Biomedical subjects

L Hertz

Publications and source records attributed to L Hertz.

At least 19 recordsLinked to original sources

PubMed2006

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.

Acetates↗

PubMed2004

Analysis of glutamine accumulation in rat brain mitochondria in the presence of a glutamine uptake inhibitor, histidine, reveals glutamine pools with a distinct access to deamidation.

Rat cerebral nonsynaptic mitochondria were incubated in medium containing 2 mM glutamine (Gln) or 2 mM glutamate (Glu), in the presence of a Gln uptake inhibitor histidine (His) as well as other basic amino acids, lysine and arginine (Lys, Arg) not inhibiting Gln uptake. Subsequently, the mitochondrial contents of Glu and Gln were determined by HPLC. Incubation in the presence of Glu alone increased the Glu content from approximately 3.5 to 15 nmol/mg protein, without affecting the Gln content. On the other hand, incubation with Gln increased the content of Gln from approximately 1.5 to approximately 12 nmol/mg, and that of Glu to 10 nmol/mg. As expected, addition of His did not alter the Glu and Gln content resulting from incubation with Glu. However, His significantly decreased to almost the preincubation level the content of Glu in mitochondria incubated with Gln, without affecting the content of Gln. No other amino acid had any effect on these parameters. The results point to the existence of distinct Gln pools, one of which is accessible to external Gln via a His-sensitive transporter and is accessible for deamidation in the mitochondria.

Amides↗

PubMed2004

Inhibition of short-term memory formation in the chick by blockade of extracellular glutamate uptake.

Injection of monosodium glutamate (40nmol/hemisphere) into the intermediate hyperstriatum ventrale of the day-old chick inhibits the formation of short-term memory for a single trial learning that discriminates between colours of beads. These experiments showed that an excess of glutamate close to learning could be damaging to memory. In the present experiments we have blocked the normal reuptake of glutamate and suggest that glutamate release plays a role in normal learning. Removal of glutamate, released from presynaptic neurones during learning, is achieved by various neuronal and astrocytic glutamate transporters. By blocking the primarily astrocytic removal of glutamate by the injection of L-aspartic acid beta-hydroxamate, we effectively increased extrasynaptic levels of glutamate and inhibited short-term memory in a similar manner to central injection of 40nmol glutamate per hemisphere. These experiments suggest that glutamate release within 2.5min of the learning experience is an important feature of short-term memory formation.

Animals↗

PubMed2004

Astrocytic adrenoceptors: a major drug target in neurological and psychiatric disorders?

Considerable attention has recently been paid to astrocyte functions, which are briefly summarized. A large amount of data is available about adrenoceptor expression and function in astrocytes, some of it dating back to the 1970's and some of it very recent. This material is reviewed in the present paper. The brain is innervated by noradrenergic fibers extending from locus coeruleus in the brain stem, which in turn is connected to a network of adrenergic and noradrenergic nuclei in the medulla and pons, contributing to the control of (nor)adrenergic, serotonergic, dopaminergic and cholinergic function, both in the central nervous system (CNS) and in the periphery. In the CNS astrocytes constitute a major target for noradrenergic innervation, which regulates morphological plasticity, energy metabolism, membrane transport, gap junction permeability and immunological responses in these cells. Noradrenergic effects on astrocytes are essential during consolidation of episodic, long-term memory, which is reinforced by beta-adrenergic activation. Glycogenolysis and synthesis of glutamate and glutamine from glucose, both of which are metabolic processes restricted to astrocytes, occur at several time-specific stages during the consolidation. Astrocytic abnormalities are almost certainly important in the pathogenesis of multiple sclerosis and in all probability contribute essentially to inflammation and malfunction in Alzheimer's disease and to mood disturbances in affective disorders. Noradrenergic function in astrocytes is severely disturbed by chronic exposure to cocaine, which also changes astrocyte morphology. Development of drugs modifying noradrenergic receptor activity and/or down-stream signaling is advocated for treatment of several neurological/psychiatric disorders and for neuroprotection. Astrocytic preparations are suggested for study of mechanism(s) of action of antidepressant drugs and pathophysiology of mood disorders.

