Cellular localization and kinetic properties of Na(V)1.9-, Na(V)1.8-, and Na(V)1.7-like channel subtypes in Helix pomatia.

This article concerns the kinetics, selectivity, and distribution of the Na(V)1.9, Na(V)1.8, and Na(V)1.7 channel subtypes in the CNS of the snail, Helix pomatia. Within the snail brain, Na(V)1.9- and Na(V)1.8-like channel subtypes are widely expressed, with particularly high levels in the pedal, cerebral, and buccal ganglia. The suboesophageal ganglion contains equal amounts of neurons labeled with Na(V)1.9, 1.8, and 1.7 antibodies. Our data show that different types of ion channels are localized to discrete neurons and regions of the neuronal membrane affecting by this way the physiology of synaptic transmission or nerve conduction. Based on the voltage dependence and kinetics, the non- or slowly inactivating currents were observed in identified and nonidentified neurons of the snail CNS attributed to separate Na-channel subtypes. These observations provide the first evidence for the presence of the composite Na-current in snail neurons. The significance of Na(V)1.9 channels in gastropod neurons is assigned to regulating the subthreshold membrane depolarization. First time, we have demonstrated that in addition to the Na(V)1.2-like channels most of the neurons contain Na(V)1.8- or 1.7-like channels carrying the composite inward sodium current. In this way, neurons containing different sets of channels differently are regulated, which allows further dynamic modulation of neuronal activity. The neuronal soma membrane revealed low ion selectivity of the Na-channels with slow kinetics, which is a general property of gastropod molluscs. In addition, the relative similarity of the biophysical properties of voltage-gated currents between vertebrates and invertebrates may reflect a structural similarity existing between Na-channel subtypes pointing to a common evolutionary origin.