Glutamine from Glial Cells Is Essential for the Maintenance of the Nerve Terminal Pool of Glutamate: Immunogold Evidence from Hippocampal Slice Cultures

Abstract

Abstract: The immunogold labeling for glutamate and glutamine was studied at the electron microscopic level in hippocampal slice cultures following inhibition of l‐glutamine synthetase [l‐glutamate:ammonia ligase (ADP‐forming); EC 6.3.1.2]. In control cultures, glutamate‐like immunoreactivity was highest in terminals, intermediate in pyramidal cell bodies, and low in glial cells. Glutamine‐like immunoreactivity was high in glial cells, intermediate in pyramidal cell bodies, and low in terminals. After inhibition of glutamine synthetase with l‐methionine sulfoximine, glutamate‐like immunoreactivity was reduced by 52% in terminals and increased nearly fourfold in glia. Glutamine‐like immunoreactivity was reduced by 66% in glia following l‐methionine sulfoximine, but changed little in other compartments. In cultures that were treated with both l‐methionine sulfoximine and glutamine (1.0 m M), glutamate‐like immunoreactivity was maintained at control levels in terminals, whereas in glia glutamate‐like immunoreactivity was increased and glutamine‐like immunoreactivity was decreased to a similar extent as in cultures treated with l‐methionine sulfoximine alone. We conclude that (a) glutamate accumulates in glia when the flux through glutamine synthetase is blocked, emphasizing the importance of this pathway for the handling of glutamate; and (b) glutamine is necessary for the maintenance of a normal level of glutamate in terminals, and neither reuptake nor de novo synthesis through pathways other than the glutaminase reaction is sufficient.