Dose-related effects of detomidine on autonomic responses in the horse

1. Detomidine is a novel veterinary sedative analgesic which is thought to act by stimulation of alpha 2 adrenoreceptors. The present study was undertaken to determine the direction, time course, and dose-response relationship of detomidine on specific autonomic responses in the unanaesthetized horse. 2. Detomidine was administered intravenously to eight adult thoroughbred racehorses at doses of 0.010-0.040 mg kg-1, according to a double-blind Latin square crossover design. Cardiac and respiratory rates, pupil diameter and rectal temperature were monitored for 180 min postinjection. 3. Detomidine produced prolonged dose-related bradycardia and bradypnoea. This was accompanied by a briefer period of dose-related mydriasis. Response duration, rather than peak was consistently increased as a function of dose. 4. Rectal temperature was not altered in a dose-dependent fashion. Low doses of detomidine produced late onset hypothermia, while high doses produced early and late onset hyperthermia. 5. These data indicate the uniqueness in autonomic response by the horse to alpha 2 adrenoreceptor stimulation.