Point mutations at L1280 in Nav1.4 channel D3-S6 modulate binding affinity and stereoselectivity of bupivacaine enantiomers

An aptamer-based depot system for sustained release of small molecule therapeutics

Delivery of hydrophilic small molecule therapeutics by traditional drug delivery systems is challenging. Herein, we have used the specific interaction between DNA aptamers and drugs to create simple and effective drug depot systems. The specific binding of a phosphorothioate-modified aptamer to drugs formed non-covalent aptamer/drug complexes, which created a sustained release system. We demonstrated the effectiveness of this system with small hydrophilic molecules, the site 1 sodium channel blockers tetrodotoxin and saxitoxin. The aptamer-based delivery system greatly prolonged the duration of local anesthesia and reduced systemic toxicity. The beneficial effects of the aptamers were restricted to the compounds they were specific to. These studies establish aptamers as a class of highly specific, modifiable drug delivery systems, and demonstrate potential usefulness in the management of postoperative pain.

Delivery of local anaesthetics by a self-assembled supramolecular system mimicking their interactions with a sodium channel

Site-1 sodium channel blockers (S1SCBs) act as potent local anaesthetics, but they can cause severe systemic toxicity. Delivery systems can be used to reduce the toxicity, but the hydrophilicity of S1SCBs makes their encapsulation challenging. Here, we report a self-assembling delivery system for S1SCBs whose design is inspired by the specific interactions of S1SCBs with two peptide sequences on the sodium channel. Specifically, the peptides were modified with hydrophobic domains so that they could assemble into nanofibres that facilitated specific binding with the S1SCBs tetrodotoxin, saxitoxin and dicarbamoyl saxitoxin. Injection of S1SCB-carrying nanofibres at the sciatic nerves of rats led to prolonged nerve blockade and to reduced systemic toxicity, with benign local-tissue reaction. The strategy of mimicking a molecular binding site via supramolecular interactions may be applicable more broadly to the design of drug delivery systems for receptor-mediated drugs.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium and calcium channels are evolutionarily related transmembrane signaling proteins that initiate action potentials, neurotransmission, excitation-contraction coupling, and other physiological processes. Genetic or acquired dysfunction of these proteins causes numerous diseases, termed channelopathies, and sodium and calcium channels are the molecular targets for several major classes of drugs. Recent advances in the structural biology of these proteins using X-ray crystallography and cryo-electron microscopy have given new insights into the molecular basis for their function and pharmacology. Here we review this recent literature and integrate findings on sodium and calcium channels to reveal the structural basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity, and complex pharmacology at the atomic level. We conclude with the theme that new understanding of the diseases and therapeutics of these channels will be derived from application of the emerging structural principles from these recent structural analyses.

Structural basis for antiarrhythmic drug interactions with the human cardiac sodium channel

Voltage-gated sodium channels play a central role in cellular excitability and are key targets for drug development. Recent breakthroughs in high-resolution cryo-electron microscopy protein structure determination, Rosetta computational protein structure modeling, and multimicrosecond molecular dynamics simulations are empowering advances in structural biology to study the atomistic details of channel−drug interactions. We used Rosetta structural computational modeling and molecular dynamics simulations to study the interactions of antiarrhythmic and local anesthetic drugs with cardiac sodium channel. Our results provide crucial atomic-scale mechanistic insights into the channel–drug interactions, necessary for the rational design of novel modulators of the human cardiac sodium channel to be used for the treatment of cardiac arrhythmias.

The human voltage-gated sodium channel, hNa_V1.5, is responsible for the rapid upstroke of the cardiac action potential and is target for antiarrhythmic therapy. Despite the clinical relevance of hNa_V1.5-targeting drugs, structure-based molecular mechanisms of promising or problematic drugs have not been investigated at atomic scale to inform drug design. Here, we used Rosetta structural modeling and docking as well as molecular dynamics simulations to study the interactions of antiarrhythmic and local anesthetic drugs with hNa_V1.5. These calculations revealed several key drug binding sites formed within the pore lumen that can simultaneously accommodate up to two drug molecules. Molecular dynamics simulations identified a hydrophilic access pathway through the intracellular gate and a hydrophobic access pathway through a fenestration between DIII and DIV. Our results advance the understanding of molecular mechanisms of antiarrhythmic and local anesthetic drug interactions with hNa_V1.5 and will be useful for rational design of novel therapeutics.

Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.

Structural modeling of human cardiac sodium channel pore domain

The pore domain of human voltage-dependent cardiac sodium channel Na_v1.5 (hNa_v1.5) is the crucial binding targets for anti-arrhythmics drugs and some local anesthetic drugs but its three-dimensional structure is still lacking. This has affected the detailed studies of the binding features and mechanism of these drugs. In this paper, we present a structural model for open-state pore domain of hNa_v1.5 built using single template ROSETTA-membrane homology modeling with the crystal structure of Na_vMs. The assembled structural models are evaluated by rosettaMP energy and locations of binding sites. The modeled structures of the pore domain of hNa_v1.5 in open state will be helpful to explore molecular mechanism of a state-dependent drug binding and help designing new drugs.

Inhibition of Sodium Ion Channel Function with Truncated Forms of Batrachotoxin

A novel family of small molecule inhibitors of voltage-gated sodium channels (Na_Vs) based on the structure of batrachotoxin (BTX), a well-known channel agonist, is described. Protein mutagenesis and electrophysiology experiments reveal the binding site as the inner pore region of the channel, analogous to BTX, alkaloid toxins, and local anesthetics. Homology modeling of the eukaryotic channel based on recent crystallographic analyses of bacterial Na_Vs suggests a mechanism of action for ion conduction block.

Bupivacaine inhibits endothelin-1-evoked increases in intracellular calcium in model sensory neurons

BACKGROUND

Endothelin-1 (ET-1) induces pain-like behavior in animals and man by activating the Gq protein-coupled receptor endothelin-A (ETA ). Activation of ETA receptors on nociceptor membranes evokes intracellular calcium transients and alters membrane Na(+) and K(+) channel and TRPV1 currents, leading to neuronal hyper-excitability manifested by spontaneous and evoked pain behaviors in vivo. In addition to blocking sodium channels, local anesthetics inhibit the Gq protein-coupled signaling of several inflammatory and pro-algesic mediators. In this study, we aimed to investigate the actions of local anesthetics on ETA -mediated increases in intracellular calcium in ND7/104 model sensory neurons.

METHODS

Increases in intracellular calcium were measured by the fluorescent indicator fura-2 in a sensory neuron-derived cell line (ND7/104), which endogenously expresses ETA receptors. Effects of lidocaine and bupivacaine, along with their respective membrane-impermeant derivatives QX-314, LEA-123 and LEA-124, on peak calcium responses to ET-1 were measured.

RESULTS

Bupivacaine suppressed ET-1 responses in a concentration-dependent and non-competitive manner with an IC50 of 3.79 ± 1.63 mM. Bupivacaine (6 mM) reduced the Emax for ET-1 from 50.07 ± 1.91 mM to 27.30 ± 2.92 mM. The actions of bupivacaine occurred quickly and were rapidly reversible. Membrane-impermeant analogs of bupivacaine (LEA-123 and LEA-124, 6 mM) were without effect, as was lidocaine (10 mM) and its quaternary derivative QX-314 (10 mM).

CONCLUSION

Bupivacaine inhibits ETA -mediated calcium transients at clinically relevant concentrations through an intracellular target. The anti-inflammatory and analgesic actions of bupivacaine may be at least partially due to its inhibitory action on Gq -coupled receptors, including ETA.

Voltage-dependent blockade by bupivacaine of cardiac sodium channels expressed in Xenopus oocytes

Bupivacaine ranks as the most potent and efficient drug among class I local anesthetics, but its high potential for toxic reactions severely limits its clinical use. Although bupivacaine-induced toxicity is mainly caused by substantial blockade of voltage-gated sodium channels (VGSCs), how these hydrophobic molecules interact with the receptor sites to which they bind remains unclear. Nav1.5 is the dominant isoform of VGSCs expressed in cardiac myocytes, and its dysfunction may be the cause of bupivacaine-triggered arrhythmia. Here, we investigated the effect of bupivacaine on Nav1.5 within the clinical concentration range. The electrophysiological measurements on Nav1.5 expressed in Xenopus oocytes showed that bupivacaine induced a voltage- and concentration-dependent blockade on the peak of I Na and the half-maximal inhibitory dose was 4.51 μmol/L. Consistent with other local anesthetics, bupivacaine also induced a use-dependent blockade on Nav1.5 currents. The underlying mechanisms of this blockade may contribute to the fact that bupivacaine not only dose-dependently affected the gating kinetics of Nav1.5 but also accelerated the development of its open-state slow inactivation. These results extend our knowledge of the action of bupivacaine on cardiac sodium channels, and therefore contribute to the safer and more efficient clinical use of bupivacaine.

