We have hypothesized that nicotine has two effects on reinforcement; it increases the probability of responses resulting in nicotine delivery (primary reinforcement) and enhances the apparent reward value of non-nicotine reinforcers (reinforcement enhancing effect). The present studies investigated two predictions generated by this hypothesis: 1) that the reinforcement enhancing effect will depend on apparent stimulus reward value, and 2) that the temporal profile of this effect would depend on the pharmacological profile of nicotine. In Experiment 1, rats were trained to lever press for one of two audio-visual stimuli that differed in their intrinsic reinforcing value and then the effect of presession nicotine (0.4 mg/kg base) or saline injections was tested. The stimulus that supported very low rates of operant responding displayed smaller increases in responding after pre-session injections of nicotine. In Experiment 2 the effect of nicotine injected 5 min before the session was compared to the effect of nicotine injected 1 h after the session using the more reinforcing stimulus condition from the first experiment. A control group received only vehicle injections. In contrast to nicotine injected just prior to the session, post-session injections of nicotine had no detectable effect on responding for the more reinforcing stimulus. These results indicate that the reinforcement enhancing action of nicotine depends on the intensity of the primary reinforcer and that enhanced reinforcement by nicotine depends on coincident access to a stimulus with reinforcing properties. IntroductionTobacco dependence is a syndrome characterized by subjective drug liking, perseverative drug taking (difficulty quitting), a persistent profile of relapse, and physiological symptoms of tolerance and withdrawal (American Psychiatric Association 2000). Although tobacco dependence has been attributed almost exclusively to the pharmacological action of nicotine (USDHHS 1988), many investigators have recently suggested "nicotine is not enough" (Rose 2006; see also Caggiula et al. 2001). This statement gains credence when the defining features of nicotine dependence are weighed against those of other drug dependence syndromes. For example, the difficulty quitting and rate of relapse associated with smoking is comparable to † Corresponding Author: Matthew I. Palmatier, Department of Psychology, 3137 Sennott Square, 210 S. Bouquet St, University of Pittsburgh, Pittsburgh, PA 15260, 412-624-7345 (Office), 412-624-8558 (Fax), mip16@pitt.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Henni...
Rationale Nicotine is widely assumed to convey reinforcing properties upon tobacco-related stimuli through associative learning. We have proposed that the reinforcement derived from these conditional stimuli can be inflated by a nonassociative "reinforcement-enhancing" effect of nicotine.
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