TWIK-1 and TREK-1 are potassium channels contributing significantly to astrocyte passive conductance in rat hippocampal slices

Expression of a linear current-voltage (I-V) relationship (passive) K ⁺ membrane conductance is a hallmark of mature hippocampal astrocytes. However, the molecular identifications of the K⁺ channels underlying this passive conductance remain unknown. We provide the following evidence supporting significant contribution of the two-pore domain K ⁺ channel (K_2P) isoforms, TWIK-1 and TREK-1, to this conductance. First, both passive astrocytes and the cloned rat TWIK-1 and TREK-1 channels expressed in CHO cells conduct significant amounts of Cs⁺ currents, but vary in their relative P_Cs/P_K permeability, 0.43, 0.10, and 0.05, respectively. Second, quinine, which potently inhibited TWIK-1 (IC₅₀=85 μM) and TREK-1 (IC₅₀=41 μM) currents, also inhibited astrocytic passive conductance by 58% at a concentration of 200 μM. Third, a moderate sensitivity of passive conductance to low extracellular pH (6.0) supports a combined expression of acid-insensitive TREK-1, and to a lesser extent, acid-sensitive TWIK-1. Fourth, the astrocyte passive conductance showed low sensitivity to extracellular Ba²⁺, and extracellular Ba²⁺ blocked TWIK-1 channels at an IC₅₀ of 960 μM and had no effect on TREK-1 channels. Finally, an immunocytochemical study showed colocalization of TWIK-1 and TREK-1 proteins with the astrocytic markers GLAST and GFAP in rat hippocampal stratum radiatum. In contrast, another K_2P isoform TASK-1 was mainly colocalized with the neuronal marker NeuN in hippocampal pyramidal neurons and was expressed at a much lower level in astrocytes. These results support TWIK-1 and TREK-1 as being the major components of the long-sought K⁺ channels underlying the passive conductance of mature hippocampal astrocytes.