Combined modifications of mexiletine pharmacophores for new lead blockers of Na(v)1.4 channels

Previously identified potent and/or use-dependent mexiletine (Mex) analogs were used as template for the rational design of new Na(v)-channel blockers. The effects of the novel analogs were tested on sodium currents of native myofibers. Data and molecular modeling show that increasing basicity and optimal alkyl chain length enhance use-dependent block. This was demonstrated by replacing the amino group with a more basic guanidine one while maintaining a proper distance between positive charge and aromatic ring (Me13) or with homologs having the chirality center nearby the amino group or the aromatic ring. Accordingly, a phenyl group on the asymmetric center in the homologated alkyl chain (Me12), leads to a further increase of use-dependent behavior versus the phenyl Mex derivative Me4. A fluorine atom in paraposition and one ortho-methyl group on the xylyloxy ring (Me15) increase potency and stereoselectivity versus Me4. Charge delocalization and greater flexibility of Me15 may increase its affinity for Tyr residues influencing steric drug interaction with the primary Phe residue of the binding site. Me12 and Me15 show limited selectivity against Na(v)-isoforms, possibly due to the highly conserved binding site on Na(v). To our knowledge, the new compounds are the most potent Mex-like Na(v) blockers obtained to date and deserve further investigation.

Lidocaine reduces the transition to slow inactivation in Na(v)1.7 voltage-gated sodium channels

BACKGROUND AND PURPOSE

The primary use of local anaesthetics is to prevent or relieve pain by reversibly preventing action potential propagation through the inhibition of voltage-gated sodium channels. The tetrodotoxin-sensitive voltage-gated sodium channel subtype Na(v)1.7, abundantly expressed in pain-sensing neurons, plays a crucial role in perception and transmission of painful stimuli and in inherited chronic pain syndromes. Understanding the interaction of lidocaine with Na(v)1.7 channels could provide valuable insight into the drug's action in alleviating pain in distinct patient populations. The aim of this study was to determine how lidocaine interacts with multiple inactivated conformations of Na(v)1.7 channels.

EXPERIMENTAL APPROACH

We investigated the interactions of lidocaine with wild-type Na(v)1.7 channels and a paroxysmal extreme pain disorder mutation (I1461T) that destabilizes fast inactivation. Whole cell patch clamp recordings were used to examine the activity of channels expressed in human embryonic kidney 293 cells.

KEY RESULTS

Depolarizing pulses that increased slow inactivation of Na(v)1.7 channels also reduced lidocaine inhibition. Lidocaine enhanced recovery of Na(v)1.7 channels from prolonged depolarizing pulses by decreasing slow inactivation. A paroxysmal extreme pain disorder mutation that destabilizes fast inactivation of Na(v)1.7 channels decreased lidocaine inhibition.

CONCLUSIONS AND IMPLICATIONS

Lidocaine decreased the transition of Na(v)1.7 channels to the slow inactivated state. The fast inactivation gate (domain III-IV linker) is important for potentiating the interaction of lidocaine with the Na(v)1.7 channel.

