Perioperative administration of the α2-adrenoceptor agonist clonidine at the site of nerve injury reduces the development of mechanical hypersensitivity and modulates local cytokine expression

Integrating mechanistic-based and classification-based concepts into perioperative pain management: an educational guide for acute pain physicians

Chronic pain begins with acute pain. Physicians tend to classify pain by duration (acute vs chronic) and mechanism (nociceptive, neuropathic and nociplastic). Although this taxonomy may facilitate diagnosis and documentation, such categories are to some degree arbitrary constructs, with significant overlap in terms of mechanisms and treatments. In clinical practice, there are myriad different definitions for chronic pain and a substantial portion of chronic pain involves mixed phenotypes. Classification of pain based on acuity and mechanisms informs management at all levels and constitutes a critical part of guidelines and treatment for chronic pain care. Yet specialty care is often siloed, with advances in understanding lagging years behind in some areas in which these developments should be at the forefront of clinical practice. For example, in perioperative pain management, enhanced recovery protocols are not standardized and tend to drive treatment without consideration of mechanisms, which in many cases may be incongruent with personalized medicine and mechanism-based treatment. In this educational document, we discuss mechanisms and classification of pain as it pertains to commonly performed surgical procedures. Our goal is to provide a clinical reference for the acute pain physician to facilitate pain management decision-making (both diagnosis and therapy) in the perioperative period.

Peripheral inflammation as a potential mechanism and preventive strategy for perioperative neurocognitive disorder under general anesthesia and surgery

General anesthesia, as a commonly used medical intervention, has been widely applied during surgical procedures to ensure rapid loss of consciousness and pain relief for patients. However, recent research suggests that general anesthesia may be associated with the occurrence of perioperative neurocognitive disorder (PND). PND is characterized by a decline in cognitive function after surgery, including impairments in attention, memory, learning, and executive functions. With the increasing trend of population aging, the burden of PND on patients and society's health and economy is becoming more evident. Currently, the clinical consensus tends to believe that peripheral inflammation is involved in the pathogenesis of PND, providing strong support for further investigating the mechanisms and prevention of PND.

Effects of Dexmedetomidine on Immune Cells: A Narrative Review

As we all know, dexmedetomidine (DEX), as a highly selective α₂ adrenergic receptor agonist, exerts sedative, anti-anxiety and hypnotic effects by inhibiting the discharge of norepinephrine neurons in locus coeruleus and GABA-related hypnotic pathways. However, the role of DEX in anti-inflammatory and immune regulation has gradually attracted the attention of researchers in recent years. The α₂ adrenergic receptor is one of the members of the adrenergic receptor family, which is widely present in a variety of immune cells and mediates the biological behavior of the inflammatory immune system. At present, there have been more and more studies on the effects of DEX on immune cells and inflammatory responses, but few studies have systematically explored the anti-inflammatory and immunomodulatory effects of DEX. Here, we comprehensively review the published human and animal studies related to DEX, summarize the effects of DEX on immune cells and its role in related diseases, and propose potential research direction.

TGF-β1 in plasma and cerebrospinal fluid can be used as a biological indicator of chronic pain in patients with osteoarthritis

Introduction

Previous studies have demonstrated that cytokines, transforming growth factor (TGF-β1), and brain-derived neurotrophic factor (BDNF) can impact the intensity of pain in rodents. However, the roles of cytokines, TGF-β1 and BDNF in humans with chronic pain in osteoarthritis remains unclear, and no comparison between plasma and central cerebral spinal fluid (CSF) has been conducted.

Methods

Patients with osteoarthritis who were scheduled to receive spinal anesthesia were enrolled. The intensity of pain was evaluated with a visual analogue scale (VAS). In addition, patients with genitourinary system (GU) diseases and without obvious pain (VAS 0–1) were included as a comparison (control) group. The levels of TGF-β1, BDNF, tumor necrosis factor-α (TNF-α), and interleukin (IL)-8 within the CSF and plasma were collected and evaluated before surgery.

Results

The plasma and CSF TGF-β1 levels were significantly lower in the osteoarthritis patients with pain (VAS ≥ 3) than in the GU control patients. Downregulation of plasma BDNF was also found in osteoarthritis patients with pain. The Spearman correlation analysis showed that the VAS pain scores were significantly negatively correlated with the levels of TGF-β1 in the CSF of patients with osteoarthritis. However, there was no significant correlations between the pain scores and the levels of BDNF, TNF-α, and IL-8 in either the CSF or plasma.

Conclusions

TGF-β1 but not BDNF, TNF-α, or IL-8 may be an important biological indicator in the CSF of osteoarthritis patients with chronic pain.

Inhibition of transient receptor potential vanilloid type 1 through α2 adrenergic receptors at peripheral nerve terminals relieves pain

The activation of α₂ adrenergic receptors contributes to analgesia not only in the central nervous system but also in the peripheral nervous system. We reported that noradrenaline inhibits the activity of transient receptor potential vanilloid 1 (TRPV1) evoked by capsaicin through α₂ receptors in cultured rat dorsal root ganglion (DRG) neurons. However, it is unclear whether activation of TRPV1 expressed in peripheral nerve terminals is inhibited by α₂ receptors and whether this phenomenon contributes to analgesia. Therefore, we examined effects of clonidine, an α₂ receptor agonist, on several types of nociceptive behaviors, which may be caused by TRPV1 activity, and subtypes of α₂ receptors expressed with TRPV1 in primary sensory neurons in rats. Capsaicin injected into hind paws evoked nociceptive behaviors and clonidine preinjected into the same site inhibited capsaicin-evoked responses. This inhibition was not observed when clonidine was injected into the contralateral hind paws. Preinjection of clonidine into the plantar surface of ipsilateral, but not contralateral, hind paws reduced the sensitivity to heat stimuli. Clonidine partially reduced formalin-evoked responses when it was preinjected into ipsilateral hind paws. The expression level of α_2C receptor mRNA quantified by real-time PCR was highest followed by those of α_2A and α_2B receptors in DRGs. α_2A and α_2C receptor-like immunoreactivities were detected with TRPV1-like immunoreactivities in the same neurons. These results suggest that TRPV1 and α₂ receptors are coexpressed in peripheral nerve terminals and that the functional association between these two molecules causes analgesia.

The Efficacy and Safety of Glucocorticoid on Periarticular Infiltration Analgesia in Total Knee Arthroplasty: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

BACKGROUND

The objective of this systematic review and meta-analysis was to examine the effect and safety of multimodal cocktail protocols implemented with or without glucocorticoids on periarticular infiltration analgesia (PIA) in patients undergoing total knee arthroplasty (TKA).

METHODS

We comprehensively searched the PubMed, Medline, Embase, and Cochrane Library databases up to November 2020 for randomized controlled trials on glucocorticoids as a component of the cocktail protocol used in PIA for patients with TKA.

