Rat ultrasonic vocalizations as a measure of the emotional component of chronic pain

In humans, chronic pain is often expressed as a spontaneous emotional response which can lead to fragmented sleep. Rat 50-kHz and 20-kHz ultrasonic vocalizations are well-established measures of positive and negative emotional states, respectively. The rat chronic constriction injury model was used to induce chronic pain, and ultrasonic vocalizations were measured in both the heterospecific rough-and-tumble play (i.e. tickling) test as well as during 24-hour home cage recordings. Rates of hedonic 50-kHz ultrasonic vocalizations during the non-stimulus periods of the tickling test, as well as the rewarding value of tickling, were reduced in chronic constriction injury rats compared to sham controls. In the 24-hour home cage recording study, chronic constriction injury animals showed a reduced amplitude in circadian activity, as well as reduced hedonic 50-kHz ultrasonic vocalizations and increased evoked and spontaneous aversive 20-kHz ultrasonic vocalizations. These data demonstrate that rat ultrasonic vocalizations can be used to capture core symptoms of chronic pain and may be useful in the elucidation of the neuronal mechanisms that underlie the affective component of pain.