Abstract
There are many studies measuring the mechanical withdrawal threshold (MWT) in animals with Randall-Selitto (RS) apparatus, however, it is not clear which tissue's threshold RS device measures. In our previous human experiment with surface anesthesia, we showed that a probe larger than 2 mm measured the pain threshold of deep tissues (possibly muscles) while a smaller probe measured the skin pain threshold. To know the influence of probe diameter on the depth of tissues MWT of which the RS apparatus measures, we examined two probes of different diameters (normal probe with tip diameter 1.3 mm originally attached to the RS device and modified probe with tip diameter 2.6 mm made by our lab), and effect of surface anesthesia (EMLA cream containing 2.5% lidocaine and 2.5% prirocaine, for 30 min) on MWT measures with these probes. For comparison, MWT was also measured with von Frey hair (VFH, 0.5 mm tip diameter). SD rats (n=27) were used in this study. Surface anesthesia increased the MWT measured with von Frey hairs and that measured with the normal probe-equipped RS device. In contrast, MWT measured with the larger probe-equipped one was not influenced by surface anesthesia. These data demonstrated that the WMT measured with VFH and normal probe-equipped RS device reflects the cutaneous pain threshold while that measured with larger probe-equipped RS apparatus reflects the pain threshold in deep tissues. [J Physiol Sci. 2007;57 Suppl:S112]
