Abstract:
Pharmacokinetic factors play a critical role in cocaine use. Previous work mainly examined externally driven pharmacokinetic manipulations, showing that conditions producing rapid fluctuations in brain cocaine levels, such as intermittent versus continuous access, promote burst-like intake, increased motivation for cocaine, heightened relapse vulnerability, and persistent drug-seeking behavior. Importantly, even under identical access conditions, substantial inter-individual variability exists in both the amount and speed of cocaine self-administration. In contrast to access-related pharmacokinetic influences, the contribution of intrinsic pharmacokinetic factors remains largely unexplored. To address this gap, male rats self-administered cocaine (0.8 mg/kg/injection; maximum 35 injections/session) for up to 26 sessions. In a final session, all animals received eight non-contingent intravenous cocaine injections at 5-min intervals before brain collection. Cocaine and its metabolites (ecgonine methyl ester, benzoylecgonine, norcocaine) were quantified in eleven sampled brain regions using liquid chromatography-tandem mass spectrometry detection and related to previous self-administration behavior. Mean brain cocaine concentration matched predictions from a widely used pharmacokinetic model, yet marked inter-individual variability was observed. Lower cocaine levels and higher metabolite-to-cocaine ratios were associated with faster self-administration patterns, but not with the total amount of cocaine previously consumed. This suggests that the observed inter-individual pharmacokinetic variability is unlikely to result from differential metabolic adaptations driven by intake and may contribute to differences in cocaine intake patterns. Overall, inter-individual variability in cocaine pharmacokinetics in the rat exist and are related to differences in cocaine intake patterns, with rapid intake representing a known risk factor for problematic cocaine use in both rodents and humans.

Abstract:
The significant heterogeneity in smoking behavior among smokers, coupled with the inconsistent efficacy of approved smoking cessation therapies, supports the presence of individual variations in the mechanisms underlying smoking. This emphasizes the need to shift from standardized to personalized smoking cessation therapies. However, informed precision medicine demands precision fundamental research. Tobacco smoking is influenced and sustained by diverse psychopharmacological interactions between nicotine and environmental stimuli. In the classical experimental rodent model for studying tobacco dependence, namely intravenous self-administration of nicotine, seeking behavior is reinforced by the combined delivery of nicotine and a discrete cue (nicotine+cue). Whether self-administration behavior is driven by the same psychopharmacological mechanisms across individual rats remains unknown and unexplored. To address this, we employed behavioral pharmacology and unbiased cluster analysis to investigate individual differences in the mechanisms supporting classical intravenous nicotine self-administration (0.04 mg/kg/infusion) in male outbred Sprague-Dawley rats. Our analysis identified two clusters: one subset of rats sought nicotine primarily for its reinforcing effects, while the second subset sought nicotine to enhance the reinforcing effects of the discrete cue. Varenicline (1 mg/kg i.p.) reduced seeking behavior in the former group, whereas it tended to increase in the latter group. Crucially, despite this fundamental qualitative difference revealed by behavioral manipulation, the two clusters exhibited quantitatively identical nicotine+cue self-administration behavior. The traditional application of rodent models to study the reinforcing and addictive effects of nicotine may mask individual variability in the underlying motivational mechanisms. Accounting for this variability could significantly enhance the predictive validity of translational research.

Abstract:
Nicotine is acknowledged as the key addictive compound of tobacco. Varenicline (Champix((R)) or Chantix((R))), mainly acting as a partial agonist at the alpha4beta2 nicotinic receptor, is an approved smoking cessation pharmacotherapy, although with efficacy limited to a portion of smokers. Smokers differ in the motives that drive their drug seeking and Varenicline might be more efficient in some groups more than others. Studies in rodents revealed that nicotine-seeking is strongly supported by complex interactions between nicotine and environmental cues, and notably the ability of nicotine to enhance the reinforcing properties of salient environmental stimuli. It is not yet understood whether the decrease of nicotine-seeking by acute Varenicline in rats results from antagonism of the primary reinforcing effects of nicotine, of the reinforcement-enhancing effect of nicotine on cues, or of a combination of both. Thanks to a protocol that allows assessment of the reinforcement-enhancing effect of nicotine on cues during self-administration in rats, we showed that Varenicline targets both nicotine reinforcing effects and reinforcement-enhancing effect of nicotine on cues. Importantly, individual variations in the latter determined the amplitude of acute Varenicline-induced decrease in seeking. These results suggest that Varenicline might be more beneficial in smokers who are more sensitive to nicotine effects on surrounding stimuli.