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The Expression of Small- and Intermediate-Conductance Calcium-Activated Potassium Channels in Astrocytes of the Mouse Brain In the mammalian brain, mature astrocytes form syncytia of hundreds of cells and carry a large, ohmic, potassium-dominated current. Moderate increases in extracellular potassium (∼5mM) dilate blood vessels1 and recently, large-conductance voltage- and calcium-activated potassium channels have been localised to astrocyte vascular endfoot processes2, where potassium efflux through these channels may play a role in the control of local vascular tone in response to alterations in neuronal activity3. Here, we investigated the expression of the small- and intermediate-conductance calcium-activated potassium channels (SKCa or SK1-3 and IKCa, respectively) in astrocytes. In a transgenic mouse line expressing green fluorescent protein (GFP) in astrocytes4, immunohistochemical data indicated immunoreactivity for IKCa on the processes of some astrocytes of the neocortex. Whole-cell patch clamp data obtained in brain slices prepared from 21-30-day-old male and female mice demonstrated the functional expression of this channel in a sub-population of GFP+ neocortical astrocytes. In the presence of 1μM intracellular calcium, the IKCa blocker TRAM 34 (1μM) produced a 41±6% inhibition of whole-cell currents in 56% of cells tested (total n = 16). Application of 100nM apamin (n = 7), the selective SKCa blocker, did not alter currents beyond the level of time-matched controls, which showed an 8±2% rundown (n = 9). Conversely, in the presence of 200nM intracellular calcium, the SKCa and IKCa opener NS309 (500nM) produced a whole-cell current increase of 183±43% in 67% of cells tested (total n = 12) whilst 30μM CyPPA (n = 9), an opener of SK2 and SK3, produced no change in current beyond time-matched control current levels, which showed a rundown of 13±5% (n = 10). Primary cultured GFP+ astrocytes were prepared from 1-day-old transgenic mice. These cells demonstrated a 4-aminopyridine-sensitive (10mM; 32±3% current inhibition, n = 8) voltage-activated component typical of immature astrocytes, and were found to be insensitive to NS309 (250nM; n = 5) TRAM 34 (1μM; n = 4) and apamin (30nM; n = 4). These data strongly suggest that a sub-population of mature astrocytes in the mouse neocortex express a functional IKCa channel, which is not present in primary cultured neonatal astrocytes. No evidence was found for the functional expression of SKCa in either developing or mature astrocytes. We speculate that the astrocytic IKCa channel may have a role in the control of local vascular tone.
1. Weston et al. (2002) BJP 136: 918-926.
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