GAT-1, A HIGH-AFFINITY GABA PLASMA-MEMBRANE TRANSPORTER, IS LOCALIZEDTO NEURONS AND ASTROGLIA IN THE CEREBRAL-CORTEX

High affinity, GABA plasma membrane transporters influence the action of GABA, the main inhibitory neurotransmitter. The cellular expression of GAT-1, a prominent GABA transporter, has been investigated in the cerebral cortex of adult rats using in situ hybridizaton with S-35-lab eled RNA probes and immunocytochemistry with affinity purified polyclo nal antibodies directed to the C-terminus of rat GAT-1. GAT-1 mRNA was observed in numerous neurons and in some glial cells. Double-labeling experiments were performed to compare the pattern of GAT-1 mRNA conta ining and GAD67 immunoreactive cells. The majority of neurons expressi ng GAT-1 mRNA also contained GAD67 immunoreactivity (ir), but GAT-1 mR NA was also observed in a few pyramidal neurons. GAT-1-ir was localize d to numerous puncta and fibers and to astrocytic processes, was not o bserved in sections incubated in GAT-1 antibodies preadsorbed with rat GAT-1 C-terminal peptide, and was observed in sections incubated in G AT-1 antibodies preadsorbed with the C-terminal portion of the related peptides rat GAT3(607-627) or rat glycine transporter-1(625-633). The highest number of GAT-1-ir puncta was in layer IV, followed by layers II-III. GAT-1 positive puncta appeared to have a preferential relatio nship to the soma and proximal dendrites of unlabeled pyramidal cells. All GAT-1 positive axon terminals formed symmetric synapses. This stu dy demonstrates that (1) GAT-1 is expressed by both neurons and astroc ytes, (2) the majority of GAT-1 expressing neurons contain GAD67, and (3) GAT-1 uptake system is more extensive than the GABA synthetizing s ystem. These observations support the hypothesis that, in addition to its role in terminating GABA action by uptake into GABAergic axon term inals, GAT-1 influences both excitatory and inhibitory transmission by modulating the ''paracrine'' spread of GABA (Isaacson et al., 1993), and suggest that astrocytes may play an important role in this process .