Pejman Ghanouni MD, PhD, Presenter: Nothing to Disclose

Deepak Behera MD, Abstract Co-Author: Nothing to Disclose

Jin Xie PhD, Abstract Co-Author: Nothing to Disclose

Xiaoyuan Chen PhD, Abstract Co-Author: Nothing to Disclose

Sandip Biswal MD, Abstract Co-Author: Nothing to Disclose

Minocycline has proven anti-nociceptive effects after peripheral nerve injury, but the mechanism by which this occurs remains unclear. Using ultrasmall superparamagnetic iron oxide (USPIO)-MRI to monitor macrophage trafficking, the purpose of this project is determine whether minocycline modulates macrophage trafficking to site of nerve injury which, in turn, results in altered pain thresholds.

Study was approved by Stanford IACUC. A model of neuropathic pain, the Spared Nerve Injury (SNI) model that involves ligation of 2 of the 3 branches of the left sciatic nerve, was employed in Sprague-Dawley rats. Animals with SNI and uninjured animals were then injected with/without USPIOs (300µmol/kg IV) and with/without minocycline (50mg/kg IP) (n=3 per group). Bilateral sciatic nerves were scanned with a volume coil in a 7T magnet 24 hours after USPIO administration. FIESTA images (TR/TE/FA 11/22/20, in-plane resolution 110µm2, 1mm slice thickness) were obtained. Using Osirix, ROIs were placed on bilateral sciatic nerves to quantify signal intensity which were normalized to background signal in the muscle. Pain behavior was measured using the Von Frey filament test at the time of MR imaging. The data was analyzed with statistical tools on an Excel spreadsheet. Significance is p<0.05. Sciatic nerves were harvested to histologically determine the presence of iron oxide-laden macrophages.

Behavioral measurements confirmed the presence of allodynia in the SNI group (threshold of 3.86+/-0.34) while the uninjured (4.85+/-0.16) and minocycline-treated SNI group (4.90+/-0.08) had significantly higher paw withdrawal thresholds (p<0.011). Decreased relative MR signal was observed in the SNI group that received USPIOs (3.3+/-0.5) compared to the minocycline-treated SNI group that received USPIOs (15.2+/-4.5) and minocycline-treated group (no USPIOs; 41.2+/-2.3) (p<0.04). Histology of harvested sciatic nerve specimens confirmed the presence USPIOs at the nerve injury site in the SNI group (no minocycline).

Animals with neuropathic pain in the left hindpaw show increased trafficking of USPIO-ladened macrophages to the site of sciatic nerve injury. Minocycline appears to retard the migration of macrophages to the nerve injury site, partly explaining its anti-nociceptive effects.

USPIO-MRI is an effective tool to study the role of macrophages in the development of neuropathic pain.

Ghanouni, P, Behera, D, Xie, J, Chen, X, Biswal, S, Minocycline Prevents Development of Neuropathic Pain by Mitigating Macrophage Recruitment to Site of Nerve Injury as Shown with USPIO-MRI.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8014134.html