Disruption, action potential

An action potential is a fast, transitory and propagating change of the resting membrane potential. Neurons and muscle cells can generate an action potentials. The initial signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. These ions pass through channels that open when a specific neurotransmitter binds to the channel, leading to opening. For example, when acetylcholine is released at the synapse between a nerve and muscle (called the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic Na⁺ channels to open. Na⁺ enters the postsynaptic cell and causes the postsynaptic membrane to depolarise. This depolarisation is called an excitatory postsynaptic potential (EPSP) and makes the postsynaptic neuron more likely to fire an action potential. These incoming ions change the membrane potential closer to 0, a process known as depolarisation. When positive ions flow into the negative cell, the cell’s polarity decreases. If it gets positive enough, it can trigger the VGSC found in the axon, then the action potential will be sent.

This process lets positively charged sodium ions flow into the negatively charged axon and depolarise the surrounding axon. Once one channel opens and lets positive ions in, it sets the stage for the channels down the axon to perform the same thing in a domino-like process. This stage is known as depolarisation, the neuron becomes positively charged as the action potential passes through. When the inactivation gates of the sodium channels close, they stop the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell compared with outside, so when these channels open, more potassium exits than enters. The cell therefore loses positively charged ions and returns back toward its resting state. This step is called repolarisation. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarises or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to re-establish the resting state.

Sodium channel gating is a well regulated process that is critical to normal neuronal function, activation and propagation of the action potential. Shape, speed of conduction and fidelity in propagation of the action potential are essential to the timing, synchrony and efficacy of neuronal communication. Waveform, timing and fidelity of the axonal action potential can be modulated, which leads to changes in presynaptic neurotransmitter release. Action potential normally develops first in the initial segment of the axon. During axonal action potential initiation, the active depolarisation propagates both towards the soma (antidromic) and down the axon (orthodromic). The conduction velocity of the antidromic action potential may have a significant impact on dendritic backpropagation. This in turn will affect spike-timing dependent plasticity i.e. the synaptic plasticity sensitive to the timing of dendritic action potentials relative to incoming synaptic information. The orthodromic velocity will affect the degree of synchrony of arrival of information at different postsynaptic targets of the same axon. In neurons, voltage-gated sodium conductances play an essential role in action potential initiation and propagation. VGSC activate and inactivate within milliseconds. As the cell membrane is depolarised, sodium channels activate, resulting in the influx of sodium ions to further depolarise the membrane. This inward current produces the upstroke of the action potential. Along with the gating of potassium channels, sodium channel inactivation participates in the action potential downstroke. Although variations in many ion channels are likely to participate in the diversity of action potential waveforms observed in neurons, differences in sodium channel subunit composition, localisation and modulation may participate in shaping a neuron’s action potential. Sodium channel subunit composition at the axon initial segment contributes to the firing properties of neurons, particularly the characteristic maximum firing frequency of a particular cell class. Therefore, at nodes of Ranvier the sodium channel subunit composition may contribute to action potential propagation fidelity. Steady-state persistent sodium currents can contribute to excitability and to the shape of an action potential. These sodium channels are active near rest potential (−65 mV) and do not inactivate even with quite strong depolarisation. Therefore, these currents can participate in cellular excitability and in setting action potential threshold (Kress and Mennerick, 2009). Alterations in VGSCs can result in changes in membrane polarisation and propagation of neuronal action potentials. Changes in neuronal excitability in glutamatergic networks are described following treatment to deltamethrin and permethrin on neuronal activity in hippocampal neuronal cultures using patch clamp and microelectrode array (MEA) recordings (Meyer et al., 2008). Cao et al. (2011) demonstrated that VGSC responses of a neuronal network to pyrethroids with an increase of intracellular calcium concentration and these responses are secondary to activation of VGSCs. The effect of pyrethroids on neurotransmitters release and neuronal excitability in glutamatergic networks are described following treatment to deltamethrin and permethrin on neuronal activity in hippocampal neuronal cultures using patch clamp and microelectrode array (MEA) recordings (Meyer et al., 2008). The distinct abilities of pyrethroids to elevate BDNF mRNA expression are consistent with the demonstration of a range of pyrethroid efficacies in the stimulation of calcium influx. In vivo, deltamethrin has been reported to increase BDNF in the cortex and hippocampus (Imamura et al., 2006; Cao et al., 2011), and both deltamethrin and permethrin alter transcription profiles of activity-dependent genes in the cortex including c-fos, Egr1, and Camk1g (Harrill et al., 2008; Cao et al., 2011). Therefore, activity-dependent changes in gene transcription after pyrethroid exposure can occur both in vitro and in vivo (Cao et al., 2011; Pitzer et al., 2019; Zhang et al., 2018).

