Contralateral treatment with lidocaine reduces spinal neuronal activity in mononeuropathic rats

Antiallodynic Effects of Endomorphin-1 and Endomorphin-2 in the Spared Nerve Injury Model of Neuropathic Pain in Mice

BACKGROUND

The spared nerve injury (SNI) model is a new animal model that can mimic several characteristics of clinical neuropathic pain. Opioids are recommended as treatment of neuropathic pain. Therefore, the present study was conducted to investigate the antinociceptive effects of endomorphin-1 (EM-1) and endomorphin-2 (EM-2) given centrally and peripherally in the SNI model of neuropathic pain in mice.

METHODS

The SNI model was made in mice by sparing the sural nerve intact, when the other 2 of 3 terminal branches of the sciatic nerve (common peroneal and tibial nerves) were tightly ligated and cut. Von Frey monofilaments were used to measure the SNI-induced mechanical allodynia-like behavior. The antiallodynic effects of EM-1 and EM-2 were determined after central and peripheral administration in the SNI model of neuropathic pain. Also, the specific opioid receptor antagonists were used to determine the opioid mechanisms of EMs involved in neuropathic pain. Values were expressed as the mean ± standard deviation.

RESULTS

Our results showed that the SNI mice developed prolonged mechanical allodynia-like behavior in ipsilateral paw after surgery, with the withdrawal threshold value being 0.061 ± 0.02 g after 14 days. EM-1 and EM-2 produced significant antiallodynic effects in ipsilateral paw after intracerebroventricular (i.c.v.) administration, more effective than that of morphine. The peak withdrawal thresholds of 10 nmol EM-1 and EM-2 determined at 5 minutes after injection were 0.92 ± 0.36 and 0.87 ± 0.33 g, respectively, higher than that of morphine (0.46 ± 0.20 g). Moreover, both EMs (10 nmol, i.c.v.) exerted significant antiallodynic effects in the contralateral paw, whereas no significant antinociceptive activity was seen after i.c.v. administration of morphine with equimolar dose. It was noteworthy that EM-1 and EM-2 produced antinociception through distinct μ1- and μ2-opioid receptor subtypes, and the EM-2-induced antiallodynia contained an additional component that was mediated by the release of endogenous dynorphin A, acting on κ-opioid receptor. In addition, the antiallodynic activities of peripheral administration of EM-1, EM-2, and morphine were also investigated. Intraplantar, but not subcutaneous administration of EM-1 and EM-2 also exhibited potent antinociception, establishing the peripheral and local effects. Both μ1- and μ2-opioid receptor subtypes, but not the δ- or κ-opioid receptors were involved in the peripheral antiallodynia of EMs.

CONCLUSIONS

The present investigation demonstrated that both EM-1 and EM-2 given centrally and peripherally produced potent antiallodynic activities in SNI mice, and differential opioid mechanisms were involved.

Pharmacologic interventions for treating phantom limb pain

BACKGROUND

This is an updated version of the original Cochrane review published in Issue 12, 2011. Phantom limb pain (PLP) is pain that arises in the missing limb after amputation and can be severe, intractable, and disabling. Various medications have been studied in the treatment of phantom pain. There is currently uncertainty in the optimal pharmacologic management of PLP.

OBJECTIVES

This review aimed to summarise the evidence of effectiveness of pharmacologic interventions in treating PLP.

SEARCH METHODS

For this update, we searched the Cochrane Central Register of Controlled Trials (CENTRAL, the Cochrane Library), MEDLINE, and Embase for relevant studies. We ran the searches for the original review in September 2011 and subsequent searches for this update up to April 2016. We sought additional studies from clinical trials databases and reference lists of retrieved papers.

SELECTION CRITERIA

We included randomised and quasi-randomised trials studying the effectiveness of pharmacologic interventions compared with placebo, another active treatment, or no treatment, in established PLP. We considered the following outcomes: change in pain intensity, function, sleep, depression or mood, quality of life, adverse events, treatment satisfaction, and withdrawals from the study.

