Electrical stimulation after mechanical conditioning in dogs for assessment of conditioned pain modulation

Objective: To assess the ability of a mechanical conditioning stimulus to reliably activate conditioned pain modulation and to establish the duration of possible inhibition of the endogenous pain system in healthy dogs.

Study design: Prospective, experimental trial.

Animals: A group of 12 adult laboratory Beagles.

Methods: Electrical nociceptive thresholds were determined three times before [baseline (BL1-3)] and up to 60 minutes after conditioning. Starting at 0.2 mA, an increasing electrical stimulus (constant current, square-wave pulse, sequence of five 1 ms pulses, delivered at 200 Hz) was applied to the plantar side of one paw. Conditioning was achieved mechanically with a constant force of 22 N for either 1 or 5 minutes at one thoracic limb. Each dog completed four test sessions, with 1 day of recovery within and 14 days between experimental weeks, resulting in 48 measurement sessions in total. For each session, a feasibility score (FS) was assigned (0-5). Data were analysed with two-way anova for repeated measures. Values of p < 0.05 were considered significant.

Results: Data from one dog were excluded. Overall, 44 measurement sessions were analysed. After conditioning, electrical thresholds increased > 10%, > 20% and > 60% in 39/44 (89%), 30/44 (68%) and 15/44 (34%) of measurements, respectively, compared with baseline values. FS values of 0 and 1 were assigned to 96% of measurements.

Conclusions and clinical relevance: Conditioned pain modulation can be activated by the chosen conditioning stimulus. However, a clinically meaningful increase in the electrical threshold of > 60% could not reliably be evoked. Individual animals responded very strongly to the conditioning (showing intense behavioural reactions such as vocalisation or appeasement gestures), which may indicate that the individual tolerance limit was exceeded. Therefore, clinical application without further adaptation of the conditioning protocol is currently not recommended.