Improving the efficiency of the formalin test

A time-sampling method that allows up to eight rats to be tested simultaneo usly in the formalin test is described and compared to the continuous ratin g method. Time sampling the behavioural response to formalin every 1 or 2 m in produces scores that are essentially identical to continuous rating for both the formalin concentration effect relationship and the morphine dose e ffect relationship, with no loss of statistical power. The most important a dvantage of the method is that it allows data on other aspects of the rats' behaviour, such as behavioural state and the side effects of drugs to be s cored during the formalin test. Formalin injection produces a dose-dependen t decrease in locomotor and exploratory activity. The activity pattern of r ats is normalized at morphine doses that produce about a 50% reduction in p ain, while morphine doses high enough to completely suppress the pain respo nse are accompanied by considerable sedation. The use of the jackknifing pr ocedure to obtain unbiased estimates of the variability of parameters estim ated from dose effect relationships is also described. (C) 1999 Internation al Association for the Study of Pain. Published by Elsevier Science B.V.