A method for studying cutaneous pain perception and analgesia in horses

Pain perception and its alteration by analgesic drugs is difficult to measure in the horse. The latency to onset of flexion of a limb in response to a noxious thermal stimulus has been used as a nociceptive end point for analgesic studies in many species. While this method has been employed in the horse, it may be confounded by the spontaneous locomotor activity observed after administration of narcotic analgesics. Consequently, an alternative method of assaying narcotic analgesia that did not involve the equine locomotor apparatus was developed. This report describes the use of the heat-evoked skin-twitch reflex as a reproducible measure of pain threshold and its alteration by the narcotic analgesic fentanyl. This method is compared with the heat-evoked hoof-withdrawal reflex, and the apparatus necessary to elicit both reflexes in the horse is described. Fentanyl, administered at intravenous doses of 0.010, 0.005, and 0.0025 mg/kg, produced a dose-related prolongation of the skin-twitch reflex but failed to alter the latency to hoof withdrawal following noxious thermal stimulation. The skin-twitch reflex is therefore a more sensitive assay of narcotic analgesia in the horse than is the hoof-withdrawal reflex.