Central fatigue and nycthemeral change of serum tryptophan and serotonin in the athletic horse.

This study investigates the daily rhythm of tryptophan (TRP) and serotonin (5-HT) in athletic horses, and its connection to central fatigue. The results revealed a significant time influence on both substances, showing notable increase in the evening (evening acrophases). This can aid in understanding an athlete horse’s “chronoperformance” and help create effective training programs that factor in this daily pattern.

Research Methodology

• To conduct this investigation, blood samples were drawn from 5 Thoroughbred mares at 4-hour intervals over a 48-hour period. This process began at 08:00 hours on day 1 and ended at 4:00 on day 2.
• The sampling was carried out via an intravenous cannula inserted into the jugular vein.
• Tryptophan and serotonin concentrations in the samples were determined using a process called High Performance Liquid Chromatography (HPLC).
• The collected data was analysed using a one-way repeated measures analysis of variance (ANOVA) and a single cosinor method.

Key Findings

• Analysis of Variance (ANOVA) showed a highly significant influence of time on both tryptophan and serotonin levels in all horses, on both days, with p values less than 0.0001, indicating a statistically significant difference.
• The cosinor analysis – a method used to analyse rhythmic variation – identified the periodic parameters and their acrophases (time point of the peak value in a day) during the monitoring period.
• Both tryptophan and serotonin showed evening acrophases, meaning their concentrations peaked in the evening.

Implications and Conclusions

• The results indicate that tryptophan and serotonin levels in the blood of athletic horses follow a daily (nycthemeral) pattern with typical evening peaks. This may be connected to the onset of central fatigue, a phenomenon where the central nervous system reduces the neural drive or motor command to working muscles, causing a decline in performance.
• Understanding these variations can contribute to the knowledge of an athlete horse’s chronoperformance – the performance variation relating to biological rhythms.
• A practical implication of these findings is the potential ability to develop more effective training programs for athletic horses. These programs could take into account the daily pattern of tryptophan and serotonin levels, optimizing the timing of training to align with an athlete horse’s biological rhythm.