Optimization of the mechanical nociceptive threshold testing with the Randall-Selitto assay

The Randall-Selitto (RS) assay is widely used for quantification of thresholds of the rat hindpaw withdrawal reflex to nociceptive pressure stimulation. Despite a report by Taiwo et al. [Brain Research 487 (1989)] that the sensitivity of the RS assay can be significantly improved by pre-training, many researchers still conduct this test in untrained rats. In part, this is because the study of Taiwo et al. employed heavily-restrained and thus very stressed animals. That study also examined bradykinin-induced hyperalgesia rather than hyperalgesia associated with persistent inflammatory models used in many other studies. Therefore, it is conceivable that pre-training may be unnecessary with a less restraining RS testing paradigm and the use of inflammation-producing agents. To resolve these issues, we re-examined the need for pre-training with the RS assay with minimal animal restraint and inflammation produced by Complete Freund's Adjuvant (CFA) injection. We also examined the sensitivity of this assay to detect analgesia induced by swim stress. We found that without training the differences between untreated, CFA-injected, swim stress-exposed, and CFA + swim stress-treated animal groups did not reach statistical significance. Four days of training, however, enlarged these differences to statistically significant levels. Furthermore, we found that the use of only the last measurement within a testing session, rather than the average of all collected measurements, may further enhance the sensitivity of the RS assay.