The mechanism of depression and anxiety in adulthood after experiencing early adverse experiences: A new two-hit model of depression

Adverse childhood experiences (ACEs) increase susceptibility to depression and anxiety disorders in adulthood. This study investigated the potential mechanisms through which ACEs enhance vulnerability to depression and anxiety in adulthood, using a novel "two-hit" mouse model by combining maternal separation (MS) with 14 or 21 days of restraint stress (RS). Behavioral assessments (sucrose preference test, tail suspension test, open field test, elevated zero maze) confirmed depressive- and anxiety-like behaviors in the MS + RS 21d group mice. Neurobiological analyses revealed hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis (elevated serum corticosterone [CORT] and adrenocorticotropic hormone [ACTH]) and dysregulation, characterized by reduced levels of monoamine neurotransmitters (5-hydroxytryptamine [5-HT], 5-hydroxyindoleacetic acid, dopamine, norepinephrine), altered mRNA expression of key genes (e.g., increased ACTH, CRH, SERT; decreased GR, brain-derived neurotrophic factor [BDNF]), and corresponding protein-level changes (e.g., increased 5-HT1AR, CRHRs; decreased BDNF, TrkB). Our findings indicate that the two-hit mouse model, combining MS with a 21-day RS, stably induces depressive- and anxiety-like behaviors in mice. The underlying mechanism may be associated with HPA axis dysfunction, serotonergic system dysregulation, and aberrant BDNF signaling within the prefrontal cortex-amygdala-hypothalamus circuit.