RESULTS

Our meta-analysis included 11 randomized controlled trial studies with 1051 primary TKAs (930 patients). Visual analog scale scores at postoperative day (POD) 1 and POD 2 in the glucocorticoid groups were significantly lower than those in the control group (95% CI [-1.01--0.10], P = .02; 95% CI [-0.51--0.13], P = .001). Furthermore, the glucocorticoid group was associated with a statistically significant improvement in the range of motion at POD 1 (95% CI [3.20-8.05] P < .00001), reduction in total morphine consumption at 24 hours, and lower levels of C-reactive protein on POD 2/3 compared with the control group. However, neither the length of hospital stays nor the long-term Knee Society Knee Score showed any differences between the two groups. In addition, the results of subgroup analyses favored triamcinolone acetonide over betamethasone, methylprednisolone, and dexamethasone regarding the postoperative visual analog scale scores and range of motion.

CONCLUSION

Glucocorticoid supplementation in PIA is effective and does not increase complications or side effects for patients with either unilateral or bilateral TKA. Moreover, triamcinolone acetonide could be recommended in multimodal cocktail protocols for glucocorticoid supplementation.

Efficacy of perineural versus intravenous dexmedetomidine as a peripheral nerve block adjunct: a systematic review

BACKGROUND

Dexmedetomidine is an effective local anesthetic adjunct for peripheral nerve blocks. The intravenous route for administering dexmedetomidine has been suggested to be equally effective to the perineural route; but comparative evidence is conflicting.

OBJECTIVES

This evidence-based review evaluated trials comparing the effects of intravenous to perineural dexmedetomidine on peripheral nerve block characteristics in adult surgical patients. Our primary aim was to evaluate the durations of sensory and motor blockade. Duration of analgesia, onset times of sensory and motor blockade, analgesic consumption, rest pain, and dexmedetomidine-related adverse events were evaluated as secondary outcomes.

EVIDENCE REVIEW

We sought randomized trials comparing the effects of intravenous to perineural dexmedetomidine on peripheral nerve block characteristics. The Cochrane Risk of Bias tool and the Grades of Recommendation, Assessment, Development, and Evaluation criteria was used to evaluate the quality of evidence for when an outcome was reported by at least three studies.

RESULTS

Ten studies compared intravenous and perineural dexmedetomidine in the setting of upper extremity blocks (seven), lower extremity blocks (two), and truncal block (one). The doses of dexmedetomidine supplementing long-acting local anesthetics varied between a predetermined dose (50 μg) and a weight-based dose (0.5 μg/kg-1.0 μg/kg). Clinical diversity precluded quantitative pooling; and evidence is presented as a systematic review. Compared with the intravenous route, moderate quality evidence found that perineural dexmedetomidine prolonged the duration of sensory blockade in four of six trials and motor blockade in five of seven trials. Perineural dexmedetomidine also hastened the onset of sensory and motor blockade in three of six trials. No differences were reported for the remaining outcomes; and intravenous dexmedetomidine was not superior for any outcome in any of the trials.

CONCLUSIONS

Moderate quality evidence appears to suggest that intravenous dexmedetomidine is an inferior peripheral nerve block adjunct compared with perineural dexmedetomidine. Perineural dexmedetomidine is associated with longer durations and faster onset of sensory and motor blockade.

Effect of intraperitoneally administered propentofylline in a rat model of postoperative pain

Background

In this study, we sought to evaluate whether systemic propentofylline (PPF) has antiallodynic effects in a rat model of postoperative pain, and to assess the mechanism involved.

Methods

After plantar incision, rats were intraperitoneally injected with various doses of PPF to evaluate its antiallodynic effect. To investigate the involved mechanism, rats were intraperitoneally injected with yohimbine, dexmedetomidine, prazosin, naloxone, atropine or mecamylamine, following the incision of the rat hind paws, and then PPF was administered intraperitoneally. The mechanical withdrawal threshold (MWT) was evaluated using von Frey filaments at various time points and serum levels of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6 were measured to determine the inflammatory response level.

Results

MWT was significantly increased after intraperitoneal injection of 30 mg/kg of PPF when compared with the control group. Injection of PPF and yohimbine, atropine or mecamylamine showed significant decreases in the MWT, while injection of PPF and dexmedetomidine showed a significant increase. Systemic administration of PPF inhibited the post-incisional increase in serum level of TNF-α and IL-1β.

Conclusions

Systemic administration of PPF following surgery presented antiallodynic effects in a rat model of postoperative pain. The antiallodynic effects against mechanical allodynia could be mediated by α-adrenergic and cholinergic receptors.

Randomized Phase 2 Trial of a Novel Clonidine Mucoadhesive Buccal Tablet for the Amelioration of Oral Mucositis in Patients Treated With Concomitant Chemoradiation Therapy for Head and Neck Cancer

PURPOSE

Oral mucositis (OM) is a frequent and painful sequela of concomitant chemoradiation (CRT) used for the treatment of head and neck cancer (HNC) for which there is no effective intervention. This randomized, placebo-controlled study evaluated the efficacy of a novel, mucoadhesive topical tablet formulation of clonidine in mitigating CRT-induced OM in patients with HNC.

METHODS AND MATERIALS

Patients with HNC undergoing adjuvant radiation therapy (60-66 Gy; 5 × 1.8-2.2 Gy/wk) with concomitant platinum-based chemotherapy received daily local clonidine at 50 μg (n = 56), 100 μg (n = 65), or placebo (n = 62) via a topical mucobuccal tablet starting 1 to 3 days before and continuing during treatment. The primary endpoint was the incidence of severe OM (severe OM [SOM], World Health Organization grade 3/4).

RESULTS

SOM developed in 45% versus 60% (P = .06) of patients treated with clonidine compared with placebo and occurred for the first time at 60 Gy as opposed to 48 Gy (median; hazard ratio, 0.75 [95% confidence interval, 0.484-1.175], P = .21); median time to onset was 45 versus 36 days. Opioid analgesic use, mean patient-reported mouth and throat soreness, and CRT compliance were not significantly different between treatment arms. Adverse events were reported in 90.8% versus 98.4%, nausea in 49.6% versus 71.0%, dysphagia in 32.8% versus 48.4%, and reversible hypotension in 6.7% versus 1.6% of patients on clonidine versus placebo, respectively.

CONCLUSIONS

Although the primary endpoint was not met, the positive trends of OM-associated outcomes suggest that the novel mucoadhesive tablet delivery of clonidine might favorably affect the course and severity of CRT-induced SOM and support further evaluation.

Effects of Intraneural Injection of Dexmedetomidine in Combination With Ropivacaine in Rat Sciatic Nerve Block

BACKGROUND AND OBJECTIVES

Dexmedetomidine is known to have neural protection effect via attenuation of inflammatory responses induced by local anesthetics. We investigated whether intraneural dexmedetomidine is effective for attenuating or preventing neural injury resulting from inadvertent intraneural injection of local anesthetic.

