The aim of this study was to investigate the influence of age on Thoroughbred horse plasma serotonin (5-HT) levels, measured in platelet poor plasma samples of clinically healthy horses. The analyses of plasma 5-HT amounts, with plasma tryptophan, the 5-HT precursor, were performed by reverse phase HPLC with electrochemical detector. Cortisol levels were also detected by ELISA kits. Both plasma 5-HT and tryptophan levels were significantly higher in foals compared to adult horses, thus suggesting a different rate of 5-HT synthesis correlated to age. No significant difference of cortisol amounts age-related was measured, probably because of the variability of plasma cortisol, which is known to be the result of many influences. In foals, the positive correlation between plasma tryptophan and cortisol (r = 0.75; P<0.05) could be linked to growth processes. In adult horses, the negative correlation (r = -0.73; P<0.05) between 5-HT and cortisol was explained on the basis of the relation of serotonergic system with hypothalamic-pituitary-adrenal axis. These results showed an influence of age on plasma 5-HT concentrations, suggesting an interaction of this biological variable with plasma 5-HT metabolism.
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