GABAERGIC NEURONS OF THE RAT DORSAL HIPPOCAMPUS EXPRESS MUSCARINIC ACETYLCHOLINE-RECEPTORS

The expression of muscarinic acetylcholine receptors (mAChRs) in gluta mic acid decarboxylase (GAD)-positive cells in the different strata of CA1, CA3, and the dentate gyrus (DG) of the dorsal hippocampus is exa mined by way of quantitative immunofluorescent double labeling employi ng M35, the monoclonal antibody raised against purified mAChR protein. Of all GAD-positive neurons, 97.5% express mAChRs. Conversely, 92.9% of the muscarinic cholinoceptive nonpyramidal neurons express GAD. The se results indicate that the vast majority of the gamma-aminobutyric a cid (GABA)ergic neurons express mAChRs. In addition to GAD, parvalbumi n (PARV) and somatostatin (SOM) are two neurochemical substances notab ly expressed in GABAergic neurons. In order to examine whether the ent ire muscarinic cholinoceptive nonpyramidal cell group can be character ized by these three GABAergic markers, a cocktail of GAD, PARV, and SO M was used in a fluorescent double-labeling experiment with M35. These results show that 97.2% of all muscarinic cholinoceptive nonpyramidal neurons can be neurochemically characterized by the content of GAD, P ARV, and SOM. In conclusion, nearly all GABAergic cells express mAChRs and, conversely, virtually the entire muscarinic cholinoceptive nonpy ramidal cell group belongs to the GABAergic cell population. This stud y, therefore, provides anatomical evidence for an extensive neuronal c onnectivity of the hippocampal muscarinic cholinoceptive nonpyramidal system and the inhibitory GABAergic circuitry.