Glycogenolytic response of primary chick and mouse cultures of astrocytes to noradrenaline across development

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.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

Although efficient compared to computers, the human brain utilizes energy at 10-fold the rate of other organs by mass. How the brain is supplied with sufficient on-demand energy to support its activity in the absence of neuronal storage capacity remains unknown. Neurons are not capable of meeting their own energy requirements, instead energy supply in the brain is managed by an oligocellular cartel composed of neurons, glia and the local vasculature (NGV), wherein glia can provide the ergogenic metabolite lactate to the neuron in a process called the astrocyte-to-neuron shuttle (ANLS). The only means of energy storage in the brain is glycogen, a polymerized form of glucose that is localized largely to astrocytes, but its exact role and conditions of use are not clear. In this computational model we show that neuromodulatory stimulation by norepinephrine induces astrocytes to recover glucosyl subunits from glycogen for use in a glycolytic process that favors the production of lactate. The ATP and NADH produced support metabolism in the astrocyte while the lactate is exported to feed the neuron. Thus, rapid energy demands by both neurons and glia in a stimulated brain can be met by glycogen mobilization.

Biphasic Regulation of Caveolin-1 Gene Expression by Fluoxetine in Astrocytes: Opposite Effects of PI3K/AKT and MAPK/ERK Signaling Pathways on c-fos

Previously, we reported that fluoxetine acts on 5-HT_2B receptor and induces epidermal growth factor receptor (EGFR) transactivation in astrocytes. Recently, we have found that chronic treatment with fluoxetine regulates Caveolin-1 (Cav-1)/PTEN/PI3K/AKT/glycogen synthase kinase 3β (GSK-3β) signaling pathway and glycogen content in primary cultures of astrocytes with bi-phasic concentration dependence. At low concentrations fluoxetine down-regulates Cav-1 gene expression, decreases membrane content of PTEN, increases PI3K activity and increases phosphorylation of GSK-3β and increases its activity; at high concentrations fluoxetine acts on PTEN/PI3K/AKT/GSK-3β in an inverse fashion. Here, we present the data indicating that acute treatment with fluoxetine at lower concentrations down-regulates c-Fos gene expression via PI3K/AKT signaling pathway; in contrast at higher concentrations fluoxetine up-regulates c-Fos gene expression via MAPK/extracellular-regulated kinase (ERK) signaling pathway. However, acute treatment with fluoxetine has no effect on Cav-1 protein content. Similarly, chronic effects of fluoxetine on Cav-1 gene expression are suppressed by inhibitor of PI3K at lower concentrations, but by inhibitor of MAPK at higher concentrations, indicating that the mechanism underlying bi-phasic regulation of Cav-1 gene expression by fluoxetine is opposing effects of PI3K/AKT and MAPK/ERK signal pathways on c-Fos gene expression. The effects of fluoxetine on Cav-1 gene expression at both lower and higher concentrations are abolished by AG1478, an inhibitor of EGFR, indicating the involvement of 5-HT_2B receptor induced EGFR transactivation as we reported previously. However, PP1, an inhibitor of Src only abolished the effect by lower concentrations, suggesting the relevance of Src with PI3K/AKT signal pathway during activation of EGFR.

Bi-phasic regulation of glycogen content in astrocytes via Cav-1/PTEN/PI3K/AKT/GSK-3β pathway by fluoxetine

OBJECTIVE

Here, we present the data indicating that chronic treatment with fluoxetine regulates Cav-1/PTEN/PI3K/AKT/GSK-3β signalling pathway and glycogen content in primary cultures of astrocytes with bi-phasic concentration dependence.

RESULTS

At lower concentrations, fluoxetine downregulates gene expression of Cav-1, decreases membrane content of PTEN, increases activity of PI3K/AKT, and elevates GSK-3β phosphorylation thus suppressing its activity. At higher concentrations, fluoxetine acts in an inverse fashion. As expected, fluoxetine at lower concentrations increased while at higher concentrations decreased glycogen content in astrocytes.

