Glucose transporters and glucose utilization in rat brain after acute ethanol administration

A critical review of ethanol effects on neuronal firing: A metabolic perspective

Ethanol metabolism is relatively understudied in neurons, even though changes in neuronal metabolism are known to affect their activity. Recent work demonstrates that ethanol is preferentially metabolized over glucose as a source of carbon and energy, and it reprograms neurons to a state of reduced energy potential and diminished capacity to utilize glucose once ethanol is exhausted. Ethanol intake has been associated with changes in neuronal firing and specific brain activity (EEG) patterns have been linked with risk for alcohol use disorder (AUD). Furthermore, a haplotype of the inwardly rectifying potassium channel subunit, GIRK2, which plays a critical role in regulating excitability of neurons, has been linked with AUD and shown to be directly regulated by ethanol. At the same time, overexpression of GIRK2 prevents ethanol-induced metabolic changes. Based on the available evidence, we conclude that the mechanisms underlying the effects of ethanol on neuronal metabolism are a novel target for developing therapies for AUD.

Ethanol metabolism: The good, the bad, and the ugly

Throughout the world, ethanol is both an important commercial commodity and a source of major medical and social problems. Ethanol readily passes through biological membranes and distributes throughout the body. It is oxidized, first to acetaldehyde and then to acetate, and finally by the citric acid cycle in virtually all tissues. The oxidation of ethanol is irreversible and unregulated, making the rate dependent only on local concentration and enzyme activity. This unregulated input of reducing equivalents increases reduction of both cytoplasmic and intramitochondrial NAD and, through the latter, cellular energy state {[ATP]/([ADP][Pi])}. In brain, this increase in energy state stimulates dopaminergic neural activity signalling reward and a sense of well being, while suppressing glutamatergic neural activity signalling anxiety and unease. These positive responses to ethanol ingestion are important to social alcohol consumption. Importantly, decreased free [AMP] decreases AMP-dependent protein kinase (AMPK) activity, an important regulator of cellular energy metabolism. Oxidation of substrates used for energy metabolism in the absence of ethanol is down regulated to accommodate the input from ethanol. In liver, chronic ethanol metabolism results in fatty liver and general metabolic dysfunction. In brain, transport of other oxidizable metabolites through the blood-brain barrier and the enzymes for their oxidation are both down regulated. For exposures of short duration, ethanol induced regulatory changes are rapid and reversible, recovering completely when the concentrations of ethanol and acetate fall again. Longer periods of ethanol exposure and associated chronic suppression of AMPK activity activates regulatory mechanisms, including gene expression, that operate over longer time scales, both in onset and reversal. If chronic alcohol consumption is abruptly ended, metabolism is no longer able to respond rapidly enough to compensate. Glutamatergic neural activity adapts to chronic dysregulation of glutamate metabolism and suppression of glutamatergic neural activity by increasing excitatory and decreasing inhibitory amino acid receptors. A point is reached (ethanol dependence) where withdrawal of ethanol results in significant metabolic energy depletion in neurons and other brain cells as well as hyperexcitation of the glutamatergic system. The extent and regional specificity of energy depletion in the brain, combined with hyperactivity of the glutamatergic neuronal system, largely determines the severity of withdrawal symptoms.

Alcohol, microbiome, and their effect on psychiatric disorders

There is accumulating evidence that alcohol consumption and especially alcohol withdrawal increase brain levels of known innate immune signaling molecules and cause neuroinflammation. It has been shown that microbiota play a pivotal role in this process and affect central neurochemistry and behavior. Disruption of or alterations in the intimate cross-talk between microbiome and brain may be a significant factor in many psychiatric disorders. Alterations in the composition of the microbiome, so called dysbiosis, may result in detrimental distortion of microbe-host homeostasis modulating the hypothalamic-pituitary-adrenal axis. A variety of pathologies are associated with changes in the community structure and function of the gut microbiota, suggesting a link between dysbiosis and disease etiology, including irritable bowel syndrome depression, anxiety disorders, schizophrenia, and alcoholism. Despite a paucity of clinical studies in alcohol-dependent humans, emerging data suggests that alcohol induced alterations of the microbiome may explain reward-seeking behaviors as well as anxiety, depression, and craving in withdrawal and increase the risk of developing psychiatric disorders.

