Glycogen in the central nervous system

Ultrastructural characterization of dark microglia during aging in a mouse model of Alzheimer's disease pathology and in human post-mortem brain samples

A diverse heterogeneity of microglial cells was previously described in Alzheimer's disease (AD) pathology, including dark microglia, a state characterized by ultrastructural markers of cellular stress. To provide novel insights into the roles of dark microglia during aging in the context of AD pathology, we performed a quantitative density and ultrastructural analysis of these cells using high-throughput scanning electron microscopy in the ventral hippocampus CA1 stratum lacunosum-moleculare of 20-month-old APP-PS1 vs C57BL/6J male mice. The density of dark microglia was significantly higher in APP-PS1 vs C57BL/6J mice, with these cells accounting for nearly half of all microglia observed near amyloid-beta (Aβ) plaques. This dark microglial state interacted more with dystrophic neurites compared to other APP-PS1 microglia and possessed glycogen granules, associated with a metabolic shift toward glycolysis, which provides the first ultrastructural evidence of their presence in microglia. Dark microglia were further observed in aging human post-mortem brain samples showing similar ultrastructural features as in mouse. Overall, our results provide a quantitative ultrastructural characterization of a microglial state associated with cellular stress (i.e., dark microglia) that is primarily restricted near Aβ plaques and dystrophic neurites. The presence of this microglial state in the aging human post-mortem brain is further revealed.

3D Anatomy of the Quail Lumbosacral Spinal Canal-Implications for Putative Mechanosensory Function

Birds are diverse and agile vertebrates capable of aerial, terrestrial, aquatic, and arboreal locomotion. Evidence suggests that birds possess a novel balance sensing organ in the lumbosacral spinal canal, a structure referred to as the "lumbosacral organ" (LSO), which may contribute to their locomotor agility and evolutionary success. The mechanosensing mechanism of this organ remains unclear. Here we quantify the 3D anatomy of the lumbosacral region of the common quail, focusing on establishing the geometric and biomechanical properties relevant to potential mechanosensing functions. We combine digital and classic dissection to create a 3D anatomical model of the quail LSO and estimate the capacity for displacement and deformation of the soft tissues. We observe a hammock-like network of denticulate ligaments supporting the lumbosacral spinal cord, with a close association between the accessory lobes and ligamentous intersections. The relatively dense glycogen body has the potential to apply loads sufficient to pre-stress denticulate ligaments, enabling external accelerations to excite tuned oscillations in the LSO soft tissue, leading to strain-based mechanosensing in the accessory lobe neurons. Considering these anatomical features together, the structure of the LSO is reminiscent of a mass-spring-based accelerometer.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'_ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

The Role of Brain Glycogen in Supporting Physiological Function

Glycogen is present in the mammalian brain but occurs at concentrations so low it is unlikely to act as a conventional energy reserve. Glycogen has the intriguing feature of being located exclusively in astrocytes, but its presence benefits neurones, suggesting that glycogen is metabolized to a conduit that is transported between the glia and neural elements. In the rodent optic nerve model glycogen supports axon conduction in the form of lactate to supplement axonal metabolism during aglycemia, hypoglycemia and during periods of increased energy demand under normoglycemic conditions. In the hippocampus glycogen plays a vital role in supplying the neurones with lactate during memory formation. The physiological processes that glycogen supports, such as learning and memory, imply an inclusive and vital role in supporting physiological brain functions.

Methodological considerations for studies of brain glycogen

Glycogen stores in the brain have been recognized for decades, but the underlying physiological function of this energy reserve remains elusive. This uncertainty stems in part from several technical challenges inherent in the study of brain glycogen metabolism. These include low glycogen content in the brain, non-homogeneous labeling of glycogen by radiotracers, rapid glycogenolysis during postmortem tissue handling, and effects of the stress response on brain glycogen turnover. Here we briefly review the aspects of the glycogen structure and metabolism that bear on these technical challenges and present ways they can be addressed.

Norepinephrine regulation of ventromedial hypothalamic nucleus metabolic transmitter biomarker and astrocyte enzyme and receptor expression: Impact of 5' AMP-activated protein kinase

The ventromedial hypothalamic energy sensor AMP-activated protein kinase (AMPK) maintains glucostasis via neurotransmitter signals that diminish [γ-aminobutyric acid] or enhance [nitric oxide] counter-regulation. Ventromedial hypothalamic nucleus (VMN) 'fuel-inhibited' neurons are sensitive to astrocyte-generated metabolic substrate stream. Norepinephrine (NE) regulates astrocyte glycogen metabolism in vitro, and hypoglycemia intensifies VMN NE activity in vivo. Current research investigated the premise that NE elicits AMPK-dependent adjustments in VMN astrocyte glycogen metabolic enzyme [glycogen synthase (GS); glycogen phosphorylase (GP)] and gluco-regulatory neuron biomarker [glutamate decarboxylase_65/67 (GAD); neuronal nitric oxide synthase (nNOS); SF-1] protein expression in male rats. We also examined whether VMN astrocytes are directly receptive to NE and if noradrenergic input regulates cellular sensitivity to the neuro-protective steroid estradiol. Intra-VMN NE correspondingly augmented or reduced VMN tissue GAD and nNOS protein despite no change in circulating glucose, data that imply that short-term exposure to NE promotes persistent improvement in VMN nerve cell energy stability. The AMPK inhibitor Compound C (Cc) normalized VMN nNOS, GS, and GP expression in NE-treated animals. NE caused AMPK-independent down-regulation of alpha₂-, alongside Cc-reversible augmentation of beta₁-adrenergic receptor protein profiles in laser-microdissected astrocytes. NE elicited divergent adjustments in astrocyte estrogen receptor-beta (AMPK-unrelated reduction) and GPR-30 (Cc-revocable increase) proteins. Outcomes implicate AMPK in noradrenergic diminution of VMN nitrergic metabolic-deficit signaling and astrocyte glycogen shunt activity. Differentiating NE effects on VMN astrocyte adrenergic and estrogen receptor variant expression suggest that noradrenergic regulation of glycogen metabolism may be mediated, in part, by one or more receptors characterized here by sensitivity to this catecholamine.

State-Dependent Changes in Brain Glycogen Metabolism

A fundamental understanding of glycogen structure, concentration, polydispersity and turnover is critical to qualify the role of glycogen in the brain. These molecular and metabolic features are under the control of neuronal activity through the interdependent action of neuromodulatory tone, ionic homeostasis and availability of metabolic substrates, all variables that concur to define the state of the system. In this chapter, we briefly describe how glycogen responds to selected behavioral, nutritional, environmental, hormonal, developmental and pathological conditions. We argue that interpreting glycogen metabolism through the lens of brain state is an effective approach to establish the relevance of energetics in connecting molecular and cellular neurophysiology to behavior.

