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.
Objectives: Experiment 1 investigated whether nicotine could establish a stimulus as a conditioned reinforcer. Using the same subjects, Experiment 2 examined whether responding for a nicotine-associated stimulus was enhanced by response-independent administration of nicotine.
Materials and methods: Self-administered nicotine (Paired group, 0.03 mg kg(1) infusion(-1)) or saline (conditional stimulus or CS-Only group) was paired with a stimulus light (CS). An Unpaired group, yoked to the Paired group, received equal exposure to nicotine and the CS, but each event was temporally separated. To test for conditioning, the CS was then made contingent upon a novel lever-pressing response. In Experiment 2, a subset of the paired rats (self-administering) continued to lever press while receiving contingent nicotine and the CS. To determine whether nicotine enhanced responding for the CS, two remaining subsets of the Paired group responded for the CS while receiving nicotine (YNIC) or saline (YSAL) yoked to the self-administering rats. All remaining control groups received response-contingent CS presentations, together with yoked nicotine or saline.
Results: Pairing self-administered nicotine with the CS promoted the acquisition of a novel response for the CS. In Experiment 2, the Paired YNIC group responded at higher rates than control groups receiving YNIC or YSAL.
Conclusions: Nicotine can establish stimuli as conditioned reinforcers for which noncontingent nicotine can enhance responding.