Block of neuronal tetrodotoxin-resistant Na+ currents by stereoisomers of piperidine local anesthetics

Levobupivacaine versus ropivacaine for brachial plexus block: A systematic review and meta-analysis of randomised controlled trials

Background and Aims

Brachial plexus block (BPB) is advantageous for elective orthopaedic or reconstructive upper limb surgery. However, the optimal local anaesthetic in BPB remains debatable. Therefore, we aim to investigate the efficacy and safety of levobupivacaine versus ropivacaine in BPB for upper limb surgery.

Methods

A systematic review and meta-analysis synthesising randomised controlled trials (RCTs), retrieved by systematically searching PubMed, EMBASE, WOS, SCOPUS, Google Scholar, and CENTRAL since inception till June 2024. Continuous and dichotomous outcome variables were pooled using mean difference (MD) and risk ratio (RR), with a 95% confidence interval (CI), using Stata v. 17. We assessed heterogeneity using the Chi-square test and I2 statistic.

Results

Sixteen RCTs and 939 patients were included. Levobupivacaine was significantly associated with a longer sensory block duration [MD: 1.66 (95% CI: 1.43, 1.89), P < 0.001] and motor block duration [MD: 1.18 (95% CI: 0.11, 2.26), P = 0.03]. However, there was no difference between both groups in time to sensory block [MD: -0.30 (95% CI: -1.31, 0.71), P = 0.56], time to motor block [MD: -0.29 (95% CI: -1.26, 0.67), P = 0.55], pain score [MD: -0.48 (95% CI: -2.13, 1.16), P = 0.56], rescue analgesia rate [RR: 0.94 (95% CI: 0.74, 1.20), P = 0.64], and complications [RR: 0.47 (95% CI: 0.20, 1.13), P = 0.09].

Conclusions

Levobupivacaine is significantly associated with a longer duration of sensory and motor block in patients undergoing BPB for upper limb surgery compared to ropivacaine, with a similar safety profile. However, there was no difference regarding the time to onset of the sensory or motor block.

Comparison of the Analgesic Efficacy between Levobupivacaine 0.25% and Ropivacaine 0.375% for PENG (Pericapsular Nerve Group) Block in the Context of Hip Fracture Surgery of Elderly Patients: A Single-Center, Randomized, and Controlled Clinical Trial

Previous studies have compared levobupivacaine versus ropivacaine in various peripheral nerve blocks in terms of block duration, quality of analgesia, and onset time, but this has not occurred in the PENG block. Here, a single-center, randomized, and controlled clinical trial is presented. One hundred and twenty patients older than 65 years suffering from hip fractures and surgically treated at our institution under spinal anesthesia were eligible for participation; of them, one hundred and eight were analyzed. Patients were randomized to receive ultrasound-guided PENG blocks using 20 mL of either 0.25% levobupivacaine or 0.375% ropivacaine (both of which are equipotent concentrations). The primary endpoint was to compare the analgesic duration (time to first rescue) and analgesic quality (pain scores using the VAS, PAINAD, and AlgoPlus scales) between the groups. Secondary endpoints included comparing the onset time, describing the need for and type of rescue analgesics, and possible associated adverse effects. There were no statistically significant differences in analgesic duration between levobupivacaine (median 861.0, IQR 960) and ropivacaine (median 1205.0, IQR 1379; p = 0.069). Likewise, the quality of analgesia and onset time were comparable among the groups. A small number of patients required opioids as rescue analgesics (4.6%). The possible associated adverse effects included postoperative infection (11.1%) and delirium (2.8%).

Efficacy of four local anaesthesia protocols for mandibular first molars with symptomatic irreversible pulpitis: A randomized clinical trial

AIM

To examine the efficacy rate of four anaesthetic protocols in mandibular first molars with symptomatic irreversible pulpitis (SIP).

METHODOLOGY

One hundred and sixty patients with a diagnosis of SIP were included in this randomized clinical trial. Patients were randomly allocated into four treatment groups (N = 40) according to the administered technique: Group 1 (IANB): standard inferior alveolar nerve block (IANB) injection; Group 2 (IANB + IO): standard IANB followed by a supplemental intraosseous infusion (IO) injection; Group 3 (IANB + PDL): standard IANB followed by a supplemental periodontal ligament (PDL) injection; Group 4 (IANB + BI): standard IANB followed by a supplemental buccal infiltration. Patients rated pain intensity using a verbal rating scale when the root canal treatment procedure was initiated, that is, during caries removal, access preparation and pulpectomy. Heart rate changes were recorded before, during and after each injection. The anaesthetic efficacy rates were analysed using chi-square tests, age differences using one-way anova, gender differences using Fischer Exact tests whilst heart rate changes were analysed using Kruskal-Wallis tests. Statistical significances were set at p < .05 level.

