Long-term modulation of glucose utilization by IL-1 alpha and TNF-alpha in astrocytes: Na+ pump activity as a potential target via distinct signaling mechanisms

Interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha) markedly stimulate glucose utilization in primary cultures of mouse cortical astrocytes. The mechanism that gives rise to this effect, which takes place several hours after application of cytokine, has remained unclear. Experiments were conducted to identify the major signaling cascades involved in the metabolic action of cytokine. First, the selective IL-1 receptor antagonist (IL-1ra) prevents the effect of IL-1alpha on glucose utilization in a concentration-dependent manner, whereas it has no effect on the action of TNF-alpha. Then, using inhibitors of three classical signaling cascades known to be activated by cytokines, it appears that the PI3 kinase is essential for the effect of both IL-1alpha and TNF-alpha, whereas the action of IL-1alpha also requires activation of the MAP kinase pathway. Participation of a phospholipase C-dependent pathway does not appear critical for both IL-1alpha and TNF-alpha. Inhibition of NO synthase by L-NAME did not prevent the metabolic response to both IL-1alpha and TNF-alpha, indicating that nitric oxide is probably not involved. In contrast, the Na(+)/K(+) ATPase inhibitor ouabain prevents the IL-1alpha- and TNF-alpha-stimulated 2-deoxyglucose (2DG) uptake. When treatment of astrocytes with a cytokine was followed 24 h later by an acute application of glutamate, a synergistic enhancement in glucose utilization was observed. This effect was greatly reduced by ouabain. These data suggest that Na(+) pump activity is a common target for both the long-term metabolic action of cytokines promoted by the activation of distinct signaling pathways and the enhanced metabolic response to glutamate.

Similar perisynaptic glial localization for the Na+,K+-ATPase alpha 2 subunit and the glutamate transporters GLAST and GLT-1 in the rat somatosensory cortex

Several isoenzymes of the Na(+),K(+)-ATPase are expressed in brain but their specific roles are poorly understood. Recently, it was suggested that an isoenzyme of the Na(+),K(+)-ATPase containing the alpha(2) subunit, together with the glutamate transporters GLAST and GLT-1, participate in a coupling mechanism between neuronal activity and energy metabolism taking place in astrocytes. To substantiate this hypothesis, we compared the distribution of alpha(2), GLAST and/or GLT-1 in the rat cerebral cortex using double immunofluorescence and confocal microscopy, and immunocytochemistry at the electron microscopic level. We also investigated the relationship between alpha(2), GLAST or GLT-1 and asymmetrical synaptic junctions (largely glutamatergic) and GABAergic nerve terminals. Results show that the alpha(2) subunit has an exclusive astroglial localization, and that it is almost completely co-distributed with GLAST and GLT-1 when evaluated by confocal microscopy. This similar distribution was confirmed at the ultrastructural level, which further showed that the vast majority of the alpha(2) staining (73% of all labelled elements), like that of GLAST and GLT-1, was located in glial leaflets surrounding dendritic spines and the dendritic and/or axonal elements of asymmetrical (glutamatergic) axo-dendritic synapses. Synapses ensheathed by alpha(2), GLAST or GLT-1 virtually never included (<or=2%) GABAergic nerve terminals or synaptic junctions. However, a subset of GABAergic nerve terminals (10-14%) were directly apposed to asymmetrical axo-dendritic junctions surrounded by alpha(2), GLAST or GLT-1. Altogether these results demonstrate that alpha(2), GLAST and GLT-1 have comparable perisynaptic distribution within cortical astrocytes most likely associated with glutamatergic synapses.

MCT2 is a major neuronal monocarboxylate transporter in the adult mouse brain

Although previous Northern blot and in situ hybridization studies suggested that neurons express the monocarboxylate transporter MCT2, subsequent immunohistochemical analyzes either failed to confirm the presence of this transporter or revealed only a low density of immunolabeled neuronal processes in vivo. The authors report that appropriate section pretreatment (brief warming episode or proteinase K exposure) leads to extensive labeling of the neuropil, which appears as tiny puncta throughout the whole mouse brain. In addition, intense MCT2 immunoreactivity was found in cerebellar Purkinje cell bodies and their processes, on mossy fibers in the cerebellum, and on sensory fibers in the brainstem. Double immunofluorescent labeling with appropriate markers and observation with epifluorescence and confocal microscopy did not show extensive colocalization of MCT2 immunoreactivity with presynaptic or postsynaptic elements, but colocalization could be observed occasionally in the cortex with the postsynaptic density protein PSD95. Observations made at the electron microscopic level in the cortex corroborated these results and showed that MCT2 immunoreactivity was associated with wide membrane segments of neuronal processes. These data provide convincing evidence that MCT2 represents a major neuronal monocarboxylate transporter in the adult mouse brain, and further suggest that mature neurons could use monocarboxylates such as lactate as additional energy substrates.

Feeding active neurons: (re)emergence of a nursing role for astrocytes

Despite unquestionable evidence that glucose is the major energy substrate for the brain, data collected over several decades with different approaches suggest that lactate may represent a supplementary metabolic substrate for neurons. Starting with the pioneering work of McIlwain in the early 1950s which showed that lactate can sustain the respiratory rate of small brain tissue pieces, this idea receives confirmation with more recent studies using nuclear magnetic resonance spectroscopy undoubtedly demonstrating that lactate is efficiently oxidized by neurons, both in vitro and in vivo. Not only is lactate able to maintain ATP levels and promote neuronal survival but it was also found to support neuronal activity, at least if low levels of glucose are present. Despite the early suggestion for a role of astrocytes in metabolic supply to neurons, it is only recently however that they have been considered as a potential source of lactate for neurons. Moreover, it has been proposed that astrocytes might provide lactate to neurons in response to enhanced synaptic activity by a well-characterized mechanism involving glutamate uptake. The description of specific transporters for lactate on both astrocytes and neurons further suggest that there exist a coordinated mechanism of lactate exchange between the two cell types. Thus it is proposed that astrocytes play a nursing role toward neurons by providing lactate as an additional energy substrate especially during periods of enhanced synaptic activity. The importance of this metabolic cooperation within the central nervous system, although not unique if compared to other organs, still remains to be explored.

Aquaporins in brain: distribution, physiology, and pathophysiology

Water homeostasis in the brain is of central physiologic and clinical importance. Neuronal activity and ion water homeostasis are inextricably coupled. For example, the clearance of K+ from areas of high neuronal activity is associated with a concomitant water flux. Furthermore, cerebral edema, a final common pathway of numerous neurologic diseases, including stroke, may rapidly become life threatening because of the rigid encasement of the brain. A water channel family, the aquaporins, facilitates water flux through the plasma membrane of many cell types. In rodent brain, several recent studies have demonstrated the presence of different types of aquaporins. Aquaporin 1 (AQP1) was detected on epithelial cells in the choroid plexus whereas AQP4, AQP5 and AQP9 were localized on astrocytes and ependymal cells. In rodent brain, AQP4 is present on astrocytic end-feet in contact with brain vessels, and AQP9 is found on astrocytic processes and cell bodies. In basal physiologic conditions, AQP4 and AQP9 appear to be implicated in brain homeostasis and in central plasma osmolarity regulation. Aquaporin 4 may also play a role in pathophysiologic conditions, as shown by the reduced edema formation observed after water intoxication and focal cerebral ischemia in AQP4-knockout mice. Furthermore, pathophysiologic conditions may modulate AQP4 and AQP9 expression. For example, AQP4 and AQP9 were shown to be upregulated after ischemia or after traumatic injuries. Taken together, these recent reports suggest that water homeostasis in the brain is maintained by regulatory processes that, by control of aquaporin expression and distribution, induce and organize water movements. Facilitation of these movements may contribute to the development of edema formation after acute cerebral insults such as ischemia or traumatic injury.

