Ion channels in spinal cord astrocytes in vitro. III. Modulation of channel expression by coculture with neurons and neuron-conditioned medium.

Abstract

1. Astrocytes cultured from rat spinal cord express voltage-activated Na+ channels in high densities (up to 8 channels per microns2). Stellate astrocytes express Na+ currents at all times in vitro. In pancake astrocytes, Na+ channel expression shows a distinct temporal pattern, an absence of channel expression at 1-3 days in vitro (DIV), and peak Na+ channel density at 7-8 DIV. 2. Coculture of spinal cord astrocytes with dorsal root ganglion (DRG) neurons substantially reduces the expression of voltage-activated Na+ channels in both spinal cord astrocyte types. In pancake spinal cord astrocytes, both the percentage of cells expressing Na+ channels and the channel density in Na+ channel-expressing cells are markedly reduced. In stellate spinal cord astrocytes, the percentage of Na+ channel-expressing cells is unchanged, but the Na+ channel density per cell is markedly reduced in coculture. 3. Culturing spinal cord astrocytes in neuron-conditioned media reduces Na+ channel expression in both spinal cord astrocyte types to levels intermediate between coculture and control, suggesting that, at least in part, neuronal effects on Na+ channel expression are mediated by a soluble factor secreted into the media by neurons. 4. As with the expression of voltage-activated Na+ channels, the expression of voltage-activated K+ channels is reduced in both spinal cord astrocyte types cocultured with DRG neurons. The effect is not mimicked by culturing cells in neuron-conditioned media, suggesting that effects on K+ channel expression are mediated by a less stable and more readily degradable factor. 5. Coculture with DRG neurons or culture in neuron-conditioned media do not alter the biophysical properties of voltage-activated Na+ currents in pancake spinal cord astrocytes. Thus steady-state activation, steady-state inactivation, and the time constants of activation and inactivation are virtually unchanged under the various culture conditions.