Jg. Hamra et al., DIURNAL-VARIATION IN PLASMA IR-BETA-ENDORPHIN LEVELS AND EXPERIMENTALPAIN THRESHOLDS IN THE HORSE, Life sciences, 53(2), 1993, pp. 121-129
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.