Caffeine and alcohol are the most consumed psychoactive substances worldwide (1, 2), profoundly impacting billions of lives. Recent findings from our group demonstrate that low doses (3mg/kg and 15 mg/kg) of caffeine significantly inhibit ethanol-induced conditioned place preference and alcohol-triggered extracellular signal-regulated kinase phosphorylation (pERK) in the Shell subregion of the Nucleus Accumbens (AcbSh) (3). Both behavioral and biochemical effects are closely regulated by the Mesolimbic Dopaminergic Pathway, a brain circuit mediating the reinforcing properties of alcohol and other addictive substances (4, 5). Building on this, we investigated whether caffeine could suppress alcohol-induced activation of this reward pathway to substantiate our previous results (3).

The Mesolimbic Pathway consists of dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens. Alcohol and other drugs of abuse enhance dopaminergic activity in the AcbSh (6). In 2021, we demonstrated that alcohol metabolism into salsolinol—formed via the condensation of dopamine with alcohol-derived acetaldehyde in the posterior VTA—plays a critical role in activating the Mesolimbic Pathway (7). Thus, our initial experiments assessed whether caffeine could prevent salsolinol formation in the VTA and the resulting increase in dopaminergic activity in the AcbSh. To simultaneously monitor salsolinol in the VTA and dopamine in the AcbSh, we implanted microdialysis probes in alcohol-naive, freely behaving rats and administered alcohol (1 g/kg) orally (Fig. 1 A). Remarkably, caffeine completely suppressed salsolinol formation in the VTA and the subsequent dopamine increase in the AcbSh (Fig. 1 A, B). As a nonselective antagonist of adenosine A1 and A2A receptors (8), caffeine’s effects were further explored using selective antagonists—DPCPX for A1 and SCH-58261 for A2A. The results highlighted A2A antagonism as the mechanism by which caffeine blocks salsolinol formation and dopaminergic activation (Fig. 1 D). Targeted perfusion of caffeine into the VTA also prevented these alcohol-induced changes, emphasizing the VTA as a critical site of caffeine action (Fig. 1 E).

Figure. 1 

To validate these findings, we performed in vitro patch-clamp recordings of VTA dopaminergic neurons. Consistent with in vivo results, caffeine and the A2A adenosine receptor antagonist SCH-58261 prevented alcohol-induced firing of these neurons (Fig. 2 A-I). Surprisingly, caffeine also inhibited neuron firing induced by direct salsolinol or morphine exposure in vitro (Fig. 2 J-Q), indicating that caffeine’s effects extend beyond inhibiting salsolinol synthesis.

Figure. 2 

To uncover additional salsolinol-independent mechanisms, we conducted untargeted metabolomics analysis of VTA lysates from alcohol-treated rats, with and without caffeine pretreatment. Both alcohol and caffeine influenced lipid abundance, but caffeine counteracted alcohol-induced alterations in key lipid species (Fig.3). Brain lipids regulate processes such as signal transduction, synaptic plasticity, and neurotransmitter release, and their role in alcohol addiction is well-established (9-11). For example, caffeine normalized levels of lipids like oleamide, PC, and Lyso-PC, which are implicated in addiction (12-13), and resulted altered by alcohol in the present study (Fig.3 C). These findings suggest lipid regulation as an additional salsolinol-independent mechanism by which caffeine modulates the Mesolimbic Pathway.

Figure 3.

In conclusion, this study demonstrates for the first time that caffeine inhibits alcohol-induced activation of the mesolimbic dopaminergic pathway. Encouragingly, naltrexone, an FDA-approved drug for alcohol use disorder, mitigates alcohol’s reinforcing effects by targeting mesolimbic dopamine transmission (14). Hence, these results highlight the potential of caffeine, and particularly of A2A receptor antagonists, for developing preventive strategies for AUD.

Link to the study: https://www.nature.com/articles/s41398-024-03112-6

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