Temporal Window of Metabolic Brain Vulnerability to Concussions

Abstract

OBJECTIVE:In the present study, we investigate the existence of a temporal window of brain vulnerability in rats undergoing repeat mild traumatic brain injury (mTBI) delivered at increasing time intervals. METHODS: Rats were subjected to two diffuse mTBIs (450 g/1 m height) with the second mTBI delivered after 1 (n ϭ 6), 2 (n ϭ 6), 3 (n ϭ 6), 4 (n ϭ 6), and 5 days (n ϭ 6) and sacrificed 48 hours after the last impact. Sham-operated animals were used as controls (n ϭ 6). Two further groups of six rats each received a second mTBI after 3 days and were sacrificed at 120 and 168 hours postinjury. Concentrations of adenine nucleotides, N-acetylated amino acids, oxypurines, nucleosides, free coenzyme A, acetyl CoA, and oxidized and reduced nicotinamide adenine dinucleotides, oxidized nicotinamide adenine dinucleotide phosphate, and reduced nicotinamide adenine dinucleotide, reduced nicotinamide adenine dinucleotide phosphate nicotinic coenzymes were measured in deproteinized cerebral tissue extracts (three right and three left hemispheres), whereas the gene expression of N-acetylaspartate acylase, the enzyme responsible for N-acetylaspartate (NAA) degradation, was evaluated in extracts of three left and three right hemispheres. RESULTS: A decrease of adenosine triphosphate, adenosine triphosphate /adenosine diphosphate ratio, NAA, N-acetylaspartylglutamate, oxidized and reduced nicotinamide adenine dinucleotide, reduced nicotinamide adenine dinucleotide, and acetyl CoA and increase of N-acetylaspartate acylase expression were related to the interval between impacts with maximal changes recorded when mTBIs were spaced by 3 days. In these animals, protracting the time of sacrifice after the second mTBI up to 1 week failed to show cerebral metabolic recovery, indicating that this type of damage is difficult to reverse. A metabolic pattern similar to controls was observed only in animals receiving mTBIs 5 days apart. CONCLUSION: This study shows the existence of a temporal window of brain vulnerability after mTBI. A second concussive event falling within this time range had profound consequences on mitochondrial-related metabolism. Furthermore, because NAA recovery coincided with normalization of all other metabolites, it is conceivable to hypothesize that NAA measurement by 1 H-NMR spectroscopy might be a valid tool in assessing full cerebral metabolic recovery in the clinical setting and with particular reference to sports medicine in establishing when to return mTBI-affected athletes to play. This study also shows, for the first time, the influence of TBI on acetyl-CoA, N-acetylaspartate acylase gene expression, and N-acetylaspartylglutamate, thus providing novel data on cerebral biochemical changes occurring in head injury.

Decrease in N-Acetylaspartate Following Concussion May Be Coupled to Decrease in Creatine

“…These experimental results have recently been confirmed 32 and strongly corroborate the finding reported in the present study, indicating temporary decrease in NAA and Cr levels and no change in Cho levels in our cohort of athletes with postconcussive head injury. Other authors indicated that patients suffering from mTBI underwent transient increase in Cr levels in the white matter and no change in NAA levels, 28,29 in contrast to the results of the present and previous studies 16,26,27,33,34 and also to a conspicuous number of preclinical studies demonstrating temporary decrease in NAA 8,10,11,21 and Cr pool following mTBI. 31,32 To explain their results, these authors concluded that an increase in total Cr (Cr and CrP) levels in the white matter may support a larger pool of high-energy phosphates (CrP and ATP), helping to restore mTBI-induced alteration of cell homeostasis through upregulation of membrane pumps and other processes of cellular repair.…”

“…3,4 Such a cascade of molecular events is considered as the determinant of the so-called state of metabolic brain vulnerability, 5,6 which transiently exposes cerebral tissue to the cumulative effect of a second mTBI occurring during this particular period. 7,8 High-energy phosphates, 9,10 coenzyme A metabolites, 11 ATP catabolites, 11 and neurotransmitters, 12,13 are some of the substances whose concentrations are temporarily affected as a result of a traumatic insult. Of particular relevance is the finding that N-acetylaspartate (NAA), an abundant brain-specific compound involved in several relevant biological functions including water homeostasis 14 and lipid myelin biosynthesis, 15 clearly mirrors TBI-induced changes in ATP, showing the same pattern of decrease and recovery following mTBI and no recovery in severe TBI or repeat mTBIs.…”

“…Of particular relevance is the finding that N-acetylaspartate (NAA), an abundant brain-specific compound involved in several relevant biological functions including water homeostasis 14 and lipid myelin biosynthesis, 15 clearly mirrors TBI-induced changes in ATP, showing the same pattern of decrease and recovery following mTBI and no recovery in severe TBI or repeat mTBIs. 8,10,11 The strict correlation between NAA and ATP ensures that this compound may be used as a valid surrogate marker to monitor cerebral energy metabolism. 8,10,11 Recently, particular attention has been given to compounds, noninvasively detectable in vivo by proton magnetic resonance (MR) spectroscopy ( 1 H MRS), that may be used as markers of brain metabolism following mTBI.…”

“…8,10,11 The strict correlation between NAA and ATP ensures that this compound may be used as a valid surrogate marker to monitor cerebral energy metabolism. 8,10,11 Recently, particular attention has been given to compounds, noninvasively detectable in vivo by proton magnetic resonance (MR) spectroscopy ( 1 H MRS), that may be used as markers of brain metabolism following mTBI. 1 H MRS allows the routine measurement of NAA, creatine (Cr), and choline (Cho) in a single set of spectral acquisition, 16 although the low magnetic field currently used in the clinical setting (1.5 or 3 T) does not allow to resolve the N-acetylaspartatylglutamate signal in the NAA peak 17 or the creatine phosphate (CrP) signal in the Cr peak, 17 or to resolve about 10 different compounds containing the choline moiety in their molecule in the Cho peak.…”

“…Differently, since the Cho peak is mainly composed of phosphocholine, glycerophosphocholine, and phosphatidylcholine, quantification of this peak area is not in direct correlation with one compound only. However, since these compounds are related to phospholipid metabolism, the Cho peak area is generally considered as a good indicator of the cell membrane turnover, 17,19 that of NAA is used as a marker of energy state and neuronal integrity, 8,10,11,17 and that of Cr is thought to be a marker of cellular energy. 17 Since the period of brain vulnerability following an mTBI is mainly characterized by evident biochemical, metabolic, and molecular changes, 3,5,6,20 it is questionable whether the tests currently adopted in the clinical practice to monitor mTBI patients (neuropsychological tests, balance tests, etc), because of their inability to evaluate cerebral biochemical changes, are of utility to determine the end of brain vulnerability.…”