Sympatho-adrenal activity and the clinical sedative effect of detomidine in horses

Thirty-nine horses were given detomidine 10 micrograms/kg bodyweight (bwt) intravenously (iv) and six horses were given a corresponding volume of saline solution before minor procedures. Venous blood samples were collected for catecholamine and cortisol determination immediately before the detomidine or saline injection and 20 mins after it. The behaviour of the horse at the time of detomidine injection and the extent of sedation were evaluated. Plasma adrenaline, noradrenaline and the catecholamine metabolites, 3,4-dihydroxyphenylglycol (DHPG) and 3,4-dihydroxyphenylacetic acid (DOPAC), and cortisol concentrations decreased significantly after administration of detomidine, but they remained unchanged in the control group. A high plasma adrenaline concentration before detomidine injection, indicative of a high level of stress, seemed to correspond with a reduced sedative effect of detomidine. The extent of sedation also was related to the concentrations of adrenaline, noradrenaline and DHPG in plasma after the detomidine injection. The reductions in the plasma catecholamine concentrations may have been caused, in part, by decreased secretion of catecholamines due to the sedative effects of detomidine; but detomidine also influences the plasma catecholamine concentrations by reducing directly sympatho-adrenal activity.