High cortisol hormone levels cause depression

The cortisol hypothesis of depression argues that depression is the result of the cessation of neuron birth in the hippocampus as well as in other neural networks related to serotonin, dopamine, and norepinephrine. Cortisol appears to be the reason for this lack of neurogenesis. Cortisol is a hormone produced by the adrenal glands and secreted by the body in response to stress. It is involved in the fight or flight response.^[1] MDD patients display a symptom called HPA-Axis hyperactivity. Over secretion of cortisol leads to reduced serotonin, dopamine, and noradrenaline in the brain. As a result, MDD patients have smaller hippocampi.^[2] Empirical evidence also revealed that cortisol dysfunction is proportionate to MDD severity.^[3] The study of Maberg et al. (2000) supports this hypothesis. Researchers concluded that neurogenesis supports the effect of antidepressants in a rat model for depression.^[4] The meta-analysis of Videbech and Ravnkilde (2004) also found up to a 10% reduction of the hippocampus in the brains of depressed patients. The shrinking of the hippocampus was significantly correlated to the number of depressive episodes.^[5] Overall, the cortisol hypothesis explains depression in terms of neuroplasticity and hormonal imbalance.