Involvement of lactate transport in two object recognition tasks that require either the hippocampus or striatum

Lactate Protects Microglia and Neurons from Oxygen-Glucose Deprivation/Reoxygenation

Lactate has received attention as a potential therapeutic intervention for brain diseases, particularly those including energy deficit, exacerbated inflammation, and disrupted redox status, such as cerebral ischemia. However, lactate roles in metabolic or signaling pathways in neural cells remain elusive in the hypoxic and ischemic contexts. Here, we tested the effects of lactate on the survival of a microglial (BV-2) and a neuronal (SH-SY5Y) cell lines during oxygen and glucose deprivation (OGD) or OGD followed by reoxygenation (OGD/R). Lactate signaling was studied by using 3,5-DHBA, an exogenous agonist of lactate receptor GPR81. Inhibition of lactate dehydrogenase (LDH) or monocarboxylate transporters (MCT), using oxamate or 4-CIN, respectively, was performed to evaluate the impact of lactate metabolization and transport on cell viability. The OGD lasted 6 h and the reoxygenation lasted 24 h following OGD (OGD/R). Cell viability, extracellular lactate concentrations, microglial intracellular pH and TNF-ɑ release, and neurite elongation were evaluated. Lactate or 3,5-DHBA treatment during OGD increased microglial survival during reoxygenation. Inhibition of lactate metabolism and transport impaired microglial and neuronal viability. OGD led to intracellular acidification in BV-2 cells, and reoxygenation increased the release of TNF-ɑ, which was reverted by lactate and 3,5-DHBA treatment. Our results suggest that lactate plays a dual role in OGD, acting as a metabolic and a signaling molecule in BV-2 and SH-SY5Y cells. Lactate metabolism and transport are vital for cell survival during OGD. Moreover, lactate treatment and GPR81 activation during OGD promote long-term adaptations that potentially protect cells against secondary cell death during reoxygenation.

History and Function of the Lactate Receptor GPR81/HCAR1 in the Brain: A Putative Therapeutic Target for the Treatment of Cerebral Ischemia

GPR81 is a G-protein coupled receptor (GPCR) discovered in 2001, but deorphanized only 7 years later, when its affinity for lactate as an endogenous ligand was demonstrated. More recently, GPR81 expression and distribution in the brain were also confirmed and the function of lactate as a volume transmitter has been suggested since then. These findings shed light on a new function of lactate acting as a signaling molecule in the central nervous system, in addition to its well-known role as a metabolic fuel for neurons. GPR81 seems to act as a metabolic sensor, coupling energy metabolism, synaptic activity, and blood flow. Activation of this receptor leads to Gi-mediated downregulation of adenylyl cyclase and subsequent reduction in cAMP levels, regulating several downstream pathways. Recent studies have also suggested the potential role of lactate as a neuroprotective agent, mainly under brain ischemic conditions. This effect is usually attributed to the metabolic role of lactate, but the underlying mechanisms need further investigation and could be related to lactate signaling via GPR81. The activation of GPR81 showed promising results for neuroprotection: it modulates many processes involved in the pathophysiology of ischemia. In this review, we summarize the history of GPR81, starting with its deorphanization; then, we discuss GPR81 expression and distribution, signaling transduction cascades, and neuroprotective roles. Lastly, we propose GPR81 as a potential target for the treatment of cerebral ischemia.

Aging and memory are altered by genetically manipulating lactate dehydrogenase in the neurons or glia of flies

