Rat ultrasound model for measuring pain resulting from intramuscularly injected antimicrobials

Rats emit ultrasonic vocalizations in situations involving anxiety or stress, defense, and submission. This study demonstrates the use of ultrasonic vocalizations as a quantitative measure of pain associated with intramuscular (i.m.) injections. Intramuscularly administered cephalosporins used in a comparative evaluation of pain in humans were chosen to validate this rat model. Three groups of four rats each received an intramuscular injection of either drug first, and then placebo, or placebo and then drug. The three drugs used in this study were cefonicid, cefoxitin, and cefamandole; saline was used as the placebo. Ultrasonic vocalizations were measured from 20 kHz to 100 kHz. The number of vocalizations ranged from 50 milliseconds to 500 milliseconds and most of the vocalizations occurred between 60 kHz and 80 kHz. Rats injected with cefoxitin, cefamandole, cefonicid, and saline vocalized an average of 36 +/- 11, 33 +/- 9, 28 +/- 7, and 11 +/- 6 times respectively. The response to all three antimicrobials differed significantly when compared with placebo (p < 0.01), however, there was no significant difference between the number of vocalizations for each antimicrobial injection. Further research may allow the correlation of the total duration of each vocalization, with the individual frequencies such as the 22 kHz and the 55 kHz components and with the amount of pain associated with the injection. This correlation could then be used to further identify differences in the amount of pain associated with each antimicrobial injection without increasing the sample size.