In our population of individuals with moderate-to-severe AUD, we found a higher prevalence of individuals with overweight and obesity (approximately 60%). However, when examining the distribution of BMI, we did not observe a significant rightward skewing, indicating a relatively balanced distribution across the range. Surprisingly, we did not find a clear difference in the distribution of ELS events between patients with AUD and with a high or low BMI. Additionally, as we identified a significantly greater length of AUD history in participants with a high BMI as compared to a low BMI within our cohort, implying that those with a low BMI are still early in their history of AUD and have yet to gain weight. These findings suggest that the relationship between BMI and ELS in the context of AUD may not be straightforward, and there may be other factors that have a stronger impact on the association between ELS and BMI in individuals with AUD.
Therefore, on investigating the effect of ELS on functional brain connectivity in individuals with AUD who differ in BMI, we observed various connectivity patterns associated with ELS, indicating connections between the left and right SMG seed regions and whole brain. Also, these predicted connections were differentially associated with BMI levels within these individuals.
As predicted, we observed the main effect of ELS as anticorrelation of the SN seed regions, bilateral SMG and AIns, with several clusters in different regions of the brain, including the somatosensory and motor coordination areas (such as the bilateral LOC, right cerebellum, left posterior middle temporal gyrus (pMTG), left postcentral gyrus, and left intracalcarine cortex), frontal, or executive, control regions (e.g., the MFG). Exposure to stress during critical periods of brain development has been demonstrated to modify connectivity patterns and heighten the risk of developing AUD. Likewise, numerous studies provide evidence that experiencing ELS has harmful effects on individuals and enhances their susceptibility to alcohol use in adulthood 2, 31–35. Moreover, exposure to a series of ELS events leads to modifications in connectivity of brain regions associated with emotion, self-regulation and cognition, including nodes of the DMN, such as the PCC, mPFC and MTG, and within the fronto-limbic networks, such as the mPFC, ACC, amygdala and orbitofrontal cortex 36–38]. Children between the ages of 9 and 16 who were exposed to various stress events, such as conventional crimes, child maltreatment, peer/sibling victimization and sexual victimization, were found to have a reduced functional connection between their SMG and PCC 39. Studies have shown that in individuals with AUD, reduced connectivity within the SN is associated with decreased self-control and an inability to restrain cravings 5, as well as impaired inhibition for salient stimuli 40. Notably our analysis revealed lower functional connectivity between left/right SMG and clusters in motor [cerebellum (right Crus 1 & 2), occipital cortex, temporal (pMTG)] and somatosensory (postcentral gyrus), as well as frontal, or executive, control regions (e.g., the MFG) that reportedly are involved in processing of emotionally salient stimuli 41–44. As such, the lower connectivity of SN nodes with emotion and executive control regions are concordant with evidence in the literature that have shown the effect of ELS exposure in altering the connectivity of brain networks involved in emotion regulation and salience of rewards 45, 46. These previous findings suggest a disengagement of executive control functions when emotionally significant rewards are being processed in adults with AUD and a history of ELS. The negative association between left SMG activity and major default mode network (DMN) nodes (PCC, mPFC), which are normally inversely associated with goal-oriented actions 47, suggests that ELS can affect the decision-making ability of individuals with AUD. Such altered emotional processing and decision-making, resulting from ELS, may be interpreted as the underlying drivers for the development of stress-related AUD 48.
Moreover, we found that the functional connectivity of SN seeds was influenced by ELS, resulting in a positive correlation between the left SMG and ACC with DMN nodes (precuneus and left ATFC) in the AUD population. It is noteworthy that exposure to acute stress has been linked with elevated functional connectivity between the default mode and salience networks in healthy adults and adolescents 39, 49. Given that the participants in our study cohort were exposed to a variety of ELS events, spanning from one to nineteen stress events, it is probable that the direction of connectivity between different regions of the network was impacted by the overall degree of ELS they experienced. Our findings align with previous research suggesting that certain nodes within the DMN, such as the temporal fusiform cortex and precuneus, play a crucial role in social and self-related cognitive processes 50, 51, supporting the interpretation that increased SMG-ATFC/precuneus coupling, as a result of ELS events, may lead to heightened self-awareness and emotional response to negative social stimuli. This, in turn, could potentially increase impulsive decision-making and drinking behaviors as a way of regulating these emotions in individuals with AUD.