The astrocyte-neuron lactate shuttle hypothesis posits that glial-generated lactate is transported to neurons to fuel metabolic processes required for long-term memory. Although studies in vertebrates have revealed that lactate shuttling is important for cognitive function, it is uncertain if this form of metabolic coupling is conserved in invertebrates or is influenced by age. Lactate dehydrogenase (Ldh) is a rate limiting enzyme that interconverts lactate and pyruvate. Here we genetically manipulated expression of Drosophila melanogaster lactate dehydrogenase (dLdh) in neurons or glia to assess the impact of altered lactate metabolism on invertebrate aging and long-term courtship memory at different ages. We also assessed survival, negative geotaxis, brain neutral lipids (the core component of lipid droplets) and brain metabolites. Both upregulation and downregulation of dLdh in neurons resulted in decreased survival and memory impairment with age. Glial downregulation of dLdh expression caused age-related memory impairment without altering survival, while upregulated glial dLdh expression lowered survival without disrupting memory. Both neuronal and glial dLdh upregulation increased neutral lipid accumulation. We provide evidence that altered lactate metabolism with age affects the tricarboxylic acid (TCA) cycle, 2-hydroxyglutarate (2HG), and neutral lipid accumulation. Collectively, our findings indicate that the direct alteration of lactate metabolism in either glia or neurons affects memory and survival but only in an age-dependent manner.

Light-intensity exercise improves memory dysfunction with the restoration of hippocampal MCT2 and miRNAs in type 2 diabetic mice

Cognitive decline associated with type 2 diabetes mellitus (T2DM) is a risk factor to impair human health. Although light-intensity exercise prevents hippocampal memory dysfunction in pre-symptomatic T2DM animals by altering hippocampal lactate transport and neurotrophic factors, the effects of light-intensity exercise in an advanced stage of T2DM animals remain unclear. Here, ob/ob mice, an animal model of T2DM, were subjected to light-intensity exercise (5.0 m/min) for 30 min/day, five days/week for four weeks. The effects of light-intensity exercise on hippocampal complications, mRNA expressions of monocarboxylate transporter (MCT), and miRNA levels were assessed. The light-intensity exercise improved hippocampal memory retention in ob/ob mice. Downregulated hippocampal Mct2 mRNA levels in T2DM were improved with light-intensity exercise. Hippocampal mRNA levels of Mct1 and Mct4 were unchanged within groups. Based on miRNA sequencing, sedentary ob/ob mice exhibited that 71 miRNAs were upregulated, and 77 miRNAs were downregulated in the hippocampus. In addition, the exercise significantly increased 24 miRNAs and decreased 4 miRNAs in the T2DM hippocampus. The exercise reversed T2DM-induced alterations of hippocampal 9 miRNAs, including miR-200a-3p. Our findings imply that miR-200a-3p/Mct2 in the hippocampus would be a possible clinical target for treating T2DM-induced memory dysfunction.

Low-Carbohydrate and High-Protein Diet Suppresses Working Memory Function in Healthy Mice

Low-carbohydrate and high-protein (LC-HP) diets are acceptable for improving physiological and metabolic parameters. However, the effects of LC-HP diets on the brain are unclear, which depend on glycometabolism for neuronal activity. Since astrocyte-neuron lactate shuttle (ANLS) is an essential pathway for maintaining brain functions, we investigated the changes in hippocampal memory function. In addition, the alteration of lactate transporter constituting ANLS and ANLS-related neurotrophic factors by feeding LC-HP diets was evaluated in healthy mice. C57BL/6 mice were divided into two groups: a group feeding LC-HP diet (24.6% carbohydrate, 57.6% protein, and 17.8% fat as percentages of calories) and a group feeding control diet (58.6% carbohydrate, 24.2% protein, and 17.2% fat as percentages of calories). Here, we found that 4 wk of LC-HP diet feeding suppressed memory function in mice evaluated by Y-maze. Hippocampal mRNA levels of lactate transporters, such as Mct1, Mct4, and Mct2, were unchanged with feeding LC-HP diets; however, LC-HP diets significantly decreased Dcx and Igf-1 receptor mRNA levels in the hippocampus. Bdnf and its related signaling in mice hippocampus exhibited no change by LC-HP diets. Although there was non-influence in the lactate-transport system, LC-HP diets would suppress hippocampal working memory with dysregulation of neuroplasticity. The current data propose the importance of food choices for maintaining hippocampal health.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Systematic Investigations on the Metabolic and Transcriptomic Regulation of Lactate in the Human Colon Epithelial Cells

