Physiological correlates associated with cribbing behaviour in horses: changes in thermal threshold, heart rate, plasma beta-endorphin and serotonin.

The aim of the present study was to obtain information on the possible mechanisms underlying cribbing behaviour in horses. To investigate the horse's responsiveness to an external stimulus, a device for telemetric measurement of thermal threshold, using the forelimb withdrawal reflex, was developed and validated. Measurements of thermal threshold took place in cribbing horses (n = 11) before and during cribbing periods. Heart rate was monitored continuously in the same horses. Blood samples were collected before and during cribbing periods as well and in age- and sex-matched control horses (n = 11). beta-endorphin and cortisol were determined in plasma using radioimmunoassay techniques, serotonin was analysed by high performance liquid chromatography. Compared with basal values, thermal threshold was significantly (P = 0.003) lower during cribbing periods. The mean difference was 4.9 degrees C. Heart rate decreased significantly (P = 0.026) and showed a mean reduction of 2.4 beats/min during cribbing. Given the fact that arousal usually is associated with an increase in nociceptive threshold and in heart rate, the decrease in both during cribbing provide evidence that cribbing may reduce stress. Cribbers showed 3 times higher basal beta-endorphin levels than controls (mean 49.5 vs. 16.2 pmol/l, P = 0.006) and there was a trend for lower basal serotonin levels (mean 201.5 vs. 414.3 nmol/l, P = 0.07). These data indicate differences in cribber's endogenous opioid and serotonergic systems.