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Exploring the effect of capsaicin-induced central sensitization on the upper limb nociceptive withdrawal reflex threshold

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Abstract

The nociceptive withdrawal reflex (NWR) threshold is commonly employed in the lower limb to assess clinical and experimentally induced pain. However, no studies to date have investigated changes in spinal nociception in the upper limb, via the NWR threshold, following experimentally induced central sensitization (CS). We tested the hypothesis that experimentally induced CS of the C5–C6 spinal segment significantly reduces NWR thresholds in muscles of the upper limb. Upper limb NWR thresholds from 20 young, healthy adults were assessed by applying noxious electrical stimuli to the right index finger and recording muscle activity from the biceps brachii (BI), triceps brachii (TRI), flexor carpi ulnaris (WF), and extensor carpi radialis longus (WE) muscles via surface electromyography. Topical cream (either 0.075% capsaicin, or control) was applied to the C5–C6 dermatome of the lateral forearm (50 cm2). NWR thresholds were compared at baseline, and four 10-min intervals after topical application. WF muscle NWR thresholds were significantly reduced in the capsaicin session compared to control, while TRI muscle NWR thresholds were significantly reduced 40 min after capsaicin application only (p < 0.05). There were no significant differences for BI or WE muscle NWR thresholds. We observed poor to moderate test–retest reliability for all upper limb NWR thresholds, a key contributor to the selective reduction in NWR thresholds among muscles. Accordingly, while our findings demonstrate some comparability to previously reported lower limb NWR studies, we concurrently report limitations of the upper limb NWR technique. Further exploration of optimal parameters for upper limb NWR acquisition is needed.

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Availability of data and material

Summary data are available upon request to the corresponding author.

Code availability

Matlab and R code used for data and statistical analysis can be requested through the corresponding author’s GitHub Repository.

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Acknowledgements

We would like to thank all of the individuals who participated in this study.

Funding

JS is supported by a Natural Sciences and Engineering Research Council of Canada Discovery grant, which contributed to this project. No other authors have funding to declare in support of this study.

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Contributions

LL was responsible for original study inception, data collection, analysis, interpretation, statistical analysis, and manuscript preparation. LB, JD, and DA contributed study inception, the development of methodology, and critically reviewed the manuscript. JS supervised all aspects of the study.

Corresponding author

Correspondence to Lukas D. Linde.

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Conflict of interest

The authors have no conflicts of interest to declare.

Ethics approval

The University of Guelph research ethics board approved all study protocols.

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All participants provided informed, written consent prior to study participation.

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All participants consented to inclusion of their data for publication.

Additional information

Communicated by Winston D Byblow.

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Linde, L.D., Bent, L.R., Dickey, J.P. et al. Exploring the effect of capsaicin-induced central sensitization on the upper limb nociceptive withdrawal reflex threshold. Exp Brain Res 239, 3405–3415 (2021). https://doi.org/10.1007/s00221-021-06216-3

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  • DOI: https://doi.org/10.1007/s00221-021-06216-3

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