Local anesthetics potently block a potential insensitive potassium channel in myelinated nerve

Perioperative intravenous lignocaine infusion for postoperative pain control in patients undergoing surgery of the spine: protocol for a systematic review and meta-analysis

INTRODUCTION

Intravenous lignocaine is an amide local anaesthetic known for its analgesic, antihyperalgesic and anti-inflammatory properties. Administration of intravenous lignocaine has been shown to enhance perioperative recovery parameters. This is the protocol for a systematic review which intends to summarise the evidence base for perioperative intravenous lignocaine administration in patients undergoing spinal surgery.

METHODS AND ANALYSIS

Our primary outcomes include: postoperative pain scores at rest and movement at predefined early, intermediate and late time points and adverse events. Other outcomes of interest include perioperative opioid consumption, composite morbidity, surgical complications and hospital length of stay. We will include randomised controlled trials, which compared intravenous lignocaine infusion vs standard treatment for perioperative analgesia. We will search electronic databases from inception to present; MEDLINE, EMBASE and Cochrane Library (Cochrane Database of Systematic Reviews and CENTRAL). Two team members will independently screen all citations, full-text articles and abstract data. The individual study risk of bias will be appraised using the Cochrane risk of bias tool. We will obtain a risk ratio or mean difference (MD) from the intervention and control group event rates based on the nature of data. We will correct for the variable measurement tools by using the standardised MD (SMD). We will use a random-effects model to synthesise data. We will conduct five subgroup analysis: major versus minor surgery, emergency versus elective surgery, patients with chronic pain conditions versus patients without, duration of lignocaine infusion and adult versus paediatric. Confidence in cumulative evidence for will be classified according to the Grading of Recommendations, Assessment, Development and Evaluation system. We will construct summary of findings tables supported detailed evidence profile tables for predefined outcomes.

ETHICS AND DISSEMINATION

Formal ethical approval is not required as primary data will not be collected. The results will be disseminated through a peer-reviewed publication.

PROSPERO REGISTRATION NUMBER

CRD420201963314.

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.

Intra- and inter-Individual variability in nerve block duration: A randomized cross-over trial in the common peroneal nerve of healthy volunteers

BACKGROUND

The reported variation in nerve block duration is considerable. To individualize nerve block therapy, knowledge of the intra- vs inter-individual variability is essential. We investigated the relative contribution of these 2 parameters to the overall nerve block duration variability.

METHODS

With ethics committee approval, we conducted a randomized cross-over trial where 20 healthy volunteers received 8 common peroneal nerve blockades with lidocaine 0.5% on 4 consecutive days. Allocations were 5 mL to either the right or left side and 10 mL to the opposite side on day 1 and 2 and vice versa on day 3 and 4. With fixed needle entry and nerve target, we repeated local anaesthetic deposition for each blockade. The primary outcome was variation in duration of sensory nerve block defined as insensitivity to a cold stimulus. Data were analysed using linear mixed model regression.

RESULTS

The mean sensory block duration of 380 (95% CI = [342; 418]) minutes on day one was 55 [33; 77] minutes longer than on day two (P < .001), but there were no differences in mean duration between days 2, 3 and 4. The ratios with 2.5; 97.5 percentiles between inter- and intra-individual variation were 2.4 [0.8; 5.2] for the 5 mL blockades and 3.0 [0.9; 6.7] for the 10 mL blockades. The probabilities of inter- to intra-individual variation-ratios >1 were 96% and 97%.

CONCLUSION

The intra-individual variability is a substantially minor contributor to the overall variability in sensory nerve block duration compared with the inter-individual variability.

The in vitro mechanisms and in vivo efficacy of intravenous lidocaine on the neuroinflammatory response in acute and chronic pain

INTRODUCTION

The neuroinflammatory response plays a key role in several pain syndromes. Intravenous (iv) lidocaine is beneficial in acute and chronic pain. This review delineates the current literature concerning in vitro mechanisms and in vivo efficacy of iv lidocaine on the neuroinflammatory response in acute and chronic pain.

DATABASES AND DATA TREATMENT

We searched PUBMED and the Cochrane Library for in vitro and in vivo studies from July 1975 to August 2014. In vitro articles providing an explanation for the mechanisms of action of lidocaine on the neuroinflammatory response in pain were included. Animal or clinical studies were included concerning iv lidocaine for acute or chronic pain or during inflammation.

RESULTS

Eighty-eight articles regarding iv lidocaine were included: 36 in vitro studies evaluating the effect on ion channels and receptors; 31 animal studies concerning acute and chronic pain and inflammatory models; 21 clinical studies concerning acute and chronic pain. Low-dose lidocaine inhibits in vitro voltage-gated sodium channels, the glycinergic system, some potassium channels and Gαq-coupled protein receptors. Higher lidocaine concentrations block potassium and calcium channels, and NMDA receptors. Animal studies demonstrate lidocaine to have analgesic effects in acute and neuropathic pain syndromes and anti-inflammatory effects early in the inflammatory response. Clinical studies demonstrate lidocaine to have advantage in abdominal surgery and in some neuropathic pain syndromes.

CONCLUSIONS

Intravenous lidocaine has analgesic, anti-inflammatory and antihyperalgesic properties mediated by an inhibitory effect on ion channels and receptors. It attenuates the neuroinflammatory response in perioperative pain and chronic neuropathic pain.

Two-pore domain potassium channels enable action potential generation in the absence of voltage-gated potassium channels

In this study, we explored the possibility that two-pore domain potassium (K2P) channels are sufficient to support action potential (AP) generation in the absence of conventional voltage-gated potassium (KV) channels. Hodgkin-Huxley parameters were used to mimic the presence of voltage-gated sodium (NaV) channels in HEK-293 cells. Recombinant expression of either TREK-1 or TASK-3 channels was then used to generate a hyperpolarised resting membrane potential (RMP) leading to the characteristic non-linear current-voltage relationship expected of a K2P-mediated conductance. During conductance simulation experiments, both TASK-3 and TREK-1 channels were able to repolarise the membrane once AP threshold was reached, and at physiologically relevant current densities, this K2P-mediated conductance supported sustained AP firing. Moreover, the magnitude of the conductance correlated with the speed of the AP rise in a manner predicted from our computational studies. We discuss the physiological impact of axonal K2P channels and speculate on the possible clinical relevance of K2P channel modulation when considering the actions of general and local anaesthetics.