Metabolism of Glycogen in Brain White Matter

Brain glycogen is a specialized energy buffer, rather than a conventional reserve. In the rodent optic nerve, a central white matter tract, it is located in astrocytes, where it is converted to lactate, which is then shuttled intercellularly from the astrocyte to the axon. This basic pathway was elucidated from non-physiological experiments in which the nerve was deprived of exogenous glucose. However, this shuttling also occurs under physiological conditions, when tissue energy demand is increased above baseline levels in the presence of normoglycemic concentrations of glucose. The signaling mechanism by which axons alert astrocytes to their increased energy requirement is likely to be elevated interstitial K⁺, the inevitable consequence of increased neuronal activity.

Brain Glycogen Structure and Its Associated Proteins: Past, Present and Future

This chapter reviews the history of glycogen-related research and discusses in detail the structure, regulation, chemical properties and subcellular distribution of glycogen and its associated proteins, with particular focus on these aspects in brain tissue.

Regional Distribution of Glycogen in the Mouse Brain Visualized by Immunohistochemistry

Considering that the brain constantly consumes a substantial amount of energy, the nature of its energy reserve is an important issue. Although the brain is rich in lipid content encompassing membranes, myelin sheath, and astrocytic lipid droplets, it is devoid of adipose tissue which serves as an energy reserve. Notably, glycogen represents the major energy store in the brain. While glycogen has been observed mainly in astrocytes for decades by electron microscopy, glycogen distribution in the brain has only been partially documented. The involvement of glycogen metabolism in memory consolidation, demonstrated by several research groups, has reiterated the functional significance of this macromolecule and the need for description of its comprehensive distribution in the brain. The combination of focused microwave-assisted brain fixation and glycogen immunohistochemistry permits assessment of glycogen distribution in the rodent brain. In this article, we describe glycogen distribution in the mouse brain using glycogen immunohistochemistry. We find heterogeneous glycogen storage patterns at multiple spatial scales. The heterogeneous glycogen distribution patterns may underlie local energy metabolism or synaptic activity, and its mechanistic understanding should extend our knowledge on brain metabolism in health and disease.

Astrocytic Coverage of Dendritic Spines, Dendritic Shafts, and Axonal Boutons in Hippocampal Neuropil

Distal astrocytic processes have a complex morphology, reminiscent of branchlets and leaflets. Astrocytic branchlets are rod-like processes containing mitochondria and endoplasmic reticulum, capable of generating inositol-3-phosphate (IP₃)-dependent Ca²⁺ signals. Leaflets are small and flat processes that protrude from branchlets and fill the space between synapses. Here we use three-dimensional (3D) reconstructions from serial section electron microscopy (EM) of rat CA1 hippocampal neuropil to determine the astrocytic coverage of dendritic spines, shafts and axonal boutons. The distance to the maximum of the astrocyte volume fraction (VF) correlated with the size of the spine when calculated from the center of mass of the postsynaptic density (PSD) or from the edge of the PSD, but not from the spine surface. This suggests that the astrocytic coverage of small and larger spines is similar in hippocampal neuropil. Diffusion simulations showed that such synaptic microenvironment favors glutamate spillover and extrasynaptic receptor activation at smaller spines. We used complexity and entropy measures to characterize astrocytic branchlets and leaflets. The 2D projections of astrocytic branchlets had smaller spatial complexity and entropy than leaflets, consistent with the higher structural complexity and less organized distribution of leaflets. The VF of astrocytic leaflets was highest around dendritic spines, lower around axonal boutons and lowest around dendritic shafts. In contrast, the VF of astrocytic branchlets was similarly low around these three neuronal compartments. Taken together, these results suggest that astrocytic leaflets preferentially contact synapses as opposed to the dendritic shaft, an arrangement that might favor neurotransmitter spillover and extrasynaptic receptor activation along dendritic shafts.

Inhibition of glycogen phosphorylase stimulates ventromedial hypothalamic nucleus AMP-activated protein kinase: Activity and neuronal nitric oxide synthase protein expression in male rats

The glucose polymer glycogen is a vital fuel reserve in the brain. The mediobasal hypothalamic energy sensor AMP‐activated protein kinase (AMPK) maintains glucostasis via neurotransmitter mechanisms that suppress [γ‐aminobutyric acid; GABA] or stimulate [nitric oxide; steroidogenic factor‐1 (SF1)] counter‐regulatory outflow. This study investigated whether glycogen‐derived fuel supply is a critical screened variable in ventromedial hypothalamic nucleus (VMN) monitoring of neuro‐metabolic stability during glucostasis and/or insulin (I)‐induced hypoglycemia. Adult male rats were pretreated by intra‐VMN infusion of the glycogen phosphorylase inhibitor 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB) before sc vehicle or I injection. Western blot analyses of micropunch‐dissected VMN tissue from euglycemic animals showed DAB augmentation of phosphoAMPK (pAMPK), neuronal nitric oxide synthase (nNOS), and SF‐1, but not glutamate decarboxylase_65/67 (GAD) protein. Combinatory DAB/I treatment did not further enhance AMPK activity but significantly amplified nNOS expression relative to DAB alone. Hypoglycemic stimulation of corticosterone, but not glucagon release was prevented by DAB. Results imply that glycogen‐derived substrate fuel provision represses VMN AMPK activity and neurotransmitter signals of metabolic deficiency. Progressive augmentation of nNOS protein by DAB/I versus DAB/V intimates that “fuel‐inhibited” nitrergic neurons may exhibit increasing sensitivity to disrupted glycogen breakdown during glucoprivation versus glucostasis. nNOS and GAD reactivity to DAB/I, but not I implies that acute glycogen utilization during hypoglycemia may be sufficiently robust to avert effects on local metabolic sensory signaling. DAB/I upregulation of GAD alongside prevention of hypercorticosteronemia suggests that indicators of metabolic sufficiency may occur secondary to local compensatory adaptations to severe restriction of glucose‐derived energy.

