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.

GABA-like and glycine-like immunoreactivities of the cochlear root nucleus in rat.

The cochlear root nucleus is part of the cochlear nuclear complex in small rodents. Its cells, the large root neurons, have a superficial resemblance to the globular neurons of the ventral cochlear nucleus. It has been a matter of debate, therefore, whether the root neurons and globular neurons represent the same or different types of cell. In the present study the two cell types with adjacent neuropil structures were compared by light microscopic, postembedding immunocytochemistry. Pairs of 0.5 microns sections of resin-embedded, glutaraldehyde-fixed material were treated with purified antisera raised against GABA- and glycine-glutaraldehyde-protein conjugates, respectively. Both types of cell were found to be immunonegative. Striking differences, however, occurred in what was interpreted as afferent nerve terminals. The globular cells appeared to receive numerous afferents with GABA- or glycine-like immunoreactivity on their somata. Immunoreactive terminals on the root neurons, on the contrary, were mostly GABA-positive and located on the dendrites. Although of unknown origin, the immunoreactive afferents were clearly different from the primary fibres as demonstrated both by the immunonegativity of the latter and by the different size and distribution of the terminals labelled anterogradely after horseradish peroxidase injections into the spiral ganglion.