METHODS

Rats were randomly divided, and left sciatic nerve was surgically exposed. The rats received no injection (control group) or intraneural injections of 0.2 mL of normal saline (saline group), 0.2 mL of 0.5% ropivacaine (ropivacaine group), or 0.2 mL of 0.5% ropivacaine and 0.5 μg/kg of dexmedetomidine (ropivacaine plus dexmedetomidine group). Interleukin (IL)-6 and IL-1β messenger RNA (mRNA) levels were detected at 60 minutes after intraneural injection in experiment 1 (5 per group). Sensory and motor functions were assessed until the return of normal sensory and motor functions, and histopathological and ultrastructure analysis were performed at 4 weeks after intraneural injection in experiment 2 (8 per group).

RESULTS

Dexmedetomidine with ropivacaine better enhanced sensory and motor blockade than ropivacaine alone. IL-6 (3.2 ± 1.0 vs 5.9 ± 2.1), IL-1β (1.1 ± 0.1 vs 2.2 ± 0.7) levels, scores of axon and myelinated fiber degeneration (1 [0-2] vs 2 [1-3]), and demyelinated fiber percentages (20.1 ± 10.4 vs 48.3 ± 12.7) were lower in the ropivacaine plus dexmedetomidine group than in the ropivacaine group. No animals showed any signs of permanent neurological deficit.

CONCLUSIONS

Intraneural dexmedetomidine has sensory and motor blockade-enhancing effects, anti-inflammatory properties, and protective effects against neural injury. These findings suggest that dexmedetomidine as an adjuvant has beneficial effects in rat when intraneural injection of local anesthetic occurs.

Sustained analgesic effect of clonidine co-polymer depot in a porcine incisional pain model

Background

Previous research suggests that the α₂ adrenergic agonist clonidine, a centrally acting analgesic and antihypertensive, may also have direct effects on peripheral pain generators. However, aqueous injections are limited by rapid systemic absorption leading to off target effects and a brief analgesic duration of action.

Purpose

The aim of this study was to examine the efficacy of a sustained-release clonidine depot, placed in the wound bed, in a pig incisional pain model.

Methods

The depot was a 15 mm ×5 mm ×0.3 mm poly(lactide-co-caprolactone) polymer film containing 3% (w/w) clonidine HCl (MDT3). Fifty-two young adult mix Landrace pigs (9–11 kg) were divided into seven groups. All subjects received a 6 cm, full-thickness, linear incision into the left lateral flank. Group 1 served as a Sham control group (Sham, n=8). Group 2 received three placebo strips (PBO, n=8), placed end-to-end in the subcutaneous wound bed before wound closure. Group 3 received one MDT3 and two PBO (n=8), Group 4 received two MDT3 and one PBO (n=8), and Group 5 received three MDT3 (n=8). Positive control groups received peri-incisional injections of bupivacaine solution (Group 6, 30 mg/day bupivacaine, n=8) or clonidine solution (Group 7, 225 µg/day, n=4).

Results

The surgical procedure was associated with significant peri-incisional tactile allodynia. There was a dose-dependent effect of MDT3 in partially reversing the peri-incisional tactile allodynia, with maximum pain relief relative to Sham at 72 hours. Daily injections of bupivacaine (30 mg), but not clonidine (up to 225 µg), completely reversed allodynia within 48 hours. There was a statistically significant correlation between the dose of MDT3 and cumulative withdrawal threshold from 4 hours through the conclusion of the study on day 7.

Conclusion

These data suggest that a sustained-release clonidine depot may be a viable nonopioid, nonamide anesthetic therapy for the treatment of acute postsurgical nociceptive sensitization.

Effect of bupivacaine and adjuvant drugs for regional anesthesia on nerve tissue oximetry and nerve blood flow

Background

Nerve blood flow has a critical role in acute and chronic pathologies in peripheral nerves. Influences of local anesthetics and adjuvants on tissue perfusion and oxygenation are deemed as relevant factors for nerve damage after peripheral regional anesthesia. The link between low tissue perfusion due to local anesthetics and resulting tissue oxygenation is unclear.

Methods

Combined tissue spectrophotometry and laser-Doppler flowmetry were used to assess nerve blood flow in 40 surgically exposed median nerves in pigs, as well as nerve tissue oximetry for 60 min. After baseline measurements, test solutions saline (S), bupivacaine (Bupi), bupivacaine with epinephrine (BupiEpi), and bupivacaine with clonidine (BupiCloni) were applied topically.

Results

Bupivacaine resulted in significant decrease in nerve blood flow, as well as tissue oximetry values, compared with saline control. Addition of epinephrine resulted in a rapid, but nonsignificant, reduction of nerve blood flow and extensive lowering of tissue oximetry levels. The use of clonidine resulted in a reduction of nerve blood flow, comparable to bupivacaine alone (relative blood flow at T60 min compared with baseline, S: 0.86 (0.67-1.18), median (25th-75th percentile); Bupi: 0.33 (0.25-0.60); BupiCloni: 0.43 (0.38-0.63); and BupiEpi: 0.41(0.30-0.54). The use of adjuvants did not result in any relevant impairment of tissue oximetry values (saturation values in percent at T60, S: 91.5 [84-95]; Bupi: 76 [61-86]; BupiCloni: 84.5 [76-91]; and BupiEpi: 91 [56-92]).

Conclusion

The application of bupivacaine results in lower nerve blood flow, but does not induce relevant ischemia. Despite significant reductions in nerve blood flow, the addition of clonidine or epinephrine to bupivacaine had no significant impact on nerve tissue oximetry compared with bupivacaine alone. Nerve ischemia due to local anesthetics is not enhanced by the adjuvants clonidine or epinephrine.

Comparison of the Systemic and Local Pharmacokinetics of Clonidine Mucoadhesive Buccal Tablets with Reference Clonidine Oral Tablets in Healthy Volunteers: An Open-Label Randomised Cross-Over Trial

INTRODUCTION

The clonidine mucoadhesive buccal tablet (MBT) is a novel delivery system resulting in high and sustained concentrations of clonidine in the oral cavity. In a phase II clinical trial, clonidine MBT reduced the incidence of severe oral mucositis (OM) compared to placebo in head and neck cancer patients undergoing chemoradiation. This study compared the pharmacokinetics (PK), safety and tolerability of clonidine MBT with a reference oral tablet (OT).

METHODS

This was a randomised, three-period, single-dose crossover study in 36 healthy subjects aged 18-50 years. Eligibility was assessed within 14 days of the first dose. IMP was administered in the fasted state on day 1 of each treatment period. PK samples were collected up to 24 h (saliva)/96 h (blood) for measurement of the clonidine concentration. Safety and tolerability were evaluated at specified times throughout the study. A washout period of at least 7 days was observed between administrations.