RESULTS

All the included patients were analysed. No differences in the efficacy rate were found in relation to the age or gender of the participants amongst the study groups (p > .05). IANB + IO injections had a significantly higher efficacy rate (92.5%) when compared to other techniques (p < .05), followed by IANB + PDL injections (72.5%), IANB + BI injections (65.0%), with no significant differences between the IANB + PDL or IANB + BI injections (p > .05). IANB injection alone had a significantly lower rate (40%) compared to the other techniques (p < .05). A transient but significant rise in the heart rate was recorded in 60% (24/40) of patients who received the IANB + IO injection compared to other groups (p < .05).

CONCLUSIONS

Inferior alveolar nerve block injection alone did not reliably permit pain-free treatment for mandibular molars with SIP. The use of an additional IO supplemental injection provided the most effective anaesthesia for patients requiring emergency root canal treatment for SIP in mandibular posterior teeth.

Epidural Administration of Ropivacaine Reduces the Amplitude of Transcranial Electrical Motor-Evoked Potentials: A Double-Blinded, Randomized, Controlled Trial

BACKGROUND

An epidurally administered local anesthetic acts primarily on the epidural nerve roots and can act directly on the spinal cord through the dural sleeve. We hypothesized that epidurally administered ropivacaine would reduce the amplitude of transcranial electrical motor-evoked potentials by blocking nerve conduction in the spinal cord. Therefore, we conducted a double-blind, randomized, controlled trial.

METHODS

Thirty adult patients who underwent lung surgery were randomly allocated to 1 of 3 groups, based on the ropivacaine concentration: the 0.2% group, the 0.375% group, and the 0.75% group. The attending anesthesiologists, neurophysiologists, and patients were blinded to the allocation. The epidural catheter was inserted at the T5-6 or T6-7 interspace by a paramedian approach, using the loss of resistance technique with normal saline. General anesthesia was induced and maintained using propofol and remifentanil. Transcranial electrical motor-evoked potentials were elicited by a train of 5 pulses with an interstimulus interval of 2 milliseconds by using a constant-voltage stimulator and were recorded from the tibialis anterior muscle. Somatosensory-evoked potentials (SSEPs) were evoked by electrical tibial nerve stimulation at the popliteal fossa. After measuring the baseline values of these evoked potentials, 10 mL of epidural ropivacaine was administered at the 0.2%, 0.375%, or 0.75% concentration. The baseline amplitudes and latencies recorded before administering ropivacaine were defined as 100%. Our primary end point was the relative amplitude of the motor-evoked potentials at 60 minutes after the epidural administration of ropivacaine. We analyzed the amplitudes and latencies of these evoked potentials by using the Kruskal-Wallis test and used the Dunn multiple comparison test as the post hoc test for statistical analysis.

RESULTS

The data are expressed as the median (interquartile range). Sixty minutes after epidurally administering ropivacaine, the motor-evoked potential amplitude was lower in the 0.75% group (7% [3%-18%], between-group difference P < .001) and in the 0.375% group (52% [43%-59%]) compared to that in the 0.2% group (96% [89%-105%]). The latency of SSEP was longer in the 0.75% group compared to that in the 0.2% group, but the amplitude was unaffected.

CONCLUSIONS

Epidurally administered high-dose ropivacaine lowered the amplitude of motor-evoked potentials and prolonged the onset latencies of motor-evoked potentials and SSEPs compared to those in the low-dose group. High-dose ropivacaine can act on the motor pathway through the dura mater.