Bupivacaine and levobupivacaine induce apoptosis in rat chondrocyte cell cultures at ultra-low doses

PURPOSE

Osteoarthritis (OA) is characterized by chondrocyte apoptosis and necrosis which play a key role during the progression of OA. Intra-articular administration of bupivacaine is a practical and effective way of postoperative pain control following various joint surgeries. 0.25 % bupivacaine showed to be safe in terms of chondrocyte toxicity. Around 200 nM of bupivacaine was shown to be effective for peripheral nerve block. This study aims to observe the possible cytotoxic effects of bupivacaine and its enantiomer levobupivacaine on chondrocyte cell culture at 7.69, 76.9, and 384.5 μM or at 0.0125, 0.0025, and 0.00025 % concentrations, respectively.

METHODS

Chondrocytes were isolated from rat articular cartilage after incubating with collagenase in RPMI-1640 medium. Cells were treated with bupivacaine and levobupivacaine at 7.69, 76.9, and 384.5 μM concentrations for 6, 24, and 48 h. Treated chondrocytes were stained with acridine orange and ethidium bromide and examined under a fluorescence microscope at a 490 nm excitation wavelength for apoptotic changes.

RESULTS

Study results suggest that both bupivacaine and levobupivacaine have dose-dependent chondrocyte toxicity, and this is significantly lesser at 7.69 μM dose. There was no significant difference in terms of chondrocyte apoptosis, (p > 0.05).

CONCLUSIONS

Clinicians should be skeptic for the serious long-term side effects of bupivacaine and its analogs, even at ultra-low doses.

Synaptic activation of bulbospongiosus motoneurons via dorsal gray commissural inputs

Ejaculation is controlled by coordinated and rhythmic contractions of bulbospongiosus (BSM) and ischiocavernosus muscles. Motoneurons that innervate and control BSM contractions are located in the dorsomedial portion of the ventral horn in the L(5-6) spinal cord termed the dorsomedial (DM) nucleus. We characterized intrinsic properties of DM motoneurons as well as synaptic inputs from the dorsal gray commissure (DGC). Electrical stimulation of DGC fibers elicited fast inhibitory and excitatory responses. In the presence of glutamate receptor antagonists, both fast GABAergic as well as glycinergic inhibitory postsynaptic potentials (IPSPs) were recorded. No slow GABA(B)-mediated inhibition was evident. In the presence of GABA(A) and glycine receptor antagonists, DGC stimulation elicited fast glutamatergic excitatory responses that were blocked by application of CNQX. Importantly, a slow depolarization (timescale of seconds) was routinely observed that sufficiently depolarized the DM motoneurons to fire "bursts" of action potentials. This slow depolarization was elicited by a range of stimulus train frequencies and was insensitive to glutamate receptor antagonists (CNQX and d-APV). The slow depolarization was accompanied by an increase in membrane resistance with an extrapolated reversal potential near the K(+) Nernst potential. It was mediated by the combination of the block of a depolarization-activated K(+) current and the activation of a QX-314-sensitive cation current. These results demonstrate that fast synaptic responses in DM motoneurons are mediated primarily by glutamate, GABA, and glycine receptors. In addition, slow nonglutamatergic excitatory postsynaptic potentials (EPSPs), generated through DGC stimulation, can elicit burstlike responses in these neurons.

Ultrasound-guided fine-needle aspiration biopsy of thyroid nodules: is it necessary to use local anesthesia for the application of one needle puncture?

Objective

This study was designed to evaluate the difference in the degree of patient pain for an ultrasound-guided fine-needle aspiration biopsy (USFNAB) of a thyroid nodule with one needle puncture with and without local anesthesia.

Materials and Methods

A total of 50 patients participated in the study. We examined prospective patients who would undergo US-FNABs of two thyroid nodules (larger than 10 mm maximum diameter), which were located in separate thyroid lobes. For one of these thyroid nodules, US-FNAB was performed following the administration of local anesthesia; for the other nodule, no anesthesia was administered. The application of anesthesia was alternatively administered between patients (either prior to the first US-FNAB procedure or prior to the second procedure). For all patients, the degree of pain during and after each US-guided FNAB was evaluated according to a 4-category verbal rating scale (VRS), an 11-point numeric rating scale (NRS) and a 100-mm visual analogue scale (VAS).

Results

The mean maximum diameters of thyroid nodules examined by US-FNAB with the use of local anesthesia and with no local anesthesia were 13.6 mm and 13.0 mm, respectively. There was no significant difference in nodule size (p > 0.05) between two groups. For the VRS, there were 27 patients with a higher pain score when local anesthesia was used and four patients with a higher pain score when no local anesthesia was administered. Nineteen patients had equivalent pain score for both treatments. This finding was statistically significant (p < 0.001). For the NRS, there were 33 patients with a higher pain score when local anesthesia was used and 10 patients with a higher pain score when no local anesthesia was administered. Seven patients had an equivalent pain score for each treatment. This finding was statistically significant (p < 0.001). For the VAS, there were 35 patients with a higher pain score when local anesthesia was used and 11 patients with a higher pain score where no local anesthesia was administered. Four patients had an equivalent pain score for both treatments. This finding was also statistically significant (p = 0.001).

