Hormonal System
Is known as the HypoThalamic-Pituitary-Adrenal Axis is active (solid arrows in diagram) in response to Stress Mounting evidence suggests that chronic overactivity of the Axis, and particularly overproduction of CorticoTropin-Releasing Factor (CRF), contributes to Depression.

For instance, application of CRF to the Brain of laboratory animals produces symptoms (above) similar to those often displayed by depressed individuals.