A unifying framework for depression: bridging the major biological and psychosocial theories through stress.

Several mechanisms for the development of depression have been proposed, and a comparative examination reveals considerable overlap. This paper begins by summarizing the conclusions drawn in the literature on the major biological theories: HPA-axis hyperactivity, the monoamine theory, the cytokine hypothesis/ macrophage theory, and structural changes to relevant brain regions and neurons. It then discusses the role of psychosocial stress as a bridge between the pathophysiology of depression and its predominantly psychosocial risk factors, touching upon theories offered in psychology and in population health. This paper further proposes a unifying framework which integrates the major theories. The multiple systems involved, and the directional complexity among them, likely help to explain the wide-ranging symptoms associated with depression, and the wide variety of comorbid medical conditions. They may also contribute to challenges in treatment, the diversity in symptoms and treatment outcomes among individuals, and the high rates of symptom persistence and relapse. The apparent bi-directionality of associations may suggest the existence of positive-feedback loops which aggravate symptoms; however, further bench research is required to confirm such phenomena. A better understanding of these interweaving associations is warranted. Additionally, given the significant influence of socioeconomic and psychosocial factors on the aetiology of depression, population-level interventions that address the social determinants of health are required. Current individual-level pharmacologic approaches are designed to treat pathophysiology once it is underway, and current individual-level non-pharmacologic interventions (such as talk therapy) are designed to moderate the relationship between psychosocial stress and pathophysiology. In contrast, a key strategy for primary prevention lies in population-level interventions that address the predominantly social causes of one of depression's most notable risk factors: chronic psychosocial stress.