From artificial cerebro-spinal fluid (aCSF) to artificial extracellular fluid (aECF): microdialysis perfusate composition effects on in vivo brain ECF glucose measurements

Experimental Insights and Recommendations for Successfully Performing Cerebral Microdialysis With Hydrophobic Drug Candidates

Cerebral microdialysis in rodents represents a robust and versatile technique for quantifying the pharmacologically relevant unbound fraction of drugs in the brain. When this unbound fraction is simultaneously determined in plasma, it facilitates the calculation of the corresponding unbound plasma-to-brain partition coefficient (Kp,uu) for a given compound in vivo. This coefficient is critical for understanding the penetration and distribution of drugs across the blood-brain barrier (BBB). However, obtaining valid and accurate microdialysis data can be particularly challenging for hydrophobic drugs due to their pronounced non-specific interactions with the components of the microdialysis system. The present study reports the outcomes of comprehensive microdialysis investigations in rodents, focusing on three hydrophobic compounds: actinomycin D, selinexor, and ulixertinib. These compounds exhibited varying degrees of non-specific binding to the surfaces of the microdialysis apparatus, leading to low recovery rates and substantial carry-over effects. To diminish these limitations, strategies such as surface coating and the use of optimized materials were employed to enhance the reliability of the microdialysis system. To ensure the robustness and reproducibility of microdialysis-related research outcomes, our experimental findings were supplemented with a narrative literature review. This review encompassed keyword-driven PubMed-indexed publications on microdialysis from 1970 to 2024, providing a broader context for the challenges and solutions associated with the technique. By integrating empirical results with practical recommendations, this study offers a comprehensive resource aimed at advancing the application of cerebral microdialysis in preclinical drug development, particularly for compounds with challenging physicochemical properties.

Diet-induced diabetes is associated with lower hippocampal glycogen and reduced glycogenolysis following local exogenous insulin

Brain fuel (specifically, glucose) supply and metabolism are well-established to be limiting factors for cognitive performance, with the largest body of data being for hippocampally mediated tasks. Consistent with this, disease states such as Alzheimer's disease and insulin-resistant diabetes, that impair cognitive metabolism, impair cognition with this being shown again most prominently for hippocampally mediated processes. In addition to glucose supplied from the blood, brain oxidative metabolism can use local glycogen stores (within astrocytes) as a fuel source via conversion to lactate; both lactate and glycogen have been shown to be important contributors to regulation of cognitive metabolism. Insulin has been shown to be a key regulator of hippocampal cognitive and metabolic processes; in the periphery, insulin facilitates glycogen synthesis and storage, but the impact on brain glycogen is unclear. Furthermore, the impact of diet-induced diabetes on hippocampal glycogen levels and/or metabolism is unknown. Here, we show that in rats with high-fat diet-induced diabetes, hippocampal glycogen is reduced and is less responsive to acute intrahippocampal administration of insulin, which significantly reduces glycogen in the hippocampi of control animals: Our data suggest that impaired fuel availability from glycogen may be a contributing factor to the cognitive impairment seen in disease states that include central insulin resistance.

Unravelling the brain metabolome: A review of liquid chromatography - mass spectrometry strategies for extracellular brain metabolomics

The analysis of the brain extracellular metabolome is of interest for numerous subdomains within neuroscience. Not only does it provide information about normal physiological functions, it is even more of interest for biomarker discovery and target discovery in disease. The extracellular analysis of the brain is particularly interesting as it provides information about the release of mediators in the brain extracellular fluid to look at cellular signaling and metabolic pathways through the release, diffusion and re-uptake of neurochemicals. In vivo samples are obtained through microdialysis, cerebral open-flow microperfusion or solid-phase microextraction. The analytes of potential interest are typically low in concentration and can have a wide range of physicochemical properties. Liquid chromatography coupled to mass spectrometry has proven its usefulness in brain metabolomics. It allows sensitive and specific analysis of low sample volumes, obtained through different approaches. Several strategies for the analysis of the extracellular fluid have been proposed. The most widely used approaches apply sample derivatization, specific stationary phases and/or hydrophilic interaction liquid chromatography. Miniaturization of these methods allows an even higher sensitivity. The development of chiral metabolomics is indispensable, as it allows to compare the enantiomeric ratio of compounds and provides even more challenges. Some limitations continue to exist for the previously developed methods and the development of new, more sensitive methods remains needed. This review provides an overview of the methods developed for sampling and liquid chromatography-mass spectrometry analysis of the extracellular metabolome.

Succimer chelation does not produce lasting reductions of blood lead levels in a rodent model of retained lead fragments

Retained lead fragments from nonfatal firearm injuries pose a risk of lead poisoning. While chelation is well-established as a lead poisoning treatment, it remains unclear whether chelation mobilizes lead from embedded lead fragments. Here, we tested whether 1) DMSA/succimer or CaNa2EDTA increases mobilization of lead from fragments in vitro, and 2) succimer is efficacious in chelating fragment lead in vivo, using stable lead isotope tracer methods in a rodent model of embedded fragments. DMSA was > 10-times more effective than CaNa2EDTA in mobilizing fragment lead in vitro. In the rodent model, succimer chelation on day 1 produced the greatest blood lead reductions, and fragment lead was not mobilized into blood. However, with continued chelation and over 3-weeks post-chelation, blood lead levels rebounded with mobilization of lead from the fragments. These findings suggest prolonged chelation will increase fragment lead mobilization post-chelation, supporting the need for long-term surveillance in patients with retained fragments.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

Brain capillary pericytes are metabolic sentinels that control blood flow through a KATP channel-dependent energy switch

Despite the abundance of capillary thin-strand pericytes and their proximity to neurons and glia, little is known of the contributions of these cells to the control of brain hemodynamics. We demonstrate that the pharmacological activation of thin-strand pericyte KATP channels profoundly hyperpolarizes these cells, dilates upstream penetrating arterioles and arteriole-proximate capillaries, and increases capillary blood flow. Focal stimulation of pericytes with a KATP channel agonist is sufficient to evoke this response, mediated via KIR2.1 channel-dependent retrograde propagation of hyperpolarizing signals, whereas genetic inactivation of pericyte KATP channels eliminates these effects. Critically, we show that decreasing extracellular glucose to less than 1 mM or inhibiting glucose uptake by blocking GLUT1 transporters in vivo flips a mechanistic energy switch driving rapid KATP-mediated pericyte hyperpolarization to increase local blood flow. Together, our findings recast capillary pericytes as metabolic sentinels that respond to local energy deficits by increasing blood flow to neurons to prevent energetic shortfalls.