We further investigated the impact of BMI on ELS-influenced connectivity patterns of the SN with different brain regions in AUD. Contrary to our expectations, individuals with AUD and low BMI exhibited a stronger anticorrelation in the connectivity of the left SMG seed with the right LOC and PCC clusters, while exhibiting a weaker positive correlation with the precuneus cluster, compared to those with high BMI. This suggests that the SN left SMG connectivity with emotion regulation and decision-making regions is not always negatively impacted by AUD and may even be strengthened in these individuals with a low BMI. On the other hand, we observed that the ELS influenced anticorrelation of the right SMG seed with the somatosensory (left postcentral gyrus) and impulse control (left MFG) clusters was weaker in AUD-low BMI individuals compared to those with high BMI, which in light of previous findings 27, 28, 52–54, suggest elevated impulsivity and poor self-control behaviors in AUD with co-occurrence of high BMI. The findings suggest two potential scenarios: either ELS contributes to overeating in individuals with AUD, or ELS-induced overeating increases their susceptibility to excessive alcohol consumption.
Upon further examination, we found that the influence of ELS on connectivity patterns with the left and right hemispheric SN SMG seeds differed based on individuals' BMI levels within our population with AUD. In particular, the magnitude of increase in BMI levels were observed to exert an impact on the connectivity of the left SMG with regions associated with the default mode network. On the other hand, a negative correlation was detected between the BMI levels of individuals with AUD and the connectivity of the right SMG with clusters in frontal regions that govern impulsive or self-control behaviors. Both pre-clinical and clinical studies have identified the contribution of ELS in increasing the risk for obesity 55–57 and AUD 32, 58, 59, which was attributed to persistent overactivation of the hypothalamic-pituitary-adrenal (HPA) axis 60, dysregulation of the mesolimbic dopamine functions 61, 62 and an imbalance in connectivity patterns of salience, emotion and somatosensory networks 63. Nonetheless, none of these studies demonstrated the relationship between ELS-influenced brain connectivity in adults with a comorbid occurrence of AUD and elevated BMI.
The present study offers intriguing insights into the intricate relationship between ELS, AUD, BMI and the connections of salience network seeds with the entire brain. These results are consistent with our hypothesis, which suggests that elevated BMI may alter the connectivity between the SN and brain regions that regulate executive and impulsive behaviors in individuals with AUD who have a history of ELS.
Limitations
There are several unanswered questions that need to be explored in future large cohort studies. One limitation of our study is the use of self-reported questionnaire to measure ELS events. This type of measure is prone to recall bias and may not provide a complete evaluation of ELS. Furthermore, our study did not disentangle the effect of each type of ELS experience, even though research shows that different adverse events may have different effects on brain structure and network connectivity. For example, deprivation and neglect are linked to changes in executive control network regions, such as the dorsolateral prefrontal cortex and parietal cortex, while threat and abuse-related exposures are linked to alterations in regions of the salience network 64. Additionally, adults who grew up in poverty exhibit reduced activation in the ventrolateral prefrontal cortex and have difficulty regulating emotions 65. In a recent study alterations in connectivity within the SN was found to mediate the effects of childhood abuse and neglect with problematic alcohol use 66. Although there are no studies that have directly compared the impact of ELS on connectivity differences based on BMI levels in the AUD population, the age at which the stress occurred 67 and the level or duration of stress exposure 68 are crucial factors that should be explored in future studies. Moreover, the correlations observed between ELS and brain connectivity in individuals with AUD at different BMI levels give rise to various conclusions. For instance, it is possible that ELS influences both alcohol abuse and excessive eating. Alternatively, it could be that ELS-driven AUD contributes to overeating, or that ELS-driven overeating increases vulnerability to alcohol overconsumption. The significance of these findings emphasizes the need for longitudinal studies instead of solely relying on cross-sectional research. It is also crucial to conduct longitudinal studies that follow individuals with AUD from an early stage, allowing the observation of potential changes in their brain patterns over time, particularly in relation to any fluctuations in BMI. Lastly, our study did not identify connectivity differences with salience network seeds in AIns and ACC, as reported in previous studies on the effects of ELS on salience network seed-based connectivity. This may be attributed to the relatively low severity of the stressors reported in our cohort, making direct comparisons with previous results difficult. In our forthcoming study, we aim to investigate potential sex effects that may be influencing the observed correlations and relationships. Furthermore, it is crucial to replicate our findings using large datasets to assess the consistency and reliability of the results, reinforcing the significance and validity of our study's outcomes.