Neuroprotective potential of astroglia

Astroglia are the homoeostatic cells of the central nervous system, which participate in all essential functions of the brain. Astrocytes support neuronal networks by handling water and ion fluxes, transmitter clearance, provision of antioxidants, and metabolic precursors and growth factors. The critical dependence of neurons on constant support from the astrocytes confers astrocytes with intrinsic neuroprotective properties. On the other hand, loss of astrocytic support or their pathological transformation compromises neuronal functionality and viability. Manipulating neuroprotective functions of astrocytes is thus an important strategy to enhance neuronal survival and improve outcomes in disease states. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Emerging Roles for Glycogen in the CNS

The ability of glycogen, the depot into which excess glucose is stored in mammals, to act as a source of rapidly available energy substrate, has been exploited by several organs for both general and local advantage. The liver, expressing the highest concentration of glycogen maintains systemic normoglycemia ensuring the brain receives a supply of glucose in excess of demand. However the brain also contains glycogen, although its role is more specialized. Brain glycogen is located exclusively in astrocytes in the adult, with the exception of pathological conditions, thus in order to benefit neurons, and energy conduit (lactate) is trafficked inter-cellularly. Such a complex scheme requires cell type specific expression of a variety of metabolic enzymes and transporters. Glycogen supports neural elements during withdrawal of glucose, but once the limited buffer of glycogen is exhausted neural function fails and irreversible injury ensues. Under physiological conditions glycogen acts to provide supplemental substrates when ambient glucose is unable to support function during increased energy demand. Glycogen also supports learning and memory where it provides lactate to neurons during the conditioning phase of in vitro long-term potentiation (LTP), an experimental correlate of learning. Inhibiting the breakdown of glycogen or intercellular transport of lactate in in vivo rat models inhibits the retention of memory. Our current understanding of the importance of brain glycogen is expanding to encompass roles that are fundamental to higher brain function.

Glycogen distribution in the microwave-fixed mouse brain reveals heterogeneous astrocytic patterns

In the brain, glycogen metabolism has been implied in synaptic plasticity and learning, yet the distribution of this molecule has not been fully described. We investigated cerebral glycogen of the mouse by immunohistochemistry (IHC) using two monoclonal antibodies that have different affinities depending on the glycogen size. The use of focused microwave irradiation yielded well-defined glycogen immunoreactive signals compared with the conventional periodic acid-Schiff method. The IHC signals displayed a punctate distribution localized predominantly in astrocytic processes. Glycogen immunoreactivity (IR) was high in the hippocampus, striatum, cortex, and cerebellar molecular layer, whereas it was low in the white matter and most of the subcortical structures. Additionally, glycogen distribution in the hippocampal CA3-CA1 and striatum had a 'patchy' appearance with glycogen-rich and glycogen-poor astrocytes appearing in alternation. The glycogen patches were more evident with large-molecule glycogen in young adult mice but they were hardly observable in aged mice (1-2 years old). Our results reveal brain region-dependent glycogen accumulation and possibly metabolic heterogeneity of astrocytes. GLIA 2016;64:1532-1545.

Sex-specific basal and hypoglycemic patterns of in vivo caudal dorsal vagal complex astrocyte glycogen metabolic enzyme protein expression

Astrocytes contribute to neurometabolic stability through uptake, catabolism, and storage of glucose. These cells maintain the major brain glycogen reservoir, which is a critical fuel supply to neurons during glucose deficiency and increased brain activity. We used a combinatory approach incorporating immunocytochemistry, laser microdissection, and Western blotting to investigate the hypothesis of divergent expression of key enzymes regulating glycogen metabolism and glycolysis during in vivo normo- and/or hypoglycemia in male versus female hindbrain astrocytes. Glycogen synthase (GS) and glycogen phosphorylase (GP) levels were both enhanced in dorsal vagal complex astrocytes from vehicle-injected female versus male controls, with incremental increase in GS exceeding GP. Insulin-induced hypoglycemia (IIH) diminished GS and increased glycogen synthase kinase-3-beta (GSK3β) expression in both sexes, but decreased phosphoprotein phosphatase-1 (PP1) levels only in males. Astrocyte GP content was elevated by IIH in male, but not female rats. Data reveal sex-dependent sensitivity of these enzyme proteins to lactate as caudal hindbrain repletion of this energy substrate fully or incompletely reversed hypoglycemic inhibition of GS and prevented hypoglycemic augmentation of GSK3β and GP in females and males, respectively. Sex dimorphic patterns of glycogen branching and debranching enzyme protein expression were also observed. Levels of the rate-limiting glycolytic enzyme, phosphofructokinase, were unaffected by IIH with or without lactate repletion. Current data demonstrating sex-dependent basal and hypoglycemic patterns of hindbrain astrocyte glycogen metabolic enzyme expression imply that glycogen volume and turnover during glucose sufficiency and shortage may vary accordingly.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Noninvasive measurement of brain glycogen by nuclear magnetic resonance spectroscopy and its application to the study of brain metabolism

Glycogen is the reservoir for glucose in the brain. Beyond the general agreement that glycogen serves as an energy source in the central nervous system, its exact role in brain energy metabolism has yet to be elucidated. Experiments performed in cell and tissue culture and animals have shown that glycogen content is affected by several factors, including glucose, insulin, neurotransmitters, and neuronal activation. The study of in vivo glycogen metabolism has been hindered by the inability to measure glycogen noninvasively, but, in the past several years, the development of a noninvasive localized (13) C nuclear magnetic resonance (NMR) spectroscopy method has allowed the study of glycogen metabolism in the conscious human. With this technique, (13) C-glucose is administered intravenously, and its incorporation into and washout from brain glycogen is tracked. One application of this method has been to the study of brain glycogen metabolism in humans during hypoglycemia: data have shown that mobilization of brain glycogen is augmented during hypoglycemia, and, after a single episode of hypoglycemia, glycogen synthesis rate is increased, suggesting that glycogen stores rebound to levels greater than baseline. Such studies suggest that glycogen may serve as a potential energy reservoir in hypoglycemia and may participate in the brain's adaptation to recurrent hypoglycemia and eventual development of hypoglycemia unawareness. Beyond this focused area of study, (13) C NMR spectroscopy has a broad potential for application in the study of brain glycogen metabolism and carries the promise of a better understanding of the role of brain glycogen in diabetes and other conditions.

