Voltage‐dependent sodium and potassium currents in cultured trout astrocytes

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Abstract
Voltage‐gated ionic currents were recorded from cultured trout astrocytes with the whole‐cell variation of the patch‐clamp technique. In a subpopulation of astrocytes depolarizations above −40 m V activated a fast transient inward current that was identified as a sodium current by ion substitution experiments, its current reversal potential, and its TTX‐sensitivity. Regarding threshold of activation, peak current voltage, and amplitude this current closely resembled those previously described for mammalian astrocytes. Voltage‐dependence of inactivation and kinetics, however, markedly differed from the “glial‐like” sodium current occurring in mammalian hippocampal or optic nerve astrocytes, since the sodium current of trout astrocytes exhibited a faster time course of activation and decay and a more depolarized steady‐state inactivation curve with midpoints close to −60 mV. During a period of 2 weeks in culture the biophysical properties of the sodium current did not change significantly, albeit a continuous decrease in current density was observed. At depolarizing voltage steps positive to −40 mV, additionally voltage‐gated potassium outward currents were evoked, which could be separated into a steady‐state current with delayed rectifier properties and an inactivating component resembling the A‐type current. Moreover, in a subpopulation of astrocytes an inward potassium current was elicited at hyperpolarizing potentials, which exhibited biophysical features consistent with the potassium inward rectifier of mammalian astrocytes.