DISTINCT INTERNEURON TYPES EXPRESS M2 MUSCARINIC RECEPTOR IMMUNOREACTIVITY ON THEIR DENDRITES OR AXON TERMINALS IN THE HIPPOCAMPUS

In previous studies m2 muscarinic acetylcholine receptor-immunoreactiv e interneurons and various types of m2-positive axon terminals have be en described in the hippocampal formation.(43) The aim of the present study was to identify the types of interneurons expressing m2 receptor and to examine whether the somadendritic and axonal m2 immunostaining labels the same or distinct cell populations. In the CA1 subfield, ne urons immunoreactive for m2 have horizontal dendrites, they are locate d at the stratum oriens/alveus border and have an axon that project to the dendritic region of pyramidal cells. In the CA3 subfield and the hilus, m2-positive neurons are multipolar and are scattered in all lay ers except stratum lacunosum-moleculare. In stratum pyramidale of the CA1 and CA3 regions, striking axon terminal staining for m2 was observ ed, surrounding the somata and axon initial segments of pyramidal cell s in a basket-like manner. The co-localization of m2 with neurochemica l markers and GABA was studied using the ''mirror'' technique and fluo rescent double-immunostaining at the light microscopic level and with double-labelling using colloidal gold-conjugated antisera and immunope roxidase reaction (diaminobenzidine) at the electron microscopic level . GABA was shown to be present in the somata of most m2-immunoreactive interneurons, as well as in the majority of m2-positive terminals in all layers. The calcium-binding protein parvalbumin was absent from pr actically all m2-immunoreactive cell bodies and dendrites. In contrast , many of the terminals synapsing on pyramidal cell somata and axon in itial segments co-localized parvalbumin and m2, suggesting a different ial distribution of m2 receptor immunoreactivity on the axonal and som adendritic membrane of parvalbumin-containing basket and axo-axonic ce lls. The co-existence of m2 receptors with the calcium-binding protein calbindin and the neuropeptides cholecystokinin and vasoactive intest inal polypeptide was rare throughout the hippocampal formation. Only c alretinin and somatostatin showed an appreciable degree of co-localiza tion with m2 (20% and 15%, respectively). Using retrograde tracing, so me of the m2-positive cells in stratum oriens were shown to project to the medial septum, accouting for 38% of all projection neurons. The p resent results demonstrate that there is a differential distribution o f m2 receptor immunoreactivity on the axonal vs the somadendritic memb ranes of distinct interneuron types and suggest that acetylcholine via m2 receptors may reduce GABA release presynaptically from the termina ls of perisomatic inhibitory cells, while it may act to increase the a ctivity of another class of interneuron, which innervates the dendriti c region of pyramidal cells. (C) 1997 IBRO. Published by Elsevier Scie nce Ltd.