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.

Adjuvant-induced arthritis in rats: a possible animal model of chronic pain.

Adjuvant-induced arthritic rats were observed clinically and behaviorally. 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 (greater than 50 days). Adjuvant arthritis induces significantly quantitatively 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-rated behaviour and therefore as a possible parameter for the study of chronic pain in animals.

A time course analysis of the changes in spontaneous and evoked behaviour in a rat model of neuropathic pain.

We have previously demonstrated that scratching was significantly increased in a rat model of polyarthritis and that this could be reversed by morphine and electrical stimulation of pain-modulating brain areas. We therefore proposed that scratching might represent a parameter of chronic pain. In this study, we examined the spontaneous behaviour of rats in a model of peripheral neuropathy induced by loosely tying 4 ligatures around the right common sciatic nerve. In half of the animals (N = 7), the ligatures were made with resorbable sutures and, in the other half (N = 7), with non-resorbable sutures of the same size. Postoperatively, scratching was significantly increased at the ligated side. This increase was already observed on the first postoperative day, and maximal effects were reached on the 3rd day. We also observed a qualitative change in the scratching behaviour; postoperatively, scratching was often a vibratory-like shaking of the hind paw in the air. The time course of the increased scratching was time-locked with the development of allodynia to thermal stimulation. No differences were found either in the time course of the increased scratching behaviour or in the time course of the thermal allodynia between the rats ligated with resorbable and with non-resorbable sutures. However, a difference in the walking pattern, as measured by the sciatic functional index (SFI), was observed between the two groups: whereas the SFI normalized after 4 weeks in rats ligated with resorbable sutures, it remained disturbed until the end of the 16-week observation period in the rats ligated with non-resorbable sutures. Morphine 1, 2 and 5 mg/kg dose-dependently reduced the increased scratching behaviour. This was not due to a general depressant effect on the rats' behaviour. This finding is discussed in light of the debate on opioid sensitivity of neuropathic pain. The present results add new evidence that scratching is a possible sign of chronic pain in the animal.