DATA COLLECTION AND ANALYSIS

We independently assessed issues of study quality and extracted efficacy and adverse event data. Due to the wide variability in the studies, we did not perform a meta-analysis for all the interventions and outcomes, but attempted to pool the results of some studies where possible. We prepared a qualitative description and narrative summary of results. We assessed clinical heterogeneity by making qualitative comparisons of the populations, interventions, outcomes/outcome measures, and methods.

MAIN RESULTS

We added only one new study with 14 participants to this updated review. We included a 14 studies (10 with low risk of bias and 4 with unclear risk of bias overall) with a total of 269 participants. We added another drug class, botulinum neurotoxins (BoNTs), in particular botulinum toxin A (BoNT/A), to the group of medications reviewed previously. Our primary outcome was change in pain intensity. Most studies did not report our secondary outcomes of sleep, depression or mood, quality of life, treatment satisfaction, or withdrawals from the study.BoNT/A did not improve phantom limb pain intensity during the six months of follow-up compared with lidocaine/methylprednisolone.Compared with placebo, morphine (oral and intravenous) was effective in decreasing pain intensity in the short term with reported adverse events being constipation, sedation, tiredness, dizziness, sweating, voiding difficulty, vertigo, itching, and respiratory problems.The N-methyl D-aspartate (NMDA) receptor antagonists ketamine (versus placebo; versus calcitonin) and dextromethorphan (versus placebo), but not memantine, had analgesic effects. The adverse events of ketamine were more serious than placebo and calcitonin and included loss of consciousness, sedation, hallucinations, hearing and position impairment, and insobriety.The results for gabapentin in terms of pain relief were conflicting, but combining the results favoured treatment group (gabapentin) over control group (placebo) (mean difference -1.16, 95% confidence interval -1.94 to -0.38; 2 studies). However, gabapentin did not improve function, depression score, or sleep quality. Adverse events experienced were somnolence, dizziness, headache, and nausea.Compared with an active control benztropine mesylate, amitriptyline was not effective in PLP, with dry mouth and dizziness as the most frequent adverse events based on one study.The findings for calcitonin (versus placebo; versus ketamine) and local anaesthetics (versus placebo) were variable. Adverse events of calcitonin were headache, vertigo, drowsiness, nausea, vomiting, and hot and cold flushes. Most of the studies were limited by their small sample sizes.

AUTHORS' CONCLUSIONS

Since the last version of this review, we identified another study that added another form of medical therapy, BoNTs, specifically BoNT/A, to the list of pharmacologic interventions being reviewed for clinical efficacy in phantom limb pain. However, the results of this study did not substantially change the main conclusions. The short- and long-term effectiveness of BoNT/A, opioids, NMDA receptor antagonists, anticonvulsants, antidepressants, calcitonins, and local anaesthetics for clinically relevant outcomes including pain, function, mood, sleep, quality of life, treatment satisfaction, and adverse events remain unclear. Based on a small study, BoNT/A (versus lidocaine/methylprednisolone) does not decrease phantom limb pain. Morphine, gabapentin, and ketamine demonstrate favourable short-term analgesic efficacy compared with placebo. Memantine and amitriptyline may not be effective for PLP. However, results must be interpreted with caution, as they were based mostly on a small number of studies with limited sample sizes that varied considerably and also lacked long-term efficacy and safety outcomes. The direction of efficacy of calcitonin, local anaesthetics, and dextromethorphan needs further clarification. Overall, the efficacy evidence for the reviewed medications is thus far inconclusive. Larger and more rigorous randomised controlled trials are needed for us to reach more definitive conclusions about which medications would be useful for clinical practice.

Active immunization with myelin-derived altered peptide ligand reduces mechanical pain hypersensitivity following peripheral nerve injury

Background

T cells have been implicated in neuropathic pain that is caused by peripheral nerve injury. Immunogenic myelin basic protein (MBP) peptides have been shown to initiate mechanical allodynia in a T cell-dependent manner. Antagonistic altered peptide ligands (APLs) are peptides with substitutions in amino acid residues at T cell receptor contact sites and can inhibit T cell function and modulate inflammatory responses. In the present study, we studied the effects of immunization with MBP-derived APL on pain behavior and neuroinflammation in an animal model of peripheral nerve injury.

