Macrophages and other monocyte derivatives, as much as pain-sensing neurons themselves, play a critical role in the generation of chronic pain conditions. Macrophages are derived from the myelo-monocytic stem cells in the bone marrow and play an important role in immunity, inflammation and tissue remodeling. In the setting of nerve injury, macrophages and microglia migrate to various tissues and, as part of the healing response to injury, elaborate a variety of inflammatory factors . In an animal model of peripheral nerve injury, for example, investigators have found that monocytes migrate into the spinal cord, dorsal root ganglia and involved spinal nerve roots, and differentiate into macrophages or microglia . Because of their pro-inflammatory properties, recruitment and activation of monocyte derivatives to the nervous system is thought to promote hypersensitivity and chronic pain states.
An in vivo magnetic resonance imaging (MRI)-based method using magnetic nanoparticles, such as superparamagnetic iron-oxide particles (SPIOs), ultrasmall SPIOs (USPIOs), monocrystalline iron-oxide particles (MIONs) and cross-linked iron oxide (CLIO) has been developed to track macrophage and T-cell migration and localization . Ultrasmall superparamagnetic iron-oxide magnetic resonance imaging (USPIO-MRI) allows monitoring of trafficking of macrophages into the central nervous system in a variety of degenerative neurological conditions . SPIOs have also been used to monitor monocytic/macrophage migration patterns in the setting of rheumatoid arthritis. After intravenous injection of SPIO particles, cells that reside in the reticuloendothelial system (RES), including macrophages, engulf the agent. Because macrophages are recruited to inflamed joints, monitoring their distribution by SPIO-based techniques can be helpful, especially during early phases of the disease. MRI can be used to study the migration of these cells from the RES to inflamed joints. Investigators have successfully documented the migration of SPIO-labeled macrophages to the synovium of a rat model of RA .
Another method to monitor T-cell traffic has been developed for MRI. T-cells isolated from a subject can be loaded with dextran-coated SPIO or similar dextran-coated CLIO [6, 7]. When exposed to SPIO, T-cells will engulf the 30 nm particles by endocytosis. The T-cells are eventually re-introduced into the subject, and the subject is scanned. On gradient-echo sequences, cells carrying this contrast agent appear low in signal intensity owing to the large susceptibility effect generated by the sequestered SPIO particles. In rat models of cardiac, renal and lung allograft rejection, migration of SPIO-labeled T-cells to the allograft has been found during rejection [8–10].
Using USPIO-MRI as a surrogate marker for macrophage recruitment, we sought 1) to detect nociception-related spatiotemporal USPIO-MRI signal changes in a peripheral nerve after injury in vivo- in a longitudinal animal model of pain, 2) to determine whether chronic pain states correlate with macrophage recruitment, and 3) to determine whether USPIO-MR can be used to monitor the known effect of the antibiotic minocycline on macrophage trafficking to the site of nerve injury and whether this in turn results in altered pain thresholds.