Adenosine A1 and A3 receptors protect astrocytes from hypoxic damage

Brain levels of adenosine are elevated during hypoxia. Through effects on adenosine receptors (A(1), A(2A), A(2B) and A(3)) on astrocytes, adenosine can influence functions such as glutamate uptake, reactive gliosis, swelling, as well as release of neurotrophic and neurotoxic factors having an impact on the outcome of metabolic stress. We have studied the roles of these receptors in astrocytes by evaluating their susceptibility to damage induced by oxygen deprivation or exposure to the hypoxia mimic cobalt chloride (CoCl(2)). Hypoxia caused ATP breakdown and purine release, whereas CoCl(2) (0.8 mM) mainly reduced ATP by causing cell death in human D384 astrocytoma cells. Further experiments were conducted in primary astrocytes prepared from specific adenosine receptor knock-out (KO) and wild type (WT) mice. In WT cells purine release following CoCl(2) exposure was mainly due to nucleotide release, whereas hypoxia-induced intracellular ATP breakdown followed by nucleoside efflux. N-ethylcarboxamidoadenosine (NECA), an unselective adenosine receptor agonist, protected from cell death following hypoxia. Cytotoxicity was more pronounced in A(1)R KO astrocytes and tended to be higher in WT cells in the presence of the A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Genetic deletion of A(2A) receptor resulted in less prominent effects. A(3)R KO glial cells were more affected by hypoxia than WT cells. Accordingly, the A(3) receptor agonist 2-chloro-N(6)-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (CL-IB-MECA) reduced ATP depletion caused by hypoxic conditions. It also reduced apoptosis in human astroglioma D384 cells after oxygen deprivation. In conclusion, the data point to a cytoprotective role of adenosine mediated by both A(1) and A(3) receptors in primary mouse astrocytes.

Unlocking the Energy Dynamics of Executive Functioning: Linking Executive Functioning to Brain Glycogen

Past work suggests that executive functioning relies on glucose as a depletable energy, such that executive functioning uses a relatively large amount of glucose and is impaired when glucose is low. Glucose from the bloodstream is one energy source for the brain, and glucose stored in the brain as glycogen is another. A review of the literature on glycogen suggests that executive functioning uses it in much the same way as glucose, such that executive functioning uses glycogen and is impaired when glycogen is low. Findings on stress, physical persistence, glucose tolerance, diabetes, sleep, heat, and other topics provide general support for this view.

High glycogen levels in the hippocampus of patients with epilepsy

During intense cerebral activation approximately half of the glucose plus lactate taken up by the human brain is not oxidized and could replenish glycogen deposits, but the human brain glycogen concentration is unknown. In patients with temporal lobe epilepsy, undergoing curative surgery, brain biopsies were obtained from pathologic hippocampus (n=19) and from apparently 'normal' cortical grey and white matter. We determined the in vivo brain glycogen level and the activity of glycogen phosphorylase and synthase. Regional differences in glycogen concentration were examined similarly in healthy pigs (n=5). In the patients, the glycogen concentration in 'normal' grey and white matter was 5 to 6 mmol/L, but much higher in the hippocampus, 13.1+/-4.3 mmol/L (mean+/-s.d.; P<0.001); the activities of glycogen phosphorylase and synthase displayed the same pattern. In normal hippocampus from pigs, glycogen was similarly higher than in grey and white matter. Consequently, in human grey and white matter and, particularly, in the hippocampus of patients with temporal lope epilepsy, glycogen constitutes a large, active energy reserve, which may be of importance for energy provision during sustained synaptic activity as epileptic seizures.

Hypoglycemia, brain energetics, and hypoglycemic neuronal death

Hypoglycemia is a common and serious problem among diabetic patients receiving treatment with insulin or other glucose-lowering drugs. Moderate hypoglycemia impairs neurological function, and severe hypoglycemia leads to death of selectively vulnerable neurons. Recent advances have shed new light on the underlying processes that cause neuronal death in hypoglycemia and the factors that may render specific neuronal populations especially vulnerable to hypoglycemia. In addition to its clinical importance, the pathophysiology of hypoglycemia is an indicator of the unique bioenergetic properties of the central nervous system, in particular the metabolic coupling of neuronal and astrocyte metabolism. This review will focus on relationships between bioenergetics and brain dysfunction in hypoglycemia, the neuronal cell death program triggered by hypoglycemia, and the role of astrocytes in these processes.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Depolarisation phenomena in traumatic and ischaemic brain injury

1. Cortical spreading depression is a non-physiological global depolarisation of neurones and astrocytes that can be initiated with varying degrees of difficulty in the normally perfused cerebral cortex in the experimental laboratory. Induction is typically with electrical stimulation, needling of the cerebral cortex, or superfusion of isotonic or more concentrated potassium chloride solution. The phenomenon propagates across the cerebral cortex at a rate of 2-5 mm per minute, and is accompanied by marked but transient increases in cerebral blood flow, in local tissue oxygen tension, and most probably in metabolic rate. 2. Peri-infarct depolarisation is also a depolarisation event affecting neurones and glia, with an electrophysiological basis similar or identical to CSD, but occurring spontaneously in the ischaemic penumbra or boundary zone in focal cerebral cortical ischaemia. Most such events arise from the edge of the ischaemic core, and propagate throughout the penumbra, at a rate similar to that of cortical spreading depression. 3. Cortical spreading depression in the normally perfused cortex does not result in histological damage whereas peri-infarct depolarisations augment neuronal damage in the penumbra, and are believed by many authors to constitute an important, or the principal, mechanism by which electrophysiological penumbra progressively deteriorates, ultimately undergoing terminal depolarisation and thus recruitment into an expanded core lesion. 4. There is some experimental evidence to suggest that under some circumstances induction of episodes of cortical spreading depression can confer protection against subsequent ischaemic insults. 5. Although cortical spreading depression and peri-infarct depolarisations have been extensively studied in the experimental in vivo models, there is now clear evidence that depolarisations also occur and propagate in the human brain in areas surrounding a focus of traumatic contusion. 6. Whether such events in the injured human brain represent cortical spreading depression or peri-infarct depolarisation is unclear. However, invasive and probably non-invasive monitoring methods are available which may serve to distinguish which event has occurred. 7. Much further work will be needed to examine the relationship of depolarisation events in the injured brain with outcome from cerebral ischaemia or head injury, to examine the factors which influence the frequency of depolarisation events, and to determine which depolarisation events in the human brain augment the injury and should be prevented.

Brain glycogen re-awakened

The mammalian brain contains glycogen, which is located predominantly in astrocytes, but its function is unclear. A principal role for brain glycogen as an energy reserve, analogous to its role in the periphery, had been universally dismissed based on its relatively low concentration, an assumption apparently reinforced by the limited duration that the brain can function in the absence of glucose. However, during insulin-induced hypoglycaemia, where brain glucose availability is limited, glycogen content falls first in areas with the highest metabolic rate, suggesting that glycogen provides fuel to support brain function during pathological hypoglycaemia. General anaesthesia results in elevated brain glycogen suggesting quiescent neurones allow glycogen accumulation, and as long ago as the 1950s it was shown that brain glycogen accumulates during sleep, is mobilized upon waking, and that sleep deprivation results in region-specific decreases in brain glycogen, implying a supportive functional role for brain glycogen in the conscious, awake brain. Interest in brain glycogen has recently been re-awakened by the first continuous in vivo measurements using NMR spectroscopy, by the general acceptance of metabolic coupling between glia and neurones involving intercellular transfer of energy substrate, and by studies supporting a prominent physiological role for brain glycogen as a provider of supplemental energy substrate during periods of increased tissue energy demand, when ambient normoglycaemic glucose is unable to meet immediate energy requirements.

