The formalin test was originally developed by Dubuisson and Dennis (1977), and has since been extensively used to assess pain-related responses. Rats and mice are the most frequently used animal models, though other species including cats, rabbits, guinea pigs, Octodon degus, domestic fowls, crocodiles, tortoises, toads and primates have also been employed. The injection of formalin into the skin of rodent hindpaws to cause spontaneous pain-related flinch behaviors is the most commonly used procedure. The resulting nociceptive response can be divided into two phases differing in timing, duration and underlying mechanisms, and is responsive to many classes of analgesic drugs (Coderre et al., 1990; Hunskaar and Hole, 1987; Rosland et al., 1990; Taylor et al., 1995). The behavioral and electrophysiological responses to formalin consist of an acute phase (Phase I) of a short-lasting response, which is believed to reflect the activity of C-fiber afferent nociceptors. After a short quiescent period, the acute phase is followed by a continuous prolonged response (Phase II), which is believed to be due to central sensitization of the spinal dorsal horn neurons as a result of the initial barrage of input from C-fiber nociceptive afferents during the early phase (Coderre et al., 1993; Dickenson and Sullivan, 1987; Raboisson et al., 1995; Shibata et al., 1989). In this respect, the formalin test has been regarded as a more satisfactory model of pain than tests producing phasic pain like the hot-plate and tail-flick tests (Abbott et al., 1981). Here, we describe the procedures for generating an efficient and reproducible formalin test in rats and mice.
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