Insights into the mechanisms of ifosfamide encephalopathy: drug metabolites have agonistic effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and induce cellular acidification in mouse cortical neurons

Therapeutic value of the alkylating agent ifosfamide has been limited by major side effects including encephalopathy. Although the underlying biochemical processes of the neurotoxic side effects are still unclear, they could be attributed to metabolites rather than to ifosfamide itself. In the present study, the effects of selected ifosfamide metabolites on indices of neuronal activity have been investigated, in particular for S-carboxymethylcysteine (SCMC) and thiodiglycolic acid (TDGA). Because of structural similarities of SCMC with glutamate, the Ca(2+)(i) response of single mouse cortical neurons to SCMC and TDGA was investigated. SCMC, but not TDGA, evoked a robust increase in Ca(2+)(i) concentration that could be abolished by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but only partly diminished by the N-methyl-D-aspartate receptor antagonist 10,11-dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK=801). Cyclothiazide (CYZ), used to prevent AMPA/kainate receptor desensitization, potentiated the response to SCMC. Because activation of AMPA/kainate receptors is known to induce proton influx, the intracellular pH (pH(i)) response to SCMC was investigated. SCMC caused a concentration-dependent acidification that was amplified by CYZ. Since H(+)/monocarboxylate transporter (MCT) activity leads to similar cellular acidification, we tested its potential involvement in the pH(i) response. Application of the lactate transport inhibitor quercetin diminished the pH(i) response to SCMC and TDGA by 43 and 51%, respectively, indicating that these compounds may be substrates of MCTs. Taken together, this study indicates that hitherto apparently inert ifosfamide metabolites, in particular SCMC, activate AMPA/kainate receptors and induce cellular acidification. Both processes could provide the biochemical basis of the observed ifosfamide-associated encephalopathy.

Brain energy metabolism in Alzheimer's disease: 99mTc-HMPAO SPECT imaging during verbal fluency and role of astrocytes in the cellular mechanism of 99mTc-HMPAO retention

The central hypothesis of the study which has been carried out as part of the NRP38 program, is that perturbations of brain energy metabolism are critically involved in the neurodegeneration occurring in Alzheimer's disease (AD) and that they may correlate with early cognitive dysfunctioning. In the present multidisciplinary study we set out to monitor brain energy metabolism using FDG-PET and HMPAO-SPECT imaging in a cohort of individuals over 65 years of age, drawn from the general population. HMPAO-SPECT imaging, which is a simpler and more widely accessible imaging procedure than FDG-PET, was performed under basal conditions and during the performance of a cognitive task (verbal fluency test). Three groups were studied. Two groups (groups I and II) included individuals age 65 or more, with no cognitive impairment and carrying an APOE4 positive or APOE4 negative phenotype, respectively; a third group (group III) included patients with clinical signs of AD. Each subject entering the study underwent an FDG-PET, an HMPAO-SPECT and an extensive battery of neuropsychological tests which assess various aspects of cognitive functioning, with a strong emphasis on working memory, divided attention and executive functions. A total of 101 participants were submitted to brain imaging and neuropsychological testing. Among these, 60 participants received the same set of imaging and neuropsychological tasks 24-36 months after the first set (phase II). In this article, we present a preliminary analysis performed on ten subjects from groups I and II and nine subjects from group III: activation (verbal fluency task) induced a specific pattern of increase in HMPAO retention (including BA 9/10, BA 18 bilaterally and right BA 17). In contrast to controls, in nine AD subjects no significant differences in HMPAO retention were observed when comparing activation and basal conditions. The cellular and molecular mechanisms that underlie the retention of HMPAO, the tracer used for single photon emission computed tomography (SPECT) imaging, has been studied in vitro in purified preparations of neurons and astrocytes with the aim of investigating the contribution of different cell types to hexamethyl-propyleneamineoxime labeled with technetium-99m (99mTc-HMPAO) retention in vitro. Results show that 99mTc-HMPAO retention predominates in astrocytes over neurons by a factor of approximately 2.5. Diethyl maleate, ethacrynic acid and buthionine sulfoximine, three agents which significantly reduce glutathione levels, also decreased 99mTc-HMPAO retention in both astrocytes and in neurons. Decrease did not always correlate with glutathione levels however, thus suggesting that other factors could be involved. The data presented indicate that astrocytes might constitute a prominent site of 99mTc-HMPAO retention and most likely contribute significantly to the SPECT signal. In addition, they also suggest that specific alterations in glial cell metabolism could explain flow-independent changes in 99mTc-HMPAO retention in the brain as observed by SPECT in certain pathologies (including Alzheimer's disease). In particular, these observations suggest a key role of astrocytes in the signal detected with the imaging procedure, which is altered in the Alzheimer's cohort subjected to the verbal fluency activation task.

Astrocyte-specific expression of aquaporin-9 in mouse brain is increased after transient focal cerebral ischemia

Aquaporin-9 (AQP9) is a new member of the aquaporin family of water-selective channels mainly expressed in liver and testis, presenting the characteristic of also being permeable to various solutes, particularly lactate. Recent data have shown the presence of AQP9 on tanycytes in the rat brain. In the current study, the authors show the expression of AQP9 in astrocytes in the mouse brain and changes in its expression after cerebral ischemia. Indeed, in control mouse, the AQP9 immunolabeling is present on astrocytic processes bordering the subarachnoid space and ventricles. The labeling also is observed on astrocytes in the white matter, hippocampus, hypothalamus, and lateral septum. After focal transient ischemia, an increase of the immunolabeling is detected on astrocytes in periinfarct areas. This AQP9 distribution study in mouse brain suggests a role of AQP9 in water homeostasis in the central nervous system. Furthermore, the overexpression of AQP9 on astrocytes surrounding an ischemic lesion suggests that AQP9 may also play a role in the regulation of postischemia edema and, in view of its permeability to monocarboxylates, in the clearance of lactate from the ischemic focus.

