Ca2+ entry through a non-selective cation channel in Aplysia bag cell neurons

Ca2+ influx through voltage-gated Ca2+ channels is a fundamental signaling event in neurons; however, non-traditional routes, such as non-selective cation channels, also permit Ca2+ entry. The present study examines the Ca2+ permeability of a cation channel that drives an afterdischarge in Aplysia bag cell neurons. The firing of these neurons induces peptide release and reproduction. Single channel-containing inside-out patches excised from cultured bag cell neurons, with the cytoplasmic face bathed in K+-aspartate and the extracellular face bathed in artificial seawater (11 mM Ca2+), had a reversal potential near +50 mV. In keeping with Ca2+ permeability, this was right-shifted to approximately +60 mV in high Ca2+ (substituted for Mg2+) and left-shifted to around +40 mV in zero Ca2+ (replaced with Mg2+). The current showed inward rectification between +30 and +90 mV, and a conductance of 29 pS in normal Ca2+, 30 pS in high Ca2+, 32 pS in Ba2+ (substituted for Ca2+), but only 21 pS in zero Ca2+. Despite a greater conductance in Ba2+, the channel did not display anomalous mol fraction in an equimolar Ca2+–Ba2+ mix. Eliminating internal Mg2+ lowered activity, but did not alter inward rectification, suggesting intracellular Mg2+ is a fast, voltage-independent blocker. Imaging bag cell neurons in Mn2+ saline (substituted for Ca2+) revealed enhanced fura-quench following cation channel activation, consistent with Mn2+ permeating as a Ca2+ surrogate. Finally, triggering the cation channel while tracking capacitance revealed a Ca2+-dependent increase in membrane surface area, consistent with vesicle fusion. Thus, the cation channel not only drives the afterdischarge, but also passes Ca2+ to potentially initiate secretion. In general, this may represent an alternate means by which neurons elicit neuropeptide release.