Analysis of the action of lidocaine on insect sodium channels

A new class of sodium channel blocker insecticides (SCBIs), which include indoxacarb, its active metabolite, DCJW, and metaflumizone, preferably block inactivated states of both insect and mammalian sodium channels in a manner similar to that by which local anesthetic (LA) drugs block mammalian sodium channels. A recent study showed that two residues in the cockroach sodium channel, F1817 and Y1824, corresponding to two key LA-interacting residues identified in mammalian sodium channels are not important for the action of SCBIs on insect sodium channels, suggesting unique interactions of SCBIs with insect sodium channels. However, the mechanism of action of LAs on insect sodium channels has not been investigated. In this study, we examined the effects of lidocaine on a cockroach sodium channel variant, BgNa(v)1-1a, and determined whether F1817 and Y1824 are also critical for the action of LAs on insect sodium channels. Lidocaine blocked BgNa(v)1-1a channels in the resting state with potency similar to that observed in mammalian sodium channels. Lidocaine also stabilized both fast-inactivated and slow-inactivated states of BgNa(v)1-1a channels, and caused a limited degree of use- and frequency-dependent block, major characteristics of LA action on mammalian sodium channels. Alanine substitutions of F1817 and Y1824 reduced the sensitivity of the BgNa(v)1-1a channel to the use-dependent block by lidocaine, but not to tonic blocking and inactivation stabilizing effects of lidocaine. Thus, similar to those on mammalian sodium channels, F1817 and Y1824 are important for the action of lidocaine on cockroach sodium channels. Our results suggest that the receptor sites for lidocaine and SCBIs are different on insect sodium channels.

Sodium channel molecular conformations and antiarrhythmic drug affinity

Class I cardiac antiarrhythmic drugs, for example, lidocaine, mexiletine, flecainide, quinidine, and procainamide, continue to play an important role in the therapy for cardiac arrhythmias because of the presence of use-dependent block. Lidocaine, as well as related drugs such as mepivacaine, bupivacaine, and cocaine, also belong to the class of medications referred to as local anesthetics. In this review, we will consider lidocaine as the prototypical antiarrhythmic drug because it continues to be widely used both as an antiarrhythmic drug (first used as an antiarrhythmic drug in 1950) as well as a local anesthetic agent. Both of these clinical uses depend upon block of sodium current (I(Na)), but it is the presence of use-dependent I(Na) block, that is, an increasing amount of block at faster heart rates, which enables a local anesthetic agent to be a useful antiarrhythmic drug. Although many early studies investigated the action of antiarrhythmic drugs on Na currents, the availability of site-directed mutant Na channels has enabled for major advances in understanding their mechanisms of action based upon molecular conformations of the Na channel.

Lidocaine partially depolarizes the S4 segment in domain IV of the sodium channel

Previous studies have shown that lidocaine and other local anesthetic drugs (LAs) cause use-dependent block of sodium current (I (Na)), i.e., block that increases with membrane depolarization by allosteric coupling between drug binding in the inner pore and the S4s in domains III and IV. MTSET protection experiments have established that LAs stabilize DIIIS4 in an outward, depolarized position. Similar tests have not been reported for the DIVS4, although LAs have been shown to reduce DIV's contribution to total gating charge by about one third and to alter its movement such that it contributes more gating charge at negative potentials around -100 mV compared to non-drug-bound sodium (Na) channels. To investigate whether lidocaine reduces the gating charge of DIVS4 by causing it to adopt either a depolarized position at rest or by restricting its outward movement upon depolarization, we performed MTSET protection experiments on I (Na) of the mutant Na channel, R1628C (R3C-DIV), in the presence and absence of 10 mM lidocaine. The results indicate that lidocaine causes the DIVS4 to assume a more depolarized position, which facilitates its movement upon depolarization leading to the excess gating charge at potentials near -100 mV.

Block of sensory neuronal Na+ channels by the secreolytic ambroxol is associated with an interaction with local anesthetic binding sites

