Coupling of AMPA receptors with the Na+/Ca2+ exchanger in cultured rat astrocytes

Astrocytes exhibit three transmembrane Ca2+ influx pathways: voltage-gated Ca2+ channels (VGCCs), the alpha -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) class of glutamate receptors, and Na+/Ca2+ exchangers. Each of these pathways is thought to be capable of mediating a significant increase in Ca2+ concentration ([Ca2+](i)); however, the relative importance of each and their interdependence in the regulation astrocyte [Ca2+](i) is not known. We demonstrate here that 100 muM AMPA in the presence of 100 muM cyclothiazide (CTZ) causes an increase in [Ca2+](i) in cultured cerebral astrocytes that requires transmembrane Ca2+ influx. This increase of [Ca2+](i) is blocked by 100 muM benzamil or 0.5 muM U-73122, which inhibit reverse-mode operation of the Na+/Ca2+ exchanger by independent mechanisms. This response does not require Ca2+ influx through VGCCs, nor does it depend upon a significant Ca2+ influx through AMPA receptors (AMPARs). Additionally, AMPA in the presence of CTZ causes a depletion of thapsigargin-sensitive intracellular Ca2+ stores, although depletion of these Ca2+ stores does not decrease the peak [Ca2+](i) response to AMPA. We propose that activation of AMPARs in astrocytes can cause [Ca2+](i) to increase through the reverse mode operation of the Na+/Ca2+ exchanger with an associated release of Ca2+ from intracellular stores. This proposed mechanism requires neither Ca2+-permeant AMPARs nor the activation of VGCCs to be effective. (C) 2000 Elsevier Science B.V. All rights reserved.