Ultrasonic sound as an indicator of acute pain in laboratory mice

In response to pain, mice may vocalize at frequencies above the range of human hearing (greater than 20 kHz). To determine whether an ultrasonic recording system is a reliable tool for assessing acute pain, we measured audible and ultrasonic vocalization in mice subjected to either nonpainful or potentially painful procedures performed routinely in animal facilities. Data were collected from 109 weanling mice (Mus musculus; B6, 129S6-Stab 5b) scheduled for 2 potentially painful procedures: DNA testing by tail snip and identification by ear notching. The mice each were assigned randomly to 1 of 4 groups: 1) actual tail snip, 2) sham tail snip, 3) actual ear notch, or 4) sham ear notch. Vocalizations during the treatments were recorded with an ultrasonic recorder. Most mice (65%; n = 55) demonstrated no vocal response to the potentially painful procedures. More mice that received actual tail snips produced audible sounds (11 of 29 mice) than did those that underwent sham tail snips (0 of 30 mice). In addition, audible vocalizations occurred more frequently during ear notch procedures (8 of 26 mice) than during sham ear-notch manipulations (2 of 24 mice). For all 20 of the mice that produced ultrasonic vocalizations, these calls were accompanied by simultaneous audible components. We conclude that ultrasonic vocalizations do not provide any more information than do audible vocalizations for assessing responses to potentially painful procedures. In addition, because many mice made no sound at all after a potentially painful stimulus, vocalizations generally are not good metrics of acute pain in laboratory mice. Alternatively, the lack of vocalizations in many of the mice may suggest that tail snipping and ear notching are not particularly painful procedures for most of these mice.