Voltage-gated Na(+) channels (Na(v)) regulate the excitability of sensory neurons and are potential targets for novel analgesics. The secreolytic ambroxol reduces pain-related behavior in rodents and alleviates pain in humans. With properties resembling those of local anesthetics, ambroxol has been reported to block Na(+) currents in sensory neurons with a preference for tetrodotoxin-resistant (TTXr) Na(+) currents encoded by Na(v)1.8. However, the molecular determinants for ambroxol-induced block of Na(+) channels and a preferential block of Na(v)1.8 opposed to tetrodotoxin-sensitive (TTXs) Na(v) alpha-subunits have not been studied in detail. By means of whole-cell voltage clamp recordings, we studied the effects of ambroxol and local anesthetics on the recombinant TTXr subunit Na(v)1.8, on TTXs Na(v) alpha-subunits and on mutants of Na(v)1.4 that are insensitive to local anesthetics. Tonic and use-dependent block by ambroxol was strongly alleviated in local anesthetic-insensitive Na(v)1.4 mutants. Use-dependent block, but not tonic block was significantly stronger on Na(v)1.8 than on TTXs channels. The TTXs subunit Na(v)1.3 displayed the least degree of use-dependent block by ambroxol. The local anesthetics mepivacaine and S(-)-bupivacaine also blocked Na(v)1.8 and TTXs channels differentially. While mepivacaine displayed a preferential use-dependent block of Na(v)1.8, S(-)-bupivacaine displayed a preference for TTXs Na(+) channels. Our data show that ambroxol acts as a typical local anesthetic on Na(+) channels interacting with specific residues in the S6 segments. This property probably meditates the analgesic effect of ambroxol. Ambroxol preferentially blocks Na(v)1.8, however shares this property with established local anesthetics like mepivacaine.

Using lidocaine and benzocaine to link sodium channel molecular conformations to state-dependent antiarrhythmic drug affinity

RATIONALE

Lidocaine and other antiarrhythmic drugs bind in the inner pore of voltage-gated Na channels and affect gating use-dependently. A phenylalanine in domain IV, S6 (Phe1759 in Na(V)1.5), modeled to face the inner pore just below the selectivity filter, is critical in use-dependent drug block.

OBJECTIVE

Measurement of gating currents and concentration-dependent availability curves to determine the role of Phe1759 in coupling of drug binding to the gating changes.

METHODS AND RESULTS

The measurements showed that replacement of Phe1759 with a nonaromatic residue permits clear separation of action of lidocaine and benzocaine into 2 components that can be related to channel conformations. One component represents the drug acting as a voltage-independent, low-affinity blocker of closed channels (designated as lipophilic block), and the second represents high-affinity, voltage-dependent block of open/inactivated channels linked to stabilization of the S4s in domains III and IV (designated as voltage-sensor inhibition) by Phe1759. A homology model for how lidocaine and benzocaine bind in the closed and open/inactivated channel conformation is proposed.

CONCLUSIONS

These 2 components, lipophilic block and voltage-sensor inhibition, can explain the differences in estimates between tonic and open-state/inactivated-state affinities, and they identify how differences in affinity for the 2 binding conformations can control use-dependence, the hallmark of successful antiarrhythmic drugs.

Comparative effects of halogenated inhaled anesthetics on voltage-gated Na+ channel function

BACKGROUND

Inhibition of voltage-gated Na channels (Na(v)) is implicated in the synaptic actions of volatile anesthetics. We studied the effects of the major halogenated inhaled anesthetics (halothane, isoflurane, sevoflurane, enflurane, and desflurane) on Na(v)1.4, a well-characterized pharmacological model for Na(v) effects.

METHODS

Na currents (I(Na)) from rat Na(v)1.4 alpha-subunits heterologously expressed in Chinese hamster ovary cells were analyzed by whole cell voltage-clamp electrophysiological recording.

RESULTS

Halogenated inhaled anesthetics reversibly inhibited Na(v)1.4 in a concentration- and voltage-dependent manner at clinical concentrations. At equianesthetic concentrations, peak I(Na) was inhibited with a rank order of desflurane > halothane approximately enflurane > isoflurane approximately sevoflurane from a physiologic holding potential (-80 mV). This suggests that the contribution of Na channel block to anesthesia might vary in an agent-specific manner. From a hyperpolarized holding potential that minimizes inactivation (-120 mV), peak I(Na) was inhibited with a rank order of potency for tonic inhibition of peak I(Na) of halothane > isoflurane approximately sevoflurane > enflurane > desflurane. Desflurane produced the largest negative shift in voltage-dependence of fast inactivation consistent with its more prominent voltage-dependent effects. A comparison between isoflurane and halothane showed that halothane produced greater facilitation of current decay, slowing of recovery from fast inactivation, and use-dependent block than isoflurane.

CONCLUSIONS

Five halogenated inhaled anesthetics all inhibit a voltage-gated Na channel by voltage- and use-dependent mechanisms. Agent-specific differences in efficacy for Na channel inhibition due to differential state-dependent mechanisms creates pharmacologic diversity that could underlie subtle differences in anesthetic and nonanesthetic actions.

