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.

Under particular circumstances like lactation and fasting, the blood-borne monocarboxylates acetoacetate, ␤-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.Glucose is the major, if not exclusive, energy substrate for the brain (1). Under certain situations, other substrates can contribute significantly to brain energy demands. Thus, immediately after birth, lactate present in high amounts in the blood following delivery provides an important source of energy for the brain in the presuckling period (2, 3). In addition, acetoacetate and ␤-hydroxybutyrate, two ketone bodies formed by the hepatic oxidation of fat contained in maternal milk, are also significant energy substrates for the brain during the preweaning period (4, 5). These energy substrates however do not cross the blood-brain barrier easily, and require a transport system to reach the brain parenchyma. Such a transport system that has been demonstrated by uptake studies with tracers across the blood-brain barrier during lactation is shared by lactate, pyruvate, and the ketone bodies (6, 7).Recently, two transporters for monocarboxylates have been cloned and their distribution in various tissues, organs, and cell types has been described (8-10). These reports made limited mention about their presence in the central nervous system, despite evidence from functional studies for the presence of a lactate transport system in different brain cell types or in tissue slices (11)(12)(13)(14). More recently, two reports have appeared, describing the presence of monocarboxylate transporters (MCT) mRNAs by Northern blot analysis (15) as well as the MCT1 protein (16) in the central nervous system. In this article...

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 K m value for lactate of 7.7 mM and a V max value of 250 nmol/(min ؋ 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 ␣-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.The transport of lactate is an essential part of the concept of metabolic coupling between neurons and glia (1, 2). It has been demonstrated that glutamate at concentrations around 200 M strongly increases the rates of glycolysis and lactate release in cultured astroglial cells (3). It has further been shown that neurons are able to take up lactate and to use this compound as an energy substrate (1, 4, 5). In the mammalian retina, direct evidence has been provided for a transfer of lactate between Mü ller glial cells and photoreceptors (6). Besides its role as an exchangeable metabolic fuel, lactate also interferes with pH and volume regulation in neural cells (7).There is a considerable debate over the types of transporters involved in the uptake and release of lactate by astroglial cells.Nedergaard and Goldman (8) characterized lactate transport in cultured astrocytes and determined a low K m value of 0.4 mM. The carrier-mediated transport could not be inhibited by ␣-cyano-3-hydroxycinnamate or pCMBS, 1 both being typical inhibitors of monocarboxylate transport in other cell types. The transport process was reversible and accompanied by a cotransport of protons. Diffusion of protonated lactate could not be detected. In contrast to these results, Tildon et al. (9) identified two carrier-mediated processes for lactate uptake, characterized by K m values of 0.5 mM and 11 mM, respectively. The maximum velocity of the low-affinity transporter was 170 nmol/(min ϫ mg of protein), whereas only 10% of this value was found for the high affinity component. Transport was only partially inhibited by ␣-cyano-4-hydroxycinnamate and mersalyl. Acidic pH strongly increased transport activity, a finding consistent with a lactate/proton cotransport mechanism. Dringen et al. (10) detected solely non-saturable lactate transport ...

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In recent years a vast array of experimental evidence has indicated the presence of functional receptors for neurotransmitters on nonneuronal cells, in particular astrocytes. The two neurotransmitters vasoactive intestinal peptide (VIP) and noradrenaline (NA) exert profound, receptor-mediated, metabolic actions on astrocytes. Thus both neurotransmitters stimulate glycogenolysis in primary astrocyte cultures, with EC₅₀s of 3 and 20 nM respectively. Astrocytes display basal glucose utilization rates ranging between 3 and 9 nmol/mg prot/min, a value that is remarkably close to glucose utilization of cerebral cortical grey matter as determined by the 2-deoxyglucose autoradiographic technique. NA markedly enhances glucose uptake and phosphorylation by astrocytes, with an EC₅₀ of 1 µM. The metabolic substrate that is released by astrocytes is predominantly lactate and not glucose. Since lactate can support neuronal activity and synaptic function in vitro, the possibility should be considered that glucose uptake by the brain parenchyma occurs predominantly into astrocytes which subsequently release lactate for the use of neurons.

