An improved method for assessing mechanical allodynia in the rat

A method has been developed for assessing mechanical nociceptive threshold and allodynia in the rat. The animal was habituated to stand on its hind pa ws while leaning against an experimenter's hand. The rat was unrestrained, but remained in the position throughout the test session. The Semmes-Weinst ein monofilaments were used to probe body areas such as the lateral edge of the hind paw and the orofacial skin. The median withdrawal response thresh old 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 peri oral (PO) skin, respectively. The injection of an inflammatory agent, compl ete Freund's adjuvant, into the hind paw, peritemporomandibular joint tissu e (PTMJ), or PO skin resulted in significant mechanical allodynia. The medi an 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 P O inflammation, respectively. The allodynia persisted for at least 2 weeks in all cases. This testing par adigm eliminates confounding factors related to weight bearing, and offers a simple, objective, and reliable approach t o assess mechanical sensitivity in rats. The method will be useful for stud ying the central mechanisms of mechanical allodynia. Importantly, this meth od can also be used in the orofacial region, and will facilitate studies on the mechanisms of persistent orofacial pain in animals. (C) 1999 Elsevier Science Inc.