Measuring cutaneous thermal nociception in group-housed pigs using laser technique-Effects of laser power output

Nociceptive testing is a valuable tool in the development of pharmaceutical products, for basic nociceptive research, and for studying changes in pain sensitivity is investigated after inflammatory states or nerve injury. However, in pigs only very limited knowledge about nociceptive processes are available, especially methodology which is applicable for pigs kept in group-housing without disturbing the daily routines of the animals. To validate a laser-based method to measure thermal nociception in group-housed pigs, we performed two experiments observing the behavioural responses toward cutaneous nociceptive stimulation from a computer-controlled CO2-laser beam applied to either the caudal part of the metatarsus on the hind legs or the shoulder region of gilts. In Exp. 1, effects of laser power output (0, 0.5. 1, 1.5 and 2 W) oil nociceptive responses toward stimulation oil the caudal aspects of the metatarsus were examined using 15 gilts kept in one group and tested in individual feeding stalls after feeding. Increasing the power output led to gradually decreasing latency to respond (P < 0.001) and affected the types of responding (less non-responding (P < 0.01), less moving leg (P = 0.07), less lifting leg (P < 0.01) and more kicking (P < 0.001)). Furthermore, the occurrence of tail flicking during laser stimulation was increased (P < 0.001). In Exp. 2, effects of laser power output (0, 0.8, 1.5, 2.2 and 3 W) on nociceptive responses toward stimulation on the shoulder region were examined in 10 gilts kept under the same conditions. Again, increasing the power Output led to gradually decreasing latency to respond (P < 0.0001) and affected the types of responding (less non-responding (P < 0.001), less moving shoulder (P < 0.001), less moving body (P < 0.001), higher occurrence of muscle twitch (P = 0.09), and higher occurrence of rubbing shoulder (P < 0.01)). In conclusion, the results of the two experiments suggest that behavioural responses to nociceptive cutaneous laser stimulation are a valid measure of nociception in group-housed gilts which are tested with a minimal disturbance of daily routines, and both when applied to the hind legs and to the shoulder region. Furthermore, porcine nociceptive responses appear to be graded, to include new types of behaviour as the nociceptive input increases and to show elements of site-specificity. (c) 2009 Elsevier B.V. All rights reserved.