Glial and neuronal glutamate transport following glutamine synthetase inhibition

Glutamate transport into striatal tissue preparations was studied following inhibition of glutamine synthetase with methionine sulfoximine (MSO). Glutamate uptake in striatal tissue prisms was elevated for up to 7 days following an intraventricular (i.c.v.) injection of MSO. Kinetic analysis of glutamate uptake revealed that a high- and a low-affinity carrier system mediated the transport of glutamate into tissue slices. MSO altered the transport of glutamate via the high-affinity carrier without changing the characteristics of low-affinity glutamate transport. MSO increased the K_m, for glutamate and the V_max at the high-affinity uptake site. The changes in the K_m and the V_max for glutamate uptake were maximal 24 hr after administration of MSO, but the transport system returned to normal by 14 days after injection. In addition, MSO increased high-affinity aspartate uptake into tissue slices, but it was without effect on leucine uptake. Glutamate uptake into striatal synaptosomes and bulk-isolated glial cells or neurons was, in all cases, mediated by a low- and high-affinity carrier. The K_m and V_max values for high-affinity glial-glutamate uptake were increased 24 hr after i.c.v. injection of MSO, while the low-affinity kinetic parameters for glial glutamate uptake were not altered by MSO. Neither highaffinity nor low-affinity glutamate uptake into bulk-isolated neurons or synaptosomes was altered by MSO 24 hr after injection. These results suggest that MSO induced alterations in glutamate transport within striatal slices may be due to changes in glial glutamate transport arising from the disruption of glutamate metabolism.