Fig. 8

Proposed mechanisms underlying reversal of pain-like behavior at day-12 and diminished relief at day-18. In a neuropathic pain state, CCI rats administered with DF-NE vehicle nanomedicine exhibit pain-like behavior (a). This is proposed to be centrally driven by the attenuation of COX-2 by celecoxib delivered to circulating monocytes by nanomedicine. The polarity of macrophages at the injured sciatic nerve is predominantly the M1 pro-inflammatory phenotype. This influences crosstalk with mast cells, and their subsequent activation. With both immune effector cell types in an activated state, local inflammation is increased, resulting in a subsequent increase in macrophage infiltration. Inflammation at the ipsilateral DRG is transmitted through the afferent nociceptors from the injured sciatic nerve via neurogenic inflammation. In nanomedicine treated rats in the day-12 group (b), there is a reduction in COX-2 positive macrophages and extracellular PGE2 via the action of nanomedicine-delivered celecoxib. There is both a reduction in macrophage infiltration to the injured sciatic nerve and a shift of macrophage phenotype from pro-inflammatory (M1) to anti-inflammatory (M2). There is no nanomedicine treatment effect of macrophage infiltration to the DRG at day-12. M2 macrophages function to repair and regenerate the injured tissue and do not crosstalk with resident mast cells. There is a return to pain-like behavior at day-18 (c) in both nanomedicine and vehicle-treated groups. Macrophage infiltration to the injured sciatic nerve at day-18 is relatively low in both treatment groups—similar to levels observed in the day-12 nanomedicine-treated group. Macrophage infiltration to the DRG is increased at day-18, compared to day-12 and there are no significant differences between treatment groups. It is proposed that inflammation is initiated at the injured nerve and propagated to the associated L4 and L5 DRG via neurogenic inflammation. This drives recruitment of macrophages—as well as influencing further mast cell degranulation—and together provides the inflammatory input to sensitize nociceptors, resulting in an increase in pain-like behavior. The relatively low percentage of nanomedicine -positive macrophages at the DRG compared to the injured nerve suggests that the initial wave of macrophage infiltration is focused to the injured nerve