RESULTS

Clonidine MBT (50 and 100 µg) applied to the upper gum resulted in a dose-proportional increase in saliva (C _max and AUC_0-t ) and plasma (C_max and AUC_0-inf) clonidine levels. Clonidine MBT was considered to mimic a continuous release of clonidine in plasma, significantly decreasing the C _max and AUC and increasing the T _max when compared with the reference clonidine HCl tablets. Clonidine MBT exhibited high and prolonged concentrations in saliva where concentrations with the clonidine HCl tablet were negligible. Clonidine MBT exhibited a favourable safety profile with significantly fewer subjects reporting AEs (dry mouth and fatigue) and a reduction in blood pressure when compared to the reference clonidine HCl tablets.

CONCLUSION

Clonidine MBT is well tolerated and exhibits proportional saliva and plasma PK over the 50-100-µg dose level. The MBT results in higher saliva concentrations and lower systemic exposure than OT, which was associated with a trend towards fewer adverse events and less dry mouth, fatigue and hypotensive effect.

FUNDING

Onxeo SA.

TRIAL REGISTRATION

ClinicalTrials.gov identifier, NCT02548806.

Bupivacaine Field Block With Clonidine for Postoperative Pain Control in Posterior Spine Approaches: A Randomized Double-Blind Trial

BACKGROUND

The synergistic effect of clonidine with bupivacaine, well established in peripheral nerve blocks, remains controversial in local field block for postoperative analgesia.

OBJECTIVE

To investigate the potential analgesic benefit of adding clonidine to bupivacaine during preincisional field block in posterior approaches for spine surgeries.

METHODS

Two hundred twenty-five patients were enrolled in this study and underwent lumbar spinal fusion (n = 80), lumbar laminectomy (n = 25), lumbar microdiscectomy (n = 94), or cervical laminectomy (n = 26). In each surgical subgroup, patients were randomly assigned in a double-blinded fashion to receive either 20 mL of 0.25% bupivacaine alone (control group, n = 109) or with 150 μg clonidine (clonidine group, n = 116) in the form of a preincisional field block. Outcome parameters included area under the curve of pain from postoperative day D0 to D8 and rescue morphine consumption from D0 to D3.

RESULTS

The area under the curve was reduced in the clonidine group, particularly in the microdiscectomy subgroup, and without reaching statistical significance in the cervical laminectomy subgroup. Total rescue morphine consumption was reduced in the clonidine group, particularly at D1-D2, a benefit that was exclusive to the lumbar stenosis and lumbar fusion subgroups. Field block with clonidine, surgical subgroup, and the presence of preoperative spinal pain were factors independently influencing postoperative wound pain in multivariate analysis.

CONCLUSION

The addition of clonidine to local preincisional field block with bupivacaine resulted in better and prolonged postoperative analgesia in posterior lumbar spine surgeries, an effect that was more pronounced in patients with no preoperative spinal pain.

Breaking barriers to novel analgesic drug development

Acute and chronic pain complaints, although common, are generally poorly served by existing therapies. This unmet clinical need reflects a failure to develop novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms, and the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities for developing novel therapeutic strategies and revisiting existing targets, including modulating ion channels, enzymes and G-protein-coupled receptors.

Modulation of formalin-induced pain-related behaviour by clonidine and yohimbine in the Speke's hinged tortoise (Kiniskys spekii)

The study was designed to investigate the involvement of noradrenergic and serotonergic receptor systems in the modulation of formalin-induced pain-related behaviour in the Speke's hinged tortoise. Intradermal injection of 100 μL of formalin at a dilution of 12.5% caused pain-related behaviour (hindlimb withdrawal) that lasted for a mean time of 19.28 min (monophasic response). Intrathecal administration of clonidine (α₂ -adrenergic receptor agonist) and yohimbine (α₂ -adrenergic receptor antagonist) at a dose of 40 μg/kg and 37.5 μg/kg or 50 μg/kg, respectively, caused a highly significant reduction in the duration of the formalin-induced pain-related behaviour. The effect of clonidine was reversed by intrathecal administration of yohimbine at a dose of 26.7 μg/kg. The effect of yohimbine at a dose of 50 μg/kg was reversed by intrathecal injection of 20 μg/kg of the serotonergic receptor antagonist methysergide maleate. When performing antagonistic reactions, the administration of the antagonist was followed immediately by that of the agonist. The study indicates that for experimental purposes, intrathecal route of drug administration through the atlanto-occipital joint is effective in tortoises. The data also suggest that testudines have noradrenergic and serotonergic systems that appear to play a role in the modulation of pain in this species.

[Perineural dexmedetomidine effects on sciatic nerve in rat]

The present study was designed to test the hypothesis that high dose dexmedetomidine would increase the duration of antinociception to a thermal stimulus in a rat model of sciatic nerve blockade without causing nerve damage. The rats were anesthetized with isoflurane. After electromyography (EMG) recordings, right sciatic nerves were explored and perineural injections were delivered: Group D (n=7), 40μgμgkg^-1 dexmedetomidine administration, Group II (n=6), (0.2mL) saline administration, Group III (n=2), only surgically exploration of the right sciatic nevre. Time to paw withdrawal latency (PAW) to a thermal stimulus for both paws and an assessment of motor function were measured every 30min after the nerve block until a return to baseline. The compound muscle action potential (CMAP) of right and left sciatic nerves were recorded 10 times per each nerve once more after perineural injections at 14 day. After EMG recordings, right and the part of left sciatic nerve were excised at a length of at minimum 15mm for histopathological examination. Comparison of right/left CMAP amplitude ratios before and 14 days after the procedure showed a statistically significant difference (p=0.000). There were no differences in perineural inflammation between the Group D, Group S, and Group E at 14 days.

Rubus occidentalis alleviates hyperalgesia induced by repeated intramuscular injection of acidic saline in rats

Background

The purpose of this study was to evaluate the antinociceptive effect of black raspberry (Rubus occidentalis) fruit extract (ROE) in a rat model of chronic muscle pain and examine the mechanisms involved.

Methods

Adult male Sprague–Dawley rats were used, and chronic muscle pain was induced by two injections of acidic saline into one gastrocnemius muscle. For the first experiment, 50 rats were randomly assigned to five groups. After the development of hyperalgesia, rats were injected intraperitoneally with 0.9 % saline or ROE (10, 30, 100, or 300 mg/kg). For the second experiment, 70 rats were randomly assigned to seven groups. Rats were injected intraperitoneally with saline, yohimbine, dexmedetomidine, prazosin, atropine, mecamylamine, or naloxone after the development of hyperalgesia. Ten minutes later, ROE (300 mg/kg) was administered intraperitoneally. For both experiments, the mechanical withdrawal threshold (MWT) was evaluated with von Frey filaments before the first acidic saline injection, 24 h after the second injection, and at 15, 30, 45, 60, 80, 100, and 120 min, 24 and 48 h after the drug administration.

