Role of sodium channel subtype in action potential generation by neocortical pyramidal neurons

Abstract

Neocortical pyramidal neurons express several distinct subtypes of voltage-gated Na channels. In mature cells, Na1.6 is the dominant channel subtype in the axon initial segment (AIS) as well as in the nodes of Ranvier. Action potentials (APs) are initiated in the AIS, and it has been proposed that the high excitability of this region is related to the unique characteristics of the Na1.6 channel. Knockout or loss-of-function mutation of the gene is generally lethal early in life because of the importance of this subtype in noncortical regions of the nervous system. Using the Cre/loxP system, we selectively deleted Na1.6 in excitatory neurons of the forebrain and characterized the excitability of Na1.6-deficient layer 5 pyramidal neurons by patch-clamp and Na and Ca imaging recordings. We now report that, in the absence of Na1.6 expression, the AIS is occupied by Na1.2 channels. However, APs are generated in the AIS, and differences in AP propagation to soma and dendrites are minimal. Moreover, the channels that are expressed in the AIS still show a clear hyperpolarizing shift in voltage dependence of activation, compared with somatic channels. The only major difference between Na1.6-null and wild-type neurons was a strong reduction in persistent sodium current. We propose that the molecular environment of the AIS confers properties on whatever Na channel subtype is present and that some other benefit must be conferred by the selective axonal presence of the Na1.6 channel.

Optical measurement of physiological sodium currents in the axon initial segment

“…4C). In summary, these measurements represent a general improvement from previous estimates of the Na + influx in the AIS, associated with APs, using the indicator ANG-2 (Miyazaki & Ross 2015;Miyazaki et al 2019) or the less sensitive indicator SBFI (Kole et al 2008;Fleidervish et al 2010;Baranauskas et al 2013;Scott et al 2014;Katz et al 2018). But the significant and critical improvement of the present measurements is that they were achieved at the unprecedented temporal resolution of 10 kHz.…”

“…Experimental approaches for the investigation of Na + influx in the intact AIS during AP generation include the voltage-clamp patch electrode technique (Yue et al 2005;Carter & Bean, 2009;Katz et al 2018) and the optical recording of fluorescence from Na + indicators (Kole et al 2008;Fleidervish et al 2010; Baranauskas et al 2013;Miyazaki & Ross, 2015;Katz et al 2018;Miyazaki et al 2019). The first approach allows recording fast Na + currents triggered by artificial somatic injections of voltage waveforms mimicking an AP, with no information on the spatial organisation of the current and limitations due to space-clamp problems (Bar-Yehuda & Korngreen, 2008).…”

“…In particular, critical mutations of Nav1.6 (Wagnon et al 2015) and Nav1.2 (Reynolds et al 2020) are correlated with infant epilepsy, autism and related severe neurological disorders. Interestingly, in L5 pyramidal neurons, Nav1.6 deficient mice are characterized by a reduced persistent Na + current (Katz et al 2018) whereas Nav1.2 deficient mice are characterized by reduced dendritic excitability and synaptic function (Spratt et al 2019).…”

Mutations in the Scn8a DIIS4 voltage sensor reveal new distinctions among hypomorphic and null Nav1.6 sodium channels

“…Without this increase in levels of WT VGSCs at the nodes of Ranvier, the Scn8a Δ9/Δ9 and Scn8a ∇3/∇3 homozygous mutants would demonstrate impaired nerve conduction. Though additional Western blotting and immunostaining would be necessary to confirm this hypothesis, previous studies have demonstrated that loss of Scn8a expression leads to the compensatory recruitment of other VGSC proteins, particularly Na v 1.2 (encoded by Scn2a ), to the mature nodes of Ranvier and axon initial segment . In further support of this hypothesis, Vega et al reported that Na v 1.2 protein levels are significantly upregulated in null Scn8a med‐Tg/med‐Tg mutants relative to WT littermates …”

Spikelets in pyramidal neurons: generating mechanisms, distinguishing properties, and functional implications

“…Interestingly, a recent study investigating AP initiation in Na V 1.6null pyramidal neurons demonstrated that the Na V 1.6 subtype is not necessary for the lower firing threshold at the AIS compared to the soma (Katz et al, 2018). Rather, the authors suggest that the hyperpolarizing shift in the activation of axonal sodium channels might be a result of molecular interactions within the AIS.…”