Effect of altitude on plasma serotonin levels in horses

The aim of this work was to carry out a preliminary study about the effect of altitude on plasma serotonin (5-HT) and tryptophan levels in horses. Subjects came from a riding school at sea level and 600 m above sea level. To evaluate animal welfare linked to physiological status, cortisol levels, blood cell count and hematochemical variables (glucose, total proteins, albumin, creatinine, urea, aspartate transaminase, creatine kinase, lactate dehydrogenase, triglycerides and total cholesterol) were measured. Comparison of mean plasma levels of 5-HT, tryptophan, and cortisol in horses coming from different altitudes were not significantly different. Plasma 5-HT levels were affected by altitude in a gender-dependent way, showing an opposing trend between mares and geldings, with the highest levels in plasma of geldings coming from farms at sea level. Both 5-HT and tryptophan were higher in mares than in geldings from 600 m above sea level. Plasma cortisol levels, which were significantly higher in mares than in geldings at sea level, were more affected by gender than altitude