WEIGHT-BEARING AS AN OBJECTIVE-MEASURE OF ARTHRITIC PAIN IN THE RAT

This study compares two methods for evaluating pain-related behavior i n an animal model with carrageenan-induced monoarthritis. Rats injecte d with lambda-carrageenan into the right tibio-tarsal joint were video filmed at various times after injection and later scored regarding the ir stance. Immediately after each videorecording session the animals w ere tested in a box constructed to register the weight load exerted by the hindpaws by means of force plates inserted in the floor. Followin g carrageenan injection (300 mu g in 50 mu L) the load on the injected paw fell from a control value of 39.3% +/- 0.4% of the body weight (m ean +/- SEM, n = 6) to a minimum of 5.1% +/- 1.8% at 6 hr and then slo wly increased to approach control levels at 72 hr. The weight load on the contralateral paw increased from a control value of 38.9% +/- 0.6% to 52.4% +/- 1.4% at 6 hr, whereafter it gradually decreased. The vid eo-based stance scores also showed a maximal impairment at 4-6 hr, wit h a gradual return towards control values at 72 hr. However, the resul ts based on the force plate measurements were less variable and more g raded. Morphine inhibited the carrageenan-induced effect in a dose-dep endent manner in both paradigms. In conclusion, the present results in dicate that measurement of weight bearing as described in the present paper is a practical, useful, and objective method to assess the degre e of arthritic pain in the rat.