Previous exposure to nicotine enhances the incentive motivational effects of amphetamine via nicotine-associated contextual stimuli

The effect of nicotine exposure on the subsequent self-administration of amphetamine, extinction of this behavior, and amphetamine-induced reinstatement of drug seeking was assessed with particular attention to the contribution of contextual stimuli paired or unpaired with nicotine during exposure. Rats were exposed to five injections, one injection every third day, of either saline or nicotine (0.4 mg/kg, IP, base) in three experiments. In one, exposure injections were administered in the home cage. In another, they were administered in the self-administration chambers with the levers retracted. In a third, nicotine was administered either explicitly paired or unpaired with the self-administration chambers using a discrimination learning procedure. Starting 13-15 days later, rats were trained to self-administer amphetamine (100 μg/kg/infusion, IV), tested under a progressive ratio (PR) schedule for 6 days, subjected to up to 20 days of extinction training, and were then tested for reinstatement by non-contingent injections of amphetamine (0, 0.2, 0.4, and 0.75 mg/kg, IP). Nicotine enhanced the self-administration of amphetamine under the PR schedule and amphetamine-induced reinstatement but only when rats were tested in the chamber in which they were previously exposed to nicotine. These effects were not observed in rats exposed to nicotine in the home cage or in rats exposed to nicotine explicitly unpaired with the self-administration chambers. Exposure to nicotine also rendered rats resistant to extinction when amphetamine was withheld but this effect was observed regardless of nicotine exposure context, suggesting a separate consequence of drug exposure. Together, these results show that previous exposure to nicotine can enhance the incentive motivational effects of other psychostimulants like amphetamine and indicate a critical role for nicotine-associated contextual stimuli in the mediation of this effect. These findings have important implications for the treatment of addictions in humans.