Minocycline Attenuates High Mobility Group Box 1 Translocation, Microglial Activation, and Thalamic Neurodegeneration after Traumatic Brain Injury in Post-Natal Day 17 Rats

In response to cell injury, the danger signal high mobility group box-1 (HMGB) is released, activating macrophages by binding pattern recognition receptors. We investigated the role of the anti-inflammatory drug minocycline in attenuating HMGB1 translocation, microglial activation, and neuronal injury in a rat model of pediatric traumatic brain injury (TBI). Post-natal day 17 Sprague-Dawley rats underwent moderate-severe controlled cortical impact (CCI). Animals were randomized to treatment with minocycline (90mg/kg, intraperitoneally) or vehicle (saline) at 10min and 20h after injury. Shams received anesthesia and craniotomy. We analyzed HMGB1 translocation (protein fractionation and Western blotting), microglial activation (Iba-1 immunohistochemistry), neuronal death (Fluoro-Jade-B [FJB] immunofluorescence), and neuronal cell counts (unbiased stereology). Behavioral assessments included motor and Morris-water maze testing. Nuclear to cytosolic translocation of HMGB1 in the injured brain was attenuated in minocycline versus vehicle-treated rats at 24h (p<0.001). Treatment with minocycline reduced microglial activation in the ipsilateral cortex, hippocampus, and thalamus (p<0.05 vs. vehicle, all regions); attenuated neurodegeneration (FJB-positive neurons) at seven days (p<0.05 vs. vehicle); and increased thalamic neuronal survival at 14 days (naive 22773 +/- 1012 cells/mm(3), CCI + vehicle 11753 +/- 464, CCI + minocycline 17047 +/- 524; p<0.001). Minocycline-treated rats demonstrated delayed motor recovery early after injury but had no injury effect on Morris-water maze whereas vehicle-treated rats performed worse than sham on the final two days of testing (both p<0.05 vs. vehicle). Minocycline globally attenuated HMGB1 translocation and microglial activation in injured brain in a pediatric TBI model and afforded selective thalamic neuroprotection. The HMGB1 translocation and thalamic injury may represent novel mechanistic and regional therapeutic targets in pediatric TBI.