Does deoxyglucose uptake in the brain reflect energy metabolism?

Novel PET Imaging of Atherosclerosis with 68Ga-Labeled NOTA-Neomannosylated Human Serum Albumin

Activated macrophages take up ¹⁸F-FDG via glucose transporters, so this compound is useful for atherosclerosis imaging by PET. However, ¹⁸F-FDG application is limited for imaging of the heart and brain, in which glucose uptake is high, and in patients with aberrant glucose metabolism. The aims of this study were to confirm that mannosylated human serum albumin (MSA) specifically binds to the mannose receptor (MR) on macrophages and to test the feasibility of ⁶⁸Ga-labeled NOTA-MSA for PET imaging of atherosclerotic plaques.

METHODS

The peritoneal macrophages of C57/B6 mice were collected, incubated with rhodamine B isothiocyanate-MSA (10 μg/mL), and evaluated by confocal microscopy and flow cytometry. The same evaluations were performed after preincubation of the macrophages with anti-CD206 MR blocking antibodies. NOTA-MSA was synthesized by conjugating 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid to MSA, followed by labeling with ⁶⁸Ga. Rabbits with atherosclerotic aorta induced by a 3-mo cholesterol diet and chronic inflammation underwent consecutive PET/CT with ¹⁸F-FDG and ⁶⁸Ga-NOTA-MSA at 2-d intervals.

RESULTS

The binding of MSA to MR and its dose-dependent reduction by preincubation with anti-CD206 MR blocking antibody were confirmed. Rhodamine B isothiocyanate and fluorescein isothiocyanate fluorescence colocalized at the atherosclerotic plaque. The ⁶⁸Ga-NOTA-MSA SUVs of the atherosclerotic aorta were significantly higher than those of the healthy arteries and inferior vena cava and were comparable to those obtained with ¹⁸F-FDG.

CONCLUSION

These findings suggest that MR-specific ⁶⁸Ga-NOTA-MSA is effective for detecting atherosclerosis in the aorta and is a promising radiopharmaceutical for imaging atherosclerosis because of the presence of M2 macrophages in atherosclerotic plaques.

Sodium MRI of glioma in animal models at ultrahigh magnetic fields

High magnetic fields expand our capability to use sodium MRI for biomedical applications. The central goal of this review is devoted to the unique features of sodium MRI in tumor animal models, mainly in glioma, performed at 9.4 and 21.1 T. The ability of sodium MRI to monitor tumor response to therapy was evaluated. It is noteworthy that sodium MRI can detect glioma response to chemotherapy earlier than diffusion MRI. Especially attractive is the ability of sodium MRI to predict tumor therapeutic resistance before therapy. The non-invasive prediction of tumor chemo-resistance by sodium MRI presents a potential to individualize strategies for cancer treatment. Specifics of sodium MRI and technical aspects of imaging are also presented.

5'-AMP-activated protein kinase increases glucose uptake independent of GLUT4 translocation in cardiac myocytes

Glucose uptake and glycolysis are increased in the heart during ischemia, and this metabolic alteration constitutes an important contributing factor towards ischemic injury. Therefore, it is important to understand glucose uptake regulation in the ischemic heart. There are primarily 2 glucose transporters controlling glucose uptake into cardiac myocytes: GLUT1 and GLUT4. In the non-ischemic heart, insulin stimulates GLUT4 translocation to the sarcolemmal membrane, while both GLUT1 and GLUT4 translocation can occur following AMPK stimulation. Using a newly developed technique involving [(3)H]2-deoxyglucose, we measured glucose uptake in H9c2 ventricular myoblasts, and demonstrated that while insulin has no detectable effect on glucose uptake, phenformin-induced AMPK activation increases glucose uptake 2.5-fold. Furthermore, insulin treatment produced no discernible effect on either Akt serine 473 phosphorylation or AMPKα threonine 172 phosphorylation, while treatment with phenformin results in an increase in AMPKα threonine 172 phosphorylation, and a decrease in Akt serine 473 phosphorylation. Visualization of a dsRed-GLUT4 fusion construct in H9c2 cells by laser confocal microscopy showed that unlike insulin, AMPK activation did not redistribute GLUT4 to the sarcolemmal membrane, suggesting that AMPK may regulate glucose uptake via another glucose transporter. These studies suggest that AMPK is a major regulator of glucose uptake in cardiac myocytes.

