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

Abstract

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.

Fig. 1

Effects of nicotine (NIC) or saline (SAL) on responding for the visual stimulus (VS) on an escalating fixed ratio schedule. Data are mean (±SEM) VS presentations earned. a 0.01 mg/kg per infusion; b 0.03 mg/kg per infusion; c 0.09 mg/kg per infusion. The schedule of reinforcement is indicated below the abscissa. *p<0.05, contingent and noncontingent NIC > SAL on an FR5 schedule

Fig. 2

Mean (+SEM) number of VS presentations earned with NIC or SAL on a progressive ratio reinforcement schedule. Right axis indicates highest ratio of responding achieved. a 0.01 mg/kg per infusion; b 0.03 mg/kg per infusion; c 0.09 mg/kg per infusion. Significant between-groups comparisons at each day are indicated as follows: *p<0.05, contingent and noncontingent NIC > SAL

Fig. 3

Mean (+SEM) number of infusions earned when responding is reinforced with NIC or SAL in the absence of a contingent VS on an escalating fixed ratio schedule (a) or a progressive ratio schedule (b, SAL + no VS group is replicated in each panel). The schedule of reinforcement is indicated below the abscissa. Significant between-groups comparisons at each day are indicated as follows: *p<0.05, all comparisons (0.09 mg/kg per infusion > saline, 0.01 and 0.03 mg/kg per infusion)

Fig. 4

Mean (+SEM) total NIC intake (mg/kg) for rats that lever pressed for a contingent NIC + VS or b contingent NIC + no VS on FR5 and progressive ratio reinforcement schedules. Significant between-groups comparisons at each day are indicated as follows: *p<0.05, all comparisons (0.09 mg/kg per infusion > saline, 0.01 and 0.03 mg/kg per infusion)