Disruption of sodium channel gating kinetics

Action potentials (AP) are a temporary shift (from negative to positive) in the neuron’s membrane potential caused by ions flowing in and out of the neuron. During the resting state, before an action potential occurs, voltage-gated sodium and potassium channels are predominantly closed. These gated channels only open once when an action potential has been triggered. They are called ‘voltage-gated’ because they are open and close depending on the voltage difference across the cell membrane. VGSCs have two gates (gate m and gate h), while the potassium channel only has one (gate n). Gate m (the activation gate) is normally closed and opens when the cell membrane potential starts to get more positive (depolarizes). Gate h (the deactivation gate) is normally open, and swings shut when the cell membrane potential gets too positive. Gate n is normally closed, but slowly opens when the cell is depolarised (very positive). VGSCs exist in one of three states: Deactivated/closed (closed), activated (open) and inactivated (closed) – at rest, channels are (Figure 1) .

Modifications of the sodium channel gating have been studied using voltage and patch clamp experiments in different models (Ruigt et al., 1987). Prolongation of the sodium current is mainly due to the reduced rate of closure of a fraction of the sodium channel population and is characterized by a ‘tail current’. In neuroblastoma cell preparations, chemical stressors including deltamethrin and other type II pyrethroids, induce a slow tail current with a relatively long time constant. The rate at which sodium channels close during the pyrethroid-induced slow tail current depends not only on pyrethroid structure, but also on the duration of exposure, temperature and membrane potential (Ruigt et al., 1987; Narahashi., 2002; Soderlund., 2002).

Figure 1.  The three existing states of the VGSCs: Deactivated (closed), activated (open) and inactivated (closed). Figure extracted from Wakeling et al., 2012).  

Typically, VGSC function is measured using electrophysiological approaches, as only these have sufficient temporal resolution to evaluate channel function. Voltage-clamp techniques typically use two microelectrodes, allowing control of the membrane potential (‘clamping’) and recording of transmembrane currents that result from ion channel opening and closing (Guan et al., 2013). Pharmacological approaches and modifications of the ionic composition of the solution are used to isolate currents passing through VGSC from other types of current in the neuron.

In the patch-clamp technique, a highly sensitive version of the voltage-clamp technique, a single glass microelectrode is attached to a neuron to form a tight seal between the glass pipette tip and the cell membrane. In this case, a single electrode controls voltage and passes current (Molleman, 2003).  Typically, the current measured is the sum of currents flowing through the entire population of channels in this patch of membrane, the ‘whole cell’ patch configuration (Hamill et al., 1981). Some configurations of patch clamp technique can measure current flowing through a single ion channel. Most studies utilizing this technique involve in vitro or ex vivo measurements.

Other approaches can be used to indirectly measure VGSC function, including radiotracer flux, fluorescent approaches, and calcium imaging. While these approaches can provide useful information in many cases, they are not direct, nor do they have sufficient resolution to fully describe VGSC function (Molleman A, 2003).

Ion channels are essential for the initiation and propagation of AP in excitable cells in both vertebrate and invertebrate species. In neurons, ion channels are essential for chemical communication between cells, or synaptic transmission. Ion channels also function to maintain membrane potential and initiate and propagate electrical impulses. VGSC are a target of natural and synthetic chemicals and disruption of the gate kinetics has been characterized in insects and mammalian cells (Soderlund et al., 2002).

For more details and references see also the description in MIE: KE 1353 Binding to Voltage Gated Sodium Channel.

Guan B, Chen X and Zhang H, 2013. Two-electrode voltage clamp. Methods in Molecular Biology, 998, 79–89. doi: 10.1007/978-1-62703-351-0_6

Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ. Pflugers. Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Archiv: European journal of physiology, 391(2), 85–100. https://doi.org/10.1007/BF00656997

Molleman A, 2003. Patch Clamping: An Introductory Guide to Patch Clamp Electrophysiology. John Wiley and Sons. DOI:10.1002/0470856521

Narahashi T. (2002). Nerve membrane ion channels as the target site of insecticides. Mini reviews in medicinal chemistry, 2(4), 419–432. https://doi.org/10.2174/1389557023405927

OECD,2023 Initial Recommendations on Evaluation of Data from the Developmental Neurotoxicity (DNT) In-Vitro Testing Battery; Series on Testing and Assessment No. 377. Available at: https://one.oecd.org/document/ENV/CBC/MONO(2023)13/en/pdf

Ruigt GS, Neyt HC, Van der Zalm JM, and Van den Bercken J,1987. Increase of sodium current after pyrethroid insecticides in mouse neuroblastoma cells. Brain research, 437(2), 309–322. https://doi.org/10.1016/0006-8993(87)91645-3

Soderlund DM, Clark JM, Sheets LP, Mullin LS, Piccirillo V J, Sargent D, Stevens JT and Weiner ML ,2002. Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment. Toxicology, 171(1), 3–59. https://doi.org/10.1016/s0300-483x(01)00569-8