Conclusion

In our study, patient pain scales were significantly lower when no local anesthesia was used prior to US-FNABs of thyroid nodules as compared to when local anesthesia was administered. Therefore, we believe that when one needle puncture is used, US-FNAB should be performed without administering local anesthesia.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Membrane interactivity of charged local anesthetic derivative and stereoselectivity in membrane interaction of local anesthetic enantiomers

With respect to the membrane lipid theory as a molecular mechanism for local anesthetics, two critical subjects, the negligible effects of charged drugs when applied extracellularly and the stereoselective effects of enantiomers, were verified by paying particular attention to membrane components, phospholipids with the anionic property, and cholesterol with several chiral carbons. The membrane interactivities of structurally-different anesthetics were determined by their induced fluidity changes of liposomal membranes. Lidocaine (3.0 μmol/mL) fluidized phosphatidylcholine membranes, but not its quaternary derivative QX-314 (3.0 μmol/mL). Similarly to the mother molecule lidocaine, however, QX-314 fluidized phosphatidylserine-containing nerve cell model membranes and acidic phospholipids-constituting membranes depending on the acidity of membrane lipids. Positively charged local anesthetics are able to act on lipid bilayers by ion-pairing with anionic (acidic) phospholipids. Bupivacaine (0.75 mol/mL) and ropivacaine (0.75 and 1.0 μmol/mL) fluidized nerve cell model membranes with the potency being S(−)-enantiomer < racemate < R(+)-enantiomer (P < 0.01, vs antipode and racemate) and cardiac cell model membranes with the potency being S(−)-ropivacaine < S(−)-bupivacaine < R(+)-bupivacaine (P < 0.01). However, their membrane effects were not different when removing cholesterol from the model membranes. Stereoselectivity is producible by cholesterol which increases the chirality of lipid bilayers and enables to discriminate anesthetic enantiomers. The membrane lipid interaction should be reevaluated as the mode of action of local anesthetics.

Inhibitory effects of opioids on compound action potentials in frog sciatic nerves and their chemical structures

An opioid tramadol more effectively inhibits compound action potentials (CAPs) than its metabolite mono-O-demethyl-tramadol (M1). To address further this issue, we examined the effects of opioids (morphine, codeine, ethylmorphine and dihydrocodeine) and cocaine on CAPs by applying the air-gap method to the frog sciatic nerve. All of the opioids at concentrations less than 10 mM reduced the peak amplitude of the CAP in a reversible and dose-dependent manner. The sequence of the CAP peak amplitude reductions was ethylmorphine>codeine>dihydrocodeine> or = morphine; the effective concentration for half-maximal inhibition (IC(50)) of ethylmorphine was 4.6 mM. All of the CAP inhibitions by opioids were resistant to a non-specific opioid-receptor antagonist naloxone. The CAP peak amplitude reductions produced by morphine, codeine and ethylmorphine were related to their chemical structures in such that this extent enhanced with an increase in the number of -CH(2) in a benzene ring, as seen in the inhibitory actions of tramadol and M1. Cocaine reduced CAP peak amplitudes with an IC(50) value of 0.80 mM. It is concluded that opioids reduce CAP peak amplitudes in a manner being independent of opioid-receptor activation and with an efficacy being much less than that of cocaine. It is suggested that the substituted groups of -OH bound to the benzene ring of morphine, codeine and ethylmorphine as well as of tramadol and M1, the structures of which are quite different from those of the opioids, may play an important role in producing nerve conduction block.

Two-pore-domain potassium channels contribute to neuronal potassium release and glial potassium buffering in the rat hippocampus

Two-pore-domain potassium (K2P) channels have been suggested to be involved in neuronal K+ release and glial K+ uptake. We studied effects of the K2P channel blockers quinine (200 or 500 microM), quinidine (500 microM), and bupivacaine (200 microM) on stimulus-induced and iontophoretically induced transient increases of the extracellular potassium concentration ([K+]o) in area CA1 of rat hippocampal slices, always in presence of AMPA/kainate and NMDA receptor antagonists. Increases in [K+]o evoked by repetitive alvear stimulation (20 Hz) were blocked by quinine and quinidine but amplitudes of population spikes were only modestly reduced. Bupivacaine suppressed both rises in [K+]o and population spikes. In contrast, iontophoretically induced rises in [K+]o were moderately augmented by quinine and quinidine while bupivacaine had no effect. Barium at concentrations of 2 mM which should block both potassium inward rectifier (Kir) and some K2P channels doubled iontophoretically induced rises in [K+]o also in presence of quinine, quinidine, and bupivacaine. The data suggest that quinine/quinidine-sensitive K2P channels mediate K+ release from neurons and possibly contribute to glial K+ buffering.

Development and verification of saphenous, tibial and common peroneal nerve block techniques for analgesia below the thigh in the nonchondrodystrophoid dog

OBJECTIVE

To document simple and reliable local, infiltrating nerve blocks for the saphenous, tibial and common peroneal nerves in the dog.

STUDY DESIGN

Laboratory technique development; in vivo blind, controlled, prospective study.

ANIMALS

Twenty canine cadavers and 18 clinically normal, client-owned dogs.

METHODS

A peripheral nerve blockade technique of the tibial, common peroneal, and saphenous nerves was perfected through anatomic dissection. Injections were planned in the caudal thigh for the tibial and common peroneal nerves, and in the medial thigh for the saphenous nerve. Cadaver limbs were injected with methylene blue dye and subsequently dissected to confirm successful dye placement. Clinically normal dogs undergoing general anesthesia for unrelated, elective procedures were randomly assigned to treatment (bupivacaine; n = 8) or control (saline; n = 8) nerve blocks of the nerves under study. Upon recovery from general anesthesia, skin sensation in selected dermatomes was evaluated for 24 hours.

RESULTS

Cadaver tibial, common peroneal, and saphenous perineural infiltrations were successful in nonchondrodystrophoid dogs (100, 100, and 97%, respectively.) Intraneural injection was rare (1%; 1/105; tibial nerve) in cadaver dogs. In the treatment group of normal dogs, duration of loss of cutaneous sensation in some dermatomes (saphenous, superficial and deep peroneal nerve) was significantly different than control dogs; the range of desensitization occurred for 1-20 hours. No clinical morbidity was detected.

