Scratching behaviour in arthritic rats: a sign of chronic pain or itch?

In a previous study, it was shown that adjuvant-induced arthritic rats present an abnormal behaviour pattern up to 60 days after inoculation with Mycobacterium butyricum. The purpose of the present study was to investigate how long the abnormal behaviour pattern continues, and whether the observed increased scratching behaviour is a parameter of chronic pain or rather a reaction to itch. Adjuvant-induced arthritic rats were observed for up to 180 days after the inoculation and their behaviour was quantitatively analysed. The following behavioural changes persisted for more than 60 days: rearing, running and climbing were decreased while grooming, scratching, biting and freezing were increased. No behavioural changes were observed 120 days after the inoculation. The increased scratching was not influenced by an antihistamine drug (astemizole). Not only morphine but also acetylsalicylate selectively depressed the increased scratching behaviour without influencing the other behaviours. These results reinforce the notion that in arthritic rats the increased scratching is a sign of chronic pain.