DIURNAL-VARIATION IN PLASMA IR-BETA-ENDORPHIN LEVELS AND EXPERIMENTALPAIN THRESHOLDS IN THE HORSE

Diurnal variation in nociceptive sensitivity and plasma immunoreactive beta-endorphin (ir-BEND) concentrations was examined in eight healthy Thoroughbred horses. Pain thresholds, ir-BEND concentrations, rectal temperature, heart rate, respiratory rate and pupil diameter were meas ured over a 24 hour period. Nociceptive sensitivity was determined usi ng two objective measures of pain: the skin-twitch reflex latency and the hoof withdrawal reflex latency. Significant variation in both noci ceptive thresholds and ir-BEND concentrations were noted over the 24 h our period, with elevated pain thresholds observed at 0900 hours and s maller secondary peaks at 1500 hours. Immunoreactive beta-endorphin co ncentrations were also elevated at 0900 hours. Cardiac rate was high a nd pupil diameter was largest at 0900 hours. These physiologic changes , along with increased pain threshold, mimic the observed effects of m orphine and other mu-agonists in the horse. The results of this study suggest that endogenous opioid peptides may modulate pain threshold as well as other physiologic parameters in the horse.