Stimulus- and cell-type-specific release of purines in cultured rat forebrain astrocytes and neurons

Adenosine is formed during conditions that deplete ATP, such as ischemia. Adenosine deaminase converts adenosine into inosine, and both adenosine and inosine can be beneficial for postischemic recovery. This study investigated adenosine and inosine release from astrocytes and neurons during chemical hypoxia or oxygen-glucose deprivation. In both cell types, 2-deoxyglucose was the most effective stimulus for depleting cellular ATP and for evoking inosine release; in contrast, oxygen-glucose deprivation evoked the greatest adenosine release. alpha,beta-Methylene ADP, an inhibitor of ecto-5'nucleotidase, significantly reduced adenosine release from astrocytes but not neurons. Dipyridamole, an inhibitor of equilibrative nucleoside transporters, inhibited both adenosine and inosine release from neurons. Erythro-9-(2-hydroxy-3-nonyl)adenine, an inhibitor of adenosine deaminase, reduced neuronal inosine release evoked by oxygen-glucose deprivation but not by 2-deoxyglucose treatment. These data indicate that (1). astrocytes release adenine nucleotides that are hydrolyzed extracellularly to adenosine, whereas neurons release adenosine per se, (2). inosine is formed intracellularly and released via nucleoside transporters, and (3). inosine is formed by an adenosine deaminase-dependent pathway during oxygen-glucose deprivation but not during 2-deoxyglucose treatment. In summary, the metabolic pathways for adenosine formation and release were cell-type dependent whereas the pathways for inosine formation were stimulus dependent.

Glycogen in the brain of Drosophila melanogaster: diurnal rhythm and the effect of rest deprivation

One function of sleep is thought to be the restoration of energy stores in the brain depleted during wakefulness. One such energy store found in mammalian brains is glycogen. Many of the genes involved in glycogen regulation in mammals have also been found in Drosophila melanogaster and rest behavior in Drosophila has recently been shown to have the characteristics of sleep. We therefore examined, in the fly, variation in the glycogen contents of the brain, the whole head and the body throughout the rest/activity cycle and after rest deprivation. Glycogen in the brain varies significantly throughout the day (p=0.001) and is highest during rest and lowest while flies are active. Glycogen levels in the whole head and body do not show diurnal variation. Brain glycogen drops significantly when flies are rest deprived for 3 h (p=0.034) but no significant differences are observed after 6 h of rest deprivation. In contrast, glycogen is significantly depleted in the body after both 3 and 6 h of rest deprivation (p<0.0001 and p<0.0001, respectively). Glycogen in the fly brain changes in relationship to rest and activity and demonstrates a biphasic response to rest deprivation similar to that observed in mammalian astrocytes in culture.

Timing of neuronal and glial ultrastructure disruption during brain slice preparation and recovery in vitro

Hippocampal slices often have more synapses than perfusion-fixed hippocampus, but the cause of this synaptogenesis is unclear. Ultrastructural evidence for synaptogenic triggers during slice preparation was investigated in 21-day-old rats. Slices chopped under warm or chilled conditions and fixed after 0, 5, 25, 60, or 180 minutes of incubation in an interface chamber were compared with hippocampi fixed by perfusion or by immersion of the whole hippocampus. There was no significant synaptogenesis in these slices compared with perfusion-fixed hippocampus, but there were other structural changes during slice preparation and recovery in vitro. Whole hippocampus and slices prepared under warm conditions exhibited an increase in axonal coated vesicles, suggesting widespread neurotransmitter release. Glycogen granules were depleted from astrocytes and neurons in 0-min slices, began to reappear by 1 hour, and had fully recovered by 3 hours. Dendritic microtubules were initially disassembled in slices, but reassembled into normal axial arrays after 5 minutes. Microtubules were short at 5 minutes (12.3 +/- 1.1 microm) but had recovered normal lengths by 3 hours (84.6 +/- 20.0 microm) compared with perfusion-fixed hippocampus (91 +/- 22 microm). Microtubules appeared transiently in 15 +/- 3% and 9 +/- 4% of dendritic spines 5 and 25 minutes after incubation, respectively. Spine microtubules were absent from perfusion-fixed hippocampus and 3-hour slices. Ice-cold dissection and vibratomy in media that blocked activity initially produced less glycogen loss, coated vesicles, and microtubule disassembly. Submersing these slices in normal oxygenated media at 34 degrees C led to glycogen depletion, as well as increased coated vesicles and microtubule disassembly within 1 minute.

Role of enteric glial cells in inflammatory bowel disease

Enteric glial cells (EGCs) represent an extensive but relatively poorly described cell population within the gastrointestinal tract. Accumulating data suggest that EGCs represent the morphological and functional equivalent of CNS astrocytes within the enteric nervous system (ENS). The EGC network has trophic and protective functions toward enteric neurons and is fully implicated in the integration and the modulation of neuronal activities. Moreover, EGCs seem to be active elements of the ENS during intestinal inflammatory and immune responses, sharing with astrocytes the ability to act as antigen-presenting cells and interacting with the mucosal immune system via the expression of cytokines and cytokine receptors. Transgenic mouse systems have demonstrated that specific ablation of EGC by chemical ablation or autoimmune T-cell targeting induces an intestinal pathology that shows similarities to the early intestinal immunopathology of Crohn's disease. EGCs may also share with astrocytes the ability to regulate tissue integrity, thereby postulating that similar interactions to those observed for the blood-brain barrier may also be partly responsible for regulating mucosal and vascular permeability in the gastrointestinal tract. Disruption of the EGC network in Crohn's disease patients may represent one possible cause for the enhanced mucosal permeability state and vascular dysfunction that are thought to favor mucosal inflammation.

