Effect of local and intravenous lidocaine on ongoing activity in injured afferent nerve fibers

Identifying risk factors for chronic postsurgical pain and preventive measures: a comprehensive update

INTRODUCTION

Chronic postsurgical pain (CPSP) is a prevalent condition that can diminish health-related quality of life, cause functional deficits, and lead to patient distress. Rates of CPSP are higher for certain types of surgeries than others (thoracic, breast, or lower extremity amputations) but can occur after even uncomplicated minimally invasive procedures. CPSP has multiple mechanisms, but always starts as acute postsurgical pain, which involves inflammatory processes and may encompass direct or indirect neural injury. Risk factors for CPSP are largely known but many, such as female sex, younger age, or type of surgery, are not modifiable. The best strategy against CPSP is to quickly and effectively treat acute postoperative pain using a multimodal analgesic regimen that is safe, effective, and spares opioids.

AREAS COVERED

This is a narrative review of the literature.

EXPERT OPINION

Every surgical patient is at some risk for CPSP. Control of acute postoperative pain appears to be the most effective approach, but principles of good opioid stewardship should apply. The role of regional anesthetics as analgesics is gaining interest and may be appropriate for certain patients. Finally, patients should be better informed about their relative risk for CPSP.

Preventive Effect of Local Lidocaine Administration on the Formation of Traumatic Neuroma

BACKGROUND

Traumatic neuroma is a common sequela of peripheral nerve injury or amputation, which often leads to severe neuropathic pain. The present study investigated the effect of local lidocaine administration on preventing the formation of traumatic neuroma.

METHODS

Forty-eight male Sprague-Dawley rats were randomly assigned to two groups. The lidocaine group underwent sciatic nerve transection, followed by an injection of lidocaine (0.5%) around the proximal of a severed sciatic nerve under ultrasound-guidance 2-7 days after neurectomy. In the control group, rats received an injection of saline following neurectomy. The autotomy score, mechanical allodynia, thermal hyperalgesia, histological assessment, expression of neuroma, and pain-related markers were detected.

RESULTS

Lidocaine treatment reduced the autotomy score and attenuated mechanical allodynia and thermal hyperalgesia. The mRNA expression of α-SMA, NGF, TNF-α, and IL-1β all significantly decreased in the lidocaine group in comparison to those in the saline control group. The histological results showed nerve fibers, demyelination, and collagen hyperplasia in the proximal nerve stump in the saline control group, which were significantly inhibited in the lidocaine group.

CONCLUSIONS

The present study demonstrated that local lidocaine administration could inhibit the formation of painful neuroma due to traumatic nerve injury.

Electrophysiological characterization of ectopic spontaneous discharge in axotomized and intact fibers upon nerve transection: a role in spontaneous pain?

Many patients experience positive symptoms after traumatic nerve injury. Despite the increasing number of experimental studies in models of peripheral neuropathy and the knowledge acquired, most of these patients lack an effective treatment for their chronic pain. One possible explanation might be that most of the preclinical studies focused on the development of mechanical or thermal allodynia/hyperalgesia, neglecting that most of the patients with peripheral neuropathies complain mostly about spontaneous forms of pains. Here, we summarize the aberrant electrophysiological behavior of peripheral nerve fibers recorded in experimental models, the underlying pathophysiological mechanisms, and their relationship with the symptoms reported by patients. Upon nerve section, axotomized but also intact fibers develop ectopic spontaneous activity. Most interestingly, a proportion of axotomized fibers might present receptive fields in the skin far beyond the site of damage, indicative of a functional cross talk between neuromatose and intact fibers. All these features can be linked with some of the symptoms that neuropathic patients experience. Furthermore, we spotlight the consequence of primary afferents with different patterns of spontaneous discharge on the neural code and its relationship with chronic pain states. With this article, readers will be able to understand the pathophysiological mechanisms that might underlie some of the symptoms that experience neuropathic patients, with a special focus on spontaneous pain.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Educating Nurses on Intravenous Lidocaine for Postoperative Pain Management

