Adjuvant-induced arthritic rats were observed clinically and behaviourally. The clinical disease has a duration of greater than 1 month and can be divided into a pre-clinical (1–10 days), an acute (15–30 days), postacute (30–50 days) and a late phase (> 50 days). Adjuvant arthritis induces significantly quantitative changes in the rats' behaviour. Two types of behavioural change merit special attention: freezing (arresting) and scratching. Freezing is significantly increased in the acute and postacute phases; it is increased by morphine, this effect being blocked by naloxone.
Scratching is significantly increased in the acute, postacute and late phases; it is decreased by morphine, this effect being blocked by naloxone. The chronic presence of scratching, and the effects of morphine and naloxone on it, allow us to consider it as a possible pain-related behaviour and therefore as a possible parameter for the study of chronic pain in animals.