Intracellular Calcium Transients and Potassium Current Oscillations Evoked by Glutamate in Cultured Rat Astrocytes

Abstract

Glutamate responses in cultured rat astrocytes from cerebella of neonatal rats were investigated using the perforated-patch configuration to record membrane currents without rundown of intracellular messenger cascades, and microfluorometric measurements to measure the intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) and intracellular pH (pH i ) with fura-2 AM and 2Ј,7Ј-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxy methylester respectively. In the perforated-patch mode, glutamate evoked single or multiple outward current transients in 82% of the cells, which disappeared when the recording technique was converted into a conventional whole-cell mode. The outward current transients were accompanied by [Ca 2ϩ ] i transients, whereas pH i fell monophasically, without any sign of oscillation. Pharmacological analysis of the glutamate-induced responses indicated that ionotropic receptor activation evoked an inward current but no outward current transients, and metabotropic receptor activation (of the mGluR1/5 type) elicited outward current transients but no inward current. The outward current transients were reduced in frequency, or even abolished, after depletion of the intracellular Ca 2ϩ -stores by the Ca 2ϩ -ATPase inhibitor cyclopiaconic acid (10 M). They reversed near Ϫ85 mV and were reduced by tetraethylammonium (10 mM), suggesting that they were caused by K ϩ channel activation. It is concluded that glutamate evoked these K ϩ outward current transients by oscillatory Ca 2ϩ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K ϩ uptake capacity and other voltage-dependent processes.

Diversity of Calcium Signaling by Metabotropic Glutamate Receptors

“…In cultured astrocytes, glutamate induces [Ca 2ϩ ] i oscillations through mGluR5 (21,22). Similar to the observations in mGluR5a-expressed HEK-293 cells (9), the PKC activator abolishes Ca 2ϩ response in cultured astrocytes (21,23).…”

Ca2+ Regulation of Dynamin-Independent Endocytosis in Cortical Astrocytes

“…Application of glutamate onto astrocytes has been shown to trigger oscillations of intracellular Ca 2ϩ via metabotropic glutamate receptors (mGluRs) (Chen et al, 1997;Fellin et al, 2006). We have confirmed in our cultured astrocytes that application of glutamate (1 mM; 5 min) evoked a significant intracellular Ca 2ϩ oscillation (Fig.…”

Regulation of Store-Operated Calcium Entry by Calcium-Independent Phospholipase A2in Rat Cerebellar Astrocytes

“…Ca 2ϩ signaling in the form of repetitive oscillations are associated with regulating a variety of physiological processes, including gene expression (Dolmetsch et al, 1998), K ϩ conductance (Chen et al, 1997), and secretion (Thomas et al, 1996). The number of spontaneously active astrocytes and the number of Ca 2ϩ transients exhibited by each astro- cyte increased by increasing the external Ca 2ϩ (Parri and Crunelli, 2003) and was reduced by inhibiting metabotropic glutamate receptors of groups I and II (Zur Nieden and Deitmer, 2006).…”