Dendritic Excitability and Synaptic Plasticity In Vitro and In Vivo

Much of our understanding of dendritic and synaptic physiology comes from in vitro experimentation, where the afforded mechanical stability and convenience of applying drugs allowed patch-clamping based recording techniques to investigate ion channel distributions, their gating kinetics, and to uncover dendritic integrative and synaptic plasticity rules. However, with current efforts to study these questions in vivo, there is a great need to translate existing knowledge between in vitro and in vivo experimental conditions. In this review, we identify discrepancies between in vitro and in vivo ionic composition of extracellular media and discuss how changes in ionic composition alter dendritic excitability and plasticity induction. Here, we argue that under physiological in vivo ionic conditions, dendrites are expected to be more excitable and the threshold for synaptic plasticity induction to be lowered. Consequently, the plasticity rules described in vitro vary significantly from those implemented in vivo.

Leptin in the Commissural Nucleus of the Tractus Solitarius (cNTS) and Anoxic Stimulus in the Carotid Body Chemoreceptors Increases cNTS Leptin Signaling Receptor and Brain Glucose Retention in Rats

Background and Objectives: The commissural nucleus of the tractus solitarius (cNTS) not only responds to glucose levels directly, but also receives afferent signals from the liver, and from the carotid chemoreceptors (CChR). In addition, leptin, through its receptors in the cNTS, regulates food intake, body weight, blood glucose levels, and brain glucose retention (BGR). These leptin effects on cNTS are thought to be mediated through the sympathetic–adrenal system. How these different sources of information converging in the NTS regulate blood glucose levels and brain glucose retention remains largely unknown. The goal of the present study was to determine whether the local administration of leptin in cNTS alone, or after local anoxic stimulation using sodium cyanide (NaCN) in the carotid sinus, modifies the expression of leptin Ob-Rb and of c-Fos mRNA. We also investigated how leptin, alone, or in combination with carotid sinus stimulation, affected brain glucose retention. Materials and Methods: The experiments were carried out in anesthetized male Wistar rats artificially ventilated to maintain homeostatic values for pO₂, pCO₂, and pH. We had four groups: (a) experimental 1, leptin infusion in cNTS and NaCN in the isolated carotid sinus (ICS; n = 10); (b) experimental 2, leptin infusion in cNTS and saline in the ICS (n = 10); (c) control 1, artificial cerebrospinal fluid (aCSF) in cNTS and NaCN in the ICS (n = 10); (d) control 2, aCSF in cNTS and saline in the ICS (n = 10). Results: Leptin in cNTS, preceded by NaCN in the ICS increased BGR and leptin Ob-Rb mRNA receptor expression, with no significant increases in c-Fos mRNA in the NTSc. Conclusions: Leptin in the cNTS enhances brain glucose retention induced by an anoxic stimulus in the carotid chemoreceptors, through an increase in Ob-Rb receptors, without persistent changes in neuronal activation.

Proton MRS on sub-microliter volume in rat brain using implantable NMR microcoils

The use of miniaturized NMR receiver coils is an effective approach for improving detection sensitivity in studies using MRS and MRI. By optimizing the filling factor (the fraction of the coil occupied by the sample), and by increasing the RF magnetic field produced per unit current, the sensitivity gain offered by NMR microcoils is particularly interesting when small volumes or regions of interest are investigated. For in vivo studies, millimetric or sub-millimetric microcoils can be deployed in tissues to access regions of interest located at a certain depth. In this study, the implementation and application of a tissue-implantable NMR microcoil with a detection volume of 850 nL is described. The RF magnetic field generated by the microcoil was evaluated using a finite element method simulation and experimentally determined by high spatial resolution MRI acquisitions. The performance of the microcoil in terms of spectral resolution and limit of detection was measured at 7 T in vitro and in vivo in rodent brains. These performances were compared with those of a conventional external detection coil. Proton MR spectra were acquired in the cortex of rat brain. The concentrations of main metabolites were quantified and compared with reference values from the literature. In vitro and in vivo results obtained with the implantable microcoil showed a gain in sensitivity greater than 50 compared with detection using an external coil. In vivo proton spectra of diagnostic value were obtained from brain regions of a few hundred nanoliters. The similarities between spectra obtained with the implanted microcoil and those obtained with the external NMR coil highlight the minimally invasive nature of the coil implantation procedure. These implantable microcoils represent new tools for probing tissue metabolism in very small healthy or diseased regions using MRS.

Carvedilol prevents impairment of the counterregulatory response in recurrently hypoglycaemic diabetic rats

AIM

It has been suggested that repeated activation of the adrenergic system during antecedent episodes of hypoglycaemia contributes to the development of counterregulatory failure. We previously reported that treatment with carvedilol, a non-specific β-blocker, prevented the development of counterregulatory failure and improved hypoglycaemia awareness in recurrently hypoglycaemic non-diabetic rats. The current study investigated whether carvedilol has similar benefits in diabetic rats.

METHODS

Recurrently hypoglycaemic streptozotocin-diabetic rats (STZ+RH) were treated with carvedilol for one week prior to undergoing a hypoglycaemic clamp. Hypoglycaemia awareness was evaluated in streptozotocin-diabetic rats made hypoglycaemia unaware using repeated injections of 2-deoxyglucose.

RESULTS

Compared to hypoglycaemia-naïve STZ-diabetic controls, exogenous glucose requirements were more than doubled in the STZ+RH animals and this was associated with a 49% reduction in the epinephrine response to hypoglycaemia. Treating STZ+RH animals with carvedilol improved the epinephrine response to hypoglycaemia. Of note, neither recurrent hypoglycaemia nor carvedilol treatment affected the glucagon response in diabetic animals. Additionally, carvedilol treatment improved the feeding response to insulin-induced hypoglycaemia in diabetic animals made 'hypoglycaemia unaware' using repeated injections of 2-deoxyglucose, suggesting the treatment improved awareness of hypoglycaemia as well.

CONCLUSION

Our data suggest that carvedilol may be useful in preventing impairments of the sympathoadrenal response and the development of hypoglycaemia unawareness during recurring episodes of hypoglycaemia in diabetic animals.

Repeated Activation of Noradrenergic Receptors in the Ventromedial Hypothalamus Suppresses the Response to Hypoglycemia

Activation of the adrenergic system in response to hypoglycemia is important for proper recovery from low glucose levels. However, it has been suggested that repeated adrenergic stimulation may also contribute to counterregulatory failure, but the underlying mechanisms are not known. The aim of this study was to establish whether repeated activation of noradrenergic receptors in the ventromedial hypothalamus (VMH) contributes to blunting of the counterregulatory response by enhancing local lactate production. The VMH of nondiabetic rats were infused with either artificial extracellular fluid, norepinephrine (NE), or salbutamol for 3 hours/day for 3 consecutive days before they underwent a hypoglycemic clamp with microdialysis to monitor changes in VMH lactate levels. Repeated exposure to NE or salbutamol suppressed both the glucagon and epinephrine responses to hypoglycemia compared to controls. Furthermore, antecedent NE and salbutamol treatments raised extracellular lactate levels in the VMH. To determine whether the elevated lactate levels were responsible for impairing the hormone response, we pharmacologically inhibited neuronal lactate transport in a subgroup of NE-treated rats during the clamp. Blocking neuronal lactate utilization improved the counterregulatory hormone responses in NE-treated animals, suggesting that repeated activation of VMH β2-adrenergic receptors increases local lactate levels which in turn, suppresses the counterregulatory hormone response to hypoglycemia.

