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

Abstract

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 α-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⁺]_iincrease on the depolarizing stimulus; and (3) after inhibiting neurotransmitter secretion in neurons by incubating the slices with tetanus neurotoxin, no [Ca²⁺]_iincrease 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.