Activation of brain lactate receptor GPR81 aggravates exercise-induced central fatigue

Brain energy metabolism: A roadmap for future research

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+, as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Lactate as a supplemental fuel for synaptic transmission and neuronal network oscillations: Potentials and limitations

Lactate shuttled from the blood circulation, astrocytes, oligodendrocytes or even activated microglia (resident macrophages) to neurons has been hypothesized to represent a major source of pyruvate compared to what is normally produced endogenously by neuronal glucose metabolism. However, the role of lactate oxidation in fueling neuronal signaling associated with complex cortex function, such as perception, motor activity, and memory formation, is widely unclear. This issue has been experimentally addressed using electrophysiology in hippocampal slice preparations (ex vivo) that permit the induction of different neural network activation states by electrical stimulation, optogenetic tools or receptor ligand application. Collectively, these studies suggest that lactate in the absence of glucose (lactate only) impairs gamma (30-70 Hz) and theta-gamma oscillations, which feature high energy demand revealed by the cerebral metabolic rate of oxygen (CMRO2, set to 100%). The impairment comprises oscillation attenuation or moderate neural bursts (excitation-inhibition imbalance). The bursting is suppressed by elevating the glucose fraction in energy substrate supply. By contrast, lactate can retain certain electric stimulus-induced neural population responses and intermittent sharp wave-ripple activity that features lower energy expenditure (CMRO2 of about 65%). Lactate utilization increases the oxygen consumption by about 9% during sharp wave-ripples reflecting enhanced adenosine-5'-triphosphate (ATP) synthesis by oxidative phosphorylation in mitochondria. Moreover, lactate attenuates neurotransmission in glutamatergic pyramidal cells and fast-spiking, γ-aminobutyric acid (GABA)ergic interneurons by reducing neurotransmitter release from presynaptic terminals. By contrast, the generation and propagation of action potentials in the axon is regular. In conclusion, lactate is less effective than glucose and potentially detrimental during neural network rhythms featuring high energetic costs, likely through the lack of some obligatory ATP synthesis by aerobic glycolysis at excitatory and inhibitory synapses. High lactate/glucose ratios might contribute to central fatigue, cognitive impairment, and epileptic seizures partially seen, for instance, during exhaustive physical exercise, hypoglycemia and neuroinflammation.

Brain-regional characteristics and neuroinflammation in ME/CFS patients from neuroimaging: A systematic review and meta-analysis

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating condition characterized by an elusive etiology and pathophysiology. This study aims to evaluate the pathological role of neuroinflammation in ME/CFS by conducting an exhaustive analysis of 65 observational studies. Four neuroimaging techniques, including magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), electroencephalography (EEG), and positron emission tomography (PET), were employed to comparatively assess brain regional structure, metabolite profiles, electrical activity, and glial activity in 1529 ME/CFS patients (277 males, 1252 females) and 1715 controls (469 males, 1246 females). Clinical characteristics, including sex, age, and fatigue severity, were consistent with established epidemiological patterns. Regional alterations were most frequently identified in the cerebral cortex, with a notable focus on the frontal cortex. However, our meta-analysis data revealed a significant hypoactivity in the insular and thalamic regions, contrary to observed frequencies. These abnormalities, occurring in pivotal network hubs bridging reason and emotion, disrupt connections with the limbic system, contributing to the hallmark symptoms of ME/CFS. Furthermore, we discuss the regions where neuroinflammatory features are frequently observed and address critical neuroimaging limitations, including issues related to inter-rater reliability. This systematic review serves as a valuable guide for defining regions of interest (ROI) in future neuroimaging investigations of ME/CFS.

Cerebral Lactate Participates in Hypoxia-induced Anapyrexia Through its Receptor G Protein-coupled Receptor 81

