Regional comparisons of brain glucose influx

Elevated Cellular Uptake of Succinimide- and Glucose-Modified Liposomes for Blood-Brain Barrier Transfer and Glioblastoma Therapy

The uptake of four liposomal formulations was tested with the murine endothelial cell line bEnd.3 and the human glioblastoma cell line U-87 MG. All formulations were composed of DPPC, cholesterol, 5 mol% of mPEG (2000 Da, conjugated to DSPE), and the dye DiD. Three of the formulations had an additional PEG chain (nominally 5000 Da, conjugated to DSPE) with either succinimide (NHS), glucose (PEG-bound at C-6), or 4-aminophenyl β-D-glucopyranoside (bound at C-1) as ligands at the distal end. Measuring the uptake kinetics at 1 h and 3 h for liposomal incubation concentrations of 100 µM, 500 µM, and 1000 µM, we calculated the liposomal uptake saturation S and the saturation half-time t1/2. We show that only succinimide has an elevated uptake in bEnd.3 cells, which makes it a very promising and so far largely unexplored candidate for BBB transfer and brain cancer therapies. Half-times are uniform at low concentrations but diversify for high concentrations for bEnd.3 cells. Contrary, U-87 MG cells show almost identical saturations for all three ligands, making a uniform uptake mechanism likely. Only mPEG liposomes stay at 60% of the saturation for ligand-coated liposomes. Half-times are diverse at low concentrations but unify at high concentrations for U-87 MG cells.

Design and Development of Nanomaterial-Based Drug Carriers to Overcome the Blood-Brain Barrier by Using Different Transport Mechanisms

Central nervous system (CNS) diseases are the leading causes of death and disabilities in the world. It is quite challenging to treat CNS diseases efficiently because of the blood-brain barrier (BBB). It is a physical barrier with tight junction proteins and high selectivity to limit the substance transportation between the blood and neural tissues. Thus, it is important to understand BBB transport mechanisms for developing novel drug carriers to overcome the BBB. This paper introduces the structure of the BBB and its physiological transport mechanisms. Meanwhile, different strategies for crossing the BBB by using nanomaterial-based drug carriers are reviewed, including carrier-mediated, adsorptive-mediated, and receptor-mediated transcytosis. Since the viral-induced CNS diseases are associated with BBB breakdown, various neurotropic viruses and their mechanisms on BBB disruption are reviewed and discussed, which are considered as an alternative solution to overcome the BBB. Therefore, most recent studies on virus-mimicking nanocarriers for drug delivery to cross the BBB are also reviewed and discussed. On the other hand, the routes of administration of drug-loaded nanocarriers to the CNS have been reviewed. In sum, this paper reviews and discusses various strategies and routes of nano-formulated drug delivery systems across the BBB to the brain, which will contribute to the advanced diagnosis and treatment of CNS diseases.

Integrated Epigenetics, Transcriptomics, and Metabolomics to Analyze the Mechanisms of Benzo[a]pyrene Neurotoxicity in the Hippocampus

Benzo[a]pyrene (B[a]P) is a common environmental pollutant that is neurotoxic to mammals, which can cause changes to hippocampal function and result in cognitive disorders. The mechanisms of B[a]P-induced impairments are complex .To date there have been no studies on the association of epigenetic, transcriptomic, and metabolomic changes with neurotoxicity after B[a]P exposure. In the present study, we investigated the global effect of B[a]P on DNA methylation patterns, noncoding RNAs (ncRNAs) expression, coding RNAs expression, and metabolites in the rat hippocampus. Male Sprague Dawley rats (SD rats) received daily gavage of B[a]P (2.0 mg/kg body weight [BW]) or corn oil for 7 weeks. Learning and memory ability was analyzed using the Morris water maze (MWM) test and change to cellular ultrastructure in the hippocampus was analyzed using electron microscope observation. Integrated analysis of epigenetics, transcriptomics, and metabolomics was conducted to investigate the effect of B[a]P exposure on the signaling and metabolic pathways. Our results suggest that B[a]P could lead to learning and memory deficits, likely as a result of epigenetic and transcriptomic changes that further affected the expression of CACNA1C, Tpo, etc. The changes in expression ultimately affecting LTP, tyrosine metabolism, and other important metabolic pathways.

Premises of plasticity - And the loneliness of the medial temporal lobe

In this perspective paper, we examine possible premises of plasticity in the neural substrates underlying cognitive change. We take the special role of the medial temporal lobe as an anchoring point, but also investigate characteristics throughout the cortex. Specifically, we examine the dimensions of evolutionary expansion, heritability, variability of morphometric change, and inter-individual variance in myelination with respect to the plastic potential of different brain regions. We argue that areas showing less evolutionary expansion, lower heritability, greater variability of cortical thickness change through the lifespan, and greater inter-individual differences in intracortical myelin content have a great extent of plasticity. While different regions of the brain show these features to varying extent, analyses converge on the medial temporal lobe including the hippocampi as the target of all these premises. We discuss implications for effects of training on brain structures, and conditions under which plasticity may be evoked.

