Depression and Monoamines
Major depressive disorder is thought to be linked to the levels of certain neurotransmitters called monoamines. Three examples of monoamines are serotonin, norepinephrine and dopamine. Norepinephrine is a stimulatory neurotransmitter that can also be thought of as adrenaline, dopamine is responsible for "pleasurable" feelings in the brain and serotonin is involved in modulating the effects of other neurotransmitters.
Role of Serotonin
Because serotonin is thought to help control the effects of other neurotransmitters, it is thought that a deficiency of serotonin can cause other pathways in the brain to work erratically, which can lead to depression. Studies have also found that stressful events, when combined with certain genetic deficiencies in serotonin production, can lead to major depressive disorder. Many antidepressants work to raise the levels of serotonin throughout the brain.
Limitations
Some scientists think the monoamine hypothesis is too simplified. Studies have not been able to link major depressive disorder with any specific dysfunction or disorder within the monoamine system, and drugs that deplete monoamines from the brain do not result in depression.
Depression and the Brain
Certain sections of the brain have been found to have abnormal activity in people with major depressive disorder. The raphe nuclei, which is the part of the brain that makes serotonin, and the suprachiasmatic nucleus, the section of the brain that regulates the body's "internal clock" or circadian rhythm, are the most strongly affected. Depression also lowers activity in the ventral tegmental area, which is the part of the brain that controls the body's perception of pleasure.
Physiology and Psychology
It is thought that in a major depressive disorder, there is a certain amount of synergy between the physiological aspects of depression and the psychological parts. Abnormal brain activity causes the feeling of being "down," as well as a lack of pleasure, which causes the patient to feel sad and withdrawn. As a result the patient begins to engage in behaviors that further heighten the feelings of depression, which in turn reinforce some of the abnormal patterns of activity in the brain.
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