Red nucleus contributes to the maintenance of neuropathic pain in male rats via CCL3-CCR5 axis

The latest evidence indicates that the red nucleus (RN) contributes not only to motor control, but also to sensory processing and emotion regulation. Here, we demonstrate that the RN participates in the maintenance of neuropathic pain via CCL3-CCR5 axis. Following spared nerve injury (SNI) in male rats, a prominent upregulation of CCL3 and its receptor CCR5, instead of CCR1, was observed in RN neurons, astrocytes, and microglia during the late maintenance phase of neuropathic pain. Knockdown of red nucleus CCL3 alleviated SNI-induced allodynia, while intrarubral administration of exogenous CCL3 in naive rats induced allodynia. Pharmacological inhibition of CCR5, rather than CCR1, effectively alleviated SNI- or CCL3-induced allodynia. Further studies revealed that red nucleus CCL3-CCR5 axis contributed to the maintenance of neuropathic pain by stimulating the secretion of chemokines CCL4 and CXCL10, and complement C3. Blockade of these inflammatory mediators alleviated CCL3-mediated neuropathic pain. Moreover, our data indicated that red nucleus NF-κB, JAK2/STAT3, and ERK pathways were involved in CCL3-induced release of these inflammatory mediators. Specifically, CCL3 could induce CCL4 expression by activating NF-κB in neurons and/or microglia, and JAK2/STAT3 and ERK in neurons, astrocytes, and/or microglia. CCL3 could induce CXCL10 expression by activating JAK2/STAT3 and ERK in neurons, astrocytes, and/or microglia. CCL3 could induce C3 expression by activating NF-κB in microglia, and ERK in astrocytes and/or microglia. Inhibition of red nucleus NF-κB, JAK2/STAT3, or ERK suppressed the upregulations of CCL4, CXCL10, and/or C3, and alleviated CCL3-mediated neuropathic pain. Collectively, these findings provide potential mechanisms by which the RN contributes to the maintenance of neuropathic pain in male rats via CCL3-CCR5 axis, and provide promising therapeutic targets for intractable neuropathic pain.