BACKGROUND: The effects of N-methyl-D-aspartic acid (NMDA) receptor antagonist on neurodegeneration in the immature brain following traumatic brain injury(TBI)are still widely unknown.OBJECTIVE:To study the effects of dizocipine maleate(MK-801),a non-competitive NMDA receptor antagonist,on mitochondrial ultramicrostructure of neurons in the ipsilateral cingulate cortex and hippocampus after TBI in neonatal rats,and to analyze the optimal time interval of MK-801 administration(1 mg/kg).DESIGN:Completely randomized controlled study. SETTING:Shanghai Jiao Tong University. MATERIALS:Eight 7-day-old neonatal SD rats,irrespective of gender,were provided by Experimental Animal Center,Medical College of Fudan University.The experiment was approved by a local ethics committee.MK-801 was provided by Sigma.A CM-120 transmission electron microscope(Philips,Holland)was used for tissue analysis.METHODS:This study was performed at the Departments of Anatomy,Neuromorphology,and Biophysics, Medical College of Shanghai,Jiaotong University,between October 2006 and January 2007.Focal models of contusion and laceration of brain were established by the free-falling impact method.Eight rats were randomly divided into a normal control group(n＝2 )and a MK-801 group(n＝6).Rats in the normal control group did not receive model establishment and administration,and they were only analyzed by an electron microscope.In the MK-801 group,the cingulate cortex was damaged using a contusion device.MK-801(1 mg/kg)was intraperitoneally injected 30 minutes before lesion,immediately after lesion,and 30 minutes after lesion(n＝2 for each time point).MAIN OUTCOME MEASURES:The cingulate cortex and hippocampal tissues from the injured side were removed 24 hours after lesion and routinely processed for analysis of neuronal ultramicrostructure using transmission electron microscopy.RESULTS:Differential therapeutic effects of MK-801(1 mg/kg)at distinct administration time points: thirty minutes before lesion,the shape of cortical and hippocampal neurons was similar to that observed during excitotoxicity-induced cell death.Organelles were enlarged,the nuclear membrane of cortical neurons was complete with gear wheel-like changes,and the nuclear chromatin was irregularly aggregated around the edge.When MK-801 was applied 30 minutes after lesion,the cingulate cortex contained apoptotic neurons in early and late stages.The nuclear membrane of hippocampal neurons displayed incisures.The chromatin shape was not similar to necrosis in an early stage.Immediate administration of MK-801 after lesion slightly altered the neuronal architecture,such that mltochondria were enlarged.The neuronal shape in the control group was normal.Effects of immediate administration of MK-801 on mitochondrial injury following TBI were that the mitochondria in cortical and hippocampal neurons were damaged to a certain degree in the MK-801 group.Mitochondrial injury was reversible,when MK-801 was applied 30 minutes before lesion and immediately after lesion.Application 30 minutes after lesion produced irreversible changes.In addition,mitochondrial injury occurred earlier than other organeUe and nuclear changes.CONCLUSION:Mitochondrial injury occurs earlier than other organelle and nuclear changes.Early administration of MK-801(1 mg/kg)can prevent or reduce necrosis following TBI,decrease the degree of neuronal injury,and protect nerve cells.