title: A potentially novel nicotinic receptor in Aplysia neuroendocrine cells.
abstract: Nicotinic receptors form a diverse group of ligand-gated ionotropic receptors with roles in both synaptic transmission and the control of excitability. In the bag cell neurons of Aplysia, acetylcholine activates an ionotropic receptor, which passes inward current to produce a long-lasting afterdischarge and hormone release, leading to reproduction. While testing the agonist profile of the cholinergic response, we observed a second current that appeared to be gated only by nicotine and not acetylcholine. The peak nicotine-evoked current was markedly smaller in magnitude than the acetylcholine-induced current, cooperative (Hill value of 2.7), had an EC50 near 500 μM, readily recovered from desensitization, showed Ca(2+) permeability, and was blocked by mecamylamine, dihydro-β-erythroidine, or strychnine, but not by α-conotoxin ImI, methyllycaconitine, or hexamethonium. Aplysia transcriptome analysis followed by PCR yielded 20 full-length potential nicotinic receptor subunits. Sixteen of these were predicted to be cation selective, and real-time PCR suggested that 15 of the 16 subunits were expressed to varying degrees in the bag cell neurons. The acetylcholine-induced current, but not the nicotine current, was reduced by double-strand RNA treatment targeted to both subunits ApAChR-C and -E. Conversely, the nicotine-evoked current, but not the acetylcholine current, was lessened by targeting both subunits ApAChR-H and -P. To the best of our knowledge, this is the first report suggesting that a nicotinic receptor is not gated by acetylcholine. Separate receptors may serve as a means to differentially trigger plasticity or safeguard propagation by assuring that only acetylcholine, the endogenous agonist, initiates large enough responses to trigger reproduction.
authors.affiliation: Department of Biomedical and Molecular Sciences, Physiology Graduate Program, Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada.
authors.forename: Sean H
authors.initials: SH
authors.lastname: White
authors.affiliation: Department of Biomedical and Molecular Sciences, Physiology Graduate Program, Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Carter
authors.affiliation: Department of Biomedical and Molecular Sciences, Physiology Graduate Program, Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada magoski@queensu.ca.
authors.forename: Neil S
authors.initials: NS
authors.lastname: Magoski
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000157:Aconitine / Q000031:analogs & derivatives / Q000494:pharmacology; D000200:Action Potentials; D000818:Animals; D001048:Aplysia; D020916:Conotoxins / Q000494:pharmacology; D004079:Dihydro-beta-Erythroidine / Q000494:pharmacology; D018738:Hexamethonium / Q000494:pharmacology; D008464:Mecamylamine / Q000494:pharmacology; D055099:Neuroendocrine Cells / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009538:Nicotine / Q000494:pharmacology; D018722:Nicotinic Agonists / Q000494:pharmacology*; D018733:Nicotinic Antagonists / Q000494:pharmacology*; D011978:Receptors, Nicotinic / Q000235:genetics / Q000378:metabolism*; D013331:Strychnine / Q000494:pharmacology; D059467:Transcriptome
chemical_list: D020916:Conotoxins; D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D011978:Receptors, Nicotinic; C054634:methyllycaconitine; D004079:Dihydro-beta-Erythroidine; D018738:Hexamethonium; D008464:Mecamylamine; D009538:Nicotine; D013331:Strychnine; D000109:Acetylcholine; D000157:Aconitine
keywords: Ca2+ permeability; acetylcholine receptor; desensitization; mollusc; α-conotoxin ImI