A patch clamp study of excitatory amino acid effects on cortical astrocyte subtypes in culture

Electrophysiological effects of the excitatory amino acids (EAAs) glutamate (Glu) and kainate (KA) on membrane properties of confluent (> 7-9 day) astrocyte cultures were examined. The whole-cell patch clamp technique was employed to measure membrane currents. Cells were subdivided antigenically, morphologically and electrophysiologically into type-1-like and type-2 astrocytes. Lucifer yellow injection showed that type-1-like, but not type-2, astrocytes were electrically coupled in type-2 astrocytes, EAAs induced a cationic current by activating an ionotropic Glu receptor. The underlying receptor mechanism was KA-preferring and was blocked by the broad-spectrum EAA receptor channel antagonist kynurenate (Kyn). The current was dose-dependent and gave a Hill coefficient close to 2 for KA. In type-1-like astrocytes, EAA effects were agonist-dependent. Glu action involved an inward current mainly carried by an electrogenic Glu uptake system. This current was suppressed by the Glu uptake blocker DL-aspartate beta hydroxamate (ABH), but was not sensitive to Kyn. On the other hand, KA activated Kyn-sensitive receptors and was still able to induce this current in the presence of ABH. In type-1-like astrocytes, application of KA on average produced no conductance change. However, application of Ringer containing 5 mM Ba2+ caused a significant increase in input resistance and KA applied in the presence of Ba2+ consistently increased input conductance. In both subtypes of astrocyte, the KA-induced current was predominantly Na(+)-dependent, although in type-2 cells a small, Na(+)-independent current was also seen. These results support recent findings that type-1-like and type-2 astrocytes possess KA-preferring ionotropic receptors and type-1-like astrocytes also possess an electrogenic Glu uptake system.