Is Depression caused by either low norepinephrine or low serotonin?
There is a considerable amount of research that supports the theory that for some people depression is caused by low norepinephrine and that for others, low serotonin. For example, MHPG, a metabolite of norepinephrine is excreted in low levels in some, but not all depressed people (Yazici et al, 1993; Maas, 1975). Similar results have been found with 5-HIAA a major metabolite of serotonin (Yazici et al, 1993; Maas, 1975). Another line of research involves administering precursors of serotonin or norepinephrine to depressed participants. If they alleviate the symptoms it would support the theory. Van Praag (1984) found that tryptophan, a precursor of serotonin, does indeed reduce symptoms of depression in some, but not all cases. Similarly, Gelenberg et al (1980) found that tyrosine, a precursor of norepinephrine, alleviates depressive symptoms in some cases. There is also evidence that norepinephrine depression is different to serotonin depression. Indeed, serotonin and norepinephrine
