Effects of intravenous oxytocin on plasma serotonin and trainability in horses.

Oxytocin (OXT) plays a crucial role in regulating the serotonin (5-HT) system and behavior. Although previous studies have reported significant relationships among OXT, 5-HT, and behavioral responses, the effects of OXT on 5-HT concentrations and behavioral traits in horses remain poorly understood. This study aimed to investigate the effects of intravenous OXT administration on plasma 5-HT concentrations and trainability in horses. A crossover design was employed using eight horses, each receiving either a 10-IU intravenous dose of OXT or normal saline. Blood samples were collected at baseline (T0) and at 30 (T1) and 60 min (T2) post-treatment. Plasma 5-HT concentrations were determined using a commercial ELISA kit. Trainability was assessed 60 min after administration using three behavioral tests that measured the time taken for horses to lower their heads and to yield their hindquarters under both mediate and direct pressure. Plasma 5-HT concentrations significantly increased at T2 compared with T0 and T1 following OXT administration. Although the time taken to complete the hindquarter-yielding test under direct pressure tended to be shorter in the OXT-treated group, this difference did not reach statistical significance. These findings suggest that OXT administration elevates plasma 5-HT concentrations and may enhance trainability in horses. This study provides a foundation for future research on how neuromodulatory mechanisms influence equine behavior.