Adrenergic Agents↗

PubMed2001

Glucose and lactate metabolism during brain activation.

The dependence of brain function on blood glucose as a fuel does not exclude the possibility that lactate within the brain might be transferred between different cell types and serve as an energy source. It has been recently suggested that 1) about 85% of glucose consumption during brain activation is initiated by aerobic glycolysis in astrocytes, triggered by demand for glycolytically derived energy for Na+ -dependent accumulation of transmitter glutamate and its amidation to glutamine, and 2) the generated lactate is quantitatively transferred to neurons for oxidative degradation. However, astrocytic glutamate uptake can be fueled by either glycolytically or oxidatively derived energy, and the extent to which "metabolic trafficking" of lactate might occur during brain function is unknown. In this review, the potential for an astrocytic-neuronal lactate flux has been estimated by comparing rates of glucose utilization in brain and in cultured neurons and astrocytes with those for lactate release and uptake. Working brain tissue and isolated brain cells release large amounts of lactate. Cellular lactate uptake occurs by carrier-mediated facilitated diffusion and is normally limited by its dependence on metabolism of accumulated lactate to maintain a concentration gradient. The rate of this process is similar in cultured astrocytes and glutamatergic neurons, and, at physiologically occurring lactate concentrations, lactate uptake corresponds at most to 25% of the rate of glucose oxidation, which accordingly is the upper limit for "metabolic trafficking" of lactate. Because of a larger local release than uptake of lactate and the necessity for rapid lactate clearance to maintain the intracellular redox state to support lactate production in the presence of normal oxygen levels, brain activation in vivo is probably, in many cases, accompanied by a substantial overflow of glycolytically generated lactate, both to different brain areas and under some conditions (spreading depression, hyperammonemia) to circulating blood.

Animals↗

PubMed2001

Guanosine-induced increase in free cytosolic calcium concentration in mouse astrocytes in primary cultures: does it act on an A3 adenosine receptor?

Purinergic receptors play an important role in the regulation of free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes. In the present study, 10 microM adenosine caused an increase in [Ca(2+)](i) in 85% of the cultures studied, i.e., primary cultures of mouse astrocytes, differentiated by culturing in the presence of dibutyryl cyclic AMP. Antagonist sensitivity and rapid desensitization suggested that it did so by acting on A3 receptors. Another biologically important purine, guanosine, also caused an increase in astrocytic [Ca(2+)](i) (at concentrations of 0.1-100 microM). Although this response did not show the same rapid desensitization as the response to adenosine, it may also have been exerted on an A3 receptor. It supports this idea that inosine also caused an increase in [Ca(2+)](i), because inosine is known to activate A3 receptors in mast cells and structurally is even more closely related to guanosine than is adenosine.

Adenosine↗

PubMed2001

Effects of L-glutamate, D-aspartate, and monensin on glycolytic and oxidative glucose metabolism in mouse astrocyte cultures: further evidence that glutamate uptake is metabolically driven by oxidative metabolism.

The hypothesis was tested that oxidative metabolism, mainly fueled by glutamate itself, provides the energy for active, Na(+),K(+)-ATPase-catalyzed Na(+) extrusion following glutamate uptake in conjunction with Na(+). This hypothesis was supported by the following observations: (i) glutamate had either no effect or caused a slight reduction in glycolytic rate, measured as deoxyglucose phosphorylation; (ii) D-aspartate, which is accumulated by the L-glutamate carrier, but cannot be metabolized by the cells, caused an increase in glycolytic rate; (iii) monensin which, like D-aspartate, stimulates the intracellular, Na(+)-activated site of the Na, K-ATPase and thus energy metabolism, but provides no metabolic substrate, stimulated both glycolysis and glucose oxidation; and (iv) oxidation of glucose was potently inhibited by glutamate, although glutamate is known to stimulate oxygen consumption in primary cultures of astrocytes, a combination showing that oxidation of a non-glucose substrate is increased in the presence of glutamate. These findings should be considered in attempts to understand metabolic interactions between neurons and astrocytes and regulation of energy metabolism in brain.

