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Peripheral Hot Spots for Local Ca²⁺ Release after Single Action Potentials in Sympathetic Ganglion Neurons

Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland

Correspondence: Address reprint requests to Martin F. Schneider. Tel.: 410-706-7812; Fax: 410-706-8297; E-mail: mschneid{at}umaryland.edu.

Ca²⁺ release from the endoplasmic reticulum (ER) contributes to Ca²⁺ transients in frog sympathetic ganglion neurons. Here we use video-rate confocal fluo-4 fluorescence imaging to show that single action potentials reproducibly trigger rapidly rising Ca²⁺ transients at 1–3 local hot spots within the peripheral ER-rich layer in intact neurons in fresh ganglia and in the majority (74%) of cultured neurons. Hot spots were located near the nucleus or the axon hillock region. Other regions exhibited either slower and smaller signals or no response. Ca²⁺ signals spread into the cell at constant velocity across the ER in nonnuclear regions, indicating active propagation, but spread with a (time)^1/2 dependence within the nucleus, consistent with diffusion. 26% of cultured cells exhibited uniform Ca²⁺ signals around the periphery, but hot spots were produced by loading the cytosol with EGTA or by bathing such cells in low-Ca²⁺ Ringer's solution. Peripheral hot spots for Ca²⁺ release within the perinuclear and axon hillock regions provide a mechanism for preferential initiation of nuclear and axonal Ca²⁺ signals by single action potentials in sympathetic ganglion neurons.

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