Brain high-energy phosphates and creatine kinase synthesis rate under graded isoflurane anesthesia: An in vivo (31) P magnetization transfer study at 11.7 tesla.

PURPOSE: The creatine kinase rate of metabolic adenosine triphosphate (ATP) synthesis is an important metabolic parameter but is challenging to measure in vivo due to limited signal-to-noise ratio and long measurement time. THEORY AND METHODS: This study reports the implementation of an accelerated (31) P Four Angle Saturation Transfer (FAST) method to measure the forward creatine kinase (CK) rate of ATP synthesis. Along with a high-field scanner (11.7 Tesla) and a small sensitive surface coil, the forward CK rate in the rat brain was measured in ∼5 min. RESULTS: Under 1.2% isoflurane, the forward CK rate constant and metabolic flux were, respectively, kf , CK =0.26 ± 0.02 s(-1) and Ff,CK =70.8 ± 4.6 µmol/g/min. As a demonstration of utility and sensitivity, measurements were made under graded isoflurane. Under 2.0% isoflurane, kf , CK =0.16 ± 0.02 s(-1) and Ff,CK =410.0 ± 4.2 µmol/g/min, corresponding to a 38% and 42% reduction, respectively, relative to 1.2% isoflurane. By contrast, the ATP and phosphocreatine concentrations were unaltered. CONCLUSION: This study demonstrated the (31) P FAST measurement of creatine kinase rate of ATP synthesis in rat brain with reasonable temporal resolution. Different isoflurane levels commonly used in animal models significantly alter the CK reaction rate but not ATP and phosphocreatine concentrations.

Quantitative cerebral blood flow measurements using MRI.

Magnetic resonance imaging can be utilized as a quantitative and noninvasive method to image cerebral blood flow. The two most common techniques used to detect cerebral blood flow are dynamic susceptibility contrast (DSC) perfusion MRI and arterial spin labeling perfusion MRI. Herein we describe the use of these two techniques to measure cerebral blood flow in rodents, including methods, analysis, and important considerations when utilizing these techniques.

Decreased in vitro mitochondrial function is associated with enhanced brain metabolism, blood flow, and memory in Surf1-deficient mice.

Recent studies have challenged the prevailing view that reduced mitochondrial function and increased oxidative stress are correlated with reduced longevity. Mice carrying a homozygous knockout (KO) of the Surf1 gene showed a significant decrease in mitochondrial electron transport chain Complex IV activity, yet displayed increased lifespan and reduced brain damage after excitotoxic insults. In the present study, we examined brain metabolism, brain hemodynamics, and memory of Surf1 KO mice using in vitro measures of mitochondrial function, in vivo neuroimaging, and behavioral testing. We show that decreased respiration and increased generation of hydrogen peroxide in isolated Surf1 KO brain mitochondria are associated with increased brain glucose metabolism, cerebral blood flow, and lactate levels, and with enhanced memory in Surf1 KO mice. These metabolic and functional changes in Surf1 KO brains were accompanied by higher levels of hypoxia-inducible factor 1 alpha, and by increases in the activated form of cyclic AMP response element-binding factor, which is integral to memory formation. These findings suggest that Surf1 deficiency-induced metabolic alterations may have positive effects on brain function. Exploring the relationship between mitochondrial activity, oxidative stress, and brain function will enhance our understanding of cognitive aging and of age-related neurologic disorders.

Methylene blue as a cerebral metabolic and hemodynamic enhancer.

By restoring mitochondrial function, methylene blue (MB) is an effective neuroprotectant in many neurological disorders (e.g., Parkinson's and Alzheimer's diseases). MB has also been proposed as a brain metabolic enhancer because of its action on mitochondrial cytochrome c oxidase. We used in vitro and in vivo approaches to determine how MB affects brain metabolism and hemodynamics. For in vitro, we evaluated the effect of MB on brain mitochondrial function, oxygen consumption, and glucose uptake. For in vivo, we applied neuroimaging and intravenous measurements to determine MB's effect on glucose uptake, cerebral blood flow (CBF), and cerebral metabolic rate of oxygen (CMRO(2)) under normoxic and hypoxic conditions in rats. MB significantly increases mitochondrial complex I-III activity in isolated mitochondria and enhances oxygen consumption and glucose uptake in HT-22 cells. Using positron emission tomography and magnetic resonance imaging (MRI), we observed significant increases in brain glucose uptake, CBF, and CMRO(2) under both normoxic and hypoxic conditions. Further, MRI revealed that MB dramatically increased CBF in the hippocampus and in the cingulate, motor, and frontoparietal cortices, areas of the brain affected by Alzheimer's and Parkinson's diseases. Our results suggest that MB can enhance brain metabolism and hemodynamics, and multimetric neuroimaging systems offer a noninvasive, nondestructive way to evaluate treatment efficacy.