Comparison of the onset time between 0.375% ropivacaine and 0.25% levobupivacaine for ultrasound-guided infraclavicular brachial plexus block: a randomized-controlled trial

At centers with pressure on rapid operating room turnover, onset time is one of the important considerations for choosing a local anesthetic drug. To hasten the onset of the block, higher concentrations of local anesthetics are sometimes used. However, the use of diluted local anesthetics may be safer. Therefore, we aimed to compare the onset times of equipotential levobupivacaine and ropivacaine at low concentrations for infraclavicular brachial plexus block. Adult patients undergoing upper extremity surgery under ultrasound-guided infraclavicular brachial plexus block at our center were randomly allocated to the levobupivacaine and ropivacaine groups. Infraclavicular brachial plexus block was induced with 0.25% levobupivacaine or 0.375% ropivacaine depending on the assigned group. The degrees of sensory and motor blockade were assessed for 40 min after the administration of local anesthetics. A total of 46 patients were included in the analysis. Infraclavicular brachial plexus block with 0.25% levobupivacaine and 0.375% ropivacaine provided sufficient surgical anesthesia. The sensory onset time of 0.375% ropivacaine was shorter than that of 0.25% levobupivacaine (group R, 15 [15.0–22.5] min; group L, 30 [17.5–35.0] min, p = 0.001). There were no significant differences in other block characteristics and clinical outcomes between the two groups. Thus, when a quicker block onset is required, 0.375% ropivacaine is a better choice than 0.25% levobupivacaine.

Trial registration ClinicalTrials.gov (NCT03679897).

Nebulized hypertonic saline triggers nervous system-mediated active liquid secretion in cystic fibrosis swine trachea

Inhaled hypertonic saline (HTS) treatment is used to improve lung health in patients with cystic fibrosis (CF). The current consensus is that the treatment generates an osmotic gradient that draws water into the airways and increases airway surface liquid (ASL) volume. However, there is evidence that HTS may also stimulate active secretion of ASL by airway epithelia through the activation of sensory neurons. We tested the contribution of the nervous system and airway epithelia on HTS-stimulated ASL height increase in CF and wild-type swine airway. We used synchrotron-based imaging to investigate whether airway neurons and epithelia are involved in HTS treatment-triggered ASL secretion in CFTR^−/− and wild-type swine. We showed that blocking parasympathetic and sensory neurons in airway resulted in ~50% reduction of the effect of HTS treatment on ASL volume in vivo. Incubating tracheal preparations with inhibitors of epithelial ion transport across airway decreased secretory responses to HTS treatment. CFTR^−/− swine ex-vivo tracheal preparations showed substantially decreased secretory response to HTS treatment after blockage of neuronal activity. Our results indicated that HTS-triggered ASL secretion is partially mediated by the stimulation of airway neurons and the subsequent activation of active epithelia secretion; osmosis accounts for only ~50% of the effect.

In silico identification of promiscuous scaffolds as potential inhibitors of 1-deoxy-d-xylulose 5-phosphate reductoisomerase for treatment of Falciparum malaria

CONTEXT

Malaria remains one of the prevalent infectious diseases worldwide. Plasmodium falciparum 1-deoxy-d-xylulose-5-phosphate reductoisomerase (PfDXR) plays a role in isoprenoid biosynthesis in the malaria parasite, making this parasite enzyme an attractive target for antimalarial drug design. Fosmidomycin is a promising DXR inhibitor, which showed safety as well as efficacy against Plasmodium falciparum malaria in clinical trials. However, due to its poor oral bioavailability and non-drug-like properties, the focus of medicinal chemists is to develop inhibitors with improved pharmacological properties.

OBJECTIVE

This study described the computational design of new and potent inhibitors for deoxyxylulose 5-phosphate reductoisomerase and the prediction of their pharmacokinetic and pharmacodynamic properties.

MATERIAL AND METHODS

A complex-based pharmacophore model was generated from the complex X-ray crystallographic structure of PfDXR using MOE (Molecular Operating Environment). Furthermore, MOE-Dock was used as docking software to predict the binding modes of hits and target enzyme.

RESULTS

Finally, 14 compounds were selected as new and potent inhibitors of PfDXR on the basis of pharmacophore mapping, docking score, binding energy and binding interactions with the active site residues of the target protein. The predicted pharmacokinetic properties showed improved permeability by efficiently crossing blood-brain barrier. While, in silico promiscuity binding data revealed that these hits also have the ability to bind with other P. falciparum drug targets.

DISCUSSION AND CONCLUSION

In conclusion, innovative scaffolds with novel modes of action, improved efficacy and acceptable physiochemical/pharmacokinetic properties were computationally identified.

