A number of polymorphisms related to serotonin production, transport, dissociation, receptor binding and post-receptor activity have been associated with depressed patients vs. non-depressed patients.
In a pediatric population at high risk for depression, Birmaher et. Al (Birmaher B, Kaufman J, Brent DA, Dahl RE, Perel JM, al-Shabbout M, et al. Neuroendocrine response to 5-hydroxy-L-tryptophan in prepubertal children at high risk of major depressive disorder. Arch Gen Psychiatry. Dec 1997;54(12):1113-9) found that these children had a similarly blunted response to intravenous 5-HT infusion as did children diagnosed with major depression, even though they were not clinically depressed. This supports the endobiogenic notion that the serotonin response is neither the pre-critical terrain nor the cause of depression and that low central serotonin is a necessary but not sufficient cause of depression.