A thermal threshold assay to measure the nociceptive response to morphine sulphate in cattle.

A thermal threshold measurer (TTM) apparatus was developed and tested in 12 dry, nonpregnant, culled cows with the purpose of measuring the thermal nociceptive threshold and of finding the response to morphine sulphate dosages. The cows received a cumulative dose (from 0.00 to 0.40 mg/kg BW) of morphine sulphate through a catheter in the jugular vein. The interval between doses was 20 min, and a nociceptive test was performed 15 min after each injection. The TTM device consisted of a 60 W halogen bulb mounted in a 15 cm PVC tube, with a 0.6 s response time probe attached to its end, connected to a thermocouple. The probe measured the response temperature on the skin over the middle phalanges on the dorsum of the forefoot. The radiating heat stimulus from the bulb was instantaneously terminated with the foot-lift response of the tested animal. The nociceptive response to the 0.00 mg/kg dose was considered the baseline and subsequent measurements were expressed in difference from it. Data were evaluated in a regression analysis using the GLM procedure. A significant elevation (P < 0.0001) in the nociceptive threshold of the cows with cumulative dosing of morphine sulphate was noticed. A high variability (P < 0.0001) in the response among animals was also detected, suggesting that a 2-step dose of morphine sulphate is necessary to achieve a certain degree of induced analgesia in all cows. The nociceptive assay described, using the TTM device, was able to detect an elevation of the thermal threshold of cows due to morphine sulphate induced analgesia. An increase in locomotory behaviour or other side effects due to morphine sulphate were not noticed.