Though many smokers report wanting to quit, very few are able to do so with traditional medical therapies. In May of this year, Pfizer received FDA approval to market its smoking cessation drug, varenicline, a partial agonist with purported selectivity for the ?2 containing nicotinic acetylcholine receptors (?2*nAChRs). An accumulation of data indicates that the ?2*nAChRs are necessary for nicotine associated dopamine (DA) release and for the primary reinforcing effects of nicotine. The ?2*nAChRs have more recently been implicated for their role in regulating conditioned reinforcement (CR); that is to say, the control that cues paired with a primary reinforcer, such as nicotine, have over behaviors, such as smoking. The primary goal of the proposed experiments is to determine whether subclasses of ?2*nAChRs, broken down by a-conotoxin MII sensitivity, differentially regulate nicotine's primary rewarding effects versus nicotine's ability to enhance CR. Like the a-conotoxin MII (a-CMII) insensitive a4?2*nAChRs, the a-CMII sensitive, a6?2?3*nAChRs, also modulate nicotine-stimulated DA release. Chiefly expressed in catecholaminergic neurons and highly enriched at striatal DA terminals, the role of a6?2?3*nAChRs in nicotine reinforcement is not known. Although the a4 subunit is highly implicated for its contributions to nicotine reward, no studies to date have specifically questioned whether the a4 subunits are required for nicotine reward.
Specific Aim 1 of these studies is to identify which nAChR subunits in combination with ?2 are necessary for the rewarding effects of nicotine as measured by non-biased nicotine conditioned place preference (CPP). Experiment 1 will test ?3 and a4 nAChR subunit knockout mice for altered levels of nicotine CPP. Studies have shown that ventral tegmental area (VTA)-infusion of dihydro-beta-erythroidine (DH?E), a selective antagonist of the ?2*nAChRs, blocks nicotine self administration. Intra-VTA infusion of a-CMII will determine whether MII sensitive receptors at the level of the VTA are required for nicotine CPP.
Specific Aim 2 of these experiments will determine whether a-CMII sensitive and insensitive subclasses of ?2*nAChRs differentially regulate baseline CR and the ability of prior chronic nicotine to enhance CR. ?3 and a4 knockout mice with different histories of nicotine exposure will undergo Pavlovian discriminative approach training followed by acquisition of a new response with a conditioned reinforcer. DA projections to the nucleus accumbens (NAc) core are critical for regulation of CR at baseline. Although the systems that regulate nicotine-mediated facilitation of CR are unknown, nucleus accumbens shell DA is essential for psychostimulant enhancement of CR. The 2nd experiment of Aim 2 will use intra-shell infusion of a-conotoxin MII and DH?E to test whether different ?2*nAChRs at the level of the accumbens regulate nicotine stimulated elevations of CR. These experiments will begin to identify the systems that support nicotine associated enhancement of CR and in doing so will identify behaviorally relevant targets for nicotine cessation. Project De RRATIVE The ?2 subunit containing nicotinic acetylcholine receptors (?2*nAChR) have the highest affinity for nicotine and are the most widely expressed in the brain. Pfizer has recently received FDA and European Commission approval to market its smoking cessation drug, varenicline, a compound that reduces activity of these receptors that are important for nicotine dependence. Though varenicline's long-term success is double that of previously approved therapies, treatments that are targeted against specific behaviors that support nicotine dependence ought to 1) lead to fewer side effects and 2) improve outcomes for smoking cessation in a greater diversity of smokers. In determining that ?2*nAChRs can be broken down by a-conotoxin sensitivity in regard to regulation of nicotine's rewarding effects versus nicotine's ability to enhance the control that cues have over behavior, the proposed experiments will identify novel, more selective targets for nicotine cessation therapy. ? ? ?
| Brunzell, Darlene H; McIntosh, J Michael (2012) Alpha7 nicotinic acetylcholine receptors modulate motivation to self-administer nicotine: implications for smoking and schizophrenia. Neuropsychopharmacology 37:1134-43 |
| Brunzell, Darlene H; Boschen, Karen E; Hendrick, Elizabeth S et al. (2010) Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors in the nucleus accumbens shell regulate progressive ratio responding maintained by nicotine. Neuropsychopharmacology 35:665-73 |
| Brunzell, Darlene H; Picciotto, Marina R (2009) Molecular mechanisms underlying the motivational effects of nicotine. Nebr Symp Motiv 55:17-30 |
| Jackson, K J; McIntosh, J M; Brunzell, D H et al. (2009) The role of alpha6-containing nicotinic acetylcholine receptors in nicotine reward and withdrawal. J Pharmacol Exp Ther 331:547-54 |
