EXPRESSION OF M1-M4 MUSCARINIC ACETYLCHOLINE-RECEPTOR PROTEINS IN RATHIPPOCAMPUS AND REGULATION BY CHOLINERGIC INNERVATION

A family of muscarinic ACh receptor genes are expressed in hippocampus , but little is known about the localization of the encoded proteins a nd their regulation by cholinergic innervation, Subtype-specific antib odies were used to localize m1-m4 proteins in the hippocampal formatio n by immunocytochemistry and to determine the alterations in the subty pes following deafferentation. Each of the receptors is differentially localized in Ammon's horn and dentate gyrus, with highly complementar y distributions. m1 is widely expressed in somata and dendrites of pyr amidal neurons and granule cells in dentate gyrus. m2 immunoreactivity is expressed mostly in nonpyramidal neurons, and in several discrete bands of fibers and puncta surrounding pyramidal neurons and other lay ers. m3 is enriched in pyramidal neurons, the neuropil in stratum lacu nosum-moleculare and the outer third of the molecular layer of dentate gyrus. m4 is enriched in nonpyramidal neurons, in fiber pathways (alv eus, fimbria, and hippocampal commissure), and in the inner third of t he molecular layer. Fimbria-fornix lesions decreased ipsilateral m2- a nd m4-immunoreactive axons in the fimbria, with no apparent changes in the distribution of any of the receptors in hippocampus. 192-IgG immu notoxin lesions of the cholinergic septohippocampal projections, which spare noncholinergic projections, produced a small decrease in m2-imm unoreactive fibers in the fimbria with no other major changes in the d istribution of subtypes. Immunoprecipitation studies at 3-28 d followi ng fimbria-fornix lesions revealed a 25% loss of m2 at 3 d in hippocam pus, and upregulation of both m1 (20-29% at 7-14 d) and m4 (44% at 28 d). Thus, the vast majority of muscarinic receptor subtypes are intrin sic to the hippocampal formation and/or nonseptal hippocampal afferent s. A subset of m2 and m4 are presynaptically localized, with m2 in cho linergic axons and m2 and m4 possibly in noncholinergic axons that com prise the septohippocampal pathway. The unique laminar and regional di stributions of m1-m4 in the hippocampus reflect differential cellular and subcellular distributions of the subtypes and/or selective associa tion of receptor subtypes with certain afferent and intrinsic connecti ons. These results indicate that each subtype likely has a different r ole in cholinergic modulation of excitatory and inhibitory hippocampal circuits.