Ropivacaine versus levobupivacaine in peripheral nerve block: A PRISMA-compliant meta-analysis of randomized controlled trials

BACKGROUND

To determine which is more potent in peripheral nerve block between ropivacaine and levobupivacaine.

METHODS

A literature search was performed in the EMBASE, Medline, the Cochrane Library, and the Web of Science. The trials that were found were then evaluated for eligibility. The Cochrane Collaboration's Review Manager software was used to perform the meta-analyses.

RESULTS

Twelve studies including 556 patients were included for final analysis. No statistically significant difference was observed between the 2 drugs with respect to onset time of surgical anesthesia, onset time of sensory block, onset time of motor block, duration of motor block, and patients overall satisfaction. Levobupivacaine provided more long-term anesthesia (weighted mean difference [WMD], -2.94; 95% confidence interval [CI], -5.56 to -0.32; I = 93%) and significantly lower incidence of postoperative rescue analgesia (odds ratio [OR], 2.11; 95% CI 1.18-3.74; I = 21%) than ropivacaine. There was a trend toward greater duration of sensory block in the levobupivacaine group (WMD, -1.16; 95% CI, -1.89 to -0.43; I = 14%).

CONCLUSION

Levobupivacaine is more potent than ropivacaine in peripheral nerve block to some extent. Otherwise, more rigorous randomized control trials are required in the future.

Crystal structure of [4-(chloro-meth-yl)phen-yl](4-hy-droxy-piperidin-1-yl)methanone

The title compound, C13H16ClNO2, crystallized with two independent mol-ecules in the asymmetric unit (A and B). The piperidinol ring in mol-ecule B is disordered over two positions with a site occupancy ratio of 0.667 (5):0.333 (5). In both mol-ecules these rings have a chair conformation, including the minor component in mol-ecule B. Their mean planes are inclined to the benzene ring by 45.57 (13)° in mol-ecule A, and by 50.5 (4)° for the major component of the piperidine ring in mol-ecule B. In the crystal, the individual mol-ecules are linked by O-H⋯O hydrogen bonds, forming chains of A and B mol-ecules along the [100] direction. The chains are inter-linked by C-H⋯O hydrogen bonds, forming ribbons.

Crystal structure of 4-bromo-2-[(E)-N-(2,2,6,6-tetra-methyl-piperidin-4-yl)carboximido-yl]phenol dihydrate

In the title hydrate, C16H23BrN2O·2H2O, the organic mol-ecule features a strong intra-molecular O-H⋯N hydrogen bond. The piperidine ring, in addition, adopts a chair conformation with the exocyclic C-N bond in an equatorial orientation. The water molecules of crystallization are disordered (each over two sets of sites with half occupancy. In the crystal, they associate into corrugated (100) sheets of (H2O)4 tetra-mers linked by O-H⋯O hydrogen bonds. The organic mol-ecules, in turn, are arranged at both sides of these sheets, linked by water-piperidine O-H⋯N hydrogen bonds.

Lipid Rescue Reverses the Bupivacaine-induced Block of the Fast Na+ Current (INa) in Cardiomyocytes of the Rat Left Ventricle

Background:

Cardiovascular resuscitation upon intoxication with lipophilic ion channel–blocking agents has proven most difficult. Recently, favorable results have been reported when lipid rescue therapy is performed, i.e., the infusion of a triglyceride-rich lipid emulsion during resuscitation. However, the mechanism of action is poorly understood.

Methods:

The authors investigate the effects of a clinically used lipid emulsion (Lipovenös® MCT 20%; Fresenius Kabi AG, Bad Homburg, Germany) on the block of the fast Na+ current (INa) induced by the lipophilic local anesthetic bupivacaine in adult rat left ventricular myocytes by using the whole cell patch clamp technique.

Results:

Bupivacaine at 10 µm decreased INa by 54% (−19.3 ± 1.9 pApF−1vs. −42.3 ± 4.3 pApF−1; n = 17; P &lt; 0.001; VPip = −40 mV, 1 Hz). Addition of 10% lipid emulsion in the presence of bupivacaine produced a 37% increase in INa (−26.4 ± 2.8 pApF−1; n = 17; P &lt; 0.001 vs. bupivacaine alone). To test whether these results could be explained by a reduction in the free bupivacaine concentration by the lipid (lipid-sink effect), the authors removed the lipid phase from the bupivacaine–lipid mixture by ultracentrifugation. Also, the resulting water phase led to an increase in INa (+19%; n = 17; P &lt; 0.001 vs. bupivacaine), demonstrating that part of the bupivacaine had been removed during ultracentrifugation. The substantially less lipophilic mepivacaine (40 µm) reduced INa by 27% (n = 24; P &lt; 0.001). The mepivacaine–lipid mixture caused a significant increase in INa (+17%; n = 24; P &lt; 0.001). For mepivacaine, only a small lipid-sink effect could be demonstrated (+8%; n = 23; P &lt; 0.01), reflecting its poor lipid solubility.