Methods

Lewis rats were immunized subcutaneously at the base of the tail with either a weakly encephalitogenic peptide of MBP (cyclo-MBP_87-99) or APL (cyclo-(87-99)[A⁹¹,A⁹⁶]MBP_87-99) in complete Freund’s adjuvant (CFA) or CFA only (control), following chronic constriction injury (CCI) of the left sciatic nerve. Pain hypersensitivity was tested by measurements of paw withdrawal threshold to mechanical stimuli, regulatory T cells in spleen and lymph nodes were analyzed by flow cytometry, and immune cell infiltration into the nervous system was assessed by immunohistochemistry (days 10 and 30 post-CCI). Cytokines were measured in serum and nervous tissue of nerve-injured rats (day 10 post-CCI).

Results

Rats immunized with the APL cyclo-(87-99)[A⁹¹,A⁹⁶]MBP_87-99 had significantly reduced mechanical pain hypersensitivity in the ipsilateral hindpaw compared to cyclo-MBP_87-99-treated and control rats. This was associated with significantly decreased infiltration of T cells and ED1+ macrophages in the injured nerve of APL-treated animals. The percentage of anti-inflammatory (M2) macrophages was significantly upregulated in the APL-treated rats on day 30 post-CCI. Compared to the control rats, microglial activation in the ipsilateral lumbar spinal cord was significantly increased in the MBP-treated rats, but was not altered in the rats immunized with the MBP-derived APL. In addition, immunization with the APL significantly increased splenic regulatory T cells. Several cytokines were significantly altered after CCI, but no significant difference was observed between the APL-treated and control rats.

Conclusions

These results suggest that immune deviation by active immunization with a non-encephalitogenic MBP-derived APL mediates an analgesic effect in animals with peripheral nerve injury. Thus, T cell immunomodulation warrants further investigation as a possible therapeutic strategy for the treatment of peripheral neuropathic pain.

Developing An Acupuncture Protocol for Treating Phantom Limb Pain: A Delphi Consensus Study

Background

Little is known about how a Traditional Chinese Medicine (TCM) approach could be used to treat phantom limb pain (PLP). There is currently no standard acupuncture protocol in the literature to treat this syndrome.

Objectives

To achieve consensus among a group of acupuncture practitioners on the pathology and recommended treatment of PLP and devise an acupuncture protocol for the treatment of this condition.

Methods

A classical Delphi approach was used using two parallel online Delphi studies. One study focused on participants with past experience of treating PLP (TPLP, n=7) and the other on practitioners with no past experience (NTPLP, n=16). Two hypothetical case studies were provided and participants were asked for responses on how they would treat these patients. Three rounds were included. Participants were also invited to rate and comment on the finalised protocol. Round 1 data were analysed using content analysis. In subsequent rounds an a priori criterion for defining consensus was set at ≤1.75 IQR. A group median of 5–6 was considered to mean ‘agree’.

Results

19 participants completed all Delphi rounds (12 NTPLP, 7 TPLP). 108 NTPLP and 76 TPLP statements were generated and circulated in round 2; 53% of the NTPLP statements and 62% of the TPLP statements met consensus in round 2 and 45% of the NTPLP statements and 44% of the TPLP statements met consensus in round 3. Participants all agreed with the final protocol developed.

Conclusions

The protocol developed does not claim to be best practice but provides a preliminary consensus from practitioners practising acupuncture for the treatment of PLP.

Peripheral neuropathy induces cutaneous hypersensitivity in chronically spinalized rats

BACKGROUND/OBJECTIVES

The present study was aimed at the issue of whether peripheral nerve injury-induced chronic pain is maintained by supraspinal structures governing descending facilitation to the spinal dorsal horn, or whether altered peripheral nociceptive mechanisms sustain central hyperexcitability and, in turn, neuropathic pain. We examined this question by determining the contribution of peripheral/spinal mechanisms, isolated from supraspinal influence(s), in cutaneous hypersensitivity in an animal model of peripheral neuropathy.

METHODS

Adult rats were spinalized at T8-T9; 8 days later, peripheral neuropathy was induced by implanting a 2-mm polyethylene cuff around the left sciatic nerve. Hind paw withdrawal responses to mechanical or thermal plantar stimulation were evaluated using von Frey filaments or a heat lamp, respectively.