Lactate, primarily produced by the gut microbiota, performs as a necessary “information transmission carrier” between the gut and the microbiota. To investigate the role of lactate in the gut epithelium cell–microbiota interactions as a metabolic signal, we performed a combinatory, global, and unbiased analysis of metabolomic and transcriptional profiling in human colon epithelial cells (Caco-2), using a lactate treatment at the physiological concentration (8 mM). The data demonstrated that most of the genes in oxidative phosphorylation were significantly downregulated in the Caco-2 cells due to lactate treatment. Consistently, the levels of fumarate, adenosine triphosphate (ATP), and creatine significantly decreased, and these are the metabolic markers of OXPHOS inhibition by mitochondria dysfunction. The one-carbon metabolism was affected and the polyol pathway was activated at the levels of gene expression and metabolic alternation. In addition, lactate significantly upregulated the expressions of genes related to self-protection against apoptosis. In conclusion, lactate participates in gut–gut microbiota communications by remodeling the metabolomic and transcriptional signatures, especially for the regulation of mitochondrial function. This work contributes comprehensive information to disclose the molecular mechanisms of lactate-mediated functions in human colon epithelial cells that can help us understand how the microbiota communicates with the intestines through the signaling molecule, lactate.

Hippocampus-sensitive and striatum-sensitive learning one month after morphine or cocaine exposure in male rats

These experiments examined whether morphine and cocaine alter the balance between hippocampal and striatal memory systems measured long after drug exposure. Male rats received injections of morphine (5 mg/kg), cocaine (20 mg/kg), or saline for five consecutive days. One month later, rats were trained to find food on a hippocampus-sensitive place task or a striatum-sensitive response task. Relative to saline controls, morphine-treated rats exhibited impaired place learning but enhanced response learning; prior cocaine exposure did not significantly alter learning on either task. Another set of rats was trained on a dual-solution T-maze that can be solved with either place or response strategies. While a majority (67%) of control rats used place solutions, morphine treatment one month prior resulted in the exclusive use of response solutions (100%). Prior cocaine treatment did not significantly alter strategy selection. Molecular markers related to learning and drug abuse were measured in the hippocampus and striatum one month after drug exposure in behaviorally untested rats. Protein levels of glial-fibrillary acidic protein (GFAP), an intermediate filament specific to astrocytes, increased significantly in the hippocampus after morphine exposure, but not after cocaine exposure. Exposure to morphine or cocaine did not significantly change levels of brain-derived neurotrophic factor (BDNF) or a downstream target of BDNF signaling, glycogen synthase kinase 3β (GSK3β), in the hippocampus or striatum. Thus, exposure to morphine resulted in a long-lasting shift from hippocampal toward striatal dominance during learning, an effect that may be associated with lasting alterations in hippocampal astrocytes. Cocaine produced changes in the same direction, suggesting that use of a higher dose or longer duration of exposure might produce effects comparable to those seen with morphine.

Long-term light and moderate exercise intervention similarly prevent both hippocampal and glycemic dysfunction in presymptomatic type 2 diabetic rats

A prediabetic population has an increased risk of cognitive decline and type 2 diabetes mellitus (T2DM). This study investigated whether the progression of memory dysfunction and dysregulated brain glycogen metabolism is prevented with 4 mo of exercise intervention from the presymptomatic stage in a T2DM rat model. Memory function and biochemical and molecular profiles were assessed in the presymptomatic stage of Otsuka-Long-Evans-Tokushima fatty (OLETF) rats, a T2DM model, with Long-Evans Tokushima (LETO) rats as genetic control. These rats were subjected to light- or moderate-intensity treadmill running for 4 mo with repetition of the same experiments. Significant hippocampal-dependent memory dysfunction was observed in the presymptomatic stage of OLETF rats, accompanied by downregulated levels of hippocampal monocarboxylate transporter 2 (MCT2), a neuronal lactate-transporter, without alteration in hippocampal glycogen levels. Four months of light or moderate exercise from the presymptomatic stage of T2DM normalized glycemic parameters and hippocampal molecular normalization through MCT2, glycogen, and brain-derived neurotrophic factor (BDNF) levels with the improvement of memory dysfunction in OLETF rats. A 4-mo exercise regimen from the presymptomatic stage of T2DM at a light and moderate intensities contributed to the prevention of the development of T2DM and the progression of cognitive decline with hippocampal lactate-transport and BDNF improvement.NEW & NOTEWORTHY Type 2 diabetes mellitus is an independent risk factor for hippocampal memory dysfunction, which would progress since the prediabetic stage. We found that 4 mo of exercise both at the light and moderate intensity prevented the progression of memory dysfunction with an improvement of hippocampal MCT2 expression in presymptomatic diabetes, implying that light intensity exercise could be a therapeutic approach, and the alteration of hippocampal MCT2 would be a therapeutic target of memory dysfunction from presymptomatic diabetes.

