Evidence for the effect of serotoninergic and dopaminergic gene variants on stress levels in horses participating in dressage and harness racing

Abstract

Eye temperature assessed with infrared thermography is an adequate tool for stress level assessment in sport horses’ competitions having a moderate heritability. Serotonin and dopamine signal transduction-linked gene variants have been associated with anxiety-related traits in several species. In this study we examined the association between 10 gene variants in BDNF, COMT, HTR1A, TPH2 and SLC6A4 genes (and the haplotypes at SLC6A4 gene) with stress level (measured with eye temperature and heart rate) in 270 animals, 135 Spanish Trotter Horses (STH) participating in trotting races and 135 ‘Pura Raza Español’ (PRE) horses in dressage. Association analyses were performed using a unified mixed model (counting for population structure and individual relatedness) for the whole population and for each horse breed. The g.43865600G > A intronic gene variant located 11.0 kb downstream from the transcription start site of SLC6A4 gene was associated with an increase in eye temperature before competition with a relative contribution of this gene variant of 38.8% (P = 0.001), 31.8% just after (P = 0.001) and 29.8% 2 h after the competition (P = 0.003). In STH, the g.43865600G > A gene variant showed the same association with eye temperature before (P = 0.001, contribution 27.2%), just after (P = 0.0003, 29.0%) and after the competition (P = 0.002, 17.5%); and the c.*111G > A gene variant located at the 3′UTR region of COMT gene was associated with eye temperature 2 h after the competition (P = 0.001, 22.3%). These results showed that SLC6A4 and COMT gene variants are associated with stress level measured as eye temperature increase during competitions, and may be promising tools for genetic testing against resistance at high stress levels in trotter horses.