Alcohol-induced activation of the mesocorticolimbic dopaminergic system and its modulation by energy drinks and caffeine: neurochemical, behavioral, and immunohistochemical analysis

The co-consumption of alcohol and caffeinated energy drinks among adolescents represents a growing public health concern, raising questions about its negative impact on the developing brain. Therefore, this PhD thesis aimed to investigate the effects of alcohol and caffeine, as well as alcohol and Red Bull® (AMED), on the responsivity of the mesocorticolimbic dopaminergic system, a neural circuit critically involved in reward processing and motivated behavior. During adolescence, this system is characterized by high vulnerability, which may lead to abnormal maturation in adulthood, resulting in impulsivity and an increased risk of compulsive drug use. Accordingly, this project was designed to investigate not only the acute effects of these substances, but also their long-term consequences on this system in adulthood following adolescent exposure. Notably, this study is among the first to employ a multidimensional approach across behavioral, neurochemical, and immunohistochemical levels, evaluating acute, subchronic, and chronic paradigms under both contingent and non-contingent conditions, providing new insights into the impact of these substances on neurodevelopment. Findings demonstrate that acute pretreatment of adult rats with caffeine, the main stimulating component of Red Bull®, prevents the alcohol-induced formation of salsolinol in the posterior ventral tegmental area and the associated increase of dopamine transmission in the nucleus accumbens shell (AcbSh). Similarly, acute administration of AMED attenuates the alcohol-stimulated expression of phosphorylated extracellular signal-regulated kinase (pERK)-positive cells and of dopaminergic transmission in the AcbSh, although it did not prevent the alcohol- dependent reduction of locomotion. Subchronic AMED exposure, using an intermittent binge-like protocol designed to model adolescent patterns of consumption, revealed that its early exposure induces a reduction in the responsiveness of mesocortical dopaminergic neurons and an increased voluntary alcohol intake in adulthood. Lastly, chronic operant self-administration of AMED, from adolescence to adulthood, resulted in enhanced acquisition of the operant response, leading to increased intake, and upregulation of mesocorticolimbic dopaminergic transmission. These findings indicate that acute exposure to caffeine and AMED may exert protective effects against some alcohol-induced neurobiological alterations. In contrast, subchronic and chronic exposure to AMED during maturation (i.e., from adolescence to adulthood) leads to a clear dysregulation of the mesocorticolimbic dopaminergic system, accompanied by increased substance intake. Moreover, the results of the experiments under contingent conditions also suggest that additional factors, such as taste, liking, wanting, and learning processes, beyond the pharmacological effects of the substances alone, contribute to the behavioral and neurochemical outcomes. Taken together, this study casts novel and critical insights into the developmental and context-dependent effects of AMED: highlights its potential to alter reward-related circuits and increase vulnerability to compulsive substance use, suggests that AMED consumption during adolescence may not be entirely safe, and provides a robust and neat foundation for future research on its long-term consequences and possible reversibility.