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Journal of Neuroscience, Vol 14, 1677-1687, Copyright © 1994 by Society for Neuroscience

ARTICLE

Astroglial modulation of transient potassium current development in cultured mouse hippocampal neurons

RL Wu and ME Barish
Division of Neurosciences, Beckman Research Institute of the City of Hope, Duarte, California 91010.

Hippocampal neurons exhibit three voltage-gated potassium currents, two transient currents and a delayed rectifier, that influence numerous aspects of electrogenesis including action potential duration and accommodation to sustained depolarization. These currents, termed A-, D- , and K-currents, respectively, can be distinguished based on kinetics, steady state inactivation characteristics, and sensitivity to 4- aminopyridine (see Wu and Barish, 1992b). We have compared the voltage- gated potassium currents in voltage-clamped pyramidally shaped cultured hippocampal neurons growing on or touching glial fibrillary acidic protein-expressing astroglia (termed on-glia or touching-glia neurons, respectively) with those in similar neurons growing directly on a coated glass substrate (termed off-glia neurons). We observed differences in the wave forms of total potassium current that correlated with the extent of astroglial contact. After 5-7 d in culture, A-current amplitude in off-glial neurons was approximately 19% of that of neurons growing in the normal (for culture) on-glia configuration. D-current amplitude tended to be larger in these off- glia neurons. Neurons in contact with astroglia had greater membrane area than off-glia neurons. Comparison of current densities (current at a fixed voltage normalized to capacitance and expressed in units of pA/pF) indicated that A-currents were the major component of transient potassium current in on- and touching-glia neurons, while D-currents were more dominant in off-glia neurons. Astroglia influenced membrane currents by a surface- or extracellular matrix-associated mechanism, rather than by free diffusion of a soluble factor, as differences were observed between closely adjacent neurons on the same coverslip. Living glia were required, as potassium currents in neurons grown on dried or methanol-fixed glia resembled those of off-glia neurons. On-glia neurons in cultures treated with an RNA synthesis inhibitor [DRB (5,6- dichloro-1-beta-D-ribofuranosylbenzimidazole)] for 5-7 d had reduced whole-cell capacitance and A-current amplitude. This effect was localized to DRB actions on underlying astroglia, not on the neurons. Action potentials elicited by current injection varied with astroglial contact. In on-glia neurons with relatively larger A-currents a delay was seen in the onset of firing after depolarization. In contrast, action potentials in off-glia neurons rose smoothly after initiation of depolarization. We conclude that astrocytes modulate the appearance of transient potassium currents in hippocampal pyramidal neurons by inducing development of A-current.(ABSTRACT TRUNCATED AT 400 WORDS)

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