As healthcare practitioners continue to search for multimodal analgesic therapies to reduce postoperative opioid requirements, surgical ward nurses need to stay up to date on the status of emerging therapies. One of these is intravenous lidocaine for postoperative pain management. Unfortunately, there is a lack of resources to assist surgical ward nurses who are unfamiliar with lidocaine infusions for pain. This article aims to review the pharmacology of intravenous lidocaine for pain management, describe an experience of a university-affiliated Veterans Affairs hospital with implementation of intravenous lidocaine on the surgical ward, and suggest practical tools that can be used to develop protocols and educational content for nurses managing intravenous lidocaine infusions in the postoperative period.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

The Analgesic and Emotional Response to Intravenous Lidocaine Infusion in the Treatment of Postherpetic Neuralgia: A Randomized, Double-Blinded, Placebo-controlled Study

OBJECTIVES

This study evaluated the analgesic efficacy and emotional response to intravenous lidocaine infusion compared with placebo in patients with postherpetic neuralgia (PHN).

METHODS

In this randomized, double-blinded study, patients with PHN received 5 mg/kg intravenous lidocaine infusion or placebo. The primary outcome was pain measured by Visual Analogue Scale, Von Frey, and area of allodynia. Moreover, emotional status of anxiety and depression were evaluated by Self-rating Anxiety Scale (SAS) and Self-rating Depression Scale (SDS). Quality of life was assessed by Short Form Health Survey 36.

RESULTS

In total, 197 patients were enrolled and eligible data were collected from 183 of those patients. The Visual Analogue Scale scores were reduced to a minimum at 2 weeks (2.74, 2.99) after infusion, but no significant difference was found between the lidocaine and placebo groups. Similar changes were also found in mechanical pain threshold and area of allodynia. However, the lidocaine group was associated with a statistically significant reduction in consumption of analgesics with a relative risk of 6.2 (95% confidence interval [CI], 2.24-17.16). Lidocaine infusion also significantly improved the anxiety and depression status; the values of mean change in anxiety and depression were 3.89 (95% CI, 1.43-6.35) and 4.3 (95% CI, 0.63-7.98), respectively, at 2 weeks. Moreover, improvement was exhibited in Short Form Health Survey 36 health status, with the mean change of 49.81 (95% CI, 28.17-71.46) at 1 week, in particular scores on vitality, physical and emotional role functioning, and mental health.

CONCLUSIONS

The analgesic response of 5 mg/kg lidocaine intravenous infusion is comparable to placebo in patients with PHN, but intravenous lidocaine infusion significantly reduced total analgesic consumption, and improved the overall emotional and health status.

Spinal glial cell line-derived neurotrophic factor infusion reverses reduction of Kv4.1-mediated A-type potassium currents of injured myelinated primary afferent neurons in a neuropathic pain model

High frequency spontaneous activity in injured primary afferents has been proposed as a pathological mechanism of neuropathic pain following nerve injury. Although spinal infusion of glial cell line-derived neurotrophic factor reduces the activity of injured myelinated A-fiber neurons after fifth lumbar (L5) spinal nerve ligation in rats, the implicated molecular mechanism remains undetermined. The fast-inactivating transient A-type potassium current (I_A) is an important determinant of neuronal excitability, and five voltage-gated potassium channel (Kv) alpha-subunits, Kv1.4, Kv3.4, Kv4.1, Kv4.2, and Kv4.3, display I_A in heterologous expression systems. Here, we examined the effect of spinal glial cell line-derived neurotrophic factor infusion on I_A and the expression of these five Kv mRNAs in injured A-fiber neurons using the in vitro patch clamp technique and in situ hybridization histochemistry. Glial cell line-derived neurotrophic factor infusion reversed axotomy-induced reduction of the rheobase, elongation of first spike duration, and depolarization of the resting membrane potential. L5 spinal nerve ligation significantly reduced the current density of I_A and glial cell line-derived neurotrophic factor treatment reversed the reduction. Among the examined Kv mRNAs, only the change in Kv4.1-expression was parallel with the change in I_A after spinal nerve ligation and glial cell line-derived neurotrophic factor treatment. These findings suggest that glial cell line-derived neurotrophic factor should reduce the hyperexcitability of injured A-fiber primary afferents by I_A recurrence. Among the five I_A-related Kv channels, Kv4.1 should be a key channel, which account for this I_A recurrence.

