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

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 microM, 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 (< or =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.