Calcium release from intracellular stores in rodent astrocytes and neurons in situ

Abstract

Endoplasmic reticular Ca 2+ stores, instrumental for intra- and intercellular calcium signalling, can be depleted by different receptor agonists. In the present study, the functional status of ER Ca 2+ stores was probed by cyclopiazonic acid (CPA, 10–30 μM, inhibitor of SERCA-dependent ER Ca 2+ uptake) and/or caffeine (20 mM, ryanodine receptor activator) in astrocytes and neurons of rat and mouse acute hippocampal brain slices ( Stratum radiatum, Stratum moleculare), and in cultured astrocytes, using confocal microscopy and conventional Ca 2+ imaging. Astrocytes and neurons in situ, identified by their Ca 2+ response in K +-free saline (Dallwig and Deitmer [J. Neurosci. Methods 116 (2002) 77]), had a resting cytosolic Ca 2+ level of 105 and 157 nM, respectively ( P<0.05). CPA evoked a Ca 2+ transient, which was faster and larger in neurons than in astrocytes, indicating larger Ca 2+ leak of neuronal Ca 2+ stores. Caffeine evoked a Ca 2+ rise in most neurons (>80%), but only in less than 40% of astrocytes. The glial Ca 2+ transients in the presence of caffeine had a large and variable delay (>50 s), as compared to those in neurons (≤10 s), and appeared to be spontaneous and/or secondary to the neuronal Ca 2+ response, leading to release of neuronal transmitters. Astrocytes in culture responded to CPA, but never to caffeine with a Ca 2+ rise. Our results indicate that astrocytes, in contrast to neurons, lack caffeine-sensitive Ca 2+ stores, and have a relatively smaller leak from CPA-sensitive Ca 2+ stores than neurons.