CONCLUSIONS

Our findings indicate that bi-phasic regulation of glycogen content via Cav-1/PTEN/PI3K/AKT/GSK-3β pathway by fluoxetine may be responsible for both therapeutic and side effects of the drug.

Role of Glycogenolysis in Memory and Learning: Regulation by Noradrenaline, Serotonin and ATP

This paper reviews the role played by glycogen breakdown (glycogenolysis) and glycogen re-synthesis in memory processing in two different chick brain regions, (1) the hippocampus and (2) the avian equivalent of the mammalian cortex, the intermediate medial mesopallium (IMM). Memory processing is regulated by the neuromodulators noradrenaline and serotonin soon after training glycogen breakdown and re-synthesis. In day-old domestic chicks, memory formation is dependent on the breakdown of glycogen (glycogenolysis) at three specific times during the first 60 min after learning (around 2.5, 30, and 55 min). The chicks learn to discriminate in a single trial between beads of two colors and tastes. Inhibition of glycogen breakdown by the inhibitor of glycogen phosphorylase 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) given at specific times prior to the formation of long-term memory prevents memory forming. Noradrenergic stimulation of cultured chicken astrocytes by a selective β₂-adrenergic (AR) agonist reduces glycogen levels and we believe that in vivo this triggers memory consolidation at the second stage of glycogenolysis. Serotonin acting at 5-HT_2B receptors acts on the first stage, but not on the second. We have shown that noradrenaline, acting via post-synaptic α₂-ARs, is also responsible for the synthesis of glycogen and our experiments suggest that there is a readily accessible labile pool of glycogen in astrocytes which is depleted within 10 min if glycogen synthesis is inhibited. Endogenous ATP promotion of memory consolidation at 2.5 and 30 min is also dependent on glycogen breakdown. ATP acts at P₂Y₁ receptors and the action of thrombin suggests that it causes the release of internal calcium ([Ca²⁺]_i) in astrocytes. Glutamate and GABA, the primary neurotransmitters in the brain, cannot be synthesized in neurons de novo and neurons rely on astrocytic glutamate synthesis, requiring glycogenolysis.

Monoaminergic Control of Cellular Glucose Utilization by Glycogenolysis in Neocortex and Hippocampus

Brainstem nuclei are the principal sites of monoamine (MA) innervation to major forebrain structures. In the cortical grey matter, increased secretion of MA neuromodulators occurs in response to a wealth of environmental and homeostatic challenges, whose onset is associated with rapid, preparatory changes in neural activity as well as with increases in energy metabolism. Blood-borne glucose is the main substrate for energy production in the brain. Once entered the tissue, interstitial glucose is equally accessible to neurons and astrocytes, the two cell types accounting for most of cellular volume and energy metabolism in neocortex and hippocampus. Astrocytes also store substantial amounts of glycogen, but non-stimulated glycogen turnover is very small. The rate of cellular glucose utilization in the brain is largely determined by hexokinase, which under basal conditions is more than 90 % inhibited by its product glucose-6-phosphate (Glc-6-P). During rapid increases in energy demand, glycogen is a primary candidate in modulating the intracellular level of Glc-6-P, which can occur only in astrocytes. Glycogenolysis can produce Glc-6-P at a rate higher than uptake and phosphorylation of glucose. MA neurotransmitter are released extrasinaptically by brainstem neurons projecting to neocortex and hippocampus, thus activating MA receptors located on both neuronal and astrocytic plasma membrane. Importantly, MAs are glycogenolytic agents and thus they are exquisitely suitable for regulation of astrocytic Glc-6-P concentration, upstream substrate flow through hexokinase and hence cellular glucose uptake. Conforming to such mechanism, Gerald A. Dienel and Nancy F. Cruz recently suggested that activation of noradrenergic locus coeruleus might reversibly block astrocytic glucose uptake by stimulating glycogenolysis in these cells, thereby anticipating the rise in glucose need by active neurons. In this paper, we further develop the idea that the whole monoaminergic system modulates both function and metabolism of forebrain regions in a manner mediated by glycogen mobilization in astrocytes.