RESULTS

Spinalized rats without cuff implantation exhibited a moderate decrease in mechanical withdrawal threshold on ~day 10 (P < 0.05) and in thermal withdrawal threshold on ~day 18 (P < 0.05). However, cuff-implanted spinalized rats developed a more rapid and significant decrease in mechanical (~day 4; P < 0.001) and thermal (~day 10; P < 0.05) withdrawal thresholds that remained significantly decreased through the duration of the study.

CONCLUSIONS

Our findings demonstrate an aberrant peripheral/spinal mechanism that induces and maintains thermal and to a greater degree tactile cutaneous hypersensitivity in the cuff model of neuropathic pain, and raise the prospect that altered peripheral/spinal nociceptive mechanisms in humans with peripheral neuropathy may have a pathologically relevant role in both inducing and sustaining neuropathic pain.

Postamputation pain: epidemiology, mechanisms, and treatment

Postamputation pain (PAP) is highly prevalent after limb amputation but remains an extremely challenging pain condition to treat. A large part of its intractability stems from the myriad pathophysiological mechanisms. A state-of-art understanding of the pathophysiologic basis underlying postamputation phenomena can be broadly categorized in terms of supraspinal, spinal, and peripheral mechanisms. Supraspinal mechanisms involve somatosensory cortical reorganization of the area representing the deafferentated limb and are predominant in phantom limb pain and phantom sensations. Spinal reorganization in the dorsal horn occurs after deafferentataion from a peripheral nerve injury. Peripherally, axonal nerve damage initiates inflammation, regenerative sprouting, and increased "ectopic" afferent input which is thought by many to be the predominant mechanism involved in residual limb pain or neuroma pain, but may also contribute to phantom phenomena. To optimize treatment outcomes, therapy should be individually tailored and mechanism based. Treatment modalities include injection therapy, pharmacotherapy, complementary and alternative therapy, surgical therapy, and interventions aimed at prevention. Unfortunately, there is a lack of high quality clinical trials to support most of these treatments. Most of the randomized controlled trials in PAP have evaluated medications, with a trend for short-term Efficacy noted for ketamine and opioids. Evidence for peripheral injection therapy with botulinum toxin and pulsed radiofrequency for residual limb pain is limited to very small trials and case series. Mirror therapy is a safe and cost-effective alternative treatment modality for PAP. Neuromodulation using implanted motor cortex stimulation has shown a trend toward effectiveness for refractory phantom limb pain, though the evidence is largely anecdotal. Studies that aim to prevent PA P using epidural and perineural catheters have yielded inconsistent results, though there may be some benefit for epidural prevention when the infusions are started more than 24 hours preoperatively and compared with nonoptimized alternatives. Further investigation into the mechanisms responsible for and the factors associated with the development of PAP is needed to provide an evidence-based foundation to guide current and future treatment approaches.

Pharmacologic interventions for treating phantom limb pain

BACKGROUND

Phantom limb pain (PLP) is pain that arises in the missing limb after amputation and can be severe, intractable and disabling. Various medications have been studied in the treatment of phantom pain. Presently there is uncertainty in the optimal pharmacologic management of PLP. 

OBJECTIVES

This review aims to summarize the evidence of effectiveness of pharmacologic interventions in treating PLP.

SEARCH METHODS

We searched the Cochrane Pain, Palliative and Supportive Care Review Group (PaPaS) Trials Register, the Cochrane Controlled Trials Register (CENTRAL, The Cochrane Library), MEDLINE and EMBASE up to September 2011 for randomised and quasi-randomised trials comparing pharmacologic treatment with placebo, another active treatment, or no treatment.

SELECTION CRITERIA

We included randomised and quasi-randomised trials studying the effectiveness of pharmacologic interventions in patients with established PLP. The outcomes considered were change in pain intensity, function, mood, sleep, quality of life, satisfaction and adverse effects.