Astrocytes as a predominant cellular site of (99m)Tc-HMPAO retention

Technetium-99m-d,l-hexamethylpropylene amine oxime ((99m)Tc-HMPAO) retention in the brain monitored by single photon emission computed tomography (SPECT) is currently used as a marker of cerebral blood flow. The purported mechanism by which (99m)Tc-HMPAO accumulates in the brain is through its intracellular conversion from a lipophilic form to more hydrophilic derivatives within the brain parenchyma. The issue of the contribution of different cell types on (99m)Tc-HMPAO retention was investigated in vitro by studying the accumulation of (99m)Tc-HMPAO in primary cultures of mouse cortical astrocytes and neurons. Results show that (99m)Tc-HMPAO retention predominates in astrocytes over neurons by a factor of approximately 2.5 (0.26 +/- 0.05 vs. 0.095 +/- 0.042 fmol/mg protein after 120 minutes, respectively). Diethyl maleate (60 micromol/L), ethacrynic acid (1 mmol/L) and buthionine sulfoximine (1 mmol/L), 3 agents which significantly reduced glutathione levels also decreased (99m)Tc-HMPAO retention in both astrocytes (29%, 3%, and 46% of control, respectively) and neurons (69%, 11% and 63% of control). Decrease did not always correlate with glutathione levels, however, which suggests that other factors could be involved. The possibility that cell energy status determines (99m)Tc-HMPAO retention was also assessed. Agents that activate (glutamate, azide) or inhibit (cytochalasin B) glucose utilization in astrocytes, as measured by the (3)H-2-deoxyglucose method, were without effect on (99m)Tc-HMPAO retention. In conclusion, the data presented indicate that astrocytes may constitute a prominent site of (99m)Tc-HMPAO retention and most likely contribute significantly to the SPECT signal. In addition, the data also suggest that specific alterations in glial cell metabolism could explain flow-independent changes in (99m)Tc-HMPAO retention in the brain as observed by SPECT in some pathologies.

Effects of glial glutamate transporter inhibitors on intracellular Na+ in mouse astrocytes

The effects of inhibitors of the glial Na+/glutamate co-transporter on the intracellular Na+ concentration ([Na+](i)) were investigated in mouse cortical astrocytes. [Na+](i) was monitored by fluorescence microscopy on single astrocytes using the Na+-sensitive probe sodium-binding benzofuran isophtalate. Application of the competitive inhibitors threo-beta-hydroxyaspartate (THA) and trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC) resulted in robust and reversible increases in [Na+](i) that were comparable in shape to the response to glutamate but about twice lower in amplitude. As previously observed with glutamate, the amplitude of the [Na+](i) response to these compounds was concentration-dependent with EC(50) values of 11.1 microM (THA) and 7.6 microM (t-PDC), as was the initial rate of [Na+](i) rise (EC(50) values of 14.8 microM for THA and 11.5 microM for t-PDC). Both compounds diminished the response to subsequent glutamate applications, possibly because of an inhibitory effect of the intracellularly-accumulated compounds. In comparison, the newly-developed compound threo-beta-benzyloxyaspartate (TBOA) alone did not cause any significant alteration of [Na+](i) up to a concentration of 500 microM . TBOA inhibited the [Na+](i) response evoked by 200 microM glutamate in a concentration-dependent manner with IC(50) values of 114 and 63 microM, as measured on the amplitude and the initial rate, respectively. The maximum inhibition of glutamate-evoked [Na+](i) increase by TBOA was approximately 70%. The residual response persisted in the presence of a non-NMDA receptor antagonist or the inhibitor of the GLT-1 glutamate transporters, dihydrokainate (DHK). In view of the complete reversibility of its effects, TBOA represents a very useful pharmacological tool for studies of glutamate transporters.

Cell-specific localization of monocarboxylate transporters, MCT1 and MCT2, in the adult mouse brain revealed by double immunohistochemical labeling and confocal microscopy

Recent evidence suggests that lactate could be a preferential energy substrate transferred from astrocytes to neurons. This would imply the presence of specific transporters for lactate on both cell types. We have investigated the immunohistochemical localization of two monocarboxylate transporters, MCT1 and MCT2, in the adult mouse brain. Using specific antibodies raised against MCT1 and MCT2, we found strong immunoreactivity for each transporter in glia limitans, ependymocytes and several microvessel-like elements. In addition, small processes distributed throughout the cerebral parenchyma were immunolabeled for monocarboxylate transporters. Double immunofluorescent labeling and confocal microscopy examination of these small processes revealed no co-localization between glial fibrillary acidic protein and monocarboxylate transporters, although many glial fibrillary acidic protein-positive processes were often in close apposition to elements labeled for monocarboxylate transporters. In contrast, several elements expressing the S100beta protein, another astrocytic marker found to be located in distinct parts of the same cell when compared with glial fibrillary acidic protein, were also strongly immunoreactive for MCT1, suggesting expression of this transporter by astrocytes. In contrast, MCT2 was expressed in a small subset of microtubule-associated protein-2-positive elements, indicating a neuronal localization. In conclusion, these observations are consistent with the possibility that lactate, produced and released by astrocytes (via MCT1), could be taken up (via MCT2) and used by neurons as an energy substrate.

A quantitative analysis of L-glutamate-regulated Na+ dynamics in mouse cortical astrocytes: implications for cellular bioenergetics

The mode of Na+ entry and the dynamics of intracellular Na+ concentration ([Na+]i) changes consecutive to the application of the neurotransmitter glutamate were investigated in mouse cortical astrocytes in primary culture by video fluorescence microscopy. An elevation of [Na+]i was evoked by glutamate, whose amplitude and initial rate were concentration dependent. The glutamate-evoked Na+ increase was primarily due to Na+-glutamate cotransport, as inhibition of non-NMDA ionotropic receptors by 6-cyano-7-nitroquinoxiline-2,3-dione (CNQX) only weakly diminished the response and D-aspartate, a substrate of the glutamate transporter, produced [Na+]i elevations similar to those evoked by glutamate. Non-NMDA receptor activation could nevertheless be demonstrated by preventing receptor desensitization using cyclothiazide. Thus, in normal conditions non-NMDA receptors do not contribute significantly to the glutamate-evoked Na+ response. The rate of Na+ influx decreased during glutamate application, with kinetics that correlate well with the increase in [Na+]i and which depend on the extracellular concentration of glutamate. A tight coupling between Na+ entry and Na+/K+ ATPase activity was revealed by the massive [Na+]i increase evoked by glutamate when pump activity was inhibited by ouabain. During prolonged glutamate application, [Na+]i remains elevated at a new steady-state where Na+ influx through the transporter matches Na+ extrusion through the Na+/K+ ATPase. A mathematical model of the dynamics of [Na+]i homeostasis is presented which precisely defines the critical role of Na+ influx kinetics in the establishment of the elevated steady state and its consequences on the cellular bioenergetics. Indeed, extracellular glutamate concentrations of 10 microM already markedly increase the energetic demands of the astrocytes.

[Functional brain imaging: role metabolic coupling between astrocytes and neurons]

Despite the technological sophistication of brain imaging techniques, the precise mechanisms and cell types involved in coupling and in generating metabolic signals are still debated. Recent experimental data on the cellular and molecular mechanisms that underlie the fluorodeoxyglucose (FDG)--based PET imaging point to a critical role of a particular brain cell type, the astrocytes, in coupling neuronal activity to glucose utilization. Indeed, astrocytes possess receptors and re-uptake sites for a variety of neurotransmitters, including glutamate, the predominant excitatory neurotransmitter in the brain. In addition, astrocytic end-feet, which surround capillaries, are enriched in the specific glucose transporter GLUT-1. These features allow astrocytes to "sense" synaptic activity and to couple it with energy metabolism. In vivo and in vitro data support the following functional model: in response to glutamate released by active neurons, glucose is predominantly taken up by astrocytic end-feet; glucose is then metabolized to lactate which provides a preferred energy substrate for neurons. These data support the notion that astrocytes markedly contribute to the FDG-PET signal.

