20 The effects of androstenone on the plasma concentration of serotonin, β-endorphin, and cortisol in horses

The neuroendocrine system is the main factor that regulates animal behavior and is generally mediated by internal or external stimuli, including pheromones. Pheromones are volatile and non-volatile chemical molecules that usually trigger behavioral changes in other animals of the same species. Androstenone is a pheromone known to change behaviors of animals. Recently, the expression of androstenone receptor was discovered in horses. Consequently, behavioral changes in response to androstenone may be regulated by the neuroendocrine system. Thus, the objective of this study was to monitor the neuroendocrinal changes induced by treatment with androstenone. A total of 8 Thoroughbred horses consisting of 5 females and 3 males were used with a crossover study design. Horses were treated with androstenone diluted with jojoba oil (10 ug/ml) and only oil for negative control. A handler applied oil with or without androstenone to horses's nostril and rubbed for 5 s wearing latex gloves. Blood samples were collected before, 15, 30, and 60 min after androstenone treatment. The plasma concentration of 5-HT, β-endorphin, and cortisol were analyzed using ELISA kits. The interaction effect of each treatment of androstenone on the level of neurotransmitters was accessed by Mixed GLM model using SAS. A Least square(LS) post-hoc test was applied to test the significant difference in the plasma concentration of neurotransmitters between treatment and control group at the same sampling time. Time-dependent hormonal changes were also compared using LS means comparison. P values of < 0.05 were considered statistically significant. The concentration of 5-HT of androstenone-treated horses remained consistent throughout the experiment, while the concentration of the control group significantly decreased over time. The plasma concentration of β-endorphin in the treatment group also remained constant, however, it was increased in the control group compared with the before treatment. Cortisol levels did not change in either treated and untreated groups. In conclusion, the androstenone treatment appears to regulate 5-HT, and β-endorphin system in horses.