Synaptic localization of GLT-1a in the rat somatic sensory cortex.

GLT-1a, the major glutamate transporter, plays an important role in both physiological and pathological conditions. Uncertainty regarding its localization in the cerebral cortex prompted us to re-examine its cellular and subcellular localization in the rat somatic sensory cortex. GLT-1a detection was sensitive to fixation; in optimal conditions approximately 25% of GLT-1a+ profiles were axon terminals. GLT-1a/VGLUT1 double-labeling and pre-embedding electron microscopy studies showed that approximately 50% of GLT-1a+ profiles were in the vicinity of asymmetric synapses. Using pre-embedding electron microscopy, we found that approximately 70% of GLT-1a located in the vicinity of asymmetric synapses was astrocytic and approximately 30% was neuronal. Post-embedding immunogold studies showed that the density of gold particles coding for GLT-1a was much higher in astrocytic processes than in axon terminals, and that in the latter they were never at the active zone. In both astrocytic processes and axon terminals most gold particles were localized in a membrane region extending for about 250 nm from active zone margin, with a peak at 140 nm for astrocytic processes and at 80 for axon terminals. We conclude that, although GLT-1a is expressed by both astrocytes and axon terminals, astrocytic GLT-1a predominates at asymmetric synapses, and that the perisynaptic localization of GLT-1a in cortex is well-suited to modulate Glu concentrations at the cleft and also to restrict Glu spillover.