Preventing Alzheimer's disease by means of natural selection

The amyloid cascade model for the origin of sporadic forms of Alzheimer's disease (AD) posits that the imbalance in the production and clearance of beta-amyloid is a necessary condition for the disease. A competing theory called the entropic selection hypothesis asserts that the primary cause of sporadic AD is age-induced mitochondrial dysregulation and the following cascade of events: (i) metabolic reprogramming—the upregulation of oxidative phosphorylation in compensation for insufficient energy production in neurons, (ii) natural selection—competition between intact and reprogrammed neurons for energy substrates and (iii) propagation—the spread of the disease due to the selective advantage of neurons with upregulated metabolism. Experimental studies to evaluate the predictions of the amyloid cascade model are being continually retuned to accommodate conflicts of the predictions with empirical data. Clinical trials of treatments for AD based on anti-amyloid therapy have been unsuccessful. We contend that these anomalies and failures stem from a fundamental deficit of the amyloid hypothesis: the model derives from a nuclear-genomic perspective of sporadic AD and discounts the bioenergetic processes that characterize the progression of most age-related disorders. In this article, we review the anomalies of the amyloid model and the theoretical and empirical support for the entropic selection theory. We also discuss the new therapeutic strategies based on natural selection which the model proposes.

Glucose transporter and folic acid receptor-mediated Pluronic P105 polymeric micelles loaded with doxorubicin for brain tumor treating

In this study, glucose transporter and folic acid (FA) receptor-mediated Pluronic P105 polymeric micelles loaded with DOX (GF-DOX) were prepared for enhancing the blood-brain barrier (BBB) transportation and improving the drug accumulation in the glioma cells. The pH-triggered DOX release of GF-DOX indicating a comparatively fast drug release at weak acidic condition and stable state of the carrier at physiological environment. The transport of GF-DOX across the in vitro BBB model showed that GF-DOX exhibited higher BBB transportation ability with the transporting ratio of 21.47% in 4 h. The carrier was internalized into C6 glioma cells upon crossing the BBB model for the combined effect of the brain targeting by transportation of glucose transporter and active tumor cell targeting by FA receptor-mediated endocytosis. Moreover, minimized weight changes and high suppression ratio of tumor growth were observed after intravenous injection of GF-DOX. In conclusion, the glucose transporter and FA dual-targeting micelles would provide a safe and effective strategy for new modalities to treat brain tumor.

Metabolic encephalopathies

Kinnier Wilson coined the term metabolic encephalopathy to describe a clinical state of global cerebral dysfunction induced by systemic stress that can vary in clinical presentation from mild executive dysfunction to deep coma with decerebrate posturing; the causes are numerous. Some mechanisms by which cerebral dysfunction occurs in metabolic encephalopathies include focal or global cerebral edema, alterations in transmitter function, the accumulation of uncleared toxic metabolites, postcapillary venule vasogenic edema, and energy failure. This article focuses on common causes of metabolic encephalopathy, and reviews common causes, clinical presentations and, where relevant, management.

The effects of hypoglycemic and alcoholic coma on the blood-brain barrier permeability

In this investigation, the effects of hypoglycemic coma and alcoholic coma on the blood-brain barrier (BBB) permeability have been compared. Female adult Wistar albino rats weighing 180-230 g were divided into three groups: Control group (n=8), Alcoholic Coma Group (n=18), and Hypoglycemic Coma group (n=12). The animals went into coma approximately 3-4 hours after insulin administration and 3-5 minutes after alcohol administration. Evans blue (4mL/kg) was injected intravenously as BBB tracer. It was observed that the alcoholic coma did not significantly increase the BBB permeability in any of the brain regions when compared to control group. Changes in BBB permeability were significantly increased by the hypoglycemic coma in comparison to the control group values (p<0.01). Our findings suggest that hypoglycemic and alcoholic coma have different effects on the BBB permeability depending on the energy metabolism.

Leptin boosts cellular metabolism by activating AMPK and the sirtuins to reduce tau phosphorylation and β-amyloid in neurons

Leptin is a pleiotropic hormone primarily secreted by adipocytes. A high density of functional Leptin receptors has been reported to be expressed in the hippocampus and other cortical regions of the brain, the physiological significance of which has not been explored extensively. Alzheimer's disease (AD) is marked by impaired brain metabolism with decreased glucose utilization in those regions which often precede pathological changes. Recent epidemiological studies suggest that plasma Leptin is protective against AD. Specifically, elderly with plasma Leptin levels in the lowest quartile were found to be four times more likely to develop AD than those in the highest quartile. We have previously reported that Leptin modulates AD pathological pathways in vitro through a mechanism involving the energy sensor, AMP-activated protein kinase (AMPK). To this end, we investigated the extent to which activation of AMPK as well as another class of sensors linking energy availability to cellular metabolism, the sirtuins (SIRT), mediate Leptin's biological activity. Leptin directly activated neuronal AMPK and SIRT in cell lines. Additionally, the ability of Leptin to reduce tau phosphorylation and β-amyloid production was sensitive to the AMPK and sirtuin inhibitors, compound C and nicotinamide, respectively. These findings implicate that Leptin normally acts as a signal for energy homeostasis in neurons. Perhaps Leptin deficiency in AD contributes to a neuronal imbalance in handling energy requirements, leading to higher Aβ and phospho-tau, which can be restored by replenishing low Leptin levels. This may also be a legitimate strategy for therapy.

