Ultrasonic vocalization (22-28 kHz) in a model of chronic pain, the arthritic rat: effects of analgesic drugs.

Adjuvant-induced arthritis in rats is considered as a chronic pain model. In a search for a relevant behavioural marker of the chronic pain state, we studied social interactions between an arthritic rat and a healthy partner in a neutral cage. During the test, arthritic rats emitted ultrasonic 22-28 kHz vocalizations (USV), whereas control rats did not, and showed less exploration and more immobility, but did not differ from controls in terms of social behaviours (social investigation, allogrooming). Aspirin (200 mg kg < > (en)1) or morphine (3 mg kg < > (en)1) injected intraperitoneally before the test significantly decreased USV without affecting the animals' behaviour. These results suggest that USV could be associated with affect related to aversive or painful stimuli and may constitute a behavioural marker of chronic pain in arthritic rats.