Extinction with varenicline and nornicotine, but not ABT-418, weakens conditioned responding evoked by the interoceptive stimulus effects of nicotine.

The interoceptive stimulus effects of nicotine acquire control over behavior. This observation, among others, suggests that the stimulus effects of nicotine are important in the development and tenacity of tobacco dependence. Despite this importance, there has been little research examining whether non-reinforced presentations (extinction) of a ligand that share stimulus effects of nicotine will weaken responding controlled by nicotine. Rats were trained to discriminate nicotine (0.4 mg/kg) from saline using a discriminated goal-tracking task in which nicotine signaled intermittent access to sucrose; sucrose was withheld on saline sessions. Experiment 1 examined substitution for nicotine by ABT-418, nornicotine, epibatidine, varenicline, or cytisine in 4-min extinction tests. Experiments 2-5 [low-dose nicotine (0.05 mg/kg), ABT-418, nornicotine, or varenicline, respectively] examined whether substitution for nicotine would persist if extinction tests were increased to 20 min and repeated daily for 6 days. Finally, generalization of this extinction back to the nicotine training stimulus was assessed. Full substitution in brief 4-min extinction tests was seen for ABT-418, nornicotine, epibatidine, varenicline, and cytisine. Low-dose nicotine, ABT-418, nornicotine, and varenicline, evoked only a partial 'nicotine-like' response in the first 20-min extinction test. With repeated extinction, only low-dose nicotine, nornicotine, and varenicline continued to substitute. Extinction with nornicotine and varenicline transferred back to nicotine as indicated by a partial conditioned response to the training stimulus. Interpretations regarding 'nicotine-like' effects of a ligand depend on the nature of the test. Understanding the processes mediating transfer of extinction learning with potential pharmacotherapies may reveal new treatment targets.