The action potential is a cycle of membrane depolarisation, hyperpolarisation and return to the resting value. To measure an action potential, the patch clamp or the intracellular recording (impale a sharp electrode into the cell cytosol) technique are generally used. For either, a glass-made microelectrode is sufficient to measure action potential. The measurement of Na+ ion concentration would not detect single action potentials but a change in bulk ion concentration over a longer time and that might depend mainly on the firing rate of the cells and the activity of Na+/K+-pumps. Patch clamp is the preferred technique for the qualification and quantification of the altered firing rate (Meyer et al., 2008). Neurotransmitter release can be evaluated in vivo using western blotting quantification or using microdialysis and analytical quantification.

Action potentials or nerve impulses are rapid and transient electrical activity that is propagated in the membrane of excitable such as neurons and muscle cells. Action potentials allow long-distance signalling in the nervous system. An action potential results from the sequential opening and closing of voltage-gated cation channels. First, opening of Na+ channels permits influx of Na+ ions for about 1 ms, causing a sudden large depolarisation of a segment of the membrane. The channel then closes and becomes unable to open (refractory) for several milliseconds, preventing further Na+ flow. Opening of K+ channels as the action potential reaches its peak permits efflux of K+ ions, which initially hyperpolarises the membrane. As these channels close, the membrane returns to its resting potential. The same basic mechanism is used by all neurons. Myelination produced by oligodendrocytes increases the velocity of impulse conduction (Lodish et al., 2000, ‘The Action Potential and Conduction of Electric Impulses’ in Molecular Cell Biology Section 21.2, New York )

Cao Z, Shafer TJ and Murray TF, 2011. Mechanisms of pyrethroid insecticide-induced stimulation of calcium influx in neocortical neurons, Journal of Pharmacology and Experimental Therapeutics, 336(1), 197–205. doi: https://doi.org/10.1124/jpet.110.171850

Harrill JA, Li Z, Wright FA, Radio NM, Mundy WR, Tornero-Velez R and Crofton KM, 2008. Transcriptional response of rat frontal cortex following acute in vivo exposure to the pyrethroid insecticides permethrin and deltamethrin. BMC Genomics, 9(1), 546. https://doi.org/10.1186/1471-2164-9-546

Imamura L, Yasuda M, Kuramitsu K, Hara D, Tabuchi A and Tsuda M, 2006. Deltamethrin, a pyrethroid insecticide, is a potent inducer for the activity-dependent gene expression of brain-derived neurotrophic factor in neurons. Journal of Pharmacology and Experimental Therapeutics, 316(1), 136–143. doi: 10.1124/jpet.105.092478

Kress GJ and Mennerick S, 2009. Action potential initiation and propagation: upstream influences on neurotransmission. Neuroscience, 158(1), 211–222.

Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D and Darnell J. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 21.2, The Action Potential and Conduction of Electric Impulses. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21668/

Meyer DA, Carter JM, Johnstone AF and Shafer TJ, 2008. Pyrethroid modulation of spontaneous neuronal excitability and neurotransmission in hippocampal neurons in culture. Neurotoxicology, 29(2), 213– 225. doi: 10.1016/j.neuro.2007.11.005.

Pitzer EM, Sugimoto C, Gudelsky GA, Huff Adams CL, Williams MT and Vorhees CV, 2019. Deltamethrin exposure daily from postnatal day 3–20 in Sprague-Dawley rats causes long-term cognitive and behavioral deficits. Toxicological Sciences, 169(2), 511–523. https://doi.org/10.1093/toxsci/kfz067 Zhang C, Xu Q, Xiao X, Li W, Kang Q,

Zhang X, … and Li Y, 2018. Prenatal deltamethrin exposureinduced cognitive impairment in offspring is ameliorated by memantine through NMDAR/BDNF signaling in hippocampus, Frontiers in Neuroscience, 12, 615. https://doi.org/10.3389/fnins.2018.00615