CONCLUSIONS

This technique for local blockade of the tibial, common peroneal, and saphenous nerves just proximal to the stifle is easy to perform, requires minimal supplies and results in significant desensitization of the associated dermatomes in clinically normal, nonchondrodystrophoid dogs.

CLINICAL RELEVANCE

This technique may be an effective tool for post-operative analgesia to the femoro-tibial joint and distal pelvic limb. Other applications, using sustained-release drugs or methods, may include anesthesia/analgesia in high-risk patients or as a treatment for chronic pelvic limb pain or self-mutilation.

Local anaesthetics block hyperpolarization-activated inward current in rat small dorsal root ganglion neurones

(1) Hyperpolarizing voltage steps evoke slowly activating inward currents in a variety of neurones and in cardiac cells. This hyperpolarization-activated inward current (I(h)) is thought to play a significant role in cell excitability, firing frequency, or in setting of the resting membrane potential in these cells. We studied the effects of lidocaine, mepivacaine, QX-314 and bupivacaine as well as its enantiomers on I(h) in the membrane of dorsal root ganglion neurones (DRG). (2) The patch-clamp technique was applied to small dorsal root ganglion neurones identified in 200 micro M thin slices of young rat DRGs. Under voltage-clamp conditions, the whole-cell I(h) current was recorded in the presence of different concentrations of the local anaesthetics. In current-clamp mode the resting membrane potential and the voltage response of DRG neurones to injected current pulses were investigated. (3) I(h) was reversibly blocked by bupivacaine, lidocaine and mepivacaine applied externally in clinically relevant concentrations. Concentration-response curves gave half-maximum inhibiting concentrations of 55, 99 and 190 micro M, respectively. Bupivacaine block of the I(h) current was not stereoselective. No significant effect was observed when QX-314 was applied to the external surface of the membrane. (4) In current-clamp experiments 60 micro M bupivacaine slightly hyperpolarized the membrane. The membrane stimulation by low-amplitude current pulses in the presence of bupivacaine showed an increase of the hyperpolarizing responses. (5) Our findings suggest an important role of the I(h)-block by local anaesthetics in the complex mechanism of drug action during epidural and spinal anaesthesia.

Mechanisms of anesthesia: towards integrating network, cellular, and molecular level modeling

The mechanisms of anesthesia are surprisingly little understood. The present article summarizes current knowledge about the function of general anesthetics at different organization levels of the nervous system. It argues that a consensus view can be constructed, assuming that general anesthetics modulate the activity of ion channels, the main targets being GABA and NMDA channels and possibly voltage-gated and background channels, thereby hyperpolarizing neurons in thalamocortical loops, which lead to disruption of coherent oscillatory activity in the cortex. Two computational cases are used to illustrate the possible importance of molecular level effects on cellular level activity. Subtle differences in the mechanism of ion channel block can be shown to cause considerable differences in the modification of the oscillatory activity in a single neuron, and consequently in an associated network. Finally, the relation between the anesthesia problem and the classical consciousness problem is discussed, and some consequences of introducing the phenomenon of degeneracy into the picture are pointed out.

Amide local anesthetics potently inhibit the human tandem pore domain background K+ channel TASK-2 (KCNK5)

Blockade of voltage-gated sodium (Na+) channels by local anesthetics represents the main mechanism for inhibition of impulse propagation. Local anesthetic-induced potassium (K+) channel inhibition is also known to influence transmission of sensory impulses and to potentiate inhibition. K+ channels involved in this mechanism may belong to the emerging family of background tandem pore domain K+ channels (2P K+ channels). To determine more precisely the effects of local anesthetics on members of this ion channel family, we heterologously expressed the 2P K+ channels TASK-2 (KCNK5), TASK-1 (KCNK3), and chimeric TASK-1/TASK-2 channels in oocytes of Xenopus laevis. TASK-2 cDNA-transfected HEK 293 cells were used for single-channel recordings. Local anesthetic inhibition of TASK-2 was dose-dependent, agent-specific, and stereoselective. The IC50 values for R-(+)-bupivacaine and S-(-)-bupivacaine were 17 and 43 micro M and for R-(+)-ropivacaine and S-(-)-ropivacaine, 85 and 236 micro M. Lidocaine (1 mM) inhibited TASK-2 currents by 55 +/- 4%, whereas its quaternary positively charged analog N-ethyl lidocaine (QX314) had no effect. Bupivacaine (100 micro M) decreased channel open probability from 20.8 +/- 1.6% to 5.6 +/- 2.2%. Local anesthetics [300 micro M R-(+)-bupivacaine] caused significantly greater depolarization of the resting membrane potential of TASK-2-expressing oocytes compared with water-injected control oocytes (15.8 +/- 2.5 mV versus 0.1 +/- 0.05 mV; p < 0.001). Chimeric TASK-1/TASK-2 2P K+ channel subunits that retained pH sensitivity demonstrated that the carboxy domain of TASK-2 mediates the greater local anesthetic sensitivity of TASK-2. These results show that clinically achievable concentrations of local anesthetics inhibit background K+ channel function and may thereby enhance conduction blockade.

Inhibition of human TREK-1 channels by bupivacaine

Human TWIK-related K(+) channels (TREK-1) stabilize the membrane potential (mp) of neurons and have a major role in the regulation of membrane excitability. In view of their physiological significance, interaction of bupivacaine with TREK-1 channels may be clinically important. Our aim was to characterize with the patch-clamp technique the properties of human TREK-1 channels and the effects of bupivacaine on these channels expressed in Chinese hamster ovary (CHO) cells. Transfection of CHO cells with TREK-1 channels (CHO(TREK-1) cells) hyperpolarized the mp from -33 +/- 13 to -78 +/- 4 mV. The channels were stimulated by intracellular acidosis. Inhibition of TREK-1 channels by bupivacaine was reversible, concentration-dependent, voltage-independent, and increased with intracellular acidosis. Bupivacaine depolarized the mp of CHO(TREK-1) cells in a reversible and concentration-dependent manner. Concentrations for channel inhibition and membrane depolarization were not linearly related (50% inhibitory concentration value for channel inhibition 370 +/- 20 micro M, Hill coefficient 1.8 +/- 0.1, n = 51; 50% inhibitory concentration value for membrane depolarization 856 +/- 14 micro M, Hill coefficient 2.4 +/- 0.1, mean +/- SEM, n = 27). The results suggest that protonated bupivacaine elicits the observed effects via a site of interaction accessible from the intracellular space. Inhibition of TREK-1 channels and consecutive depolarization of the cell membrane by bupivacaine may contribute to blockade of neuronal signal conduction during regional anesthesia.