DATA COLLECTION AND ANALYSIS

Three review authors independently assessed the methodologic quality of the studies and extracted the data. We reported continuous and dichotomous data on change in pain intensity, function, mood/depression scores, sleep, quality of life, satisfaction for each study, where available. Because of the wide variability in the studies, we did not perform a meta-analysis for all the interventions and outcomes but we attempted to pool the results of some studies where possible. We prepared a qualitative description and narrative summary of results and described adverse effects. We assessed clinical heterogeneity by making qualitative comparisons in terms of the populations, interventions, outcomes/outcome measures and methods.

MAIN RESULTS

From 583 references/publications, we selected 13 studies involving 255 participants. Six groups of medications were reviewed, namely, N-methyl D-aspartate (NMDA) receptor antagonists, antidepressants, anticonvulsants, anaesthetics, opioids, and calcitonin. Ten studies were of high quality and three were of moderate quality based on both Jadad and Van Tulder criteria. Because of the wide variation (heterogeneity) in the pharmacologic interventions, outcome measures, analyses, reporting of results, duration of follow-ups and study designs, it was not possible to pool the results for most of the interventions and outcomes. Morphine (oral and intravenous) was effective in decreasing pain intensity in the short-term with reported adverse effects being constipation, sedation, tiredness, dizziness, sweating, voiding difficulty, vertigo, itching, and respiratory problems. The NMDA receptor antagonists, ketamine and dextromethorphan but not memantine, had analgesic effects. The adverse effects of ketamine were more serious and included loss of consciousness, sedation, hallucinations, hearing and position impairment, and insobriety. The results for gabapentin in terms of pain relief were conflicting but combining the results showed a trend towards benefit. Gabapentin, however, did not improve function, depression score and sleep quality. Side effects experienced were somnolence, dizziness, headache and nausea. Amitryptiline was not effective in PLP with dry mouth and dizziness as side effects based on one study. The findings for calcitonin and anaesthetics were variable. Adverse effects of calcitonin were headache, vertigo, drowsiness, nausea, vomiting, and hot and cold flushes. Most of the studies were limited by their small sample sizes.

AUTHORS' CONCLUSIONS

The short- and long-term effectiveness of opioids, NMDA receptor antagonists, anticonvulsants, antidepressants, calcitonins, and anaesthetics for clinically relevant outcomes that include pain, function, mood, sleep, quality of life, satisfaction and adverse effects remains unclear. Morphine, gabapentin and ketamine demonstrate trends towards short-term analgesic efficacy. Memantine and amitriptyline were ineffective for PLP. Results, however, are to be interpreted with caution as these were based mostly on a small number of studies with limited sample sizes that varied considerably and also lacked long-term efficacy and safety outcomes. The direction of efficacy of calcitonin, anaesthetics and dextromethorphan need further clarification. Larger and more rigorous randomised controlled trials are needed to make stronger recommendations about which medications would be useful for clinical practice.

Contralateral input modulates the excitability of dorsal horn neurons involved in noxious signal processes. Potential role in neuronal sensitization

Wide Dynamic Range (WDR) neurons in the spinal cord receive inputs from the contralateral side that, under normal conditions, are ineffective in generating an active response. These inputs are effective when the target WDRs change their excitability conditions. To further reveal the mechanisms supporting this effectiveness shift, we investigated the weight of the excitation of the contralateral neurons on the target WDR responses. In the circuit of presynaptic (sending) and postsynaptic (receiving) neurons in crossed spinal connections the fibres that form the presynaptic neurons impinge on postsynaptic neurons can be considered the final relay of this contralateral pathway. The enhancement of the presynaptic neuron excitability may thus modify the efficacy of the contralateral input. Pairs of neurons each on a side of the spinal cord, at the L5-L6 lumbar level were simultaneously recorded in intact, anaesthetized, paralysed rats. The excitatory aminoacid NMDA and strychnine, the antagonist of the inhibitory aminoacid glycine, were iontophoretically administrated to presynaptic neurons to increase their excitability. Before and during the drug administration, spontaneous and noxious-evoked activities of the neurons were analysed. During the iontophoresis of the two substances we found that noxious stimuli applied to the receptive field of presynaptic neurons activated up to 50% of the previously unresponsive postsynaptic neurons on the opposite side. Furthermore, the neurons on both sides of the spinal cord showed significantly increased spontaneous activity and amplified responses to ipsilateral noxious stimulation. These findings indicate that the contralateral input participates in the circuit dynamics of spinal nociceptive transmission, by modulating the excitability of the postsynaptic neurons. A possible functional role of such a nociceptive transmission circuit in neuronal sensitization following unilateral nerve injury is hypothesized.

