adgrl3.1 knockout disrupts cortisol regulation and stress reactivity, linking externalizing and internalizing...

The adhesion G protein-coupled receptor L3 (ADGRL3) protein has been implicated in various externalizing disorders (EDs), including ADHD and substance abuse disorder. Previous work has shown that zebrafish with a homozygous knockout of adgrl3.1 (adgrl3.1^-/-) not only show altered externalizing behaviors, but also altered internalizing behaviors, such as anxiety. Here, we aimed to understand the functional significance of adgrl3.1 across both externalizing and internalizing behaviors, to understand if there was an intrinsic link between externalizing disorders and comorbid internalizing symptoms. To further understand the role of this gene in anxiety and stress reactivity, we investigated the behavioral and physiological responses of adgrl3.1 knockouts to several anxiety-related behavioral tasks, and when challenged with an acute stressor, conspecific alarm substance (CAS). This included measuring stress-related markers both at baseline and during stress, including cortisol levels and expression of the genes bdnf and gr. We found that adgrl3.1^-/- exhibited increase in bottom-dwelling in the novel tank diving test and greater preference for the dark zone in the light/dark task, both indicative of heightened anxiety-like behavior. These animals also showed a disrupted stress response, characterized by lower baseline cortisol levels together with increased cortisol response to CAS. adgrl3.1^-/- also exhibited increased repetitions in the FMP Y-maze, which, when combined with stress, reflected a shift toward more rigid behavioral strategies and impaired cognitive flexibility. Our findings suggest that the adgrl3.1 gene plays an important role in regulating stress reactivity and cognitive function. Overall, our study provides novel evidence of the functional significance of adgrl3.1 in both externalizing and internalizing behaviors, particularly via its role in modulating physiological responses to stress.