Fluoride Alteration of [3H]Glucose Uptake in Wistar Rat Brain and Peripheral Tissues

The present study was designed to investigate the role of postnatal fluoride intake on [3H]glucose uptake and transport in rat brain and peripheral tissues. Sodium fluoride (NaF) in a concentration of 10 or 50 ppm was added to the drinking water of adult Wistar rats. The control group received distilled water. After 4 weeks, respective plasma fluoride levels were 0.0541 ± 0.0135 μg/ml (control), 0.0596 ± 0.0202 μg/ml (10 ppm), and 0.0823 ± 0.0199 μg/ml (50 ppm). Although plasma glucose levels were not altered in any group, the plasma insulin level in the fluoride (50 ppm) group was elevated (0.72 ± 0.13 μg/ml) versus the control group (0.48 ± 0.24 μg/ml) and fluoride (10 ppm) group. In rats receiving fluoride for 4 weeks at 10 ppm in drinking water, [3H]glucose uptake was unaltered in all tested parts of the brain. However, in rats receiving fluoride at 50 ppm, [3H]glucose uptake in cerebral cortex, hippocampus, and thalamus with hypothalamus was elevated, versus the saline group. Fluoride intake had a negligible effect on [3H]glucose uptake by peripheral tissues (liver, pancreas, stomach, small intestine, atrium, aorta, kidney, visceral tissue, lung, skin, oral mucosa, tongue, salivary gland, incisor, molars, and jawbone). In neither fluoride group was glucose transporter proteins 1 (GLUT 1) or 3 (GLUT 3) altered in frontal cortex and striatum versus control. On the assumption that increased glucose uptake (by neural tissue) reasonably reflects neuronal activity, it appears that fluoride damage to the brain results in a compensatory increase in glucose uptake and utilization without changes in GLUT 1 and GLUT 3 expression.

The metabolic syndrome in patients with alcohol dependency: Current research and clinical implications

The relationship between alcohol dependency and disorders such as liver disease and cancer has been thoroughly researched. However, the effects of alcohol on cardiometabolic health remain controversial. Several reports found low to moderate alcohol consumption to be associated with a lower risk for cardiometabolic disorders. In contrast, excessive alcohol consumption has been related to an increased risk. Most of these studies were performed in non-clinical populations, therefore limiting the explanatory power to non-dependent patients. Only a few studies examined cardiovascular disorders and cardiovascular risk factors, in particular the metabolic syndrome (MetS), in alcohol dependent patients. We here present a narrative review of studies performed so far on the MetS in alcohol dependency, and provide current hypotheses on the association of alcohol dependency, appetite regulation and the development of the MetS.

Cerebral hypoperfusion and glucose hypometabolism: Key pathophysiological modulators promote neurodegeneration, cognitive impairment, and Alzheimer's disease

Aging, hypertension, diabetes, hypoxia/obstructive sleep apnea (OSA), obesity, vitamin B12/folate deficiency, depression, and traumatic brain injury synergistically promote diverse pathological mechanisms including cerebral hypoperfusion and glucose hypometabolism. These risk factors trigger neuroinflammation and oxidative-nitrosative stress that in turn decrease nitric oxide and enhance endothelin, Amyloid-β deposition, cerebral amyloid angiopathy, and blood-brain barrier disruption. Proinflammatory cytokines, endothelin-1, and oxidative-nitrosative stress trigger several pathological feedforward and feedback loops. These upstream factors persist in the brain for decades, upregulating amyloid and tau, before the cognitive decline. These cascades lead to neuronal Ca²⁺ increase, neurodegeneration, cognitive/memory decline, and Alzheimer's disease (AD). However, strategies are available to attenuate cerebral hypoperfusion and glucose hypometabolism and ameliorate cognitive decline. AD is the leading cause of dementia among the elderly. There is significant evidence that pathways involving inflammation and oxidative-nitrosative stress (ONS) play a key pathophysiological role in promoting cognitive dysfunction. Aging and several comorbid conditions mentioned above promote diverse pathologies. These include inflammation, ONS, hypoperfusion, and hypometabolism in the brain. In AD, chronic cerebral hypoperfusion and glucose hypometabolism precede decades before the cognitive decline. These comorbid disease conditions may share and synergistically activate these pathophysiological pathways. Inflammation upregulates cerebrovascular pathology through proinflammatory cytokines, endothelin-1, and nitric oxide (NO). Inflammation-triggered ONS promotes long-term damage involving fatty acids, proteins, DNA, and mitochondria; these amplify and perpetuate several feedforward and feedback pathological loops. The latter includes dysfunctional energy metabolism (compromised mitochondrial ATP production), amyloid-β generation, endothelial dysfunction, and blood-brain-barrier disruption. These lead to decreased cerebral blood flow and chronic cerebral hypoperfusion- that would modulate metabolic dysfunction and neurodegeneration. In essence, hypoperfusion deprives the brain from its two paramount trophic substances, viz., oxygen and nutrients. Consequently, the brain suffers from synaptic dysfunction and neuronal degeneration/loss, leading to both gray and white matter atrophy, cognitive dysfunction, and AD. This Review underscores the importance of treating the above-mentioned comorbid disease conditions to attenuate inflammation and ONS and ameliorate decreased cerebral blood flow and hypometabolism. Additionally, several strategies are described here to control chronic hypoperfusion of the brain and enhance cognition. © 2016 Wiley Periodicals, Inc.

Glucose Transporters at the Blood-Brain Barrier: Function, Regulation and Gateways for Drug Delivery

Glucose transporters (GLUTs) at the blood-brain barrier maintain the continuous high glucose and energy demands of the brain. They also act as therapeutic targets and provide routes of entry for drug delivery to the brain and central nervous system for treatment of neurological and neurovascular conditions and brain tumours. This article first describes the distribution, function and regulation of glucose transporters at the blood-brain barrier, the major ones being the sodium-independent facilitative transporters GLUT1 and GLUT3. Other GLUTs and sodium-dependent transporters (SGLTs) have also been identified at lower levels and under various physiological conditions. It then considers the effects on glucose transporter expression and distribution of hypoglycemia and hyperglycemia associated with diabetes and oxygen/glucose deprivation associated with cerebral ischemia. A reduction in glucose transporters at the blood-brain barrier that occurs before the onset of the main pathophysiological changes and symptoms of Alzheimer's disease is a potential causative effect in the vascular hypothesis of the disease. Mutations in glucose transporters, notably those identified in GLUT1 deficiency syndrome, and some recreational drug compounds also alter the expression and/or activity of glucose transporters at the blood-brain barrier. Approaches for drug delivery across the blood-brain barrier include the pro-drug strategy whereby drug molecules are conjugated to glucose transporter substrates or encapsulated in nano-enabled delivery systems (e.g. liposomes, micelles, nanoparticles) that are functionalised to target glucose transporters. Finally, the continuous development of blood-brain barrier in vitro models is important for studying glucose transporter function, effects of disease conditions and interactions with drugs and xenobiotics.

Ethanol-induced transcriptional activation of programmed cell death 4 (Pdcd4) is mediated by GSK-3β signaling in rat cortical neuroblasts

Ingestion of ethanol (ETOH) during pregnancy induces grave abnormalities in developing fetal brain. We have previously reported that ETOH induces programmed cell death 4 (PDCD4), a critical regulator of cell growth, in cultured fetal cerebral cortical neurons (PCNs) and in the cerebral cortex in vivo and affect protein synthesis as observed in Fetal Alcohol Spectrum Disorder (FASD). However, the mechanism which activates PDCD4 in neuronal systems is unclear and understanding this regulation may provide a counteractive strategy to correct the protein synthesis associated developmental changes seen in FASD. The present study investigates the molecular mechanism by which ethanol regulates PDCD4 in cortical neuroblasts, the immediate precursor of neurons. ETOH treatment significantly increased PDCD4 protein and transcript expression in spontaneously immortalized rat brain neuroblasts. Since PDCD4 is regulated at both the post-translational and post-transcriptional level, we assessed ETOH's effect on PDCD4 protein and mRNA stability. Chase experiments demonstrated that ETOH does not significantly impact either PDCD4 protein or mRNA stabilization. PDCD4 promoter-reporter assays confirmed that PDCD4 is transcriptionally regulated by ETOH in neuroblasts. Given a critical role of glycogen synthase kinase 3β (GSK-3β) signaling in regulating protein synthesis and neurotoxic mechanisms, we investigated the involvement of GSK-3β and showed that multifunctional GSK-3β was significantly activated in response to ETOH in neuroblasts. In addition, we found that ETOH-induced activation of PDCD4 was inhibited by pharmacologic blockade of GSK-3β using inhibitors, lithium chloride (LiCl) and SB-216763 or siRNA mediated silencing of GSK-3β. These results suggest that ethanol transcriptionally upregulates PDCD4 by enhancing GSK-3β signaling in cortical neuroblasts. Further, we demonstrate that canonical Wnt-3a/GSK-3β signaling is involved in regulating PDCD4 protein expression. Altogether, we provide evidence that GSK-3β/PDCD4 network may represent a critical modulatory point to manage the protein synthetic anomalies and growth aberrations of neural cells seen in FASD.