Evidence supporting the existence of an activity-dependent astrocyte-neuron lactate shuttle

Mounting evidence from in vitro experiments indicates that lactate is an efficient energy substrate for neurons and that it may significantly contribute to maintain synaptic transmission, particularly during periods of intense activity. Since lactate does not cross the blood-brain barrier easily, blood-borne lactate cannot be a significant source. In vitro studies by several laboratories indicate that astrocytes release large amounts of lactate. In 1994, we proposed a mechanism whereby lactate could be produced by astrocytes in an activity-dependent, glutamate-mediated manner. Over the last 2 years we have obtained further evidence supporting the notion that a transfer of lactate from astrocytes to neurons might indeed take place. In this article, we first review data showing the presence of mRNA encoding for two monocarboxylate transporters, MCT1 and MCT2, in the adult mouse brain. Second, by using monoclonal antibodies selectively directed against the two distinct lactate dehydrogenase isoforms, LDH1 and LDH5, a specific cellular distribution between neurons and astrocytes is revealed which suggests that a population of astrocytes is a lactate 'source' while neurons may be a lactate 'sink'. Third, we provide biochemical evidence that lactate is interchangeable with glucose to support oxidative metabolism in cortical neurons. This set of data is consistent with the existence of an activity-dependent astrocyte-neuron lactate shuttle for the supply of energy substrates to neurons.

Protein targeting to glycogen mRNA expression is stimulated by noradrenaline in mouse cortical astrocytes

Brain glycogen levels are dynamically regulated by certain neurotransmitters, including noradrenaline (NA) and vasoactive intestinal peptide (VIP). In particular, glycogen synthesis involves activation by NA and VIP of the transcription factors C/EBPbeta and -delta as well as the induction of glycogen synthase. Glycogen accumulation is found in a variety of neuropathological conditions, including reactive astrocytosis after CNS lesions, as well as in Alzheimer's disease. Protein targeting to glycogen (PTG) belongs to a family of proteins that play a key role in glycogen synthesis in peripheral tissues. In this study, we report the presence of PTG mRNA in adult mouse brain, as well as in astrocytes, a non-neuronal cell type that contains most of brain glycogen. Using primary cultures of mouse cortical astrocytes, we observed that NA leads to time- and concentration-dependent induction of PTG mRNA expression. This effect, concomitant to an enhancement of glycogen synthesis in these cells, depends on the activation of beta(1)-adrenergic receptors. Induction of PTG mRNA expression was mimicked by the adenylate cyclase activator forskolin and by dibutyryl cAMP, suggesting the involvement of the cAMP-dependent signal transduction cascade. Among other neuroactive substances known to elevate glycogen levels in astrocytes, VIP had a comparable effect to that of noradrenaline, whereas insulin and glutamate were without effect on PTG mRNA expression. These data suggest that increased PTG expression by neurotransmitters such as noradrenaline and VIP could represent a major event leading to enhancement of glycogen levels in astrocytes.

Opposite regulation of calbindin and calretinin expression by brain-derived neurotrophic factor in cortical neurons

Regulation of calbindin and calretinin expression by brain-derived neurotrophic factor (BDNF) was examined in primary cultures of cortical neurons using immunocytochemistry and northern blot analysis. Here we report that regulation of calretinin expression by BDNF is in marked contrast to that of calbindin. Indeed, chronic exposure of cultured cortical neurons for 5 days to increasing concentrations of BDNF (0.1-10 ng/ml) resulted in a concentration-dependent decrease in the number of calretinin-positive neurons and a concentration-dependent increase in the number of calbindin-immunoreactive neurons. Consistent with the immunocytochemical analysis, BDNF reduced calretinin mRNA levels and up-regulated calbindin mRNA expression, providing evidence that modifications in gene expression accounted for the changes in the number of calretinin- and calbindin-containing neurons. Among other members of the neurotrophin family, neurotrophin-4 (NT-4), which also acts by activating tyrosine kinase TrkB receptors, exerted effects comparable to those of BDNF, whereas nerve growth factor (NGF) was ineffective. As for BDNF and NT-4, incubation of cortical neurons with neurotrophin-3 (NT-3) also led to a decrease in calretinin expression. However, in contrast to BDNF and NT-4, NT-3 did not affect calbindin expression. Double-labeling experiments evidenced that calretinin- and calbindin-containing neurons belong to distinct neuronal subpopulations, suggesting that BDNF and NT-4 exert opposite effects according to the neurochemical phenotype of the target cell.

Differential messenger RNA distribution of lactate dehydrogenase LDH-1 and LDH-5 isoforms in the rat brain

The role of lactate in brain energy metabolism has recently received renewed attention. Although blood-borne monocarboxylates such as lactate poorly cross the blood-brain barrier in the adult brain, lactate produced within the brain parenchyma may be a suitable substrate for brain cells. Lactate dehydrogenase is crucial for both the production and utilization of lactate. In this article, we report the regional distribution of the messenger RNAs for lactate dehydrogenase isoforms 1 and 5 in the adult rat brain using in situ hybridization histochemistry with specific [alpha-(35)S]dATP 3' end-labeled oligoprobes. The autoradiographs revealed that the lactate dehydrogenase-1 messenger RNA is highly expressed in a variety of brain structures, including the main olfactory bulb, the piriform cortex, several thalamic and hypothalamic nuclei, the pontine nuclei, the ventral cochlear nucleus, the trigeminal nerve and the solitary tractus nucleus. In addition, the granular and Purkinje cell layers of the cerebellum showed a strong labeling. The neocortex (e.g., cingular, retrosplenial and frontoparietal cortices) often exhibits a marked laminar pattern of distribution of lactate dehydrogenase-1 messenger RNA (layers II/III, IV and VI being most strongly labeled). In contrast, expression of the lactate dehydrogenase-5 messenger RNA generally seemed more diffusely distributed across the different brain regions. Expression was particularly strong in the hippocampal formation (especially in Ammon's horn and dentate gyrus) and in the cerebral cortex, where no laminar pattern of distribution was observed. Overall, these data are consistent with the emerging idea that lactate is an important energy substrate produced and consumed by brain cells.

Pro-inflammatory cytokines induce the transcription factors C/EBPbeta and C/EBPdelta in astrocytes

The transcription factors CCAAT/enhancer binding protein (C/EBP)-beta and -delta are key regulators for the expression of the acute phase genes in the liver, such as complement component C3 and antichymotrypsin. In the brain, these acute phase proteins are produced in response to pro-inflammatory cytokines by the reactive astrocytes, in particular those surrounding the amyloid plaques of Alzheimer's disease brains. Here we show that lipopolysaccharides (LPS), IL-1beta, and TNFalpha induce the expression of the c/ebpbeta and -delta genes in mouse primary astrocytes. This induction precedes the expression of the acute phase genes coding for the complement component C3 and the mouse homologue of antichymotrypsin. The induction of these two acute phase genes by LPS is blocked by cycloheximide, whereas this protein synthesis inhibitor does not affect the expression of the c/ebp genes. Altogether, our data support a role as immediate-early genes for c/ebpbeta and -delta, whose expression is induced by pro-inflammatory cytokines in mouse cortical astrocytes. In the liver, these transcription factors are known to play an important role in inflammation and energy metabolism regulation. Therefore, C/EBPbeta and -delta could be pivotal transcription factors involved in brain inflammation, in addition to their previously demonstrated role in brain glycogen metabolism regulation (Cardinaux and Magistretti. J Neurosci 16:919-929, 1996).

