GLIAL-CELLS IN THE MOUSE HIPPOCAMPUS EXPRESS AMPA RECEPTORS WITH AN INTERMEDIATE CA2+ PERMEABILITY

Recently, we could demonstrate that 'complex' glial cells in mouse hip pocampal slices express glutamate receptor channels of the lpha-amino- 3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate subtypes. In the present study, we further characterized this glial receptor. Si nce voltage-clamp control is imperfect and diffusion barriers hinder t he quantitative analysis of the receptor currents in situ, the patch-c lamp technique was applied to glial cells acutely isolated from the mo use hippocampal CA1 stratum radiatum subregion. A concentration-clamp technique was used which enabled very fast exchange of the extracellul ar solutions. Thus, it was possible to characterize the transient rece ptor currents with high time resolution. Application of L-glutamate, A MPA and L-homocysteate induced rapidly activating and fast desensitizi ng receptor currents in the suspended glial cells. In contrast, kainat e induced non-desensitizing currents. The corresponding dose-response curve revealed a half-maximum of current activation at 350 mu M. The c urrent/voltage relationship of the kainate-evoked response was linear, with a reversal potential at similar to 9 mV. Analysis of the reversa l potential in solutions containing high concentrations of CaCl2 confi rmed earlier in situ data by demonstrating significant Ca2+ permeabili ty of the glial glutamate receptor channels in the hippocampus. The ka inate-induced receptor currents were markedly increased by cyclothiazi de, a substance which selectively potentiates glutamate receptors of t he AMPA subtype. We conclude that glial cells of the juvenile hippocam pus mainly express heteromeric high-affinity AMPA receptors. Most prob ably, the receptor channels are assembled from the low Ca2+-permeable glutamate receptor-2 subunit together with Ca2+-permeable AMPA-preferr ing subunits.