FUNCTIONAL SPECIALIZATION AND TOPOGRAPHIC SEGREGATION OF HIPPOCAMPAL ASTROCYTES

Astrocytes have been suggested to play several roles in the complex co ntrol of brain microenvironment. However, they have been generally con sidered to constitute a homogeneous population of cells. Here we show that at least three electrophysiologically distinct types of astrocyte s can be found in the mature hippocampus, These subpopulations of glia were characterized by expression of different ion currents. In astroc ytes exposed to elevated K+, Cs+ prevented influx of K+ only in cells with inwardly rectifying currents (I-IR). The topographic distribution of glia with Cs+-sensitive inward rectifying currents (involved in K buffering) was nonuniform. Cs+-sensitive astrocytes were predominantl y found in CA3 radiatum, whereas most CA1 astrocytes were Cs+-insensit ive. Functional significance of the spatial segregation of glial cells with inward rectification was addressed in slices that were bathed in Cs+- containing media. Under these conditions, neuronal stimulation i nduced spontaneous epileptiform activity, which first appeared in CA3 and was then synaptically propagated to CA1. Intracellular labeling of astrocytes with biocytin revealed that CA1 astrocytes are characteriz ed by a high degree of cell-to-cell coupling; in contrast, cell labeli ng in CA3 revealed smaller groups and occasionally individual cells. T hree individual biocytin-labeled cells had electrophysiological proper ties indistinguishable from Cs+-sensitive astrocytes but had morpholog y typical of oligodendroglia, These results provide evidence for a rol e of K+ uptake via I-IR into astrocytes, The segregated expression of potassium channels in a subpopulation of astrocytes suggests that func tionally specialized cell types are involved in K+ homeostasis.