Depression is a mood disorder related to biochemical disturbances in the brain. These disturbances manifests as abnormal levels of chemical substances known as neurotransmitters. Scientists have identified three main neurotransmitters related to depression: norepinephrine, dopamine, and serotonin. Adjustment of these neurotransmitter levels can sometimes alleviate feelings of depression.
Background
Scientists cannot easily examine neurotransmitter function in the brain. Much of their knowledge of the causes of depression has evolved through indirect measures, such as observation of response to specific classes of medications, measurement of neurotransmitters in cerebrospinal fluid and autopsy examination of depressed individuals, according to the Brain Explorer website. Through these methods, scientists have pinpointed neurotransmitters that seem to be integral to mood regulation in the brain.
Norepinephrine
Norepinephrine, also known as noradrenalin, belongs to a class of neurotransmitters known as monoamines. Scientists have suspected its role in depression since discovering that drugs that deplete norepinephrine in the brain can lead to depression, according to the Brain Explorer website. When scientists developed tricyclic antidepressants, they discovered that tricyclics inhibit neurons from reabsorbing extra norepinephrine and serotonin, thus allowing more of these neurotransmitters to travel on to other neurons.
Serotonin
The other major monoamine neurotransmitter, serotonin, is of great interest in depression research. Evidence for serotonin's important role in mood regulation comes from autopsies of individuals who committed suicide; serotonin levels were found to be significantly lower in these patients than in non-depressed individuals who died from other causes, according to the Brain Explorer website.
The exact role of serotonin in depression remains unclear, but two major theories exist. One theory is that a deficit of serotonin activity may directly cause depression. Others believe that a serotonin deficit serves as a risk factor for depression, but is not a direct cause, according to the American College of Neuropsychopharmacology. Many scientists acknowledge that the depression-serotonin link is probably more complex than a simple response to a deficit of this neurotransmitter, according to Mark Moran in the May 2, 2003 edition of the Psychiatric News.
Dopamine
Because scientists suspect that no single neurotransmitter causes depression, examination and development of treatment for several neurotransmitters continues. Several classes of antidepressants regulate the neurotransmitter dopamine. Recent studies of cerebrospinal fluid of depressed patients has revealed an abnormally low level of a dopamine metabolite, according to the Primary Psychiatry website. Imaging studies also suggest that depressed individuals have a deficiency of dopamine passing between cells of the nervous system.
How Antidepressants Affect Neurotransmitters
Scientists only partially understand how antidepressants affect neurotransmitters in the brain. Tricyclic antidepressants, selective serotonin reuptake inhibitors, or SSRIs, and dopamine norepinephrine reuptake inhibitors (DNRIs) all seem to work in a similar fashion: to prevent brain cells from reabsorbing leftover neurotransmitters, thus allowing them to circulate to other areas of the brain. Monoamine oxidase inhibitors, or MAOIs, block monoamine oxidase, an enzyme that breaks down serotonin and norepinephrine, thus allowing more of these neurotransmitters to circulate throughout the brain, according to the Brain Explorer website.
