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On the role of voltage-dependent calcium channels in calcium signaling of astrocytes in situ.
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 alpha-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.[1]References
- On the role of voltage-dependent calcium channels in calcium signaling of astrocytes in situ. Carmignoto, G., Pasti, L., Pozzan, T. J. Neurosci. (1998) [Pubmed]
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