IMPLICATIONS

The interaction of bupivacaine with human TREK-1 channels was studied with the patch-clamp technique. Bupivacaine inhibited TREK-1 channels and depolarized the membrane potential of cells expressing TREK-1 channels in a concentration-dependent and reversible manner. Both effects may contribute to conductance block caused by bupivacaine.

Structural effects of the local anesthetic bupivacaine hydrochloride on the human erythrocyte membrane and molecular models

The interaction of the local anesthetic bupivacaine with the human erythrocyte membrane and molecular models is described. The latter consisted of isolated unsealed human erythrocyte membranes (IUM), large unilamellar vesicles (LUV) of dimyristoylphosphatidylcholine (DMPC), and phospholipid multilayers built-up of DMPC and dimyristoylphosphatidylethanolamine (DMPE), representatives of phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. Optical and scanning electron microscopy revealed that bupivacaine induced erythrocyte spheroechinocytosis. According to the bilayer couple hypothesis, this result implied that bupivacaine inserted in the outer monolayer of the erythrocyte membrane. Experiments performed on IUM and DMPC LUV by fluorescence spectroscopy and X-ray diffraction on DMPC and DMPE multilayers confirmed this result. Changes in the molecular organization of membranes alter lipid-protein interactions and induce functional perturbation of membrane proteins such as Na(+) channels. Since local anesthetics may control the influx of Na(+) into the human erythrocyte, in order to relate the structural perturbations induced by bupivacaine in these systems to Na(+) transport, the interaction of this anesthetic with isolated toad skin was also studied. Electrophysiological measurements indicated a significant decrease in the potential difference and in the short-circuit current of the skin after the application of the anesthetic, reflecting inhibition of the active transport of ions. These results suggest that bupivacaine-induced conformational changes of the lipid molecules alter the lipid-protein boundaries of the outer moiety of the erythrocyte membrane, thus interfering with the function of neighboring sodium channels.

Enhancement of delayed-rectifier potassium conductance by low concentrations of local anaesthetics in spinal sensory neurones

Combining the patch-clamp recordings in slice preparation with the 'entire soma isolation' method we studied action of several local anaesthetics on delayed-rectifier K(+) currents in spinal dorsal horn neurones. Bupivacaine, lidocaine and mepivacaine at low concentrations (1 - 100 microM) enhanced delayed-rectifier K(+) current in intact neurones within the spinal cord slice, while exhibiting a partial blocking effect at higher concentrations (>100 microM). In isolated somata 0.1 - 10 microM bupivacaine enhanced delayed-rectifier K(+) current by shifting its steady-state activation characteristic and the voltage-dependence of the activation time constant to more negative potentials by 10 - 20 mV. Detailed analysis has revealed that bupivacaine also increased the maximum delayed-rectifier K(+) conductance by changing the open probability, rather than the unitary conductance, of the channel. It is concluded that local anaesthetics show a dual effect on delayed-rectifier K(+) currents by potentiating them at low concentrations and partially suppressing at high concentrations. The phenomenon observed demonstrated the complex action of local anaesthetics during spinal and epidural anaesthesia, which is not restricted to a suppression of Na(+) conductance only.

Bupivacaine, but not tetracaine, protects against the in vitro ischemic insult of rat hippocampal CA1 neurons

Neuroprotective actions of local anesthetics, bupivacaine and tetracaine, against the irreversible membrane dysfunction induced by in vitro ischemia were investigated. Intracellular recordings were made from hippocampal CA1 neurons in rat brain slice preparations. Oxygen and glucose deprivation (in vitro ischemia) produced a rapid depolarization after approximately 5 min of exposure. When oxygen and glucose were reintroduced, the membrane depolarized further and reached at 0 mV: the membrane showed no functional recovery (irreversible membrane dysfunction). Pretreatment with tetracaine or bupivacaine significantly prolonged the latency of rapid depolarization. Bupivacaine, but not tetracaine, restored the membrane potential after the reintroduction of oxygen and glucose. Tetracaine and bupivacaine depressed both field postsynaptic potentials and presynaptic volleys. The drugs also reduced the dV/dt of Ca(2+)-dependent spikes and the rapid rise of [Ca(2+)](i) induced by in vitro ischemia. Compared with tetracaine, bupivacaine markedly suppressed the resting K(+) conductance and the ATP-sensitive and Ca(2+)-dependent K(+) conductances. Moreover, in the presence of tetraethylammonium (TEA), a majority of CA1 neurons impaled with Cs acetate-filled electrodes showed complete or partial recovery of the membrane potential after reintroducing oxygen and glucose. These results suggest that the neuroprotective action of bupivacaine is mainly due to the suppression of the K(+) conductances.

