. So far.     

A novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury

Partial nerve injury is the main cause of causalgiform pain disorders in humans. We present here a novel animal model of this condition. In rats we unilaterally ligated about half of the sciatic nerve high in the thigh. Within a few hours after the operation, and for several months thereafter, the rats developed guarding behavior of the ipsilateral hind paw and licked it often, suggesting the possibility of spontaneous pain. The plantar surface of the foot was evenly hyperesthetic to non-noxious and noxious stimuli. None of the rats autotomized. There was a sharp decrease in the withdrawal thresholds bilaterally in response to repetitive Von Frey hair stimulation at the plantar side. After a series of such stimuli in the operated side, light touch elicited aversive responses, suggesting allodynia to touch. The withdrawal thresholds to CO2 laser heat pulses were markedly lowered bilaterally. Suprathreshold noxious heat pulses elicited exaggerated responses unilaterally, suggesting thermal hyperalgesia. Pin-prick evoked such exaggerated responses bilaterally (mechanical hyperalgesia). In a companion report, we show that these abnormalities critically depend on the sympathetic outflow. Based on the immediate onset and long-lasting perpetuation of similar symptoms, such as touch-evoked allodynia and hyperalgesia, and the resemblance of the contralateral phenomena to 'mirror image' pains in some humans with causalgia, we suggest that this preparation may serve as a model for syndromes of the causalgiform variety that are triggered by partial nerve injury and maintained by sympathetic activity.     

Genetic ablation of delta opioid receptors in nociceptive sensory neurons increases chronic pain and abolishes opioid analgesia

Opioid receptors are major actors in pain control and are broadly distributed throughout the nervous system. A major challenge in pain research is the identification of key opioid receptor populations within nociceptive pathways, which control physiological and pathological pain. In particular, the respective contribution of peripheral vs. central receptors remains unclear, and it has not been addressed by genetic approaches. To investigate the contribution of peripheral delta opioid receptors in pain control, we created conditional knockout mice where delta receptors are deleted specifically in peripheral Na_V1.8-positive primary nociceptive neurons. Mutant mice showed normal pain responses to acute heat and to mechanical and formalin stimuli. In contrast, mutant animals showed a remarkable increase of mechanical allodynia under both inflammatory pain induced by complete Freund adjuvant and neuropathic pain induced by partial sciatic nerve ligation. In these 2 models, heat hyperalgesia was virtually unchanged. SNC80, a delta agonist administered either systemically (complete Freund adjuvant and sciatic nerve ligation) or into a paw (sciatic nerve ligation), reduced thermal hyperalgesia and mechanical allodynia in control mice. However, these analgesic effects were absent in conditional mutant mice. In conclusion, this study reveals the existence of delta opioid receptor-mediated mechanisms, which operate at the level of Na_V1.8-positive nociceptive neurons. Delta receptors in these neurons tonically inhibit mechanical hypersensitivity in both inflammatory and neuropathic pain, and they are essential to mediate delta opioid analgesia under conditions of persistent pain. This delta receptor population represents a feasible therapeutic target to alleviate chronic pain while avoiding adverse central effects.     

Distribution of Artemin and GFRα3 Labeled Nerve Fibers in the Dura Mater of Rat

(Headache 2010;50:442‐450) Objective.— We examined the distribution of artemin and its receptor, glial cell line‐derived neurotrophic factor family receptor α3 (GFRα3), in the dura mater of rats. Background.— Artemin, a member of the glial cell line‐derived neurotrophic factor family, is a vasculature‐derived growth factor shown to regulate migration of sympathetic neuroblasts and targeting of sympathetic innervation. The artemin receptor, GFRα3, is present in both sympathetic efferents and a subpopulation of nociceptive afferents. Recent evidence has shown that artemin may contribute to inflammatory hyperalgesia. The extent to which artemin is present in the dural vasculature and its relationship to GFRα3 containing fibers have yet to be investigated. Methods.— We used retrograde labeling, double and triple labeling with immunohistochemistry on the dura mater and trigeminal ganglia of female Sprague‐Dawley rats. Results.— Artemin‐like immunoreactivity (‐LI) was detected in the smooth muscle of dural vasculature. GFRα3‐LI was present in nerve fibers that closely associated with tyrosine hydroxylase or calcitonin gene‐related peptide (CGRP). CGRP‐LI and transient receptor potential ion channel 1 (TRPV1)‐LI were present in all GFRα3‐positive dural afferents, which constituted 22% of the total population of dural afferents. Conclusions.— These anatomical results support the hypothesis that artemin contributes to dural afferent activity, and possibly migraine pain, through modulation of both primary afferent and sympathetic systems.     