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.

Axotomy-induced up-regulation of tumor necrosis factor-alpha in the dorsal root ganglia

OBJECTIVES

Neuropathic pain is a chronic pain syndrome associated with drug, injury or disease-induced damage or destruction of sensory afferent fibers of the dorsal root ganglia (DRG). Although the exact underlying pathologic mechanisms are not known, pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) are recognized as potential modulators of peripheral and central nervous system inflammatory responses. They play a crucial role in injury and the pathologic development of chronic pain syndromes such as neuropathic pain.

METHODS

Twenty-four rats were divided into a naive control (n=6), sham (surgery exposing sciatic nerve, n=6), and peripheral nerve lesion group (unilateral axotomy of sciatic nerve, n=12).

RESULTS

The results of this study demonstrate a transient up-regulation of TNF-alpha expression within ipsi- and contralateral DRG following complete unilateral sciatic nerve axotomy as confirmed by immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Elevated expression of TNF-alpha was noted to occur within the first 7 days post-axotomy, which subsequently normalized to baseline levels by day 14. This transient up-regulation was also associated with a switch in cellular source from predominant satellite cell expression at baseline to that involving satellite cells and abundant numbers of sensory neurons.

DISCUSSION

These results support the role of TNF-alpha in the upstream cascade of cellular events involved in the underlying pathogenesis of neuropathic pain. Strategies targeting the early attenuation of TNF-alpha within the DRG during the first week post-injury may have significant clinical impact in preventing the downstream cascade of events involved in the underlying cellular pathology of neuropathic pain.

DAMGO and 6beta-glycine substituted 14-O-methyloxymorphone but not morphine show peripheral, preemptive antinociception after systemic administration in a mouse visceral pain model and high intrinsic efficacy in the isolated rat vas deferens

Peripheral micro-opioid receptors (MOR) have emerged as important components of inhibitory nociceptive pathways. Here, the antinociceptive effects of MOR agonists, the 6beta-glycine derivative of 14-O-methyloxymorphone (HS-731), DAMGO and morphine were evaluated in a mouse model of visceral pain. The abdominal acetic acid-induced writhing test was used to examine the peripheral, preemptive antinociceptive opioid action on visceral nociception. HS-731 administered subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) dose-dependently and completely inhibited writhing, being 24-598-fold more potent, depending on the administration route, than two selective MOR agonists, the enkephalin analogue [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin (DAMGO) and morphine. A longer duration of action (2-3 h) was induced by HS-731 given before acetic acid, while shorter effect was produced by morphine (30-60 min) and DAMGO (30-45 min). The antinociceptive effects of systemic opioids were reversed by the s.c. opioid antagonist, naloxone. Blocking of central MOR by the selective MOR antagonist D-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP, i.c.v.) resulted in a significant reduction of antinociception of s.c. morphine, whereas it completely failed to antagonize the effects of systemic HS-731 or DAMGO. In in vitro studies, HS-731 and DAMGO, but not morphine showed high intrinsic efficacy, naltrexone-sensitive agonist effect at MOR of the rat vas deferens. These data demonstrate that selective activation of peripheral MOR by systemic s.c. HS-731 or DAMGO produces potent peripheral, preemptive visceral antinociception, while morphine's effects are mediated primarily through central mechanisms. Our findings support the role of peripheral MOR in the pathology of pain states involving sensitization of peripheral nociceptors.

Effects of traditional “Juci” (contralateral acupuncture) on orofacial nociceptive behavior in the rat

PURPOSE

"Juci", one of the traditional acupuncture techniques, means contralateral acupuncture; i.e., implanting a needle into an acupoint to treat a given disease or disorder, but on the side of the body opposite to the diseased side. The aim of this study was: (1) to assess acupuncture effects on formalin-induced nociceptive behavior in the orofacial region in the rat, and (2) to evaluate the efficacy of Juci in the orofacial formalin test.