Disrupted energy metabolism and neuronal circuit dysfunction in cognitive impairment and Alzheimer's disease

Epidemiological, neuropathological, and functional neuroimaging evidence implicates global and regional disruptions in brain metabolism and energetics in the pathogenesis of cognitive impairment. Nerve cell microcircuits are modified by excitatory and inhibitory synaptic activity and neurotrophic factors. Ageing and Alzheimer's disease cause perturbations in cellular energy metabolism, level of excitation or inhibition, and neurotrophic factor release, which overwhelm compensatory mechanisms and result in dysfunction of neuronal microcircuits and brain networks. A prolonged positive energy balance impairs the ability of neurons to adapt to oxidative and metabolic stress. Results from experimental studies in animals show how disruptions caused by chronic positive energy balance, such as diabetes, lead to accelerated cognitive ageing and Alzheimer's disease. Therapeutic interventions to allay cognitive dysfunction that target energy metabolism and adaptive stress responses (such as neurotrophin signalling) have been effective in animal models and in preliminary studies in humans.

In vitro and in vivo investigation of glucose-mediated brain-targeting liposomes

New glycosyl derivative of cholesterol was synthesized as a material for preparing novel liposome to overcome the ineffective delivery of normal drug formulations to brain by targeting the (glucose transporters) GLUTs on the BBB. Coumarin-6 was used as fluorescent probe. The results have shown that the cytotoxicity for the brain capillary endothelial cells (BCECs) of the glucose-mediated brain targeting liposome containing coumarin-6 was less than that of conventional liposome. The BBB model in vitro was established by coculturing of BCECs and astrocytes (ACs) of rat to test the transendothelial ability crossing the BBB. The transendothelial ability was confirmed strengthen alone with the amount of the new glycosyl derivative of cholesterol used in liposome. After i.v. administration of LIP, control liposome (CLP), and GLP-4, the AUC(0-t) of coumarin-6 for GLP-4 was 2.85 times higher than that of LIP, and 3.33 times higher than that of CLP. The C(max) of CLP-4 was 1.43 times higher than that of LIP, and 3.10 times higher than that of CLP. Both pharmacokinetics and distribution in mice were also investigated to show that this novel brain targeting drug delivery system was promising.

In vivo proton MRS to quantify anesthetic effects of pentobarbital on cerebral metabolism and brain activity in rat

To quantitatively investigate the effects of pentobarbital anesthesia on brain activity, brain metabolite concentrations and cerebral metabolic rate of glucose, in vivo proton MR spectra, and electroencephalography were measured in the rat brain with various doses of pentobarbital. The results show that (1) the resonances attributed to propylene glycol, a solvent in pentobarbital injection solution, can be robustly detected and quantified in the brain; (2) the concentration of most brain metabolites remained constant under the isoelectric state (silent electroencephalography) with a high dose of pentobarbital compared to mild isoflurane anesthesia condition, except for a reduction of 61% in the brain glucose level, which was associated with a 37% decrease in cerebral metabolic rate of glucose, suggesting a significant amount of "housekeeping" energy for maintaining brain cellular integrity under the isoelectric state; and (3) electroencephalography and cerebral metabolic activities were tightly coupled to the pentobarbital anesthesia depth and they can be indirectly quantified by the propylene glycol resonance signal at 1.13 ppm. This study indicates that in vivo proton MR spectroscopy can be used to measure changes in cerebral metabolite concentrations and cerebral metabolic rate of glucose under varied pentobarbital anesthesia states; moreover, the propylene glycol signal provides a sensitive biomarker for quantitatively monitoring these changes and anesthesia depth noninvasively.

Impact of energy intake and expenditure on neuronal plasticity

The Roman poet Horace was among the first to recognize that when "clogged with yesterday's excess, the body drags the mind down with it." Although considerable attention has been paid in neuroscience to the enhancement of neuronal function by wheel running and caloric restriction, far less is known about the other side of this issue. What are the consequences of unhealthy habits to central nervous system function? Prolonged exposure to excessive caloric intake impairs neuronal function and also contributes to obesity and other risk factors for diabetes. Diabetes, a disease characterized by reduced sensitivity to glucose and insulin, is also associated with deficits in brain structure and function. In contrast, enhancement of somatic metabolism by wheel running or caloric restriction improves central neuroplasticity. Generalizing across studies reveals a relationship between global metabolic efficiency and neuroplasticity in the hippocampus, a brain region that is essential for learning and memory. The specific principles upheld by these findings are suggestive of a continuum, with global metabolic alterations fluctuating in concert with neuroplasticity in the hippocampus.

Reversible diffusion-weighted imaging changes in the splenium of the corpus callosum and internal capsule associated with hypoglycemia - case report -

A 63-year-old man presented with hypoglycemia-induced hemiparesis manifesting as diffusion-weighted magnetic resonance (MR) imaging changes in the splenium of the corpus callosum and internal capsule which disappeared after glucose administration. Clinicians should be aware that hypoglycemia can cause reversible splenium abnormalities on MR imaging, although the underlying mechanism still remains unclear, as this may be helpful in the differential diagnosis of hypoglycemia-induced hemiparesis and stroke.

Prolonged cerebellar ataxia: an unusual complication of hypoglycemia

A 51-year-old male with a history of insulin-dependent diabetes and polysubstance abuse presented after overdose on insulin. Soon after resuscitation, he displayed a severe ataxia in all 4 limbs and was unable to walk; all of which persisted for at least 5 days. Laboratory testing was unrevealing, including relatively normal brain magnetic resonance imaging. He had recovered full neurologic function 3 months after the event. This report describes a case of reversible cerebellar ataxia as a rare complication of severe hypoglycemia that may occur in patients with abnormal cerebellar glucose metabolism. Thus, this phenomenon should be included in the differential diagnosis of patients with a history of hypoglycemia who present with ataxia. In this context, the differential diagnosis of cerebellar ataxia is discussed, as is the proposed mechanism for hypoglycemia-induced cerebellar dysfunction.

Hypoglycemic brain damage

Hypoglycemia was long considered to kill neurons by depriving them of glucose. We now know that hypoglycemia kills neurons actively from without, rather than by starvation from within. Hypoglycemia only causes neuronal death when the EEG becomes flat. This usually occurs after glucose levels have fallen below 1 mM (18 mg/dl) for some period, depending on body glycogen reserves. At the time that abrupt brain energy failure occurs, the excitatory amino acid aspartate is massively released into the limited brain extracellular space and floods the excitatory amino acid receptors located on neuronal dendrites. Calcium fluxes occur and membrane breaks in the cell lead rapidly to neuronal necrosis. Significant neuronal necrosis occurs after 30 min of electrocerebral silence. Other neurochemical changes include energy depletion to roughly 25% of control, phospholipase and other enzyme activation, tissue alkalosis and a tendency for all cellular redox systems to shift towards oxidation. The neurochemistry of hypoglycemia thus differs markedly from ischemia. Hypoglycemia often differs from ischemia in its neuropathologic distribution, a phenomenon applicable in forensic practice. The border-zone distribution of global ischemia is not seen, necrosis of the dentate gyrus of the hippocampus can occur and a predilection for the superficial layers of the cortex is sometimes seen. Cerebellum and brainstem are universally spared in hypoglycemic brain damage. Hypoglycemia constitutes a unique metabolic brain insult.

Synthesis, Characterization, and Metal Coordinating Ability of Multifunctional Carbohydrate-Containing Compounds for Alzheimer's Therapy

Dysfunctional interactions of metal ions, especially Cu, Zn, and Fe, with the amyloid-beta (A beta) peptide are hypothesized to play an important role in the etiology of Alzheimer's disease (AD). In addition to direct effects on A beta aggregation, both Cu and Fe catalyze the generation of reactive oxygen species (ROS) in the brain further contributing to neurodegeneration. Disruption of these aberrant metal-peptide interactions via chelation therapy holds considerable promise as a therapeutic strategy to combat this presently incurable disease. To this end, we developed two multifunctional carbohydrate-containing compounds N,N'-bis[(5-beta-D-glucopyranosyloxy-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL1) and N,N'-bis[(5-beta-D-glucopyranosyloxy-3-tert-butyl-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL2) for brain-directed metal chelation and redistribution. Acidity constants were determined by potentiometry aided by UV-vis and 1H NMR measurements to identify the protonation sites of H2GL1,2. Intramolecular H bonding between the amine nitrogen atoms and the H atoms of the hydroxyl groups was determined to have an important stabilizing effect in solution for the H2GL1 and H2GL2 species. Both H2GL1 and H2GL2 were found to have significant antioxidant capacity on the basis of an in vitro antioxidant assay. The neutral metal complexes CuGL1, NiGL1, CuGL2, and NiGL2 were synthesized and fully characterized. A square-planar arrangement of the tetradentate ligand around CuGL2 and NiGL2 was determined by X-ray crystallography with the sugar moieties remaining pendant. The coordination properties of H2GL1,2 were also investigated by potentiometry, and as expected, both ligands displayed a higher affinity for Cu2+ over Zn2+ with H2GL1 displaying better coordinating ability at physiological pH. Both H2GL1 and H2GL2 were found to reduce Zn2+- and Cu2+- induced Abeta1-40 aggregation in vitro, further demonstrating the potential of these multifunctional agents as AD therapeutics.

Statistical analysis of metabolic pathways of brain metabolism at steady state

The estimation of metabolic fluxes for brain metabolism is important, among other things, to test the validity of different hypotheses which have been proposed in the literature. The metabolic model that we propose considers, in addition to the blood compartment, the cytosol, and mitochondria of both astrocyte and neuron, including detailed metabolic pathways. In this work we use a recently developed methodology to perform a statistical Flux Balance Analysis (FBA) for this model. The methodology recasts the problem in the form of Bayesian statistical inference and therefore can take advantage of qualitative information about brain metabolism for the simultaneous estimation of all reaction fluxes and transport rates at steady state. By a Markov Chain Monte Carlo (MCMC) sampling method, we are able to provide for each reaction flux and transport rate a distribution of possible values. The analysis of the histograms of the reaction fluxes and transport rates provides a very useful tool for assessing the validity of different hypotheses about brain energetics proposed in the literature, and facilitates the design of the pathways network that is in accordance with what is understood of the functioning of the brain. In this work, we focus on the analysis of biochemical pathways within each cell type (astrocyte and neuron) at different levels of neural activity, and we demonstrate how statistical tools can help implement various bounds suggested by experimental data.

A quantitative overview of glucose dynamics in the gliovascular unit

While glucose is constantly being "pulled" into the brain by hexokinase, its flux across the blood brain barrier (BBB) is allowed by facilitative carriers of the GLUT family. Starting from the microscopic properties of GLUT carriers, and within the constraints imposed by the available experimental data, chiefly NMR spectroscopy, we have generated a numerical model that reveals several hidden features of glucose transport and metabolism in the brain. The half-saturation constant of glucose uptake into the brain (K(t)) is close to 8 mM. GLUT carriers at the BBB are symmetric, show accelerated-exchange, and a K(m) of zero-trans flux (K(zt)) close to 5 mM, determining a ratio of 3.6 between maximum transport rate and net glucose flux (T(max)/CMR(glc)). In spite of the low transporter occupancy, the model shows that for a stimulated hexokinase to pull more glucose into the brain, the number or activity of GLUT carriers must also increase, particularly at the BBB. The endothelium is therefore predicted to be a key modulated element for the fast control of energy metabolism. In addition, the simulations help to explain why mild hypoglycemia may be asymptomatic and reveal that [glucose](brain) (as measured by NMR) should be much more sensitive than glucose flux (as measured by PET) as an indicator of GLUT1 deficiency. In summary, available data from various sources has been integrated in a predictive model based on the microscopic properties of GLUT carriers.

Diffusion-weighted MRI predicts prognosis in severe hypoglycemic encephalopathy

A 20-year-old woman presented unconscious due to hypoglycemia after a self-administered insulin injection. Diffusion-weighted MRI (DWI), performed 5 days after admission, demonstrated heterogeneous high-intensity signal areas in both the cortex and subcortex but sparing the motor and sensory centers. On the 11th day after admission, she began making incomprehensible verbal sounds, eye opening spontaneously and moving her extremities with pyramidal tract signs. Three months later, she had aphasia, agnosia and apraxia but a normal gait without pyramidal tract signs or ataxia. DWI is thus considered useful to predict the functional outcome of patients with severe hypoglycemia.

Adenosine treatment delays postischemic hippocampal CA1 loss after cardiac arrest and resuscitation in rats

Resuscitation from cardiac arrest results in reperfusion injury that leads to increased postresuscitation mortality and delayed neuronal death. One of the many consequences of resuscitation from cardiac arrest is a derangement of energy metabolism and the loss of adenylates, impairing the tissue's ability to regain proper energy balance. In this study, we investigated the effects of adenosine (ADO) on the recovery of the brain from 12 min of ischemia using a rat model of cardiac arrest and resuscitation. Compared to the untreated group, treatment with adenosine (7.2 mg/kg) initiated immediately after resuscitation increased the proportion of rats surviving to 4 days and significantly delayed hippocampal CA1 neuronal loss. Brain blood flow was increased significantly in the adenosine-treated rats 1 h after cardiac arrest and resuscitation. Adenosine-treated rats exhibited less edema in cortex, brainstem and hippocampus during the first 48 h of recovery. Adenosine treatment significantly lowered brain temperature during recovery, and a part of the neuroprotective effects of adenosine treatment could be ascribed to adenosine-induced hypothermia. With this dose, adenosine may have a delayed transient effect on the restoration of the adenylate pool (AXP = ATP + ADP + AMP) 24 h after cardiac arrest and resuscitation. Our findings suggested that improved postischemic brain blood flow and ADO-induced hypothermia, rather than adenylate supplementation, may be the two major contributors to the neuroprotective effects of adenosine following cardiac arrest and resuscitation. Although adenosine did not prevent eventual CA1 neuronal loss in the long term, it did delay neuronal loss and promoted long-term survival. Thus, adenosine or specific agonists of adenosine receptors should be evaluated as adjuncts to broaden the window of opportunity in the treatment of the reperfusion injury following cardiac arrest and resuscitation.

Glucose-targeted niosomes deliver vasoactive intestinal peptide (VIP) to the brain

The aim of this study was to evaluate glucose-bearing niosomes as a brain targeted delivery system for the vasoactive intestinal peptide (VIP). To this end, VIP/125I-VIP-loaded glucose-bearing niosomes were intravenously injected to mice. Brain uptake was determined by measuring the radioactivity of 125I-labeled VIP using gamma-counting, after intravenous administration of VIP in solution or encapsulated in glucose-bearing niosomes or in control niosomes. VIP integrity was assessed by reversed-phase HPLC analysis of brain extracts. Distribution of 125I-VIP derived radioactivity was examined from serial brain slices. HPLC analysis confirmed the presence of intact VIP in brain after administration of VIP-loaded niosomes, but not after administration of VIP solution. Encapsulation within glucose-bearing niosomes mainly allowed a significantly higher VIP brain uptake compared to control niosomes (up to 86%, 5min after treatment). Brain distribution of intact VIP after injection of glucose-bearing niosomes, indicated that radioactivity was preferentially located in the posterior and the anterior parts of the brain, whereas it was homogeneously distributed in the whole brain after the administration of control vesicles. In conclusion, this novel vesicular formulation of VIP delivers intact VIP to particular brain regions in mice. Glucose-bearing vesicles might be therefore a novel tool to deliver drugs across the blood-brain barrier (BBB).

Renormalization of regional brain blood flow during prolonged mild hypoxic exposure in rats

In this study, we measured regional brain blood flow (BF) in rats during hypoxic exposure. Our data show the hypoxia-induced brain blood flow increase returned to baseline within 1 week as blood hemoglobin increased. Because this return to baseline occurred before capillary angiogenesis, this result suggests that the mechanism for brain blood flow renormalization during prolonged hypoxia exposure is more likely related to the increased systemic arterial oxygen carrying capacity than to local tissue hypoxia that persists for at least 2 weeks.

Comparison of glucose influx and blood flow in retina and brain of diabetic rats

Diabetes is associated with extensive microvascular pathology and decreased expression of the glucose transporter (GLUT-1) in retina, but not brain. To explore the basis of these differences, the authors simultaneously measured glucose influx (micromol x g(-1) x min(-1)) and blood flow (mL x g(-1) x min(-1)) in retina and brain cortex of nondiabetic control rats (normoglycemic and acute-hyperglycemic) and in rats with streptozotocin-induced diabetes (with or without aminoguanidine (AMG) treatment) using a single-pass, dual-label indicator method. In addition, tissue glucose and adenosine triphosphate (nmol/mg dry weight) levels were measured. Glucose influx in retina exceeded that of cortex by about threefold for both the nondiabetic and diabetic groups. In contrast, blood flow in retina was significantly lower than in cortex by about threefold for each group. Retinal and cortical glucose influx in the diabetic rats was lower than in the nondiabetic acute-hyperglycemic group, but not in the normoglycemic group. Blood flow in these tissues remained relatively unchanged with glycemic conditions. The glucose levels in the diabetic retina (aminoguanidine untreated and aminoguanidine treated) were fourfold to sixfold greater than the nondiabetic retina. The cortical glucose levels remained unchanged in all groups. These data suggest that the accumulation of glucose in the diabetic retina cannot be explained by increased endothelial-glucose uptake or increased vascular membrane permeability.

Dual responses of tissue partial pressure of oxygen after functional stimulation in rat somatosensory cortex

To compare the spatial heterogeneity of brain tissue partial pressure of oxygen (pO(2)) among local brain regions, we focused on functional and anatomical variations in rat somatosensory cortex. Tissue pO(2) was measured by using an oxygen microelectrode with high spatio-temporal resolution, and investigated in three somatosensory areas including hindlimb (HL), forelimb (FL), and trunk region (Tr). Their anatomical structures were determined with histological techniques (Nissl stain). In addition to the measurement of baseline tissue pO(2), we examined temporal shifts in tissue pO(2) distribution elicited by functional stimulation using the brushing stimulation to the hindlimb, forelimb, and trunk regions of the body. We observed that average tissue pO(2) in the Tr (14+/-10 Torr) was significantly lower than those in the HL (25+/-13 Torr) and FL (24+/-13 Torr). Such regional differences in tissue pO(2) were closely related to the cytoarchitectonic variations among these three areas. In addition, the functional stimulation enlarged the regional differences in the pO(2) depending on each somatosensory area; the pO(2) in the HL increased by 3.6+/-2.9% after the stimulation to hindlimb, whereas that in the Tr decreased by -2.9+/-2.5% after the stimulation to trunk region. Such dual responses of tissue pO(2) (i.e. increase or decrease) after the functional stimulation to the corresponding body regions may provide a criterion to clinically predict regions susceptible to tissue hypoxia, because considerable decrease in tissue pO(2) occurred in the Tr showing the lowest baseline pO(2).

Magnesium sulfate attenuates increased blood-brain barrier permeability during insulin-induced hypoglycemia in rats

Magnesium probably protects brain tissue against the effects of cerebral ischemia, brain injury and stroke through its actions as a calcium antagonist and inhibitor of excitatory amino acids. The effects of magnesium sulfate on cerebrovascular permeability to a dye, Evans blue, were studied during insulin-induced hypoglycemia with hypothermia in rats. Hypoglycemia was induced by an intramuscular injection of insulin. After giving insulin, each animal received MgSO4 (270 mg/kg) ip, followed by a 27 mg/kg dose every 20 min for 2.5 h. Plasma glucose and Mg2+ levels of animals were measured. Magnesium concentrations increased in the serum following MgSO4 administration (6.05 ± 0.57 vs. 2.58 ± 0.14 mg/dL in the Mg2+ group, and 7.14 ± 0.42 vs. 2.78 ± 0.06 mg/dL in the insulin + Mg2+ group, P < 0.01). Plasma glucose levels decreased following hypoglycemia (4 ± 0.66 vs. 118 ± 2.23 mg/dL in the insulin group, and 7 ± 1.59 vs. 118 ± 4.84 mg/dL in the insulin + Mg2+ group, P < 0.01). Blood-brain barrier permeability to Evans blue considerably increased in hypoglycemic rats (P < 0.01). In contrast, blood-brain barrier permeability to Evans blue was significantly reduced in treatment of hypoglycemic rats with MgSO4 (P < 0.01). These results indicate that Mg2+ greatly reduced the passage of exogenous vascular tracer bound to albumin into the brain during hypoglycemia with hypothermia. Mg2+ could have protective effects on blood-brain barrier permeability against insulin-induced hypoglycemia.Key words: blood-brain barrier, hypoglycemia, Mg2+, Evans-blue.

Plasma glucose alone does not predict neurologic dysfunction in hypoglycemic nondiabetic subjects

STUDY OBJECTIVE

To assess the value of plasma glucose concentration alone as a predictor of neurologic dysfunction in nondiabetic subjects with normal baseline neurologic examination and electroencephalographic (EEG) findings.

METHODS

Neurologic function and EEG results were evaluated in 17 subjects before and during insulin-induced hypoglycemia using relevant and reliable clinical tools for bedside use.

RESULTS

Hypoglycemia (mean nadir concentration, 30 mg/dL) was without effect on level of consciousness or cranial nerve, motor, sensory, vestibulocerebellar, language, or simple visuospatial functions. Attention was minimally impaired in all subjects, but memory in only 3. EEG results remained normal in 5 subjects; minimal to moderate nonspecific changes occurred in the rest. All patients manifested signs of sympathetic stimulation from hypoglycemia, including tremor, tachycardia, and diaphoresis. The manifestations of neuroglycopenia did not correlate significantly with nadir plasma glucose or duration of hypoglycemia.

CONCLUSION

Moderately severe hypoglycemia of short duration can be neurologically occult, or subtle inattention can be its first and only clinical manifestation. Our findings are at variance with reports in the emergency medicine literature in which marked deficits are universally present at glucose concentrations equal to those attained in this study. This discrepancy suggests that the expression of neuroglycopenia is multifactorially determined and that plasma glucose concentration alone does not predict neurologic dysfunction in nondiabetic subjects with normal baseline neurologic examinations.

Synthesis of glucose-chlorambucil derivatives and their recognition by the human GLUT1 glucose transporter

A limitation of the use of chemotherapeutic agents against intracerebral tumors lies on their poor uptake into the central nervous system. An approach to enhance brain delivery is to design agents that are transported into the brain by one of the saturable nutrient carriers of the blood-brain barrier, the highly efficient brain and erythrocyte glucose transporter isoform GLUT1. Since the GLUT1 hexose transporter of the blood-brain barrier is also present on erythrocytes, new compounds designed to be transported by the GLUT1 transporter were studied on human erythrocytes, which represent unique, easily accessible human GLUT1 expressing cells. In this paper we describe the synthesis of four glucose-chlorambucil derivatives, namely methyl 6-O-4[bis(2-chloroethyl)amino]benzenebut anoyl-beta-D-glucopyranosi de (3), 6-O-4-[bis(2-chloroethyl)amino]benzenebu tanoyl-D-glucopyranose (6), methyl 6-[4-[bis(2-chloroethyl)amino]benzenebut anoylamido]-6-deoxy-beta-D-glucopyranoside (9) and 6-[4-[bis(2-chloroethyl)amino]benzenebut anoyl amido]-6-deoxy-D-glucopyranose (10), and the study of their interactions with the GLUT1 transporter of the human erythrocytes. All four compounds were able to inhibit [14C]glucose uptake in a concentration-dependent manner. One of them, compound 6, exhibited an approximately 160-fold higher inhibition of [14C]glucose uptake by the GLUT1 transporter than glucose itself. Compound 6 was also able to inhibit [3H]cytochalasin B binding to erythrocytes with approximately 1000-fold higher efficacy than does glucose. The inhibition of glucose uptake was entirely reversible, indicating that it was not due to alkylation of a nucleophilic group of the hexose transporter. The above results suggested specific interactions of compound 6 with the hexose transporter protein. Uptake studies of [14C]compound 6 indicated, in addition, some non-specific interactions with intact and open erythrocyte membranes: only a small amount of the bound [14C]compound 6 can be displaced by cytochalasin B. Collectively, these findings led us to conclude that the interactions of compound 6 with GLUT1 are presumably that of a non-transported inhibitor.

Severe transient hypoglycemia causes reversible change in the apparent diffusion coefficient of water

BACKGROUND AND PURPOSE

The aim of this study was to determine the effects of temporary severe hypoglycemia on the apparent diffusion coefficient (ADC) acquired by diffusion-weighted MRI of brain water with the use of serial multislice ADC mapping in rats. Severe hypoglycemia reduces the extracellular space volume, as does ischemia. Demonstrating a reduction of ADC with hypoglycemia should increase our understanding of the mechanisms underlying ADC changes in ischemia and other conditions.

METHODS

Fasted rats were given regular insulin (15 IU/kg IP). Rats were subjected to 15 minutes (n = 5) and 50 minutes (n = 5) of temporary severe hypoglycemia, causing a transiently isoelectric electroencephalogram (EEG). ADC mapping was performed every 30 seconds beginning at the onset of isoelectricity for 8.5 minutes. ADC maps were also obtained later during the isoelectric EEG period and 10, 20, 30, and 40 minutes after glucose infusion. Control images were obtained from a separate group of animals suffering cardiac arrest (n = 5).

RESULTS

Abnormal ADC values were not observed before the onset of cerebral isoelectricity, except for isolated areas in the cortex and periventricular regions. Cortical ADC values globally declined at the onset of EEG isoelectricity. The ADC decline spread to subcortical regions within a few minutes. During the isoelectric period, significant declines of ADC values (27% to 45%) occurred in the entire brain. Glucose infusion normalized most of the ADC changes, even after a 50-minute period of isoelectricity.

CONCLUSIONS

ADC mapping during hypoglycemia clearly demonstrates changes likely related to energy depletion. Most of these ADC declines were reversible. Hypoglycemia is a condition known to be associated with shrinkage of the extracellular space. These observations support the hypothesis that ADC reductions observed in ischemia are also related to shifts of water from the extracellular to the intracellular compartment.

Diabetes and the nervous system

Hyperglycemia and its vascular complications affect the entire nervous system, contributing to increased morbidity and mortality. Chronic hyperglycemia is not only a known and major risk factor for cerebral vascular diseases but also the presence of hyperglycemia at the time of a cerebrovascular event may adversely influence the outcome. It also affects the treatment of some neurodegenerative disorders, and there are suggestions that diabetes may in fact suffer from a "chronic diabetic encephalopathy." Its varied effects on the peripheral nervous system result in several forms of diabetic neuropathies, the exact pathogenesis of which is still obscure. There is, however, some new information that may link metabolic and vascular hypotheses.

Bilateral temporal lobe MRI changes in uncomplicated hypoglycemic coma

We report bilateral temporal lobe MRI findings in a patient following an episode of prolonged hypoglycemia uncomplicated by coexisting anoxia, hypotension, acidosis, drug intoxication, infection, or status epilepticus. The MRI findings are discussed in relation to the experimental and human data on hypoglycemic neuronal injury.