Functional impact of glycogen degradation on astrocytic signalling

Astrocytic glycogen degradation is an important factor in metabolic support of brain function, particularly during increased neuronal firing. In this context, glycogen is commonly thought of as a source for the provision of energy substrates, such as lactate, to neurons. However, the signalling pathways eliciting glycogen degradation inside astrocytes are themselves energy-demanding processes, a fact that has been emphasized in recent studies, demonstrating dependence of these signalling mechanisms on glycogenolytic ATP.

Brain glycogen-new perspectives on its metabolic function and regulation at the subcellular level

Glycogen is a complex glucose polymer found in a variety of tissues, including brain, where it is localized primarily in astrocytes. The small quantity found in brain compared to e.g., liver has led to the understanding that brain glycogen is merely used during hypoglycemia or ischemia. In this review evidence is brought forward highlighting what has been an emerging understanding in brain energy metabolism: that glycogen is more than just a convenient way to store energy for use in emergencies—it is a highly dynamic molecule with versatile implications in brain function, i.e., synaptic activity and memory formation. In line with the great spatiotemporal complexity of the brain and thereof derived focus on the basis for ensuring the availability of the right amount of energy at the right time and place, we here encourage a closer look into the molecular and subcellular mechanisms underlying glycogen metabolism. Based on (1) the compartmentation of the interconnected second messenger pathways controlling glycogen metabolism (calcium and cAMP), (2) alterations in the subcellular location of glycogen-associated enzymes and proteins induced by the metabolic status and (3) a sequential component in the intermolecular mechanisms of glycogen metabolism, we suggest that glycogen metabolism in astrocytes is compartmentalized at the subcellular level. As a consequence, the meaning and importance of conventional terms used to describe glycogen metabolism (e.g., turnover) is challenged. Overall, this review represents an overview of contemporary knowledge about brain glycogen and its metabolism and function. However, it also has a sharp focus on what we do not know, which is perhaps even more important for the future quest of uncovering the roles of glycogen in brain physiology and pathology.

Effects of adrenergic agents on intracellular Ca2+ homeostasis and metabolism of glucose in astrocytes with an emphasis on pyruvate carboxylation, oxidative decarboxylation and recycling: implications for glutamate neurotransmission and excitotoxicity

Glucose and glycogen are essential sources of energy for maintaining glutamate homeostasis as well as glutamatergic neurotransmission. The metabolism of glycogen, the location of which is confined to astrocytes, is affected by norepinephrine (NE), and hence, adrenergic signaling in the astrocyte might affect glutamate homeostasis with implications for excitatory neurotransmission and possibly excitotoxic neurodegeneration. In order to study this putative correlation, cultured astrocytes were incubated with 2.5 mM [U-(13)C]glucose in the presence and absence of NE as a time course for 1 h. Employing mass spectrometry, labeling in intracellular metabolites was determined. Moreover, the involvement of Ca(2+) in the noradrenergic response was studied. In unstimulated astrocytes, the labeling pattern of glutamate, aspartate, malate and citrate confirmed important roles for pyruvate carboxylation and oxidative decarboxylation in astrocytic glucose metabolism. Importantly, pyruvate carboxylation was best visualized at 10 min of incubation. The abundance and pattern of labeling in lactate and alanine indicated not only an extensive activity of malic enzyme (initial step for pyruvate recycling) but also a high degree of compartmentalization of the pyruvate pool. Stimulating with 1 μM NE had no effect on labeling patterns and glycogen metabolism, whereas 100 μM NE increased glutamate labeling and decreased labeling in alanine, the latter supposedly due to dilution from degradation of non-labeled glycogen. It is suggested that further experiments uncovering the correlation between adrenergic and glutamatergic pathways should be performed in order to gain further insight into the role of astrocytes in brain function and dysfunction, the latter including excitotoxicity.