Rethinking the causes of pain in herpes zoster and postherpetic neuralgia: the ectopic pacemaker hypothesis

INTRODUCTION

Pain in herpes zoster (HZ) and postherpetic neuralgia (PHN) is traditionally explained in terms of 2 processes: irritable nociceptors in the rash-inflamed skin and, later, deafferentation due to destruction of sensory neurons in one virally infected dorsal root ganglion.

OBJECTIVES AND METHODS

Consideration of the evidence supporting this explanation in light of contemporary understanding of the pain system finds it wanting. An alternative hypothesis is proposed as a replacement.

RESULTS

This model, the ectopic pacemaker hypothesis of HZ and PHN, proposes that pain in both conditions is driven by hyperexcitable ectopic pacemaker sites at various locations in primary sensory neurons affected by the causative varicella zoster virus infection. This peripheral input is exacerbated by central sensitization induced and maintained by the ectopic activity.

CONCLUSIONS

The shift in perspective regarding the pain mechanism in HZ/PHN has specific implications for clinical management.

Pain Management in Horses

There has been great progress in the understanding of basic neurobiologic mechanisms of pain, but this body of knowledge has not yet translated into new and improved analgesics. Progress has been made regarding pain assessment in horses, but more work is needed until sensitive and accurate pain assessment tools are available for use in clinical practice. This review summarizes and updates the knowledge concerning the cornerstones of pain medicine (understand, assess, prevent, and treat). It highlights the importance of understanding pain mechanisms and expressions to enable a rational approach to pain assessment, prevention, and management in the equine patient.

Perioperative Use of Intravenous Lidocaine

Perioperative lidocaine infusion improves analgesia and recovery after some surgical procedures, possibly through systemic antiinflammatory effects. This commentary provides the clinician with evidence for rational use of perioperative lidocaine infusion in procedures where it is of demonstrated benefit.

Antinociceptive effects of topical mepivacaine in a rat model of HIV-associated peripheral neuropathic pain

Background

A consequence of HIV infection is sensory neuropathy, a debilitating condition that degrades the quality of life of HIV patients. Furthermore, life-extending antiretroviral treatment may exacerbate HIV sensory neuropathy. Analgesics that relieve other neuropathic pains show little or no efficacy in ameliorating HIV sensory neuropathy. Thus, there is a need for analgesics for people with this particular pain. While lidocaine is used in the management of painful peripheral neuropathies, another local anesthetic mepivacaine, with a potentially improved bioavailability, could be utilized for the management of HIV neuropathic pain.

Methods

The efficacy of topical anesthetics was evaluated in a preclinical rodent model of painful peripheral neuropathy induced by epineural administration of the HIV envelope protein gp120 delivered using saturated oxidized cellulose implanted around the sciatic nerve. Beginning at 2 weeks following gp120 administration, the effects of local anesthetics topically applied via gauze pads were tested on heat and mechanical hyperalgesia in the hind paw. Rats were tested using several concentrations of mepivacaine or lidocaine during the following 2 weeks.