Brain glycogen decreases with increased periods of wakefulness: implications for homeostatic drive to sleep

Sleep is thought to be restorative in function, but what is restored during sleep is unclear. Here we tested the hypothesis that increased periods of wakefulness will result in decreased levels of glycogen, the principal energy store in brain, and with recovery sleep levels of glycogen will be replenished, thus representing a homeostatic component of sleep drive. Using a high-energy focused microwave irradiation method to kill animals and thereby snap-inactivate glycogen-producing and -metabolizing enzymes, we determined, with accuracy and precision, levels of brain glycogen and showed these levels to decrease significantly by approximately 40% in brains of rats deprived of sleep for 12 or 24 hr. Recovery sleep of 15 hr duration after 12 hr of sleep deprivation reversed the decreases in glycogen. Using a novel histochemical method to stain brain glycogen, we found glycogen to be concentrated in white matter; this finding was confirmed biochemically in white matter dissected from rats killed with microwave irradiation. Levels of glycogen, as determined histochemically, were significantly decreased in gray and white matter with sleep deprivation, and these decreases were reversed with recovery sleep. The observed decreases in levels of brain glycogen may be a consequence of increased wakefulness and/or a component integral to the homeostatic drive to sleep.

Blocked gap junctional coupling increases glutamate-induced neurotoxicity in neuron-astrocyte co-cultures

Gap junctional communication is likely one means by which neurons can endure glutamate cytotoxicity associated with CNS insults (i.e. ischemia). To examine this neuroprotective role of gap junctions, we employed gap junctional blockers to neuronal and astrocytic co-cultures during exposure to a high concentration of extracellular glutamate. Co-cultures were treated with the blocking agents carbenoxolone (CBX; 25 microM), 18alpha-glycyrrhetinic acid (AGA; 10 microM), vehicle or the inactive blocking analogue glycyrrhizic acid (GZA; 25 microM). Twenty-four hours following the insult, cell mortality was analyzed and quantified by the release of lactate dehydrogenase (LDH) into the media, the cells' inability to exclude propidium iodide, and terminal dUTP nick end labeling (TUNEL). Measurement of LDH release revealed that the glutamate insult was detrimental to the co-cultures when gap junctions were blocked with CBX and AGA. Based on propidium iodide and TUNEL labeling, the glutamate insult caused significant cell death compared to sham vehicle and mortality was amplified in the presence of CBX and AGA. Since blockers were not themselves toxic and did not affect astrocytic uptake of glutamate, it is likely that blocked gap junctions lead to the increased glutamate cytotoxicity. These findings support the hypothesis that gap junctions play a neuroprotective role against glutamate cytotoxicity.

Astrocytic clasmatodendrosis in hippocampal organ culture

Mechanisms by which astrocytes are irreversibly injured from ischemic brain injury remain incompletely defined. More than 90 years ago Alzheimer showed that astrocytes lose their distal processes (i.e., undergo "clasmatodendrosis") when irreversibly injured by a reduction in blood flow, a process shown by Friede and van Houten (1961) to be due to energy failure and acidosis. Such alterations in astrocytic morphology can relate directly to changes in cell function. However, astrocytic clasmatodendrosis has largely been lost to the modern literature, perhaps because of a inability to study it under controlled conditions. In the present study, novel four-dimensional (4D)and digital deblurring imaging of glial fibrillary acidic protein (GFAP) immunostaining changes in hippocampal organ cultures (HOTCs) were used to establish an in vitro model of astrocytic clasmatodendrosis. Also, astrocytes in primary culture were transfected with green fluorescent protein (GFP) to show the occurrence of clasmatodendrosis via a parallel and separate means. In HOTCs, a significant reduction in astrocytic process length occurred 15 min (and remained for 60 min) after exposure to acidic Ringer's and mitochondrial inhibition in the pyramidal cell body layer. Time-lapsed images of primary cultures showed thinning of cell processes within 15 min of exposure to acidic Ringer's and mitochondrial inhibition. Distal processes subsequently broke away but retained their fluorescence for minutes before disintegrating along with their parent cell bodies. This report shows the spatiotemporal occurrence of clasmatodendrosis in astrocytes of HOTCs closely parallels that seen in vivo. Thus, HOTCs, where microenvironmental conditions can be controlled and single, identified cells can be followed in space and time, can be applied to study the interrelations between energy metabolism and pH that result in clasmatodendrosis.

L-Glutamate and insulin enhance glycogen synthesis in cultured astrocytes from the rat brain through different intracellular mechanisms

The effects of L-glutamate and insulin on glycogen synthesis in astrocytes were examined. L-Glutamate and insulin both stimulated glycogen synthesis in primary cultures of rat astrocytes in a dose-dependent manner, as measured by the incorporation of 14C from [14C]glucose into glycogen. D-Aspartate also increased the incorporation of 14C into glycogen. When insulin and L-glutamate were added together, the glycogen synthesis as well as glycogen content of the cells was additively increased. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, had little effect on glycogen synthesis induced by L-glutamate, whereas it suppressed the insulin-induced glycogen synthesis. These results suggest that the insulin- and L-glutamate-induced glycogen syntheses are mediated by different intracellular mechanisms. In fact, insulin stimulated the conversion of glycogen synthase b to glycogen synthase a, which was suppressed by wortmannin. L-Glutamate and D-aspartate, however, did not increase the level of glycogen synthase a activity. By contrast, L-glutamate increased 2-deoxy-D-[3H]glucose uptake by the astrocytes, whereas insulin did not affect the uptake. These results suggest that insulin stimulates glycogen synthesis in astrocytes by activating glycogen synthase, which is dependent on a wortmannin-sensitive signaling pathway. L-Glutamate, however, enhances the glucose uptake, which contributes to the increase in glycogen synthesis in the cells.

[U-13C] aspartate metabolism in cultured cortical astrocytes and cerebellar granule neurons studied by NMR spectroscopy

The metabolism of [U-13C]aspartate was studied in cultured cortical astrocytes and cerebellar granule neurons in the presence of glucose and during inhibition of glycolysis. Redissolved, lyophilized cell extracts and incubation media were analyzed by 13C nuclear magnetic resonance spectroscopy for the determination of metabolites labeled from aspartate. Uniformly labeled lactate was prominent in control media of astrocytes and cerebellar granule neurons. In both cell types, aspartate entered the tricarboxylic acid (TCA) cycle, as shown by labeling patterns in glutamate and, in astrocytes, in glutamine. From the complex labeling patterns in aspartate in astrocytic perchloric acid extracts it was clear that acetylcoenzyme A (acetyl-CoA) derived from aspartate via oxaloacetate and pyruvate could enter the TCA cycle. Such "recycling," however, could not be detected in cerebellar granule neurons. Inhibition of glycolysis reduced aspartate uptake and metabolism in both cell types. Most notably, lactate derived from aspartate showed a large reduction, and in astrocytes, incorporation of labeled acetyl-CoA into the TCA cycle was significantly reduced. Thus, astrocytes and cerebellar granule neurons differ in their handling of aspartate. Furthermore, inhibition of glycolysis clearly affected aspartate metabolism by such cells.

Anoxic and ischemic injury of myelinated axons in CNS white matter: from mechanistic concepts to therapeutics

White matter of the brain and spinal cord is susceptible to anoxia and ischemia. Irreversible injury to this tissue can have serious consequences for the overall function of the CNS through disruption of signal transmission. Myelinated axons of the CNS are critically dependent on a continuous supply of energy largely generated through oxidative phosphorylation. Anoxia and ischemia cause rapid energy depletion, failure of the Na(+)-K(+)-ATPase, and accumulation of axoplasmic Na+ through noninactivating Na+ channels, with concentrations approaching 100 mmol/L after 60 minutes of anoxia. Coupled with severe K+ depletion that results in large membrane depolarization, high [Na+]i stimulates reverse Na(+)-Ca2+ exchange and axonal Ca2+ overload. A component of Ca2+ entry occurs directly through Na+ channels. The excessive accumulation of Ca2+ in turn activates various Ca(2+)-dependent enzymes, such as calpain, phospholipases, and protein kinase C, resulting in irreversible injury. The latter enzyme may be involved in "autoprotection," triggered by release of endogenous gamma-aminobutyric acid and adenosine, by modulation of certain elements responsible for deregulation of ion homeostasis. Glycolytic block, in contrast to anoxia alone, appears to preferentially mobilize internal Ca2+ stores; as control of internal Ca2+ pools is lost, excessive release from this compartment may itself contribute to axonal damage. Reoxygenation paradoxically accelerates injury in many axons, possibly as a result of severe mitochondrial Ca2+ overload leading to a secondary failure of respiration. Although glia are relatively resistant to anoxia, oligodendrocytes and the myelin sheath may be damaged by glutamate released by reverse Na(+)-glutamate transport. Use-dependent Na+ channel blockers, particularly charged compounds such as QX-314, are highly neuroprotective in vitro, but only agents that exist partially in a neutral form, such as mexiletine and tocainide, are effective after systemic administration, because charged species cannot penetrate the blood-brain barrier easily. These concepts may also apply to other white matter disorders, such as spinal cord injury or diffuse axonal injury in brain trauma. Moreover, whereas many events are unique to white matter injury, a number of steps are common to both gray and white matter anoxia and ischemia. Optimal protection of the CNS as a whole will therefore require combination therapy aimed at unique steps in gray and white matter regions, or intervention at common points in the injury cascades.

A possible mechanism for the hypoxia-hypoglycemia-induced release of excitatory amino acids from cultured hippocampal astrocytes

In order to elucidate the mechanism of release of excitatory amino acid (EAA) induced by hypoxia-hypoglycemia (in vitro ischemia) from cultured hippocampal astrocytes, we compared the EAA release by in vitro ischemia with those by other treatments. The EAA release induced by in vitro ischemia treatment was rapid and reversible. The amount of released aspartate was comparable to that of glutamate, although the endogenous content of aspartate was one sixth that of glutamate. High-K (100 mM) treatment and the addition of 5 mM NaCN induced a rapid EAA release and the glutamate release was much greater than aspartate. Addition of 5 mM iodoacetate, a glycolysis inhibitor, induced a slow EAA release, and the amount of released aspartate was much higher than that of glutamate. On the other hand, the in vitro ischemia treatment and the addition of 5 mM NaCN induced only 20% reduction in ATP content for initial 5 min, whereas the addition of 5 mM iodoacetate induced a marked reduction. Our data suggest that ischemia-induced EAA release from astrocytes is a complex process in which local energy failure, inhibition of glycolysis, and depolarization of the cell membrane are involved.

Colocalization of fructose-1,6-bisphosphatase and glial fibrillary acidic protein in rat brain

Immunofluorescence studies of rat brain sections demonstrated an exclusive colocalization of the gluconeogenic key enzyme fructose-1,6-bisphosphatase (FBPase) with the astroglial marker glial fibrillary acidic protein, indicating FBPase in brain as an astrocyte-specific enzyme. This conclusion was supported by the presence of FBPase activity in astroglia-rich but not neuron-rich primary cultures derived from rat brain.

Significant amounts of glycogen are synthesized from 3-carbon compounds in astroglial primary cultures from mice with participation of the mitochondrial phosphoenolpyruvate carboxykinase isoenzyme

The incorporation was studied of the gluconeogenic substrates lactate, alanine, aspartate and glutamate into glycogen of astroglial primary cultures derived from mouse brain. The incorporation was inhibited by 3-mercaptopicolinate, an inhibitor of one of the characteristic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase. Only the mitochondrial isoenzyme of phosphoenolpyruvate carboxykinase was detectable in the astroglial primary cultures. After the incubation of glucose-starved cells with medium containing a mixture of [6-3H]glucose and [U-14C]glucose, the newly synthesized glycogen showed a 3H/14C ratio which was approximately 15% less than the isotope ratio for the medium. The decrease of the isotope ratio was not significantly inhibited by 3-mercaptopicolinate, indicating a cycling of approximately 15% of the glucose to the level of the triose phosphates before its incorporation into astroglial glycogen. During the initial phase of glycogen resynthesis, the contribution of the gluconeogenic substrates appeared to be higher. This was in agreement with the accumulation of fructose 2,6-bisphosphate during refeeding. A participation of gluconeogenic substrates in glycogen metabolism was also detectable when the glycogen content was not changing significantly.

Glial glycogen stores affect neuronal survival during glucose deprivation in vitro

Glia perform several energy-dependent functions that may aid neuronal survival under pathological conditions. Glycogen is the major energy reserve in brain, and it is localized almost exclusively to astrocytes. Using murine cortical cell cultures containing both glia and neurons, we examined the effect of altered glial glycogen stores on neuronal survival following glucose deprivation. As previously reported, cultures exposed for several hours to media lacking glucose developed widespread neuronal degeneration without glial degeneration. If glial astrocyte glycogen content was increased to 2-3 times control levels by a 24-h pretreatment with 1 microM insulin or 0.5 mM methionine sulfoximine (MSO), glucose deprivation-induced neuronal degeneration was attenuated. These protective effects were blocked if glycogen levels were reduced back to control levels by a 30-min exposure to 1 mM dibutyryl cyclic AMP or 20 microM norepinephrine prior to glucose deprivation. Astrocyte glycogen stores may be an important factor influencing neuronal survival under conditions of energy substrate limitation.

