Although astrocytes constitute the major cell type in the mammalian central nervous system (CNS), their functional properties in maintaining homeostatis during neuronal activity have been reconsidered, with the ability to prepare homogeneous cell cultures. Using these cultures, recent studies indicate that this previously considered electrically silent cell possesses voltage-sensitive channels for sodium, calcium, potassium and chloride. Voltage-sensitive channels meditate changes in membrane permeability to selected ions in response to a change in membrane voltage. This project deals specifically with types of voltage-gated sodium channels in astrocytes. In electrically excitable tissue, two distinct types of sodium channels have been found. These types of voltage-gated channels have separate binding sites for saxitoxin and tetrodotoxin. Rat brain astrocytes also have these types of sodium channels. They are present in differing proportions at early and late stages of astrocyte maturity. Using neurotoxin-activated 22Na+ flux, intracellular and whole-cell recording techniques and toxin binding methods, physiological and pharmacological properties of both types of astrocyte sodium channels will be compared to properties in excitable membranes; the density of sodium channels will be discovered at different stages of morphological differentiation; electrical excitability will be determined in early and late astrocyte cultures; and factors that trigger or retard the expression of toxin-insensitive channels and toxin-sensitive channels will be explored. In addition, an astrocyte membrane preparation will be incorporated into planar lipid bilayers in order to study the biophysical characteristics of single toxin-insensitive and toxin-sensitive sodium channels and to compare their properties with those from electrically excitable tissue. The results of this proposal should provide a new basis for understanding some of the complex interactions between neurons and glia and whether astrocyte sodium channels are similar to those in nerve and muscle.
