Reduction in locomotor activity of arthritic rats as parameter for chronic pain: effect of morphine, acetylsalicylic acid and citalopram.

Arthritis was induced in rats by plantar injection of a suspension of killed mycobacteria in Viscoleo. Non-treated rats served as controls. The locomotor activities of the rats were measured weekly during the following four weeks. The exploratory activity was determined in the daytime in activity cages and spontaneous locomotor activity was measured during the night in the same cages. From one week to four weeks after treatment in one hind paw the arthritic rats showed significantly lower exploratory activity than the control rats, whereas the spontaneous locomotor activity in the arthritic rats was not significantly reduced. After treatment in two hind paws both activities were significantly smaller in the arthritic group than in the control group. The low exploratory activity level of the arthritic rats was not changed by acetylsalicylic acid but was significantly increased by morphine and citalopram in doses which did not influence the exploratory activity of the control rats. The results suggest that reduction of the locomotor activity of arthritic rats might be a reliable parameter for chronic pain.