Muscle Sensory Dysfunction in a Rat Model of Peripheral Arterial Disease: the Role of Macrophage Activation in Chronic Limb Pain

Abstract

IntroductionPeripheral arterial disease (PAD) is associated with muscle sensory dysfunction. Here, we provide novel evidence demonstrating that macrophage activation in peripheral dorsal root ganglia (DRG) plays a critical role in this process. We hypothesized that inhibition of macrophage activation prevents the reduction in pain threshold in PAD rats.MethodsThe PAD model was developed by unilateral chronic femoral artery occlusion. We examined time‐dependent macrophage activation associated with altered nociceptive protein expression such as transient receptor potential vanilloid 1 (TRPV1) receptor, purinergic 2X3 receptor (P2X3) and vesicular glutamate transporter 1 (vGlut1) in the ipsilateral L4/L5 DRGs of PAD rats. We co‐cultured Lipopolysaccharide (LPS)‐pretreated macrophage (activated macrophages) with 50B11 DRG neurons. The manual monofilament von Frey test was used to evaluate the mechanical pain threshold. To prevent macrophage activation, minocycline was applied to animals 2 days prior to the surgery and lasted for 6 weeks.ResultsCompared to sham rats, the number of Iba‐1‐positive (a macrophage / microglia marker) cells in L4/L5 DRGs of PAD rats was significantly increased at 3 days and lasted at least 2 months post PAD. TRPV1 and P2X3 receptors were initially elevated in the acute PAD phase but significantly decreased in the chronic PAD phase, suggesting a dynamic change of these receptors in PAD. Interestingly, vGlut1 expression was changed in the opposite direction compared to TRPV1 and P2X3 receptors at each stage of PAD. In vitro, 72‐hour co‐culture of LPS‐pretreated macrophages with 50B11 DRG neurons significantly downregulated TRPV1 and P2X3 and increased vGlut1 protein expression in DRG neurons. Compared with baseline, the mechanical pain threshold to the von Frey test was significantly lower at 3 days post PAD and lasted for 6 weeks. Pretreatment with minocycline prevented the decreased mechanical pain threshold in the chronic stage of PAD (e.g., 5 weeks post PAD, PAD + minocycline vs. PAD, 6.62±1.50 vs. 2.26±0.43 g, P<0.05; 6 weeks post PAD, 6.55±1.43 vs. 2.53±0.37 g, P<0.05) but not in the acute stage (e.g., 3 days post PAD, PAD + minocycline vs. PAD, 3.88 ± 0.61 vs. 2.17±0.57 g, P>0.05).ConclusionOur data suggest that macrophage activation mediates muscle nociceptor afferent dysfunction in chronic PAD.Support or Funding InformationSupported in part by grants from the National Heart, Lung, and Blood Institute (1R01HL121012‐01 and R01 HL116608‐01)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.