Hypoglycaemia: brain neurochemistry and neuropathology

The widespread use of insulin and oral hypoglycaemic agents has increased the incidence of hypoglycaemic brain damage due to accidental, suicidal, or homicidal overdose. Hypoglycaemia is capable of damaging the brain in the face of intact cardiac function, but neuronal necrosis occurs only when the electroencephalogram (EEG) becomes isoelectric. Neurochemical changes are distinct from ischaemia, and cerebral blood flow is actually increased, in contrast to cerebral ischaemia. Salient neurochemical changes include an arrest of protein synthesis in many but not all brain regions, a shift of brain redox equilibria towards oxidation, incomplete energy failure, loss of ion homeostasis, cellular calcium influx, intracellular alkalosis, and a release of neuroactive amino acids, especially aspartate, into the extracellular space of the brain. The metabolic release of aspartate, and to a lesser extent glutamate, into the interstitial space of the brain produces histopathological patterns of neuronal death that can be distinguished from ischaemic brain damage in experimental brain tissue and, occasionally, in brains from human autopsies after hypoglycaemic brain damage. The excitatory amino acids released during profound hypoglycaemia bind to neuronal dendrites and perikarya, but not to other cell types in the nervous system, thus giving rise to selective neuronal death. The absence of acidosis, and an adequate blood supply during hypoglycaemia, protect the brain against pan-necrosis or infarction. However, the neurons die more quickly during hypoglycaemic brain damage than after cerebral ischaemia. Hypoglycaemic brain damage thus falls into the newly defined class of cerebral 'excitotoxic' neuropathologies, where neurons are selectively killed by an extracellular overflow of excitatory amino acids produced by the brain itself. The pathogenesis of hypoglycaemic brain damage is thus rather more novel and intriguing than was thought even a decade ago, when it was believed that glucose starvation and simple energy failure resulted directly in neuronal catabolism.

Regional blood-brain lactate influx

Regional blood-to-brain lactate transport was studied in chloral hydrate anesthetized rats using the single pass, dual-label, indicator fractionation, right atrial injection method. Lactate influx was resolved into two components, a saturable, stereospecific (to the L-enantiomer) component and a non-saturable, non-stereospecific diffusional component. The saturable component was found to have a low efficiency and moderate capacity with transport affinity coefficients between 6 and 14 mM and transport maxima of 23-40 mumol/100 g/min in the various brain regions. Lactate transport was not inhibited by probenecid. The diffusional component was determined from D-lactate influx measurements and the regional linear diffusion coefficients ranged from 0.020 to 0.036 ml/g/min. At the usual levels of plasma lactate (1-1.5 mM) these two influx components were about equal. The relative contribution of the non-stereospecific diffusional component was increased at higher plasma lactate concentrations. Lactate clearance, estimated by the total apparent permeability x surface area products was between 6 and 8 ml/100 g/min.

Regional density of perfused capillaries and cerebral blood flow in untreated short-term and long-term streptozotocin diabetes

The regional density of perfused cerebral capillaries (rDPC) and regional cerebral blood flow (rCBF) were measured in 12 selected brain regions in rats after 3 and 20 weeks of streptozotocin-induced diabetes and in control groups. After 3 weeks of diabetes, both rCBF and rDPC were unchanged in the diabetic group compared to the control group. A diabetes duration of 20 weeks causing bilateral cataracts induced a significant (p less than 0.05) reduction in rCBF in two structures in the visual system compared to the control group (visual cortex: 105 versus 129 ml 100 g-1 min-1; lateral geniculate body: 106 versus 128 ml 100 g-1 min-1) and in the pontine reticular nucleus (82 versus 128 ml 100 g min-1), whereas rDPC remained unchanged. A highly significant correlation between rCBF and rDPC was found in both control groups (r = 0.8, p less than 0.005) whereas the correlation was more scattered in the diabetic groups (r = 0.6, p less than 0.05). The present results show that during chronic diabetes, a reduction of rCBF does not affect the number of perfused capillaries.

Effects of internal carotid administration of MPTP on rat brain and blood-brain barrier

Unlike primates, rats are resistant to systemic 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) neurotoxicity, but direct infusion of MPTP into rat substantia nigra causes specific destruction of dopaminergic neurons. We now demonstrate that rats are resistant to MPTP neurotoxicity even when MPTP is injected directly into the brain circulation. Injection of 1-3.5 mg of MPTP into the internal carotid artery of Wistar rats causes no behavioral or motor abnormalities and small, but significant, dopamine loss in the ipsilateral striatum. MPTP caused no changes in the levels of norepinephrine or serotonin in the cerebral cortex. Higher doses of intracarotid MPTP were lethal. Pretreatment with pargyline, a monoamine oxidase inhibitor, did not alter the mortality but prevented dopamine depletion. The high uptake and retention of MPTP by rat brain, yet its failure to cause major dopaminergic toxicity suggest that MPTP is rapidly metabolized in brain capillaries to 1-methyl-4-phenylpyridinium (MPP+) and other polar metabolites that have difficulty in traversing the blood-brain barrier. Sequestration of MPTP metabolites in brain capillary endothelial cells could result in their dysfunction. However, we found no defects in the ability of the blood-brain barrier to prevent the entry of vascular aminoisobutyric acid or horseradish peroxidase into brain in spite of morphologic evidence of endothelial changes and astrocytic swelling after intracarotid MPTP injections. Our results provide further evidence that the rat's resistance to systemic MPTP neurotoxicity is probably due to its unique blood-brain barrier properties.