Increased expression of CGRP in sensory afferents of arthritic mice--effect of genetic deletion of the vanilloid receptor TRPV1

Abstract

The neuropeptide calcitonin gene-related peptide (CGRP), expressed by nociceptive sensory afferents in joints, is an important mediator in the pathogenesis of arthritis. Capsaicin causes neurons in the dorsal root ganglia (DRG) to release CGRP from their central and/or peripheral axons, suggesting a functional link between CGRP and the capsaicin receptor TRPV1. The expression of both TRPV1 and CGRP have been reported to increase in several models of arthritis but the specific involvement of TRPV1-expressing articular afferents that can release CGRP remains unclear. We here wanted to ascertain whether the increase in the number of CGRP-positive primary afferents during arthritis may be affected by genetic deletion of TRPV1. For this, we quantified the expression of CGRP in primary afferent neurons in DRG in wild type mice (WT) vs. TRPV1-KO mice with adjuvant-induced arthritis (AIA), using immunohistochemistry. We found that the fraction of DRG neurons that were immunopositive for CGRP (1) was higher in naïve TRPV1-KO mice than in naïve WT mice, (2) increased progressively 3-21 days after induction of AIA, and (3) this increase was bilateral but significantly greater on the complete Freund's adjuvant-injected side than on the incomplete Freund's adjuvant-injected side in TRPV1-KO mice. The increased expression of CGRP in AIA may reflect a phenotypic switch of primary afferents from non-peptidergic to peptidergic and the larger increase in TRPV1-KO mice may represent a plastic change to compensate for the missing receptor in a major sensory circuit.

Figure 1

Immunofluorescent staining for CGRP in DRG from sections of wild type (WT) and TRPV1-KO mice: the number of labeled neuronal profiles appears greater on the CFA-injected side than on the IFA-injected side 21 days after induction of AIA. In every group (WT and TRPV1-KO), the sections of are from the left and right L5 DRG of the same mouse. Scale bar, 100 μm.

Figure 2

The expression of CGRP was higher in TRPV1-KO (KO) than in wild type (WT) mice with AIA: the fraction (mean ± standard error of the mean) of immunostained neuronal profiles was higher in naïve KO mice than in naïve WT mice (time point “0”), increased progressively 3–21 days after induction of AIA, and at days 7 and 14, was significantly greater on the CFA-injected side than on the IFA-injected side in TRPV1-KO mice.