Altered kinetics of sodium channel leads to Altered, Action Potential

The falling phase of the action potential caused by the inactivation of the VGSCs and the opening of voltage-gated potassium channels allowing K⁺ to leave the cell. The efflux of K⁺ ions results in hyperpolarisation (undershoot phase) of the membrane. Ultimately the voltage-gated K⁺ channels close and the membrane potential returns to its resting state. It is therefore biologically plausible that changing the dynamic of VGSCs leads to a series of complex cellular events resulting in alteration of the firing rate as a final consequence. Type II pyrethroids cause stimulus dependent membrane depolarisation and conduction block. Expression of VGSC are spatial and temporal dependent; however, it is biological plausible that also in developing brain pyrethroids would bind to VGSC isoforms and disrupt the channel gating kinetic (Shafer et al., 2005; Soderlund et al., 2002).

Effect on the neuronal electrical activity using the type II pyrethroid deltamethrin is reported in Meyer et al. (2008) when using hippocampal cultures from postnatal day 2–4 pups. The electrical changes indicate neuron depolarisation and conduction block consequent to disruption of action potential generation with a dose-dependent inhibition of spontaneous glutamate release from hippocampal neurons. Deltamethrin inhibits spontaneous glutamate release from hippocampal neurons as measured by a decrease in sEPSC frequency during bursting release activity (Meyer et al., 2008). The effect is considered presynaptic because the decrease in sEPSC frequency following treatment with deltamethrin was not accompanied by changes in amplitude (Meyer et al., 2008). These data support the fact that deltamethrin decreases neuronal excitation by inhibition of the firing rate (inhibition of the spontaneous spiking activity) and the subsequent release of glutamate from the synapse.

Alterations of calcium dynamics are also reported for pyrethroids (Soderlund et al., 2002; Cao et al., 2011). Extracellular calcium, rather than calcium release from the intracellular calcium stores, is the likely source for pyrethroid-induced elevation of calcium in neocortical neurons (Cao et al., 2011). The same paper demonstrates that L-type VGCCs, NMDA receptors, and the sodium/calcium exchanger accounted for most pyrethroid-induced calcium entry. TTX completely abolished pyrethroid-induced calcium entry, indicating that these pathways were activated as a result of pyrethroid actions on VGSCs. For L-type VGCCs, activation by deltamethrin is likely to have been secondary to depolarisation of the cell membrane as a result of VGSC activation. Although it was not measured, it is likely that the depolarisation and calcium entry resulted in glutamate release, which then activated NMDA receptors, resulting in additional calcium entry. Finally, sodium entry through VGSC may have caused sodium loading of the neurons, which can result in a reversal of sodium/calcium exchange, which accounts for the contribution of this component to pyrethroid-induced calcium entry (Cao et al., 2011).

Dose Concordance

Changes in VGSC kinetic and disruption of the action potential are reported in vitro at concentration between 0.01 to 1 mM, in hippocampal or neocortical neurons from postnatal day 2–4 pups (Meyer et al., 2008; Cao et al., 2011).

Temporal Concordance

The two KEs were observed immediately following exposure to deltamethrin when measured in vitro up to 800 seconds recording in Cao et al. (2007) and immediately following exposure to deltamethrin when measured in vitro up to 9 minutes recording in Mayer et al., 2008.

The mechanistic understanding of the generation of membrane potentials, based on Na, K, Cl and Ca ions is broadly accepted and extensive documentation is also available. However, some uncertainties can be detected. The uncertainties and inconsistencies detected in the Meyer et al., 2008 are also applicable for this KER.

The events investigated by Cao et al. (2011) e.g. depolarisation and calcium entry, glutamate release, activation of NMDA receptors and additional calcium entry, were not directly measured in the study. Moreover, as reported also for VGSCs, the action of pyrethroids on calcium channel is temperature dependent and may have an impact on the deltamethrin-induced calcium influx in neocortical neurons. in the study from Cao et al. (2011) the temperature at which the experiment was carried out is not reported. 9 out of 11 pyrethroids tested were able to produce a concentration-dependent elevation in intracellular calcium concentration in neocortical neurons which occurred secondary to activation of VGSCs. The nine pyrethroids that stimulated calcium influx displayed distinct efficacies. The rank order of efficacy for calcium influx was similar to that for sodium influx (Cao et al., 2009) with the exception of S-bioallethrin, which is the least efficacious compound on calcium influx. Deltamethrin, the prototype stressor for this AOP, is in position 6 (out of 9) in terms of potency.

It should be further noted that other ionic channels may have an impact on the action potential generation and in this regard the knowledge is limited.

Also, in this case, some inconsistencies can be observed in experimental studies. They can be associated with the electrophysiological technique used to study ionic currents in individual isolated living cells, tissue sections or patches of cells. The solution used in the bath can be similar to cytoplasm composition or completely different, they can be changed by adding ions or drugs to study the ion channels under different conditions.