Depression and bipolar disorder are commonly referred to as mood (or affective) disorders.

  • Major hallmarks of depression include a persistent low or sad mood, decreased or absent interest in almost all activities, loss of self-confidence, and a feeling of worthlessness.
  • Most people with bipolar disorder (formerly known as manic-depressive illness) experience alternating episodes of both depression and mania. Mania, which can be thought of as the opposite of depression, is characterized by an elated or elevated mood, increased activity, an overblown self-image, and an exaggerated sense of self-confidence.

Usually, both depression and bipolar disorder are episodic—that is, bouts of illness are separated by symptom-free periods characterized by feelings of relative well-being.

Causes of Depression and Bipolar Disorder

The exact causes of depression and bipolar disorder are not well understood, but some combination of genetic predisposition and psychological and medical factors appears to play a role in these mood disorders.

Changes in the Brain

When people get depressed, chemical changes occur in the brain, and researchers believe that these changes are linked to the symptoms of mood disorders, especially depression.

The brain is composed of distinct regions, each made up of networks of nerve cells called neurons that transmit messages throughout the nervous system. Individual neurons are separated by small gaps at each end called synaptic clefts. Chemicals called neurotransmitters bridge the synaptic clefts and pass messages from one neuron to the next. Imbalances in three particular neurotransmitters—serotonin, norepinephrine, and dopamine—appear to contribute to depression and bipolar disorder, although less is known about changes in the brain that occur during the manic phase of bipolar disorder.

One specific brain region thought to be involved in depression is the limbic system, which affects our emotional behavior. An area within this system, the hypothalamus, regulates the pituitary gland, which in turn regulates key hormones and may be involved in the hormonal imbalances sometimes associated with depression.

Publication Review By: Karen L. Swartz, M.D.

Published: 01 Mar 2011

Last Modified: 28 Jan 2015