Diurnal variation in plasma ir-beta-endorphin levels and experimental pain 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 measured over a 24 hour period. Nociceptive sensitivity was determined using two objective measures of pain: the skin-twitch reflex latency and the hoof withdrawal reflex latency. Significant variation in both nociceptive thresholds and ir-BEND concentrations were noted over the 24 hour period, with elevated pain thresholds observed at 0900 hours and smaller secondary peaks at 1500 hours. Immunoreactive beta-endorphin concentrations were also elevated at 0900 hours. Cardiac rate was high and pupil diameter was largest at 0900 hours. These physiologic changes, along with increased pain threshold, mimic the observed effects of morphine 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.