Ethanol, not detectably metabolized in brain, significantly reduces brain metabolism, probably via action at specific GABA(A) receptors and has measureable metabolic effects at very low concentrations

Ethanol is a known neuromodulatory agent with reported actions at a range of neurotransmitter receptors. Here, we measured the effect of alcohol on metabolism of [3-¹³C]pyruvate in the adult Guinea pig brain cortical tissue slice and compared the outcomes to those from a library of ligands active in the GABAergic system as well as studying the metabolic fate of [1,2-¹³C]ethanol. Analyses of metabolic profile clusters suggest that the significant reductions in metabolism induced by ethanol (10, 30 and 60 mM) are via action at neurotransmitter receptors, particularly α4β3δ receptors, whereas very low concentrations of ethanol may produce metabolic responses owing to release of GABA via GABA transporter 1 (GAT1) and the subsequent interaction of this GABA with local α5- or α1-containing GABA(A)R. There was no measureable metabolism of [1,2-¹³C]ethanol with no significant incorporation of ¹³C from [1,2-¹³C]ethanol into any measured metabolite above natural abundance, although there were measurable effects on total metabolite sizes similar to those seen with unlabelled ethanol.

Acute alcohol intoxication decreases glucose metabolism but increases acetate uptake in the human brain

Alcohol intoxication results in marked reductions in brain glucose metabolism, which we hypothesized reflect not just its GABAergic enhancing effects but also the metabolism of acetate as an alternative brain energy source. To test this hypothesis we separately assessed the effects of alcohol intoxication on brain glucose and acetate metabolism using Positron Emission Tomography (PET). We found that alcohol intoxication significantly decreased whole brain glucose metabolism (measured with FDG) with the largest decrements in cerebellum and occipital cortex and the smallest in the thalamus. In contrast, alcohol intoxication caused a significant increase in [1-(11)C]acetate brain uptake (measured as standard uptake value, SUV), with the largest increases occurring in the cerebellum and the smallest in the thalamus. In heavy alcohol drinkers [1-(11)C]acetate brain uptake during alcohol challenge tended to be higher than in occasional drinkers (p<0.06) and the increases in [1-(11)C]acetate uptake in cerebellum with alcohol were positively associated with the reported amount of alcohol consumed (r=0.66, p<0.01). Our findings corroborate a reduction of brain glucose metabolism during intoxication and document an increase in brain acetate uptake. The opposite changes observed between regional brain metabolic decrements and regional increases in [1-(11)C]acetate uptake support the hypothesis that during alcohol intoxication the brain may rely on acetate as an alternative brain energy source and provides preliminary evidence that heavy alcohol exposures may facilitate the use of acetate as an energy substrate. These findings raise the question of the potential therapeutic benefits that increasing plasma acetate concentration (i.e. ketogenic diets) may have in alcoholics undergoing alcohol detoxification.

Daily ethanol exposure during late ovine pregnancy: physiological effects in the mother and fetus in the apparent absence of overt fetal cerebral dysmorphology

High levels of ethanol (EtOH) consumption during pregnancy adversely affect fetal development; however, the effects of lower levels of exposure are less clear. Our objectives were to assess the effects of daily EtOH exposure (3.8 USA standard drinks) on fetal-maternal physiological variables and the fetal brain, particularly white matter. Pregnant ewes received daily intravenous infusions of EtOH (0.75 g/kg maternal body wt over 1 h, 8 fetuses) or saline (8 fetuses) from 95 to 133 days of gestational age (DGA; term ∼145 DGA). Maternal and fetal arterial blood was sampled at 131-133 DGA. At necropsy (134 DGA) fetal brains were collected for analysis. Maternal and fetal plasma EtOH concentrations reached similar maximal concentration (∼0.11 g/dl) and declined at the same rate. EtOH infusions produced mild reductions in fetal arterial oxygenation but there were no changes in maternal oxygenation, maternal and fetal Pa(CO(2)), or in fetal mean arterial pressure or heart rate. Following EtOH infusions, plasma lactate levels were elevated in ewes and fetuses, but arterial pH fell only in ewes. Fetal body and brain weights were similar between groups. In three of eight EtOH-exposed fetuses there were small subarachnoid hemorrhages in the cerebrum and cerebellum associated with focal cortical neuronal death and gliosis. Overall, there was no evidence of cystic lesions, inflammation, increased apoptosis, or white matter injury. We conclude that daily EtOH exposure during the third trimester-equivalent of ovine pregnancy has modest physiological effects on the fetus and no gross effects on fetal white matter development.

Inhibitory effects of alcohol on glucose transport across the blood-brain barrier leads to neurodegeneration: preventive role of acetyl-L: -carnitine

PURPOSE

Evidence shows that alcohol intake causes oxidative neuronal injury and neurocognitive deficits that are distinct from the classical Wernicke-Korsakoff neuropathy. Our previous findings indicated that alcohol-elicited blood-brain barrier (BBB) damage leads to neuroinflammation and neuronal loss. The dynamic function of the BBB requires a constant supply and utilization of glucose. Here we examined whether interference of glucose uptake and transport at the endothelium by alcohol leads to BBB dysfunction and neuronal degeneration.

MATERIAL AND METHODS

We tested the hypothesis in cell culture of human brain endothelial cells, neurons and alcohol intake in animal by immunofluorescence, Western blotting and glucose uptake assay methods.

RESULTS

We found that decrease in glucose uptake correlates the reduction of glucose transporter protein 1 (GLUT1) in cell culture after 50 mM ethanol exposure. Decrease in GLUT1 protein levels was regulated at the translation process. In animal, chronic alcohol intake suppresses the transport of glucose into the frontal and occipital regions of the brain. This finding is validated by a marked decrease in GLUT1 protein expression in brain microvessel (the BBB). In parallel, alcohol intake impairs the BBB tight junction proteins occludin, zonula occludens-1, and claudin-5 in the brain microvessel. Permeability of sodium fluorescein and Evans Blue confirms the leakiness of the BBB. Further, depletion of trans-endothelial electrical resistance of the cell monolayer supports the disruption of BBB integrity. Administration of acetyl-L: -carnitine (a neuroprotective agent) significantly prevents the adverse effects of alcohol on glucose uptake, BBB damage and neuronal degeneration.

CONCLUSION

These findings suggest that alcohol-elicited inhibition of glucose transport at the blood-brain interface leads to BBB malfunction and neurological complications.

Deep thiopental anesthesia alters steady-state glucose homeostasis but not the neurochemical profile of rat cortex

Barbiturates are regularly used as an anesthetic for animal experimentation and clinical procedures and are frequently provided with solubilizing compounds, such as ethanol and propylene glycol, which have been reported to affect brain function and, in the case of (1)H NMR experiments, originate undesired resonances in spectra affecting the quantification. As an alternative, thiopental can be administrated without any solubilizing agents. The aim of the study was to investigate the effect of deep thiopental anesthesia on the neurochemical profile consisting of 19 metabolites and on glucose transport kinetics in vivo in rat cortex compared with alpha-chloralose using localized (1)H NMR spectroscopy. Thiopental was devoid of effects on the neurochemical profile, except for the elevated glucose at a given plasma glucose level resulting from thiopental-induced depression of glucose consumption at isoelectrical condition. Over the entire range of plasma glucose levels, steady-state glucose concentrations were increased on average by 48% +/- 8%, implying that an effect of deep thiopental anesthesia on the transport rate relative to cerebral glucose consumption ratio was increased by 47% +/- 8% compared with light alpha-chloralose-anesthetized rats. We conclude that the thiopental-induced isoelectrical condition in rat cortex significantly affected glucose contents by depressing brain metabolism, which remained substantial at isoelectricity.

Effect of prenatal manganese intoxication on [(3)H]glucose uptake in the brain of rats lesioned as neonates with 6-hydroxydopamine

In the present study we examined the effects of prenatal manganese (Mn) intoxication on [(3)H]glucose uptake in the brain of rats lesioned as neonates with 6-hydroxydopamine (6-OHDA). MnCl(2) . 4H(2)O (10,000 ppm) was added to the drinking water of pregnant Wistar rats for the duration of pregnancy. On the day of parturition, Mn was discontinued as an additive to the drinking water. The control group consisted of rats that consumed water without Mn. Three days after birth, rats in both groups (control and Mn) were pretreated with desipramine hydrochloride (20 mg/kg) and pargyline hydrochloride (50 mg/kg) and injected bilaterally icv with one of three doses of 6-OHDA hydrobromide (15 mug, 30 mug or 67 mug base form in saline on each side) or with saline (control). 6-[(3)H]-D-glucose (500 muCi/kg, ip) was administered to male offspring in adulthood; after 15 min, brain specimens were taken (frontal cortex, hippocampus, striatum, thalamus with hypothalamus, pons and cerebellum) for determination of radioactivity in a liquid scintillation counter. Low dose 6-OHDA (15 mug icv) increased [(3)H]glucose uptake in all brain regions (p < 0.05) in both control and Mn-intoxicated animals. In rats lesioned with a moderate dose of 6-OHDA (30 mug icv), [(3)H]glucose uptake was unaltered in both control and Mn-exposed rats. High dose 6-OHDA (67 mug icv) reduced [(3)H]glucose uptake in all brain regions of Mn-exposed rats (except for cerebellum) compared with the saline group (all, p < 0.05). There was no change in regional brain uptake of [(3)H]glucose in control rats. In conclusion, this study shows that mild neuronal insult (15 mug icv 6-OHDA) increased glucose uptake in the brain while severe damage (concomitant 60 mug icv 6-OHDA and Mn treatment) significantly diminished this process.

Acute ethanol exposure in pregnancy alters the insulin-like growth factor axis of fetal and maternal sheep

Maternal ethanol intake during pregnancy impairs fetal growth, but mechanisms are not clearly defined. Reduced IGF abundance or bioavailability in the fetus and/or mother may contribute to this growth restriction. We hypothesized that an episode of acute ethanol exposure, mimicking binge drinking would restrict fetal growth and perturb the maternal and fetal IGF axes. Pregnant sheep were infused intravenously with saline or ethanol (1 g/kg maternal wt) over 1 h, on days 116, 117, and 118 of gestation (start of 1st infusion = time 0, term is 147 days). Maternal and fetal plasma IGF and IGF-binding protein (IGFBP) concentrations were measured before and after each infusion. Compared with controls, ethanol exposure reduced fetal weight at day 120 by 19%, transiently reduced maternal plasma IGF-I (-35%) at 30 h, and decreased fetal plasma IGF-II (-28%) from 24 to 54 h after the first infusion. Ethanol exposure did not alter maternal or fetal plasma concentrations of IGFBP-2 and IGFBP-3, measured by Western ligand blotting. We conclude that suppression of maternal and fetal IGF abundance may contribute to fetal growth restriction induced by acute or binge ethanol exposure.

Microarray analysis of mouse brain gene expression following acute ethanol treatment

Alterations in gene expression are thought to help mediate certain effects of alcohol in the brain. We have analyzed the expression of approximately 24,000 genes using oligonucleotide microarrays to examine the brain expression profiles in two strains of inbred mice, C57BL/6J and DBA/2J, following exposure to an acute dose of ethanol. Our screen identified 61 genes responding to the ethanol treatment beyond a 1.5-fold threshold, with 46 genes altered in both mouse strains and 15 altered in only one strain. Approximately 25% of the genes were selected for confirmation by reverse transcriptase polymerase chain reaction with an 87% success rate. The genes identified have roles in cell signaling, gene regulation, and homeostasis/stress response. Although some of the genes were previously known to be ethanol responsive, we have for the most part identified novel genes involved in the acute murine brain response to ethanol. Such genes have the potential to represent candidate genes in the search to elucidate the molecular pathways mediating ethanol's effects in the brain.