METHODS

Forty-four adult male Wistar rats were used in the present study. A 1.0% formalin solution (25 microl s.c., diluted in saline) was injected into the right upper lip. The rats were randomly assigned to five groups. (1) The control group (n = 9), which received formalin injection without acupuncture pretreatment; (2) the ipsilateral Ho-ku (see note below) acupuncture group (n = 10); (3) the contralateral Ho-ku acupuncture group (n = 11); (4) the acupuncture plus naloxone group (n = 9), where intraperitoneal naloxone (1.0 mgxkg(-1)) was injected immediately before acupuncture pretreatment; and (5) the sham acupuncture group (n = 5). "Ho-ku" is the term used for the "Large Intestine 4" acupoint, located between the first and second metacarpal bones.

RESULTS

The injection of formalin produced the characteristic biphasic behavioral response. Acupuncture significantly inhibited the response in the early and late phases. Naloxone significantly reversed these effects. There were no statistically significant differences between the ipsilateral and Juci acupuncture groups. Sham acupuncture did not exert any significant effect on the formalin-induced behavior.

CONCLUSION

Our results showed that the degree of effectiveness of Juci was similar to that of the ipsilateral acupuncture technique. Therefore, the Juci technique is also useful for the treatment of orofacial pain.

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.

Contralateral cytokine gene induction after peripheral nerve lesions: dependence on the mode of injury and NMDA receptor signaling

There is increasing evidence that unilateral nerve injury evokes contralateral responses, but the underlying mechanisms are largely unknown. In the present investigation, we analyzed cytokine and chemokine gene induction in contralateral, non-lesioned nerves after sciatic nerve crush and chronic constriction injury (CCI) by quantitative reverse transcriptase polymerase chain reaction in mice. After sciatic nerve crush, contralateral changes in cytokine gene expression were restricted to interleukin (IL)-1beta, which showed a monophasic peak at the first postoperative day. Following CCI, contralateral transcripts for IL-1beta, IL-10 and monocyte chemoattractant protein-1 (MCP-1) were significantly increased already at day 1 and upregulation persisted over the next 4 weeks. In contrast, tumor necrosis factor alpha (TNF-alpha) levels remained unchanged. Contralateral gene induction was restricted to the homonymous opposite sciatic nerve, but spared the femoral nerve. NMDA receptor blockade completely abolished contralateral cytokine expression after CCI on the mRNA level. In contralateral dorsal root ganglia, only IL-10 mRNA levels were modified after nerve injury. Sham operation significantly increased the cytokine and chemokine gene expression at the ipsilateral side, but could not mediate contralateral effects. Our study confirms that nerve injury evokes contralateral responses and identifies NMDA-mediated signaling as one underlying mechanism.

Contralateral, ipsilateral and bilateral treatments with the kappa-opioid receptor agonist U-50,488H in mononeuropathic rats

The effect of repeated contralateral administration of the kappa-opioid receptor agonist U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate) on nociceptive behaviour was investigated and compared with ipsilateral and bilateral treatments in a rat model of peripheral unilateral neuropathy (chronic constriction of the common sciatic nerve). Administration of 0.3 mg U-50,488H into the contralateral hindpaw on days 6 and 10 after induction of mononeuropathy increased hindpaw withdrawal latency to mechanical but not to thermal stimulation compared to saline-treated rats. No difference in pain-related behaviour was found between different peripheral (contralateral, ipsilateral and bilateral) treatments with 0.3 mg U-50,488H. Autotomy behaviour was reduced for 6 weeks after sciatic nerve ligation in rats treated contralaterally with the opioid receptor agonist. Antinociceptive effects of contralaterally administered U-50,488H were abolished by the peripherally acting opioid receptor antagonist naloxone methiodide. Our findings indicate that contralateral treatment with U-50,488H attenuates nociceptive behaviour in mononeuropathic rats. These antinociceptive effects are mediated via peripheral opioid receptors.

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

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

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

ISSLS prize winner: Erythropoietin inhibits spinal neuronal apoptosis and pain following nerve root crush

STUDY DESIGN

The authors investigated the association of L5 proximal nerve root injury with spinal cord neuronal apoptosis (histologic) and whether exogenous erythropoietin therapy might reduce apoptosis/or pain (behavioral).