Results

Compared with the control group, the MWT significantly increased up to 45 min after injection of ROE 100 mg/kg and up to 60 min after injection of ROE 300 mg/kg, respectively. Injection of ROE together with yohimbine or mecamylamine significantly decreased the MWT compared with the effect of ROE alone, while ROE together with dexmedetomidine significantly increased the MWT.

Conclusions

ROE showed antinociceptive activity against induced chronic muscle pain, which may be mediated by α2-adrenergic and nicotinic cholinergic receptors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-016-1192-z) contains supplementary material, which is available to authorized users.

Neurotoxicity of common peripheral nerve block adjuvants

PURPOSE OF REVIEW

This review outlines the analgesic role of perineural adjuvants for local anesthetic nerve block injections, and evaluates current knowledge regarding whether adjuvants modulate the neurocytologic properties of local anesthetics.

RECENT FINDINGS

Perineural adjuvant medications such as dexmedetomidine, clonidine, buprenorphine, dexamethasone, and midazolam play unique analgesic roles. The dosing of these medications to prevent neurotoxicity is characterized in various cellular and in-vivo models. Much of this mitigation may be via reducing the dose of local anesthetic used while achieving equal or superior analgesia. Dose-concentration animal models have shown no evidence of deleterious effects. Clinical observations regarding blocks with combined bupivacaine-clonidine-buprenorphine-dexamethasone have shown beneficial effects on block duration and rebound pain without long-term evidence of neurotoxicity. In-vitro and in-vivo studies of perineural clonidine and dexmedetomidine show attenuation of perineural inflammatory responses generated by local anesthetics.

SUMMARY

Dexmedetomidine added as a peripheral nerve blockade adjuvant improves block duration without neurotoxic properties. The combined adjuvants clonidine, buprenorphine, and dexamethasone do not appear to alter local anesthetic neurotoxicity. Midazolam significantly increases local anesthetic neurotoxicity in vitro, but when combined with clonidine-buprenorphine-dexamethasone (sans local anesthetic) produces no in-vitro or in-vivo neurotoxicity. Further larger-species animal testing and human trials will be required to reinforce the clinical applicability of these findings.

Perineural dexmedetomidine effects on sciatic nerve in rat

The present study was designed to test the hypothesis that high dose dexmedetomidine would increase the duration of antinociception to a thermal stimulus in a rat model of sciatic nerve blockade without causing nerve damage. The rats were anesthetized with isoflurane. After electromyography (EMG) recordings, right sciatic nerves were explored and perineural injections were delivered: Group D (n=7), 40μgμgkg^-1 dexmedetomidine administration, Group II (n=6), (0.2mL) saline administration, Group III (n=2), only surgically exploration of the right sciatic nevre. Time to paw withdrawal latency (PAW) to a thermal stimulus for both paws and an assessment of motor function were measured every 30min after the nerve block until a return to baseline. The compound muscle action potential (CMAP) of right and left sciatic nerves were recorded 10 times per each nerve once more after perineural injections at 14 day. After EMG recordings, right and the part of left sciatic nerve were excised at a length of at minimum 15mm for histopathological examination. Comparison of right/left CMAP amplitude ratios before and 14 days after the procedure showed a statistically significant difference (p=0.000). There were no differences in perineural inflammation between the Group D, Group S, and Group E at 14 days.

High-frequency transcutaneous electrical nerve stimulation attenuates postsurgical pain and inhibits excess substance P in rat dorsal root ganglion

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is a common therapeutic modality for pain management, but its effectiveness in skin/muscle incision and retraction (SMIR)-evoked pain is unknown. We aimed to examine the effects of TENS on postoperative pain and the levels of substance P (SP), N-methyl-D-aspartate receptor 1 (NR1), and interleukin 1β (IL-1β) in rat dorsal root ganglion (DRG).

METHODS

High-frequency (100 Hz) TENS was administered daily beginning on postoperative day 1 (POD1) and continued until animal subjects were killed for tissues. Mechanical sensitivity to von Frey stimuli (6g and 15g) and the levels of NR1, SP, and IL-1β in DRG were assessed in the sham-operated, SMIR-operated, TENS after SMIR surgery, and placebo-TENS after SMIR surgery groups.

RESULTS

Skin/muscle incision and retraction rats exhibited a significant hypersensitivity to von Frey stimuli on POD3. In contrast with SMIR rats, SMIR-operated rats receiving TENS therapy demonstrated a rapid recovery of mechanical hypersensitivity. The SMIR-operated rats showed an up-regulation of NR1, SP, and IL-1β in DRG on PODs 14 and 28, whereas the SMIR-operated rats after TENS administration reversed this up-regulation. By contrast, the placebo-TENS after SMIR operation did not alter postsurgical pain nor the levels of NR1, SP, and IL-1β.

CONCLUSIONS

Our data demonstrated that TENS intervention reduced persistent postoperative pain caused by SMIR operation. Up-regulation of NR1, SP, and IL-1β in DRG, activated after SMIR surgery, is important in the development of prolonged postincisional pain. The TENS pain relief may be related to the suppression of NR1, SP, and IL-1β in DRG of SMIR rats.

Transcutaneous electrical nerve stimulation attenuates postsurgical allodynia and suppresses spinal substance P and proinflammatory cytokine release in rats

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is often used for management of chronic pain.

OBJECTIVE

The purpose of this study was to investigate whether TENS altered postincisional allodynia, substance P, and proinflammatory cytokines in a rat model of skin-muscle incision and retraction (SMIR).

DESIGN

This was an experimental study.

METHODS

High-frequency (100-Hz) TENS therapy began on postoperative day 3 and was administered for 20 minutes daily to SMIR-operated rats by self-adhesive electrodes delivered to skin innervated via the ipsilateral dorsal rami of lumbar spinal nerves L1-L6 for the next 27 days. The expressions of substance P, tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1beta (IL-1β) in the spinal cord and mechanical sensitivity to von Frey stimuli (4g and 10g) were evaluated.

RESULTS

The SMIR-operated rats displayed a marked hypersensitivity to von Frey stimuli on postoperative day 3. In contrast to the SMIR-operated rats, SMIR-operated rats after TENS administration showed a quick recovery of mechanical hypersensitivity. On postoperative days 3, 16, and 30, SMIR-operated rats exhibited an upregulation of substance P and cytokines (TNF-α, IL-6, and IL-1β) in the spinal cord, whereas SMIR-operated rats after TENS therapy inhibited that upregulation. By contrast, the placebo TENS following SMIR surgery did not alter mechanical hypersensitivity and the levels of spinal substance P, TNF-α, IL-6, and IL-1β.

LIMITATIONS

The experimental data are limited to animal models and cannot be generalized to postoperative pain in humans.

CONCLUSIONS

The results revealed that TENS attenuates prolonged postoperative allodynia following SMIR surgery. Increased levels of spinal substance P and proinflammatory cytokines, activated after SMIR surgery, are important in the processing of persistent postsurgical allodynia. The protective effect of TENS may be related to the suppression of spinal substance P and proinflammatory cytokines in SMIR-operated rats.