Trans-inhibition of glutamate transport prevents excitatory amino acid-induced glycolysis in astrocytes

Previous studies have demonstrated that activation of glutamate transporters promotes glycolysis in astrocytes. Current evidence indicates that compounds such as threo-beta-hydroxyaspartate (THA) are both competitive inhibitors and substrates for glutamate transporters. In this study, we have analyzed the effect of THA on excitatory amino acid (EAA) transport and on EAA-induced glycolysis in mouse primary astrocyte cultures. In agreement with previous studies in rat astrocytes, THA competitively inhibited 3H-D-aspartate (3H-D-Asp) uptake with an IC50 of 319 microM (Ki = 36.6 microM). In contrast, it did not prevent D-aspartate-induced 3H-2-deoxyglucose (2DG) uptake in these conditions. Preexposure of cells to THA for at least 15 min revealed another form of glutamate transport inhibition. This effect was concentration-dependent with an apparent IC50 of 47.7 microM and showed kinetic characteristics consistent with a mechanism of trans-inhibition. Preincubation with THA also inhibited D-aspartate-induced 3H-2DG uptake in a concentration-dependent manner with an apparent IC50 of 59.8 microM. Comparison with other transportable analogues reveals that they share with THA the ability to cause trans-inhibition of glutamate transport and to prevent glutamate-stimulated glycolysis; THA, however, is unique in that it has no effect alone on glucose utilization after preexposure. These data indicate that trans-inhibition of glutamate transport may be a mechanism by which certain glutamate transport inhibitors can prevent the stimulation of aerobic glycolysis by glutamate in astrocytes.

Deep hypothermia and rewarming alters glutamate levels and glycogen content in cultured astrocytes

BACKGROUND

Deep hypothermia has been associated with an increased incidence of postoperative neurologic dysfunction after cardiac surgery in children. Recent studies suggest an excitotoxic mechanism involving overstimulation of glutamate receptors. Extracellular glutamate uptake occurs primarily by astrocytes. Astrocytes also store glycogen, which may be used to sustain the energy-consuming glutamate uptake. Extracellular glutamate and glycogen content were studied during temperature changes mimicking cardiopulmonary bypass in vivo.

METHODS

Primary cultures of cerebral cortical astrocytes were used in a specially designed incubator allowing continuous changes of temperature and ambient gas concentrations. The sequence of events was as follows: normothermia, rapid cooling (2.8 degrees C/min) followed by 60 min of deep hypothermia (15 degrees C), followed by rewarming (3.0 degrees C/min) and subsequent 5 h of mild hyperthermia (38.5 degrees C). Two different conditions of oxygenation were studied: (1) normoxia (25% O2, 70% N2, 5% CO2); or (2) hyperoxia (95% O2, 5% CO2). The extracellular glutamate concentrations and intracellular glycogen levels were measured at nine time points.

RESULTS

One hundred sixty-two cultures were studied in four independent experiments. The extracellular concentration of glutamate in the normoxic group increased significantly from 35+/-10 nM/mg protein at baseline up to 100+/-15 nM/mg protein at the end of 5 h of mild hyperthermia (P < 0.05). In contrast, extracellular glutamate levels did not vary from control in the hyperoxic group. Glycogen levels decreased significantly from 260+/-85 nM/mg protein at baseline to < 25+/-5 nM/mg protein at the end of 5 h in the normoxic group (P < 0.05) but returned to control levels after rewarming in the hyperoxic group. No morphologic changes were observed in either group.

CONCLUSION

The extracellular concentration of glutamate increases, whereas the intracellular glycogen content decreases when astrocytes are exposed to a sequence of deep hypothermia and rewarming. This effect of hypothermia is prevented when astrocytes are exposed to hyperoxic conditions.

Assessing multiple sclerosis activity: is the in vitro production of tumor necrosis factor-alpha, interleukins 2, 6, 4, and 10, and immunoglobulin G of value?

Tumor necrosis factor (TNF) alpha, interleukins (IL) 2, 4, 6, and 10, and IgG oligoclonal bands (IgG OB) in vitro production was assessed, after whole-blood stimulation with lipopolysaccharide or concanavalin A, in 61 patients presenting with relapsing-remitting, relapsing-progressive, or chronic progressive multiple sclerosis. Multiple sclerosis patients were receiving no treatment or azathioprine (AZA), cyclosporin, cyclophosphamide, subcutaneous interferon (IFN) beta 1 a, or corticosteroids (CST). Statistical correlations significantly showed that: (a) AZA lowers TNF-alpha (P = 0.002) and increases IL-4 production (P = 0.0024), and IFN-beta 1 a increases TNF-alpha and decreases IL-4 levels; (b) CST has a negative effect on TNF-alpha, IL-6, and IL-4 synthesis; and (c) AZA, IFN-beta 1 a, and CST diminish IgG OB synthesis (P = 0.001). Although our study of the dynamics of TNF-alpha, IL-2, IL-4, IL-6, and IL-10 in vitro production generally found no statistically significant correlations (partly explained by the limited number of values in the various groups), IL-6 was shown to drop during the periods surrounding relapse (P = 0.05) in the absence of treatment, while TNF-alpha (P = 0.04) and IL-6 (P < 0.05) dropped before exacerbation in the presence of AZA. In vitro production of TNF-alpha was closely and positively correlated with that of IL-6, independently of clinical features. The enhanced production of IL-10 detected before or at relapse with AZA and IFN-beta 1 a (trends) may interfere with initiation of the immune reaction and with the development of new CNS lesions. Some discrepancies with previously published results stress the difficulties in studying the state of stimulation of different populations of leukocytes by using a variety of in vitro stimuli and in establishing a correlation between mRNA studies and the amount of final or active protein produced.

Astrocytes Couple Synaptic Activity to Glucose Utilization in the Brain

Astrocytes have functional characteristics that make them particularly well suited to couple glutamate uptake from the synaptic cleft to Na(+)-K(+)-ATPase activation and glucose utilization. The changes in glucose metabolism associated with these processes may provide signals detected by positron emission tomography.

