ABSTRACT: Hippocampal inhibitory interneurons are a diverse population of cells widely scattered in the hippocampus, where they regulate hippocampal circuit activity. The hippocampus receives cholinergic projections from the basal forebrain, and functional studies have suggested the presence of different subtypes of nicotinic acetylcholine receptors (AChRs) on gamma-aminobutyric acid (GABA)ergic interneurons. Single-cell polymerase chain reaction analysis had confirmed that several nAChR subunit mRNAs are co-expressed with glutamate decarboxylase 67 (GAD67), the marker for GABAergic interneurons. In this anatomical study, we systematically investigated the co-expression of GAD67 with different nAChR subunits by using double in situ hybridization with a digoxigenin-labeled GAD67 probe and (35)S-labeled probes for nAChR subunits (alpha2, alpha3, alpha4, alpha5, alpha6, alpha7, beta2, beta3, and beta4). The results revealed that most GAD67-positive interneurons expressed beta2, and 67 % also expressed alpha7 mRNA. In contrast, mRNA expression of other subunits was limited; only 13 % of GAD67-positive neurons co-expressed alpha4, and less than 10% expressed transcripts for alpha2, alpha3, alpha5, or beta4. Most GAD67/alpha2 co-expression was located in CA1/CA3 stratum oriens, and GAD67/alpha5 co-expression was predominantly detected in CA1/CA3 stratum radiatum/lacunosum moleculare and the dentate gyrus. Expression of alpha6 and beta3 mRNAs was rarely detected in the hippocampus, and mRNAs were not co-expressed with GAD67. These findings suggest that the majority of nicotinic responses in GABAergic interneurons should be mediated by a homomeric alpha7 or heteromeric alpha7*-containing nAChRs. Other possible combinations such as alpha2beta2*, alpha4beta2*, or alpha5beta2* heteromeric nAChRs could contribute to functional nicotinic response in subsets of GABAergic interneurons but overall would have a minor role.
