Nicotine enhances operant responding for qualitatively distinct reinforcers under maintenance and extinction conditions

Abstract

Rationale: Nicotine enhancement of reward has been implicated as an important contributor to tobacco addiction. Despite the attention that reward enhancement has received, the behavioral mechanisms whereby nicotine enhances operant responding remain largely unknown. The present study sought to extend previous work by evaluating the effects of nicotine on responding for two qualitatively different rewards (visual stimulation (VS) and 4% sucrose solution) under fixed-ratio (FR) maintenance and extinction conditions.

Method: Sprague–Dawley rats were trained to press an active lever for VS (Experiment 1) or 4% sucrose solution (Experiment 2) and evaluated over 15 sessions on a FR5 schedule of reinforcement. Nicotine (0.4 mg base/kg, SC) or saline were administered 5 min before each session; the alternate solution was given in the home cage after the session. The effects of nicotine on extinction responding were then assessed over 5 sessions and rats were divided into 4 groups based on drug of injection received during FR-maintenance and extinction phases (maintenance–extinction): Nic–Nic, Nic–Sal, Sal–Sal, and Sal–Nic.

Results: Nicotine increased active lever response rates for both VS and 4% sucrose under FR5 maintenance conditions. Nicotine also increased response rates in the Nic–Nic group relative to all other groups under extinction conditions in both experiments, though this effect had greater longevity following VS maintenance conditions than sucrose. Enhancement of responding during extinction does not appear dependent upon locomotor activation by nicotine.

Figure 1

Total responses on each lever and locomotor activity across the 15-session VS-reinforcer maintenance phase of Experiment 1. Lever pressing data (Panel A) from the nicotine and saline conditions are represented as circles or triangles, respectively. Filled shaped represent active lever pressing, whereas open shapes represent inactive lever pressing. Locomotor activity data (Panel B) is represented as filled circles for the nicotine condition and as open circles for the saline condition. All data presented as mean ± SEM.

Figure 2

Active lever pressing (Panel A) and inactive lever pressing (Panel B) across the 5-session extinction phase between the Nic-Nic (filled circles), Nic-Sal (open circles), Sal-Sal (filled squares), and Sal-Nic (open squares) groups from Experiment 1. All data presented as mean ± SEM.

Figure 3

Locomotor activity counts across the 5-session extinction phase between the Nic-Nic (filled circles), Nic-Sal (open circles), Sal-Sal (filled squares), and Sal-Nic (open squares) groups from Experiment 1. All data presented as mean ± SEM.

Figure 4

Total responses on each lever and locomotor activity across the 15-session sucrose-reinforcer maintenance phase of Experiment 2. Lever pressing data (Panel A) from the nicotine and saline conditions are represented as circles or triangles, respectively. Filled shaped represent active lever pressing, whereas open shapes represent inactive lever pressing. Locomotor activity data (Panel B) is represented as filled circles for the nicotine condition and as open circles for the saline condition. All data presented as mean ± SEM.

Figure 5

Active lever pressing (Panel A) and inactive lever pressing (Panel B) across the 5-session extinction phase between the Nic-Nic (filled circles), Nic-Sal (open circles), Sal-Sal (filled squares), and Sal-Nic (open squares) groups from Experiment 2. All data presented as mean ± SEM.

Figure 6

Locomotor activity counts across the 5-session extinction phase between the Nic-Nic (filled circles), Nic-Sal (open circles), Sal-Sal (filled squares), and Sal-Nic (open squares) groups from Experiment 2. All data presented as mean ± SEM.