Role of the P2 × 4 Receptor in Sympathetic-mediated Inflammatory Low Back Pain

The sympathetic nervous system mediates various pain conditions including inflammatory low back pain. To better understand the underlying mechanisms, we developed a surgical procedure named localized microsympathectomy (mSYMPX) in rats and mice, which consists of cutting the grey rami containing postganglionic sympathetic fibers as they enter the spinal nerves near the lumbar dorsal root ganglia (DRG). We reported that mSYMPX attenuated pain behaviors in several preclinical pain models including a mouse model of chemotherapy-induced peripheral neuropathy (CIPN model). Under pathological conditions release of ATP from sympathetic nerve endings locally in the DRG activates purinergic receptor P2 × 4 in macrophages resulting in a more M1/pro-inflammatory polarization. In this study we asked whether the macrophage-enriched P2 × 4 receptors are involved in sympathetic-mediated inflammatory low back pain/radiculopathy produced by intravertebral injection of the immune activator zymosan, causing mechanical hypersensitivity (LID model). Our previous microarray study in rats showed upregulation of the macrophage cell marker Iba1 and purinergic receptor (P2rx4) after LID. As previously reported in rats, we found that mice showed mechanical hypersensitivity assessed by calibrated von Frey filaments and transcriptional upregulation of Iba1 and P2rx4 three days after LID. Interestingly, mSYMPX prevented mechanical hypersensitivity, and also caused significant transcriptional upregulation of Iba1 and a trend towards downregulation of P2rx4 three days after LID. The enhancement of P2 × 4 signaling with the allosteric modulator ivermectin reduced expression of the M2 marker in cultured peritoneal macrophages. Conversely, blocking P2 × 4 signaling with PSB12062 in M1 skewed macrophages mitigated the expression of M1 marker. Furthermore, the mechanical hypersensitivity in the LID model was reversed by intrathecal injection of the P2 × 4 antagonist (PSB12062) three days after LID. These findings suggest P2 × 4 receptor as a potential therapeutic target for managing inflammatory low back pain/radiculopathy. Funded by NS045594 and the HEAL supplement to NINDS grant NS045594 (J-M Z).