Background/Purpose:

We have previously shown that activation of vanilloid receptors by intra-articular (IA) Capsaicin (CAP) injection normalizes Evoked Pain Scores (EPS) and Dynamic Weight Bearing (DWB) measures in carrageenan-induced acute inflammatory arthritis. Resiniferatoxin (RTX) is an ultrapotent CAP analogue that has a similar mechanism of action, and may have a greater efficacy on carrageenan-induced arthritis when administered intra-articularly. We hypothesized that mice with acute arthritis would have measurable changes in DWB and EPS due to joint pain, and that these changes could be prevented by pre-treating with IA RTX.

Methods:

C57Bl6 mice were used for all experiments. Acute inflammatory arthritis was produced by IA injection of 10µl of 2.5% carrageenan into the left knee 3 hours prior to pain behavior testing. Two groups of mice were injected with different doses of IA RTX (10µl of 0.001% and 10µl of 0.0003%) 7 days prior to induction of arthritis. Similarly, another group of mice was injected with 10µl of 0.01% IA CAP 7 days prior to induction of arthritis. DWB was measured with a Dynamic Weight Bearing apparatus (Bioseb, Vitrolles, France). Evoked pain behavior was measured by tallying fights + vocalizations/1 min with repeated firm palpation of the knee.

Results:

Arthritis pain can be clearly and reproducibly indicated by increased EPS and reduced DWB measures in the affected limb of arthritic mice. Naïve mice demonstrated low EPS scores (1.01) and equal left to right DWB ratios for weight (1.01) and time (1.00). Induction of acute arthritis by IA Carrageenan resulted in a significantly increased EPS (6.25) and a significant decrease in left to right DWB ratios for weight (0.64) and time (0.89) when compared with controls. Pretreatment with IA CAP 7 days prior to IA Carrageenan resulted in significant improvement in EPS (3.25) and near normalization of left to right DWB ratios for weight (0.975) and time (1.00). Pretreatment with the high dose and the low dose IA RTX 7 days prior to IA Carrageenan lead to significantly improved EPS (1.5 & 1.5, respectively) and left to right DWB ratios for weight (0.85 & 0.82, respectively) and time (0.99 & 0.96, respectively) when compared with the acute arthritis model. IA administration of CAP alone and RTX alone did not have a significant impact on EPS or DWB ratios after 7 days.

Conclusion:

Pain can be quantitated in murine arthritis models using DWB and EPS. IA Carrageenan administration resulted in a significant increase in EPS and a significant decrease in DWB measures in the affected limb. IA RTX pretreatment in these mice clearly improved pain measures as assessed by EPS and DWB measures and these results were comparable to those previously reported for IA CAP. The potential advantages RTX may have over CAP include a possibly lower therapeutic dose and longer duration of effect. These factors represent important directions for future studies.