serotonin Serotonin 84 nonhuman 80 animal experiment 72 article 64 Animals 58 controlled study 58 rat 58 priority journal 50 Male 40 Rats 40 Female 38 animal cell 38 male 38 female 30 animal tissue 24 Animal 22 animal 22 melatonin 22 noradrenalin 22 5 hydroxyindoleacetic acid 20 unclassified drug 20 Melatonin 18 Support, Non-U.S. Gov't 18 central nervous system 18 drug effect 16 in vitro study 16 metabolism 16 protein expression 16 Article 14