The amygdala (AMY) is a key brain region of the limbic system that plays a critical role in emotion processing and stress response. Stress exposure is a major precipitating factor of MDD episodes which are associated with AMY hyperactivity. It remains unknown if chronic hyperactivation of the amygdala (cHOA) is sufficient to induce chronic stress-like deficits or is a susceptibility factor.
Background: The amygdala (AMY) is a key brain region of the limbic system that plays a critical role in emotion processing and stress response. Functional magnetic resonance imaging (fMRI) studies identified abnormal AMY activation in psychiatric illnesses including major depressive disorder (MDD). Stress exposure is a major precipitating factor of MDD episodes which are associated with AMY hyperactivity. Preclinical studies using of pharmacologic, opto- and chemogenetic approaches to activate AMY neurons have consistently demonstrated that acute AMY hyperactivation induces anxiety-like behaviors in mice. However, it remains unknown if chronic hyperactivation of the amygdala (cHOA) is sufficient to induce chronic stress-like deficits or is a susceptibility factor for chronic stress-induced behavioral, volumetric and synaptic deficits. Methods: Using designer receptor exclusively activated by designer drug (DREADD) approach, basolateral amygdala (BLA) neurons of Camk2a-cre mice infected with a virus driving the expression of the Gq-coupled DREADD were activated with clozapine-N-oxide (in drink water for 5 weeks). Mice were then exposed to chronic restraint stress (CRS; 1X/day for 1hr) for 2 weeks. All mice were behaviorally assessed in the Phenotyper (PT), and sucrose consumption tests (SCT) each week and in the novelty supressed feeding (NSF, once at the end of the experiment). Animals were then perfused for ex vivo-MRI and puncta density analysis. Results: We found that mice with cHOA displayed a progressive increase in baseline anxiety-like deficits in the PT test and slightly more marked deficits following CRS compared to controls, but not statistically different from animals subjected to CRS alone. Also, cHOA did not exacerbate CRS effect in the NSF. No significant cAH effect was found in the SCT before or after CRS. MRI analysis revealed no statistical charges between groups, while increased synaptic puncta density was found in cHOA mice subjected to CRS compared to cHOA or CRS alone. Conclusion: We demonstrate that cAH is sufficient to induce anxiety and may exacerbate CRS effects on anxiety and synaptic measures. Results also suggest that cHOA was not sufficient to induce depressive-like behavior and was not a vulnerability factor for stress-induced depressive-like behavior in mice. Altogether, our findings imply that a strong causal link between AMY hyperactivity and elevated anxiety, but not depressive-like behaviors and provide critical information to clinical research focused on using AMY activity level as a biomarker in stress-related illnesses.