Voltage‐dependent sodium and potassium currents in cultured trout astrocytes

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 mV 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.

Glutamate‐activated ionic currents in cultured astrocytes from trout: Evidence for the occurrence of non‐N‐methyl‐D‐aspartate receptors

“…Hence it is not clear if glutamate receptors represent a specialized feature of mammalian astrocytes or if they also occur in glial cells of phylogenetically older vertebrate species including fishes. Therefore in the present study, we performed whole-cell patch-clamp recordings of glutamate-induced currents from cultured then characterized and identified astrocytes of trout CNS, after we had previously characterized voltage-sensitive ionic currents in glial cultures of this fish species (Glassmeier et al, 1992(Glassmeier et al, , 1994.…”

Molecular structure and expression of shaker type potassium channels in glial cells of trout CNS

“…The electrophysiological properties of this component are difficult to predict at the moment, since no immediate structural homologue for this channel subunit could be identified within the shaker subfamily. The occurrence of only a single potassium channel subtype in trout astrocytes was surprising, since two components of outward potassium currents had been previously revealed by whole cell recording (Glassmeier et al, 1994). The occurrence of additional subtypes of shaker-related channels at very low abundance, however, cannot be entirely ruled out.…”

“…Astrocytes were strongly stained with anti-goldfish GFAP antibodies (Nona et al, 1989) and exhibited a morphological heterogeneity ranging from a flat polygonal structure to a spindle-shaped morphology. Regarding their antigenic phenotype and their electrophysiological features, the cells in these cultures have been previously characterized in detail Glassmeier et al, 1992Glassmeier et al, , 1994.…”

Voltage-gated sodium and potassium channels in radial glial cells of trout optic tectum studied by patch clamp analysis and single cell RT-PCR

“…When taken in culture radial glial cells largely retained their characteristic morphology, allowing us to discriminate them from type I astrocytes that were concomitantly present in the cultures. Furthermore, both subtypes of glial cells exhibited distinct electrophysiological properties, in that almost all radial glial cells expressed voltage-gated sodium currents, while only 10% of type I astrocytes possessed this current (Glassmeier et al, 1994). Finally, voltage-gated potassium currents of type I astrocytes showed a much higher proportion of inactivating component than those of radial glial cells, and astrocytes were endowed with the tsha2 subtype of shaker channels (Nguyen and Jeserich, 1998), which was not detected in radial glial cells.…”

“…Regarding the kinetics of inactivation, however, a more depolarized hϱ curve was determined, indicating ''neuronal-like'' characteristics of this current, which is in line with its fast kinetics of activation and inactivation. Astrocytes and oligodendroglia progenitor cells cultured from whole brain of trout were recently shown to possess similar kinetics of inactivation (Glassmeier et al, 1994;Glassmeier and Jeserich, 1995), and there has been no further evidence for the expression of ''glia-like'' sodium channels in the CNS of fish. The physiological role of glial sodium channels is still not well understood.…”