GABA plasma membrane transporters, GAT-1 and GAT-3, display different distributions in the rat hippocampus

This study evaluates the distribution of two high affinity gamma-aminobutyric acid (GABA) transporters (GAT-1 and GAT-3) in the rat hippocampus using immunocytochemistry and affinity purified antibodies. GAT-1 immunoreactivity was prominent in punctate structures and axons in all layers of the dentate gyrus. In Ammon's horn, immunoreactive processes were concentrated around the somata of pyramidal cells, particularly at their basal regions. The apical and basal dendritic fields of pyramidal cells also displayed numerous GAT-1 immunoreactive punctate structures and axons. The zone of termination of the mossy fibers that includes both the hilus of the dentate gyrus and stratum lucidum of the CA3 area was the lightest immunolabeled region of the hippocampal complex. Electron microscopic preparations demonstrated that GAT-1 immunoreactive axon terminals form symmetric synapses with somata, axon initial segments, and dendrites of granule and pyramidal cells in the dentate gyrus and Ammon's horn, respectively. Immunoreactivity was localized to the plasma membrane and the cytoplasm of axon terminals. The somata of previously described local circuit neurons in the dentate gyrus and Ammon's horn contained GAT-1 immunoreactivity associated with the Golgi complex. Light, diffuse GAT-3 immunoreactivity was present throughout the hippocampal formation. Thin, astrocytic glial processes displayed GAT-1 and GAT-3 immunoreactivity. This localization of GAT-1 and GAT-3 indicates that they are involved in the uptake of GABA from the extracellular space into GABAergic axon terminals and astrocytes.