Antimicrobial Essential Oil Formulation: Chitosan Coated Nanoemulsions for Nose to Brain Delivery

Brain infections as meningitis and encephalitis are attracting a great interest. Challenges in the treatment of these diseases are mainly represented by the blood brain barrier (BBB) that impairs the efficient delivery of even very potent drugs to reach the brain. The nose to the brain administration route, is a non-invasive alternative for a quick onset of action, and enables the transport of numerous medicinal agents straight to the brain thus workarounding the BBB through the highly vascularized olfactory region. In this report, Thymus vulgaris and Syzygium aromaticum essential oils (EOs) were selected to be included in chitosan coated nanoemulsions (NEs). The EOs were firstly analyzed to determine their chemical composition, then used to prepare NEs, that were deeply characterized in order to evaluate their use in intranasal administration. An in vitro evaluation against a collection of clinical isolated bacterial strains was carried out for both free and nanoemulsioned EOs. Chitosan coated NEs showed to be a potential and effective intranasal formulation against multi-drug resistant Gram-negative bacteria such as methicillin-susceptible Staphylococcus aureus and multi-drug resistant Gram-negative microorganisms including carbapenem-resistant Acinetobacter baumannii and Klebsiella pneumoniae.

Investigation of Early Death-Induced Changes in Rat Brain by Solid Phase Microextraction via Untargeted High Resolution Mass Spectrometry: In Vivo versus Postmortem Comparative Study

Analysis of brain samples obtained postmortem remains a standard approach in neuroscience, despite often being suboptimal for inferring roles of small molecules in the pathophysiology of brain diseases. Sample collection and preservation further hinders conclusive interpretation of biomarker analysis in autopsy samples. We investigate purely death-induced changes affecting rat hippocampus in the first hour of postmortem interval (PMI) by means of untargeted liquid chromatography-mass spectrometry-based metabolomics. The unique possibility of sampling the same brain area of each animal both in vivo and postmortem was enabled by employing solid phase microextraction (SPME) probes. Four millimeter probes coated with mixed mode extraction phase were used to sample awake, freely roaming animals, with 2 more sampling events performed after death. Significant changes in brain neurochemistry were found to occur as soon as 30 min after death, further progressing with increasing PMI, evidenced by relative changes in levels of metabolites and lipids. These included species from several distinct groups, which can be classified as engaged in energy metabolism-related processes, signal transduction, neurotransmission, or inflammatory response. Additionally, we perform thorough analysis of interindividual variability in response to death, which provides insights into how this aspect can obscure conclusions drawn from an untargeted study at single metabolite and pathway level. The results suggest high demand for systematic studies examining the PMI time course with in vivo sampling as a starting point to eliminate artifacts in the form of neurochemical changes assumed to occur in vivo.

Leptin in the Commissural Nucleus Tractus Solitarii Increases the Glucose Responses to Carotid Chemoreceptors Activation by Cyanide

Leptin is a protein hormone that plays a key role in the regulation of energy balance and glucose homeostasis. Leptin and all leptin receptor isoforms are present in the carotid bodies, but its precise function in glucose regulation and metabolism is not yet known. The aim of this study was to determine whether exogenous leptin, microinjected into the commissural nucleus tractus solitarii (cNTS), preceding sodium cyanide (NaCN) injection into the circulatory isolated carotid sinus (ICS), in vivo, modifies hyperglycemic reflex (HR) and brain glucose retention (BGR). In anesthetized Wistar rats (sodium pentobarbital, i.p. 3.3 mg/100 g/saline, Pfizer, Mex), arterial and venous blood samples were collected from silastic catheters implanted in the abdominal aorta and jugular sinus. Exogenous leptin (50 ng/20 nL of aCSF) or leptin vehicle (20 nL of aCSF) microinjected (stereotaxically) into the cNTS 4 min before NaCN (5 μg/100 g/50 μL saline into ICS) (experimental 1 [E1] and control 1[C1] groups, respectively) significantly increased HR and BGR compared with their basal values, but the increase was bigger in the E1 group. When leptin or aCSF were injected into the cNTS before saline (E2 and C2 groups, respectively) glucose responses did not vary when compared with their basal levels. Leptin and its receptors in the cNTS cells probably contribute to their sensitization during hypoxia.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Mental Work Requires Physical Energy: Self-Control Is Neither Exception nor Exceptional

The brain’s reliance on glucose as a primary fuel source is well established, but psychological models of cognitive processing that take energy supply into account remain uncommon. One exception is research on self-control depletion, where debate continues over a limited-resource model. This model argues that a transient reduction in self-control after the exertion of prior self-control is caused by the depletion of brain glucose, and that self-control processes are special, perhaps unique, in this regard. This model has been argued to be physiologically implausible in several recent reviews. This paper attempts to correct some inaccuracies that have occurred during debate over the physiological plausibility of this model. We contend that not only is such limitation of cognition by constraints on glucose supply plausible, it is well established in the neuroscience literature across several cognitive domains. Conversely, we argue that there is no evidence that self-control is special in regard to its metabolic cost. Mental processes require physical energy, and the body is limited in its ability to supply the brain with sufficient energy to fuel mental processes. This article reviews current findings in brain metabolism and seeks to resolve the current conflict in the field regarding the physiological plausibility of the self-control glucose-depletion hypothesis.

Chitosan Glutamate-Coated Niosomes: A Proposal for Nose-to-Brain Delivery

The aim of this in vitro study is to prepare and characterize drug free and pentamidine loaded chitosan glutamate coated niosomes for intranasal drug delivery to reach the brain through intranasal delivery. Mucoadhesive properties and stability testing in various environments were evaluated to examine the potential of these formulations to be effective drug delivery vehicles for intranasal delivery to the brain. Samples were prepared using thin film hydration method. Changes in size and ζ-potential of coated and uncoated niosomes with and without loading of pentamidine in various conditions were assessed by dynamic light scattering (DLS), while size and morphology were also studied by atomic force microscopy (AFM). Bilayer properties and mucoadhesive behavior were investigated by fluorescence studies and DLS analyses, respectively. Changes in vesicle size and ζ-potential values were shown after addition of chitosan glutamate to niosomes, and when in contact with mucin solution. In particular, interactions with mucin were observed in both drug free and pentamidine loaded niosomes regardless of the presence of the coating. The characteristics of the proposed systems, such as pentamidine entrapment and mucin interaction, show promising results to deliver pentamidine or other possible drugs to the brain via nasal administration.

