NEURONAL REGULATION OF GLUTAMATE TRANSPORTER SUBTYPE EXPRESSION IN ASTROCYTES

GLT-1, GLAST, and EAAC1 are high-affinity, Na+-dependent glutamate tra nsporters identified in rat forebrain. The expression of these transpo rter subtypes was characterized in three preparations: undifferentiate d rat cortical astrocyte cultures, astrocytes cocultured with cortical neurons, and astrocyte cultures differentiated with dibutyryl cyclic AMP (dBcAMP). The undifferentiated astrocyte monocultures expressed on ly the GLAST subtype. Astrocytes cocultured with neurons developed a s tellate morphology and expressed both GLAST and GLT-1; neurons express ed only the EAAC1 transporter, and rare microglia in these cultures ex pressed GLT-1. Treatment of astrocyte cultures with dBcAMP induced exp ression of GLT-1 and increased expression of GLAST. These effects of d BcAMP on transporter expression were qualitatively similar to those re sulting from coculture with neurons, but immunocytochemistry showed th e pattern of transporter expression to be more complex in the cocultur e preparations. Compared with astrocytes expressing only GLAST, the dB cAMP-treated cultures expressing both GLAST and GLT-1 showed an increa se in glutamate uptake V-max but no change in the glutamate K-m and no increased sensitivity to inhibition by dihydrokainate. Pyrrolidine-2, 4-dicarboxylic acid and threo-beta-hydroxyaspartic acid caused relativ ely less inhibition of transport in cultures expressing both GLAST and GLT-1, suggesting a weaker effect at GLT-1 than at GLAST. These studi es show that astrocyte expression of glutamate transporter subtypes is influenced by neurons, and that dBcAMP can partially mimic this influ ence. Manipulation of transporter expression in astrocyte cultures may permit identification of factors regulating the expression and functi on of GLAST and GLT-1 in their native cell type.