Microglial metabolic flexibility: emerging roles for lactate

Microglia, the resident macrophages of the central nervous system (CNS), play important functions in the healthy and diseased brain. In the emerging field of immunometabolism, progress has been made in understanding how cellular metabolism can orchestrate the key responses of tissue macrophages, such as phagocytosis and inflammation. However, very little is known about the metabolic control of microglia. Lactate, now recognized as a crucial metabolite and a central substrate in metabolic flexibility, is emerging not only as a novel bioenergetic fuel for microglial metabolism but also as a potential modulator of cellular function. Parallels with macrophages will help in understanding how microglial lactate metabolism is implicated in brain physiology and pathology, and how it could be targeted for therapeutic purposes.

Lactate transporters in the rat barrel cortex sustain whisker-dependent BOLD fMRI signal and behavioral performance

Lactate is an efficient neuronal energy source, even in presence of glucose. However, the importance of lactate shuttling between astrocytes and neurons for brain activation and function remains to be established. For this purpose, metabolic and hemodynamic responses to sensory stimulation have been measured by functional magnetic resonance spectroscopy and blood oxygen level-dependent (BOLD) fMRI after down-regulation of either neuronal MCT2 or astroglial MCT4 in the rat barrel cortex. Results show that the lactate rise in the barrel cortex upon whisker stimulation is abolished when either transporter is down-regulated. Under the same paradigm, the BOLD response is prevented in all MCT2 down-regulated rats, while about half of the MCT4 down-regulated rats exhibited a loss of the BOLD response. Interestingly, MCT4 down-regulated animals showing no BOLD response were rescued by peripheral lactate infusion, while this treatment had no effect on MCT2 down-regulated rats. When animals were tested in a novel object recognition task, MCT2 down-regulated animals were impaired in the textured but not in the visual version of the task. For MCT4 down-regulated animals, while all animal succeeded in the visual task, half of them exhibited a deficit in the textured task, a similar segregation into two groups as observed for BOLD experiments. Our data demonstrate that lactate shuttling between astrocytes and neurons is essential to give rise to both neurometabolic and neurovascular couplings, which form the basis for the detection of brain activation by functional brain imaging techniques. Moreover, our results establish that this metabolic cooperation is required to sustain behavioral performance based on cortical activation.

Time-dependent changes in hippocampal and striatal glycogen long after maze training in male rats

Long-lasting biological changes reflecting past experience have been studied in and typically attributed to neurons in the brain. Astrocytes, which are also present in large number in the brain, have recently been found to contribute critically to learning and memory processing. In the brain, glycogen is primarily found in astrocytes and is metabolized to lactate, which can be released from astrocytes. Here we report that astrocytes themselves have intrinsic neurochemical plasticity that alters the availability and provision of metabolic substrates long after an experience. Rats were trained to find food on one of two versions of a 4-arm maze: a hippocampus-sensitive place task and a striatum-sensitive response task. Remarkably, hippocampal glycogen content increased while striatal levels decreased during the 30 days after rats were trained to find food in the place version, but not the response version, of the maze tasks. A long-term consequence of the durable changes in glycogen stores was seen in task-by-site differences in extracellular lactate responses activated by testing on a working memory task administered 30 days after initial training, the time when differences in glycogen content were most robust. These results suggest that astrocytic plasticity initiated by a single experience may augment future availability of energy reserves, perhaps priming brain areas to process learning of subsequent experiences more effectively.

