Proton nuclear magnetic resonance (1H NMR) spectroscopy is a noninvasive technique that can provide information on a wide range of metabolites. Marked abnormalities of 1H NMR brain spectra have been reported in patients with neurological disorders, but their neurochemical implications may be difficult to appreciate because NMR data are obtained from heterogeneous tissue regions composed of several cell populations. The purpose of this study was to examine the 1H NMR profile of major neural cell types. This information may be helpful in understanding the metabolic abnormalities detected by 1H NMR spectroscopy. Extracts of cultured cerebellar granule neurons, cortical astrocytes, oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, oligodendrocytes, and meningeal cells were analyzed. The purity of the cultured cells was > 95% with all the cell lineages, except for neurons (approximately 90%). Although several constituents (creatine, choline-containing compounds, lactate, acetate, succinate, alanine, glutamate) were ubiquitously detectable with 1H NMR, each cell type had distinctive qualitative and/or quantitative features. Our most unexpected finding was a large amount of N-acetyl-aspartate (NAA) in O-2A progenitors. This compound, consistently detected by 1H NMR in vivo, was previously thought to ne present only in neurons. The finding that meningeal cells have an alanine:creatine ratio three to four times higher than astrocytes, neurons, or oligodendrocytes is in agreement with observations that meningiomas express a higher alanine:creatine ratio than gliomas. The data suggest that each individual cell type has a characteristic metabolic pattern that can be discriminated by 1H NMR, even by looking at only a few metabolites (e.g., NAA, glycine, beta-hydroxybutyrate).(ABSTRACT TRUNCATED AT 250 WORDS)

To test the specificity of N-acetylaspartate (NAA) as a neuronal marker for proton nuclear magnetic resonance (1H NMR) spectroscopy, purified and characterized cultured cells were analyzed for their NAA content using both 1H NMR and HPLC. Cell types studied included cerebellar granule neurons, type-1 astrocytes, meningeal cells, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, and oligodendrocytes. A high concentration of NAA was found in extracts of cerebellar granule neurons (approximately 12 nmol/mg of protein), whereas NAA remained undetectable in purified type-1 astrocytes, meningeal cells, and mature oligodendrocytes. However, twice the neuronal level of NAA was found in O-2A progenitors grown in vitro. In addition significant levels of NAA were also detected in cultures of immature oligodendrocytes. Our data partly support previous suggestions that NAA may be a useful neuronal marker for 1H NMR spectroscopic examination of the adult brain. However, they also raise the further possibility that alterations of NAA associated with some specific brain disorders, particularly disorders seen in newborn and young children, may reflect abnormalities in the development of oligodendroglia or their precursors.

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Traumatic brain injury (TBI) increases extracellular levels of the excitatory amino acid glutamate and aspartate, and N-methyl-D aspartate (NMDA)-receptor antagonists protect against experimental TBI. These two findings have led to the prevalent hypothesis that excitatory amino acid efflux is a major contributor to the development of neuronal damage subsequent to traumatic injury. However, as with stroke, the hypothesis that high extracellular glutamate is the key to excitotoxicity in TBI conflicts with important data. For example, the initial increase in extracellular glutamate is cleared within 5 min after moderate TBI, whereas antagonists of glutamate receptors and the so- called presynaptic glutamate release inhibitors remain effective when administered 30 min after insult. In this article, we argue that the current concept of excitotoxicity in TBI, centered on high extracellular glutamate, does not withstand scientific scrutiny. As alternatives to explain the beneficial actions of glutamate antagonists in experimental TBI, we propose abnormalities of glutamatergic neurotransmission, such as deficient Mg2+ block of NMDA-receptor ionophore complexes, and phenomena such as spreading depression, which requires activation of glutamate receptors and is detrimental to neurons in damaged/vulnerable brain regions. Finally, we introduce the notion that beneficial effects of glutamate receptor antagonists in experimental models of neurological disorders do not necessarily imply the occurrence of excitotoxic processes. Indeed, glutamate-receptor blockade may be protective by reducing the energy demand required to counterbalance Na+ influx associated with glutamatergic synaptic transmission. In other words, glutamate receptor antagonists (and blockers of voltage-gated Na+-channels) may help nervous tissue to cope with increased permeability of the cellular membrane to ions and reduced efficacy of Na+ extrusion, and thus prevent the decay of transmembrane ionic concentrations gradients.

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