Results

By 2 weeks following epineural gp120 implantation, the ipsilateral hind paw developed significant hypersensitivity to noxious pressure and heat hyperalgesia. A short-lasting, concentration-dependent amelioration of pressure and heat hyperalgesia was observed following topical application of mepivacaine to the ipsilateral plantar hind paw. By contrast, topical lidocaine ameliorated heat hyperalgesia in a concentration-dependent manner but not pressure hyperalgesia. Equipotent concentrations of mepivacaine and lidocaine applied topically to the tail of mice significantly increased tail withdrawal latencies in the tail flick test, demonstrating that both local anesthetics attenuate responding to a brief noxious stimulus.

Conclusion

These findings showed that mepivacaine, rather than lidocaine, consistently attenuated two distinct symptoms of neuropathic pain and suggest that topical formulations of this local anesthetic could have utility in the alleviation of clinical HIV neuropathic pain.

Effects on hemodynamic variables and echocardiographic parameters after a stellate ganglion block in 15 healthy volunteers

BACKGROUND

The sympathetic nervous system has an important role in generating pain. Various pathomechanisms are involved that respond well to the application of local anesthetics (LA), for example to the stellate ganglion block (SGB).

OBJECTIVES

We wanted to know more about the effects of SGB on cardiovascular parameters.

METHODS

We included 15 healthy volunteers; another 15 healthy volunteers as a control group (sham injection of LA). In order to produce a more precise SGB, we employed only a small volume of LA (3mL), a LA with a lower permeability (procaine 1%), and a modified injection technique. Systolic and diastolic blood pressure (SBP, DBP), heart rate (HR), and echocardiographic parameters were recorded before and after SGB. We also investigated whether there are side differences (left and right SBG).

RESULTS

At baseline all parameters were within the normal range. After performing right and left SGB DBP significantly increased (on the right side from 68.73±8.61 to 73.53±11.10, p=0.015; on the left side from 70.66±13.01 to 77.93±10.40, p=0.003). In the control group no increase in DBP was observed. No side-specific differences were found, except a significant reduction in the maximum velocity of myocardial contraction during the systole with left-sided SGB.

CONCLUSIONS

Even with our methods we could not prevent the simultaneous occurrence of a partial parasympatholytic effect. For this reason, the SGB has only minor hemodynamic effects, which is desirable as it enhances the safety of the SGB.

Spontaneous activity in C-fibres after partial damage to the saphenous nerve in mice: Effects of retigabine

BACKGROUND

Spontaneous pain is the most devastating positive symptom in neuropathic pain patients. Recent data show a direct relationship between spontaneous discharges in C-fibres and spontaneous pain in neuropathic patients. Unfortunately, to date there is a lack of experimental animal models for drug testing.

METHODS

We recorded afferent fibres from a new experimental model in vitro. The preparation contains a neuroma formed in a peripheral branch of the saphenous nerve together with the undamaged branches, which maintain intact terminals in a skin flap.

RESULTS

Fibres with stable rates of ectopic spontaneous discharges were found among axotomized (5 A- and 18 C-fibres, mean discharge 0.48 ± 0.08 Hz) and 'putative intact' fibres (12 C-fibres, mean discharge 0.28 ± 0.08 Hz). A proportion (~9%) of axotomized fibres had mechanical receptive fields in the skin far beyond the site of injury. Collision experiments demonstrated that action potentials evoked from neuroma and skin travelled by the same fibre, indicating functional cross-talk between neuromatose and putative intact fibres. Retigabine, the specific Kv7 channel opener, depressed spontaneous discharges by 70% in 15/18 units tested. In contrast, responses to mechanical stimulation of the skin were unaltered by retigabine.

CONCLUSIONS

Partial damage to a peripheral nerve may increase the incidence of spontaneous activity in C-fibres. Retigabine reduced spontaneous activity but not stimulus-evoked activity, suggesting an important role for ion channels in the control of spontaneous pain and demonstrating the utility of the model for the testing of compounds in clinically relevant variables. WHAT DOES THIS STUDY ADD?: Our in vitro experimental model of peripheral neuropathy allows for pharmacological characterization of spontaneously active fibres. Using this model, we show that retigabine inhibits aberrant spontaneous discharges without altering physiological responses in primary afferents.

