Circadian variation in plasma serotonin level in equines submitted to transport stress

Stress triggers a set of adaptative reactions that can compromise the homeostasis of the organism, such as transport from one environment to another. The aim of the present study was to determine whether there is a periodic variation in serotonin levels following transport stress. We used a databank to obtain data on plasma serotonin levels of healthy adult horses collected at regular eight-hour intervals for three consecutive days on two different occasions: before and after transport. The results were tabulated and submitted to Hurst statistical analysis procedures. Curves were created of the variation in serotonin levels as a function of time using the cosinor method. Plasma serotonin levels exhibited circadian variations independently of sex and age. The circadian variation pattern was significantly affected when the animals experienced transport stress (p < 0.05). Thus, plasma serotonin levels can be used as an indicator of physiological disorders and the promotion of animal wellbeing.