Conclusion:

The authors demonstrate lipid rescue on the single-cell level and provide evidence for a lipid-sink mechanism.

Discriminative sensory characteristics of the lateral femoral cutaneous nerve after mepivacaine-induced block

Background and objectives

Unmyelinated C-fibres comprise the largest group of somatic afferents and have demonstrated a crucial role not only in the perception of high-threshold mechanically, thermally or chemically induced pain, but also in non-harmful low-threshold mechanical stimuli [1,2]. The objective of our study was to characterize differential sensitivity changes of C-fibre related subclasses of high-threshold and low-threshold polymodal nociceptors and low-threshold mechanoreceptors to the local anaesthetic (LA) mepivacaine during nerve block of the purely sensory lateral femoral cutaneous nerve (LFCN) in human. We assumed a diverse response of different classes of afferents to the two different concentrations of the LA mepivacaine (Scandicaine).

Methods

In a double-blind randomized experimental setting, an ultrasound-guided nerve block of the LFCN was performed in 10 healthy male subjects, each with two different concentrations of mepivacaine (0.5 and 1%). Responsiveness of afferent nerve fibres to different noxious and non-noxious stimuli was tested by Quantitative Sensory Testing (QST) 30, 180, and 300 min after nerve block. Both LA concentrations of mepivacaine were compared for time course of the areas of anaesthesia for the tested sensory modalities.

Results

Initial extension of anaesthetic areas at 30 min did not differ between both LA concentrations. At 180 min only the anaesthetic areas to nociceptive stimuli were reduced at the site of lower mepivacaine injection (260mN: 204mm2 (18; 244; median difference and 95% confidence interval; p < 0.05), heat: 276mm2 (3; 305)). In contrast, no significant differences were found between the two concentration when non-nociceptive stimuli were used (100mN: 187mm2 (4; 240), p >0.05, brush: 159mm2 (–59; 202)).

Conclusion

Equal initial sizes of anaesthesia areas for all sensory modalities can be explained by supramaximal perineural LA molecule concentration in both administered mepivacaine dosages. Upon washout of the LA nociceptive function is restored faster as compared to non-nociceptive sensation and higher concentration of the LA are required to maintain the analgesia. Quantitative sensory testing is able to detect different susceptibility of low threshold mechanosensors and subtypes of nociceptive C-fibres to mepivacaine. Using painful mechanical, heat and electrical stimulation different classes of nociceptors will be activated. The analgesic areas to electrical stimulation were particularly small; one might therefore hypothesize that the proposed protocol allows to also differentiate mechano-insensitive (“silent”) and mechanosensitive (“polymodal”) nociceptors.

Implications

QST is a non-invasive method to functionally examine sensory modalities and their pharmacological modulation in humans. The method is sufficiently sensitive to differentiate the analgesic properties of mepivacaine at 0.5 and 1% and might also be adequate to different classes of nociceptors. Further development of nociceptive stimuli including supra-threshold encoding characteristics will enable to investigate peripheral analgesic effects more specifically and thus might help to design new analgesics with preferential effect on high frequency discharge of nociceptors.

[Peridural application of ropivacaine and clonidine for pain therapy after prostatectomy]

OBJECTIVE

The aim of this study was to find out if ropivacaine with clonidine can be successfully used after radical retropubic prostatectomy.

MATERIAL AND METHODS

A total of 86 patients were randomized into two groups each consisting of 43 patients. Patients in group 1 received epidural bupivacaine/morphine/clonidine and those in group 2 ropivacaine and clonidine. If necessary analgesic, antiemetic or antihistamine drugs were administered. Pain on a visual analogue scale (VAS), motoric and sensoric disturbances, sedation, itching, nausea and vomiting, hypotension, need of antihistamines, naloxon, antiemetics, ephedrine, and analgesics were also documented.

