Nociceptive behavioural thresholds for cutaneous mechanical and heat stimulation of the tail root in pigs

Background and aims: The use of pigs in translational pain research is increasing. However, only few methods are available to behaviourally quantify porcine nociceptive thresholds. Our aim was to determine baseline mechanical and thermal nociceptive thresholds using behavioural assays in pig with the tail root as stimulation site.
Methods: Eight female pigs were used (50±5 kg, 15 weeks of age) and habituated to procedures before testing. Two mechanical (von Frey filaments (vF), pressure application measurement device (PAM)) and one thermal assay (CO2 laser) were used. Behavioural responses to stimulation at a 2 x 2 cm test area on the dorsal tail base were measured during one test session while the pigs were kept in a test cage and accompanied by a pen mate for social support. Within the test session each assay involved three stimulations per pig.
Results: All assays provided robust values for nociceptive thresholds, with a mean of 5±1 sec (CO2); 422±35 gf (PAM) and 73±11 gf (vF). The responses to the 24 single stimulations ranged from 1-17 sec, 212-890 gf and 26-300 gf for CO2, PAM and vF. The observed behavioural responses were tail flicks (88, 8 and 29 %), tail clamps (0, 0 and 63 %), rump flinches (4, 83 and 0 %) and body movements (8, 8 and 8 %) respectively.
Conclusions: Using these experimental approaches and behavioural assays already described for rodents, it was possible to quantify mechanical and thermal nociceptive thresholds on the tail roots of standing pigs.