Animals↗

PubMed2001

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.

Animals↗

PubMed2001

Cloning and identification of differentially expressed transcripts in primary culture of GABAergic neurons.

A RNA based arbitrarily primed polymerase chain reaction (RAP-PCR) was used to identify differentially expressed transcripts in primary cultures of cerebral cortical neurons prepared from E16 mouse cerebral cortex. The majority of neurons found in this culture preparation are known to be GABAergic. Different primer combinations were used, and the PCR products were separated on PAGE. Visualization by silver staining revealed a high resolution RNA fingerprint pattern with a total of about 200 transcripts. Six differentially expressed cDNA fragments were recovered, cloned and sequenced. The results of a NCBI database search showed that 6 clones were highly homologous to known genes and expressed sequence tags (ESTs), and that they were either up-regulated or down-regulated during development. Among these clones, Clone 3.1.7 shared 99% sequence homology to mouse Reelin, a neuronal migration and positioning related protein. Clone 4.6.2 shared 91% homology to Rat prepro bone morphogenetic protein-3 mRNA. Clone 6.10.2 had 90% homology to a novel orphan gene of calcium-independent alpha-latrotoxin receptor, which stimulates presynaptic neurotransmitter release. Northern blot analysis confirmed the up-regulated expression profile of Clone 6.10.2 in neuron from Day 2 to 7 during stages of differentiation and development.

Animals↗

PubMed2000

Involvement of non-neuronal brain cells in AVP-mediated regulation of water space at the cellular, organ, and whole-body level.

Vasopressin (AVP) influences non-neuronal brain cells in cell-type specific manners: (1) it regulates water balance at the cellular level of brain parenchyma by adjusting astrocytic water permeability; (2) it contributes to the control of extracellular K(+) concentration ([K(+)](e)) in brain by stimulation of K(+) transfer from blood to brain, due to activation of an inwardly directed Na(+),K(+),Cl(-) cotransporter at the luminal membrane of capillary endothelial cells and opening of K(+) channels at their abluminal membrane; (3) it decreases formation of cerebrospinal fluid (CSF) by decreasing Cl(-) secretion into CSF by epithelial cells of the choroid plexus, probably by inhibition of Cl(-)/HCO(-)(3) exchange at their basolateral membrane; (4) it contributes to regulation of intracellular volume within the brain by regulation of water permeability in ependymal cells and subpial astrocytes; and (5) it exerts effects on specialized astrocytes in circumventricular organs, their adjacent glia limitans, and the neural pituitary, which regulate AVP release to the systemic circulation by altering the spatial relationship between neurons and their adjacent glial cells. A unified mechanism is proposed, which integrates most of the effects of AVP and may be of considerable importance for neuronal excitability and, thus, for behavior.

Animals↗

PubMed2000

Receptor subtype and dose dependence of dexmedetomidine-induced accumulation of [14C]glutamine in astrocytes suggests glial involvement in its hypnotic-sedative and anesthetic-sparing effects.

Dexmedetomidine, a selective alpha(2)-adrenergic agonist, increases accumulation of [14C]glutamine and its labeled metabolites in primary cultures of mouse astrocytes. The concentration dependence is biphasic and identical to that previously described for dexmedetomidine's effect on free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes, and both effects are exerted on the alpha(2A) subtype of the alpha(2) receptor, suggesting a Ca(2+)-mediated effect. The concentration corresponding to the most potent effect is similar to that with which dexmedetomidine exerts its anesthetic-sparing activity in vivo, and the second peak corresponds to its hypnotic-sedative effect. It is suggested that both effects may be caused by decreased glutamatergic neurotransmission, secondary to reduced availability of glutamine as a glutamate precursor in glutamatergic neurons.