Ionic mechanisms underlying repetitive high-frequency burst firing in supragranular cortical neurons

Neocortical neurons in awake, behaving animals can generate high-frequency (>300 Hz) bursts of action potentials, either in single bursts or in a repetitive manner. Intracellular recordings of layer II/III pyramidal neurons were obtained from adult ferret visual cortical slices maintained in vitro to investigate the ionic mechanisms by which a subgroup of these cells generates repetitive, high-frequency burst discharges, a pattern referred to as "chattering." The generation of each but the first action potential in a burst was dependent on the critical interplay between the afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs) that followed each action potential. The spike-afterdepolarization and the generation of action potential bursts were dependent on Na(+), but not Ca(2+), currents. Neither blocking of the transmembrane flow of Ca(2+) nor the intracellular chelation of free Ca(2+) with BAPTA inhibited the generation of intrinsic bursts. In contrast, decreasing the extracellular Na(+) concentration or pharmacologically blocking Na(+) currents with tetrodotoxin, QX-314, or phenytoin inhibited bursting before inhibiting action potential generation. Additionally, a subset of layer II/III pyramidal neurons could be induced to switch from repetitive single spiking to a burst-firing mode by constant depolarizing current injection, by raising extracellular K(+) concentrations, or by potentiation of the persistent Na(+) current with the Na(+) channel toxin ATX II. These results indicate that cortical neurons may dynamically regulate their pattern of action potential generation through control of Na(+) and K(+) currents. The generation of high-frequency burst discharges may strongly influence the response of postsynaptic neurons and the operation of local cortical networks.

Ionic mechanisms underlying repetitive high-frequency burst firing in supragranular cortical neurons

Neocortical neurons in awake, behaving animals can generate high-frequency (>300 Hz) bursts of action potentials, either in single bursts or in a repetitive manner. Intracellular recordings of layer II/III pyramidal neurons were obtained from adult ferret visual cortical slices maintained in vitro to investigate the ionic mechanisms by which a subgroup of these cells generates repetitive, high-frequency burst discharges, a pattern referred to as "chattering." The generation of each but the first action potential in a burst was dependent on the critical interplay between the afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs) that followed each action potential. The spike-afterdepolarization and the generation of action potential bursts were dependent on Na(+), but not Ca(2+), currents. Neither blocking of the transmembrane flow of Ca(2+) nor the intracellular chelation of free Ca(2+) with BAPTA inhibited the generation of intrinsic bursts. In contrast, decreasing the extracellular Na(+) concentration or pharmacologically blocking Na(+) currents with tetrodotoxin, QX-314, or phenytoin inhibited bursting before inhibiting action potential generation. Additionally, a subset of layer II/III pyramidal neurons could be induced to switch from repetitive single spiking to a burst-firing mode by constant depolarizing current injection, by raising extracellular K(+) concentrations, or by potentiation of the persistent Na(+) current with the Na(+) channel toxin ATX II. These results indicate that cortical neurons may dynamically regulate their pattern of action potential generation through control of Na(+) and K(+) currents. The generation of high-frequency burst discharges may strongly influence the response of postsynaptic neurons and the operation of local cortical networks.

Neurobiology: the acid test for resting potassium channels

The K(+) channels that generate the resting potential of mammalian neurons have been difficult to identify and characterize. Recent experiments on hypoglossal motoneurons and cerebellar granule cells suggest that the resting current in these neurons is carried by TASK-1, a member of the twin-pore family of K(+) channels.

Pharmacological isolation of the synaptic and nonsynaptic components of the GABA-mediated biphasic response in rat CA1 hippocampal pyramidal cells

High-frequency stimulation (HFS) applied to stratum radiatum of a rat hippocampal slice in the presence of ionotropic glutamate receptor antagonists evokes a biphasic GABA(A) receptor-dependent response in CA1 pyramidal neurons, with a brief hyperpolarizing IPSP (hIPSP) followed by a long-lasting depolarization. We show now that it is possible to pharmacologically separate the hIPSP and late depolarization from one another. In neurons intracellularly perfused for 1-2 hr with F(-) as the major anion and no ATP, the hIPSP (and the corresponding current, hIPSC) evoked by HFS was blocked, whereas neither the late depolarization nor its underlying current was attenuated. In contrast, internal perfusion with a high concentration (5 mM) of the impermeant lidocaine derivative QX-314 selectively abolished the depolarizing component of the biphasic response and also strongly reduced depolarizations evoked by extracellular microinjection of K(+). Bath application of quinine (0. 2-0.5 mM) or quinidine (0.1 mM) resulted in a pronounced inhibition of the HFS-induced extracellular K(+) concentration ([K(+)](o)) transient but not of the bicarbonate-dependent alkaline shift in extracellular pH. The attenuation of the [K(+)](o) transient was closely paralleled by a suppression of the HFS-evoked depolarization but not of the hIPSP. Quini(di)ne did not affect depolarizations induced by exogenous K(+) either. These data provide direct pharmacological evidence for the view that the HFS-induced biphasic response of the pyramidal neuron is composed of mechanistically distinct components: a direct GABA(A) receptor-mediated phase, which is followed by a slow, nonsynaptic [K(+)](o)-mediated depolarization. The bicarbonate-dependent, activity-induced [K(+)](o) transient can be blocked by quini(di)ne, whereas its depolarizing action in the pyramidal neuron is inhibited by internal QX-314. The presence of fundamentally distinct components in GABA(A) receptor-mediated actions evoked by HFS calls for further investigations of their functional role(s) in standard experimental maneuvers, such as those used in studies of synaptic plasticity and induction of gamma oscillations.

Pharmacological isolation of the synaptic and nonsynaptic components of the GABA-mediated biphasic response in rat CA1 hippocampal pyramidal cells