Structural determinants of drugs acting on the Nav1.8 channel

The aim of this work is to study the role of pore residues on drug binding in the Na(V)1.8 channel. Alanine mutations were made in the S6 segments, chosen on the basis of their roles in other Na(V) subtypes; whole cell patch clamp recordings were made from mammalian ND7/23 cells. Mutations of some residues caused shifts in voltage dependence of activation and inactivation, and gave faster time course of inactivation, indicating that the residues mutated play important roles in both activation and inactivation in the Na(V)1.8 channel. The resting and inactivated state affinities of tetracaine for the channel were reduced by mutations I381A, F1710A, and Y1717A (for the latter only inactivated state affinity was measured), and by mutation F1710A for the Na(V)1.8-selective compound A-803467, showing the involvement of these residues for each compound, respectively. For both compounds, mutation L1410A caused the unexpected appearance of a complete resting block even at extremely low concentrations. Resting block of native channels by compound A-803467 could be partially removed ("disinhibition") by repetitive stimulation or by a test pulse after recovery from inactivation; the magnitude of the latter effect was increased for all the mutants studied. Tetracaine did not show this effect for native channels, but disinhibition was seen particularly for mutants L1410A and F1710A. The data suggest differing, but partially overlapping, areas of binding of A-803467 and tetracaine. Docking of the ligands into a three-dimensional model of the Na(V)1.8 channel gave interesting insight as to how the ligands may interact with pore residues.

Access and binding of local anesthetics in the closed sodium channel

Local anesthetics (LAs) are known to bind Na+ channels in the closed, open, and inactivated states and reach their binding sites via extracellular and intracellular access pathways. Despite intensive studies, no atomic-scale theory is available to explain the diverse experimental data on the LA actions. Here we attempt to contribute to this theory by simulating access and binding of LAs in the KcsA-based homology model of the closed Na+ channel. We used Monte Carlo minimizations to model the channel with representative local anesthetics N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium (QX-314), cocaine, and tetracaine. We found the nucleophilic central cavity to be a common binding region for the ammonium group of LAs, whose aromatic group can extend either along the pore axis (vertical binding mode) or to the III/IV domain interface (horizontal binding mode). The vertical mode was earlier predicted for the open channel, but only the horizontal mode is consistent with mutational data on the closed-channel block. To explore hypothetical access pathways of the permanently charged QX-314, we pulled the ligand via the selectivity filter, the closed activation gate, and the III/IV domain interface. Only the last pathway, which leads to the horizontal binding mode, did not impose steric obstacles. The LA ammonium group mobility within the central cavity was more restricted in the vertical mode than in the horizontal mode. Therefore, occupation of the selectivity-filter DEKA locus by a Na+ ion destabilizes the vertical mode, thus favoring the horizontal mode. LA binding in the closed channel requires the resident Na+ ion to leave the nucleophilic central cavity through the selectivity filter, whereas the LA egress should be coupled with reoccupation of the cavity by Na+. This hypothesis on the coupled movement of Na+ and LA in the closed channel explains seemingly contradictory data on how the outer-pore mutations as well as tetrodotoxin and micro-conotoxin binding affect the ingress and egress of LAs.

Sodium channels: ionic model of slow inactivation and state-dependent drug binding

Inactivation is a fundamental property of voltage-gated ion channels. Fast inactivation of Na(+) channels involves channel block by the III-IV cytoplasmic interdomain linker. The mechanisms of nonfast types of inactivation (intermediate, slow, and ultraslow) are unclear, although the ionic environment and P-loops rearrangement appear to be involved. In this study, we employed a TTX-based P-loop domain model of a sodium channel and the MCM method to investigate a possible role of P-loop rearrangement in the nonfast inactivation. Our modeling predicts that Na(+) ions can bind between neighboring domains in the outer-carboxylates ring EEDD, forming an ordered structure with interdomain contacts that stabilize the conducting conformation of the outer pore. In this model, the permeant ions can transit between the EEDD ring and the selectivity filter ring DEKA, retaining contacts with at least two carboxylates. In the absence of Na(+), the electrostatic repulsion between the EEDD carboxylates disrupts the permeable configuration. In this Na(+)-deficient model, the region between the EEDD and DEKA rings is inaccessible for Na(+) but is accessible for TMA. Taken together, these results suggest that Na(+)-saturated models are consistent with experimental characteristics of the open channels, whereas Na(+)-deficient models are consistent with experimentally defined properties of the slow-inactivated channels. Our calculations further predict that binding of LAs to the inner pore would depend on whether Na(+) occupies the DEKA ring. In the absence of Na(+) in the DEKA ring, the cationic group of lidocaine occurs in the focus of the pore helices' macrodipoles and would prevent occupation of the ring by Na(+). Loading the DEKA ring with Na(+) results in the electrostatic repulsion with lidocaine. Thus, there are antagonistic relations between a cationic ligand bound in the inner pore and Na(+) in the DEKA ring.

