Effects of Novel Environmental Stimuli on Neuroendocrine Activity in Thoroughbred Horses

Abstract

Emotional stimuli have been shown to suppress vasopressin and facilitate oxytocin release from the posterior pituitary in some mammalian species. However, it is not clear in horses. Here we examined effects of novel environmental stimuli on plasma concentrations of vasopressin, oxytocin, cortisol, adrenaline and noradrenaline in Thoroughbred horses. To impose novel environmental stimuli, horses were introduced to an experimental stall (4.5 × 5.0 m) with side walls of black rubber board, in which a yellow balloon (80 cm in diameter) was hung from the ceiling. Horses were exposed to the novel environmental stimuli for 3 min. Heart rates were significantly increased during the stimuli. Plasma concentrations of adrenaline and noradrenaline in the blood collected immediately after the stimuli were twice as high as those at rest and decreased to the basal levels within 10 min after the stimuli. Plasma cortisol concentration increased after the stimuli and reached the peak 20 min later. However, plasma concentrations of vasopressin and oxytocin did not significantly change throughout the experiments. These data suggest that the novel environmental stimuli do not significantly influence the hypothalamo-neurohypophysial system while the stimuli activate the sympatho-adrenal medulla and the hypothalamo-pituitary-adrenal cortical axes in horses.