Glutamine from glial cells is essential for the maintenance of the nerve terminal pool of glutamate: immunogold evidence from hippocampal slice cultures.
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 four-fold 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 mM), 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.
Laake
J H
JH
Department of Anatomy, Faculty of Medicine, University of Oslo, Norway.
Slyngstad
T A
TA
Haug
F M
FM
Ottersen
O P
OP
D000818:Animals; D046508:Culture Techniques; D018698:Glutamic Acid / Q000378:metabolism*; D005973:Glutamine / Q000502:physiology*; D006624:Hippocampus / Q000378:metabolism* / Q000648:ultrastructure; D007106:Immune Sera; D007150:Immunohistochemistry; D008297:Male; D008854:Microscopy, Electron; D009411:Nerve Endings / Q000378:metabolism*; D009457:Neuroglia / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D007106:Immune Sera; D005973:Glutamine; D018698:Glutamic Acid