Physical exercise affects serotoninergic system in horse leukocytes

Abstract Serotonin (5-HT) may induce metabolic effects in different cell types, including leukocytes. In horses, 5-HT is involved in physiological and behavioral functions. Physical exercise is known to increase the amounts of 5-HT both in brain and periphery, but so far the signal mechanism in response to exercise is not known. The aim of the study was to investigate the effect of a racehorse intensive training session on plasma 5-HT levels, serotonin transporter (SERT), 5HT1A, 5-HT2A, 5-HT1B, 5-HT7 receptor, interleukin-1 beta and tumor necrosis factor-alpha expression in horse peripheral blood mononuclear cells (PBMC). In particular, the research was carried out on 12 trained horses performing daily training. Plasma 5-HT levels were analyzed in platelet poor plasma fraction by ELISA assay at T0, T1, T2 (pre-, 30’, 2h post-training session) respectively. PBMC were isolated to perform Real Time PCR for the evaluation of SERT, 5-HT receptor and cytokine mRNA levels. Results showed significant increased levels of plasma 5-HT, 5HT1A and 5-HT2A and significant decreased levels of SERT, 5-HT1B, 5-HT7, and both cytokine mRNAs in PBMC at T1, compared to T0 and T2. Results were confirmed by in vitro experiment. Training may induce a lower degree of 5-HT storage and therefore a higher plasma 5-HT concentrations. Leukocyte 5-HT receptor mRNAs seem strongly influenced by the exercise. Observed changes suggest a transient neuroendocrinological response to the exercise. A better understanding of the influence of physical exercise on serotoninergic system could have potential application for the implementation of training protocols in racing horses.