Mental disorders pose a significant global health burden, with prevalence on the rise. A major contributing factor is the increased level of acute stress in modern society, particularly in the workplace. While prolonged stress exposure increases vulnerability to psychopathology, individuals exhibit considerable heterogeneity in their responses. Most individuals demonstrate resilience, experiencing minimal or transient disruptions in functioning. However, others develop substantial stress-related psychopathology, including anxiety and depression. Despite significant interest in understanding stress-related psychopathology, the biological basis for human stress resilience remains largely unknown. This study aims to identify a neurobiological mechanism associated with stress resilience, potentially improving prediction and treatment of stress-related psychopathology. Decades of animal research have linked vulnerability to prolonged stress with locus coeruleus (LC-NE) hyper-responsivity, the noradrenergic arousal system. The LC-NE, a small pontine nucleus with widespread projections throughout the central nervous system, plays a vital role in the central stress circuitry. Its function is to upregulate physiological processes that mobilize energy and promote autonomic adaptation to stress. Sustained LC-NE hyper-responsivity contributes to chronic anxiety and depression, fear, PTSD, hypertension, and cardiovascular disease. Functional coupling between LC-NE and the amygdala, a region associated with emotional intensity and threat perception, promotes stress-induced anxiety-like behavior in mice. However, human research on the LC-NE's role in stress vulnerability is limited, with most studies examining brain activity after stressors have occurred, thus unable to identify predisposing mechanisms or predict individual resilience. This study uses a prospective design to measure individual LC-NE responsivity in a laboratory task and predict subsequent real-world stress responses. Behavioral measures, pupil dilation (a marker of LC-NE firing and cholinergic activity), and fMRI data of LC-NE responses and functional coupling between LC-NE and amygdala were acquired. The emotional Stroop task, which induces affective conflict, was used to assess LC-NE responsivity. Conflict resolution in this task involves processing costs and increased arousal, associated with noradrenergic release via the LC-NE. The study focuses on a high-risk population: medical students before their first internship, as medical internships are a known risk factor for stress-related disorders. Levels of depression and anxiety were assessed before the internship and at 3 and 6 months during the internship to measure symptom changes from baseline.

Extensive research in animal models has established a strong link between the locus coeruleus-norepinephrine (LC-NE) system and stress responses. Studies have consistently shown that hyper-responsivity of the LC-NE system is associated with increased vulnerability to stress and the development of stress-related disorders. These studies have used various stressors in rodent and primate models, demonstrating a consistent pattern of LC-NE activation in response to acute and chronic stress. Furthermore, research has explored the interaction between the LC-NE system and other brain regions, particularly the amygdala, in mediating stress-related behaviors. Animal studies have shown that functional coupling between LC-NE and amygdala plays a significant role in stress-induced anxiety. Despite this evidence from animal models, human research has lagged behind. Most human studies have investigated LC-NE activity after the onset of stress-related symptoms, thus hindering the ability to identify pre-existing vulnerability markers and mechanisms of resilience. This gap in knowledge highlights the need for prospective studies linking pre-stress LC-NE activity to the development of psychopathology in response to real-world stressors. This study bridges this gap by using a prospective design with a high risk population, allowing researchers to examine the predictive value of pre-stress LC-NE activity for later stress-related disorders.

This prospective study involved 48 medical students (28 women, mean age 24 years) who completed an fMRI session and questionnaires before their first medical internship. The fMRI session included an emotional Stroop task, designed to elicit affective conflict and assess LC-NE responsivity. Participants categorized faces according to emotional expression (happy vs. fearful), while ignoring overlaid emotionally congruent or incongruent words. The conflict arises from the incompatibility between task-relevant and irrelevant stimuli. The CI > II contrast, comparing incongruent trials preceded by congruent trials (CI) to those preceded by incongruent trials (II), isolates neural processes involved in noradrenergic upregulation of cognitive control. Pupil dilation was measured concurrently with fMRI using an MR-compatible eye-tracker, serving as a peripheral marker of LC-NE firing and cholinergic activity. Anxiety and depression symptoms were assessed using the State-Trait Anxiety Inventory (STAI) and Patient Health Questionnaire (PHQ-9), respectively, at three time points: before the internship (T0), and at 3 and 6 months into the internship (T1 and T2). Changes in symptom severity were calculated by subtracting baseline scores (T0) from scores at T1 and T2. This design allows for the analysis of individual differences in stress resilience. Data analysis included behavioral analyses of reaction times and accuracy, fMRI analyses using general linear models (GLMs) in SPM8 to identify brain regions involved in the conflict response, psychophysiological interaction (PPI) analysis to examine functional coupling between LC-NE and the amygdala during the upregulation response, and analyses of pupil dilation data. Leave-one-subject-out (LOSO) and leave-two-subjects-out (LTSO) cross-validation techniques were used to assess predictive validity of the biomarkers. Multiple regression models were employed to compare the predictive power of various parameters, including LC-NE responsivity, pupil dilation, LC-amygdala connectivity, and the behavioral congruency-sequence effect. Statistical significance was assessed using various methods including t-tests, ANOVAs, correlations, and permutation tests. The study employed several methodological approaches to improve the reliability and specificity of the fMRI data. These include using a smaller, more robust LC mask, controlling for physiological noise using principal component analysis of CSF signals, and comparing results with data from other brainstem nuclei. The study also analyzed the congruency effects on pupil dilation to ensure that light reflex responses did not confound results.

