Project Summary Nicotine is the key addictive ingredient of tobacco. The FDA is considering a regulatory strategy aiming to reduce the nicotine content of tobacco products to “non-addictive” levels to prevent the development of addiction among youth experimenting with tobacco products. The implementation of such a strategy would require strong scientific support from a range of preclinical and clinical studies. However, there are gaps in our current knowledge about the actual threshold for nicotine's addictive effects and whether this threshold varies across individuals or with flavors that are commonly added to inhaled tobacco products. Studies aiming to address these gaps are hampered by difficulty in delivering accurate doses of nicotine through cigarette smoking or e-cigarettes. Further, separating nicotine's effects from multiple other chemicals that are inhaled with nicotine remains a challenge. Among pure nicotine options, the intravenous (IV) route is the gold standard for preclinical research and is increasingly used in clinical studies as well. IV route approximates the inhaled delivery for nicotine pharmacokinetics, and it also produces reinforcing and subjective-rewarding effects. In a series of pilot studies, we refined our IV nicotine procedure to better model puff-sized nicotine delivered by smoking cigarettes or e-cigarettes and to allow assessment of nicotine's dose-dependent effect on multiple addiction-related outcomes, including reinforcement, drug discrimination, and subjective-rewarding effects. Using this method, we propose to determine benchmarks regarding the addictive threshold of nicotine and the impact of sweeteners on this threshold. Specifically, we will 1) determine the nicotine threshold dose(s) for reinforcement, discrimination, and subjective-rewarding effects in smokers (Aim 1) and 2) evaluate whether these threshold doses change with inhaled sweetener (sucralose), administered by e-cigarettes concurrent with IV nicotine infusions (Aim 2). For both Aims, we will use a placebo-controlled study that will recruit male and female cigarette smokers. For Aim 1, in each of the 4 test sessions, a different nicotine dose (0.1, 0.05, 0.025, and 0.0125 mg nicotine/pulse) will be compared to saline for reinforcement, nicotine discrimination, and subjective effects. Concurrently with the pulsed infusions, participants will inhale unflavored e-cigarettes, which will allow a closer matching of the sensory aspects of inhaled tobacco use. To assess the effect of sweeteners in Aim 2, participants in a placebo-controlled study will have 6 test sessions. Each Test Session will include one of the 3 nicotine doses (selected from Aim 1) vs. saline, all combined with or without the sweetener sucralose. Sweetener and control solution will be administered by e-cigarettes concurrent with IV infusions. By providing novel information on the addictive threshold of nicotine and the impact of sweeteners on this threshold, the results of this s...