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1 Neural and Vascular Biology, Ordway Research Institute, Albany, New York, United States
* To whom correspondence should be addressed. E-mail: gschools{at}ordwayresearch.org.
Gap junction communication between astrocytes is prevalent and has been proposed to be involved in several astrocyte functions. However, little is known about the developmental time course of gap junction coupling and how much the syncytium affects whole cell measurements of ion currents. Here we correlated post-whole cell recording immunohistochemistry for GLAST and the spread of injected dye from the recorded cell with the measured electrophysiological phenotype to quantify cell coupling of astrocytes and the type of astrocyte coupled, in the rat hippocampus. We found that passive astrocytes, which predominate after 3 postnatal weeks, have much lower membrane resistances (Rm), and are more frequently dye-coupled and to more cells, than outwardly and variably rectifying astrocytes which predominate in early postnatal development. Dye-coupling in GLAST(+) cells was first detected in the first postnatal week and the degree of coupling peaked before the complete transition to the low Rm, passive electrophysiological type. Also, the degree of dye coupling did not correlate with the passive electrophysiological phenotype. Passive cells were also detected after pre-treatment with a gap junction inhibitor. Further evidence that cell coupling does not contribute to the mature astrocyte electrophysiological phenotype came from recording of excised membrane patches, which predominantly corresponded to the ion channel expression profiles of their cells of origin. These findings imply that in the hippocampus inter-astrocyte cell coupling likely contributes little to the overall whole-cell current profile of diverse glia, and the electrophysiological passivity reflects the intrinsic ion channel expression of the mature astrocyte.
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