High-frequency stimulation (HFS) applied to stratum radiatum of a rat hippocampal slice in the presence of ionotropic glutamate receptor antagonists evokes a biphasic GABA(A) receptor-dependent response in CA1 pyramidal neurons, with a brief hyperpolarizing IPSP (hIPSP) followed by a long-lasting depolarization. We show now that it is possible to pharmacologically separate the hIPSP and late depolarization from one another. In neurons intracellularly perfused for 1-2 hr with F(-) as the major anion and no ATP, the hIPSP (and the corresponding current, hIPSC) evoked by HFS was blocked, whereas neither the late depolarization nor its underlying current was attenuated. In contrast, internal perfusion with a high concentration (5 mM) of the impermeant lidocaine derivative QX-314 selectively abolished the depolarizing component of the biphasic response and also strongly reduced depolarizations evoked by extracellular microinjection of K(+). Bath application of quinine (0. 2-0.5 mM) or quinidine (0.1 mM) resulted in a pronounced inhibition of the HFS-induced extracellular K(+) concentration ([K(+)](o)) transient but not of the bicarbonate-dependent alkaline shift in extracellular pH. The attenuation of the [K(+)](o) transient was closely paralleled by a suppression of the HFS-evoked depolarization but not of the hIPSP. Quini(di)ne did not affect depolarizations induced by exogenous K(+) either. These data provide direct pharmacological evidence for the view that the HFS-induced biphasic response of the pyramidal neuron is composed of mechanistically distinct components: a direct GABA(A) receptor-mediated phase, which is followed by a slow, nonsynaptic [K(+)](o)-mediated depolarization. The bicarbonate-dependent, activity-induced [K(+)](o) transient can be blocked by quini(di)ne, whereas its depolarizing action in the pyramidal neuron is inhibited by internal QX-314. The presence of fundamentally distinct components in GABA(A) receptor-mediated actions evoked by HFS calls for further investigations of their functional role(s) in standard experimental maneuvers, such as those used in studies of synaptic plasticity and induction of gamma oscillations.

Mechanisms of bupivacaine action on Na+ and K+ channels in myelinated axons of Xenopus laevis

The local anaesthetic bupivacaine has recently been proposed to inhibit Na+ channels indirectly by making the resting potential less negative. To test this hypothesis we analysed the effects of bupivacaine on voltage and current clamped nodes of Ranvier. Contrary to the hypothesis, the leak current and the resting potential were unaffected. The Na+ and K+ channels were, however, affected at relatively low concentrations (33 microM). Steady-state activation curves were decreased without notable shift effects, whereas the Na+ inactivation curve was decreased and shifted in negative direction. The effect on the Na+ current was tentatively explained by a single-site, state-dependent binding model (Kd = 44 microM), while that on the K+ current was explained by two population-specific mechanisms, one open-state dependent (Kd = 550 microM) and one state independent (Kd = 59 microM). The binding stoichiometry was higher than 1:1 for the main sites of action. In conclusion, bupivacaine exerts its main anaesthetic action on myelinated nerve axons by a direct modification of Na+ channels.

Fundamental properties of local anesthetics: half-maximal blocking concentrations for tonic block of Na+ and K+ channels in peripheral nerve

Local anesthetics suppress excitability by interfering with ion channel function. Ensheathment of peripheral nerve fibers, however, impedes diffusion of drugs to the ion channels and may influence the evaluation of local anesthetic potencies. Investigating ion channels in excised membrane patches avoids these diffusion barriers. We investigated the effect of local anesthetics with voltage-dependent Na+ and K+ channels in enzymatically dissociated sciatic nerve fibers of Xenopus laevis using the patch clamp method. The outside-out configuration was chosen to apply drugs to the external face of the membrane. Local anesthetics reversibly blocked the transient Na+ inward current, as well as the steady-state K+ outward current. Half-maximal tonic inhibiting concentrations (IC50), as obtained from concentration-effect curves for Na+ current block were: tetracaine 0.7 microM, etidocaine 18 microM, bupivacaine 27 microM, procaine 60 microM, mepivacaine 149 microM, and lidocaine 204 microM. The values for voltage-dependent K+ current block were: bupivacaine 92 microM, etidocaine 176 microM, tetracaine 946 microM, lidocaine 1118 microM, mepivacaine 2305 microM, and procaine 6302 microM. Correlation of potencies with octanol:buffer partition coefficients (logP0) revealed that ester-bound local anesthetics were more potent in blocking Na+ channels than amide drugs. Within these groups, lipophilicity governed local anesthetic potency. We conclude that local anesthetic action on peripheral nerve ion channels is mediated via lipophilic drug-channel interactions.

IMPLICATIONS

Half-maximal blocking concentrations of commonly used local anesthetics for Na+ and K+ channel block were determined on small membrane patches of peripheral nerve fibers. Because drugs can directly diffuse to the ion channel in this model, these data result from direct interactions of the drugs with ion channels.

Cardiovascular and central nervous system effects of intravenous levobupivacaine and bupivacaine in sheep

Commercially available bupivacaine is an equimolar mixture of R(+)- and S(-)-bupivacaine. S(-)-bupivacaine (i.e., levobupivacaine) is currently undergoing preclinical evaluation. Cross-over studies with i.v. levobupivacaine and bupivacaine were conducted in two groups of seven conscious sheep. Doses were chosen to avoid convulsions (smaller dose 6.25-37.5 mg/min) or to be potentially toxic (larger dose 75-200 mg/3 min). In subconvulsive doses, both drugs produced similar time- and dose-dependent depression of left ventricular systolic contractility (dP/dt(max)). Convulsions occurred consistently with > or = 75 mg of bupivacaine and > or = 100 mg of levobupivacaine, producing an abrupt reversal of dP/dt(max) depression. Subconvulsive doses produced minor cardiovascular effects on heart rate and blood pressure, whereas both were increased by convulsions. Cardiac output and myocardial blood flow were decreased with larger doses of both drugs. Doses > 75 mg of bupivacaine or > 100 mg of levobupivacaine induced QRS widening and ventricular arrhythmias, but significantly fewer and less deleterious arrhythmias were induced by levobupivacaine. Three animals died after 150, 150, and 200 mg of bupivacaine from the sudden onset of ventricular fibrillation. These doses of levobupivacaine produced nonfatal arrhythmias that automatically returned to sinus rhythm. We conclude that levobupivacaine could offer a greater margin of clinical safety than bupivacaine.

IMPLICATIONS

Levobupivacaine comprises 50% of commercially available bupivacaine and is being considered for use in its own right. Local anesthetics can cause toxicity to the cardiovascular and central nervous systems. As a part of a preclinical evaluation of levobupivacaine, this study compared the toxic effects of levobupivacaine and bupivacaine in sheep.