Neem oil nanoemulsions: characterisation and antioxidant activity

The aim of the present work is to develop nanoemulsions (NEs), nanosized emulsions, manufactured for improving the delivery of active pharmaceutical ingredients. In particular, nanoemulsions composed of Neem seed oil, contain rich bioactive components, and Tween 20 as nonionic surfactant were prepared. A mean droplet size ranging from 10 to 100 nm was obtained by modulating the oil/surfactant ratio. Physicochemical characterisation was carried out evaluating size, ζ-potential, microviscosity, polarity and turbidity of the external shell and morphology, along with stability in simulated cerebrospinal fluid (CSF), activity of Neem oil alone and in NEs, HEp-2 cell interaction and cytotoxicity studies. This study confirms the formation of NEs by Tween 20 and Neem oil at different weight ratios with small and homogenous dimensions. The antioxidant activity of Neem oil alone and in NEs was comparable, whereas its cytotoxicity was strongly reduced when loaded in NEs after interaction with HEp-2 cells.

Insulin modulates hippocampally-mediated spatial working memory via glucose transporter-4

The insulin-regulated glucose transporter, GluT4, is a key molecule in peripheral insulin signaling. Although GluT4 is abundantly expressed in neurons of specific brain regions such as the hippocampus, the functional role of neuronal GluT4 is unclear. Here, we used pharmacological inhibition of GluT4-mediated glucose uptake to determine whether GluT4 mediates insulin-mediated glucose uptake in the hippocampus. Consistent with previous reports, we found that glucose utilization increased in the dorsal hippocampus of male rats during spontaneous alternation (SA), a hippocampally-mediated spatial working memory task. We previously showed that insulin signaling within the hippocampus is required for processing this task, and that administration of exogenous insulin enhances performance. At baseline levels of hippocampal insulin, inhibition of GluT4-mediated glucose uptake did not affect SA performance. However, inhibition of an upstream regulator of GluT4, Akt, did impair SA performance. Conversely, when a memory-enhancing dose of insulin was delivered to the hippocampus prior to SA-testing, inhibition of GluT4-mediated glucose transport prevented cognitive enhancement. These data suggest that baseline hippocampal cognitive processing does not require functional hippocampal GluT4, but that cognitive enhancement by supra-baseline insulin does. Consistent with these findings, we found that in neuronal cell culture, insulin increases glucose utilization in a GluT4-dependent manner. Collectively, these data demonstrate a key role for GluT4 in transducing the procognitive effects of elevated hippocampal insulin.

Novel Roles for the Insulin-Regulated Glucose Transporter-4 in Hippocampally Dependent Memory

The insulin-regulated glucose transporter-4 (GluT4) is critical for insulin- and contractile-mediated glucose uptake in skeletal muscle. GluT4 is also expressed in some hippocampal neurons, but its functional role in the brain is unclear. Several established molecular modulators of memory processing regulate hippocampal GluT4 trafficking and hippocampal memory formation is limited by both glucose metabolism and insulin signaling. Therefore, we hypothesized that hippocampal GluT4 might be involved in memory processes. Here, we show that, in male rats, hippocampal GluT4 translocates to the plasma membrane after memory training and that acute, selective intrahippocampal inhibition of GluT4-mediated glucose transport impaired memory acquisition, but not memory retrieval. Other studies have shown that prolonged systemic GluT4 blockade causes insulin resistance. Unexpectedly, we found that prolonged hippocampal blockade of glucose transport through GluT4-upregulated markers of hippocampal insulin signaling prevented task-associated depletion of hippocampal glucose and enhanced both working and short-term memory while also impairing long-term memory. These effects were accompanied by increased expression of hippocampal AMPA GluR1 subunits and the neuronal GluT3, but decreased expression of hippocampal brain-derived neurotrophic factor, consistent with impaired ability to form long-term memories. Our findings are the first to show the cognitive impact of brain GluT4 modulation. They identify GluT4 as a key regulator of hippocampal memory processing and also suggest differential regulation of GluT4 in the hippocampus from that in peripheral tissues.

SIGNIFICANCE STATEMENT

The role of insulin-regulated glucose transporter-4 (GluT4) in the brain is unclear. In the current study, we demonstrate that GluT4 is a critical component of hippocampal memory processes. Memory training increased hippocampal GluT4 translocation and memory acquisition was impaired by GluT4 blockade. Unexpectedly, whereas long-term inhibition of GluT4 impaired long-term memory, short-term memory was enhanced. These data further our understanding of the molecular mechanisms of memory and have particular significance for type 2 diabetes (in which GluT4 activity in the periphery is impaired) and Alzheimer's disease (which is linked to impaired brain insulin signaling and for which type 2 diabetes is a key risk factor). Both diseases cause marked impairment of hippocampal memory linked to hippocampal hypometabolism, suggesting the possibility that brain GluT4 dysregulation may be one cause of cognitive impairment in these disease states.

Epileptogenesis in organotypic hippocampal cultures has limited dependence on culture medium composition

Rodent organotypic hippocampal cultures spontaneously develop epileptiform activity after approximately 2 weeks in vitro and are increasingly used as a model of chronic post-traumatic epilepsy. However, organotypic cultures are maintained in an artificial environment (culture medium), which contains electrolytes, glucose, amino acids and other components that are not present at the same concentrations in cerebrospinal fluid (CSF). Therefore, it is possible that epileptogenesis in organotypic cultures is driven by these components. We examined the influence of medium composition on epileptogenesis. Epileptogenesis was evaluated by measurements of lactate and lactate dehydrogenase (LDH) levels (biomarkers of ictal activity and cell death, respectively) in spent culture media, immunohistochemistry and automated 3-D cell counts, and extracellular recordings from CA3 regions. Changes in culture medium components moderately influenced lactate and LDH levels as well as electrographic seizure burden and cell death. However, epileptogenesis occurred in any culture medium that was capable of supporting neural survival. We conclude that medium composition is unlikely to be the cause of epileptogenesis in the organotypic hippocampal culture model of chronic post-traumatic epilepsy.

Consensus statement from the 2014 International Microdialysis Forum

Microdialysis enables the chemistry of the extracellular interstitial space to be monitored. Use of this technique in patients with acute brain injury has increased our understanding of the pathophysiology of several acute neurological disorders. In 2004, a consensus document on the clinical application of cerebral microdialysis was published. Since then, there have been significant advances in the clinical use of microdialysis in neurocritical care. The objective of this review is to report on the International Microdialysis Forum held in Cambridge, UK, in April 2014 and to produce a revised and updated consensus statement about its clinical use including technique, data interpretation, relationship with outcome, role in guiding therapy in neurocritical care and research applications.

Membrane Potential Dynamics of Spontaneous and Visually Evoked Gamma Activity in V1 of Awake Mice

Cortical gamma activity (30–80 Hz) is believed to play important functions in neural computation and arises from the interplay of parvalbumin-expressing interneurons (PV) and pyramidal cells (PYRs). However, the subthreshold dynamics underlying its emergence in the cortex of awake animals remain unclear. Here, we characterized the intracellular dynamics of PVs and PYRs during spontaneous and visually evoked gamma activity in layers 2/3 of V1 of awake mice using targeted patch-clamp recordings and synchronous local field potentials (LFPs). Strong gamma activity patterned in short bouts (one to three cycles), occurred when PVs and PYRs were depolarizing and entrained their membrane potential dynamics regardless of the presence of visual stimulation. PV firing phase locked unconditionally to gamma activity. However, PYRs only phase locked to visually evoked gamma bouts. Taken together, our results indicate that gamma activity corresponds to short pulses of correlated background synaptic activity synchronizing the output of cortical neurons depending on external sensory drive.

Gamma activity, an important component of brain dynamics, is driven by synaptic background activity and synchronizes distinct cortical cell types differently depending on visual input.

The neocortex is the main substrate of cognitive activity of the mammalian brain. During active wakefulness, it exhibits an oscillatory activity in the gamma range (30–80Hz), which is believed to play an important functional role and is altered in schizophrenic patients. Experimental studies have shown that gamma activity arises from the interaction of excitatory pyramidal neurons, the main neuronal type of the cortex, and local inhibitory neurons expressing the protein parvalbumin (PV). However, how these neuronal types behave during gamma activity remains largely unknown. Here, we recorded the intracellular activity of pyramidal and PV-expressing neurons in the visual cortex of awake mice while acquiring Local Field Potentials (LFPs)—extracellular voltage fluctuations within a small volume of the cortex—to monitor gamma activity. We found that gamma activity arises when PV-expressing neurons synchronize their output in response to a correlated input, reflecting the general activation of the local cortical network. This happens even in the absence of visual input. On the other hand, the output of pyramidal neurons only becomes entrained to gamma activity when the mice are exposed to visual stimulation. Thus, our results suggest that gamma activity synchronizes pyramidal neurons specifically when the cortex is engaged in processing external inputs.

Recurrent Hypoglycemia Increases Anxiety and Amygdala Norepinephrine Release During Subsequent Hypoglycemia

Recurrent hypoglycemia (RH) is a common and debilitating side effect of therapy in patients with both type 1 and, increasingly, type 2 diabetes. Previous studies in rats have shown marked effects of RH on subsequent hippocampal behavioral, metabolic, and synaptic processes. In addition to impaired memory, patients experiencing RH report alterations in cognitive processes that include mood and anxiety, suggesting that RH may also affect amygdala function. We tested the impact of RH on amygdala function using an elevated plus-maze test of anxiety together with in vivo amygdala microdialysis for norepinephrine (NEp), a widely used marker of basolateral amygdala cognitive processes. In contrast to findings in the hippocampus and prefrontal cortex, neither RH nor acute hypoglycemia alone significantly affected plus-maze performance or NEp release. However, animals tested when hypoglycemic who had previously experienced RH had elevated amygdala NEp during plus-maze testing, accompanied by increased anxiety (i.e., less time spent in the open arms of the plus-maze). The results show that RH has widespread effects on subsequent brain function, which vary by neural system.

Characterization of conjugated polymer actuation under cerebral physiological conditions

Conjugated polymer actuators have potential use in implantable neural interface devices for modulating the position of electrode sites within brain tissue or guiding insertion of neural probes along curved trajectories. The actuation of polypyrrole (PPy) doped with dodecylbenzenesulfonate (DBS) is characterized to ascertain whether it can be employed in the cerebral environment. Microfabricated bilayer beams are electrochemically cycled at either 22 or 37 °C in aqueous NaDBS or in artificial cerebrospinal fluid (aCSF). Nearly all the ions in aCSF are exchanged into the PPy-the cations Na(+) , K(+) , Mg(2+) , Ca(2+) , as well as the anion PO4 (3-) ; Cl(-) is not present. Nevertheless, deflections in aCSF are comparable to those in NaDBS and they are monotonic with oxidation level: strain increases upon reduction, with no reversal of motion despite the mixture of ionic charges and valences being exchanged. Actuation depends on temperature. Upon warming, the cyclic voltammograms show additional peaks and an increase of 70% in the consumed charge. Bending is, however, much less affected: strain increases somewhat (6%-13%) but remains monotonic, and deflections shift (up to 20%). These results show how the actuation environment must be taken into account, and demonstrate proof of concept for actuated implantable neural interfaces.

Assessment of blood-brain barrier function and the neuroinflammatory response in the rat brain by using cerebral open flow microperfusion (cOFM)

Blood-brain barrier (BBB) impairment in systemic inflammation leads to neuroinflammation. Several factors including cytokines, chemokines and signal transduction molecules are implicated in BBB dysfunction in response to systemic inflammation. Here, we have adopted a novel in vivo technique; namely, cerebral open flow microperfusion (cOFM), to perform time-dependent cytokine analysis (TNF-alpha, IL-6 and IL-10) in the frontal cortex of the rat brain in response to a single peripheral administration of lipopolysaccharide (LPS). In parallel, we monitored BBB function using sodium fluorescein as low molecular weight reporter in the cOFM sample. In response to the systemic LPS administration, we observed a rapid increase of TNF-alpha in the serum and brain, which coincides with the BBB disruption. Brain IL-6 and IL-10 synthesis was delayed by approximately 1 h. Our data demonstrate that cOFM can be used to monitor changes in brain cytokine levels and BBB disruption in a rat sepsis model.

Long-term implanted cOFM probe causes minimal tissue reaction in the brain

This study investigated the histological tissue reaction to long-term implanted cerebral open flow microperfusion (cOFM) probes in the frontal lobe of the rat brain. Most probe-based cerebral fluid sampling techniques are limited in application time due to the formation of a glial scar that hinders substance exchange between brain tissue and the probe. A glial scar not only functions as a diffusion barrier but also alters metabolism and signaling in extracellular brain fluid. cOFM is a recently developed probe-based technique to continuously sample extracellular brain fluid with an intact blood-brain barrier. After probe implantation, a 2 week healing period is needed for blood-brain barrier reestablishment. Therefore, cOFM probes need to stay in place and functional for at least 15 days after implantation to ensure functionality. Probe design and probe materials are optimized to evoke minimal tissue reaction even after a long implantation period. Qualitative and quantitative histological tissue analysis revealed no continuous glial scar formation around the cOFM probe 30 days after implantation and only a minor tissue reaction regardless of perfusion of the probe.

Simultaneous recording of hippocampal oxygen and glucose in real time using constant potential amperometry in the freely-moving rat

Amperometric sensors for oxygen and glucose allow for real time recording from the brain in freely-moving animals. These sensors have been used to detect activity- and drug-induced changes in metabolism in a number of brain regions but little attention has been given over to the hippocampus despite its importance in cognition and disease. Sensors for oxygen and glucose were co-implanted into the hippocampus and allowed to record for several days. Baseline recordings show that basal concentrations of hippocampal oxygen and glucose are 100.26±5.76 μM and 0.60±0.06 mM respectively. Furthermore, stress-induced changes in neural activity have been shown to significantly alter concentrations of both analytes in the hippocampus. Administration of O2 gas to the animals' snouts results in significant increases in hippocampal oxygen and glucose and administration of N2 gas results in a significant decrease in hippocampal oxygen. Chloral hydrate-induced anaesthesia causes a significant increase in hippocampal oxygen whereas treatment with the carbonic anhydrase inhibitor acetazolamide significantly increases hippocampal oxygen and glucose. These findings provide real time electrochemical data for the hippocampus which has been previously impossible with traditional methods such as microdialysis or ex vivo analysis. As such, these sensors provide a window into hippocampal function which can be used in conjunction with behavioural and pharmacological interventions to further elucidate the functions and mechanisms of action of the hippocampus in normal and disease states.

Elevated glucose metabolism in the amygdala during an inhibitory avoidance task

There is a long-standing debate as to whether the memory process of consolidation is neurochemically similar to or the same as the set of processes involved in retrieval and reconsolidation of that memory. In addition, although we have previously shown that initial memory processing in the hippocampus causes a drainage of hippocampal glucose because of increased local metabolic demand, it is unknown what metabolic changes occur elsewhere in the brain or during subsequent processing of a previously consolidated memory. Male Sprague Dawley rats (3 months old) were implanted with unilateral microdialysis cannulae and in vivo microdialysis of amygdala extracellular fluid (ECF) was performed during both (i) initial learning and (ii) retrieval 24 h later of an aversively motivated avoidance memory task. ECF samples were analyzed for glucose, lactate, pyruvate and glutamate. Results showed close similarity between increases in local glycolysis seen during both consolidation and retrieval, but also suggested that there may perhaps be a difference in amygdalar oxidative phosphorylation stimulated by the two processes. Hence, our data suggest that memory formation places similar metabolic demands across neural systems, and that consolidation may be metabolically different from retrieval.

Hippocampal insulin microinjection and in vivo microdialysis during spatial memory testing

Glucose metabolism is a useful marker for local neural activity, forming the basis of methods such as 2-deoxyglucose and functional magnetic resonance imaging. However, use of such methods in animal models requires anesthesia and hence both alters the brain state and prevents behavioral measures. An alternative method is the use of in vivo microdialysis to take continuous measurement of brain extracellular fluid concentrations of glucose, lactate, and related metabolites in awake, unrestrained animals. This technique is especially useful when combined with tasks designed to rely on specific brain regions and/or acute pharmacological manipulation; for example, hippocampal measurements during a spatial working memory task (spontaneous alternation) show a dip in extracellular glucose and rise in lactate that are suggestive of enhanced glycolysis, and intrahippocampal insulin administration both improves memory and increases hippocampal glycolysis. Substances such as insulin can be delivered to the hippocampus via the same microdialysis probe used to measure metabolites. The use of spontaneous alternation as a measure of hippocampal function is designed to avoid any confound from stressful motivators (e.g. footshock), restraint, or rewards (e.g. food), all of which can alter both task performance and metabolism; this task also provides a measure of motor activity that permits control for nonspecific effects of treatment. Combined, these methods permit direct measurement of the neurochemical and metabolic variables regulating behavior.

Caffeine prevents weight gain and cognitive impairment caused by a high-fat diet while elevating hippocampal BDNF

Obesity, high-fat diets, and subsequent type 2 diabetes (T2DM) are associated with cognitive impairment. Moreover, T2DM increases the risk of Alzheimer's disease (AD) and leads to abnormal elevation of brain beta-amyloid levels, one of the hallmarks of AD. The psychoactive alkaloid caffeine has been shown to have therapeutic potential in AD but the central impact of caffeine has not been well-studied in the context of a high-fat diet. Here we investigated the impact of caffeine administration on metabolism and cognitive performance, both in control rats and in rats placed on a high-fat diet. The effects of caffeine were significant: caffeine both (i) prevented the weight-gain associated with the high-fat diet and (ii) prevented cognitive impairment. Caffeine did not alter hippocampal metabolism or insulin signaling, likely because the high-fat-fed animals did not develop full-blown diabetes; however, caffeine did prevent or reverse a decrease in hippocampal brain-derived neurotrophic factor (BDNF) seen in high-fat-fed animals. These data confirm that caffeine may serve as a neuroprotective agent against cognitive impairment caused by obesity and/or a high-fat diet. Increased hippocampal BDNF following caffeine administration could explain, at least in part, the effects of caffeine on cognition and metabolism.

Antibody-enhanced microdialysis collection of CCL2 from rat brain

Chemokine(C-C motif) Ligand 2 (CCL2 or MCP-1) is a signaling protein that is released under various conditions. In this study we demonstrate the first microdialysis collection of CCL2 from rat brain tissue using antibody-enhanced microdialysis. A monoclonal antibody to CCL2 was included in the dialysis perfusion fluid as an affinity agent to enhance the recovery of CCL2 both in vitro and in vivo. In vitro it was found that the use of antibody affinity agent increases the relative recovery of CCL2 from 9.6±3.4% to 37.5±10.2% and 64.8±11.7% (n=10) at flow rates of 2μL/min and 1μL/min, respectively. Following the in vitro observation, CCL2 was collected from rat brain with microdialysis sampling using both control and antibody-included perfusion fluids. The in vivo data showed that relative recovery was increased at all but the first time point. This shows that the use of free antibody in the perfusion fluid increases the relative recovery of CCL2 and this enhanced microdialysis method may be applicable to other cytokines.

Lactate Effectively Covers Energy Demands during Neuronal Network Activity in Neonatal Hippocampal Slices

Although numerous experimental data indicate that lactate is efficiently used for energy by the mature brain, the direct measurements of energy metabolism parameters during neuronal network activity in early postnatal development have not been performed. Therefore, the role of lactate in the energy metabolism of neurons at this age remains unclear. In this study, we monitored field potentials and contents of oxygen and NAD(P)H in correlation with oxidative metabolism during intense network activity in the CA1 hippocampal region of neonatal brain slices. We show that in the presence of glucose, lactate is effectively utilized as an energy substrate, causing an augmentation of oxidative metabolism. Moreover, in the absence of glucose lactate is fully capable of maintaining synaptic function. Therefore, during network activity in neonatal slices, lactate can be an efficient energy substrate capable of sustaining and enhancing aerobic energy metabolism.

Hippocampal memory processes are modulated by insulin and high-fat-induced insulin resistance

Insulin regulates glucose uptake and storage in peripheral tissues, and has been shown to act within the hypothalamus to acutely regulate food intake and metabolism. The machinery for transduction of insulin signaling is also present in other brain areas, particularly in the hippocampus, but a physiological role for brain insulin outside the hypothalamus has not been established. Recent studies suggest that insulin may be able to modulate cognitive functions including memory. Here we report that local delivery of insulin to the rat hippocampus enhances spatial memory, in a PI-3-kinase dependent manner, and that intrahippocampal insulin also increases local glycolytic metabolism. Selective blockade of endogenous intrahippocampal insulin signaling impairs memory performance. Further, a rodent model of type 2 diabetes mellitus produced by a high-fat diet impairs basal cognitive function and attenuates both cognitive and metabolic responses to hippocampal insulin administration. Our data demonstrate that insulin is required for optimal hippocampal memory processing. Insulin resistance within the telencephalon may underlie the cognitive deficits commonly reported to accompany type 2 diabetes.

Establishing a physiological environment for visualized in vitro brain slice recordings by increasing oxygen supply and modifying aCSF content

Our insights into the basic characteristics of neuronal function were significantly advanced by combining the in vitro slice technique with the visualization of neurons and their processes. The visualization through water immersion objectives requires keeping slices submerged in recording chambers where delivering artificial cerebro-spinal fluid (aCSF) at flow rates of 2-3 ml/min results in a limited oxygen supply [Hájos N, Ellender TJ, Zemankovics R, Mann EO, Exley R, Cragg SJ, et al. Maintaining network activity in submerged hippocampal slices: importance of oxygen supply. Eur J Neurosci 2009;29:319-27]. Here we review two methods aimed at providing sufficient oxygen levels to neurons in submerged slices to enable high energy consuming processes such as elevated firing rates or network oscillations. The use of these methods may also influence the outcome of other electrophysiological experiments in submerged slices including the study of intercellular signaling pathways. In addition, we also emphasize the importance of various aCSF constituents used in in vitro experiments.

Regional difference in stainability with calcium-sensitive acetoxymethyl-ester probes in mouse brain slices

Loading neurons with membrane permeable Ca2+ indicators is a core experimental procedure in functional multineuron Ca2+ imaging (fMCI), an optical technique for monitoring multiple neuronal activities. Although fMCI has been applied to several brain networks, including cerebral cortex, hippocampus, and cerebellum, no studies have systematically addressed the dye-loading efficiency in different brain regions. Here, we describe the stainability of Oregon Green 488 BAPTA-1AM in mouse acute brain slice preparations. The data are suggestive of the potential usability of fMCI in many brain regions, including olfactory bulb, thalamus, dentate gyrus, habenular nucleus, and pons.

Spontaneous plasticity of multineuronal activity patterns in activated hippocampal networks

Using functional multineuron imaging with single-cell resolution, we examined how hippocampal networks by themselves change the spatiotemporal patterns of spontaneous activity during the course of emitting spontaneous activity. When extracellular ionic concentrations were changed to those that mimicked in vivo conditions, spontaneous activity was increased in active cell number and activity frequency. When ionic compositions were restored to the control conditions, the activity level returned to baseline, but the weighted spatial dispersion of active cells, as assessed by entropy-based metrics, did not. Thus, the networks can modify themselves by altering the internal structure of their correlated activity, even though they as a whole maintained the same level of activity in space and time.

Spike Ca2+ influx upmodulates the spike afterdepolarization and bursting via intracellular inhibition of KV7/M channels

In principal brain neurons, activation of Ca(2+) channels during an action potential, or spike, causes Ca(2+) entry into the cytosol within a millisecond. This in turn causes rapid activation of large conductance Ca(2+)-gated channels, which enhances repolarization and abbreviates the spike. Here we describe another remarkable consequence of spike Ca(2+) entry: enhancement of the spike afterdepolarization. This action is also mediated by intracellular modulation of a particular class of K(+) channels, namely by inhibition of K(V)7 (KCNQ) channels. These channels generate the subthreshold, non-inactivating M-type K(+) current, whose activation curtails the spike afterdepolarization. Inhibition of K(V)7/M by spike Ca(2+) entry allows the spike afterdepolarization to grow and can convert solitary spikes into high-frequency bursts of action potentials. Through this novel intracellular modulatory action, Ca(2+) spike entry regulates the discharge mode and the signalling capacity of principal brain neurons.

Active hippocampal networks undergo spontaneous synaptic modification

The brain is self-writable; as the brain voluntarily adapts itself to a changing environment, the neural circuitry rearranges its functional connectivity by referring to its own activity. How the internal activity modifies synaptic weights is largely unknown, however. Here we report that spontaneous activity causes complex reorganization of synaptic connectivity without any external (or artificial) stimuli. Under physiologically relevant ionic conditions, CA3 pyramidal cells in hippocampal slices displayed spontaneous spikes with bistable slow oscillations of membrane potential, alternating between the so-called UP and DOWN states. The generation of slow oscillations did not require fast synaptic transmission, but their patterns were coordinated by local circuit activity. In the course of generating spontaneous activity, individual neurons acquired bidirectional long-lasting synaptic modification. The spontaneous synaptic plasticity depended on a rise in intracellular calcium concentrations of postsynaptic cells, but not on NMDA receptor activity. The direction and amount of the plasticity varied depending on slow oscillation patterns and synapse locations, and thus, they were diverse in a network. Once this global synaptic refinement occurred, the same neurons now displayed different patterns of spontaneous activity, which in turn exhibited different levels of synaptic plasticity. Thus, active networks continuously update their internal states through ongoing synaptic plasticity. With computational simulations, we suggest that with this slow oscillation-induced plasticity, a recurrent network converges on a more specific state, compared to that with spike timing-dependent plasticity alone.

ATP-sensitive K(+) channels regulate the release of GABA in the ventromedial hypothalamus during hypoglycemia

OBJECTIVE-To determine whether alterations in counterregulatory responses to hypoglycemia through the modulation of ATP-sensitive K(+) channels (K(ATP) channels) in the ventromedial hypothalamus (VMH) are mediated by changes in GABAergic inhibitory tone in the VMH, we examined whether opening and closing K(ATP) channels in the VMH alter local GABA levels and whether the effects of modulating K(ATP) channel activity within the VMH can be reversed by local modulation of GABA receptors. RESEARCH DESIGN AND METHODS-Rats were cannulated and bilateral guide cannulas inserted to the level of the VMH. Eight days later, the rats received a VMH microinjection of either 1) vehicle, 2) the K(ATP) channel opener diazoxide, 3) the K(ATP) channel closer glybenclamide, 4) diazoxide plus the GABA(A) receptor agonist muscimol, or 5) glybenclamide plus the GABA(A) receptor antagonist bicuculline methiodide (BIC) before performance of a hypoglycemic clamp. Throughout, VMH GABA levels were measured using microdialysis. RESULTS-As expected, diazoxide suppressed glucose infusion rates and increased glucagon and epinephrine responses, whereas glybenclamide raised glucose infusion rates in conjunction with reduced glucagon and epinephrine responses. These effects of K(ATP) modulators were reversed by GABA(A) receptor agonism and antagonism, respectively. Microdialysis revealed that VMH GABA levels decreased 22% with the onset of hypoglycemia in controls. Diazoxide caused a twofold greater decrease in GABA levels, and glybenclamide increased VMH GABA levels by 57%. CONCLUSIONS-Our data suggests that K(ATP) channels within the VMH may modulate the magnitude of counterregulatory responses by altering release of GABA within that region.

