DIFFERENTIAL DEVELOPMENTAL EXPRESSION OF THE 2 RAT-BRAIN GLUTAMATE TRANSPORTER PROTEINS GLAST AND GLT

The extracellular concentration of the excitatory neurotransmitter glu tamate is kept low by the action of glutamate transporters in the plas ma membranes of both neurons and glial cells, These transporters may p lay important roles, not only in the adult brain, but also in the deve loping brain, as glutamate is thought to modulate the formation and el imination of synapses as well as neuronal migration, proliferation and apoptosis. Here we demonstrate the developmental changes in the expre ssion of two glutamate transporters, GLAST and GLT, by quantitative im munoblotting and by light and electron microscopic immunocytochemistry . At birth, GLT is not detectable, but GLAST is present at significant concentrations both in the forebrain and in the cerebellum. GLT is fi rst detected in the forebrain and cerebellum in the second and third w eek, respectively. Both transporters reach adult levels by postnatal w eek 5. The development of the total glutamate uptake activity in the f orebrain, as determined by solubilization and reconstitution of the tr ansporters in liposomes, parallels that of GLT, in agreement with the observation that GLT is the predominant transporter in the adult brain . The regional distributions of both GLAST and GLT in the tissue are s imilar in young and adult rats. Only GLAST is detectable in the extern al germinal layer of the cerebellar cortex, Electron microscopical inv estigation demonstrated GLAST and GLT exclusively in glial cells in yo ung as well as in adult animals.