Adjuncts should always be used in pediatric regional anesthesia

A number of different adjuncts to local anesthetics can be used to prolong and optimize postoperative pain relief following regional anesthesia in children. The present text provides a slightly opinionated but evidence-based argument in favor of this practice.

Alpha-adrenoceptor modulation in central nervous system trauma: pain, spasms, and paralysis--an unlucky triad

Many researchers have attempted to pharmacologically modulate the adrenergic system to control locomotion, pain, and spasms after central nervous system (CNS) trauma, although such efforts have led to conflicting results. Despite this, multiple studies highlight that α-adrenoceptors (α-ARs) are promising therapeutic targets because in the CNS, they are involved in reactivity to stressors and regulation of locomotion, pain, and spasms. These functions can be activated by direct modulation of these receptors on neuronal networks in the brain and the spinal cord. In addition, these multifunctional receptors are also broadly expressed on immune cells. This suggests that they might play a key role in modulating immunological responses, which may be crucial in treating spinal cord injury and traumatic brain injury as both diseases are characterized by a strong inflammatory component. Reducing the proinflammatory response will create a more permissive environment for axon regeneration and may support neuromodulation in combination therapies. However, pharmacological interventions are hindered by adrenergic system complexity and the even more complicated anatomical and physiological changes in the CNS after trauma. This review is the first concise overview of the pros and cons of α-AR modulation in the context of CNS trauma.

Antinociceptive effects of venlafaxine in a rat model of peripheral neuropathy: role of alpha2-adrenergic receptors

This study was designed to determine whether acute or chronic venlafaxine administration was effective in alleviating symptoms of neuropathic pain in a rat model of neuropathic pain, and whether the effect of venlafaxine involved manipulation of α2-adrenoceptors,by determining the effect of yohimbine, a α2-adrenoceptor antagonist on its actions. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve in the rats that resulted in stimulus-evoked thermal hyperalgesia, tactile mechanical and cold allodynia. Acute venlafaxine injections (20 and 40 mg/kg i.p.) on the 7th, 14th and 21st postoperative days could not reduce tactile and cold hypersensitivity significantly compared to CCI group. But in these groups venlafaxine (40 mg/kg i.p.) blocked heat hyperalgesia. When venlafaxine (10 and 20mg/kg i.p.) administration was started on the first day after CCI and given daily until the 14th day, tactile hypersensitivity and heat hyperalgesia considerably were attenuated. But when venlafaxine (20mg/kg i.p.) treatment was initiated on the 10th day after CCI, once the model had been fully established, and given daily for 11 days, no differences in withdrawal thresholds were observed compared with CCI group however heat hyperalgesia significantly has been blocked. Also the effect of venlafaxine on heat hyperalgesia was reversed by pretreatment with yohimbine at all-time intervals. These results indicate that venlafaxine, when administered immediately after nerve injury, and for a sufficient period of time, can prevent the development and expression of neuropathic pain. Also we conclude that α2-adrenoceptors participate in the antinociceptive effects of venlafaxine.

Antinociceptive activity of methanolic extract of Muntingia calabura leaves: further elucidation of the possible mechanisms

Background

Muntingia calabura (Elaecoparceae) is a medicinal plant traditionally used, particularly, by the Peruvian people to alleviate headache and cold, pain associated with gastric ulcers or to reduce the prostate gland swelling. Following the recent establishment of antinociceptive activity of M. calabura leaf, the present study was performed to further elucidate on the possible mechanisms of antinociception involved.

Methods

The methanol extract of M. calabura (MEMC) was prepared in the doses of 100, 250 and 500 mg/kg. The role of bradykinin, protein kinase C, pottasium channels, and various opioid and non-opioid receptors in modulating the extract’s antinociceptive activity was determined using several antinociceptive assays. Results are presented as Mean ± standard error of mean (SEM). The one-way ANOVA test with Dunnett's multiple comparison was used to analyze and compare the data, with P < 0.05 as the limit of significance.

Results

The MEMC, at all doses, demonstrated a significant (p < 0.05) dose-dependent antinociceptive activity in both the bradykinin- and phorbol 12-myristate 13-acetate (PMA)-induced nociception. Pretreatment of the 500 mg/kg MEMC with 10 mg/kg glibenclamide (an ATP-sensitive K⁺ channel inhibitor), the antagonist of μ-, δ- and κ-opioid receptors (namely 10 mg/kg β-funaltrexamine, 1 mg/kg naltrindole and 1 mg/kg nor-binaltorphimine), and the non-opioid receptor antagonists (namely 3 mg/kg caffeine (a non-selective adenosinergic receptor antagonist), 0.15 mg/kg yohimbine (an α₂-noradrenergic antagonist), and 1 mg/kg pindolol (a β-adrenoceptor antagonist)) significantly (p < 0.05) reversed the MEMC antinociception. However, 10 mg/kg atropine (a non-selective cholinergic receptor antagonist), 0.15 mg/kg prazosin (an α₁-noradrenergic antagonist) and 20 mg/kg haloperidol (a non-selective dopaminergic antagonist) did not affect the extract's antinociception. The phytochemicals screening revealed the presence of saponins, flavonoids, tannins and triterpenes while the HPLC analysis showed the presence of flavonoid-based compounds.

Conclusions

The antinociceptive activity of MEMC involved activation of the non-selective opioid (particularly the μ-, δ- and κ-opioid) and non-opioid (particularly adenosinergic, α₂-noradrenergic, and β-adrenergic) receptors, modulation of the ATP-sensitive K⁺ channel, and inhibition of bradikinin and protein kinase C actions. The discrepancies in MEMC antinociception could be due to the presence of various phytochemicals.

Analgesic treatment with pregabalin does not prevent persistent pain after peripheral nerve injury in the rat

Reducing the risk of chronic postoperative pain through preventive analgesia is an attractive therapeutic concept. Because peripheral nerve lesions are a major cause of chronic pain after surgery, we tested in rats whether analgesic treatment with pregabalin (PGB) has the capacity to mitigate the development of persistent neuropathic pain-like behavior. Starting on the day of spared nerve injury or 1week later, we treated rats with a continuous intrathecal infusion of PGB (300 or 900μg/24hours) or vehicle for up to 28days. Rats receiving early PGB treatment had almost normal withdrawal thresholds for punctate mechanical stimuli and were clearly less sensitive to pinprick or cold stimulation. The responses to punctate mechanical and cold stimulation were still reduced for a brief period after the infusion was terminated, but the difference from vehicle-treated rats was minor. Essentially, the analgesic effect of PGB was limited to the duration of the infusion, whether analgesia started at the time of surgery or with a delay of 1week, independently of the length of the treatment. PGB did not suppress the activation of spinal microglia, indicating that analgesia alone does not eliminate certain pain mechanisms even if they depend, at least partially, on nociceptive input. Unexpectedly, intrathecal infusion of PGB did not inhibit the nerve injury-induced accumulation of its binding target, the voltage-gated calcium channel subunit α2δ1, at primary afferent terminals in the spinal cord. Interference with the synaptic trafficking of α2δ1 is not required to achieve analgesia with PGB.

