The Glutamate Transporter GLT1a Is Expressed in Excitatory Axon Terminals of Mature Hippocampal Neurons

Abstract

GLT1 is the major glutamate transporter of the brain and has been thought to be expressed exclusively in astrocytes. Although excitatory axon terminals take up glutamate, the transporter responsible has not been identified. GLT1 is expressed in at least two forms varying in the C termini, GLT1a and GLT1b. GLT1 mRNA has been demonstrated in neurons, without associated protein. Recently, evidence has been presented, using specific C terminus-directed antibodies, that GLT1b protein is expressed in neurons in vivo. These data suggested that the GLT1 mRNA detected in neurons encodes GLT1b and also that GLT1b might be the elusive presynaptic transporter. To test these hypotheses, we used variant-specific probes directed to the 3Ј-untranslated regions for GLT1a and GLT1b to perform in situ hybridization in the hippocampus. Contrary to expectation, GLT1a mRNA was the more abundant form. To investigate further the expression of GLT1 in neurons in the hippocampus, antibodies raised against the C terminus of GLT1a and against the N terminus of GLT1, found to be specific by testing in GLT1 knock-out mice, were used for light microscopic and EM-ICC. GLT1a protein was detected in neurons, in 14 -29% of axons in the hippocampus, depending on the region. Many of the labeled axons formed axo-spinous, asymmetric, and, thus, excitatory synapses. Labeling also occurred in some spines and dendrites. The antibody against the N terminus of GLT1 also produced labeling of neuronal processes. Thus, the originally cloned form of GLT1, GLT1a, is expressed as protein in neurons in the mature hippocampus and may contribute significantly to glutamate uptake into excitatory terminals.

Increased expression of the astrocytic glutamate transporter GLT‐1 in the prefrontal cortex of schizophrenics

“…In the present and in previous studies (Minelli et al, 2001;Melone et al, 2001), we observed GLT-1 ir exclusively in astrocytic processes, in line with the results of most reports (Danbolt, 2001). Recently, Chen et al (2004), using the same antibody used here, reported that in the hippocampus of animals perfused without glutaraldehyde only 65-70% of immunoreactivity was astrocytic, the remaining being neuronal. Even though these findings await confirmation in neocortical areas of humans, they raise the possibility that the changes reported here may not depend on altered astrocytic GLT-1 expression alone.…”

The mGluR2/3 Agonist LY379268 Blocks the Effects of GLT-1 Upregulation on Prepulse Inhibition of the Startle Reflex in Adult Rats

“…and perfused transcardially with a flush (B1 min) of saline, followed by 4% paraformaldehyde in phosphate buffer (PB). The brains were removed, postfixed in the same fixative for 7 days (Chen et al, 2004), and cut on a Vibratome in 50-mm parasagittal sections, which were collected in PB in serial groups of 5 and then used for immunocytochemistry (Minelli et al, 2001;Chen et al, 2004;Melone et al, 2005). Sections were incubated in a solution containing a mixture of GLT-1 (1 : 1000; Millipore; AB1783; Bragina et al, 2006) and mGlurR-2/3 (1 : 50; Millipore; AB1553; Testa et al, 1998) primary antibodies.…”

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

“…However, more recent studies have established that GLT1 is far more abundant than GLT1b in both neurons and astrocytes, indicating that GLT1b is neither unique to neurons, nor is it the predominant isoform in neurons. 54,55 In retrospect, it is possible that the use of various immunohistochemical techniques, such as employing ultrathin sections and fixation without glutaraldehyde, may have produced the confusing results observed in 2002.…”