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The Journal of Neuroscience, June 15, 1998, 18(12):4637-4645

On the Role of Voltage-Dependent Calcium Channels in Calcium Signaling of Astrocytes In Situ

Giorgio Carmignoto, Lucia Pasti, and Tullio Pozzan

Department of Experimental Biomedical Sciences and Consiglio Nazionale delle Ricerche Center for the Study of Biomembranes, University of Padova, 35121 Padova, Italy

Calcium ions play crucial roles in a large variety of cell functions. The recent proposal that changes in the intracellular calcium concentration ([Ca²⁺]_i) in astrocytes underline a reciprocal communication system between neurons and astrocytes encourages the interest in the definition of the various components participating in this novel Ca²⁺ signaling system. We investigate here whether functional voltage-operated calcium channels (Ca²⁺ VOCs), which are clearly expressed in cultured astrocytes, participate in the regulation of [Ca²⁺]_i also in astrocytes in situ. Depolarization with 40-60 mM K⁺ was used to analyze the activity of Ca²⁺ VOCs in Indo-1-loaded astrocytes in acute slices from the visual cortex and the CA1 hippocampal region of developing rats. We demonstrate here that the depolarization-induced [Ca²⁺]_i increases in astrocytes are solely attributed to the activation of metabotropic receptors by neurotransmitters, such as glutamate, released by synaptic terminals on depolarization. In fact, (1) the K⁺-induced [Ca²⁺]_i increases in astrocyte [Ca²⁺]_i were potently reduced by -methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor competitive inhibitor; (2) after emptying intracellular Ca²⁺ stores with cyclopiazonic acid, none of the astrocytes displayed a [Ca²⁺]_i increase on the depolarizing stimulus; and (3) after inhibiting neurotransmitter secretion in neurons by incubating the slices with tetanus neurotoxin, no [Ca²⁺]_i increase on K⁺ stimulation was observed in astrocytes. Finally, patch-clamp whole-cell recordings from hippocampal astrocytes in acute brain slices failed to reveal any voltage-dependent calcium currents. On the basis of these results, the various roles proposed for astrocyte Ca²⁺ VOCs in the CNS should be reconsidered.

Key words: calcium channels; glutamate; astrocyte-neuron interactions; confocal microscope; patch-clamp; brain slices; rat

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Copyright © 1998 Society for Neuroscience  0270-6474/98/18124637-09$05.00/0

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