PubMed1993
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.