Depression is a prevalent mental illness marked by persistent sadness and lack of interest in previously enjoyable activities. It can take several forms, including major depression, persistent depressive disorder, and bipolar I and II disorders. Symptoms range from emotional changes like chronic worry to physical changes like sleep disturbances and suicidal thoughts. From a neurobiological perspective, depression is believed to be triggered by abnormalities in the brain's prefrontal cortex, amygdala, and hippocampus. These regions play crucial roles in emotions, memory, and decision-making.

Several theories attempt to explain the root causes of depression. The monoamine theory suggests that a deficiency of certain neurotransmitters — monoamines — involved in mood regulation in the brain may trigger depression. On the other hand, the neurotrophic factors and plasticity theory associates depression with diminished neurotrophic support and reduced neurogenesis (the process of forming new neurons). The neuroendocrine theory links depression to hormonal imbalances, particularly elevated cortisol levels and thyroid dysregulation. Additionally, irregularities in glutamate neurotransmission and the hypothalamic-pituitary-adrenal (HPA) axis are also implicated in the onset of depression. Despite the abundance of theories, depression still remains a complex disorder that requires continued research for a more comprehensive understanding and development of effective treatment strategies.