ASTROGLIAL MODULATION OF TRANSIENT POTASSIUM CURRENT DEVELOPMENT IN CULTURED MOUSE HIPPOCAMPAL-NEURONS

Hippocampal neurons exhibit three voltage-gated potassium currents, tw o transient currents and a delayed rectifier, that influence numerous aspects of electrogenesis including action potential duration and acco mmodation 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-aminop yridine (see Wu and Barish, 1992b). We have compared the voltage-gated potassium currents in voltage-clamped pyramidally shaped cultured hip pocampal neurons growing on or touching glial fibrillary acidic protei n-expressing astroglia (termed on-glia or touching-glia neurons, respe ctively) with those in similar neurons growing directly on a coated gl ass substrate (termed off-glia neurons). We observed differences in th e wave forms of total potassium current that correlated with the exten t of astroglial contact. After 5-7 d in culture, A-current amplitude i n off-glia neurons was approximately 19% of that of neurons growing in the normal (for culture) on-glia configuration. D-current amplitude t ended 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 capacit ance and expressed in units of pA/pF) indicated that A-currents were t he 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 solub le factor, as differences were observed between closely adjacent neuro ns on the same coverslip. Living glia were required, as potassium curr ents in neurons grown on dried or methanol-fixed glia resembled those of off-glia neurons. On-glia neurons in cultures treated with an RNA s ynthesis inhibitor [DRB 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazo le)] for 5-7 d had reduced whole-cell capacitance and A-current amplit ude. This effect was localized to DRB actions on underlying astroglia, not on the neurons. Action potentials elicited by current injection v aried with astroglial contact. In on-glia neurons with relatively larg er A-currents a delay was seen in the onset of firing after depolariza tion. In contrast, action potentials in off-glia neurons rose smoothly after initiation of depolarization. We conclude that astrocytes modul ate the appearance of transient potassium currents in hippocampal pyra midal neurons by inducing development of A-current. Increased A-curren t amplitude was associated with an increase in membrane area; this obs ervation suggests the possibility that glia promote insertion of A-cur rent-rich membrane. Signals appear to pass from astroglia to neurons b y contact or short-range diffusion. We propose astroglial-induced plas ticity of A-current as a mechanism for long-term modulation of hippoca mpal neuron excitability.