Abbreviations used: BBB, blood-brain barrier; CMR ace , cerebral metabolic rate of acetate; NAA, N-acetyl-aspartate; PC, pyruvate carboxylase; TCA, tricarboxylic acid. AbstractAcetate, a glial-specific substrate, is an attractive alternative to glucose for the study of neuronal-glial interactions. The present study investigates the kinetics of acetate uptake and utilization in the rat brain in vivo during infusion of [2-13 C]acetate using NMR spectroscopy. When plasma acetate concentration was increased, the rate of brain acetate utilization (CMR ace ) increased progressively and reached close to saturation for plasma acetate concentration > 2-3 mM, whereas brain acetate concentration continued to increase. The Michaelis-Menten constant for brain acetate utilization (K util M = 0.01 ± 0.14 mM) was much smaller than for acetate transport through the blood-brain barrier (BBB) (K t M = 4.18 ± 0.83 mM). The maximum transport capacity of acetate through the BBB (V t max = 0.96 ± 0.18 lmol/g/min) was nearly twofold higher than the maximum rate of brain acetate utilization (V util max = 0.50 ± 0.08 lmol/g/min). We conclude that, under our experimental conditions, brain acetate utilization is saturated when plasma acetate concentrations increase above 2-3 mM. At such high plasma acetate concentration, the rate-limiting step for glial acetate metabolism is not the BBB, but occurs after entry of acetate into the brain.

Recent studies with a conditional mouse model of spinocerebellar ataxia type 1 (SCA1) suggest that neuronal dysfunction is reversible and neurodegeneration preventable with early interventions. Success of such interventions will depend on early detection of neuronal and glial abnormalities before cell loss and availability of objective methods to monitor progressive neurodegeneration. Cerebellar concentrations of N-acetylaspartate (NAA), myo-inositol, and glutamate as measured by magnetic resonance spectroscopy (MRS) correlate with ataxia scores of patients with SCA1, indicating their potential as reliable biomarkers of neurodegeneration. Here we investigated whether neurochemical levels are altered by early, presymptomatic disease and whether they gauge disease progression in a mouse model of SCA1. Cerebellar neurochemical profiles of transgenic mice that overexpress the mutant human ataxin-1 (the SCA1[82Q] line) were measured longitudinally up to 1 year by MRS at 9.4 T and compared to those of transgenic mice that overexpress the normal human ataxin-1 (the SCA1[30Q] line) and wild-type controls. Multiple neurochemicals distinguished the SCA1[82Q] mice from controls with no overlap at all ages. Six neurochemicals were significantly different in SCA1[82Q] mice at 6 weeks, before major pathological and neurological changes. Alterations in NAA, myo-inositol, and glutamate progressively worsened and were significantly correlated ( p Ͻ 0.0001) with disease progression as assessed by histology (molecular layer thickness and an overall severity score). Therefore, the neurochemicals that correlate with clinical status in patients reflected progressive pathology in the mouse model. These data demonstrate that presymptomatic and progressive neurodegeneration in SCA1 can be noninvasively monitored using MRS.

All13 C NMR studies of brain glycogen to date relied on observing the incorporation of 13 C label into glycogen, and thus interpretation was potentially affected by changes in 13 C label turnover rates. The goal of this study was to quantify total brain glycogen concentration under conditions of hypoglycemia or normoglycemia using biochemical methods. Rats were sacrificed using a focused microwave fixation device. The results showed that metabolism of brain glycogen was Glc-and insulin-sensitive and that insulin-induced hypoglycemia promoted a gradual glycogenolysis. Moreover, we show that there are very mild effects of isoflurane and a-chloralose anesthesia on brain glycogen concentration. Altogether these results show that total brain glycogen serves as a substantial source of glucosyl units during insulin-induced moderate hypoglycemia and therefore may be neuroprotective. Finally we also conclude that previous interpretation of 13 C NMR spectroscopy data accurately reflected the changes in total brain glycogen content. #