The study found that the locus coeruleus (LC) was strongly recruited during conflict-induced upregulation, as indicated by the CI > II contrast which revealed robust activity in the midbrain/brainstem. Region-of-interest analysis in the LC confirmed significant CI > II responses, confirming the role of the noradrenergic arousal system in the conflict response. Participants who experienced greater increases in anxiety and depression symptoms during their internship also showed significantly stronger LC-NE responsivity in response to the conflict task (CI > II). This relationship held true for both anxiety and depression symptoms and was robust across different LC masks. Furthermore, individual LC-NE responsivity correlated significantly with anxiety and depression symptom changes measured at 3 and 6 months, and with the mean symptom changes. Smaller conflict responses in the LC were associated with less symptom change and more resilience. Analyses comparing the LC with other brainstem nuclei demonstrated that the LC was uniquely associated with both anxiety and depression symptom changes. The predictive validity of LC-NE upregulation was further confirmed through out-of-sample analyses (LTSO), which showed above-chance prediction of symptom changes. Functional coupling between LC-NE and the amygdala during the upregulation response also correlated significantly with mean anxiety and depression symptom changes, confirming prior findings in rodents. Again, LTSO analyses confirmed the predictive validity of this connectivity measure for symptom changes. Pupil dilation, a peripheral physiological marker of LC-NE firing, also reflected the conflict response. Pupil dilation was significantly enhanced for incongruent trials compared to congruent trials. Trial-sequence effects were observed, with pre-trial pupil dilation for CI trials being higher than for II trials, and post-trial pupil dilation for II trials being lower than for CI trials. This suggests a noradrenergic carry-over effect from previous trials. Analyses showed a negative correlation between the increase in pre-trial pupil dilation and the subsequent CI > II pupil dilation difference and the CI > II difference in LC-NE BOLD responses. Pupil dilation distance (PDD), reflecting the difference between current and pre-trial CI > II pupil dilation, also correlated significantly with mean anxiety and depression changes. While anxiety predictions were only marginally above chance, out-of-sample predictions of depression change using PDD were significantly above chance. Finally, comprehensive regression models indicated that LC-NE upregulation and connectivity significantly improved predictions of anxiety and depression symptom changes compared to the baseline model that only included self-report questionnaires. The most parsimonious model for anxiety included LC-amygdala connectivity, LC responsivity, pupil dilation, and the behavioral congruency-sequence effect, while for depression, the most parsimonious model included LC responsivity and the baseline PHQ score. These results highlight the robustness and predictive power of LC-NE activation and connectivity as biomarkers for stress resilience.

This study makes a significant contribution to the understanding of stress resilience by identifying prospective neurobiological markers that predict individual differences in response to real-world stressors. The findings demonstrate a robust association between LC-NE responsivity during emotional conflict and subsequent levels of anxiety and depression following prolonged real-life stress. The results support the hypothesis that a hyper-responsive LC-NE system predisposes individuals to develop stress-related psychopathology, while a less reactive system is associated with resilience. The inclusion of pupil dilation data provides additional evidence supporting the involvement of the noradrenergic system and highlights the potential of using pupil dilation as a readily accessible, cost-effective biomarker for stress resilience. The strong relationship between LC-amygdala connectivity and anxiety levels suggests that the interaction between these brain regions might play a crucial role in the development of stress-related psychopathology. This study addresses several methodological limitations of previous research by using a prospective design, real-world stressors, and rigorous cross-validation techniques. The use of a high-risk population enhances the generalizability of the findings to at-risk groups.

This study provides compelling evidence that the locus coeruleus's responsivity to emotional conflict is a robust predictor of stress resilience in humans. The findings highlight the potential of LC-NE activation and connectivity as biomarkers for predicting individual vulnerability to stress-related psychopathology. Future research should focus on refining these biomarkers, exploring potential interventions targeting LC-NE activity, and extending these findings to other at-risk populations and stress contexts. The development of targeted interventions, possibly utilizing neurofeedback or emotion regulation training, might help reduce LC-NE hyper-responsivity and improve stress resilience.

The study's limitations include the relatively small sample size and the focus on a specific population (medical students). The generalizability of the findings to other populations and occupational contexts needs further investigation. While pupil dilation is correlated with LC-NE activity, it is not a specific marker and may reflect activity in multiple neuromodulatory systems. Future studies could use more sophisticated imaging techniques such as high-field fMRI to enhance the signal-to-noise ratio and improve the spatial resolution for investigating the brainstem. The relatively low level of anxiety and depression symptoms in the current sample compared to previous studies might reflect differences in stress exposure or selection bias in the studied cohort. Finally, the study primarily focused on the absence or presence of symptoms rather than the full range of resilience constructs, such as individual coping strategies.