Background: Anti-inflammatory corticosteroids are a common treatment for different conditions involving chronic pain and inflammation. Clinically used steroids target the glucocorticoid receptor (GR) for its anti-inflammatory effects. We previously reported that GR in sensory neurons may play central roles in some pain models and that GR immunoreactivity signal in dorsal root ganglia (DRG) decreased after local inflammation of the DRG (a model of low back pain). In the current study, we aimed to determine if similar changes in GR signal also exist in a skin inflammation model, the complete Freund's adjuvant (CFA) model (a model of peripheral inflammatory pain), in which the terminals of the sensory neurons rather than the somata are inflamed.
Methods: A low dose of CFA was injected into the hind paw to establish the peripheral inflammation model in Sprague-Dawley rats of both sexes, as confirmed by measurements of behavior and paw swelling. Immunohistochemical and western blotting techniques were used to determine the expression pattern of the GR in the inflamed hind paw and the DRGs. Plasma corticosterone levels were measured with radioimmunoassay.
Results: The immunohistochemical staining revealed that GR is widely expressed in the normal DRG and skin tissues. Paw injection with CFA caused upregulation of the GR in the skin tissue on postinjection day 1, mostly detected in the dermis area. However, paw inflammation significantly reduced the GR signal in the L5 DRG 1 day after the injection. The GR downregulation was still evident 14 days after CFA inflammation. On day 1, western blotting confirmed this downregulation and showed that it could also be observed in the contralateral L5 DRG, as well as in the L2 DRG (a level which does not innervate the paw). Plasma corticosterone levels were elevated in both sexes on day 14 after CFA compared to day 1, suggesting autologous downregulation of the GR by corticosterone may have contributed to the downregulation observed on day 14 but not day 1.
Conclusions: There are distinctive patterns of GR activation under different pain conditions, depending on the anatomical location. The observed downregulation of the GR in sensory neurons may have a significant impact on the use of steroids as treatment in these conditions and on the regulatory effects of endogenous glucocorticoids.