OBJECTIVES

The first objective was to determine whether nerve root crush induces specific programmed cell death of spinal neurons in the dorsal and ventral horn and whether this is correlated with pain behaviors. The second objective was to determine if exogenous erythropoietin might reduce apoptosis and/or pain.

SUMMARY OF BACKGROUND DATA

Whether spinal neuronal apoptosis is correlated with pain behaviors following nerve root injury remains unknown. It has been hypothesized that neuroprotective factors may alleviate pain behaviors by protecting neurons from death. Erythropoietin is a hematopoietic growth factor that recently has been demonstrated as a potent neuroprotective factor against ischemic damage in the brain. The effects of erythropoietin on pain and spinal cord neurons have not been examined.

METHODS

Sprague-Dawley rats received a L5 proximal nerve root crush injury or sham operation and were separated into two treatment groups for subcutaneous injection: 1) vehicle; 2) recombinant human erythropoietin, 2680 U/kg. The rats were sacrificed, and spinal cords were removed for apoptotic and immunohistochemical analysis at 0, 1, and 3 days after surgery. To determine whether recombinant human erythropoietin prevented mechanical allodynia in animals with nerve root crushes (n = 12/group), both treatment groups were tested for pain behaviors using the von Frey test at -1, -2, -3, 1, 3, 7, 11, and 14 days after surgery.

RESULTS

After nerve root injury, apoptotic neurons increased by 80% in the ipsilateral spinal cord and moderately in contralateral spinal cord in vehicle-treated animals compared to uninjured controls. Recombinant human erythropoietin reduced (P < 0.05) neuronal apoptosis in both superficial dorsal and ventral horns of the spinal cord. This corresponded with identification of erythropoietin and its receptors on spinal neurons and reductions in TNF-alpha colocalization in ventral horn neurons. Mechanical allodynia developed in the corresponding ipsilateral hind paw within 1 day and was sustained until day 14. Recombinant human erythropoietin-treated animals demonstrated faster recovery from mechanical allodynia compared with vehicle-treated controls (P < 0.05).

CONCLUSIONS

Our findings indicated that L5 proximal nerve root crush increased neuronal apoptosis in the superficial dorsal and ventral horn that correlated with mechanical allodynia. Exogenous recombinant human erythropoietin facilitated receptor-mediated neuroprotection of spinal cord neurons and faster recovery from mechanical allodynia. Erythropoietin may be a potential therapeutic factor for patients with low back pain by providing pain relief and neuroprotection.

Pharmacological characterisation of the spared nerve injury model of neuropathic pain

The spared nerve injury (SNI) model involves a lesion of two of the three terminal branches of the sciatic nerve (tibial and common peroneal nerves) leaving the sural nerve intact. The changes in pain-like sensation of the injured animals appear to correlate with a number of symptoms presented in human patients with neuropathic pain syndromes. In order to characterise the SNI model pharmacologically, reflex nociceptive responses to mechanical and cold stimulation were measured after systemic administration of morphine, mexiletine, gabapentin and the glutamate receptor antagonists, MK-801 and NS1209. We observed that injection of morphine (6 mg/kg, s.c.) in non-sedative doses significantly attenuated mechanical hypersensitivity in response to von Frey hair and pin prick stimulation and cold hypersensitivity in response to ethyl chloride. The sodium-channel blocker, mexiletine (37.5 mg/kg, i.p.), relieved both cold allodynia and mechanical hyperalgesia, but the most distinct and prolonged effect was observed on mechanical allodynia. Gabapentin (100 mg/kg, i.p.) significantly alleviated mechanical allodynia for at least 3h, while no significant effects were observed for either mechanical hyperalgesia or cold allodynia. In contrast, the NMDA receptor antagonist MK-801 (0.1 mg/kg, i.p.) and the AMPA receptor antagonist NS1209 (6 mg/kg, i.p.) did not relieve any of the pain-like behaviours of the SNI animals. The present study has shown that a variety of drugs with proven analgesic potency in other models of chronic pain, have differing analgesic profiles in the SNI model of neuropathic pain.