The role of alpha-2 adrenoceptor subtype in the antiallodynic effect of intraplantar dexmedetomidine in a rat spinal nerve ligation model

The purpose of this study was to examine the effects of intraplantar dexmedetomidine to relieve neuropathic pain and determine the role of peripheral α2-adrenoceptors. Neuropathic pain was induced by ligating the L5 and L6 spinal nerves in male Sprague-Dawley rats, and mechanical allodynia was assessed using von Frey filaments. Several antagonists were injected into the hindpaws to evaluate the mechanisms of action of dexmedetomidine, a nonselective α2-adrenoceptor antagonist yohimbine, an α2A-adrenoceptor antagonist BRL 44408, an α2B-adrenoceptor antagonist ARC 239, and a α2C-adrenoceptor antagonist JP 1302. The expression of α2A-adrenoceptor, α2B-adrenoceptor, and α2C-adrenoceptor genes in the lumbar segment of the spinal cord and the plantar skin of the nerve-injured leg was detected by reverse transcription-polymerase chain reaction. Ipsilateral intraplantar injection of dexmedetomidine produced dose-dependent antiallodynia. Ipsilateral, but not contraleral, intraplantar injection of yohimbine reversed the antinociception of dexmedetomidine. Intraplantar BRL 44408, ARC 239, and JP 1302 reversed the antinociception of dexmedetomidine. The expression levels of α2-adrenoceptor genes in the lumbar spinal cord did not differ between rats with neuropathic pain and naïve rats. The expression levels of α2B-adrenoceptor and α2C-adrenoceptor genes of plantar skin were upregulated significantly in the model group, whereas α2A-adrenoceptor expression was unchanged. These results suggest that intraplantar injection of dexmedetomidine produced an antiallodynic effect in spinal nerve ligation-induced neuropathic pain. All three types of peripheral α2A, α2B, and α2C-adrenoceptors were involved in the antiallodynic mechanism of dexmedetomidine.

Management of chronic pain following nerve injuries/CRPS type II

Chronic pain affects quality of life and adversely affects functional outcomes. Chronic postoperative pain is a frustrating problem for the surgeon because it ruins a technically perfect procedure, and the surgeon may be unsure of treatment strategies. There is much information on chronic pain and its treatment, but it is often published outside of surgery and diffusion of this information across disciplines is slow. This article synthesizes some of this literature and provides a systematic presentation of the evidence on pain associated with peripheral nerve injury. It highlights the use of perioperative and early intervention to decrease this debilitating problem.

Guided paravertebral blocks with versus without clonidine for women undergoing breast surgery: a prospective double-blinded randomized study

BACKGROUND

Paravertebral blocks (PVBs) have been introduced as an alternative to general anesthesia for breast cancer surgeries. The addition of clonidine as an adjuvant in PVBs may enhance quality and duration of analgesia and significantly reduce the consumption of analgesics after breast surgery. In this prospective randomized double-blind study, we assessed the significance of adding clonidine to the anesthetic mixture for women undergoing mastectomy.

METHODS

Sixty patients were randomized equally into 2 groups, both of which received PVB block, either with or without clonidine. Analgesic consumption was noted up to 2 weeks after the operation. A visual analog scale was used to assess pain postoperatively during the hospital stay, and a numeric rating scale was used when patients were discharged.

RESULTS

Analgesic consumption was significantly lower in the clonidine group 48 hours postoperatively with 95% confidence interval (CI) for the difference (-69.5% to -6.6%). Pain scores at rest showed significant reduction in the clonidine group during the period from 24 to 72 hours postoperatively with 95% CI for the ratios of 2 means (1.09-3.61), (2.04-9.04), and (2.54-16.55), respectively, with shoulder movement at 24, 48, and 72 hours postoperatively 95% CI for the ratio of 2 means (1.10-3.15), (1.32-6.38), and (1.33-8.42), respectively. The time needed to resume daily activity was shorter in the clonidine group compared with the control group with 95% CI for the ratio of 2 means (1.14-1.62).

CONCLUSION

The addition of clonidine enhanced the analgesic efficacy of PVB up to 3 days postoperatively for patients undergoing breast surgery.

Perioperative interventions to reduce chronic postsurgical pain

Approximately 10% of patients following a variety of surgeries develop chronic postsurgical pain. Reducing chronic postoperative pain is especially important to reconstructive surgeons because common operations such as breast and limb reconstruction have even higher risk for developing chronic postsurgical pain. Animal studies of posttraumatic nerve injury pain demonstrate that there is a critical time frame before and immediately after nerve injury in which specific interventions can reduce the incidence and intensity of chronic neuropathic pain behaviors-so called "preventative analgesia." In animal models, perineural local anesthetic, systemic intravenous local anesthetic, perineural clonidine, systemic gabapentin, systemic tricyclic antidepressants, and minocycline have each been shown to reduce pain behaviors days to weeks after treatment. The translation of this work to humans also suggests that brief perioperative interventions may protect patients from developing new chronic postsurgical pain. Recent clinical trial data show that there is an opportunity during the perioperative period to dramatically reduce the incidence and severity of chronic postsurgical pain. The surgeon, working with the anesthesiologist, has the ability to modify both early and chronic postoperative pain by implementing an evidence-based preventative analgesia plan.

Causes and prevention of chronic postsurgical pain

PURPOSE OF REVIEW

Surgical incision invariably causes some measure of nerve damage and inflammatory response that, in most cases, heals quickly without long-term negative consequence. However, a subset of these patients go on to develop lasting neuropathic pain that is difficult to treat and, in many cases, prevents the return to normal activities of life. It remains unknown why two patients with identical surgical interventions may go on to develop completely divergent pain phenotypes or no pain at all. Aggressive, early analgesic therapy has been shown to reduce the incidence of chronic postsurgical pain (CPSP), but no specific regional anesthetic technique or systemic pharmacologic therapy has been shown to prevent CPSP.

RECENT FINDINGS

Inflammation and glial cell activation have recently been shown to be just as important in the transition from normal acute pain to pathologic chronic pain as nerve injury itself and that central sensitization may not be solely due to repetitive nociceptive firing at the time of nerve injury. This has opened a number of new therapeutic possibilities for prevention of CPSP.

SUMMARY

Here, we discuss the causes of CPSP and current useful preventive strategies in the perioperative period. We also discuss future potential disease-modifying treatments of CPSP.

