Hypoxic-ischemic injury in the immature brain : excitatory aminoacids and mitochondrial function during reperfusion

Abstract

The aims were to study excitatory amino acid dependent mechanisms that contribute to evolution of brain damage after hypoxia-ischemia (HI) and to evaluate the neurotoxicity of N-methyl-D-aspartate receptor (NMDAR) antagonists in the immature brain.HI was induced in 7-day-old rats by unilateral carotid occlusion and 1-2 h of hypoxia. Posttreatment with the NMDAR antagonist MK-801 and the AMPA receptor antagonist NBQX, and pre- or posttreatment with the voltage dependent Na+ channel blocker BW1003C87 was evaluated by morphology and hemispheric weight 3 and 14 days after HI. Cerebral energy metabolites, energy utilization, glucose utilization (RCGU), mitochondrial respiration, morphology and blood flow (CBF) were studied after HI in controls and in MK-801 posttreated rats.The possible neurotoxicity of NMDAR antagonists was evaluated by RCGU and heat shock protein 72 (HSP72) expression.MK-801 (0.3-0.5 mg/kg) and NBQX (40 mg/kg) reduced brain injury by 38-61% and 27%, respectively. MK-801 neuroprotection was not related to hypothermia. Pretreatment with BW1003C87 (10 mg/kg) reduced brain injury by 45% whereas posttreatment was ineffective. Between 2-6 h after HI, the energy utilization and CBF were unaltered, the high energy phosphate levels recovered to 75% of control and mitochondrial respiration decreased by 46%. The RCGU and tissue lactate levels were increased. Posttreatment with MK-801 reduced energy utilization by 43% and RCGU by 30 % and improved mitochondrial function by 42% but did not affect the tissue levels of ATP and phosphocreatine at 3 h. Between 8-24 h, regions with low RCGU and loss of the neuronal marker microtubule associated protein 2 replaced regions with high RCGU. A secondary deterioration of high energy phosphates was prevented by MK-801. CBF declined by 80% between 24 and 48 h after HI. MK-801 or MgSO4 did not increase RCGU or induce HSP72 expression. In conclusion, evolution of brain damage is related to NMDAR and AMPA receptor dependent processes during reperfusion. Activation of NMDARs sustains the energy demand and impairs the energy production which may be critical for the survival of the tissue.