"A decrease in serotonin neurons is associated with depression and suicide, and neurons show evidence of neurodegeneration in autism, Alzheimer's disease, Parkinson's disease, frontal lobe dementia and Lewy‐body dementia."
http://www.els.net/WileyCDA/ElsArticle/refId-a0000124.html
http://www.els.net/WileyCDA/ElsArticle/refId-a0000124.html