Physical exercise affects serotoninergic system in horse leucocytes

Serotonin (5-hydroxytryptamine [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 enzyme-linked immunosorbent assay at T0, T1, and T2 (pretraining, 30 minutes post-training, and 2 hours post-training session), respectively. Peripheral blood mononuclear cells were isolated to perform real-time polymerase chain reaction for the evaluation of SERT, 5-HT receptor, and cytokine mRNA levels. The results showed significantly increased levels of plasma 5-HT, 5HT1A, and 5-HT2A and significantly decreased levels of SERT, 5-HT1B, 5-HT7, and both cytokine mRNAs in PBMC at T1, compared with T0 and T2. The 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.