M1 muscarinic receptor signaling in mouse hippocampus and cortex

The five subtypes (M1-M5) of muscarinic acetylcholine receptors signal through Galphaq or Galphai/Galphao. M1, M3 and M5 receptors couple through Galphaq and function predominantly as postsynaptic receptors in the central nervous system. M1 and M3 receptors are localized to brain regions involved in cognition, such as hippocampus and cortex. but their relative contribution to function has been difficult to ascertain due to the lack of subtype specific ligands. A functional and genetic approach was used to identify the predominant muscarinic receptor subtype(s) mediating responses in mouse hippocampus and cortex, is well as the relative degree of spare muscarinic receptors in hippocampus. The nonselective muscarinic agonist oxotremorine-M stimulated Galphaq/11-specific GTP-gamma-S-35 binding in a concentration dependent manner with a Hill slope near unity in wild type mouse hippocampus and cortex. Muscarinic receptor stimulated Galphaq/11-specific GTP-gamma-S-35 binding was virtually abolished in both the hippocampus and cortex of M1 receptor knockout (KO) mice. In contrast, there was no loss of signaling in M3 receptor KO ruice in either brain region. Muscarinic receptor reserve in wildtype mouse hippocampus was measured by Furchgott analysis after partial receptor alkylation with propylbenzylcholine mustard. Occupation of just 15% of the MI receptors in mouse hippocampus was required for maximal efficacy of oxotrernorine-M -stimulated GTP- S bind ng indicating a substantial level of spare receptors. These findings support a role for the M1 receptor subtype as the primary Gaq/11-coupled muscarinic receptor in mouse hippocampus and cortex. (C) 2002 Elsevier Science B.V. All rights reserved.