Hypoxia-induced anapyrexia is thought to be a regulated decrease in body core temperature (Tcore), but the underlying mechanism remains unclear. Recent evidence suggests that lactate, a glycolysis product, could modulate neuronal excitability through the G protein-coupled receptor 81 (GPR81). The present study aims to elucidate the role of central lactate and GPR81 in a rat model of hypoxia-induced anapyrexia. The findings revealed that hypoxia (11.1% O2, 2 h) led to an increase in lactate in cerebrospinal fluid (CSF) and a decrease in Tcore. Injection of dichloroacetate (DCA, 5 mg/kg, 1 μL), a lactate production inhibitor, to the third ventricle (3 V), alleviated the increase in CSF lactate and the decrease in Tcore under hypoxia. Immunofluorescence staining showed GPR81 was expressed in the preoptic area of hypothalamus (PO/AH), the physiological thermoregulation integration center. Under normoxia, injection of GPR81 agonist 3-chloro-5-hydroxybenzoic acid (CHBA, 0.05 mg/kg, 1 μL) to the 3 V, reduced Tcore significantly. In addition, hypoxia led to a dramatic increase in tail skin temperature and a decrease in interscapular brown adipose tissue skin temperature. The number of c-Fos+ cells in the PO/AH increased after exposure to 11.1% O2 for 2 h, but administration of DCA to the 3 V blunted this response. Injection of CHBA to the 3 V also increased the number of c-Fos+ cells in the PO/AH under normoxia. In light of these, our research has uncovered the pivotal role of central lactate-GPR81 signaling in anapyrexia, thereby providing novel insights into the mechanism of hypoxia-induced anapyrexia.

Versatile lactate signaling via HCAR1: a multifaceted GPCR involved in many biological processes

G-coupled protein receptors (GPCRs) are the ultimate refuge of pharmacology and medicine as more than 40% of all marketed drugs are directly targeting these receptors. Through cell surface expression, they are at the forefront of cellular communication with the outside world. Metabolites among the conveyors of this communication are becoming more prominent with the recognition of them as ligands for GPCRs. HCAR1 is a GPCR conveyor of lactate. It is a class A GPCR coupled to Gαi which reduces cellular cAMP along with the downstream Gβγ signaling. It was first found to inhibit lipolysis, and lately has been implicated in diverse cellular processes, including neural activities, angiogenesis, inflammation, vision, cardiovascular function, stem cell proliferation, and involved in promoting pathogenesis for different conditions, such as cancer. Other than signaling from the plasma membrane, HCAR1 shows nuclear localization with different location-biased activities therein. Although different functions for HCAR1 are being discovered, its cell and molecular mechanisms are yet ill understood. Here, we provide a comprehensive review on HCAR1, which covers the literature on the subject, and discusses its importance and relevance in various biological phenomena.

Monocarboxylate transporter-dependent mechanism is involved in the adaptability of the body to exercise-induced fatigue under high-altitude hypoxia environment

Under high-altitude hypoxia environment, the body is more prone to fatigue, which occurs in both peripheral muscles and the central nervous system (CNS). The key factor determining the latter is the imbalance of brain energy metabolism, which makes it difficult to maintain the central nervous system to send peripheral nerve impulse continuously. During strenuous exercise, lactate released from astrocytes is taken up by neurons stored for energy to maintain synaptic transmission, a process mediated by monocarboxylate transporters (MCTs) in CNS. The present study investigated the correlation among the adaptability to exercise-induced fatigue, brain lactate metabolism and neuronal hypoxia injury under high-altitude hypoxia environment. Rats were subjected to exhaustive incremental load treadmill exercise under either normal pressure and normoxic conditions or simulated high-altitude low pressure and hypoxic conditions, with subsequent evaluation of the average exhaustive time as well as the expression of monocarboxylate transporters 2 (MCT2), MCT4, the average neuronal density in the cerebral motor cortex, and the lactate content in rat brain. At the early stage of simulated high-altitude environment, the average exhaustive time and neuronal density of rats decreased rapidly, then gradually recovered to some extent with the extension of altitude acclimatization time. The expression of MCT2, MCT4 and the lactate content in rat brain also increased gradually with the extension of altitude acclimatization time. After the application of lactate transport inhibitor, the recovery of exercise capacity of rats after altitude acclimatization was quickly blocked, and the neuronal injury in the cerebral motor cortex of rats was also significantly aggravated. These findings demonstrate that MCT-dependent mechanism is involved in the adaptability of the body to central fatigue, and provide a potential basis for medical intervention for exercise-induced fatigue under high-altitude hypoxia environment.

A monocarboxylate transporter-dependent mechanism confers resistance to exercise-induced fatigue in a high-altitude hypoxic environment

The body is more prone to fatigue in a high-altitude hypoxic environment, in which fatigue occurs in both peripheral muscles and the central nervous system (CNS). The key factor determining the latter is the imbalance in brain energy metabolism. During strenuous exercise, lactate released from astrocytes is taken up by neurons via monocarboxylate transporters (MCTs) as a substrate for energy metabolism. The present study investigated the correlations among the adaptability to exercise-induced fatigue, brain lactate metabolism and neuronal hypoxia injury in a high-altitude hypoxic environment. