Hypo-osmotic swelling-activated release of organic osmolytes in brain slices: implications for brain oedema in vivo

High resolution 1H NMR-based metabolomics indicates a neurotransmitter cycling deficit in cerebral tissue from a mouse model of Batten disease

The neuronal ceroid lipofuscinoses (NCLs) constitute a range of progressive neurological disorders primarily affecting children. Although six of the causative genes have been characterized, the underlying disease pathogenesis for this family of disorders is unknown. Using a metabolomics approach based on high resolution 1H NMR spectroscopy of the cortex, cerebellum, and remaining regions of the brain in conjunction with statistical pattern recognition, we report metabolic deficits associated with juvenile NCL in a Cln3 knock-out mouse model. Tissue from Cln3 null mutant mice aged 1-6 months was characterized by an increased glutamate concentration and a decrease in -amino butyric acid (GABA) concentration in aqueous extracts from the three regions of the brain. These changes are consistent with the reported altered expression of genes involved in glutamate metabolism in older mice and imply a change in neurotransmitter cycling between glutamate/glutamine and the production of GABA. Further variations in myo-inositol, creatine, and N-acetyl-aspartate were also identified. These metabolic changes were distinct from the normal aging/developmental process. Together, these changes represent the first documented pre-symptomatic symptoms of the Cln3 mouse at 1 month of age and demonstrate the versatility of 1H NMR spectroscopy as a tool for phenotyping mouse models of disease.

NMDA-receptor mediated efflux of N-acetylaspartate: physiological and/or pathological importance?

N-Acetylaspartate (NAA) is a largely neuron specific dianionic amino acid present in high concentration in vertebrate brain. Many fundamental questions concerning N-acetylaspartate in brain remain unanswered. One such issue is the predominantly neuronal synthesis and largely glial catabolism which implies the existence of a regulated efflux from neurons. Here we show that transient (5 min) NMDA-receptor activation (60 microM) induces a long lasting Ca2+ -dependent efflux of N-acetylaspartate from organotypic slices of rat hippocampus. The NMDA-receptor stimulated efflux was unaffected by hyper-osmotic conditions (120 mM sucrose) and no efflux of N-acetylaspartate was evoked by high K+ -depolarization (50 mM) or kainate (300 microM). These results indicate that the efflux induced by NMDA is not related directly to either cell swelling or depolarization but is coupled to Ca2+ -influx via the NMDA-receptor. The efflux of N-acetylaspartate persisted at least 20 min after the omission of NMDA, similar to the efflux of the organic anions glutathione and phosphoethanolamine. The efflux of taurine and hypotaurine was also stimulated by NMDA but returned more quickly to basal levels. The NMDA-receptor stimulated efflux of N-acetylaspartate, glutathione, phosphoethanolamine, taurine and hypotaurine correlated with delayed nerve cell death measured 24 h after the transient NMDA-receptor stimulation. However, exogenous administration of high concentrations of N-acetylaspartate to the culture medium was non-toxic. The results suggest that Ca2+ -influx via the NMDA-receptor regulates the efflux of N-acetylaspartate from neurons which may have both physiological and pathological importance.

Magnetic resonance spectroscopy in cognitive research

The neurophysiological basis of cognition is relatively unexplained, with most studies reporting weak relationships between cognition and measures of brain function, such as event-related potentials, brain size and cerebral blood flow. Magnetic resonance spectroscopy (MRS) is an in vivo method used to detect neurochemicals within the brain that are relevant to certain brain processes. The most widely used methods are 1H-MRS and 31P-MRS, which detect compounds that contain hydrogen and phosphorus, respectively. Recent studies have shown that the absolute concentrations or ratios of these neurochemicals, in particular N-acetyl aspartate (NAA), which is associated with neuronal viability, correlate with performance on neuropsychological tests or other measures of cognitive function in normal subjects. Many studies in adults and children have shown a relationship between neurometabolite values and cognitive status or extent of cognitive dysfunction in various neurological and neuropsychiatric disorders. We review these studies and conclude that MRS has potential applications for the study of cognitive processes in health and disease and may be used clinically for differential diagnosis, the early detection of pathology and the examination of longitudinal change.

Taurine enhances volume regulation in hippocampal slices swollen osmotically

Cell volume regulation has been studied in neuronal and glial cultures but little is known about volume regulation in brain tissue with an intact extracellular space. We investigated volume regulation in hippocampal slices maintained in an interface chamber and exposed to hypo-osmotic medium. Relative changes in intracellular and extracellular volume were measured respectively as changes in light transmittance and extracellular resistance. Slices exposed to hypo-osmotic medium (200-240 mOsm/L) showed a decrease in light transmittance, which occasionally was preceded by a brief transient increase. However, hypo-osmotic exposure was always accompanied by a monotonic increase in extracellular resistance. Peak changes in light transmittance and extracellular resistance occurred at 15-20 min following exposure to hypo-osmotic medium. Optical evidence of volume regulation (RVD) was observed in six of 12 slices and occurred over the next 60-90 min. We hypothesized that the relatively low incidence of RVD was related to depletion of taurine, an osmolyte known to play an important role in volume regulation, during preparation of the slices. Indeed, taurine levels in freshly prepared slices were <50% of those reported in intact hippocampus. Incubation of slices in 1 mM taurine restored taurine to levels observed in situ and increased both the likelihood and magnitude of RVD in hypo-osmotic medium. Inhibition of taurine flux with 100 microM 5-nitro-2-(3 phenylpropylamino) benzoic acid blocked both RVD and the transient undershoot of volume commonly associated with return of swollen slices to iso-osmotic medium. Taurine treatment had no effect on levels of several other amino acids but preserved slice potassium content. The results indicate a critical role for cellular taurine during hypo-osmotic volume regulation in hippocampal slices. Inconsistencies between optical measurements of cellular volume changes and electrical measurements of extracellular space are likely to result from the complex nature of light transmittance in the interface slice preparation.

Developmental and regional distribution of aspartoacylase in rat brain tissue

The function of N-acetyl-aspartate (NAA), a predominant molecule in the brain, has not yet been determined. However, NAA is commonly used as a putative marker of viable neurones. To investigate the possible function of NAA, we determined the anatomical, developmental and cellular distribution of aspartoacylase, which catalyses the hydrolysis of NAA. Levels of aspartoacylase activity were measured during postnatal development in several brain regions. The differential distribution of aspartoacylase activity in purified populations of cells derived from the rat CNS was also investigated. The developmental and anatomical distribution of aspartoacylase correlated with the maturation of white matter tracts in the rat brain. Activity increased markedly after 7 days and coincided with the time course for the onset of myelination in the rat brain. Gray matter showed little activity or developmental trend. There was a 60-fold excess in optic nerve (a white matter tract) when compared with cortex at 21 days of development. In the adult brain there was a 18-fold difference in corpus callosum compared with cortex (stripped of corpus callosum). Cellular studies demonstrated that purified cortical neurons and cerebellar granular neurones have no activity. Primary O-2A progenitor cells had moderate activity, with three-fold higher activity in immature oligodendrocyte and 13-fold increase in mature oligodendrocytes (myelinating cells of the CNS). The highest activity was seen in type-2 astrocytes (20-fold difference compared with O-2A progenitors) derived from the same source. Aspartoacylase activity increased with time in freshly isolated astrocytes, with significantly higher activity after 15 days in culture. We conclude that aspartoacylase activity in the developing postnatal brain corresponds with maturation of myelination, and that the cellular distribution is limited to glial cells.