Although a comprehensive understanding of the causes of depression and anxiety has not been reached, researchers have identified certain biological changes that may predispose a person to these disorders. The following is a summary of some of the theories that are currently under exploration.
Neurotransmitters in Depression and Anxiety
One area of study is the role of neurotransmitters—chemical messengers that carry information between neurons (nerve cells) in the brain. Decreased availability of certain neurotransmitters (such as norepinephrine, serotonin, and dopamine) may result in depression. Anxiety is thought to result from problems with the neurotransmitter gamma-aminobutyric acid (GABA), which normally suppresses the action of neurons.
Structures of the Brain
Much current research also focuses on particular structures in the brain that may be responsible for depression and anxiety. In general, it is believed that depression and anxiety may largely result from chemical imbalances affecting the brain's limbic system—a ring of structures (including the cerebral cortex, thalamus, hypothalamus, and hippocampus) that appears to be the seat of human emotions such as depression, fear, and rage.
The Endocrine System
Depression can also affect the endocrine system—the network of glands that regulates body functions by releasing various hormones into the bloodstream. However, these changes seem to be effects rather than causes of depression. On the other hand, some endocrine disorders can directly cause depression or mania. An example is Cushing's disease, in which the adrenal glands produce too much of the hormone cortisol. And some experts believe seasonal affective disorder (SAD) may stem from disordered release of the hormone melatonin by the pineal gland during winter.