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Depression is a psychoneurotic disorder characterized by lingering sadness, inactivity, and difficulty in thinking and concentration. A significant increase or decrease in appetite and time spent sleeping, feelings of dejection and hopelessness, and sometimes suicidal tendencies may also be present. It is one of the most common psychiatric conditions encountered, and affects up to 25% of women and 12% of men. Depression differs from grief, bereavement, or mourning, which are appropriate emotional responses to the loss of loved persons or objects.

Depression has many forms and is very responsive to treatment. Dysthymia, or minor depression, is the presence of a depressed mood for most of the day for two years with no more than two months' freedom from symptoms. Bipolar disorder (manic-depressive disorder) is characterized by recurrent episodes of mania and major depression. Manic symptoms consist of feelings of inflated self-esteem or grandiosity, a decreased need for sleep, unusual loquacity, an unconnected flow of ideas, distractibility, or excessive involvement in pleasurable activities that have a high potential for painful consequences, such as buying sprees or sexual indiscretions. Cyclothymia is a chronic mood disturbance and is a milder form of bipolar disorder.

Chemically speaking, depression is apparently caused by reduced quantities or reduced activity of the monoamine neurotransmitters serotonin and norepinephrine within the brain. Neurotransmitters are chemical agents released by neurons (nerve cells) to stimulate neighboring neurons, thus allowing electrical impulses to be passed from one cell to the next throughout the nervous system. They transmit signals between nerve cells in the brain.

Introduced in the late 1950s, antidepressant drugs have been used most widely in the management of major mental depression. All antidepressants accomplish their task by inhibiting the body's re-absorption of these neurotransmitters, thus allowing them to accumulate and remain in contact longer with their receptors in nerve cells. These changes are important in elevating mood and relieving depression.

Antidepressants are typically one of three chemical types: a tricyclic antidepressant (so called because its molecules are composed of three rings), a monoamine oxidase (MAO) inhibitor, or a serotonin reuptake inhibitor. The tricyclic antidepressants act by inhibiting the inactivation of norepinephrine and serotonin within the brain.

The MAOs apparently achieve their effect by interfering with the action of monoamine oxidase, an enzyme that causes the breakdown of norepinephrine, serotonin, and dopamine within nerve cells.

In the 1980s a new type of antidepressant called a serotonin reuptake inhibitor proved markedly successful. Its chemical name is fluoxetine, and it apparently achieves its therapeutic effect by interfering solely with the reabsorption of serotonin within the brain, thus allowing that neurotransmitter to accumulate there. Fluoxetine often relieves cases of depression that have failed to respond to tricyclics or MAOs, and it also produces fewer and less serious side effects than those drugs. It had thus become the single most widely used antidepressant by the end of the twentieth century. The most commonly used serotonin reuptake inhibitors are Prozac, Paxil, and Zoloft.

Medical experts agree that antidepressants are only a part of the therapeutic process when treating depression. Some form of psychotherapy is also needed in order to reduce the incidence for chronic recidivism of the illness.

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