Passive glial cells, fact or artifact?

Astrocytes that are recorded in acute tissue slices of rat hippocampus usin g whole-cell patch-clamp, commonly exhibit voltage-activated Na+ and K+ cur rents. Some reports have described astrocytes that appear to lack voltage-a ctivated currents and proposed that these cells constitute a subpopulation of electrophysiologically passive astrocytes. We show here that these cells can spontaneously change during a recording unmasking expression of previo usly suppressed voltage-activated currents, suggesting that such cells do n ot represent a subpopulation of passive astrocytes. Superfusion of a low Ca 2+/EGTA solution was able to reversibly suppress voltage-activated K+ curre nts in cultured astrocytes. Currents were restored upon addition of normal bath Ca2+. These effects of Ca2+ on both outward and inward K+ currents wer e dose- and time-dependent, with increasing concentrations of Ca2+ (from 0 to 800 mu M) leading to a gradual unmasking of voltage-dependent outward an d inward K+ currents. The transition from an apparently passive cell to one exhibiting prominent voltage-activated currents was not associated with an y changes in membrane capacitance or access resistance. By contrast, in cel ls in which low access resistance or poor seal accounted for the absence of voltage-activated currents, improvement of cell access was always accompan ied by changes in series resistance and membrane capacitance. We propose that spillage of pipette solution containing low Ca2+/EGTA durin g cell approach in slice recordings and/or poor cell access, lead to a tran sient masking of voltage-activated currents even in astrocytes that express prominent voltage-activated currents. These cells, however, do not constit ute a subpopulation of electrophysiologically passive astrocytes.