Abstract
Subtypes of nicotinic acetylcholine receptors (nAChR) containing α6 subunits comprise 25 to 30% of the presynaptic nAChRs expressed in striatal dopaminergic terminals in rodents and 70% in monkeys. This class of receptors, potentially important in nicotine addiction, binds α-conotoxin MII (α-CtxMII) with high affinity and is heterogeneous, consisting of several subtypes in mice, possibly an important consideration for the design of compounds that selectively activate or antagonize the α6 subclass of nAChRs. Selected-null mutant mice were bred to generate isolated subtypes of α6β2* nAChRs expressed in vivo for assessing pharmacology of α6β2* nAChRs. Binding to striatal membranes and function in synaptosomes from (α4–/–)(β3+/+) and (α4–/–)(β3–/–) mice were measured and compared with wild-type (α4+/+)(β3+/+) mice. Gene deletions (α4 and β3) decreased binding of 125I-α-CtxMII without affecting affinity for α-CtxMII or inhibition of α-CtxMII binding by epibatidine or nicotine. Deletion of the α4 subunit substantially increased EC50 values for both nicotine- and cytisine-stimulated α-CtxMII-sensitive dopamine release from striatal synaptosomes. A further increase in EC50 values was seen upon the additional deletion of the β3 subunit. The data indicate that one α-CtxMII-sensitive nAChR subtype, prevalent on wild-type dopaminergic terminals, has the lowest EC50 for a nicotine-mediated function so far measured in mice. In conclusion, the gene deletion strategy enabled isolation of α6* subtypes, and these nAChR subtypes exhibited differential activation by nicotine and cytisine.
Footnotes
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This work was supported by National Institute on Drug Abuse grants DA12242 (to M.J.M.) and DA03194 (to A.C.C), National Institute of Mental Health grant MH53631 (to J.M.M.), Colorado Tobacco Research Grant 3F-034 (to O.S.), and an Emil Aaltonen Foundation grant (to O.S.). Production of the null mutant mice was supported by animal resources grant DA015663 (to A.C.C.).
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A portion of this work has been presented as an abstract: Salminen OS, Marks MJ, McIntosh JM, Collins AC, and Grady SR. (2005) Characterization of a novel presynaptic nicotinic acetylcholine receptor subtype (62) on mouse brain dopaminergic terminals. Soc Neurosci Abstr31:954.7
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
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doi:10.1124/mol.106.031492.
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ABBREVIATIONS: nAChR, nicotinic acetylcholine receptor; α-CtxMII, α-conotoxin MII; ACh, acetylcholine iodide; A-85380, 3-((2S)-azetidinylmethoxy)pyridine dihydrochloride; BSA, bovine serum albumin; DHβE, dihydro-β-erythroidine; MLA, methyllycaconitine; ST, striatum; OT, olfactory tubercle; NAcc, nucleus accumbens; SC, superior colliculus.
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↵1 Current affiliation: Division of Pharmacology and Toxicology, Faculty of Pharmacy, University of Helsinki, Helsinki, Finland.
- Received October 4, 2006.
- Accepted March 6, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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