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 Ca²⁺ concentration ([Ca²⁺]_i) and intracellular pH (pH_i) with fura-2 AM and 2′,7′-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 [Ca²⁺]_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 Ca²⁺-stores by the Ca²⁺-ATPase inhibitor cyclopiaconic acid (10 μm). They reversed near −85 mV and were reduced by tetraethylammonium (10 mm), suggesting that they were caused by K⁺ channel activation. It is concluded that glutamate evoked these K⁺ outward current transients by oscillatory Ca²⁺ release mediated by mGluR activation. The corresponding membrane potential waves across the astroglial syncytium could provide spatial and temporal dynamics to the glial K⁺ uptake capacity and other voltage-dependent processes.