Kinetic quantitation of cerebral PET-FDG studies without concurrent blood sampling: statistical recovery of the arterial input function

Kinetic quantitation of dynamic positron emission tomography (PET) studies via compartmental modeling usually requires the time-course of the radio-tracer concentration in the arterial blood as an arterial input function (AIF). For human and animal imaging applications, significant practical difficulties are associated with direct arterial sampling and as a result there is substantial interest in alternative methods that require no blood sampling at the time of the study. A fixed population template input function derived from prior experience with directly sampled arterial curves is one possibility. Image-based extraction, including requisite adjustment for spillover and recovery, is another approach. The present work considers a hybrid statistical approach based on a penalty formulation in which the information derived from a priori studies is combined in a Bayesian manner with information contained in the sampled image data in order to obtain an input function estimate. The absolute scaling of the input is achieved by an empirical calibration equation involving the injected dose together with the subject's weight, height and gender. The technique is illustrated in the context of (18)F -Fluorodeoxyglucose (FDG) PET studies in humans. A collection of 79 arterially sampled FDG blood curves are used as a basis for a priori characterization of input function variability, including scaling characteristics. Data from a series of 12 dynamic cerebral FDG PET studies in normal subjects are used to evaluate the performance of the penalty-based AIF estimation technique. The focus of evaluations is on quantitation of FDG kinetics over a set of 10 regional brain structures. As well as the new method, a fixed population template AIF and a direct AIF estimate based on segmentation are also considered. Kinetics analyses resulting from these three AIFs are compared with those resulting from radially sampled AIFs. The proposed penalty-based AIF extraction method is found to achieve significant improvements over the fixed template and the segmentation methods. As well as achieving acceptable kinetic parameter accuracy, the quality of fit of the region of interest (ROI) time-course data based on the extracted AIF, matches results based on arterially sampled AIFs. In comparison, significant deviation in the estimation of FDG flux and degradation in ROI data fit are found with the template and segmentation methods. The proposed AIF extraction method is recommended for practical use.

Nonparametric Residue Analysis of Dynamic PET Data With Application to Cerebral FDG Studies in Normals

Kinetic analysis is used to extract metabolic information from dynamic positron emission tomography (PET) uptake data. The theory of indicator dilutions, developed in the seminal work of Meier and Zierler (1954), provides a probabilistic framework for representation of PET tracer uptake data in terms of a convolution between an arterial input function and a tissue residue. The residue is a scaled survival function associated with tracer residence in the tissue. Nonparametric inference for the residue, a deconvolution problem, provides a novel approach to kinetic analysis-critically one that is not reliant on specific compartmental modeling assumptions. A practical computational technique based on regularized cubic B-spline approximation of the residence time distribution is proposed. Nonparametric residue analysis allows formal statistical evaluation of specific parametric models to be considered. This analysis needs to properly account for the increased flexibility of the nonparametric estimator. The methodology is illustrated using data from a series of cerebral studies with PET and fluorodeoxyglucose (FDG) in normal subjects. Comparisons are made between key functionals of the residue, tracer flux, flow, etc., resulting from a parametric (the standard two-compartment of Phelps et al. 1979) and a nonparametric analysis. Strong statistical evidence against the compartment model is found. Primarily these differences relate to the representation of the early temporal structure of the tracer residence-largely a function of the vascular supply network. There are convincing physiological arguments against the representations implied by the compartmental approach but this is the first time that a rigorous statistical confirmation using PET data has been reported. The compartmental analysis produces suspect values for flow but, notably, the impact on the metabolic flux, though statistically significant, is limited to deviations on the order of 3%-4%. The general advantage of the nonparametric residue analysis is the ability to provide a valid kinetic quantitation in the context of studies where there may be heterogeneity or other uncertainty about the accuracy of a compartmental model approximation of the tissue residue.

Human brain glucose metabolism may evolve during activation: findings from a modified FDG PET paradigm

In human brain, short-term physiological stimulation results in dramatic and proportional increase in blood flow and metabolic rate of glucose but minimal change in oxygen utilization, however, with continuing stimulation, we have observed that blood flow response diminishes and oxygen utilization increases. Given the temporal limitation of conventional methods to measure glucose metabolism in the human brain, we modified [(18)F]fluorodeoxyglucose (FDG) PET paradigm to evaluate the short-term and long-term effects of visual stimulation on human brain glucose metabolism. In the present study, seven healthy volunteers each underwent three dynamic FDG PET studies: at rest and after 1 min and 15 min of visual stimulation (using reversing black-white checkerboard) which continued for only 5 min after FDG injection. We found that increase in FDG uptake in the visual cortex was attenuated by 28% when preceded by 15 min of continuous visual stimulation (p<0.001). This decline in metabolism occurred in the absence of any behavior changes in task performance. The similarity in behavior of blood flow and glucose metabolism over time supports the hypothesis that, in activated brain, blood flow is modulated by changes in cytosolic free NADH/NAD(+) ratio related to increased glycolysis. Furthermore, the observed decline in glucose metabolism may reflect a shift from glycolytic to oxidative glucose metabolism with continued activation.

A revised mathematical model of cerebral microischemia

Cerebral microinfarcts (lacunes) are considered to cause dementia. Experimental studies of this kind of infarct may bring us closer to an understanding of, and thus, a treatment for, cerebral infarction. However, the study of experimental cerebral microischemia is complex because of the high resolution needed. Comparing experimental with theoretical results should improve our knowledge of the subject. In the present work, a theoretical model of cerebral microischemia was improved and some results are presented. This study confirms that the glucose supply in some cases limits the energy turnover. Furthermore, the results show that increasing the oxygen supply (increasing Khem, and increasing the partial pressure of oxygen in blood) should at maximum only improve the energy turnover by about 5%. The present model may be an aid in the interpretation of experimental results and may be used to predict benefits of various treatments.

Developmental regulation of genes mediating murine brain glucose uptake

We examined the molecular mechanisms that mediate the developmental increase in murine whole brain 2-deoxyglucose uptake. Northern and Western blot analyses revealed an age-dependent increase in brain GLUT-1 (endothelial cell and glial) and GLUT-3 (neuronal) membrane-spanning facilitative glucose transporter mRNA and protein concentrations. Nuclear run-on experiments revealed that these developmental changes in GLUT-1 and -3 were regulated posttranscriptionally. In contrast, the mRNA and protein levels of the mitochondrially bound glucose phosphorylating hexokinase I enzyme were unaltered. However, hexokinase I enzyme activity increased in an age-dependent manner suggestive of a posttranslational modification that is necessary for enzymatic activation. Together, the postnatal increase in GLUT-1 and -3 concentrations and hexokinase I enzymatic activity led to a parallel increase in murine brain 2-deoxyglucose uptake. Whereas the molecular mechanisms regulating the increase in the three different gene products may vary, the age-dependent increase of all three constituents appears essential for meeting the increasing demand of the maturing brain to fuel the processes of cellular growth, differentiation, and neurotransmission.

A model for the coupling between cerebral blood flow and oxygen metabolism during neural stimulation

A general mathematical model for the delivery of O2 to the brain is presented, based on the assumptions that all of the brain capillaries are perfused at rest and that all of the oxygen extracted from the capillaries is metabolized. The model predicts that disproportionately large changes in blood flow are required in order to support small changes in the O2 metabolic rate. Interpreted in terms of this model, previous positron emission tomography (PET) studies of the human brain during neural stimulation demonstrating that cerebral blood flow (CBF) increases much more than the oxygen metabolic rate are consistent with tight coupling of flow and oxidative metabolism. The model provides a basis for the quantitative interpretation of functional magnetic resonance imaging (fMRI) studies in terms of changes in local CBF.

Effect of TAK-147, a novel AChE inhibitor, on cerebral energy metabolism

Effect of TAK-147, a novel acetylcholinesterase (AChE) inhibitor, on cerebral energy metabolism was investigated using an in vivo 31P-magnetic resonance spectroscopy (31P-MRS) technique and the autoradiographic 2-deoxy-[14C]-D-glucose method in aged Fischer 344 rats. We revealed that high-energy phosphate metabolites, phosphocreatine (PCr) and ATP, in the brain decreased gradually with aging and that significant decrement of cerebral PCr and ATP was observed from 13- and 8.5-month-old in comparison with those of 2.5-month-old rats, respectively. Daily oral administration of TAK-147 (1 mg/kg) for 40 days increased PCr and ATP levels in aged rats (29-month-old). To determine the site at which TAK-147 acts to increase high-energy phosphate metabolism, we investigated the rate of local cerebral glucose utilization (LCGU) in various brain regions. The rate of LCGU decreased in almost all brain regions in aged rats (28 months of age), and the decrease was significant in 29 out of the 35 regions. When TAK-147 was administered orally to the aged rats, the levels were dose dependently increased, especially in the auditory cortex. These results indicate that TAK-147 increases cerebral energy metabolism in aged rats.

Cerebral transport and metabolism of 1-11C-D-glucose during stepped hypoglycemia

Attempts to measure blood-to-brain glucose transport and cerebral glucose metabolism with 11C-glucose have been hampered by methods that require jugular venous sampling or do not adequately account for the efflux of labeled metabolites from the brain. We performed eight positron emission tomography studies with 1-11C-D-glucose in macaques at arterial plasma glucose concentrations of 8.43 to 1.51 mumol ml-1 (152-27 mg dl-1) using a model that includes a fourth rate constant to account for regional egress of all 11C-metabolites. Values for blood-to-brain glucose influx, cerebral glucose metabolism, and brain free glucose concentration agreed closely with values obtained in mammals by other investigators. Values for net extraction fraction corresponded closely to simultaneously measured arteriovenous values. We demonstrated that utilization of a model that includes a fourth rate constant to account for regional egress of all 11C-metabolites with positron emission tomography and 1-11C-D-glucose provides accurate measurements of blood-to-brain glucose transport and cerebral glucose metabolism in vivo without need for jugular venous sampling, even under conditions of severe hypoglycemia.

Improved synthesis of 1-[11C]D-glucose

An improved synthesis of 1-[11C]D-glucose is described. The major improvement is achieved when a 0.033 M borate buffer at pH 8.1 is used to effect the condensation of d-arabinose with NH4(11)CN. Subsequent reduction of the 1-[11C]D-aldonitriles gives the epimeric sugars 1-[11C]D-glucose and 1-[11C]D-mannose in a ratio of 1.8 +/- 0.57 as the major products. The decay corrected radiochemical yield is about 30% for the mixture of sugars. The overall synthesis, starting with the production of NH4(11)CN, is conducted in a dedicated remote system. The remote gantry was easy to build with commercially available valves and glassware, and has been practically trouble-free after more than 2 years of use. Improved purification and quality control of the final product uses ion chromatography and a more efficient resin, and is also described. A preliminary PET study on a macaque has been conducted using 1-[11C]D-glucose obtained with this new improved synthesis.

Glucose utilization rates in single neurons and neuropil determined by injecting nontracer amounts of 2-deoxyglucose

A nontracer amount of 2-deoxyglucose (DG) was intravenously injected into rats, which were frozen 2 and 4 min later in liquid nitrogen. The freeze-dried samples of cell bodies of anterior horn cells, dorsal root ganglion cells, and cerebellar Purkinje cells, as well as the neuropil adjacent to anterior horn cell bodies, were prepared. Their contents of glucose, glucose 6-phosphate, DG, and 2-deoxyglucose 6-phosphate were microassayed using an enzymatic amplification reaction. NADP cycling. Based on the resulting data and theoretical equations previously described, glucose utilization rate (GUR) and apparent distribution volumes (DVs) of glucose and DG were determined. Anterior horn cell bodies had the highest GUR and their neuropil the lowest, although apparent DVs of glucose and DG were similar in both. This indicates that the glucose supply was equally balanced in all, but that the cell bodies had higher functional activity supported by hexokinase (and other enzymes) related to their energy demands. Dorsal root ganglion cells showed the lowest 2-deoxyglucose 6-phosphate formation rate, but their GUR was slightly higher than that of neuropil because of their markedly large DV of glucose, thus demonstrating that the abundant glucose supply supports the neuronal function. Purkinje cells indicated GUR and apparent DVs similar to molecular and granular layers.

Effects of 1,3-di-o-tolylguanidine (DTG), a sigma ligand, on local cerebral glucose utilization in rat brain

The 2-deoxy-D-[1-14C]glucose ([14C]DG) method was used to examine the effects of the relatively selective sigma ligand 1,3-di-o-tolylguanidine (DTG) on cerebral metabolism in freely moving rats. Each animal received an i.p. injection of DTG (0.2, 1, or 5 mg/kg) or normal saline 20 min prior to the infusion of [14C]DG. DTG induced dose-dependent changes in local cerebral glucose utilization (LCGU) in several motor and limbic structures. Most structures showed increases in LCGU, with a maximum effect at 1 mg/kg. The most profound increases in LCGU were observed in brain regions that are rich in sigma receptors. These included cerebellar and related nuclei (interpositus, lateral and medial cerebellar n., vestibular n., olivary n.), ambiguus n., superior colliculus (superior layers), hippocampus (CA2, CA3, DG), n. basalis of Meynert interpeduncular n., and the substantia nigra pars compacta and pars reticulata. No significant decreases in glucose utilization were observed at any dose. Although the areas affected by DTG are similar to those previously reported for other sigma ligands, future studies employing a range of doses for additional selective sigma ligands must be carried out in order to confirm whether these changes in LCGU were sigma-mediated.

Learning-induced change in neural activity during acquisition and consolidation of a passive avoidance response in the rat

Time-dependent alterations in neural activity have been established during the acquisition and consolidation of a stepdown passive avoidance paradigm. Change in neural activity was established by administering a glucose analogue, [3H]2-deoxyglucose, 50min prior to sacrifice and estimating perchloric acid soluble counts in nine hand dissected brain regions. Change in [3H]2-deoxyglucose uptake was closely paralleled in both trained and yoked animals for up to 40min following task acquisition however the striatum was the only area to exhibit a task-specific increase in [3H]2-deoxyglucose uptake at 20-30min after training. Longterm changes in neural activity were also apparent as the amygdala and brainstem showed increased [3H]2-deoxyglucose uptake at the 24 h time point. No further paradigm-specific changes were apparent at 48 h. These findings are concluded to suggest that the striatum is involved in the early events of acquiring a passive avoidance response and the amygdala and brainstem during the later events.

Measurement of cerebral glucose utilization from brain uptake of [14C]2-deoxyglucose and [3H]3-O-methylglucose in the mouse

Glucose utilization (GU) in the mouse brain in vivo was measured by the simultaneous use of [14C]2-deoxyglucose (2DG), the glucose analogue that can be phosphorylated in the brain, and [3H]3-O-methylglucose (3MG), the nonmetabolizable glucose analogue. Originally, this method was developed by Gjedde et al. (1985) in the rat and in humans. The present study examined the validity of this method in the mouse brain. The effects of urethane and pentobarbital (PB) on GU were also studied. Whereas the distribution volume of 3MG reached a constant value under each condition after 10 min of the tracer circulation, the apparent volume of distribution of 2DG, which increased with time in the awake mice, did not increase so greatly in the anesthetized and hypothermic mice, indicating that the net rate of 2DG phosphorylation is lowered under these conditions. These data were fitted for the conventional three-compartmental model and the values of rate constants of influx (K1), efflux (k2), phosphorylation, and dephosphorylation for 2DG, and K1 and k2 for 3MG were computed by nonlinear least square regression method. No significant difference in the value of K1/k2, the distribution volume of the precursor pool, was observed between 2DG and 3MG, indicating that 3MG can be used to estimate the distribution volume of unmetabolized 2DG in the brain. The values of GU calculated from the values of estimated parameters were not significantly different from those calculated from blood and tissue radioactivities obtained during 10 min after the injection of the tracers in both awake and PB-anesthetized mice. These findings indicate that the double tracer technique is useful for measuring GU in a short duration experiment.

Glucose phosphorylation rate in rat parietal cortex during normoglycemia, hypoglycemia, acute hyperglycemia, and in diabetes-prone rats

Cerebral metabolic rate for glucose (CMRglc) was studied in rats using [6-14C]glucose. After intravenous injection, the radioactivity of the parietal cortex was corrected for loss of labeled CO2 and divided by the integral of the arterial plasma glucose concentration, determined during tracer circulation. Treatment with insulin, resulting in plasma glucose concentrations less than 2.6 mmol/l, reduced CMRglc to 64% of the values found in control animals. CMRglc did not change in animals with acute hyperglycemia produced by intraperitoneal injection of a glucose solution or in diabetes-prone rats with or without insulin treatment.

Role of dialysable solutes in the mediation of uremic encephalopathy in the rat

This study addresses mechanisms of the clinical, encephalopathic uremic illness and its suppression by dialysis. Renoprival rats were treated with peritoneal dialysis (8 exchanges per day, 30 min dwell), or untreated (attrition group), and their EEG's were automatically sampled overnight and subjected to power spectrum analysis as an index of encephalopathy. As in man the background rhythm of the quantified EEG (Q.EEG) in the attrition group slowed with time as extracellular fluid composition became increasingly abnormal; these changes were normalized by therapeutic dialysis (TD) using standard, commercial dialysate. However, Q.EEG slowing was only partially normalized by solute-specific dialysis using "mock uremic dialysate" (M-UD), prepared from laboratory chemicals to equal plasma concentrations in preterminal uremic rats of urea, creatinine, potassium, phosphorus, calcium, magnesium, bicarbonate, sodium, and chloride. When only phosphate was added to TD, the Q.EEG slowed to the same level achieved after M-UD. We conclude that uremic encephalopathy in this model is produced by an unknown neurotoxin and augmented by one or more of the M-UD solutes, phosphate being a likely candidate. To localize the encephalopathic effect, regional brain glucose uptake was estimated in 20 discrete brain areas. Significance of reduced uptake in three areas is discussed.

Michaelis-Menten constraints improved cerebral glucose metabolism and regional lumped constant measurements with [18F]fluorodeoxyglucose

In the three-compartment model of transfer of native glucose and [18F]fluorodeoxyglucose (FDG) into brain, both transport across the blood-brain barrier and phosphorylation by hexokinase can be described by the Michaelis-Menten equation. This permits the use of fixed transport (tau = K*1/K1) and phosphorylation (psi = k*3/k3) ratios and a common partition volume (Ve = K1/k2) for tracer and glucose. By substituting transfer constants of FDG for those of glucose, using tau and psi, the lumped constant was determined directly by positron tomography. The same constraints also eliminated k*2 and k*3 from the model, thus limiting the parameters to K* [equivalent to K*1k*3/(k*2 + k*3)], K*1, and the cerebral vascular volume (Vo). In six healthy elderly men (aged 61 +/- 5 years), time-activity records of cerebral cortical regions were analyzed with tau = 1.1 and psi = 0.3. The results were compared with those of the conventional FDG method. At 20 min, the goodness of fit by the new equation was as good as that of the conventional method at 45 min. The estimates obtained by the constrained method had stable coefficients of variation. After 20 min, regional differences between the estimates were independent of time, although we observed steady decreases of K* and (k*3). The decrease strongly suggested dephosphorylation of FDG-6-phosphate, particularly after 20 min. All estimates of variables with the constrained method were more accurate than those of the conventional method, including the cerebral glucose metabolic rate itself, as well as physiologically more meaningful, particularly with respect to k*2 and k*3.