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.

[Microdialysis and brain injury: importance of animal study]

Brain microdialysis allows the exploration of brain extracellular medium. This review discusses the main contribution of brain microdialysis for the knowledge of the pathophysiology of brain ischemia and trauma. We describe fundamental principle of microdialysis, limits, and validated metabolic parameters as the lactate/pyruvate ratio or glycerol. The interest to use microdialysis for testing metabolic hypothesis and potential scientific research ways will also be discussed.

Blockade of GABA(A) receptors in the ventromedial hypothalamus further stimulates glucagon and sympathoadrenal but not the hypothalamo-pituitary-adrenal response to hypoglycemia

Hypoglycemia provokes a multifaceted counterregulatory response involving the sympathoadrenal system, stimulation of glucagon secretion, and the hypothalamo-pituitary-adrenal axis that is commonly impaired in diabetes. We examined whether modulation of inhibitory input from gamma-aminobutyric acid (GABA) in the ventromedial hypothalamus (VMH), a major glucose-sensing region within the brain, plays a role in affecting counterregulatory responses to hypoglycemia. Normal Sprague-Dawley rats had carotid artery and jugular vein catheters chronically implanted, as well as bilateral steel microinjection guide cannulas inserted down to the level of the VMH. Seven to 10 days following surgery, the rats were microinjected with artificial extracellular fluid, the GABA(A) receptor agonist muscimol (1 nmol/side), or the GABA(A) receptor antagonist bicuculline methiodide (12.5 pmol/side) before being subjected to a hyperinsulinemic-hypoglycemic (2.5 mmol/l) glucose clamp for 90 min. Following VMH administration of bicuculline methiodide, glucose infusion rates were significantly suppressed, whereas muscimol raised glucose infusion rates significantly compared with controls. Glucagon and epinephrine responses were elevated with the antagonist and suppressed with the agonist compared with controls. Corticosterone responses, however, were unaffected by either administration of the agonist or antagonist into the VMH. These data demonstrate that modulation of the GABAergic system in the VMH alters both glucagon and sympathoadrenal, but not corticosterone, responses to hypoglycemia. Our findings are consistent with the hypothesis that GABAergic inhibitory tone within the VMH can modulate glucose counterregulatory responses.

Differential effects of low glucose concentrations on seizures and epileptiform activityin vivoandin vitro

In vivo, severe hypoglycemia is frequently associated with seizures. The hippocampus is a structure prone to develop seizures and seizure-induced damage. Patients with repeated hypoglycemic episodes have frequent memory problems, suggesting impaired hippocampal function. Here we studied the effects of moderate hypoglycemia on primarily generalized flurothyl-induced seizures in vivo and, using EEG recordings, we determined involvement of the hippocampus in hypoglycemic seizures. Moderate systemic hypoglycemia had proconvulsant effects on flurothyl-induced clonic (forebrain) seizures. During hypoglycemic seizures, seizure discharges were recorded in the hippocampus. Thus, we continued the studies in combined entorhinal cortex-hippocampus slices in vitro. However, in vitro, decreases in extracellular glucose from baseline 10 mM to 2 or 1 mM did not induce any epileptiform discharges. In fact, low glucose (2 and 1 mM) attenuated preexisting low-Mg2+-induced epileptiform activity in the entorhinal cortex and hippocampal CA1 region. Osmolarity compensation in low-glucose solution using mannitol impaired slice recovery. Additionally, using paired-pulse stimuli we determined that there was no impairment of GABAA inhibition in the dentate gyrus during glucopenia. The data strongly indicate that, although forebrain susceptibility to seizures is increased during moderate in vivo hypoglycemia and the hippocampus is involved during hypoglycemic seizures, glucose depletion in vitro contributes to an arrest of epileptiform activity in the system of the entorhinal cortex-hippocampus network and there is no impairment of net GABAA inhibition during glucopenia.

Methods of measuring metabolism during surgery in humans: focus on the liver-brain relationship

PURPOSE OF REVIEW

The purpose of this work is to review recent advances in setting methods and models for measuring metabolism during surgery in humans. Surgery, especially solid organ transplantation, may offer unique experimental models in which it is ethically acceptable to gain information on difficult problems of amino acid and protein metabolism.

RECENT FINDINGS

Two areas are reviewed: the metabolic study of the anhepatic phase during liver transplantation and brain microdialysis during cerebral surgery. The first model offers an innovative approach to understand the relative role of liver and extrahepatic organs in gluconeogenesis, and to evaluate whether other organs can perform functions believed to be exclusively or almost exclusively performed by the liver. The second model offers an insight to intracerebral metabolism that is closely bound to that of the liver.

SUMMARY

The recent advances in metabolic research during surgery provide knowledge immediately useful for perioperative patient management and for a better control of surgical stress. The studies during the anhepatic phase of liver transplantation have showed that gluconeogenesis and glutamine metabolism are very active processes outside the liver. One of the critical organs for extrahepatic glutamine metabolism is the brain. Microdialysis studies helped to prove that in humans there is an intense trafficking of glutamine, glutamate and alanine among neurons and astrocytes. This delicate network is influenced by systemic amino acid metabolism. The metabolic dialogue between the liver and the brain is beginning to be understood in this light in order to explain the metabolic events of brain damage during liver failure.

Regional glucose metabolism and glutamatergic neurotransmission in rat brain in vivo

Multivolume (1)H-[(13)C] NMR spectroscopy in combination with i.v. [1,6-(13)C(2)]glucose infusion was used to detect regional glucose metabolism and glutamatergic neurotransmission in the halothane-anesthetized rat brain at 7 T. The regional information was decomposed into pure cerebral gray matter, white matter, and subcortical structures by means of tissue segmentation based on quantitative T(1) relaxation mapping. The (13)C turnover curves of [4-(13)C]glutamate, [4-(13)C]glutamine, and [3-(13)C]glutamate + glutamine were fitted with a two-compartment neuronal-astroglial metabolic model. The neuronal tricarboxylic acid cycle fluxes in cerebral gray matter, white matter, and subcortex were 0.79 +/- 0.15, 0.20 +/- 0.11, and 0.42 +/- 0.09 micromol/min per g, respectively. The glutamate-glutamine neurotransmitter cycle fluxes in cerebral gray matter, white matter, and subcortex were 0.31 +/- 0.07, 0.02 +/- 0.04, and 0.18 +/- 0.12 micromol/min per g, respectively. The exchange rate between the mitochondrial and cytosolic metabolite pools was fast relative to the neuronal tricarboxylic acid cycle flux for all cerebral tissue types.