<prompt> Increase in suicidal behaviour is due toIncrease in serotoninDecrease in serotoninIncrease in noradrenalineReactive depression  <response>: Karolinska Institute in Sweden first noted that low concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid 5-HIAA in the lumbar cerebrospinal fluid CSF were associated with suicidal behavior. This finding has been replicated many times and in different diagnostic groups.CSF, neurochemical, and receptor studies suppo the hypothesis that reduced central serotonin is associated with suicide. Reference: Synopsis of Psychiatry, page 766, 767. 