Neural Circuitry and Neuroplasticity in Mood Disorders: Insights for Novel Therapeutic Targets

Major depressive disorder and bipolar disorder are severe mood disorders that affect the lives and functioning of millions each year. The majority of previous neurobiological research and standard pharmacotherapy regimens have approached these illnesses as purely neurochemical disorders, with particular focus on the monoaminergic neurotransmitter systems. Not altogether surprisingly, these treatments are inadequate for many individuals afflicted with these devastating illnesses. Recent advances in functional brain imaging have identified critical neural circuits involving the amygdala and other limbic structures, prefrontal cortical regions, thalamus, and basal ganglia that modulate emotional behavior and are disturbed in primary and secondary mood disorders. Growing evidence suggests that mechanisms of neural plasticity and cellular resilience, including impairments of neurotrophic signaling cascades as well as altered glutamatergic and glucocorticoid signaling, underlie the dysregulation in these circuits. The increasing ability to monitor and modulate activity in these circuits is beginning to yield greater insight into the neurobiological basis of mood disorders. Modulation of dysregulated activity in these affective circuits via pharmacological agents that enhance neuronal resilience and plasticity, and possibly via emerging nonpharmacologic, circuitry-based modalities (for example, deep brain stimulation, magnetic stimulation, or vagus nerve stimulation) offers promising targets for novel experimental therapeutics in the treatment of mood disorders.