Evidence for the existence of non-GABAergic, cholinergic interneurons in the rodent hippocampus

Previous studies have revealed a small number of hippocampal interneurons i mmunoreactive for choline acetyltransferase, the acetylcholine-synthesizing enzyme. It remained an open question, however, whether these neurons repre sented a subgroup of inhibitory GABAergic neurons co-localizing acetylcholi ne. In this study, we have combined immunocytochemistry for choline acetylt ransferase and in situ hybridization for glutamate decarboxylase messenger RNA, the GABA-synthesizing enzyme. None of the choline acetyltransferase-im munoreactive neurons in the various layers of the hippocampus proper and fa scia dentata were found to co-localize glutamate decarboxylase messenger RN A. The lack of an in situ hybridization signal in these neurons is unlikely to result from the combination of the two labeling techniques. When combin ing in situ hybridization for glutamate decarboxylase messenger RNA with im munostaining for parvalbumin, a calcium-binding protein expressed by many G ABAergic hippocampal interneurons, numerous double-labeled cells were obser ved. These data provide neurochemical evidence for the existence of non-GABAergi c, supposedly cholinergic non-principal cells in the hippocampus. (C) 2000 IBRO. Published by Elsevier Science Ltd.