Sciatica: detection and confirmation by new method

We need to overcome limitations of present assessment and also integrate newer research in our work about sciatica. Inflammation induces changes in the DRG and nerve root. It sensitizes the axons. Nociceptor is a unique axon. It is pseudo unipolar: both its ends, central and peripheral, behave in similar fashion. The nerve in periphery which carries these axons may selectively become sensitive to mechanical pressure--"mechanosensitized," as we coin the phrase. Many pain questionnaires are used and are effective in identifying neuropathic pain solely on basis of descriptors but they do not directly physically correlate nerve root and pain. A thorough neurological evaluation is always needed. Physical examination is not direct pain assessment but testing mobility of nerve root and its effect on pain generation. There is a dogmatic dominance of dermatomes in assessment of leg pain. They are unreliable. Images may not correlate with symptoms and pathology in about 28% of cases. Electrophysiology may be normal in purely inflamed nerve root. Palpation may help in such inflammatory setting to refine our assessment further. Confirmation of sciatica is done by selective nerve root block (SNRB) today but it is fraught with several complications and needs elaborate inpatient and operating room set up. We have used the unique property of the pseudo unipolar axon that both its ends have similar functional properties and so inject along its peripheral end sodium channel blockers to block the basic cause of the mechanosensitization namely upregulated sodium channels in the root or DRG. Thus using palpation we may be able to detect symptomatic nerve in stage of inflammation and with distal end injection, along same inflamed nerve we may be able to abolish and so confirm sciatica. Discussions of sciatica pain diagnosis tend to immediately shift and centre on the affected disc rather than the nerve. Theoretically it may be possible to detect the affected nerve by palpating the nerve and relieve pain moment we desensitize the nerve.

De novo expression of Nav1.7 in injured putative proprioceptive afferents: Multiple tetrodotoxin-sensitive sodium channels are retained in the rat dorsal root after spinal nerve ligation

Tetrodotoxin-sensitive (TTX-s) spontaneous activity is recorded from the dorsal roots after peripheral nerve injury. Primary sensory neurons in the dorsal root ganglion (DRG) express multiple TTX-s voltage-gated sodium channel α-subunits (Navs). Since Nav1.3 increases, whereas all other Navs decrease, in the DRG neurons after peripheral nerve lesion, Nav1.3 is proposed to be critical for the generation of these spontaneous discharges and the contributions of other Navs have been ignored. Here, we re-evaluate the changes in expression of three other TTX-s Navs, Nav1.1, Nav1.6 and Nav1.7, in the injured 5th lumbar (L5) primary afferent components following L5 spinal nerve ligation (SNL) using in situ hybridization histochemistry and immunohistochemistry. While the overall signal intensities for these Nav mRNAs decreased, many injured DRG neurons still expressed these transcripts at clearly detectable levels. All these Nav proteins accumulated at the proximal stump of the ligated L5 spinal nerve. The immunostaining patterns of Nav1.6 and Nav1.7 associated with the nodes of Ranvier were maintained in the ipsilateral L5 dorsal root. Interestingly, putative proprioceptive neurons characterized by α3 Na+/K+ ATPase-immunostaining specifically lacked Nav1.7 mRNA in naïve DRG but displayed de novo expression of this transcript following SNL. Nav1.7-immunoreactive fibers were significantly increased in the ipsilateral gracile nucleus where central axonal branches of the injured A-fiber afferents terminated. These data indicate that multiple TTX-s channel subunits could contribute to the generation and propagation of the spontaneous discharges in the injured primary afferents. Specifically, Nav1.7 may cause some functional changes in sensory processing in the gracile nucleus after peripheral nerve injury.

Intraoperative systemic infusion of lidocaine reduces postoperative pain after lumbar surgery: a double-blinded, randomized, placebo-controlled clinical trial

BACKGROUND CONTEXT

Analgesic effect of lidocaine infusion on postoperative pain.