Anesthetics↗

PubMed2000

Vasopressin increases [Ca(2+)](i) in differentiated astrocytes by activation of V1b/V3 receptors but has no effect in mature cortical neurons.

Vasopressin (AVP) plays an important role in regulation of astrocytic, but not neuronal, water content and cell volume during hydro-osmotic challenge. To investigate the intracellular mechanism(s) signaling this response, [Ca(2+)](i) was measured fluorometrically in cultured cerebrocortical astrocytes and neurons, obtained from neonatal and fetal mouse brains, and matured during the culturing period. In astrocytes, [Ca(2+)](i) increased with an EC(50) of between 10(-10) and 10(-9) M AVP, the maximum increase was approximately 100 nM, and the response was independent of extracellular Ca(2+), identifying the receptor as being of the V1b/V3 subtype. In contrast, AVP had no effect on [Ca(2+)](i) in cortical neurons. This cellular difference is consistent with the ability of AVP to increase water permeability in astrocytes but not in neurons.

Animals↗

PubMed2000

Nordidemnin potently inhibits inositol uptake in cultured astrocytes and dose-dependently augments lithium's proconvulsant effect in vivo.

It has been suggested that inositol uptake across the cell membrane is of importance for maintenance of the inositol pool involved in lithium's therapeutic effect in bipolar disease and in the lithium-pilocarpine seizure test in freely moving rats (measuring the latency of a normally subconvulsive concentration of pilocarpine to seizure induction in the additional presence of lithium). We have tested this hypothesis by: 1) demonstrating an extremely high potency of nordidemnin as an inhibitor of myo-inositol uptake in primary cultures of mouse astrocytes; and 2) determining the dose-response correlation of a nordidemnin-induced decrease in the latency before appearance of seizures in the lithium-pilocarpine test after intracerebroventricular injection of minute samples (10 microl) of virtually isotonic saline solution.

Animals↗

PubMed2000

Chronic treatment of human astrocytoma cells with lithium, carbamazepine or valproic acid decreases inositol uptake at high inositol concentrations but increases it at low inositol concentrations.

Inositol uptake was measured at concentrations of 25, 40 and 50 microM in human astrocytoma cell cultures treated for 1-3 weeks with pharmacologically relevant concentrations of LiCl, valproic acid or carbamazepine as well as in control cultures that had not been treated with any drug. After at least 2 weeks of treatment, each of these 3 conventional anti-bipolar drugs increased the uptake significantly at 25 microM inositol, had no effect at 40 microM, and decreased it at 50 microM inositol. Reduction of the drug concentrations by 50% abolished the stimulation of uptake at 25 microM inositol by lithium and valproic acid and reduced that by carbamazepine. These findings may contribute to an understanding of the mechanisms of action for anti-bipolar medication, and explain the controversy in the literature whether or not brain inositol is reduced after chronic administration of lithium.

Antimanic Agents↗

PubMed2000

Stimulation of Na+,K+-ATPase activity, increase in potassium uptake, and enhanced production of ouabain-like compounds in ammonia-treated mouse astrocytes.

Active potassium (K+) uptake and Na+,K+-ATPase activity were measured in primary cultures of mouse astrocytes. Both parameters were virtually unaffected by acute ammonia treatment but increased after chronic exposure to pathophysiologically relevant concentrations of ammonia (0.3 or 3 mM) for 1-4 days. The increased Na+,K+-ATPase activity after chronic treatment with ammonia was further enhanced in the acute presence of 12 mM K+. Based on these observations and literature data it was hypothesized that the direct effect of ammonia is formation of easily diffusible compound(s) with ouabain-like effect, that upregulation occurs of Na+,K+-ATPase activity and K+ uptake in response to the resulting ATPase inhibition, and that the washing procedure preceding the uptake experiments and the determination of Na+,K+-ATPase activity unmasks the upregulation. To test this hypothesis, the content of compounds with ouabain-like action was measured in media in which astrocytes had been incubated in the presence of 3 mM ammonia for 4 days and in controls to which an additional 3 mM NaCl had been added instead of ammonia. An endogenous, compound with ouabain-like activity was demonstrated both under control conditions and in the ammonia-treated cultures, and the content of this compound was increased by 50% in the ammonia-treated cultures. Preliminary experiments showed that at least part of the released ouabain-like compounds cross-react with authentic ouabain.

