Dose‐related effects of ethylketazocine on nociception, behaviour and autonomic responses in the horse

Sensitive methods for measuring the analgesic, physiological and behavioural effects of opioids in the horse have recently been developed. Fentanyl, a prototypic μ‐opiate receptor agonist, has been previously shown to produce a syndrome characterized by marked analgesia and locomotor stimulation as well as tachycardia, tachypnoea and behavioural arousal. To determine whether other opiate receptors mediate some of the actions of the narcotic analgesics in the horse, an agent with activity at κ and to a lesser extent μ‐receptors was studied using a vigorous experimental protocol. Like fentanyl, ethylketazocine (EKC) (1.0025–1.012 mg kg⁻¹ i.v.) produced marked dose‐related analgesia to noxious thermal stimuli. Modest dose‐related increases in locomotor activity, pupil diameter and rectal temperature were also observed. However, in contrast to fentanyl, EKC failed to produce any change in cardiac or respiratory rates and produced behavioural sedation rather than arousal. These data suggest that μ‐ and possibly κ‐receptors can mediate the actions of narcotics in the horse.