The glial glutamate transporter GLT-1 is localized both in the vicinity of and at distance from axon terminals in the rat cerebral cortex.
Glutamate transporter-1 (GLT-1) is responsible for the largest proportion of glutamate transport in the brain and the density of GLT-1 molecules inserted in the plasma membrane is highest in regions of high demand. Previous electron microscopic studies in the hippocampus and cerebellum have shown that GLT-1 is concentrated both in the vicinity of and at considerable distance from the synaptic cleft [Chaudry et al., Neuron 15 (1995) 711-721], but little is known about its distribution in the neocortex. We therefore studied the spatial relationships between elements expressing the presynaptic marker synaptophysin and those containing GLT-1 in the rat cerebral cortex using confocal microscopy. Preliminary studies confirmed that GLT-1 positive puncta were exclusively astrocytic processes; moreover, they showed that in most cases GLT-1 positive processes either completely surrounded asymmetric synapses or had no apparent relationship with synapses; occasionally, they were apposed to terminals containing pleomorphic vesicles. In sections double-labeled for GLT-1 and synaptophysin, codistribution analysis revealed that 61.2% of pixels detecting fluorescent emission for GLT-1 immunoreactivity overlapped with pixels detecting synaptophysin. The percentages of GLT-1/synaptophysin codistribution were significantly different from controls. In sections double-labeled for GLT-1 and the vesicular GABA transporter, codistribution analysis revealed that 27% of pixels detecting GLT-1 overlapped with those revealing the vesicular GABA transporter.The remarkable 'synaptic' localization of GLT-1 provides anatomical support for the hypothesis that in the cerebral cortex GLT-1 contributes to shaping fast, point-to-point, excitatory synaptic transmission. Moreover, the considerable fraction of GLT-1 immunoreactivity localized at sites distant from axon terminals supports the notion that glutamate spillout occurs also in the intact brain and suggests that 'extrasynaptic' GLT-1 regulates the diffusion of glutamate escaped from the cleft.
Minelli
A
A
Istituto di Fisiologia Umana, Università di Ancona, Italy.
Barbaresi
P
P
Reimer
R J
RJ
Edwards
R H
RH
Conti
F
F
D000818:Animals; D001253:Astrocytes / Q000378:metabolism; D002540:Cerebral Cortex / Q000378:metabolism* / Q000648:ultrastructure; D027342:Excitatory Amino Acid Transporter 2 / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D007124:Immunoenzyme Techniques; D018613:Microscopy, Confocal; D009457:Neuroglia / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism*; D051381:Rats; D013569:Synapses / Q000378:metabolism; D016708:Synaptophysin / Q000378:metabolism; D014018:Tissue Distribution
D027342:Excitatory Amino Acid Transporter 2; D016708:Synaptophysin