Sodium channel subtypes are differentially localized to pre‐ and post‐synaptic sites in rat hippocampus

Abstract

Voltage-gated Na+ channels (Nav) modulate neuronal excitability, but the roles of the various Nav subtypes in specific neuronal functions such as synaptic transmission are unclear. We investigated expression of the three major brain Nav subtypes (Nav 1.1, Nav 1.2, Nav 1.6) in area CA1 and dentate gyrus of rat hippocampus. Using light and electron microscopy, we found labeling for all three Nav subtypes on dendrites, dendritic spines, and axon terminals, but the proportion of pre- and post-synaptic labeling for each subtype varied within and between subregions of CA1 and dentate gyrus. In the central hilus (CH) of the dentate gyrus, Nav 1.1 immunoreactivity was selectively expressed in presynaptic profiles, while Nav 1.2 and Nav 1.6 were expressed both pre- and post-synaptically. In contrast, in the stratum radiatum (SR) of CA1, Nav 1.1, Nav 1.2, and Nav 1.6 were selectively expressed in postsynaptic profiles. We next compared differences in Nav subtype expression between CH and SR axon terminals and between CH and SR dendrites and spines. Nav 1.1 and Nav 1.2 immunoreactivity was preferentially localized to CH axon terminals compared to SR, and in SR dendrites and spines compared to CH. No differences in Nav 1.6 immunoreactivity was found between axon terminals of CH and SR or between dendrites and spines of CH and SR. All Nav subtypes in both CH and SR were preferentially associated with asymmetric synapses rather than symmetric synapses. These findings indicate selective presynaptic and postsynaptic Nav expression in glutamatergic synapses of CH and SR supporting neurotransmitter release and synaptic plasticity.

The autism-associated gene Scn2a plays an essential role in synaptic stability and learning

“…Subsequent investigation in hippocampal pyramidal cells found that Na V 1.2 co-localized with the presynaptic terminal marker VGlut1 using diffraction-limited immunofluorescence techniques, and further showed that Na V 1.6 is expressed throughout dendrites using freeze-fracture immuno-electron microscopy (Lorincz and Nusser, 2010). By contrast, more recent immuno-electron microscopy identified Na V 1.2 throughout dendrites, with an enrichment near postsynaptic densities in spines (Johnson et al, 2017). Whether such apposition with presynaptic terminals accounts for VGlut1 co-localization is unclear.…”

Differential Inhibition of Neuronal Sodium Channel Subtypes by the General Anesthetic Isoflurane

“…These differences in inactivation gating lead to greater isoflurane-induced fast inactivation and inhibition of peak I Na for Na v 1.2 and Na v 1.6 compared with Na v 1.1 at a physiologic holding potential. The greater anesthetic sensitivity of Na v 1.2 and Na v 1.6 provides a plausible mechanism for brain region-and neurotransmitterselective effects of isoflurane on synaptic transmission due to differential expression of Na v subtypes between neuron subtypes and within neuronal compartments (Wood and Baker, 2001;Johnson et al, 2017).…”

“…Neuronal excitability and synaptic transmission show physiologic and pharmacological differences between brain regions and between neurons of different neurotransmitter phenotype (Pinheiro and Mulle, 2008;Spruston, 2008). Some of this heterogeneity could derive from differences in relative expression of various neuronal Na v subtypes having distinct voltage-dependent gating properties, with physiologic and pharmacological implications (Lai and Jan, 2006;Ogiwara et al, 2007;Lorincz and Nusser, 2008b;Johnson et al, 2017). Specific Na v subtypes are selectively expressed on presynaptic and postsynaptic glutamatergic synapses in the hippocampus, consistent with subtype-specific roles in neurotransmitter release and synaptic plasticity (Johnson et al, 2017).…”

“…Some of this heterogeneity could derive from differences in relative expression of various neuronal Na v subtypes having distinct voltage-dependent gating properties, with physiologic and pharmacological implications (Lai and Jan, 2006;Ogiwara et al, 2007;Lorincz and Nusser, 2008b;Johnson et al, 2017). Specific Na v subtypes are selectively expressed on presynaptic and postsynaptic glutamatergic synapses in the hippocampus, consistent with subtype-specific roles in neurotransmitter release and synaptic plasticity (Johnson et al, 2017). Most excitatory neurons express a high density of Na v 1.6 and Na v 1.2 (Whitaker et al, 2000;Tian et al, 2014), whereas Na v 1.1 is preferentially expressed in inhibitory GABAergic interneurons (Ogiwara et al, 2007;Lorincz and Nusser, 2008a).…”

“…For example, isoflurane inhibits release of glutamate more potently than release of GABA from cultured hippocampal neurons (Westphalen and Hemmings, 2006;Baumgart et al, 2015) and differentially inhibits neurotransmitter release between brain regions and the spinal cord (Westphalen et al, 2010(Westphalen et al, , 2011. Na v subtypes show subcellular, regional, and neurotransmitter-selective expression in the central nervous system (CNS) (Lai and Jan, 2006;Ogiwara et al, 2007;Lorincz and Nusser, 2008b;Johnson et al, 2017). Given their sensitivity to volatile anesthetics, the differential effects of anesthetics on neurotransmitter release might be explained by their selective inhibition of specific presynaptic Na v subtypes.…”