l-Lactate: Food for Thoughts, Memory and Behavior

More and more evidence shows how brain energy metabolism is the linkage between physiological and morphological synaptic plasticity and memory consolidation. Different types of memory are associated with differential inputs, each with specific inputs that are upstream diverse molecular cascades depending on the receptor activity. No matter how heterogeneous the response is, energy availability represents the lowest common denominator since all these mechanisms are energy consuming and the brain networks adapt their performance accordingly. Astrocytes exert a primary role in this sense by acting as an energy buffer; glycogen granules, a mechanism to store glucose, are redistributed at glance and conveyed to neurons via the Astrocyte–Neuron Lactate Shuttle (ANLS). Here, we review how different types of memory relate to the mechanisms of energy delivery in the brain.

Ultrastructural Evidence for a Role of Astrocytes and Glycogen-Derived Lactate in Learning-Dependent Synaptic Stabilization

Long-term memory formation (LTM) is a process accompanied by energy-demanding structural changes at synapses and increased spine density. Concomitant increases in both spine volume and postsynaptic density (PSD) surface area have been suggested but never quantified in vivo by clear-cut experimental evidence. Using novel object recognition in mice as a learning task followed by 3D electron microscopy analysis, we demonstrate that LTM induced all aforementioned synaptic changes, together with an increase in the size of astrocytic glycogen granules, which are a source of lactate for neurons. The selective inhibition of glycogen metabolism in astrocytes impaired learning, affecting all the related synaptic changes. Intrahippocampal administration of l-lactate rescued the behavioral phenotype, along with spine density within 24 hours. Spine dynamics in hippocampal organotypic slices undergoing theta burst-induced long-term potentiation was similarly affected by inhibition of glycogen metabolism and rescued by l-lactate. These results suggest that learning primes astrocytic energy stores and signaling to sustain synaptic plasticity via l-lactate.

Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats

Glucose potently enhances cognitive functions whether given systemically or directly to the brain. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. Because there were no task-related differences in glucose responses, the glucose results were pooled across tasks to form combined trained groups. During the first 1-3 min of training for food reward, glucose levels in extracellular fluid (ECF) declined significantly in the hippocampus and striatum; the declines were not seen in untrained, rewarded rats. When trained for water reward, similar decreases were observed in both brain areas, but these findings were less consistent than those seen with food rewards. After the initial declines in ECF glucose levels, glucose increased in most groups, approaching asymptotic levels ∼15-30 min into training. Compared to untrained food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. The increased brain uptake of glucose may provide additional neuronal metabolic substrate for metabolism or provide astrocytic substrate for production of glycogen and lactate.

Spontaneous object-location memory based on environmental geometry is impaired by both hippocampal and dorsolateral striatal lesions

We examined the role of the hippocampus and the dorsolateral striatum in the representation of environmental geometry using a spontaneous object recognition procedure. Rats were placed in a kite-shaped arena and allowed to explore two distinctive objects in each of the right-angled corners. In a different room, rats were then placed into a rectangular arena with two identical copies of one of the two objects from the exploration phase, one in each of the two adjacent right-angled corners that were separated by a long wall. Time spent exploring these two objects was recorded as a measure of recognition memory. Since both objects were in different locations with respect to the room (different between exploration and test phases) and the global geometry (also different between exploration and test phases), differential exploration of the objects must be a result of initial habituation to the object relative to its local geometric context. The results indicated an impairment in processing the local geometric features of the environment for both hippocampus and dorsolateral striatum lesioned rats compared with sham-operated controls, though a control experiment showed these rats were unimpaired in a standard object recognition task. The dorsolateral striatum has previously been implicated in egocentric route-learning, but the results indicate an unexpected role for the dorsolateral striatum in processing the spatial layout of the environment. The results provide the first evidence that lesions to the hippocampus and dorsolateral striatum impair spontaneous encoding of local environmental geometric features.