Glutamate increases glycogen content and reduces glucose utilization in primary astrocyte culture

The glycogen content of primary cultured astrocytes was approximately doubled by incubation with 1 mM L-glutamate or L-aspartate. Other amino acids and excitatory neurotransmitters were without effect. The increase in glycogen level was not blocked by the glutamate receptor antagonist kynurenic acid but was completely blocked by the glutamate uptake inhibitor threo-3-hydroxy-D,L-aspartate and by removal of Na+ from the medium. Incubation with radiolabeled glucose and glutamate revealed that the increased glycogen content was derived almost entirely from glucose. Glutamate at 1 mM was also found to cause a 53 +/- 12% decrease in glucose utilization and a 112 +/- 69% increase in glucose-6-phosphate levels. These results suggest that the glycogen content of astrocytes is linked to the rate of glucose utilization and that glucose utilization can, in turn, be affected by the availability of alternative metabolic substrates. These relationships suggest a mechanism by which brain glycogen accumulation occurs during decreased neuronal activity.

A small subset of cortical astrocytes in culture accumulates glycogen

We are interested in identifying the target cells for norepinephrine in cerebral cortex and in characterizing the effects of norepinephrine on these target cells. Norepinephrine inhibits the incorporation of tritiated glucose into glycogen in rat cerebral cortex in dissociated cell culture. To identify which cells store glycogen in these cultures we combined glycogen cytochemistry with glial fibrillary acidic protein immunocytochemistry. Using this technique we show that cytochemically detectable glycogen is restricted to a small subset of astrocytes as well as an unidentified cell type which does not contain glial fibrillary acidic protein. These results demonstrate that only a minority of astrocytes in cortical cultures accumulate glycogen. Therefore cortical astrocytes are differentiated with respect to glycogen accumulation, an important metabolic function. We do not know if glycogen accumulation in astrocytes is a constitutive or facultative property. In either case the subset of astrocytes which accumulates glycogen might be one of the major cellular targets for norepinephrine in cerebral cortex.

Glycogen accumulation of the aging human brain

We describe light- and electron-microscopically a new type of intracytoplasmatic inclusions within cell processes of the cerebral cortex and the underlying white matter. These structures measure 5-50 micron in diameter and consist almost exclusively of densely packed alpha- or beta-glycogen granules, which never occur together in any single structure. Within their periphery, electron-dense amorphous spots and cell organelles, especially mitochondria, were seen. No membrane-bound glycogen was observed. We propose to call them granular glycogen bodies. They occur in 4 of 7 examined postmortem specimens of the cerebral cortex of people older than 60 years of age. They were not found in 4 younger controls aged 26-48. Their appearance may reflect a distinct turnover disorder of carbohydrate metabolism, which becomes manifest under diverse pathologic conditions and in the normal aging process.

Glycogen, its transient occurrence in neurons of the rat CNS during normal postnatal development

Progressive changes in the postnatal incidence, distribution and duration of glycogen in neurons of the pons, medulla and spinal cord were studied by light and electron microscopy using cytochemical and quantitative methods. Albino rats of 11 ages ranging from newborn to adult were used for this investigation. Methacrylate sections, stained with periodic acid-Schiff-dimedone (PAS) were surveyed to identify nerve cell groups containing the polysaccharide, glycogen. The PAS reaction was positive in neuronal cell groups of the hypoglossal nucleus, the mesencephalic nucleus of V, nucleus ambiguus, the abducens nucleus, the facial motor nucleus and anterior horn cells of the spinal cord. The intensity and duration of the PAS reaction appeared greatest in the hypoglossal nucleus. Neurons of the mesencephalic nucleus of V demonstrated a reaction of moderate intensity and duration. The remaining nerve cell groups exhibited a weak, diffuse reaction of brief duration. Postnatal differences in the incidence and patterns of disposition of glycogen were quantified using ultrathin sections of the hypoglossal nucleus, the site richest in glycogen. The presence of glycogen was verified by the periodic acid-thiosemicarbizide-silver proteinate (PA-TSC-SP) ultracytochemical stain. The incidence of glycogen in neuronal perikarya of hypoglossal nuclei was related to age. All neurons contained some glycogen during the first postnatal week. By 24 days postnatal (dpn), the majority of hypoglossal neurons lacked glycogen and all neurons of adult rats were glycogen-free.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycogenolysis induced by serotonin in brain: identification of a new class of receptor

Serotonin-containing neurones in brain have been proposed to have a role in the control of physiological mechanisms such as sleep, thermoregulation, pain perception and endocrine secretions as well as in the physiopathology of migraine or depressive illness. One difficulty in testing these possibilities lies in the scarcity of pharmacological agents able to interact selectively with the probably multiple classes of serotonin receptors in the central nervous system. Development of such agents would be facilitated by simple in vitro models in which biological responses to serotonin in mammalian brain could be quantified. Thus a serotonin-sensitive adenylate cyclase has been characterized in rat brain, but the response to serotonin is weak in newborn and practically absent in adult animals. In addition, two pharmacologically distinct classes of serotoninergic binding site have been identified using 3H-serotonin and 3H-spiperone as ligands, but their identification as receptors remains to be established. More recently, serotonin has been shown to stimulate phosphorylation of a neuronal protein in slices from the facial motor nucleus, although the receptors mediating this action were not characterized. We now report that serotonin stimulates glycogen hydrolysis in slices of cerebral cortex, that this action is mediated by a novel class of receptors and that tricyclic antidepressants are among the best competitive antagonists of the indolamine.

Catecholaminergic axo-axonic synapses in the nucleus of the tractus solitarius (pars commissuralis) of the cat: possible relation to presynaptic regulation of baroreceptor reflexes

A study was performed on adult cats to investigate synaptic structures of nerve terminals in the commissural portion of the nucleus of the tractus solitarius (NTS); a site where the cardiovascular afferent fibers terminate and where a dense plexus of adrenergic element also resides. Synaptic contacts observed in the commissural portion were predominantly axo-dendritic but a smaller number of axo-somatic synapses were also recognized. A third type of junction, axo-axonic, appears to be unique to the commissural portion of the nucleus, since no axo-axonic contacts were encountered in the medial and ventrolateral portions of the NTS. By labeling with the 'false' neurotransmitter 5-hydroxydopamine (5-OHDA), three types of axo-axonic contacts with different presynaptic components could be identified; namely, axon varicosities with small clear spherical vesicles, axon varicosities containing flat synaptic vesicles and others with small cord vesicles labeled with 5-OHDA. Postsynaptic components were always axon varicosities with clear spherical vesicles. After surgical denervation of the IXth and Xth cranial nerves, no degenerated axon varicosities were observed in the presynaptic side in the axo-axonic contact. The present study clearly demonstrates that the commissural portion of the NTS contains axo-axonic synapses and some of the adrenergic axons in this area provide the presynaptic components for this type of synapse. Possible roles of adrenergic nerves to presynaptic regulation of baroreceptor reflexes are discussed.