PURPOSE

The aim of this study was to evaluate the analgesic effect of lidocaine infusion on postoperative pain after lumbar microdiscectomy.

STUDY DESIGN

This study used a prospective, randomized, double-blinded, and placebo-controlled clinical trial.

PATIENT SAMPLE

Fifty-one patients participated in this randomized, double-blinded study.

OUTCOME MEASURES

The primary outcome was the visual analog scale (VAS) (0-100 mm) pain score at 4 hours after surgery. The secondary outcomes were the VAS pain score at 2, 8, 12, 24, and 48 hours after surgery, the frequency with which patients pushed the button (FPB) of the patient-controlled analgesia system, and the fentanyl consumption at 2, 4, 8, 12, 24, and 48 hours after surgery. Other outcomes were satisfaction scores regarding pain control and the overall recovery process, incidence of postoperative nausea and vomiting (PONV), and length of hospital stay (HS).

METHODS

Preoperatively and throughout the surgery, Group L received intravenous lidocaine infusion (a 1.5-mg/kg bolus followed by a 2-mg/kg/h infusion until the end of the surgical procedure) and Group C received normal saline infusion as a placebo.

RESULTS

The VAS scores and fentanyl consumption were significantly lower in Group L compared with Group C except at 48 h after surgery (p<.05). Total fentanyl consumption, total FPB, length of HS, and satisfaction scores were also significantly lower in Group L compared with Group C (p<.05).

CONCLUSIONS

Intraoperative systemic infusion of lidocaine decreases pain perception during microdiscectomy, thus reducing the consumption of opioid and the severity of postoperative pain. This effect contributes to reduce the length of HS.

Effects of various antiepileptics used to alleviate neuropathic pain on compound action potential in frog sciatic nerves: comparison with those of local anesthetics

Antiepileptics used for treating neuropathic pain have various actions including voltage-gated Na(+) and Ca(2+) channels, glutamate-receptor inhibition, and GABA(A)-receptor activation, while local anesthetics are also used to alleviate the pain. It has not been fully examined yet how nerve conduction inhibitions by local anesthetics differ in extent from those by antiepileptics. Fast-conducting compound action potentials (CAPs) were recorded from frog sciatic nerve fibers by using the air-gap method. Antiepileptics (lamotrigine and carbamazepine) concentration dependently reduced the peak amplitude of the CAP (IC50 = 0.44 and 0.50 mM, resp.). Carbamazepine analog oxcarbazepine exhibited an inhibition smaller than that of carbamazepine. Antiepileptic phenytoin (0.1 mM) reduced CAP amplitude by 15%. On the other hand, other antiepileptics (gabapentin, sodium valproate, and topiramate) at 10 mM had no effect on CAPs. The CAPs were inhibited by local anesthetic levobupivacaine (IC50 = 0.23 mM). These results indicate that there is a difference in the extent of nerve conduction inhibition among antiepileptics and that some antiepileptics inhibit nerve conduction with an efficacy similar to that of levobupivacaine or to those of other local anesthetics (lidocaine, ropivacaine, and cocaine) as reported previously. This may serve to know a contribution of nerve conduction inhibition in the antinociception by antiepileptics.

Inhibition by local bupivacaine-releasing microspheres of acute postoperative pain from hairy skin incision

BACKGROUND

Acute postoperative pain causes physiological deficits and slows recovery. Reduction of such pain by local anesthetics that are delivered for several days postoperatively is a desirable clinical objective, which is approached by a new formulation and applied in animal studies reported here.

