An IP3-assisted form of Ca2+-induced Ca2+ release in neocortical neurons

Calcium increases induced by single action potentials in rat visual cortex layer II/III pyramidal neurons were shown to be augmented by muscarinic ace tylcholine receptor (mAchR) stimulation. This augmentation was drastically reduced by intracellular injection of heparin but not ruthenium red, theref ore involving inositol-1,4,5-trisphosphate (IP3)-sensitive rather than ryan odine-sensitive calcium stores. Only the calcium increase induced by the se cond or later spike of a spike train, but not that induced by the first spi ke, was augmented, indicating the requirement of both spike-induced calcium increase and mAchR activation. The calcium store depletor thapsigargin abo lished this augmentation use-dependently. These findings suggest a neocorti cal occurrence of calcium-induced calcium release from IP3-sensitive calciu m stores that have been sensitized beforehand by IP3 through mAchR-mediated mechanisms. NeuroReport 11:535-539 (C) 2000 Lippincott Williams & Wilkins.