Increase in "pain sensitivity" induced by exercise applied during the onset of arthritis in a model of monoarthritis in the rat.

We have recently developed, in the rat, a model with a limited arthritic process for chronic pain studies. Intra-articular injection (0.05 ml) of complete adjuvant containing 300 micrograms Mycobacterium butyricum in the tibio-tarsal joint produces a predictable monoarthritis stable clinically and behaviourly from weeks 2 through 6 post-injection. This model appears to be a suitable alternative for the polyarthritic rat for chronic studies based on both its ethical and scientific advantages. In the present work we report results of experiments on the effects of exercise on the pain behaviour and development of arthritis in this model. A group of rats prepared with the above protocol was submitted at 2 weeks post-inoculation to mild exercise (swimming [water 37 degrees C] three times per week) increasing from 5 to 15 min during 4 weeks. As revealed by analyses of the arthritis score, the stiffness score and the mobility score, no aggravation of arthritis occurred in these rats. However the threshold for struggle in response to paw pressure was further decreased (as compared to control arthritic rats) in these animals. These results are discussed in view of observations made in human studies.