Voltage-dependent blockade of normal and mutant muscle sodium channels by benzylalcohol

Tolerance to anesthesia depends on synaptic proteins

The hypnotic effects of anesthetics are caused by their interactions with neuronal components vital for proper signaling. An understanding of the adaptive mechanisms that lead to the development of anesthetic tolerance can offer insight into the regulation of neuroexcitability and plasticity that alter behavioral output. Here we use genetic and pharmacological manipulation of Drosophila to investigate the mechanisms of tolerance to benzyl alcohol. The mutants tested were temperature-sensitive paralytics that interfere with neuronal signaling: two mutations in dynamin that affect vesicle recycling, shi (ts1) and shi (ts2), and one that affects the voltage-activated Na(+) channel, para (ts1). We also used N-ethylmaleimide (NEM) to pharmacologically interfere with synaptic function. We found that blocking the generation of action potentials using a temperature-sensitive paralytic mutation does not induce nor prevent the development of functional tolerance to benzyl alcohol, but that disruption of synaptic signaling using mutations in the dynamin gene or by NEM treatment inhibits the induction of tolerance.

Binding of sodium channel inhibitors to hyperpolarized and depolarized conformations of the channel

Sodium channels are inhibited by a chemically diverse group of compounds. In the last decade entirely new structural classes with superior properties have been discovered, and novel therapeutic uses of sodium channel inhibitors (SCIs) have been suggested. Many promising novel drug candidates have been described and characterized. Published structure-activity relationship studies, pharmacophore models, and mutagenesis studies seem to lag behind, dealing with only a limited group of inhibitor compounds. The abundance of novel compounds requires an organized comparison of drug potencies. The affinity of sodium channel inhibitors can vary typically ten- to thousand-fold depending on the voltage protocol; therefore comparison of electrophysiology data is difficult. In this study we describe a method for standardization of these data with the help of a simple model of state-dependence. We derived hyperpolarized (resting) and depolarized (generally termed "inactivated") state affinities for the studied drugs, which made the measurements comparable. We show a rank order of SCIs based on resting and inactivated affinity values. In an attempt to define basic chemical requirements for sodium channel inhibitor activity we investigated the dependence of both resting and inactivated state affinities on individual chemical descriptors. Lipophilicity (most often expressed by the logP value) is the single most important determinant of SCI potency. We investigated the independent impact of several other calculated chemical properties by standardizing drug potencies for logP values. By combining these two approaches: standardization of affinity values, and standardization of potencies, we concluded that while resting affinity is mostly determined by lipophilicity, inactivated state affinity is determined by a more complex interaction of chemical properties, including hydrogen bond acceptors, aromatic rings, and molecular weight.

Topical antiseptics for the treatment of sore throat block voltage-gated neuronal sodium channels in a local anaesthetic-like manner

Lozenges for the treatment of sore throat provide relief of discomfort in cases of oral inflammation. This effect has not been fully explained so far. Here, we have examined the proposition that key components of pharmaceutical preparations for the treatment of sore throat which are routinely regarded antiseptics might have sodium channel-blocking, i.e. local anaesthetic-like effects. We investigated the effects of hexylresorcinol, amylmetacresol and dichloro-benzylalcohol on voltage-operated neuronal (Na(V)1.2) sodium channels heterologously expressed in HEK 293 cells in vitro. Hexylresorcinol, amylmetacresol and dichloro-benzylalcohol reversibly blocked depolarisation-induced whole-cell sodium inward currents. The half-maximum blocking concentrations (EC(50)) at -150 mV were 23.1, 53.6 and 661.6 microM, respectively. Block induced by hexylresorcinol and amylmetacresol was increased at depolarised potentials and use-dependent during trains of depolarisations applied at high frequency (100 Hz) indicating that both drugs bind more tightly to inactivated conformations of the channel. Estimates for the inactivated state affinity were 1.88 and 35 microM for hexylresorcinol and amylmetacresol, respectively. Hexylresorcinol and amylmetacresol are 10-20 fold more potent than the local anaesthetic lidocaine in blocking sodium inward current. Both drugs show an increased effect at depolarised membrane potentials or in conditions of high-frequency discharges.

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.

N-salicyloyltryptamine, a new anticonvulsant drug, acts on voltage-dependent Na+, Ca2+, and K+ ion channels

1. The aim of this work was to study the effects of N-salicyloyltryptamine (STP), a novel anticonvulsant agent, on voltage-gated ion channels in GH3 cells. 2. In this study, we show that STP at 17 microM inhibited up to 59.2+/-10.4% of the Ito and 73.1+/-8.56% of the IKD K+ currents in GH3 cells. Moreover, the inhibitory activity of the drug STP on K+ currents was dose-dependent (IC50=34.6+/-8.14 microM for Ito) and partially reversible after washing off. 3. Repeated stimulation at 1 Hz (STP at 17 microM) led to the total disappearance of Ito current, and an enhancement of IKD. 4. In the cell-attached configuration, application of STP to the bath increased the open probability of large-conductance Ca2+-activated K+ channels. 5. STP at 17 microM inhibited the L-type Ca2+ current by 54.9+/-7.50% without any significant changes in the voltage dependence. 6. STP at 170 microM inhibited the TTX-sensitive Na+ current by 22.1+/-2.41%. At a lower concentration (17 microM), no effect on INa was observed. 7. The pharmacological profile described here might contribute to the neuroprotective effect exerted by this compound in experimental 'in vivo' models.

