Properties of the caffeine-sensitive intracellular calcium stores in mammalian neurons

Abstract

Using indo-1- and fura-2-based microfluorometry for measuring the cytoplasmic free calcium concentration ([Ca2+]in), the properties of caffeine-induced Ca2+ release from internal stores were studied in rat cultured central and peripheral neurons, including dorsal root ganglion (DRG) neurons, neurons from then. cuneatus, CA1 and CA3 hippocampal regions, and pyramidal neocortical neurons. Under resting conditions, the Ca2+ content of internal stores in DRG neurons was high enough to produce caffeine-triggered [Ca2+]in transients. Prolonged exposure of caffeine depleted the caffeine-sensitive stores of releasable Ca2+; the degree of this depletion depended on caffeine concentration. The depletion of the caffeine-sensitive internal stores to some extent was linked to calcium extrusion via La3+-sensitive plasmalemmal Ca2+-ATPases. Caffeine-induced Ca2+ release deprived internal stores in DRG neurons, but they refilled themselves spontaneously within 10 min. Pharmacological manipulation with caffeine-sensitive stores interferred with the depolarization-induced [Ca2+]in transients. In the presence of low caffeine concentration (0.5–1.0 mM) in the extracellular solution, the rate of rise of the depolarization-triggered [Ca2+]in transients significantly increased (by a factor of 2.15 ± 0.29) suggesting the occurrence of Ca2+-induced Ca2+ release. When the caffeine-sensitive stores were emptied by prolonged application of caffeine, the amplitude and rate of rise of the depolarization-induced [Ca2+]in transients decreased. These findings suggest the involvement of internal caffeine-sensitive calcium stores in generation of calcium signal in sensory neurons. In contrast, in all types of central neurons tested the resting Ca2+ content of internal stores was low, but the stores could be charged by transmembrane Ca2+ entry through voltage-operated calcium channels. After charging, the stores in central neurons spontaneously lost releasable calcium content and within 10 min they became completely empty again. We suggest that internal Ca2+ stores in peripheral and central neurons, although having similar pharmacological characteristics, handle Ca2+ ions in a different manner. Calcium stores in sensory neurons are continuously filled by releasable calcium and after discharging they can be spontaneously refilled, whereas in central neurons internal calcium stores can be charged by releasable calcium only transiently. Caffeine-evoked [Ca2+]in transients in all types of neurons were effectively blocked by 10 mM ryanodine, 5 mM procaine, 10 mM dantrolene, or 0.5 mM Ba2+, thus sharing the basic properties of the Ca2+-induced Ca2+ release from endoplasmic reticulum.