title: Two distinct classes of muscarinic action on hippocampal inhibitory synapses: M2-mediated direct suppression and M1/M3-mediated indirect suppression through endocannabinoid signalling.
abstract: The cholinergic system in the CNS plays important roles in higher brain functions, primarily through muscarinic acetylcholine receptors. At cellular levels, muscarinic activation produces various effects including modulation of synaptic transmission. Here we report that muscarinic activation suppresses hippocampal inhibitory transmission through two distinct mechanisms, namely a cannabinoid-dependent and cannabinoid-independent mechanism. We made paired whole-cell recordings from cultured hippocampal neurons of rats and mice, and monitored inhibitory postsynaptic currents (IPSCs). When cannabinoid receptor type 1 (CB1) was blocked, oxotremorine M (oxo-M), a muscarinic agonist, suppressed IPSCs in a subset of neuron pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the M(2)-preferring antagonist gallamine, and was totally absent in neuron pairs from M(2)-knockout mice. When CB1 receptors were not blocked, oxo-M suppressed IPSCs in a gallamine-resistant manner in cannabinoid-sensitive pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the CB1 antagonist AM281, and was completely eliminated in neuron pairs from M(1)/M(3)-compound-knockout mice. Our immunohistochemical examination showed that M(2) and CB1 receptors were present at inhibitory presynaptic terminals of mostly different origins. These results indicate that two distinct mechanisms mediate the muscarinic suppression. In a subset of synapses, activation of M(2) receptors at presynaptic terminals suppresses GABA release directly. In contrast, in a different subset of synapses, activation of M(1)/M(3) receptors causes endocannabinoid production and subsequent suppression of GABA release by activating presynaptic CB1 receptors. Thus, the muscarinic system can influence hippocampal functions by controlling different subsets of inhibitory synapses through the two distinct mechanisms.
authors.affiliation: Department of Cellular Neurophysiology, Graduate School of Medical Science, Kanazawa University, 13-1 Takara-machi, Kanazawa 920-8640, Japan.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Fukudome
authors.forename: Takako
authors.initials: T
authors.lastname: Ohno-Shosaku
authors.forename: Minoru
authors.initials: M
authors.lastname: Matsui
authors.forename: Yuko
authors.initials: Y
authors.lastname: Omori
authors.forename: Masahiro
authors.initials: M
authors.lastname: Fukaya
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Tsubokawa
authors.forename: Makoto M
authors.initials: MM
authors.lastname: Taketo
authors.forename: Masahiko
authors.initials: M
authors.lastname: Watanabe
authors.forename: Toshiya
authors.initials: T
authors.lastname: Manabe
authors.forename: Masanobu
authors.initials: M
authors.lastname: Kano
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D048588:Benzoxazines; D015153:Blotting, Western / Q000379:methods; D001921:Brain / Q000033:anatomy & histology / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D063385:Cannabinoid Receptor Modulators / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D063388:Endocannabinoids; D050485:GABA Plasma Membrane Transport Proteins; D005703:Gallamine Triethiodide / Q000494:pharmacology; D006579:Heterozygote; D006624:Hippocampus / Q000166:cytology / Q000502:physiology*; D007150:Immunohistochemistry / Q000379:methods; D066298:In Vitro Techniques; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D051379:Mice; D018345:Mice, Knockout; D009025:Morpholines / Q000494:pharmacology; D018721:Muscarinic Agonists / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D018733:Nicotinic Antagonists; D010095:Oxotremorine / Q000494:pharmacology; D018408:Patch-Clamp Techniques / Q000379:methods; D011720:Pyrazoles / Q000494:pharmacology; D051381:Rats; D043884:Receptor, Cannabinoid, CB1 / Q000378:metabolism; D043643:Receptor, Muscarinic M1 / Q000502:physiology; D043585:Receptor, Muscarinic M2 / Q000235:genetics / Q000502:physiology*; D043587:Receptor, Muscarinic M3 / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D013569:Synapses / Q000187:drug effects / Q000502:physiology*
chemical_list: D048588:Benzoxazines; D002121:Calcium Channel Blockers; D063385:Cannabinoid Receptor Modulators; D002352:Carrier Proteins; D063388:Endocannabinoids; D050485:GABA Plasma Membrane Transport Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009025:Morpholines; D018721:Muscarinic Agonists; D009281:Naphthalenes; D018733:Nicotinic Antagonists; D011720:Pyrazoles; D043884:Receptor, Cannabinoid, CB1; D043643:Receptor, Muscarinic M1; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; C070417:(3R)-((2,3-dihydro-5-methyl-3-((4-morpholinyl)methyl)pyrrolo-(1,2,3-de)-1,4-benzoxazin-6-yl)(1-naphthalenyl))methanone; D010095:Oxotremorine; D005703:Gallamine Triethiodide; C109925:AM 281