title: Identification of rat brain muscarinic M4 receptors coupled to cyclic AMP using the selective antagonist muscarinic toxin 3.
abstract: In membranes of olfactory tubercle and striatum, the selective muscarinic M4 receptor antagonist muscarinic toxin 3 completely antagonized the acetylcholine-induced inhibition of forskolin- and dopamine D1 receptor-stimulated cyclic AMP formation with Ki values of 7 and 4 nM, respectively. In olfactory bulb, where acetylcholine stimulated basal adenylyl cyclase activity and inhibited forskolin-stimulated enzyme activity, muscarinic toxin 3 caused a partial antagonism of both acetylcholine effects with high potencies (Ki values = 4-6 nM). In frontal cortex, muscarinic toxin 3 counteracted the acetylcholine-induced potentiation of corticotropin-releasing hormone-stimulated cyclic AMP with a Ki of 58 nM, which is close to the toxin affinity for the muscarinic M1 receptor. In the same brain region, the acetylcholine inhibition of forskolin-stimulated enzyme activity was not affected by muscarinic toxin 3. In microdissected regions of the hippocampus, a significant portion (33-48%) of the acetylcholine inhibition of forskolin-stimulated adenylyl cyclase activity was blocked by muscarinic toxin 3 with Ki values (6-8 nM) consistent with the involvement of muscarinic M4 receptors. These data show that muscarinic toxin 3 discriminates between adenylyl cyclase-coupled muscarinic receptors and demonstrate the utility of the toxin in identifying the relative contribution by the muscarinic M4 receptor subtype.
authors.affiliation: Department of Neuroscience, University of Cagliari, Italy.
authors.forename: M C
authors.initials: MC
authors.lastname: Olianas
authors.forename: A
authors.initials: A
authors.lastname: Adem
authors.forename: E
authors.initials: E
authors.lastname: Karlsson
authors.forename: P
authors.initials: P
authors.lastname: Onali
mesh_terms: D000262:Adenylyl Cyclases / Q000032:analysis; D000818:Animals; D001921:Brain / Q000502:physiology*; D002540:Cerebral Cortex / Q000187:drug effects; D000242:Cyclic AMP / Q000502:physiology*; D003627:Data Interpretation, Statistical; D006624:Hippocampus / Q000187:drug effects; D066298:In Vitro Techniques; D036341:Intercellular Signaling Peptides and Proteins; D008297:Male; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D009830:Olfactory Bulb / Q000187:drug effects; D009833:Olfactory Pathways / Q000187:drug effects; D010455:Peptides / Q000494:pharmacology*; D051381:Rats; D011976:Receptors, Muscarinic / Q000502:physiology*
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D018727:Muscarinic Antagonists; D010455:Peptides; D011976:Receptors, Muscarinic; C089528:muscarinic toxin 3; D000242:Cyclic AMP; D000262:Adenylyl Cyclases