Morphine reduces joint hypersensitivity when administered locally into the knee joint in a rat model of chronic inflammation

Iain Strickland, Susan Bond, Daniel McQueen. University of Edinburgh, Edinburgh, United Kingdom.

Studies have shown that intra-articular morphine provided excellent analgesia to arthritis patients without any side-effects (Stein et al. 1999). The aim of this study was to test the hypothesis that functional opioid receptors are expressed on rat knee joint nociceptors in a model of inflammatory knee joint hypersensitivity. Using an incapacitance tester, weight distribution (WD) assessments were taken as a measure of how the animals preferentially distributed their body weight over their hind limbs. The ‘weight’ carried by the FCA-injected ipsilateral limb was divided by that carried by the un-injected contralateral limb to provide a weight distribution (WD) ratio. On day 0, following baseline WD assessments, a unilateral joint arthritis was induced in the left knee of adult male Wistar rats (n=32, 150-200g) by a 150μl intra-articular injection of Freund’s complete adjuvant (FCA; 1mg.ml-1 Mycobacterium tuberculosis, Sigma, UK). Rats were transiently anaesthetised using 3% isoflurane in O2 and FCA was injected into the knee joint space. Further behavioural assessments were made on days 4, 7, 11, 14, 18 and 21 post-FCA. Following day 14 measurements, animals were randomised into four dosing groups (n=8) and 100μl of morphine sulphate (10mg.ml-1, 3mg.ml-1 and 1mg.ml-1) or vehicle (saline) was administered blind into the FCA-injected knee joint under brief anaesthesia as described above. Animals were allowed to recover in their cages before further WD measures were made at 30, 60, 90, 120 and 180 minutes post drug injection. Mean (± SEM) WD ratios were calculated for each treatment group and area under the curve (AUC) analysis was performed on the results from the day 14 time course study. Data were analysed using an ANOVA where P<0.05 was considered statistically significant. Baseline assessments on day 0 showed that the mean amount of weight placed through both the left and right hind limb was 53 ± 1g, giving a calculated WD ratio of 1.00 ± 0.01 (n=32). On day 14 post-FCA animals placed more weight through the limb contralateral to the FCA-injected knee (132 ± 2g) than the FCA-injected limb (53 ± 2g), with the mean WD ratio prior to dosing being 0.39 ± 0.02. AUC results showed the 3 and 10mg.ml-1 morphine dosed groups had significantly higher WD ratios than the vehicle dosed group as animals placed more weight through the FCA-injected limb (P<0.001, one-way ANOVA). Furthermore, assessments taken on day 18 post-FCA show that there was still a significant difference between the 3 and 10mg.ml-1 morphine dosed groups compared to the vehicle treated group (P<0.05, two-way ANOVA, n=8). These results support the hypothesis that functional opioid receptors are associated with peripheral nociceptive afferent nerve fibres innervating the inflamed rat knee joint. Peripherally restricted opioid receptor agonists may play an important role in long term pain management, with the benefit of avoiding the undesirable centrally mediated side-effects seen following systemic opioid administration.