Daily variations of plasma serotonin levels in 2-year-old horses

Serotonin (5-HT) is a neurotransmitter implicated in the regulation of circadian rhythms. Synthesis and release of 5-HT are subjected to daily fluctuations, but there are no data on plasma serotonin levels in 2-year-old horses. Aim of this study was to investigate the daily variations in 5-HT levels in platelet-poor plasma from 2-year-old horses, so as to understand whether plasma 5-HT could be considered as an additional variable to assess the natural adaptability of horses to training. The research was carried out on 6 clinically healthy crossbred mares. Data showed higher levels of platelet-poor plasma 5-HT at 5 p.m. (P < 0.001) than at 1 p.m. and lower values at 5 a.m. (P < 0.001) than at 1 a.m. One-way repeated-measures analysis of variance showed a significant effect of sampling time (P < 0.0001) on plasma 5-HT changes. Results suggest that the increase of 5-HT levels could be related to an enhanced 5-HT output and release, and that their daily variations could be considered as an additional variable to assess the natural adaptability of horses for being trained. Tryptophan levels, the essential amino acid precursor of 5-HT, seemed to hint at slight fluctuations at regular intervals, with the highest amounts at 9 a.m. and at 9 p.m. and lowest at 1 a.m. and at 1 p.m., in agreement with data reported in adult horses. Cortisol levels showed wide fluctuations, with the highest amounts at 1 a.m. and the lowest amounts at 9.00 p.m.