Ice water immersion does not activate diffuse noxious inhibitory controls of spinal reflexes in sedated or anaesthetised dogs (Canis familiaris): a pilot study

Abstract

Introduction: Diffuse noxious inhibitory controls (DNIC) may be impaired in human subjects with osteoarthritis (OA) pain. Spontaneously occurring OA in dogs is considered a valuable model of human OA; however, methodology for assessing DNIC in dogs has not been fully developed. The aim of this study was to develop a suitable DNIC protocol using ice water immersion, similar to protocols used in humans.

Objective: This study objective was to create an experimental protocol for inducing DNIC in sedated or anesthetized dogs, ensuring it has face validity for future assessments of DNIC in studies involving the spontaneous canine OA model. We hypothesized that inducing DNIC in healthy dogs would result in a reduced electromyographic (EMG) response to a specific nociceptive stimulus.

Methods: Electromyographic (EMG) responses of the cranial tibial muscle to test electrical stimuli and interdigital skin temperature were recorded in seven healthy dogs before and during a 20-min duration conditioning ice water immersion of the distal forelimb. The protocol was repeated for each dog using three different states: sedation with acepromazine or alfaxalone or anaesthesia with alfaxalone.

Results: Ice water immersion caused a decrease of interdigital skin temperature in dogs in all three groups with the nadir (4.9-13.6°C) at 10 min following immersion. Skin temperatures remained significantly higher (p = 0.018) in alfaxalone sedated compared to acepromazine sedated dogs and returned to baseline more quickly than in acepromazine sedated dogs. Magnitudes of EMG responses were significantly larger in acepromazine sedated dogs compared to alfaxalone treated dogs (p < 0.001). DNIC was not induced, as the EMG magnitude did not significantly change over time for either the early (p = 0.07) or late responses (p = 0.27), and no significant interactions were observed between time and anaesthetic state in relation to EMG magnitude.

Conclusion: Our data suggest that a cold conditioning stimulus failed to elicit DNIC. It is possible that the magnitude of the conditioning stimulus was not sufficient to recruit DNIC in dogs.

Keywords: CPM; DNIC; canine; ice-water bath; pain.

Figure 1

(A) immersion of the forelimb in ice water; (B) paired stimulating needle electrodes (blue leads) were introduced into the plantar dermal tissues of the distal phalanx of the fourth digit, immediately proximal to the proximal edge of the digital pad, of the left pelvic limb; (C) paired stainless steel needle recording electrodes (blue and red leads) were placed transcutaneously (18 mm inter-electrode distance) into the belly of the cranial tibial muscle of the left pelvic limb.

Figure 2

Interdigital skin temperature changes over time following immersion of the distal forelimb in ice water (from 0 to 20 min) and subsequent removal from the ice water bath (20–30 min). Acepromazine sedation is represented by X, alfaxalone anaesthesia by ▾ and alfaxalone sedation by □. Recorded values are represented by small symbols, mean results are represented by larger symbols connected by lines.

Figure 3

Early (0–100 ms) cranial tibial responses. Time point 0 represents the baseline (mean of responses at T-15, -10, -5, and 0). Acepromazine sedation is represented by X, alfaxalone anaesthesia by ▾ and alfaxalone sedation by □. Recorded values are represented by small symbols, mean results are represented by larger symbols connected by lines. Variation with time was not significant.

Figure 4

Late (100–500 ms) cranial tibial responses. Time point 0 represents the baseline (mean of responses at T-15, -10, -5, and 0). Acepromazine sedation is represented by X, alfaxalone anaesthesia by ▾ and alfaxalone sedation by □. Recorded values are represented by small symbols, mean results are represented by larger symbols connected by lines. Variation with time was not significant.

Figure 5

Early (0–100 ms) and late (100–500 ms) reflex responses in the ipsilateral cranial tibial muscle to high frequency electrical stimulation (5 × 5 mA, 1 ms duration, 100 Hz) of the distal phalanx of the fourth digit. Time relative to immersion of the distal left forelimb in iced water (0°C–4°C) is indicated to the right of each EMG recording.