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.
Carmignoto
G
G
Department of Experimental Biomedical Sciences and Consiglio Nazionale delle Ricerche Center for the Study of Biomembranes, University of Padova, 35121 Padova, Italy.
Pasti
L
L
Pozzan
T
T
D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D004553:Electric Conductivity; D004594:Electrophysiology; D007425:Intracellular Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000502:physiology*
D015220:Calcium Channels; D011188:Potassium; D002118:Calcium