The model of the Nociceptive Withdrawal Reflex in horses

Abstract

Summary: In equine pain research, the Nociceptive Withdrawal Reflex (NWR) model has been proposed as an alternative to classical thermal, mechanical and electrical models to investigate the physiology of nociception. This model does not only allow a more reliable definition of the nociceptive threshold through neurophysiological characterization of the elicited response , but also consents plotting a stimulus-response curve as well as the evaluation of temporal summation mechanisms. Aim of this report is to summarize the experience and results obtained using the NWR model in horses over the last 15 years, firstly with the purpose of describing its physiological characteristics and secondly with the aim of evaluating its pharmacological modulation through analgesic and anaesthetic compounds. The NWR can be elicited in horses through electrical transcutaneous stimulation of a peripheral sensory nerve and is recorded through electromyographic electrodes from the muscles involved in the withdrawal reaction. As the non-nociceptive fibres have lower threshold to electrical stimuli than the nociceptive one, it is impossible to differentiate between the two groups on the stimulus side. So, the only guidance to assess the quality of the stimulus and the recruitment of nociceptive afferents should rely on the characteristics of the electromyographic response, most of all on the timing of its various components. A reflex response can be considered to be a true NWR only if it appears within a time epoch compatible with the conduction velocity of the A fibers. Furthermore a clearly recognizable EMG burst in the epoch of interest has to be accompanied by a consistent aversive behavioural reaction, thus allowing the definition of a reliable NWR threshold. Responses to stimulation intensities below and above thresholds can be quantified to draw a stimulus response curve. Furthermore, through repeated stimulation at subthreshold intensities, it is possible to elicit the phenomenon of temporal summation, considered to be the early phase of the wind up phenomenon and of relevant interest to study central integrative mechanisms of pain processing. Alpha-2 agonists, opioids, systemic lidocaine, ketamine and inhalation anaesthetics have been investigated using the NWR model in horses. Drug-specific modulation of NWR threshold, stimulus-response curves and temporal summation are summarized for each tested compound.

Comparison of Threshold and Tolerance Nociceptive Withdrawal Reflexes in Horses

“…The reflex termed R2 in the present investigation corresponds to the classically described NWR, as it appears at the threshold stimulation level in the epoch 80–250 ms after stimulation onset [ 28 ]. As previously described in equine NWR recruitment trials, R2’s amplitude and duration increased, whereas its latency decreased for higher stimulation intensities [ 7 , 8 ].…”

“…As expected, increasing stimulation intensities exerted considerable modulation of both the behavioral pattern and the electromyographic characteristics of the evoked responses. Although the NWR threshold is a well-established endpoint for the NWR model in horses [ 28 ], NWR tolerance has never been characterized in detail to date. Tolerance was defined as the maximal stimulation intensity judged to be acceptable by an individual horse restrained in stocks.…”

“…Not only did R1 appear more likely at tolerance than at threshold, but it also showed increased amplitude, shorter latency and longer duration. Remarkably, the increase in amplitude of R1 was more pronounced than the increase in amplitude of R2, classically considered as NWR (a nine-fold increase in the median relative amplitude for R1 from threshold to tolerance, as opposed to a four-fold increase for R2) [ 28 ]. Conventionally, reflex activity within the 80-millisecond epoch following stimulation is considered to be conducted by Aβ fibers with a conduction velocity of 50 to 60 m/s [ 8 , 37 , 38 , 39 ].…”

“…In addition, the main EMG burst had to last at least 10 milliseconds and had to be followed by a reaction scored ≥2. Tolerance was defined as the stimulation intensity accompanied by a reaction score of 5 (vigorous whole-body reaction and immediate lifting of the limb) during the NWR recruitment [ 8 , 28 ].…”

“…In horses, it has been shown that it is possible to elicit and record reliable nociceptive reflexes [ 28 ]. In previous studies evaluating stimulus–response curves, the maximal stimulation intensity applied corresponded to 1.3 times the threshold [ 3 , 7 ].…”