Pii: S0306-4522(97)00146-2

–The GABAA receptor is a ligand-operated chloride channel. It has a pentameric structure. In mammalian brain different subunits are recruited from four gene subfamilies. Using immunocytochemistry, we investigated the distribution of the 13 GABAA receptor subunits in the hippocampus of the rat. GABAA receptor subunits were heterogeneously distributed within different hippocampal subfields. High concentrations of á1-, á2-, á4-, â3-, ã2and ä-immunoreactivities were observed within the molecular layer of the dentate gyrus, representing the dendritic area of the granule cells. In the hippocampus proper, the predominant GABAA receptor subunits were á1, á2, á5, â3 and ã2 that were located throughout the strata radiatum and oriens of CA1 to CA3. Immunocytochemical staining was there less prominent for á4-, â1-, â2-, ã3and äsubunits. In the hippocampus proper, the â1 subunit was preferentially located in CA2. The á4and ä-subunits were somewhat more abundant in CA1 than in CA3. Numerous local circuit neurons in the hippocampus proper and the hilus of the dentate gyrus contained á1-, â2-, ã2and/or ä-subunits. á3 and ã1 were present only in minute amounts and no á6-IR was detected in the hippocampal formation. The distribution of the GABAA receptor subunits indicates the existence of heterogenously constituted GABAA receptor complexes within various hippocampal subfields, which may exert different physiological or pharmacological properties upon stimulation by GABA or its agonists. ? 1997 IBRO. Published by Elsevier Science Ltd.