Behavioral and Molecular Mechanisms of Cocaine Addiction Vulnerability: Functional Evidence from Rodent Models

Cocaine addiction is a complex, chronically relapsing disorder shaped by individual vulnerability, neuroadaptive changes, and limited treatment options. Although substantial progress has been made in understanding the reinforcing properties of psychostimulants, the neurobehavioral mechanisms distinguishing recreational use from compulsive, addiction-like behavior remain incompletely understood. This thesis aimed to (i) identify behavioral phenotypes predictive of addiction vulnerability, (ii) examine molecular adaptations underlying compulsive drug use, and (iii) evaluate the therapeutic potential of pharmacological interventions using translationally-relevant rodent models. The first objective focused on phenotypic predictors of addiction-like behavior. In Study 1A, the role of incentive salience attribution, measured through Pavlovian Conditioned Approach behavior, was evaluated within the 3-CRIT model of cocaine addiction. Contrary to the expectations of incentive sensitization theory, sign- and goal-tracking phenotypes failed to predict the development of addiction-like behavior. However, sign-trackers exhibited greater resistance to punishment. Study 1B examined sex differences in the 3-CRIT model. Although fewer females met the addiction-like criteria, those that did (i. e. , “3crit” rats) exhibited more severe behavioral profiles despite lower overall drug intake. These findings highlight the importance of both sex and individual phenotype in modeling addiction vulnerability. The second aim addressed the molecular mechanisms of addiction. In Study 2A, cocaine self-administration dynamically regulated the expression of metabotropic glutamate receptor 2 (mGluR2, gene: Grm2) across cortico-striatal pathways. Extinction training, but not abstinence, robustly upregulated Grm2 expression, and pharmacological activation of mGluR2/3 with LY379268 suppressed cue-induced cocaine seeking. Changes in mGluR2 expression followed a region- and experience-specific pattern: initial downregulation was observed in the prelimbic cortex after short-term exposure, shifting to the infralimbic cortex with prolonged cocaine use. Interestingly, addiction-vulnerable animals (3crit), despite comparable drug intake to 0crit animals, showed elevated Grm2 levels in the infralimbic cortex and dorsal striatum, likely reflecting compensatory responses. In Study 2B, a genetic dopamine transporter (DAT) impairment (Slc6a3N157K) prevented the acquisition and maintenance of cocaine self-administration, emphasizing the necessity of intact DAT function for cocaine reinforcement. The final part evaluated novel pharmacological interventions and methodological refinements. In Study 3A, the effects of psilocybin on extinction learning and cue-induced rein- statement in cocaine-experienced rats and mice were examined. Although behavioral outcomes were not significantly altered, the study provided important insight into the feasibility and limitations of applying psychedelic-based interventions in preclinical addiction models. In Study 3B, a rat adaptation of a mouse-based paradigm intended to assess reality testing was developed. While mediated aversion failed to emerge, direct aversion was consistently observed, underscoring both the challenges of cross-species translation and the importance of refining behavioral tools to probe drug-induced changes in cognition. In summary, this thesis advances understanding of the behavioral, molecular, pharmacological, and methodological factors underlying cocaine addiction. By integrating phenotypic variability, sex differences, and molecular adaptations in glutamatergic and dopaminergic systems, together with exploratory evaluations of serotonergic psychedelics and refined behavioral paradigms, the work underscores the multifaceted nature of addiction vulnerability and the limitations of oversimplified models. Region- and experience-specific regulation of Grm2 expression highlights dynamic glutamatergic plasticity associated with different stages of cocaine exposure and withdrawal. Additionally, the demonstration that impaired DAT function disrupts the acquisition and maintenance of cocaine self-administration emphasizes the necessity of intact dopaminergic signaling for cocaine reinforcement. Finally, the evaluation of psilocybin, together with efforts to refine cognitive assays, illustrates both therapeutic opportunities and methodological challenges. Together, these findings reinforce the need for refined preclinical approaches in addiction research.