Role of β-adrenoceptors in glucose uptake in astrocytes using β-adrenoceptor knockout mice

BACKGROUND AND PURPOSE

β(1) -, β(2) - and β(3) -adrenoceptors determined by functional, binding and reverse transcription polymerase chain reaction (RT-PCR) studies are present in chick astrocytes and activation of β(2) - or β(3) -adrenoceptors increase glucose uptake. The aims of the present study are to identify which β-adrenoceptor subtypes are present in mouse astrocytes, the signal transduction mechanisms involved and whether β-adrenoceptor stimulation regulates glucose uptake.

EXPERIMENTAL APPROACH

Astrocytes were prepared from four mouse strains: FVB/N, DBA/1 crossed with C57BL/6J, β(3) -adrenoceptor knockout and β(1) β(2) -adrenoceptor knockout mice. RT-PCR and radioligand binding studies were used to determine β-adrenoceptor expression. Glucose uptake and cAMP were assayed to elucidate the signalling pathways involved.

KEY RESULTS

mRNAs for all three β-adrenoceptors were identified in astrocytes from wild-type mice. Radioligand binding studies identified that β(1) - and β(3) -adrenoceptors were predominant. cAMP studies showed that β(1) - and β(2) -adrenoceptors coupled to G(s) whereas β(3) -adrenoceptors coupled to both G(s) and G(i) . However, activation of any of the three β-adrenoceptors increased glucose uptake in mouse astrocytes. Interestingly, there was no functional compensation for receptor subtype loss in knockout animals.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that although β(1) -adrenoceptors are the predominant β-adrenoceptor in mouse astrocytes and are primarily responsible for cAMP production in response to β-adrenoceptor stimulation, β(3) -adrenoceptors are also present in mouse astrocytes and activation of β(2) - and β(3) -adrenoceptors increases glucose uptake in mouse astrocytes.

Energy metabolism and memory processing: role of glucose transport and glycogen in responses to adrenoceptor activation in the chicken

From experiments using a discriminated bead task in young chicks, we have defined when and where adrenoceptors (ARs) are involved in memory modulation. All three ARs subtypes (alpha(1)-, alpha(2)- and beta-ARs) are found in the chick brain and in regions associated with memory. Glucose and glycogen are important in the role of memory consolidation in the chick since increasing glucose levels improves memory consolidation while inhibiting glucose transporters (GLUTs) or glycogen breakdown inhibits memory consolidation. The selective beta(3)-AR agonist CL316243 enhances memory consolidation by a glucose-dependent mechanism and the administration of the non-metabolized glucose analogue 2-deoxyglucose reduces the ability of CL316243 to enhance memory. Agents that reduce glucose uptake by GLUTs and its incorporation into the glycolytic pathway also reduce the effectiveness of CL316243, but do not alter the dose-response relationship to the beta(2)-AR agonist zinterol. However, beta(2)-ARs do have a role in memory related to glycogen breakdown and inhibition of glycogenolysis reduces the ability of zinterol to enhance memory. Both beta(2)- and beta(3)-ARs are found on astrocytes from chick forebrain, and the actions of beta(3)-ARs on glucose uptake, and beta(2)-ARs on the breakdown of glycogen is consistent with an effect on astrocytic metabolism at the time of memory consolidation 30 min after training. We have shown that both beta(2)- and beta(3)-ARs can increase glucose uptake in chick astrocytes but do so by different mechanisms. This review will focus on the role of ARs on memory consolidation and specifically the role of energy metabolism on AR modulation of memory.

d-Lactate inhibition of memory in a single trial discrimination avoidance task in the young chick

L-Lactate is a metabolite possibly able to meet some neuronal energy demands. However, a clear role for L-lactate in behaviour remains elusive. Administration of the inactive isomer D-lactate (1.75 mM; ic), immediately post-training, resulted in a persistent retention loss from 40 min post-training when used in conjuction with a single trial discrimination avoidance task designed for the young chick. Furthermore, 1mM noradrenaline (ic) administered 20 min post-training overcame the retention loss induced by D-lactate. Although not directly demonstrated in the current study, it is plausible that D-lactate inhibited memory processing by competing with L-lactate for uptake into neurons. The time of onset of the retention loss induced by D-lactate is in accord with findings where the action of noradrenaline is inhibited. The successful challenge of D-lactate inhibition by a high concentration of noradrenaline may suggest a relationship by some unidentified mechanism.

Inhibition of glycogenolysis in astrocytes interrupts memory consolidation in young chickens

Glycolysis and glycogenolysis are involved in memory processing in day-old chickens and, aside from the provision of energy for neuronal and astrocytic energy metabolism these pathways enable astrocytes to supply neurones with precursor for transmitter glutamate by glucose-based de novo synthesis. We have previously shown that memory processing for bead discrimination learning is dependent on glycolysis; however, the metabolic inhibitor used, iodoacetate, inhibits pyruvate formation from both glucose and glycogen. At specific time points after training transient reductions in brain glycogen content occur, mirrored by increases in glutamate/glutamine content. In the present study, we used intracerebral injection of a glycogen phosphorylase inhibitor, 1,4-dideoxy-1,4-imino-D-arabinitol (DAB), which does not affect glucose breakdown, to evaluate the role of glycogen metabolism in memory consolidation. Dose-dependent inhibition of learning occurred when DAB was administered at specific time periods in relation to training: (i) 5 min before training, (ii) around 30 min posttraining, and (iii) 55 min posttraining. After injection at either of the two earlier periods, memory disappeared after consolidation 30 min postlearning, and after injection 55 min after learning memory was absent at 70 min. The memory loss caused by early administration could be prevented after training by central injection of the glutamate precursor glutamine or the astrocyte-specific substrate acetate together with aspartate, substituting for pyruvate carboxylation. Thus, glycogenolysis is essential for learning in this paradigm and, aside from energy supply considerations, we suggest that an important role for glycogenolysis is to provide neurones with glutamine as the precursor for neuronal glutamate and GABA.

Astrocytic energy metabolism consolidates memory in young chicks

In a single trial discrimination avoidance learning task, chicks learn to distinguish between beads of two colors, which are dipped in either a strong or weak tasting aversant (methyl anthranilate) to induce strongly-reinforced and weakly-reinforced learning, respectively. Consolidation of strongly-reinforced learning can be prevented by inhibitors of glycolysis, such as 2-deoxyglucose and iodoacetate and by inhibitors of oxidative metabolism and the consolidation of weakly-reinforced learning can be promoted by administration of glucose. In the present study we show that bilateral, intracerebral injection of 30 nmol acetate can act like glucose to consolidate labile memory and to restore memory impaired by 2-deoxyglucose administration. Acetate is a metabolic substrate that feeds into the tricarboxylic acid cycle, it is oxidized in astrocytes, but not in neurones. Our data suggest that effects of glucose administered 15-25 min post-training on memory consolidation are mediated via astrocytes not neurons.

Effects of glucose and 2-deoxyglucose on memory formation in the chick: interaction with beta(3)-adrenoceptor agonists

