A new method for measuring auricular inflammation in the mouse

Abstract

The croton oil ear test is widely used to identify prospective topical antiinflammatory drugs. Ear inflammation is produced by applying a 2% solution of croton oil on the ears of mice or rats. The effectiveness of the drug that is dissolved in the croton oil solution can be gauged by comparing the croton oil treated ears with the croton oil plus drug treated ears. The effect is measured following sacrifice of the animal by weighing either the excised ear (Tonelli et al., 1965; Glenn et al.,1978; Swingle et al., 1981; Soliman et al., 1983; Mantione and Rodriguez, 1990) or a plug taken from the ear (Tubaro et al., 1985; Davis et al., 1989a; Davis et at., 1989b). Use of this technique for the generation of a time-course evaluation of antiinflammatory activity requires a large amount of the chemical to be tested and the sacrifice of many animals. In other assays, ear thickness has been measured by caliper (Carlson et al., 1985; Maloff et al., 1989) or by dial micrometer (Griswold et al., 1987), which allow multiple measurements to be made, but the pressure on the ear was not reported. In a recent review of pharmacological methods, Chang and Lewis (1989) caution that using calipers to measure ear thickness is subject to operator error and bias. Furthermore, they emphasize care must be taken to not leave the calipers in contact with the ear too long, as it is possible to squeeze substantial amounts of edema fluid out of the ear tissue. To address these limitations, this report describes a device for evaluating inflammation by measuring the thickness of the ear while using only precisely reproducible light pressure that does not vary with the operator of the instrument. The device allows a time sequence of measurements on the same animal, thus substantially reducing the amount of material and number of animals required for evaluation.