CHOLINERGIC AND GABAERGIC NEURONS IN THE RAT MEDIAL SEPTUM EXPRESS MUSCARINIC ACETYLCHOLINE-RECEPTORS

This study describes the cellular distribution of muscarinic acetylcho line receptors (mAChRs) in the medial septum (MS), employing the monoc lonal antibody M35 raised against purified mAChR-protein. mAChR-positi ve neurons are found throughout the MS, but are predominantly located in the midline area and in the lateral compartments. The labeled cell bodies are variable in shape and size (largest diameter ranging from 1 0-30 mu m), while both soma and the associated dendritic processes are densely stained for mAChRs. Astrocytes immunoreactive for mAChRs were frequently found associated with the large blood vessels in the midli ne area. To study the neurotransmitter nature of the mAChR-positive ce lls, immunofluorescence double-labeling experiments were performed for mAChRs and GABAergic and cholinergic markers. GABAergic cells were id entified immunocytochemically using antisera against glutamic acid dec arboxylase (GAD), parvalbumin (PARV) or calbindin protein (CaBP). The cholinergic transmitter nature of the mAChR-positive cells was studied using adjacent 8 mu m thick serial sections stained immunocytochemica lly for choline acetyltransferase (ChAT), or histochemically for acety lcholinesterase (AChE). These experiments showed that approximately ha lf (52.3%) of all mAChR-positive cells contain GAD, whereas the other half is cholinergic. Conversely, nearly all GABAergic (98.6%) and chol inergic (96.9%) cells are endowed with mAChRs. GAD-positive terminals were found surrounding mAChR-positive perikarya which were either GAD- positive or GAD-negative, indicating GABAergic innervation on both GAB Aergic and cholinergic MS neurons. In general, the staining intensity for mAChRs appeared to be considerably higher in GABAergic than in cho linergic neurons, suggesting a stronger cholinergic impact upon the GA BAergic neurons. The current anatomical findings contribute to the con cept that the MS neurons form a firmly interconnected cell group, in w hich cholinergic neurotransmission mediated through mAChRs seems to pl ay a significant role.