PAIN-RELATED DISABILITY AND EFFECTS OF CHRONIC MORPHINE IN THE ADJUVANT-INDUCED ARTHRITIS MODEL OF CHRONIC PAIN

Functional disability has been identified as one of the most important aspects of chronic pain, yet modeling pain-related disability has rec eived little attention. Adjuvant-induced arthritis was induced, and on e group of arthritic rats was implanted with SC 75-mg morphine pellets 1 week postadjuvant, and reimplanted every 2 weeks thereafter. The re sults confirm that the rodent adjuvant induced arthritis model of seve re chronic pain can be used to model pain-related disability: spontane ous activity levels and ambulatory function were reduced in arthritic rats and they exhibited substantial weight loss. The results of the pr esent study suggest that the operant delayed nonmatching-to-position t ask can be used as a measure of pain-related disability, which may be especially relevant to the effects of chronic pain on performance in a work setting. The delayed nonmatching-to-position operant bar-pressin g task is an ''apical'' test that is sensitive to deficits across a wi de range of behavioral functions: motor ability, attention, motivation , learning, and memory, and arthritic rats were severely impaired in t his task. In addition, analgesic treatments that impair functional abi lities in normal healthy rats may actually improve the performance of rats exhibiting pain-related disability. Previous work demonstrated th at acute morphine injections of only 4 mg/kg impaired performance in t he delayed matching-to-position task. The results of the present study demonstrate that chronic morphine attenuates the degree of pain-relat ed disability exhibited by arthritic rats in the test of ambulatory fu nction and the delayed nonmatching-to-position bar-pressing test. Thes e results demonstrate that novel analgesic treatments can be screened preclinically, both With respect to their direct analgesic effects, an d with respect to their ability to reduce pain-related disability. (C) 1997 Elsevier Science Inc.