Acrobatic exercise recovers object recognition memory impairment in hypoxic-ischemic rats

Neonatal hypoxia-ischemia (HI) can lead to cognitive impairments and motor dysfunction. Acrobatic exercises (AE) were proposing as therapeutic option to manage HI motor deficits, however, the cognitive effects after this treatment are still poorly understood. Therefore, we evaluated the effects of AE protocol on memory impairments and brain plasticity markers after Rice-Vannucci HI rodent model. Wistar rats on the 7th postnatal day (PND) were submitted to HI model and after weaning (PND22) were trained for 5 weeks with AE protocol, then subsequently submitted to cognitive tests. Our results showed recovery in novel object recognition (NOR) memory, but not, spatial Morris Water Maze (WM) memory after AE treatment in HI rats. BDNF and synaptophysin neuroplasticity markers indicate plastic alterations in the hippocampus and striatum, with maintenance of synaptophysin despite the reduction of total volume tissue, besides, hippocampal HI-induced ipsilateral BDNF increased, and striatum contralateral BDNF decreased were noted. Nevertheless, the exercise promoted functional recovery and seems to be a promising strategy for HI treatment, however, future studies identifying neuroplastic pathway for this improvement are needed.

Neuronal and astroglial monocarboxylate transporters play key but distinct roles in hippocampus-dependent learning and memory formation

Brain lactate formation, intercellular exchange and utilization has been implicated in memory formation. However, the individual role of either neuronal or astroglial monocarboxylate transporters for the acquisition and consolidation of information remains incomplete. Using novel transgenic mice and a viral vector approach to decrease the expression of each transporter in a cell-specific manner within the dorsal hippocampus, we show that both neuronal MCT2 and astroglial MCT4 are required for spatial information acquisition and retention (at 24 h post-training) in distinct hippocampus-dependent tasks. Intracerebral infusion of lactate rescued spatial learning in mice with reduced levels of astroglial MCT4 but not of neuronal MCT2, suggesting that lactate transfer from astrocytes and utilization in neurons contribute to hippocampal-dependent learning. In contrast, only neuronal MCT2 was shown to be required for long-term (7 days post training) memory formation. Interestingly, reduced MCT2 expression levels in mature neurons result in a heterologous effect as it blunts hippocampal neurogenesis associated with memory consolidation. These results suggest important but distinct contributions of both neuronal MCT2 and astroglial MCT4 in learning and memory processes, going beyond a simple passive role as alternative energy substrate suppliers or in waste product disposal.

Aging is not equal across memory systems

The present experiments compared the effects of aging on learning several hippocampus- and striatum-sensitive tasks in young (3-4 month) and old (24-28 month) male Fischer-344 rats. Across three sets of tasks, aging was accompanied not only by deficits on hippocampal tasks but also by maintained or even enhanced abilities on striatal tasks. On two novel object recognition tasks, rats showed impaired performance on a hippocampal object location task but enhanced performance on a striatal object replacement task. On a dual solution task, young rats predominately used hippocampal solutions and old rats used striatal solutions. In addition, on two maze tasks optimally solved using either hippocampus-sensitive place or striatum-sensitive response strategies, relative to young rats, old rats had impaired learning on the place version but equivalent learning on the response version. Because glucose treatments can reverse deficits in learning and memory across many tasks and contexts, levels of available glucose in the brain may have particular importance in cognitive aging observed across tasks and memory systems. During place learning, training-related rises in extracellular glucose levels were attenuated in the hippocampus of old rats compared to young rats. In contrast, glucose levels in the striatum increased comparably in young and old rats trained on either the place or response task. These extracellular brain glucose responses to training paralleled the impairment in hippocampus-sensitive learning and the sparing of striatum-sensitive learning seen as rats age, suggesting a link between age-related changes in learning and metabolic substrate availability in these brain regions.