The neuron-specific K-Cl cotransporter, KCC2, is highly expressed in the vicinity of excitatory synapses in pyramidal neurons, and recent in vitro data suggest that this protein plays a role in the development of dendritic spines. The in vivo relevance of these observations is, however, unknown. Using in utero electroporation combined with post hoc iontophoretic injection of Lucifer Yellow, we show that premature expression of KCC2 induces a highly significant and permanent increase in dendritic spine density of layer 2/3 pyramidal neurons in the somatosensory cortex. Whole-cell recordings revealed that this increased spine density is correlated with an enhanced spontaneous excitatory activity in KCC2-transfected neurons. Precocious expression of the N-terminal deleted form of KCC2, which lacks the chloride transporter function, also increased spine density. In contrast, no effect on spine density was observed following in utero electroporation of a point mutant of KCC2 (KCC2-C568A) where both the cotransporter function and the interaction with the cytoskeleton are disrupted. Transfection of the C-terminal domain of KCC2, a region involved in the interaction with the dendritic cytoskeleton, also increased spine density. Collectively, these results demonstrate a role for KCC2 in excitatory synaptogenesis in vivo through a mechanism that is independent of its ion transport function.

Rationale The pharmacological actions of most antidepressants are ascribed to the modulation of serotonergic and/or noradrenergic transmission in the brain. During therapeutic treatment for major depression, fluoxetine, one of the most commonly prescribed selective serotonin reuptake inhibitor (SSRI) antidepressants, accumulates in the brain, suggesting that fluoxetine may interact with additional targets. In this context, there is increasing evidence that astrocytes are involved in the pathophysiology of major depression. Objectives The aim of this study was to examine the effects of fluoxetine on the expression of neurotrophic/growth factors that have antidepressant properties and on glucose metabolism in cultured cortical astrocytes. Results Treatment of astrocytes with fluoxetine and paroxetine, another SSRI antidepressant, upregulated brainderived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), and VGF mRNA expression. In contrast, the tricyclic antidepressants desipramine and imipramine did not affect the expression of these neurotrophic/growth factors. Analysis of the effects of fluoxetine on glucose metabolism revealed that fluoxetine reduces glycogen levels and increases glucose utilization and lactate release by astrocytes. Similar data were obtained with paroxetine, whereas imipramine and desipramine did not regulate glucose metabolism in this glial cell population. Our results also indicate that the effects of fluoxetine and paroxetine on glucose utilization, lactate release, and expression of BDNF, VEGF, and VGF are not mediated by serotonin-dependent mechanisms. Conclusions These data suggest that, by increasing the expression of specific astrocyte-derived neurotrophic factors and lactate release from astrocytes, fluoxetine may contribute to normalize the trophic and metabolic support to neurons in major depression.

Brain-derived neurotrophic factor (BDNF) promotes the biochemical and morphological differentiation of selective populations of neurons during development. In this study we examined the energy requirements associated with the effects of BDNF on neuronal differentiation. Because glucose is the preferred energy substrate in the brain, the effect of BDNF on glucose utilization was investigated in developing cortical neurons via biochemical and imaging studies. Results revealed that BDNF increases glucose utilization and the expression of the neuronal glucose transporter GLUT3. Stimulation of glucose utilization by BDNF was shown to result from the activation of Na+/K+-ATPase via an increase in Na+ influx that is mediated, at least in part, by the stimulation of Na+-dependent amino acid transport. The increased Na+-dependent amino acid uptake by BDNF is followed by an enhancement of overall protein synthesis associated with the differentiation of cortical neurons. Together, these data demonstrate the ability of BDNF to stimulate glucose utilization in response to an enhanced energy demand resulting from increases in amino acid uptake and protein synthesis associated with the promotion of neuronal differentiation by BDNF.