Mares seem to respond to higher altitude with greater 5-HT values compared to those housed at sea level (Table 2.2), consistent with observations in cows (Bruschetta et al. 2010) and rats (Awabdy et al. 2003). At higher altitude, platelet function could be downregulated, producing a decrease of platelet serotonin uptake and increased circulating serotonin levels. On the contrary, in geldings, there was a significant increase in 5-HT at sea level. Moreover, a gender-dependent effect on plasma 5-HT levels was detected in horses at sea level, which was slight in horses at 600 m asl. An analysis using a larger sample size could support these results and clarify the existence of an interaction among different variables that may have opposite effects on plasma 5-HT levels. Further, significant findings could come from the comparison of horses coming from farms or riding schools at sea level with horses farmed at altitudes higher than 1,000 m asl.

The major changes in tryptophan values were observed in mares. These data could suggest the existence of sexual dimorphism, which could be deeper investigated using a greater number of horses. The higher concentration of cortisol observed in mares in both groups (Table 2.2) confirms the existence of a regulation of the hypothalamic–pituitary–adrenal axis of horses in a gender-dependent manner, as already observed in humans (Stroud et al. 2011) and rats (Viau et al. 2005). With a larger sample size, this could be of interest for geldings farmed at 600 m asl.