Consolidation of a weakly reinforced memory that would otherwise fade after 30 min can be achieved by central or peripheral injection of the selective beta(3)-adrenoceptor agonist CL316243 as well as the beta(2)-adrenoceptor agonist zinterol and the alpha(1)-adrenoceptor antagonist prazosin in the day-old chick. The effect of the beta(3)-adrenoceptor agonist is mimicked by peripheral or central injection of glucose that is effective in enhancing memory from 25 min before to 25 min after training. Glucose uptake into various cell types has been described following activation of beta(3)-adrenoceptors and in this paper we demonstrate that activation of beta(3)-adrenoceptors by CL316243 facilitates the effect of a dose of glucose that does not normally enhance memory, whereas a beta(2)-adrenoceptor agonist and an alpha(1)-adrenoceptor antagonist have no effect. Administration of the glucose uptake inhibitor 2-deoxyglucose prevented the consolidation of strongly reinforced training. The beta(3)-adrenoceptor agonist facilitated the effect of a non-amnestic dose of 2-deoxyglucose to inhibit memory. There are two time periods relative to the learning trial where memory is vulnerable to interference by centrally administered 2-deoxyglucose: one related to short-term memory and one at the time of consolidation into long-term memory. Peripheral injection of 2-deoxyglucose is only effective at the time of consolidation. The action of the beta(3)-adrenoceptor agonist to facilitate the action of 2-deoxyglucose only occurs at the time of consolidation. We suggest that a noradrenergic agonist acting at beta(3)-adrenoceptors enhances memory formation by facilitation of glucose uptake at the time of memory consolidation. This may represent a novel mechanism that would be beneficial for developing compounds for the facilitation of memory in diseases with cognitive deficits.

Signaling and gene expression in the neuron-glia unit during brain function and dysfunction: Holger Hydén in memoriam

Holger Hydén demonstrated almost 40 years ago that learning changes the base composition of nuclear RNA, i.e. induces an alteration in gene expression. An equally revolutionary observation at that time was that a base change occurred in both neurons and glia. From these findings, Holger Hydén concluded that establishment of memory is correlated with protein synthesis, and he demonstrated de novo synthesis of several high-molecular protein species after learning. Moreover, the protein, S-100, which is mainly found in glial cells, was increased during learning, and antibodies towards this protein inhibited memory consolidation. S-100 belongs to a family of Ca(2+)-binding proteins, and Holger Hydén at an early point realized the huge importance of Ca(2+) in brain function. He established that glial cells show more marked and earlier changes in RNA composition in Parkinson's disease than neurons. Holger Hydén also had the vision and courage to suggest that "mental diseases could as well be thought to depend upon a disturbance of processes in glia cells as in the nerve cells", and he showed that antidepressant drugs cause profound changes in glial RNA. The importance of Holger Hydén's findings and visions can only now be fully appreciated. His visionary concepts of the involvement of glia in neurological and mental illness, of learning being associated with changes in gene expression, and of the functional importance of Ca(2+)-binding proteins and Ca(2+) are presently being confirmed and expanded by others. This review briefly summarizes highlights of Holger Hydén's work in these areas, followed by a discussion of recent research, confirming his findings and expanding his visions. This includes strong evidence that glial dysfunction is involved in the development of Parkinson's disease, that drugs effective in mood disorders alter gene expression and exert profound effects on astrocytes, and that neuronal-astrocytic interactions in glutamate signaling, NO synthesis, Ca(2+) signaling, beta-adrenergic activity, second messenger production, protein kinase activities, and transcription factor phosphorylation control the highly programmed events that carry the memory trace through the initial, signal-mediated short-term and intermediate memory stages to protein synthesis-dependent long-term memory.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Astrocyte-neuron interaction during one-trial aversive learning in the neonate chick

During two specific stages of the Gibbs-Ng model of one-trial aversive learning in the neonate chick, we have recently found unequivocal evidence for a crucial involvement of astrocytes. This evidence is metabolic (utilization of the astrocyte-specific energy store, glycogen, during normal learning and inhibition of memory formation by the astrocyte specific metabolic inhibitors, fluoroacetate and methionine sulfoximine) as well as physiological (abolition of memory formation in the presence of ethacrynic acid, an astrocyte-specific inhibitor of cellular reaccumulation of potassium ions). These findings are discussed in the present review in the framework of a more comprehensive description of metabolic and physiological neuronal-astrocytic interactions across an interstitial (extracellular) space bounded by minute processes from either cell type.