Lidocaine and octanol have different modes of action at tetrodotoxin-resistant Na(+) channels of peripheral nerves

Local anesthetics and alcohols block impulse conduction in peripheral nerves by inhibiting Na(+) currents. In small peripheral nerve fibers, tetrodotoxin-resistant (TTX-r) Na(+) channels play an important role in impulse generation. We investigated the effects of lidocaine and the alcohol octanol on TTX-r Na(+) channels. Currents were recorded with the whole-cell patch-clamp method from enzymatically isolated rat dorsal root ganglion cells (data evaluation: nonlinear least-squares fitting). Lidocaine and octanol blocked the TTX-r Na(+) current in a reversible and concentration-dependent manner (50% inhibitory concentration values: 177 +/- 25 and 455 +/- 25 microM, respectively). Lidocaine additionally produced a strong use-dependent block. Both drugs showed a strong dynamic block (i.e., block developed during the time course of current activation and inactivation). Double-pulse protocols showed a slow dissociation of lidocaine from the channel during repolarization (time constant: 1763 +/- 63 ms; 300 microM). The dissociation of octanol was too quick to be distinguished from normal current repriming kinetics of 2.2 ms. Lidocaine and octanol acted noncompetitively in the Na(+) channel. Lidocaine and octanol have different blocking properties on the TTX-r Na(+) current and bind to different channel sites.

IMPLICATIONS

Lidocaine and octanol have different inhibitory effects on the function of tetrodotoxin-resistant Na(+) channels in rat dorsal root ganglion cells, as well as noncompetitive modes of action, as investigated by the whole-cell patch-clamp method, and therefore are likely to have different binding sites on the channel.

Structural requirements of phenol derivatives for direct activation of chloride currents via GABA(A) receptors

Propofol directly activates gamma-aminobutyric acid (GABA(A)) receptors in the absence of the natural agonist. This mechanism is supposed to contribute to its sedative-hypnotic actions. We studied the effects of seven structurally related phenol derivatives on chloride inward currents via rat alpha1beta2gamma2 GABA(A) receptors, heterologously expressed in HEK 293 cells in order to find structural determinants for this direct agonistic action. Only compounds with the phenolic hydroxyl attached directly to the benzene ring and with aliphatic substituents in ortho position to the phenolic hydroxyl activated chloride currents in the absence of GABA. Concentrations required for half-maximum effect were 980 microM for 2-methylphenol, 230 microM for 2,6-dimethylphenol, 200 microM for thymol, and 23 microM for propofol. Drug-induced chloride currents showed no desensitisation during the 2-s application. These results show that the position of the aliphatic substituents with respect to the phenolic hydroxyl group is the crucial structural feature for direct GABA(A) activation by phenol derivatives.

Structural requirements for voltage-dependent block of muscle sodium channels by phenol derivatives

We have studied the effects of four different phenol derivatives, with methyl and halogen substituents, on heterologously expressed human skeletal muscle sodium channels, in order to find structural determinants of blocking potency. All compounds blocked skeletal muscle sodium channels in a concentration-dependent manner. The methylated phenol 3-methylphenol and the halogenated phenol 4-chlorophenol blocked sodium currents on depolarization from -100 mV to 0 mV with IC(50) values of 2161 and 666 microM respectively. Methylation of the halogenated compound further increased potency, reducing the IC(50) to 268 microM in 2-methyl-4-chlorophenol and to 150 microM in 3,5-dimethyl-4-chlorophenol. Membrane depolarization before the test depolarization increased sodium channel blockade. When depolarizations were started from -70 mV or when a 2.5 s prepulse was introduced before the test pulse inducing slow inactivation, the IC(50) was reduced more than 3 fold in all compounds. The values of K(D) for the fast-inactivated state derived from drug-induced shifts in steady-state availability curves were 14 microM for 3,5-dimethyl-4-chlorophenol, 19 microM for 2-methyl-4-chlorophenol, 26 microM for 4-chlorophenol and 115 microM for 3-methylphenol. All compounds accelerated the current decay during depolarization and slowed recovery from fast inactivation. No relevant frequency-dependent block after depolarizing pulses applied at 10, 50 and 100 Hz was detected for any of the compounds. All the phenol derivatives that we examined are effective blockers of skeletal muscle sodium channels, especially in conditions that are associated with membrane depolarization. Blocking potency is increased by halogenation and by methylation with increasing numbers of methyl groups.