Association of plasma concentrations of oxytocin, vasopressin, and serotonin with docility and friendliness of horses

Oxytocin (OXT) and vasopressin (AVP) have been associated with social interaction and affiliative behavior in animals. Additionally, AVP is known to affect socially aggressive behavior. In addition, serotonin has an association with aggressive behaviors. The objectives of this study were (1) to evaluate OXT, AVP, and serotonin concentrations in the plasma of horses of different breeds, sexes, and ages and (2) to determine if the neurotransmitters are associated with horse docility and friendliness. This study was performed at Sangju International Equestrian Center. Blood samples were collected from 23 horses, including 6 Thoroughbreds (11 to 18 yr), 6 Warmbloods (15 to 26 yr), 6 ponies (8 to 17 yr), and 5 Quarter Horses (4 to 12 yr). The group of horses consisted of 13 mares and 10 geldings. The plasma concentrations of OXT and AVP were measured by enzyme-linked immunosorbent assay, and the serum concentration of serotonin was measured by high-performance liquid chromatography. The characteristics of each horse were surveyed by 3 horse trainers. The effects of breed, sex, and age on the concentration of each neurotransmitter were assessed by a 3-way ANOVA with LSD post-hoc analysis. Linear regression analysis was performed to determine if the concentration of neurotransmitters is related to the docility and friendliness of horses. As a result, the concentrations of OXT and AVP did not vary with the breed, sex, or age of horses. However, the serotonin concentration varied depending on the breed and age of horses. Interestingly, there was a trend toward the existence of a correlation between docility and OXT in Thoroughbred horses. However, AVP and serotonin concentrations had no correlation with the docility and friendliness of horses. In conclusion, the docility and friendliness of Thoroughbred might be related with the blood OXT concentration.