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 concentr ation ([Ca2+](i)) in astrocytes underline a reciprocal communication s ystem between neurons and astrocytes encourages the interest in the de finition of the various components participating in this novel Ca2+ si gnaling system. We investigate here whether functional voltage-operate d calcium channels (Ca2+ VOCs), which are clearly expressed in culture d astrocytes, participate in the regulation of [Ca2+](i) also in astro cytes 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 fro m the visual cortex and the CA1 hippocampal region of developing rats. We demonstrate here that the depolarization-induced [Ca2+](i) increas es in astrocytes are solely attributed to the activation of metabotrop ic receptors by neurotransmitters, such as glutamate, released by syna ptic terminals on depolarization. In fact, (1) the K+-induced [Ca2+](i ) increases in astrocyte [Ca2+](i) were potently reduced by alpha-meth yl-4-carboxyphenylglycine, a metabotropic glutamate receptor competiti ve inhibitor; (2) after emptying intracellular Ca2+ stores with cyclop iazonic 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. F inally, patch-clamp whole-cell recordings from hippocampal astrocytes in acute brain slices failed to reveal any voltage-dependent calcium c urrents. On the basis of these results, the various roles proposed for astrocyte Ca2+ VOCs in the CNS should be reconsidered.