Cellular mechanisms of brain energy metabolism and their relevance to functional brain imaging

Despite striking advances in functional brain imaging, the cellular and molecular mechanisms that underlie the signals detected by these techniques are still largely unknown. The basic physiological principle of functional imaging is represented by the tight coupling existing between neuronal activity and the associated local increase in both blood flow and energy metabolism. Positron emission tomography (PET) signals detect blood flow, oxygen consumption and glucose use associated with neuronal activity; the degree of blood oxygenation is currently thought to contribute to the signal detected with functional magnetic resonance imaging, while magnetic resonance spectroscopy (MRS) identifies the spatio-temporal pattern of the activity-dependent appearance of certain metabolic intermediates such as glucose or lactate. Recent studies, including those of neurotransmitter-regulated metabolic fluxes in purified preparations and analyses of the cellular localization of enzymes and transporters involved in energy metabolism, as well as in vivo microdialysis and MRS approaches have identified the neurotransmitter glutamate and astrocytes, a specific type of glial cell, as pivotal elements in the coupling of synaptic activity with energy metabolism. Astrocytes are ideally positioned to sense increases in synaptic activity and to couple them with energy metabolism. Indeed they possess specialized processes that cover the surface of intraparenchymal capillaries, suggesting that astrocytes may be a likely site of prevalent glucose uptake. Other astrocyte processes are wrapped around synaptic contacts which possess receptors and reuptake sites for neurotransmitters. Glutamate stimulates glucose uptake into astrocytes. This effect is mediated by specific glutamate transporters present on these cells. The activity of these transporters, which is tightly coupled to the synaptic release of glutamate and operates the clearance of glutamate from the extracellular space, is driven by the electrochemical gradient of Na+. This Na(+)-dependent uptake of glutamate into astrocytes triggers a cascade of molecular events involving the Na+/K(+)-ATPase leading to the glycolytic processing of glucose and the release of lactate by astrocytes. The stoichiometry of this process is such that for one glutamate molecule taken up with three Na+ ions, one glucose molecule enters an astrocyte, two ATP molecules are produced through aerobic glycolysis and two lactate molecules are released. Within the astrocyte, one ATP molecule fuels one 'turn of the pump' while the other provides the energy needed to convert glutamate to glutamine by glutamine synthase. Evidence has been accumulated from structural as well as functional studies indicating that, under aerobic conditions, lactate may be the preferred energy substrate of activated neurons. Indeed, in the presence of oxygen, lactate is converted to pyruvate, which can be processed through the tricarboxylic acid cycle and the associated oxidative phosphorylation, to yield 17 ATP molecules per lactate molecule. These data suggest that during activation the brain may transiently resort to aerobic glycolysis occurring in astrocytes, followed by the oxidation of lactate by neurons. The proposed model provides a direct mechanism to couple synaptic activity with glucose use and is consistent with the notion that the signals detected during physiological activation with 18F-deoxyglucose (DG)-PET may reflect predominantly uptake of the tracer into astrocytes. This conclusion does not question the validity of the 2-DG-based techniques, rather it provides a cellular and molecular basis for these functional brain imaging techniques.

BDNF stimulates expression, activity and release of tissue-type plasminogen activator in mouse cortical neurons

Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor involved in neuronal development and synaptic plasticity. Although the physiological effects of BDNF have been examined in detail, target proteins which mediate its actions remain largely unknown. Here, we report that BDNF stimulates the expression of tissue-type plasminogen activator (tPA) in primary cultures of cortical neurons in a time- and concentration-dependent manner. Among the other members of the neurotrophin family, neurotrophin-4 (NT-4) and to a lesser extent neurotrophin-3 (NT-3) also increased tPA mRNA expression, while nerve growth factor (NGF) was devoid of any effect. Induction of tPA expression by BDNF is accompanied by an increase in the proteolytic activity of tPA associated with cortical neurons and a release of tPA into the extracellular space. Release of tPA induced by BDNF depends on extracellular Ca2+ since it is markedly reduced in the presence of ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). Up-regulation of tPA expression by BDNF is followed by the induction of plasminogen activator inhibitor 2 (PAI-2), an inhibitor of tPA. Together these results suggest that activation of tPA by BDNF may contribute to structural changes associated with neuronal development or synaptic plasticity.

VIP and PACAP in the CNS: regulators of glial energy metabolism and modulators of glutamatergic signaling

VIP neurons are a homogeneous population of intracortical bipolar cells. They receive excitatory synapses from afferent circuits to the cortex and exert effects on neurons, astrocytes, and capillaries. Effects on the two latter cell types imply that VIP neurons can translate incoming neuronal signals into local metabolic actions. Indeed, VIP tightly regulates glycogen metabolism in astrocytes. In this cell type VIP regulates the expression of a number of genes related to energy metabolism, such as glycogen synthase. These effects of VIP involve the transcription factor family C/EBP and result in the induction of at least seven new proteins by astrocytes. The actions of VIP on neurons appear to be of a modulatory nature: thus VIP enhances glutamate-mediated neurotransmission by potentiating the effects of glutamate on arachidonic acid formation and on the induction of c-fos and on BDNF expression. These effects indicate that VIP can actually increase the strength of glutamate-mediated neurotransmission.

In vitro cytokine profiles as indicators of relapse activity and clinical course in multiple sclerosis

In vitro production of tumor necrosis factor-alpha (TNF-alpha), interleukin-2 (IL-2), IL-4, IL-6, IL-10 and oligoclonal IgG (IgG OB) was evaluated in the aim to investigate their profile in correlation with multiple sclerosis (MS) clinical activity and clinical course. Whole blood stimulation with lipopolysaccharide or concanavalin A was performed in 61 patients presenting with relapsing-remitting, relapsing-progressive or chronic progressive MS; treatments received were: none, azathioprine (AZA), cyclosporin, cyclophosphamide, subcutaneous interferon-beta 1a (IFN-beta 1a) and corticosteroids (CST). The cinetics of cytokine production showed that (i) in the absence of treatment, TNF-alpha and IL-6 dropped respectively after and during the periods surrounding relapse, while IL-4 was increasing before and IL-10 after relapse; (ii) with AZA, TNF-alpha and IL-6 lowered before exacerbation, IL-4 prolonged high levels after and IL-10 before relapse; (iii) with IFN-beta 1a, IL-10 was already increasing before relapse, and TNF-alpha was higher after relapse. When cytokine levels were analysed independently from MS clinical activity, the use of AZA inhibited IgG OB and TNF-alpha synthesis (P = 0.002) but increased IL-4 (P = 0.0024), whereas IFN-beta 1a stimulated TNF-alpha and inhibited IgG OB and IL-4 production. CST inhibited TNF-alpha, IL-6, IL-4 and IgG OB synthesis. This study stresses both the weight of clinical parameters and of methodology used in results obtained in cytokine analysis in MS.

Expression of monocarboxylate transporter mRNAs in mouse brain: support for a distinct role of lactate as an energy substrate for the neonatal vs. adult brain

Under particular circumstances like lactation and fasting, the blood-borne monocarboxylates acetoacetate, beta-hydroxybutyrate, and lactate represent significant energy substrates for the brain. Their utilization is dependent on a transport system present on both endothelial cells forming the blood-brain barrier and on intraparenchymal brain cells. Recently, two monocarboxylate transporters, MCT1 and MCT2, have been cloned. We report here the characterization by Northern blot analysis and by in situ hybridization of the expression of MCT1 and MCT2 mRNAs in the mouse brain. In adults, both transporter mRNAs are highly expressed in the cortex, the hippocampus and the cerebellum. During development, a peak in the expression of both transporters occurs around postnatal day 15, declining rapidly by 30 days at levels observed in adults. Double-labeling experiments reveal that the expression of MCT1 mRNA in endothelial cells is highest at postnatal day 15 and is not detectable at adult stages. These results support the notion that monocarboxylates are important energy substrates for the brain at early postnatal stages and are consistent with the sharp decrease in blood-borne monocarboxylate utilization after weaning. In addition, the observation of a sustained intraparenchymal expression of monocarboxylate transporter mRNAs in adults, in face of the seemingly complete disappearance of their expression on endothelial cells, reinforces the view that an intercellular exchange of lactate occurs within the adult brain.

