Ca2+ entry through a non-selective cation channel in Aplysia bag cell neurons.
Ca(2+) influx through voltage-gated Ca(2+) channels is a fundamental signaling event in neurons; however, non-traditional routes, such as non-selective cation channels, also permit Ca(2+) entry. The present study examines the Ca(2+) 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 Ca(2+)), had a reversal potential near +50 mV. In keeping with Ca(2+) permeability, this was right-shifted to approximately +60 mV in high Ca(2+) (substituted for Mg(2+)) and left-shifted to around +40 mV in zero Ca(2+) (replaced with Mg(2+)). The current showed inward rectification between +30 and +90 mV, and a conductance of 29 pS in normal Ca(2+), 30 pS in high Ca(2+), 32 pS in Ba(2+) (substituted for Ca(2+)), but only 21 pS in zero Ca(2+). Despite a greater conductance in Ba(2+), the channel did not display anomalous mol fraction in an equimolar Ca(2+)-Ba(2+) mix. Eliminating internal Mg(2+) lowered activity, but did not alter inward rectification, suggesting intracellular Mg(2+) is a fast, voltage-independent blocker. Imaging bag cell neurons in Mn(2+) saline (substituted for Ca(2+)) revealed enhanced fura-quench following cation channel activation, consistent with Mn(2+) permeating as a Ca(2+) surrogate. Finally, triggering the cation channel while tracking capacitance revealed a Ca(2+)-dependent increase in membrane surface area, consistent with vesicle fusion. Thus, the cation channel not only drives the afterdischarge, but also passes Ca(2+) to potentially initiate secretion. In general, this may represent an alternate means by which neurons elicit neuropeptide release.
Geiger
J E
JE
Department of Physiology, Queen's University, 4th Floor, Botterell Hall, 18 Stuart Street, Kingston, ON, Canada K7L 3N6.
Hickey
C M
CM
Magoski
N S
NS
D000818:Animals; D001048:Aplysia / Q000378:metabolism*; D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent; D002478:Cells, Cultured; D017952:Ganglia, Invertebrate / Q000166:cytology; D015640:Ion Channel Gating; D007473:Ion Channels / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques
D002413:Cations, Divalent; D007473:Ion Channels; D001464:Barium; D008274:Magnesium; D002118:Calcium