http://forum.bodybuilding.com/showthread.php?t=121212081&page=12
References about serotonin’s role:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC492609/?page=1
http://jpet.aspetjournals.org/content/267/2/1002.abstract
http://jap.physiology.org/content/77/1/277.abstract