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.

Inhibition of Nav1.7 and Nav1.4 sodium channels by trifluoperazine involves the local anesthetic receptor

The calmodulin (CaM) inhibitor trifluoperazine (TFP) can produce analgesia when given intrathecally to rats; however, the mechanism is not known. We asked whether TFP could modulate the Na(v)1.7 sodium channel, which is highly expressed in the peripheral nervous system and plays an important role in nociception. We show that 500 nM and 2 muM TFP induce major decreases in Na(v)1.7 and Na(v)1.4 current amplitudes and that 2 muM TFP causes hyperpolarizing shifts in the steady-state inactivation of Na(v)1.7 and Na(v)1.4. CaM can bind to the C-termini of voltage-gated sodium channels and modulate their functional properties; therefore we investigated if TFP modulation of sodium channels was due to CaM inhibition. However, the TFP inhibition was not replicated by whole cell dialysis of a calmodulin inhibitory peptide, indicating that major effects of TFP do not involve a disruption of CaM-channel interactions. Rather, our data show that TFP inhibition is state dependent and that the majority of the TFP inhibition depends on specific amino-acid residues in the local anesthetic receptor site in sodium channels. TFP was also effective in vivo in causing motor and sensory blockade after subfascial injection to the rat sciatic nerve. The state-dependent block of Na(v)1.7 channels with nanomolar concentrations of TFP raises the possibility that TFP, or TFP analogues, might be useful for regional anesthesia and pain management and could be more potent than traditional local anesthetics.

Cardiac Na+ channels as therapeutic targets for antiarrhythmic agents

There are many factors that influence drug block of voltage-gated Na+ channels (VGSC). Pharmacological agents vary in conformation, charge, and affinity. Different drugs have variable affinities to VGSC isoforms, and drug efficacy is affected by implicit tissue properties such as resting potential, action potential morphology, and action potential frequency. The presence of polymorphisms and mutations in the drug target can also influence drug outcomes. While VGSCs have been therapeutic targets in the management of cardiac arrhythmias, their potential has been largely overshadowed by toxic side effects. Nonetheless, many VGSC blockers exhibit inherent voltage- and use-dependent properties of channel block that have recently proven useful for the diagnosis and treatment of genetic arrhythmias that arise from defects in Na+ channels and can underlie idiopathic clinical syndromes. These defective channels suggest themselves as prime targets of disease and perhaps even mutation specific pharmacological interventions.

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.

Molecular modeling of local anesthetic drug binding by voltage-gated sodium channels

Voltage-gated sodium (Na+) channels are targets for local anesthetic (LA) drugs that bind in the inner pore of the channel with affinities related to the channel gating states. Our core model of the sodium channel (P loops and S5 and S6 segments from each of the four domains) was closed because it was developed using coordinates from the KcsA channel crystallographic structure. We developed a model of the activated, open channel based on the structure of the open MthK channel, which was characterized by bends at the S6 glycine or serine residues. This created a conformation that allowed energetically appropriate docking of the LA drugs. The alkylamino head of ionizable LA molecules was docked closer to the selectivity filter and in association with Phe-1579 of IVS6 and Leu-1280 of IIIS6 (Nav1.4), and the aromatic ring interacted with Tyr-1586 of IVS6 and Asn-434 of IS6. Comparison of multiple LA drugs showed relative binding affinities in the model consistent with experimental studies. The ionizable LA alkylamino heads interact primarily by van der Waals forces that position the charge so as to create a positive electrostatic barrier for cation permeation. Permanently uncharged benzocaine could be docked in the closed conformation as well, stabilizing the closed conformation. The structurally different anticonvulsant lamotrigine and one of its derivatives have a binding site that fully overlaps with that of the LA drugs. The open, activated channel creates the high-affinity binding site for these sodium channel blocker drugs, and block may be mainly electrostatic.

