Comparison of Threshold and Tolerance Nociceptive Withdrawal Reflexes in Horses

Abstract

The nociceptive withdrawal reflex (NWR) is used to investigate nociception in horses. The NWR threshold is a classical model endpoint. The aims of this study were to determine NWR tolerance and to compare threshold and tolerance reflexes in horses. In 12 horses, the NWR was evoked through electrical stimulation of the digital nerve and recorded via electromyography from the deltoid. Behavioral reactions were scored from 0 to 5 (tolerance). First, the individual NWR threshold was defined, then stimulation intensity was increased to tolerance. The median NWR threshold was 7.0 mA, whereas NWR tolerance was 10.7 mA. Upon visual inspection of the records, two main reflex components R1 (median latency 44 ms) and R2 (median latency 81 ms) were identified at threshold. Increasing stimulation intensity to tolerance led to a significant increase in the amplitude and duration of R1 and R2, whereas their latency decreased. At tolerance, a single burst of early, high-amplitude reflex activity, with a median latency of 39 ms, was detected in 15 out of 23 stimulations (65%). The results of this study suggest that (1) it is feasible to determine NWR tolerance in horses and (2) high-intensity stimuli initiate ultrafast bursts of reflex activity, which is well known in practice and has now been quantified using the NWR model.

Keywords: electromyography; equine; horse; nociception; nociceptive withdrawal reflex; tolerance.

Figure 1

Median value (and interquartile range) of different variables for R1 and R2 at different stimulation intensities (0.9, 1.0, 1.1, 1.2 and 1.3 times the nociceptive withdrawal reflex threshold value). (a) relative amplitude, (b) onset latency (ms), (c) reflex duration (ms).

Figure 2

Electromyograms recorded from the right deltoid muscle after electrical stimulation of the right lateral palmar digital nerve in two horses (a,b). Stimulation intensity ranged from 0.9 times NWR threshold to NWR tolerance (TO), a stimulation accompanied by a reaction score of 5. Stimulation intensity was increased in steps of 10%. In the illustrations, the x-axis represents time (ms) and the y-axis represents amplitude (mV). Stimulation onset was at time 0. Where present, R1 is marked with black arrows and R2 with gray arrows. (a) The deflection from baseline seen at NWR threshold is R2. In the recording at 1.1 times NWR threshold R1 is preceding R2. R1 and R2 can be distinguished in all recordings, but they become closer for increased stimulation intensity, due to a shortening in the latency of R2. (b) At NWR threshold, R1 and R2 can be distinguished from each other, whereas at NWR tolerance only one reflex burst can be observed.