Effect of genetic deletion of the vanilloid receptor TRPV1 on the expression of Substance P in sensory neurons of mice with adjuvant-induced arthritis

Abstract

The neuropeptide Substance P (SP), expressed by nociceptive sensory afferents in joints, plays an important role in the pathogenesis of arthritis. Capsaicin causes neurons in the dorsal root ganglia (DRG) to release SP from their central and peripheral axons, suggesting a functional link between SP and the capsaicin receptor, the transient receptor potential vanilloid 1 (TRPV1). The expression of both TRPV1 and SP have been reported to increase in several models of arthritis but the specific involvement of TRPV1-expressing articular afferents that can release SP is not completely understood. We here wanted to ascertain whether the increase in the number of SP-positive primary afferents in arthritis may be affected by genetic deletion of TRPV1. For this, we used immunohistochemistry to quantify the expression of SP in primary afferent neurons in wild-type mice (WT) vs. TRPV1-knockout (KO) mice with adjuvant-induced arthritis (AIA). We found that the expression of SP in DRG (1) increased significantly over naïve level in both WT and KO mice 3 weeks after AIA, (2) was significantly higher in KO mice than in WT mice in naïve mice and 2-3 weeks after AIA, (3) was significantly higher on the side of AIA than on the contralateral, vehicle-injected side at all time points in WT mice, but not in KO mice, and (4) increased predominantly in small-size neurons in KO mice and in small- and medium-size neurons in WT mice. Since the size distribution of SP-positive DRG neurons in arthritic TRPV1-KO mice was not significantly different from that in naïve mice, we speculate that the increased expression of SP is unlikely to reflect recruitment of A-fiber primary afferents and that the higher expression of SP in KO mice may represent a plastic change to compensate for the missing receptor in a major sensory circuit.

Fig. 1

Immunofluorescent staining for SP in DRG sections of wild type (WT) and TRPV1-KO (KO) mice 21 days after induction of AIA. In each group (WT and KO), the sections are from the left and right L5 DRG of the same mouse. Scale bar, 100 μm.

Fig. 2

The expression of SP was higher in TRPV1-KO (KO) than in wild type (WT) mice: the fraction (mean ± standard error of mean) of immunostained neuronal profiles was higher in KO mice than WT naïve mice (time point “0”) and 14-21 days after induction of AIA. It was also significantly higher on the side of CFA than on the contralateral, IFA-injected side at all time points in WT mice, but not in KO mice. *significantly different compared to naïve values for the same genotype; †significantly different compared to WT values for the same time point; p<0.05, ANOVA.

Fig. 3

Size distribution of SP-positive neuronal profiles (NP) in DRG of wild type (WT) and TRPV1-KO (KO) mice before (naïve) and 21 days after injection of CFA. Data suggest a shift towards larger cells, particularly in the 200-400 μm² bins, between naïve and arthritic mice for WT but not for KO mice. In the insert, histogram suggests a pronounced increase in the number of medium-size neurons (300-600 μm²) in WT mice (solid bars), whereas in KO mice (open bars), the increase occurred mostly in small neurons (<300 μm²). There were no significant changes in size distribution of SP-negative NP (not shown for clarity).