Interactions of local anesthetics with voltage-gated Na+ channels

Voltage-gated Na+ channels are dynamic transmembrane proteins responsible for the rising phase of the action potential in excitable membranes. Local anesthetics (LAs) and structurally related antiarrhythmic and anticonvulsant compounds target specific sites in voltage-gated Na+ channels to block Na+ currents, thus reducing excitability in neuronal, cardiac, or central nervous tissue. A high-affinity LA block is produced by binding to open and inactivated states of Na+ channels rather than to resting states and suggests a binding site that converts from a low- to a high-affinity conformation during gating. Recent findings using site-directed mutagenesis suggest that multiple S6 segments together form an LA binding site within the Na+ channel. While the selectivity filter may form the more extracellular-located part of this binding site, the role of the fast inactivation gate in LA binding has not yet been resolved. The receptor of the neurotoxin batrachotoxin (BTX) is adjacent to or even overlaps with the LA binding site. The close proximity of the LA and BTX binding sites to residues critical for inactivation, together with gating transitions through S6 segments, might explain the strong impact of LAs and BTX on inactivation of voltage-gated Na+ channels and might help elucidate the mechanisms underlying voltage- and frequency-dependent LA block.

Block of inactivation-deficient Na+ channels by local anesthetics in stably transfected mammalian cells: evidence for drug binding along the activation pathway

According to the classic modulated receptor hypothesis, local anesthetics (LAs) such as benzocaine and lidocaine bind preferentially to fast-inactivated Na⁺ channels with higher affinities. However, an alternative view suggests that activation of Na⁺ channels plays a crucial role in promoting high-affinity LA binding and that fast inactivation per se is not a prerequisite for LA preferential binding. We investigated the role of activation in LA action in inactivation-deficient rat muscle Na⁺ channels (rNav1.4-L435W/L437C/A438W) expressed in stably transfected Hek293 cells. The 50% inhibitory concentrations (IC₅₀) for the open-channel block at +30 mV by lidocaine and benzocaine were 20.9 ± 3.3 μM (n = 5) and 81.7 ± 10.6 μM (n = 5), respectively; both were comparable to inactivated-channel affinities. In comparison, IC₅₀ values for resting-channel block at −140 mV were >12-fold higher than those for open-channel block. With 300 μM benzocaine, rapid time-dependent block (τ ≈ 0.8 ms) of inactivation-deficient Na⁺ currents occurred at +30 mV, but such a rapid time-dependent block was not evident at −30 mV. The peak current at −30 mV, however, was reduced more severely than that at +30 mV. This phenomenon suggested that the LA block of intermediate closed states took place notably when channel activation was slow. Such closed-channel block also readily accounted for the LA-induced hyperpolarizing shift in the conventional steady-state inactivation measurement. Our data together illustrate that the Na⁺ channel activation pathway, including most, if not all, transient intermediate closed states and the final open state, promotes high-affinity LA binding.

Effects of the local anesthetic benzocaine on the human erythrocyte membrane and molecular models

The interaction of the local anesthetic benzocaine 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 dimyristoylphospatidylcholine (DMPC), and phospholipid multilayers of DMPC and dimyristoylphospatidyletanolamine (DMPE), representatives of phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. Optical and scanning electron microscopy of human erythrocytes revealed that benzocaine induced the formation of echinocytes. Experiments performed on IUM and DMPC LUV by fluorescence spectroscopy showed that benzocaine interacted with the phospholipid bilayer polar groups and hydrophobic acyl chains. X-ray diffraction analysis of DMPC confirmed these results and showed that benzocaine had no effects on DMPE. The effect on sodium transport was also studied using the isolated toad skin. Electrophysiological measurements indicated a significant decrease in the potential difference (PD) and in the short-circuit current (Isc) after the application of benzocaine, reflecting inhibition of active ion transport.