An in vivo method for the quantitative evaluation of local anesthetics

We describe a mouse model for evaluation of skin anesthesia after infiltration of local anesthetic. The method involves subcutaneous injection of the anesthetic over the abdomen, and monitoring the vocalization response to electrical stimulus as a measure of analgesia. Prior to drug injection, the vocalization threshold was determined. Mice that vocalized at < or = 8 mA were included in the study. The model was tested using representative agents of the two classes of local anesthetics, bupivacaine, an amide, and chloroprocaine, an ester. The time course and dose response were assessed after injection. The median analgesic time was 15, 40, and 55 min for 0.015%, 0.0625%, and 0.25% bupivacaine and 30, 50, and 55 min for 0.125%, 0.25%, and 2.0% chloroprocaine, respectively. Statistical analysis of the data showed that this method is sufficiently sensitive to detect differences between the dose and duration of local anesthesia (p<0.05, by log rank test of the survival curves). To further validate the model, we compared the duration of anesthesia between the 0.5% bupivacaine and a new long-acting liposomal formulation of 2% bupivacaine. The results showed that the new formulation significantly prolonged the duration of anesthesia (p<0.05). This simple and reliable method may facilitate research on the pharmacology of infiltration anesthesia and the development of new local anesthetics and/or formulations.