Self-administered and noncontingent nicotine enhance reinforced operant responding in rats: impact of nicotine dose and reinforcement schedule

Rationale: Nicotine infusions that are self-administered (contingent) or response-independent (noncontingent) increase lever pressing for a reinforcing nonpharmacological stimulus in rats, suggesting that in addition to primary reinforcement, nicotine self-administration may result from nicotine enhancing the reinforcement derived from nonnicotine stimuli. Objectives: Based on our previous research, in this study, we tested the hypothesis that contingent and noncontingent nicotine would equally elevate responding for a moderately reinforcing visual stimulus, across a range of nicotine doses on both fixed ratio and progressive ratio reinforcement schedules. Materials and methods: The rats lever pressed for a visual stimulus with contingent nicotine, noncontingent nicotine, or contingent saline. Separate groups responded for saline or nicotine without the visual stimulus. Three doses of nicotine (0.01, 0.03, and 0.09 mg/kg per infusion, free base) were tested in a between-groups design. After responding on an escalating fixed ratio reinforcement schedule, the rats were tested on a progressive ratio schedule. Results: Compared to responding for the visual stimulus with saline, both contingent and noncontingent nicotine equally elevated lever pressing for the stimulus at each dose on fixed and progressive ratio schedules. In the absence of the stimulus, only the highest nicotine dose sustained self-administration. Conclusions: The ability of noncontingent nicotine to elevate responding for a moderately reinforcing visual stimulus occurs across a range of doses, and both self-administered and noncontingent nicotine equally increase motivation to obtain the stimulus, as reflected by performance on a progressive ratio schedule. In the absence of a contingent stimulus, primary reinforcement from nicotine only supports self-administration at high nicotine doses in rats.