Striatal dopamine in motor activation and reward-mediated learning: steps towards a unifying model

On the basis of behavioural evidence, dopamine is found to be involved in two higher-level functions of the brain: reward-mediated learning and motor activation. In these functions dopamine appears to mediate synaptic enhancement in the corticostriatal pathway. However, in electrophysiological studies, dopamine is often reported to inhibit corticostriatal transmission. These two effects of dopamine seem incompatible. The existence of separate populations of dopamine receptors, differentially modulating cholinergic and glutamatergic synapses, suggests a possible resolution to this paradox. The synaptic enhancement which occurs in reward-mediated learning may also be involved in dopamine-mediated motor activation. The logical form of reward-mediated learning imposes constraints on which mechanisms can be considered possible. Dopamine D1 receptors may mediate enhancement of corticostriatal synapses. On the other hand, dopamine D2 receptors on cholinergic terminals may mediate indirect, inhibitory effects of dopamine on striatal neurons.

Local cerebral glucose utilization during status epilepticus in newborn primates

The effect of bicuculline-induced status epilepticus (SE) on local cerebral metabolic rates for glucose (LCMRglc) was studied in 2-wk-old ketamine-anesthetized marmoset monkeys, using the 2-[14C]-deoxy-D-glucose autoradiographical technique. To estimate LCMRglc in cerebral cortex and thalamus during SE, the lumped constant (LC) for 2-deoxy-D-glucose (2-DG) and the rate constants for 2-DG and glucose were calculated for these regions. The control LC was 0.43 in frontoparietal cortex, 0.51 in temporal cortex, and 0.50 in thalamus; it increased to 1.07 in frontoparietal cortex, 1.13 in temporal cortex, and 1.25 in thalamus after 30 min of seizures. With control LC values, LCMRglc in frontoparietal cortex, temporal cortex, and dorsomedial thalamus appeared to increase four to sixfold. With seizure LC values, LCMRglc increased 1.5- to 2-fold and only in cortex. During 45-min seizures, LCMRglc in cortex and thalamus probably increases 4- to 6-fold initially and later falls to the 1.5- to 2-fold level as tissue glucose concentrations decrease. Together with our previous results demonstrating depletion of high-energy phosphates and glucose in these regions, the data suggest that energy demands exceed glucose supply. The long-term effects of these metabolic changes on the developing brain remain to be determined.

The deoxyglucose method in the ferret brain. I. Methodological considerations

In the brain of the anesthetized ferret, the 2-deoxyglucose (2-DG) transfer rate constants required to determine cerebral glucose utilization by the deoxyglucose method were calculated from regional gray matter time-radioactivity curves measured for 180 min after tracer injection. Results suggest that loss of metabolized tracer from brain occurs at a rate of about 1%/min for the first 180 min after injection if the rate constant of the rate-limiting step for loss of metabolized tracer (k4*) represents a first-order kinetic process. A simulation experiment shows that, whether k4* is assumed to be 0 or 0.01 min-1, has a negligible influence on glucose utilization rates obtained in conventional 45 min autoradiographic experiments provided that the entire analysis, including lumped constant determination, is carried out in a consistent way. The 2-DG lumped constant for k4* = 0 is 0.54, and 0.68 for k4* = 0.01 min-1.