METHODS

We subcutaneously injected a new formulation of poly-lactic-co-glycolic acid polymer microspheres, which provides steady drug release for 96+ hours into rats at the dorsal region 2 hours before surgery. A single 1.2-cm-long skin incision was followed by blunt dissection of skin away from the underlying fascia, and closed by 2 sutures, followed by 14 days of testing. Microspheres containing 5, 10, 20, and 40 mg bupivacaine were injected locally 2 hours before surgery; bupivacaine-free microspheres were the vehicle control, and bupivacaine HCl solution (0.5%), the positive control. Mechanical sensitivity was determined by the frequency of local muscle contractions to repeated pokes with nylon monofilaments (von Frey hairs) exerting 4 and 15 g forces, testing, respectively, allodynia and hyperalgesia, and by pinprick.

RESULTS

Injection of bupivacaine microspheres (40 mg drug) into intact skin reduced responses to 15 g von Frey hairs for 6 hours and to pinprick for 36 hours. Respective reductions from bupivacaine HCl lasted for 3 and 2 hours. Skin incision and dissection alone caused mechanical allodynia and hyperalgesia for 14 days. Microspheres containing 20 or 40 mg bupivacaine suppressed postoperative hypersensitivity for up to 3 days, reduced integrated allodynia (area under curve of response versus time) over postoperative days 1 to 5 by 51% ± 20% (mean ± SE) and 78% ± 12%, and reduced integrated hyperalgesia by 55% ± 13% and 64% ± 11%, for the respective doses. Five and ten milligrams bupivacaine in microspheres and the 0.5% bupivacaine solution were ineffective in reducing postoperative hypersensitivity, as were 40 mg bupivacaine microspheres injected contralateral to the incision.

CONCLUSIONS

Significant suppression of postoperative pain by the slow-release bupivacaine preparation outlasts its anesthetic action on intact skin. These findings demonstrate preventive analgesia and indicate the importance of acute processes in the development of chronic postoperative pain.

Differential effects of a 5% lidocaine medicated patch in peripheral nerve injury

INTRODUCTION

We examined the effect of topical lidocaine on the function of small and large fibers in patients with peripheral neuropathic pain due to traumatic or postoperative nerve injury.

METHODS

In an open-label study, 24 patients were treated with a 5% lidocaine patch for up to 12 weeks. We recorded contact heat evoked potentials (CHEPs) and performed quantitative sensory testing (QST) before and after treatment with the contralateral side as control.

RESULTS

Twenty-one patients (mean age 47.6 ± 13.5 years) completed the study. Lidocaine increased cold pain threshold (P = 0.04) and reduced CHEP amplitude (P = 0.007) with no effect on other QST parameters. Patients responding to treatment had less cold detection deficit on the affected side and had a larger increase in cold pain detection threshold following treatment than nonresponders.

CONCLUSIONS

Controlled trials are warranted to further understand the mechanisms mediating the effects of topical lidocaine.

Transient tactile allodynia following intrathecal puncture in mouse: contributions of Toll-like receptor signaling

Studies of spinal drug action in mice often involve percutaneous intrathecal drug administration delivered in a lightly anesthetized animal. A successful lumbar intrathecal (IT) needle stick of a lightly anesthetized (isoflurane) mouse evokes a tail flick, which is an indication of local spinal nerve stimulation. Immediately upon arousal, a hind paw tactile allodynia, as measured with von Frey hairs (pre 1.55±0.11 g vs. injected 0.66±0.08 g) lasts 3-4 h. In a similarly anesthetized mouse without the needle stick, a 1-h allodynia was noted. In studies on spinal Toll-like receptor (TLR) signaling, we observed that following intrathecal puncture and mechanical stimulation of the nerve roots mice deficient in TLR down-stream signaling (Myd88(-/-)/Trif(lps2)), displayed only the transient (1-h) allodynia otherwise observed following isoflurane alone. These data suggest that the extended period of hyperalgesia observed with needle penetration of the dura and mechanical stimulation of the nerve roots requires signaling through the MyD88/TRIF pathways and supports the intrinsic role of Toll-like receptors in the allodynia secondary to the minor nerve activation occurring during the intradural puncture.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.