title: Carbachol-evoked suppression of excitatory neurotransmission in guinea-pig olfactory cortex slices is unlikely to involve an M4-muscarinic receptor subtype.
abstract: Depression of the electrically-evoked surface-negative field potential (N-Wave) by bath-superfusion of carbachol was measured in guinea-pig olfactory cortex slices maintained in vitro. The possibility that this response, previously proposed to be mediated via a presynaptic M1: muscarinic receptor, might in fact be due to M4 receptor activation, was investigated by testing the effectiveness of himbacine (a proposed M4-selective antagonist) on our cortical preparation. Himbacine (100 nM-1 microM) had no effect on the N-wave potential alone, but it induced a clear competitive-type inhibition of carbachol effects. Schild plot analysis (regression slope constrained to unity) of pooled data yielded a pA2 value of 7.2 for this antagonist (n = 7 slices). This value accords more with that expected for the interaction of himbacine with M1 receptors (approximately 7.2) than with functionally expressed M4 receptors (approximately 8.5-8.5). We therefore conclude that M4-type muscarinic receptors are unlikely to be involved in mediating this presynaptic carbachol response.
authors.affiliation: Department of Pharmacology, School of Pharmacy, London, UK.
authors.forename: B
authors.initials: B
authors.lastname: Das
authors.forename: V
authors.initials: V
authors.lastname: Libri
authors.forename: A
authors.initials: A
authors.lastname: Constanti
mesh_terms: D000470:Alkaloids / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D002217:Carbachol / Q000008:administration & dosage / Q000494:pharmacology*; D002540:Cerebral Cortex / Q000187:drug effects* / Q000502:physiology; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects; D005663:Furans; D006168:Guinea Pigs; D009281:Naphthalenes; D009833:Olfactory Pathways / Q000502:physiology; D010880:Piperidines; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects
chemical_list: D000470:Alkaloids; D005663:Furans; D009281:Naphthalenes; D010880:Piperidines; D011976:Receptors, Muscarinic; D002217:Carbachol; C048172:himbacine