Substance P and glutamate receptor antagonists improve the anti-arthritic actions of dexamethasone in rats

Abstract

Background and purpose: Current single drug treatments for rheumatoid arthritis have problems of limited efficacy and/or high toxicity. This study investigates the benefits of individual and combined treatments with dexamethasone and substance P and glutamate receptor antagonists in a rat model of arthritis.

Experimental approach: Arthritis was induced in rats by unilateral intra-articular injection of Freund's complete adjuvant. Separate groups of rats were subjected to the following treatments 15 min before induction of arthritis: (i) control with no drug treatment; (ii) single intra-articular injection of a NK(1) receptor antagonist RP67580; (iii) single intra-articular injection of a NMDA receptor antagonist AP7 plus a non-NMDA receptor antagonist CNQX; (iv) daily oral dexamethasone; and (v) combined treatment with dexamethasone and all of the above receptor antagonists. Knee joint allodynia, swelling, hyperaemia and histological changes were examined over a period of 7 days.

Key results: Treatment with dexamethasone suppressed joint swelling, hyperaemia and histological changes that include polymorphonuclear cell infiltration, synovial tissue proliferation and cartilage erosion in the arthritic rat knees. Treatment with RP67580 or AP7 plus CNQX did not attenuate hyperaemia or histological changes, but reduced joint allodynia and swelling. Co-administration of dexamethasone with these receptor antagonists produced greater inhibition on joint allodynia and swelling than their individual effects.

Conclusions and implications: The data suggest substance P and glutamate contribute to arthritic pain and joint swelling. The efficacy of dexamethasone in reducing arthritic pain and joint swelling can be improved by co-administration of substance P and glutamate receptor antagonists.

Figure 1

Individual and combined effects of antagonists of substance P receptors (A), glutamate receptors (B) and dexamethasone (C and D) on knee joint allodynia of Freund's complete adjuvant (FCA)-induced arthritic rats. Combining dexamethasone with the receptor antagonists produced better inhibition of allodynia than dexamethasone alone. Bonferroni post hoc test: *P < 0.05, **P < 0.01, ***P < 0.001 (significantly different from FCA alone); ♦♦P < 0.01, ♦♦♦P < 0.001 (significantly different from FCA + Dex). ^aP < 0.05, ^bP < 0.01, ^cP < 0.001 (two-factor anova). n≥ 7. Dex, dexamethasone; RP, RP67580.

Figure 2

Individual and combined effects of antagonists of substance P receptors (A), glutamate receptors (B), and dexamethasone (C and D) on knee joint swelling of Freund's complete adjuvant (FCA)-induced arthritic rats. Combining dexamethasone with the receptor antagonists produced slightly better inhibition of joint swelling than dexamethasone alone. Bonferroni post hoc test: *P < 0.05, **P < 0.01, ***P < 0.001 (significantly different from FCA alone); ♦P < 0.05 (significantly different from FCA + Dex). ^aP < 0.05, ^bP < 0.01, ^cP < 0.001 (two-factor anova). n≥ 7. Dex, dexamethasone; RP, RP67580.

Figure 3

Individual and combined effects of antagonists of substance P receptors, glutamate receptors and dexamethasone on knee joint blood flow (A) and blood pressure (B) of Freund's complete adjuvant (FCA)-induced arthritic rats. Dexamethasone produced the most pronounced inhibition of FCA-induced hyperaemia. All drug treatments had no significant effect on blood pressure. Significance level: *P < 0.05 (paired Wilcoxon test); ♦P < 0.05 (unpaired Mann–Whitney test). n≥ 6. Dex, dexamethasone; RP, RP67580.

Figure 4

Micrographs showing typical effects of dexamethasone alone and dexamethasone combined with substance P and glutamate receptor antagonists on histological changes in Freund's complete adjuvant (FCA)-induced arthritic rat knees. FCA-injected knee showed marked cell infiltration, tissue proliferation and cartilage erosion compared with contralateral saline-injected knee. Dexamethasone and the combined treatment produced similar inhibition on these histological changes. C, cartilage; Combined, dexamethasone (0.5 mg·kg⁻¹·day⁻¹) plus 10 nmol of RP67580, AP7 and CNQX; Dex, dexamethasone (0.5 mg·kg⁻¹·day⁻¹); F, fat cells; Fe, femur; M, meniscus; S, synovial space.

Figure 5

Individual and combined effects of receptor antagonists of substance P, glutamate and dexamethasone on cell infiltrate (A), tissue proliferation (B) and cartilage erosion (C) in Freund's complete adjuvant (FCA)-induced arthritic rat knees. The substance P antagonist had no effect, glutamate receptor antagonists slightly reduced tissue proliferation and dexamethasone reduced all histological changes in the arthritic knees, but its effect was not significantly altered by combined treatment with the receptor antagonists. Data are shown as histological changes in an arbitrary scale of severity: 0 = no change; 1 = mild; 2 = moderate; 3 = marked. Significance level: *P < 0.05, **P < 0.01 (paired Wilcoxon test); ♦P < 0.05, ♦♦P < 0.01, ♦♦♦P < 0.001 (unpaired Mann–Whitney test). n≥ 7. Dex, dexamethasone; RP, RP67580.