The deoxyglucose method in the ferret brain. II. Glucose utilization images and normal values

To measure cerebral glucose utilization with the autoradiographic deoxyglucose method, the tracer transfer rate constants and lumped constants must be known. 2-Deoxyglucose (2-DG) and fluorodeoxyglucose (FDG) constants were determined in 18 gray and white matter brain structures of the anesthetized ferret. The ferret is a domestic carnivore particularly suitable for deoxyglucose studies because of its small brain size and low body weight. The average gray matter rate constants for tracer transfer across the blood-brain barrier are similar for 2-DG and FDG in the ferret brain (K*1 = 0.21 ml/g/min and k*2 = 0.39 min-1). The rate constant for the rate-limiting step of tracer phosphorylation, k*3, is 1.6 times higher for FDG than for 2-DG (0.21 vs. 0.13 min-1). Loss of metabolized tracer is about 1-1.5%/min throughout the ferret brain for both tracers as estimated for a 180 min experimental period. Taking into account this loss, the lumped constant is 0.92 for FDG and 0.68 for 2-DG. Glucose utilization values in the brain of the anesthesized ferret range from 33 mumol/100 g/min in the corpus callosum to 104 mumol/100 g/min in the caudate nucleus. Representative glucose utilization images of coronal sections of the ferret brain are shown. Brain structures are identified on the same slices counterstained with thionin.

Local cerebral glucose utilization in the autoimmune New Zealand black (NZB) mouse

By means of the [14C]-2-deoxyglucose method the local cerebral glucose utilization (LCGU) was measured in 41 brain regions in autoimmune New Zealand Black (NZB) mice and in Carworth Farm Winkelmann (CFW) mice, which served as the control strain. At the age of 6 months, the mean LCGU of all measured areas and brain stem nuclei was 67.7 mumol glucose/(100 g x min) in the nonautoimmune CFW mice. These LCGU values are within the limits published by other observers. In contrast, in the aged-matched NZB mice the glucose use was markedly reduced, the mean LCGU of all measured areas being 37.7 mumol glucose/(100 g x min). These findings suggest that the immunological, morphological and behavioural abnormalities in the aged NZB mouse correlate with a reduced functional activity of the central nervous system, measured as reduced cerebral glucose utilization.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

The determination of the local cerebral glucose utilization with the 2-deoxyglucose method

In the adult mammalian brain, the energy metabolism is almost entirely dependent on glucose. Furthermore, a close relationship between the energy metabolism and the functional activity could be shown. Thus, the functional activity of the brain or parts thereof can be quantified by measuring the cerebral metabolic rate for glucose. Studying in vivo the fate of a radioactive labeled analogue of glucose, the 2-deoxy-D-[1-14C]glucose, and using quantitative autoradiographic techniques, it is possible to estimate the cerebral glucose utilization of every discrete brain region. The advantage of the 2-deoxyglucose method is, that the local cerebral glucose utilization represents a "metabolic encephalography" (Sokoloff 1982).

Blood flow and metabolism in heterotopic cerebellar grafts during hypoglycemia

Hypoglycemia-induced disturbances of brain metabolism and neuronal injury exhibit a distinct predilection for forebrain structures, in particular the caudate-putamen, hippocampus and cerebral cortex, whereas the cerebellum is remarkably resistant. In an attempt to assess the biological basis of this differential regional vulnerability, we have used a neural transplantation technique to compare hemodynamic and metabolic changes in cerebellum during severe hypoglycemia with those in heterotopic cerebellar grafts. To this end, the cerebellar anlage of fetal rat brain (day 15 of gestation) was stereotactically transplanted into the vulnerable caudate-putamen. Following a differentiation period of 8 weeks the grafts had developed into an organotypic population of mature cells with laminar histoarchitecture. Host animals were then subjected to insulin-induced hypoglycemia. After 15 min of isoelectric EEG, blood flow was increased throughout the brain but residual glucose consumption was significantly higher in cerebellum (0.29 mumol/g per min) and cerebellar grafts (0.22 mumol/g per min) as a result of increased glucose extraction. Hypoglycemia caused a depletion of ATP in all brain structures except cerebellum where normal levels were maintained. Correlation of local ATP content and glucose utilization revealed a threshold-like decline of ATP at a glucose utilization rate of 0.27 mumol/g per min. ATP, in consequence, was normal in cerebellum but partially depleted in cerebellar grafts. It is concluded that the resistance of cerebellum to hypoglycemia is due to its capacity for higher glucose extraction at low blood glucose levels, and that this unique intrinsic property is preserved after heterotopic transplantation.