VIP and PACAP potentiate the action of glutamate on BDNF expression in mouse cortical neurones

In view of the neurotrophic effect of vasoactive intestinal peptide (VIP), the regulation of brain-derived neurotrophic factor (BDNF) expression by VIP and the related peptide pituitary adenylate cyclase-activating polypeptide (PACAP) was analysed by Northern blot in primary cultures of cortical neurones. Results reported in this article demonstrate that VIP and PACAP stimulate the expression of BDNF mRNA in primary cultures of cortical neurones and astrocytes. In primary cultures of cortical neurones, induction of BDNF mRNA by VIP and PACAP is completely inhibited by the N-methyl-D-aspartate (NMDA) receptor antagonists MK-801 and AP5, therefore indicating that VIP and PACAP do not stimulate BDNF expression directly but rather by potentiating the effect of glutamate tonically released by neurones and acting at NMDA receptors. In addition to its neurotrophic effects, BDNF has been shown to be involved in neuronal plasticity and results reported here suggest that by stimulating BDNF expression, VIP and PACAP could modulate synaptic plasticity in the cerebral cortex.

IL-1-alpha and TNF-alpha differentially regulate CD4 and Mac-1 expression in mouse microglia

The regulatory effects of the proinflammatory cytokines, interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha) were investigated on CD4 and Mac-1 expression in mouse microglial cultures. The identity of the microglia in cultures was confirmed by multiple indices including morphology, uptake of acetylated low-density lipoprotein and lectin RCA 120 staining. Microglia growing on a monolayer of astrocytes (astrocyte-supported microglia) were both CD4- and Mac-1 positive (out of 94.5 % Mac-1-positive cells, 85.3% were also CD4 positive). When astrocyte-supported microglia were replated directly onto culture dishes (plate-supported microglia), the percentage of CD4- and Mac-1-positive cells decreased to 12-29 and 20-25% respectively. The addition of IL-1alpha or TNF-alpha to plate-supported microglia led to an upregulation of Mac-1 expression in a time- and dose-dependent manner with different EC50s (0.5 ng/ml for IL-1alpha and 2 ng/ml for TNF-alpha) but exhibited similar time-to-peak responses (over 12 h). The addition of IL-1alpha, but not TNF-alpha, also led to an increase in CD4 expression on plate-supported microglia with a similar dose response and time course. IL-1alpha treatment gave rise to an increase in the level of CD4 mRNA as assessed by RT-PCR. The possibility that cell proliferation was responsible for the observed effects on microglia was excluded by an analysis of 3H-thymidine incorporation. Our results suggest that cultured mouse microglia express CD4 molecules which can be upregulated by IL-1alpha while Mac-1 can be upregulated by both IL-1alpha and TNF-alpha.

Comparison of lactate transport in astroglial cells and monocarboxylate transporter 1 (MCT 1) expressing Xenopus laevis oocytes. Expression of two different monocarboxylate transporters in astroglial cells and neurons

The transport of lactate is an essential part of the concept of metabolic coupling between neurons and glia. Lactate transport in primary cultures of astroglial cells was shown to be mediated by a single saturable transport system with a Km value for lactate of 7.7 mM and a Vmax value of 250 nmol/(min x mg of protein). Transport was inhibited by a variety of monocarboxylates and by compounds known to inhibit monocarboxylate transport in other cell types, such as alpha-cyano-4-hydroxycinnamate and p-chloromercurbenzenesulfonate. Using reverse transcriptase-polymerase chain reaction and Northern blotting, the presence of mRNA coding for the monocarboxylate transporter 1 (MCT1) was demonstrated in primary cultures of astroglial cells. In contrast, neuron-rich primary cultures were found to contain the mRNA coding for the monocarboxylate transporter 2 (MCT2). MCT1 was cloned and expressed in Xenopus laevis oocytes. Comparison of lactate transport in MCT1 expressing oocytes with lactate transport in glial cells revealed that MCT1 can account for all characteristics of lactate transport in glial cells. These data provide further molecular support for the existence of a lactate shuttle between astrocytes and neurons.

Glutamate uptake stimulates Na+,K+-ATPase activity in astrocytes via activation of a distinct subunit highly sensitive to ouabain

The excitatory amino acid glutamate was previously shown to stimulate aerobic glycolysis in astrocytes by a mechanism involving its uptake through an Na+-dependent transporter. Evidence had been provided that Na+,K+-ATPase might be involved in this process. We have now measured the activity of Na+,K+-ATPase in cultured astrocytes, using ouabain-sensitive 86Rb uptake as an index. L-Glutamate increases glial Na+,K+-ATPase activity in a concentration-dependent manner with an EC50 = 67 microM. Both L- and D-aspartate, but not D-glutamate, produce a similar response, an observation that is consistent with an uptake-related effect rather than a receptor-mediated one. Under basal conditions, concentration-dependent inhibition of Na+,K+-ATPase activity in astrocytes by ouabain indicates the presence of a single catalytic site with a low affinity for ouabain (K0.5 = 113 microM), compatible with the presence of an alpha1 isozyme. On stimulation with glutamate, however, most of the increased activity is inhibited by low concentrations of ouabain (K0.5 = 20 nM), thus revealing a high-affinity site akin to the alpha2 isozyme. These results suggest that astrocytes possess a glutamate-sensitive isoform of Na+,K+-ATPase that can be mobilized in response to increased neuronal activity.

Brain-derived neurotrophic factor stimulates phosphorylation of stathmin in cortical neurons

We have identified by two-dimensional polyacrylamide gel electrophoresis a protein known as stathmin which is phosphorylated in a time- and concentration-dependent manner in response to brain-derived neurotrophic factor (BDNF) in primary cultures of cortical neurons. We show that stathmin phosphorylation is preceded by the activation of mitogen-activated protein kinase (MAPK) isoforms p44 and p42. Moreover, the MAPK kinase inhibitor PD 098059, which inhibits MAPK activation, also markedly reduces BDNF-stimulated phosphorylation of stathmin, therefore suggesting that phosphorylation of stathmin is triggered by the activation of MAPK. Phosphorylation of stathmin is specific for BDNF since nerve growth factor does not stimulate MAPK and stathmin phosphorylation in cultured cortical neurons. Taken together, these results identify stathmin as a new target protein of BDNF, possibly involved in the development of cortical neurons.

