WEIGHT-BEARING OF THE LIMB AS A CONFOUNDING FACTOR IN ASSESSMENT OF MECHANICAL ALLODYNIA IN THE RAT

Effect of weight bearing of the hindlimbs on the assessment of mechani cally-induced hindlimb withdrawal threshold was determined in intact r ats and in rats with various pathophysiological conditions causing all odynia or hyperalgesia. Hindlimb withdrawal was elicited by applying a series of calibrated monofilaments to the plantar or the dorsal surfa ce of the paw. During testing the;at was either in a restraint tube wi th hindlimbs hanging semi-extended without weight bear,ng or it was st anding on a metal grid (bearing its own weight). In intact rats, the w ithdrawal thresholds were significantly lower when the stimulus site w as the dorsal hairy skin rather than the plantar glabrous skin. Also, thresholds were significantly lower when the hindlimbs were not bearin g weight. Following carrageenan-induced unilateral inflammation of the plantar paw or a tibial nerve cut ther-was a marked threshold decreas e to test stimuli applied to plantar or dorsal paw, respectively, ipsi lateral to the pathological condition in standing rats. However, when the hindlimbs were not weight bearing the unilateral threshold decreas e was markedly attenuated (carrageenan-treated rats) or completely abo lished (tibial cut). In contrast, in rats with a unilateral spinal ner ve ligation the threshold decrease ipsilateral to the nerve lesion was highly significant independent of the weight bearing of the hindlimbs . The results indicate that weight bearing of hindlimbs is an importan t confounding factor in the assessment of tactile allodynia in rats. ( C) 1998 International Association for the Study of Pain. Published by Elsevier Science B.V.