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Depression manifests in several forms: major depression, persistent depressive disorder, and bipolar I and II disorders. Each form presents with varying severity and duration of symptoms. Major depression involves intense episodes of sadness and disinterest, while persistent depressive disorder is a chronic, milder form lasting years. Bipolar disorders include alternating episodes of depression and elevated mood.

Depression presents with emotional symptoms including persistent sadness, disinterest in previously enjoyable activities, and low self-esteem. Biological symptoms include decreased cognition, reduced appetite, and sleep disturbances. These symptoms often occur together, affecting both mental and physical functioning. The combination of emotional and biological manifestations distinguishes depression from temporary mood changes.

Depression is associated with abnormalities in three key brain regions: the prefrontal cortex, amygdala, and hippocampus. The prefrontal cortex regulates decision-making and emotional control. The amygdala processes emotions and fear responses. The hippocampus manages memory formation. Dysfunction in these regions contributes to the emotional and cognitive symptoms observed in depression.

The monoamine theory postulates that depression results from a deficiency of monoamine neurotransmitters that regulate mood in the brain. These neurotransmitters include serotonin, norepinephrine, and dopamine. When monoamine levels are reduced, mood regulation becomes impaired, leading to depressive symptoms. This theory has guided development of antidepressant drugs tricyclics SSRIs and SNRIs targeting monoamine systems.

The neurotrophic factors and plasticity theory links depression to diminished neurotrophic support and reduced neurogenesis, the process of forming new neurons. Neurotrophic factors promote neuron survival and growth. When these factors are reduced, neurogenesis decreases, impairing the brain's ability to adapt and repair. This neurobiological change contributes to depression's cognitive and emotional symptoms.

The neuroendocrine theory links depression to hormonal imbalances, particularly elevated cortisol levels and thyroid dysregulation. Cortisol, a stress hormone, is often elevated in depression, disrupting normal stress response mechanisms. Thyroid dysfunction affects metabolism and mood regulation. These hormonal abnormalities interact with brain chemistry to perpetuate depressive symptoms.

Beyond monoamines and hormones, irregularities in glutamate neurotransmission and disruptions in the hypothalamic-pituitary-adrenal (HPA) axis are implicated in depression. Glutamate abnormalities affect excitatory signaling in the brain. HPA axis dysfunction impairs the body's stress response regulation. Despite multiple theories, depression remains complex, requiring continued research to develop more refined intervention strategies.