Elsevier

Physiology & Behavior

Volume 67, Issue 5, November 1999, Pages 711-716
Physiology & Behavior

Articles
An Improved Method for Assessing Mechanical Allodynia in the Rat

https://doi.org/10.1016/S0031-9384(99)00136-5Get rights and content

Abstract

A method has been developed for assessing mechanical nociceptive threshold and allodynia in the rat. The animal was habituated to stand on its hind paws while leaning against an experimenter's hand. The rat was unrestrained, but remained in the position throughout the test session. The Semmes–Weinstein monofilaments were used to probe body areas such as the lateral edge of the hind paw and the orofacial skin. The median withdrawal response threshold was found to be 62.9 g for both hind paws (95% confidence interval, CL, 61.4–66.7 g, n = 69). The median response thresholds of the orofacial skin were 62.9 g (95% CL: 55.7–68.7 g, n = 45), and 25.6 g (95% CL: 22.6–36.3 g, n = 45), for the skin above the temporomandibular joint (PT) and the perioral (PO) skin, respectively. The injection of an inflammatory agent, complete Freund's adjuvant, into the hind paw, peritemporomandibular joint tissue (PTMJ), or PO skin resulted in significant mechanical allodynia. The median response thresholds were reduced to 0.09 g (p < 0.01, n = 6), 5.60 g (p < 0.001, n = 9), and 3.24 g (p < 0.001, n = 9), after hind paw, PTMJ, and PO inflammation, respectively. The allodynia persisted for at least 2 weeks in all cases. This testing paradigm eliminates confounding factors related to weight bearing, and offers a simple, objective, and reliable approach to assess mechanical sensitivity in rats. The method will be useful for studying the central mechanisms of mechanical allodynia. Importantly, this method can also be used in the orofacial region, and will facilitate studies on the mechanisms of persistent orofacial pain in animals.

Section snippets

Materials and methods

Male Sprague–Dawley rats (250–350 g, Harlan, Indianapolis) were used. The rat was habituated to stand on its hind paws and lean against the experimenter's hand wearing a regular leather work glove (Fig. 1). Instead of standing on a meshed metal, or grid surface, the rat stands on a soft pad. The habituation required no more than normal petting of the rat, and it can be achieved within half an hour. The smell that developed on the glove through the handling of animals appeared to facilitate the

Results

During the test session, the rat was calm and exhibited normal grooming behavior. Occasional petting by the investigator ensured the rat to maintain its position. In most cases, the nociceptive response threshold was determined without interruption.

Discussion

The present study provides an improved and simple method for assessing the mechanical nociceptive threshold in naive rats and mechanical allodynia in CFA-inflamed rats. The key to this method is that although there was no restraining involved, the rat stays calmly in one place. The mechanical probes can be delivered to the targeted areas repeatedly. Because the animal was not walking during the testing, false responses due to random stepping were eliminated.

Compared to the plantar probing

Acknowledgements

The author thanks Dr. R. Dubner and Dr. J. D. Greenspan for critical reviewing of an earlier draft of this manuscript, and Ms. E. Wade and Ms S. Zou for technical assistance. This work was supported by DE 11964, DA10275.

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