A Store-Operated Ca2+Influx Pathway in the Bag Cell Neurons ofAplysia

Abstract

Although store-operated Ca2+influx has been well-studied in nonneuronal cells, an understanding of its nature in neurons remains poor. In the bag cell neurons of Aplysia californica, prior work has suggested that a Ca2+entry pathway can be activated by Ca2+store depletion. Using fura-based imaging of intracellular Ca2+in cultured bag cell neurons, we now characterize this pathway as store-operated Ca2+influx. In the absence of extracellular Ca2+, the endoplasmic reticulum Ca2+-ATPase inhibitors, cyclopiazonic acid (CPA) or thapsigargin, depleted intracellular stores and elevated intracellular free Ca2+. With the subsequent addition of extracellular Ca2+, a prominent Ca2+influx was observed. The ryanodine receptor agonist, chloroethylphenol (CEP), also increased intracellular Ca2+but did not initiate store-operated Ca2+influx, despite overlap between CEP- and CPA-sensitive stores. Bafilomycin A, a vesicular H+-ATPase inhibitor, liberated intracellular Ca2+from acidic stores and attenuated subsequent Ca2+influx, presumably by replenishing CPA-depleted stores. Store-operated Ca2+influx was partially blocked by low concentrations of La3+or BTP2, and strongly inhibited by either 1-[b-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365) or a high concentration of Ni2+. Regarding IP3receptor blockers, 2-aminoethyldiphenyl borate, but not xestospongin C, prevented store-operated Ca2+influx. However, jasplakinolide, an actin stabilizer reported to inhibit this pathway in smooth muscle cell lines, was ineffective. The bag cell neurons initiate reproductive behavior through a prolonged afterdischarge associated with intracellular Ca2+release and neuropeptide secretion. Store-operated Ca2+influx may serve to replenish stores depleted during the afterdischarge or participate in the release of peptide that triggers behavior.