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

Voltage-gated Na+ channels (Na-v) modulate neuronal excitability, but the roles of the various Na-v subtypes in specific neuronal functions such as synaptic transmission are unclear. We investigated expression of the three major brain Na-v subtypes (Na(v)1.1, Na(v)1.2, Na(v)1.6) in area CA1 and dentate gyrus of rat hippocampus. Using light and electron microscopy, we found labeling for all three Na-v 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, Na(v)1.1 immunoreactivity was selectively expressed in presynaptic profiles, while Na(v)1.2 and Na(v)1.6 were expressed both pre- and post-synaptically. In contrast, in the stratum radiatum (SR) of CA1, Na(v)1.1, Na(v)1.2, and Na(v)1.6 were selectively expressed in postsynaptic profiles. We next compared differences in Na-v subtype expression between CH and SR axon terminals and between CH and SR dendrites and spines. Na(v)1.1 and Na(v)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 Na(v)1.6 immunoreactivity were found between axon terminals of CH and SR or between dendrites and spines of CH and SR. All Na-v subtypes in both CH and SR were preferentially associated with asymmetric synapses rather than symmetric synapses. These findings indicate selective presynaptic and postsynaptic Na-v expression in glutamatergic synapses of CH and SR supporting neurotransmitter release and synaptic plasticity.