Freshly isolated hippocampal CA1 astrocytes comprise two populations differing in glutamate transporter and AMPA receptor expression

We have shown previously that process-bearing GFAP+ astrocytes freshly isolated from rat hippocampus CA1 and CA3 regions are heterogeneous in ion channel expression and K+ uptake capabilities, such that two distinct populations of astrocytes can be described (Zhou and Kimelberg, 2000). In the present study, we report that glutamate transporter (GT) currents can only be measured from one type of these freshly isolated hippocampal CA1 astrocytes [variably rectifying astrocytes (VRAs)] but were not detectable in the second type of astrocyte [outwardly rectifying astrocytes (ORAs)]. The GT currents showed a strict Na+ dependency and high affinity for glutamate (EC50 of 4 +/- 1.1 muM). The astrocytes lacking GT currents (ORAs) showed an AMPA receptor current density (55 pA/pF) that was 42-fold higher than VRAs (1.3 pA/pF). In contrast, the GABA(A) currents were of comparable current density in both types. The specificity of these differences makes it unlikely that they are attributable to preparative damage. Therefore, these findings strongly indicate that, within a single region of the hippocampus, GFAP+ astrocytes comprise a functionally diverse population that are qualitatively different in their functional glutamate transporter and quantitatively different in their functional AMPA receptor expression. This heterogeneity implies that GFAP+ astrocytes may participate in or modulate glutamate synaptic transmission differently.