β2-subunit-containing nicotinic acetylcholine receptors are involved in nicotine-induced increases in conditioned reinforcement but not progressive ratio responding for food in C57BL/6 mice

Rationale: Nicotine administration potentiates conditioned reinforcement in rats, an effect that persists for weeks after chronic exposure. Little is known regarding the nicotinic receptor subtypes that may mediate this effect. Objective: The purpose of this study was to determine whether beta 2-subunit-containing nicotinic acetylcholine receptors (beta 2*nAChRs) are necessary for lasting effects of nicotine on conditioned and primary reinforcement in mice. Methods: beta 2 knockout (beta 2KO) and wild-type (WT) mice received 14 days of nicotine exposure (NIC, 200 mu g/ml in 2% saccharin) or saccharin alone (SAC) in their drinking water. Five days later, mice received paired presentations of a conditioned stimulus (CS) with water unconditioned stimulus (US) or explicitly unpaired presentations of the CS and US during Pavlovian discriminative approach training. Training was followed by two conditioned reinforcement tests. Mice were subsequently tested for food-reinforced responding in the absence of explicit cues followed by a progressive ratio test. Results: During conditioned reinforcement testing, only mice in the paired condition showed increased responding in the CS-reinforced aperture over inactive apertures. WT-NIC mice showed enhanced conditioned reinforcement compared to WT-SAC animals. beta 2KO-SAC mice showed elevated conditioned reinforcement compared to WT-SAC subjects, but beta 2KO-NIC and beta 2KO-SAC mice did not differ in responding with conditioned reinforcement. Prior nicotine exposure did not alter food-reinforced responding but resulted in elevated break points for food in both genotypes. Conclusion: These data show that nicotine exposure enhances conditioned reinforcement in mice and indicate that beta 2*nAChRs are necessary for nicotine-dependent enhancement of incentive aspects of motivation but not motivation for primary reinforcement measured by progressive ratio responding.