Janus molecule I: dichotomous effects of COMT in neuropathic vs nociceptive pain modalities

The enzyme catechol-O-methyltransferase (COMT) has been shown to play a critical role in pain perception by regulating levels of epinephrine (Epi) and norepinephrine (NE). Although the key contribution of catecholamines to the perception of pain has been recognized for a long time, there is a clear dichotomy of observations. More than a century of research has demonstrated that increasing adrenergic transmission in the spinal cord decreases pain sensitivity in animals. Equally abundant evidence demonstrates the opposite effect of adrenergic signaling in the peripheral nervous system, where adrenergic signaling increases pain sensitivity. Viewing pain processing within spinal and peripheral compartments and determining the directionality of adrenergic signaling helps clarify the seemingly contradictory findings of the pain modulatory properties of adrenergic receptor agonists and antagonists presented in other reviews. Available evidence suggests that adrenergic signaling contributes to pain phenotypes through α(1/2) and β(2/3) receptors. While stimulation of α(2) adrenergic receptors seems to uniformly produce analgesia, stimulation of α(1) or β receptors produces either analgesic or hyperalgesic effects. Establishing the directionality of adrenergic receptor modulation of pain processing, and related COMT activity in different pain models are needed to bring meaning to recent human molecular genetic findings. This will enable the translation of current findings into meaningful clinical applications such as diagnostic markers and novel therapeutic targets for complex human pain conditions.

The influence of adjuvants used in regional anesthesia on lidocaine-induced neurotoxicity in vitro

BACKGROUND

Neurotoxic properties of local anesthetics can rarely lead to irreversible neuronal damage as in cauda equina syndrome. Clinically, local anesthetics are often combined with adjuvants to improve or prolong the anesthetic effect, whereas the impact of such adjuvants on lidocaine-induced apoptosis is unclear. Therefore, we investigated the influence of different adjuvants on the neurotoxicity of lidocaine.

METHODS

Human neuroblastoma cells and primary rat astrocytes were incubated for 24 hrs with lidocaine at a toxic concentration alone and in combination with morphine, sufentanil, clonidine, epinephrine, neostigmine, ketamine, and midazolam. Subsequently, the rates of cell death and early apoptosis were measured by flow cytometry in neuroblastoma cells, whereas astrocyte viability was analyzed by mitochondrial activity assay. In addition, isobolograms were calculated to describe the additive effects of lidocaine with ketamine or midazolam, respectively.

RESULTS

Coadministration of lidocaine with sufentanil, clonidine, epinephrine, and neostigmine did not alter the rates of cell death compared with cells treated with lidocaine alone. Morphine improved the viability of astrocytes only at concentrations beyond those occurring clinically. In contrast, coincubation of lidocaine with ketamine or midazolam led to significantly increased rates of cell death. The combined toxicity of ketamine and lidocaine was additive, whereas the combined toxicity of midazolam and lidocaine was subadditive.

CONCLUSIONS

Sufentanil, clonidine, epinephrine, and neostigmine do not influence the neurotoxicity of lidocaine in vitro. Morphine may have some cytoprotective effect at concentrations greater than those seen intrathecally in humans. In contrast, ketamine and midazolam increase the neurotoxicity of lidocaine in vitro, presumably by additive induction of mitochondrial apoptosis.

Prevention of chronic pain after surgical nerve injury: amputation and thoracotomy

Although techniques for acute pain management have improved in recent years, a dramatic reduction in the incidence and severity of chronic pain following surgery has not occurred. Amputation and thoracotomy, although technically different, share the commonalities of unavoidable nerve injury and the frequent presence of persistent postsurgical neuropathic pain. The authors review the risk factors for the development of chronic pain following these surgeries and the current evidence that supports analgesic interventions. The inconclusive results from many preemptive analgesic studies may require us to reconceptualize the perioperative treatment period as a time of gradual neurologic remodeling.

Effects of clonidine on bilateral pain behaviors and inflammatory response in rats under the state of neuropathic pain

This study was conducted to investigate the effects of clonidine on bilateral pain behaviors and inflammatory responses in neuropathic pain induced by partial sciatic nerve ligation (PSNL), and to better understand whether the antinociception of clonidine was related to α(2)-adrenoceptor mechanisms. Rats were divided randomly into five groups: sham-operation with saline, i.p.; PSNL with clonidine (0.2mg/kg) or saline, i.p.; PSNL with yohimbine (2mg/kg) followed by clonidine (0.2mg/kg), i.p.; and PSNL with naloxone (0.3mg/kg) followed by clonidine (0.2mg/kg), i.p. On post-operative days 1, 3, 7, 14, and 21, both ipsilateral and contralateral pain behaviors were measured. In rats receiving antagonists, bilateral behavioral changes were measured on day 14. Bilateral paw pressure threshold and paw withdrawal latencies were measured, and the extent of glial activation was dertermined by measuring macrophage antigen complex-1 (Mac-1) and glial fibrillary acidic protein (GFAP). Additionally, the levels of tumor necrosis factor α (TNF-α) and interleukin (IL)-6 were determined. PSNL induced bilateral behavioral hyperalgesia, with the ipsilateral level displaying a higher extent of behavior changes than the contralateral side. In addition, the glial activation markers and cytokine production were augmented bilaterally. Clonidine caused significant attenuation of bilateral mechanical allodynia and thermal hyperalgesia, accompanied by inhibition of glial activation and the expression of cytokines. The effects of clonidine were blocked by the α(2)-adrenoceptor antagonist yohimbine and partially reversed by the μ-opioid receptor antagonist naloxone. These data suggest that the bilateral antinoceptive effects of clonidine might mediate through immunomodulation by acting on α(2)-adrenoceptor in rats undergoing neuropathic pain.

Toll-like receptor 2 contributes to chemokine gene expression and macrophage infiltration in the dorsal root ganglia after peripheral nerve injury

Background

We have previously reported that nerve injury-induced neuropathic pain is attenuated in toll-like receptor 2 (TLR2) knock-out mice. In these mice, inflammatory gene expression and spinal cord microglia actvation is compromised, whereas the effects in the dorsal root ganglia (DRG) have not been tested. In this study, we investigated the role of TLR2 in inflammatory responses in the DRG after peripheral nerve injury.

Results

L5 spinal nerve transection injury induced the expression of macrophage-attracting chemokines such as CCL2/MCP-1 and CCL3/MIP-1 and subsequent macrophage infiltration in the DRG of wild-type mice. In TLR2 knock-out mice, however, the induction of chemokine expression and macrophage infiltration following nerve injury were markedly reduced. Similarly, the induction of IL-1β and TNF-α expression in the DRG by spinal nerve injury was ameliorated in TLR2 knock-out mice. The reduced inflammatory response in the DRG was accompanied by attenuation of nerve injury-induced spontaneous pain hypersensitivity in TLR2 knock-out mice.

Conclusions

Our data show that TLR2 contributes to nerve injury-induced proinflammatory chemokine/cytokine gene expression and macrophage infiltration in the DRG, which may have relevance in the reduced pain hypersensitivity in TLR2 knock-out mice after spinal nerve injury.