Progress and challenges in research of the mechanisms of anhedonia in major depressive disorder.

There is an increasing heavy disease burden of major depressive disorder (MDD) globally. Both high diagnostic heterogeneity and complicated pathological mechanisms of MDD pose significant challenges. There is much evidence to support anhedonia as a core feature of MDD. In the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, anhedonia is further emphasised as a key item in the diagnosis of major depression with melancholic features. Anhedonia is a multifaceted symptom that includes deficits in various aspects of reward processing, such as anticipatory anhedonia, consummatory anhedonia, and decision-making anhedonia. Anhedonia is expected to become an important clinicopathological sign for predicting the treatment outcome of MDD and assisting clinical decision making. However, the precise neurobiological mechanisms of anhedonia in MDD are not clearly understood. In this paper, we reviewed (1) the current understanding of the link between anhedonia and MDD; (2) the biological basis of the pathological mechanism of anhedonia in MDD; and (3) challenges in research on the pathological mechanisms of anhedonia in MDD. A more in-depth understanding of anhedonia associated with MDD will improve the diagnosis, prediction, and treatment of patients with MDD in the future.