RESULTS

Differences in sedation, itching, nausea and vomiting, motoric and sensoric disturbances, need of antihistamines, antiemetics, and ketoprofen were significant, the other parameters did not show any significant differences.

CONCLUSIONS

We recommend the use of ropivacaine with clonidine in the postoperative pain management after radical retropubic prostatectomy.

The toxicity of local anesthetics: the place of ropivacaine and levobupivacaine

PURPOSE OF REVIEW

Ropivacaine and levobupivacaine were developed after evidence of bupivacaine-related severe toxicity. Despite a comparable analgesic profile, quantitative differences become evident with regard to their specific rate of systemic toxicity. The present article provides a concise review of the toxic potencies of levobupivacaine and ropivacaine.

RECENT FINDINGS

As lipophilicity is known to be a major determinant in local anesthetic toxicity, the clinical safety profile of ropivacaine seems to be more favorable than that of levobupivacaine. Experimental studies and case reports confirm this hypothesis, showing that ropivacaine is characterized by fewer (cardio) toxic effects and, most probably, a greater margin of safety. Both agents also may dose dependently damage neurons and skeletal muscle tissue at the injection site. Although their specific rate of neurotoxicity appears to be rather low, levobupivacaine is characterized by an outstanding myotoxic potential.

SUMMARY

Compared with bupivacaine, both agents may be considered as 'more well tolerated' but not as 'totally well tolerated', as they are still capable of inducing systemic and local toxicity. However, ropivacaine seems to have the greatest margin of safety of all long-acting local anesthetics at present.

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.

A novel conotoxin from Conus striatus, mu-SIIIA, selectively blocking rat tetrodotoxin-resistant sodium channels

Mu-conotoxin SIIIA, a novel blocker of tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels (VGSCs) has been identified from the fish-hunting cone snail, Conus striatus. The deduced sequence consists of a 20-residue signal peptide, a 31-residue pro-peptide, and a 20-residue mature toxin with its N-terminal Gln cyclized and C-terminus amidated. Mu-SIIIA shares the common cysteine arrangement with other mu-conotoxins. Besides, it exhibits high sequence homology with mu-SmIIIA, a toxin recently characterized from C. stercusmuscarum which potently blocks the TTX-R VGSCs in frog neurons. With whole-cell recording, mu-SIIIA potently and selectively inhibits the TTX-R VGSCs of dissociated adult rat small-diameter dorsal root ganglia (DRG) neurons with a dose- and time-dependent property and irreversibly. Homology-based modeling of mu-PIIIA, SIIIA and SmIIIA implies that they share a common backbone conformation except at the N termini. The hydroxyl-proline residue only present in mu-PIIIA is absent and substituted by an Asp residue in mu-SIIIA and SmIIIA. Similarly, one crucial basic residue (Arg12 in mu-PIIIA) is replaced by serine in the latter two toxins. Such differences might endow them with the capacity to selectively inhibit TTX-S or TTX-R VGSCs. Considering that TTX-R VGSCs predominantly expressed in DRG neurons play pivotal roles in the nociceptive information transmission and that their specific antagonists are still lacking, mu-SIIIA might provide a useful tool for functional studies of these channels, and potentially be developed as an efficient pain killer.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Bupivacaine, levobupivacaine and ropivacaine: are they clinically different?

Two new, long-acting local anaesthetics have been developed after the evidence of bupivacaine-related severe toxicity: levobupivacaine and ropivacaine. Both these agents are pure left-isomers and, based on their three-dimensional structure, they have less toxic potential both on the central nervous system and on the heart. Several clinical studies have evaluated their toxicology and clinical profiles: theoretically and experimentally, some differences can be seen, but the reflections of these characteristics into clinical practice have not been evident. Evaluating randomised, controlled trials that have compared these three local anaesthetics, this chapter supports the evidence that both levobupivacaine and ropivacaine have a clinical profile similar to that of racemic bupivacaine, and that the minimal differences observed between the three agents are mainly related to the slightly different anaesthetic potency, with racemic bupivacaine>levobupivacaine>ropivacaine. However, the reduced toxic potential of the two pure left-isomers supports their use in those clinical situations in which the risk of systemic toxicity related to either overdosing or unwanted intravascular injection is high, such as during epidural or peripheral nerve blocks.

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.