Ammonia↗

PubMed2000

A model of inositol compartmentation in astrocytes based upon efflux kinetics and slow inositol depletion after uptake inhibition.

Intracellular compartmentation of inositol was demonstrated in primary cultures of mouse astrocytes, incubated in isotonic medium, by determination of efflux kinetics after "loading" with [3H]inositol. Three kinetically different compartments were delineated. The largest and most slowly exchanging compartment had a halflife of approximately 9 hr. This slow release leads to retention of a sizeable amount of pre-accumulated inositol in the tissue 24 hr after the onset of uptake inhibition, as confirmed by the observation that the inositol uptake inhibitor fucose caused a larger inhibition of unidirectional inositol uptake than of inositol pool size, measured as accumulated [3H]inositol after 24 hr of combined exposure to the inhibitor and the labeled isotope. Based upon the present observations and literature data, it is suggested that the large, slowly exchanging compartment is largely membrane-associated and participating in signaling via the phosphatidylinositide second messenger system, whereas inositol functioning as an osmolyte is distributed in the cytosol and located in one or both of the compartments showing a faster release.

Animals↗

PubMed2000

A correlation between dexmedetomidine-induced biphasic increases in free cytosolic calcium concentration and energy metabolism in astrocytes.

UNLABELLED: The alpha(2)-adrenergic agonist, dexmedetomidine, increases free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes, but not in neurons. The present study was performed to characterize the origin of the increased Ca(2+) in mouse astrocytes cultured from the cerebral cortex, the dose dependence of the effect, and its functional consequences. The increase in [Ca(2+)](i) was independent of extracellular Ca(2+), but was inhibited by dantrolene, showing that it is derived from intracellular stores; two peaks in [Ca(2+)](i) were demonstrated-one around 100 nM dexmedetomidine and the other in the low micromolar range. A similar dose dependence was found for pyruvate dehydrogenation, the initial metabolic reaction of oxidative degradation of pyruvate, suggesting that the these events are interrelated. The alpha(2)-adrenergic antagonist, yohimbine, abolished the metabolic stimulation at both peaks. However, whereas the increase in [Ca(2+)] (i) at 100 nM is abolished by yohimbine, increase in the micromolar range was partly inhibited by yohimbine and partly by idazoxan, an inhibitor at the imidazoline-preferring site. The stimulation of energy metabolism in cerebrocortical astrocytes may explain the repeated finding that dexmedetomidine does not decrease oxidative metabolism in the brain in vivo. The functional importance of the additional imidazoline receptor-mediated increase in [Ca(2+)](i) at large dexmedetomidine concentrations is unknown. IMPLICATIONS: Cytosolic calcium concentration and metabolism were measured in cultured astrocytes, the predominant glial cells. The results suggest that dexmedetomidine may owe its anesthetic effects to a Ca(2+)-dependent increase in astrocytic energy metabolism, allowing these cells to more effectively remove extracellular glutamate and potassium ions, and thus, decreasing neuronal excitability.

Adjuvants, Anesthesia↗

PubMed2000

Dexmedetomidine-induced stimulation of glutamine oxidation in astrocytes: a possible mechanism for its neuroprotective activity.

Dexmedetomidine is a highly specific alpha2-adrenergic agonist, which is used clinically as an anesthetic adjuvant and in animal experiments has a neuroprotective effect during ischemia. The current study showed that dexmedetomidine enhances glutamine disposal by oxidative metabolism in astrocytes. This effect occurs at pharmacologically relevant concentrations. It is exerted on alpha2-adrenergic receptors and not on imidazoline-preferring sites, and it is large enough to reduce the availability of glutamine as a precursor of neurotoxic glutamate.

Adrenergic alpha-Agonists↗