A neuro-transmitter chemical hormone which transmits messages between nerve cells it is known as the ‘’happy hormone’’ for its mood stabilizing and anti-depressant effects in humans. In horses it is a candidate for influencing anxiety, aggression and fatigue and is believed to affect the normal circadian (night/day) rhythms. Stereotypic behavior may also be influenced by serotonin but as yet any mechanisms, if present, are unclear. There is also interest in this hormone as a part of the so called gut / brain axis with potential for behavioral patterns to be influenced by the gut microbiota mediated by effects on the autonomic nervous system.
The amino acid Tryptophan is a precursor for Serotonin formation and is touted for use as a calmative for horses on that basis, though sound evidence for any such effect is lacking and tryptophan deficiency has not been recorded in horses.