Graded, irreversible changes in crayfish giant axon as manifestations of lidocaine neurotoxicity in vitro

High concentrations of lidocaine induce irreversible conduction block with little effect on resting membrane potential (Em). We assumed the mechanism of persistent neurologic deficit caused by local anesthetics may result from neural death, as represented by the loss of Em. We investigated the effects of lidocaine on Em and action potential (AP) in single crayfish giant axons in vitro. Axons were perfused with two doses of lidocaine for either 15 or 30 min, and they were continuously washed. No axons exposed to 80 mM lidocaine for 30 min showed recovery of AP and Em. Those exposed to 40 mM for 30 min and 80 mM for 15 min showed a return to baseline for Em, but no recovery of AP. Those exposed to 40 mM lidocaine for 15 min showed full recovery of Em and AP immediately after washing. The membrane depolarization was significantly greater during exposure to 80 mM lidocaine for 30 min than in other groups. We conclude that lidocaine has a direct neurotoxic effect on crayfish giant axons and that the generation of AP is more vulnerable than the maintenance of Em. The irreversibility of AP and Em is dose- and time-dependent.

IMPLICATIONS

Highly concentrated lidocaine induced an irreversible conduction block and a complete loss of resting membrane potential in crayfish giant axons in vitro. Our results may represent a possible explanation for various grades of local anesthetic-induced neurotoxicity in clinical cases if the same toxicity occurs in mammalian nerves in vivo.

An open rectifier potassium channel with two pore domains in tandem cloned from rat cerebellum

Tandem pore domain K+ channels represent a new family of ion channels involved in the control of background membrane conductances. We report the structural and functional properties of a TWIK-related acid-sensitive K+ channel (rTASK), a new member of this family cloned from rat cerebellum. The salient features of the primary amino acid sequence include four putative transmembrane domains and, unlike other cloned tandem pore domain channels, a PDZ (postsynaptic density protein, disk-large, zo-1) binding sequence at the C terminal. rTASK has distant overall homology to a putative Caenorhabditis elegans K+ channel and to the mammalian clones TREK-1 and TWIK-1. rTASK expression is most abundant in rat heart, lung, and brain. When exogenously expressed in Xenopus oocytes, rTASK currents activate instantaneously, are noninactivating, and are not gated by voltage. Because rTASK currents satisfy the Goldman-Hodgkin-Katz current equation for an open channel, rTASK can be classified an open rectifier. Activation of protein kinase A produces inhibition of rTASK, whereas activation of protein kinase C has no effect. rTASK currents were inhibited by extracellular acidity. rTASK currents also were inhibited by Zn2+ (IC50 = 175 microM), the local anesthetic bupivacaine (IC50 = 68 microM), and the anti-convulsant phenytoin ( approximately 50% inhibition at 200 microM). By demonstrating open rectification and open probability independent of voltage, we have established that rTASK is a baseline potassium channel.

An open rectifier potassium channel with two pore domains in tandem cloned from rat cerebellum

Tandem pore domain K+ channels represent a new family of ion channels involved in the control of background membrane conductances. We report the structural and functional properties of a TWIK-related acid-sensitive K+ channel (rTASK), a new member of this family cloned from rat cerebellum. The salient features of the primary amino acid sequence include four putative transmembrane domains and, unlike other cloned tandem pore domain channels, a PDZ (postsynaptic density protein, disk-large, zo-1) binding sequence at the C terminal. rTASK has distant overall homology to a putative Caenorhabditis elegans K+ channel and to the mammalian clones TREK-1 and TWIK-1. rTASK expression is most abundant in rat heart, lung, and brain. When exogenously expressed in Xenopus oocytes, rTASK currents activate instantaneously, are noninactivating, and are not gated by voltage. Because rTASK currents satisfy the Goldman-Hodgkin-Katz current equation for an open channel, rTASK can be classified an open rectifier. Activation of protein kinase A produces inhibition of rTASK, whereas activation of protein kinase C has no effect. rTASK currents were inhibited by extracellular acidity. rTASK currents also were inhibited by Zn2+ (IC50 = 175 microM), the local anesthetic bupivacaine (IC50 = 68 microM), and the anti-convulsant phenytoin ( approximately 50% inhibition at 200 microM). By demonstrating open rectification and open probability independent of voltage, we have established that rTASK is a baseline potassium channel.

Local anaesthetic effects on tetrodotoxin-resistant Na+ currents in rat dorsal root ganglion neurones

Besides the fast tetrodotoxin-sensitive Na+ current, small dorsal root ganglion neurones of rats also possess a slower tetrodotoxin-resistant Na+ current. The blocking effect of commonly used local anaesthetics upon the tetrodotoxin-resistant Na+ current was investigated in the present paper. Dorsal root ganglia were dissected from adult rats and cells were enzymatically isolated. The whole-cell patch clamp technique was then used to measure inward Na+ currents of small dorsal root ganglion neurones. Externally applied local anaesthetics reversibly blocked the tetrodotoxin-resistant Na+ current in a dose-dependent manner. Half-maximal blocking concentrations for tonic block were: lignocaine, 326 microM; prilocaine, 253 microM; mepivacaine, 166 microM; etidocaine, 196 microM bupivacaine, 57 microM procaine, 518 microM benzocaine, 489 microM; tetracaine, 21 microM; and dibucaine, 23 microM. Blocking of the current by lignocaine was independent of temperature. The quaternary lignocaine derivative OX-314 did not have any effect upon the tetrodotoxin-resistant Na+ current when applied externally. High concentrations of tetrodotoxin also blocked the tetrodotoxin-resistant Na+ current with a half-maximal blocking concentration of 115 microM. The block by high tetrodotoxin concentrations did not compete with the lignocaine block, suggesting that there were two independent blocking mechanisms for the two substances. The tetrodotoxin-resistant Na+ currents also showed a marked sensitivity to phasic (use-dependent) block by local anaesthetics.