Understanding the stimulus effects of nicotine and bupropion in a drug-drug discriminated goal-tracking task.

RATIONALE: Bupropion is a non-nicotine medication for smoking cessation that has overlapping stimulus effects with nicotine as demonstrated in drug discrimination studies. Whether these shared stimulus effects will alter acquisition or maintenance of a discrimination between nicotine and bupropion is unknown. OBJECTIVE: We sought to test this possibility using the drug discriminated goal-tracking (DGT) task and whether discrimination training history affected generalization and substitution tests. METHODS: Sixty adult Sprague-Dawley rats (30M/30F) were equally split into three discrimination training groups: SAL-0.4NIC, 10BUP-0.4NIC, and 20BUP-0.4NIC. On nicotine days, all rats were administered subcutaneously 0.4 mg/kg nicotine and had intermittent access to liquid sucrose. On intermixed non-reinforced days, rats were administered intraperitoneally saline, 10 or 20 mg/kg bupropion. Upon completion, a range of nicotine and bupropion doses were assessed before substitution tests with varenicline and sazetidine-A were conducted. RESULTS: The SAL-0.4NIC and 10BUP-0.4NIC groups readily discriminated by session 8, as evidenced by increased dipper entries (goal-tracking) on nicotine days. The 20BUP-0.4NIC group was slower to acquire the discrimination. Female rats, regardless of group, had higher conditioned responding evoked by the lowest dose of nicotine (0.025 mg/kg) in the dose-effect curve. The discrimination required rats to learn to withhold responding to the training dose of bupropion. This withholding of excitatory dipper entries generalized to other doses. Varenicline and sazetidine-A partially substituted for the nicotine stimulus, and this pattern did not differ with training history. CONCLUSIONS: We are the first to study a drug-drug discrimination using the DGT task. Females appeared to have a lower discrimination threshold for nicotine that was not impacted by the learning history. Further work on the importance of sex as a biological variable and how the complex interoceptive stimulus effects of nicotine can vary with training histories is needed.

Menthol blunts the interoceptive discriminative stimulus effects of nicotine in female but not male rats.

RATIONALE: Menthol is a widely used tobacco constituent that has shown to enhance nicotine's reinforcing effects. OBJECTIVE: To determine whether injected menthol also alters nicotine's stimulus effects, we used a drug discrimination task. METHODS: A total of 57 adult Sprague-Dawley rats (28M, 29F) received 20 positive and 20 negative days (intermixed) of discrimination training. On positive days, rats received a group-specific menthol and nicotine injection (VEH + 0.1 NIC, 1 M + 0.1 NIC, 5 M + 0.1 NIC, VEH + 0.4 NIC, 1 M + 0.4 NIC, 5 M + 0.4 NIC; mg/kg) before eight 15-s cue light presentations (conditioned stimulus (CS)), each followed by 4-s sucrose access. On negative days, all rats were injected with vehicle and saline before eight non-reinforced CS presentations. Next, rats underwent generalization testing with 30 dose combinations of menthol and nicotine. The change in drug-mediated anticipatory goal tracking during the CS was calculated as a difference score (CS minus pre-CS responding). RESULTS: All groups readily acquired drug discrimination. However, difference scores for the 5M + 0.1 NIC group were lower for females. Additionally, females had lower scores for 0.05, 0.1, and 0.4 mg/kg nicotine generalization tests. The lowest nicotine dose discriminable from saline was 0.05 mg/kg for females but 0.025 mg/kg for males. Co-administration with 5 or 10 mg/kg menthol weakened discrimination performance between 0.1 and 0.4 mg/kg and between 0.1 and 0.05 mg/kg nicotine for 0.1 mg/kg nicotine training groups. CONCLUSIONS: Female rats that were trained with 0.1 mg/kg nicotine were more sensitive to menthol's modulatory effects on nicotine's stimulus effects. This highlights the importance of taking sex and training dose into account when evaluating the interoceptive stimulus effects of nicotine and menthol.

Investigating the interoceptive stimulus effects of injected menthol in rats.

Menthol is a commonly used tobacco constituent that also modulates nicotine reinforcement and metabolism. Little is known about the stimulus effects of menthol that mediate the behavior associated with reinforcement-learning. Our present research explored the interoceptive stimulus effects of intraperitoneally administered menthol in a drug discrimination task. For Experiment 1, Sprague-Dawley rats (N = 20) received IP menthol (0.0183 or 5 mg/kg) or vehicle. For positive sessions, rats were given menthol before receiving 8 15-s light presentations, each followed by 4-s access to liquid sucrose. For intermixed negative sessions, rats were given vehicle before receiving 8 light presentations without sucrose delivery. After 32 sessions, rats previously receiving 0.0183 mg/kg menthol were switched to 15 mg/kg menthol. After 16 sessions, the injection-to-placement-interval was switched from 5 min to 15 min for 16 additional sessions. Lastly, a subset of rats (n = 10) received nicotine discrimination training for 40 sessions, with 0.4 mg/kg nicotine pretreatment on positive days and saline on negative days. In Experiment 2, naïve rats (N = 7) received nicotine discrimination training. Later sessions assessed nicotine discrimination performance in combination with 5 mg/kg menthol or vehicle. Menthol-vehicle discrimination was not evident regardless of dose or injection-to-placement interval in Experiment 1. However, rats that underwent nicotine training developed robust drug discrimination. In Experiment 2, co-exposure with menthol or vehicle did not modulate nicotine discrimination performance. These data suggest that menthol does not acquire control of responding in a drug discrimination task. Additional research is needed to further explore the interoceptive stimulus effects of menthol and nicotine combined. (PsycINFO Database Record (c) 2020 APA, all rights reserved).