Localized inflammation of lumbar dorsal root ganglia (DRG) may contribute to low back pain. Local injections of corticosteroids used for low back pain are sometimes ineffective. Many corticosteroids activate not only the target glucocorticoid receptor (GR) but also the mineralocorticoid receptor (MR), which may have proinflammatory effects countering the effects of GR activation. A low back pain model was implemented in rats (n = 6 to 10 per group) by locally inflaming the L5 DRG. Sensory neuron excitability and mechanical hypersensitivity of the hind paws were measured. Tested steroids were applied locally to the inflamed DRG or orally. The selective MR blocker eplerenone reduced pain behaviors when given orally starting at the time of surgery, or starting 7 days later. The highly GR-selective agonist fluticasone, applied locally to the inflamed DRG, was much more effective in reducing mechanical hypersensitivity. The MR/GR agonist 6-α methylprednisolone, commonly injected for low back pain, reduced mechanical hypersensitivity when applied locally to the DRG but was less effective than fluticasone. Its effectiveness was improved by combining it with local eplerenone. All tested steroids reduced hyperexcitability of myelinated sensory neurons (n = 71 to 220 cells per group) after inflammation, particularly abnormal spontaneous activity. This preclinical study indicates the MR may play an important role in low back pain involving inflammation. Some MR effects may occur at the level of the sensory neuron. It may be useful to consider the action of clinically used steroids at the MR as well as at the GR.
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