High potency of the orally-active NMDA-receptor antagonist CGP 40 116 in inhibiting excitatory postsynaptic potentials of rat basolateral amygdala neurones in vitro

Conventional intracellular recordings were used to monitor postsynaptic potentials of basolateral amygdala neurones (BLA) in brain slices comprising the BLA, the entorhinal cortex (EC) and the hippocampus, in which the EC-BLA connections were preserved. Stimulation of the BLA with a bipolar electrode elicited complex postsynaptic potentials consisting of alpha-amino-3-hydroxy-5-methyl-isoxazoleproprionic acid (AMPA) receptor-mediated fast excitatory postsynaptic potentials (fast EPSPs), gamma-amino-butyric acid [GABA(A)] receptor-mediated fast inhibitory postsynaptic potentials (fast IPSPs) and GABAB receptor-mediated slow IPSPs. Bath application of 10 microM of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and of 10 microM of the GABA(A) receptor antagonist bicuculline methiodide (BMI) revealed a N-methyl-D-aspartate (NMDA) receptor-mediated slow EPSPs, which was occasionally followed by a GABAB receptor-mediated slow IPSPs. Under these conditions, the log concentration-response curve for D-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 40 116), a newly developed drug with proposed NMDA-receptor antagonist properties, was compared to that obtained with the 'classic' antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) in inhibiting the NMDA-mediated postsynaptic potentials. CGP 40 116 (IC50: 130 nM) was over 30 times more potent than D-AP5 (IC50: 4100 nM) in reducing NMDA-mediated slow EPSP. In conclusion, the present study indicates that CGP 40 116, a new orally-active NMDA antagonist, shows a very high potency on NMDA receptors in the amygdala and may therefore be a valuable tool for studying the behavioural effect of NMDA-receptor mediated transmission in this structure.

Regulation of energy metabolism by neurotransmitters in astrocytes in primary culture and in an immortalized cell line

Evidence suggests that astrocytes might play an important role in cerebral energy metabolism. A recently developed cell line, called DI TNC1, displays several characteristic features of astrocytes. Thus, we have investigated in these cells a number of parameters related to energy metabolism. First, glycogen, the major energy reserve in the brain, is present in these cells and its levels are influenced by the glucose content of the growth medium and the presence of serum. Second, several neurotransmitters including noradrenaline and vasoactive intestinal peptide (VIP) induce a glycogenolytic response. Their effect on glycogen is paralleled by a similar effect on the formation of cyclic AMP, which is presumably the second messenger involved. Third, noradrenaline stimulates glucose utilization (as reflected by 2-deoxyglucose uptake) in DI TNC1 cells, an effect which is mimicked by the second messenger arachidonate. Interestingly, two actions of neurotransmitters, which are well characterized in primary astrocytes, are absent in DI TNC1 cells. These are the noradrenaline- and VIP-induced resynthesis of glycogen and the glutamate-stimulated glycolysis. In summary, the observations reported here lend further support to the concept that astrocytes are important for the control of brain energy metabolism. In addition, DI TNC1 cells might represent an interesting preparation to help decipher some of the astrocytic functions related to energy metabolism.

Noradrenaline modulates glutamate-mediated neurotransmission in the rat basolateral amygdala in vitro

The entorhinal cortex and the amygdala are interconnected structures of the limbic system in which paroxysmal activity occurs during temporal lobe epilepsy. Conflicting evidence shows that noradrenaline (i) inhibits the spreading to other parts of the limbic system of paroxysmal activity generated in the amygdala or the entorhinal cortex, but also (ii) increases glutamatergic transmission in the basolateral amygdala. Given our previous work on the inhibitory effect of noradrenaline on entorhinal cortex neurons, we developed an in vitro slice preparation to study the synaptic transmission in the basolateral amygdala and its modulation by noradrenaline. Noradrenaline reduced the fast excitatory postsynaptic potential (EPSP) by approximately 40% at 100 microM and the slow EPSP by approximately 50% at 50 microM. A similar effect was obtained with the alpha2-agonist UK 14304 at 100 and 50 microM respectively. In contrast, the beta-agonist isoproterenol increased the fast EPSP by approximately 40% at 100 microM and the slow EPSP by approximately 20% at 50 microM. Accordingly, the effect of noradrenaline on the EPSPs was blocked by the alpha2-antagonist yohimbine (10 microM) but not by the alpha1-antagonist prazosine (10 microM) and the beta-antagonist propranolol (10 microM). Noradrenaline (50-100 microM) was ineffective on most (14/16) of the isolated inhibitory postsynaptic potentials (IPSPs). These experiments provide evidence that noradrenaline inhibits the excitatory synaptic response of basolateral amygdala neurons. A pharmacological analysis revealed that the noradrenergic modulation of the excitatory transmission in the basolateral amygdala can be dissected into a predominant alpha2-adrenoreceptor-mediated inhibition and a beta-adrenoreceptor-mediated excitation.

beta-Adrenergic stimulation promotes homocysteic acid release from astrocyte cultures: evidence for a role of astrocytes in the modulation of synaptic transmission

The sulfur-containing amino acid homocysteic acid (HCA) is present in and released from nervous tissue, exerting excitatory effects on neurons by predominantly activating NMDA receptors. It is interesting that HCA appears to be exclusively localized in glial cells, not in neurons. This profile of glial localization and excitatory action on neurons has led to the hypothesis that HCA could participate in intercellular communication in the brain as a "gliotransmitter." To test this hypothesis further, we searched for specific, receptor-mediated stimuli that could induce release of HCA from cultured astrocytes. For this reason we tested the effect of noradrenaline and vasoactive intestinal peptide, two transmitters known to interact with specific receptors on astrocytes, on the release of HCA from these cells. Noradrenaline and the beta-adrenergic agonist isoproterenol induced an efflux of HCA from astrocyte cultures. Further stressing the beta-adrenergic mediation of this effect is the blockade by atenolol of the HCA release evoked by isoproterenol. The stimulation of HCA release from astrocytes was not observed with the alpha-noradrenergic agonist methoxamine and with vasoactive intestinal peptide. These results taken together further strengthen the role of HCA as a gliotransmitter. Its efflux from glia could be controlled by noradrenaline, activating beta-adrenergic receptors on astrocytes. The present study provides the first evidence for an influence of beta-adrenergic receptor activation on the release of an excitatory amino acid from astrocytes and further supports the notion that glial-neuronal interactions play a role in synaptic transmission.

AMPA/kainate receptor activation blocks K+ currents via internal Na+ increase in mouse cultured stellate astrocytes

Cultured mouse cortical astrocytes of the stellate type were studied by using the patch-clamp technique in whole-cell configuration. The astrocytes express at least two types of outwardly rectifying K+ channels which mediate a transient and a sustained current. Activation of AMPA receptors by kainate leads to a substantial blockade of both types of K+ currents. The blockade is absent when Na+ is withdrawn from the external medium, suggesting that it is caused by constant Na+ influx through AMPA receptors. The presence of high Na+ solutions in the pipette induces a blockade of both K+ currents which is very similar to the blockade induced by kainate, supporting thus the view that the mechanism of the blockade of K+ channels by kainate involves Na+ increases in the submembrane area. The blockade occurs between 20 and 40 mM [Na+]i, which is within the physiological range of [Na+]i in astrocytes. The data may suggest that the blockade of K+ channels by high [Na+]i conditions could provide a mechanism to prevent K+ leakage from the astrocytes into the extracellular space during periods of intense neuronal activity.