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.
Department of Physiology, Queen's University, 4th Floor, Botterell Hall, 18 Stuart Street, Kingston, ON, Canada K7L 3N6.
J E
JE
Geiger
C M
CM
Hickey
N S
NS
Magoski
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