Intrathecal, but not intravenous adenosine reduces allodynia in patients with neuropathic pain

Intrathecal adenosine reduces allodynia from intradermal capsaicin in human volunteers, and reduces hypersensitivity to mechanical stimuli in animals with nerve injury. Although both intrathecal and intravenous adenosine have been reported to relieve pain in patients with neuropathic pain, there are no controlled trials of this therapy. In order to determine the effect of adenosine, seven patients with chronic neuropathic pain and stable areas of mechanical hyperalgesia and allodynia were recruited. Using a double-blind, cross-over design, patients were studied on two occasions - once with intrathecal adenosine, 2 mg and once with intravenous adenosine, 2 mg, using saline by the alternate route. Areas of hyperalgesia and allodynia and pain from von Frey stimulation in the area of allodynia were determined up to 24 h after drug injection. Intrathecal, but not intravenous adenosine reduced area of allodynia by approximately 25% (P<0.05) from 2 to 24 h after injection. Intrathecal adenosine reduced pain from von Frey filament stimulation in the area of allodynia by approximately 20% (P<0.05). Ongoing pain was unaffected by adenosine by either route. Intrathecal, but not intravenous adenosine, caused backache in five of seven patients, lasting 6 h. These results indicate that intrathecal adenosine reduces allodynia and pain from stimulation in the area of allodynia, whereas the same dose of adenosine intravenously was ineffective. Given the modest effect and common side effects, the role for intrathecal adenosine as a sole agent for the treatment of neuropathic pain may be limited.     

Vasodilatation in hyperalgesic rat skin evoked by stimulation of afferent A beta-fibers: further evidence for a role of dorsal root reflexes in allodynia.

In areas of secondary hyperalgesia, innocuous mechanical stimuli evoke pain (allodynia). We have proposed that this is produced by a central pre-synaptic interaction whereby A beta-fibers evoke spike activity (dorsal root reflexes) in nociceptive afferents (Pain, 68 (1996) 13). This activity should conduct centrally, evoking allodynia, and peripherally, evoking neurogenic vasodilatation. Here we tested this hypothesis by examining the effects of electrical stimulation of A beta-fibers on cutaneous blood flow before and after producing secondary hyperalgesia in anesthetized rats. Cutaneous blood flow was recorded in the hind paw skin innervated by the sural nerve using a laser Doppler flowmeter. The sural nerve was prepared for electrical stimulation, and the evoked activity was recorded from the sciatic nerve in continuity. Electrical stimulation (1 Hz, 4 x 0.2 ms pulses, 20 s) was applied to the sural nerve at 2T (A beta-fibers only) and 4T and 6T (A beta + A delta-fibers). Flux was recorded at baseline and after capsaicin or mustard oil application outside the sural nerve territory. The effects of intravenous administration of the calcitonin gene-related peptide (CGRP) receptor antagonist, alpha-CGRP(8-37), or of section of the sciatic nerve or of the L4-L6 dorsal roots were examined. Selective activation of the sural nerve A beta-fibers reliably evoked increases in cutaneous blood flow close to areas of chemical irritation or skin damage. A beta-fiber-evoked vasodilatation was abolished by sciatic nerve or dorsal root section and had a spatial arrangement and optimal stimulation pattern suggesting a central synaptic interaction similar to that responsible for dorsal root reflexes. The flux increases were dose-dependently and reversibly inhibited by alpha-CGRP(8-37), indicating that the A beta-fiber-evoked vasodilatation resulted from the antidromic activation of nociceptive cutaneous afferent fibers. These results support our hypothesis by showing activation of nociceptive primary afferents by A beta-fibers in areas of allodynia in a manner consistent with a pre-synaptic interaction evoking dorsal root reflexes.     

Does the epidermal nerve fibre density measured by skin biopsy in patients with peripheral neuropathies correlate with neuropathic pain?

The different neuropathic pain types (eg, ongoing burning pain and allodynia) are frequent and disabling complaints in patients with peripheral neuropathies. Although the reference standard technique for diagnosing painful small-fibre neuropathies is nerve fibre density assessment by skin biopsy, the relationship between the epidermal nerve fibre (ENF) density and neuropathic pain is still unclear. In a clinical and skin biopsy study designed to investigate whether changes in ENF density are directly related to pain, we enrolled 139 consecutive patients with distal symmetric peripheral neuropathy. All patients underwent clinical examination. The Neuropathic Pain Symptom Inventory was used to distinguish the different neuropathic pain types. A skin biopsy was conducted, and ENFs were immunostained with the antiprotein gene product 9.5, and their linear density was quantified with bright-field microscopy. No difference was found in ENF density between patients with and without neuropathic pain, nor between patients with and without ongoing burning pain. Conversely, ENF density was higher in patients with provoked pains (including mechanical dynamic allodynia) than in those without. The variable association between ENF density and symptoms of neuropathic pain supports the idea that neuropathic pain symptoms arise through distinct underlying mechanisms. The lack of relationship between ongoing burning pain and ENF density suggests that this type of pain reflects factors other than loss of nociceptive afferents. The association between ENF density and provoked pain (including mechanical dynamic allodynia) suggests that this type of pain might be mediated by spared and sensitised nociceptive afferents.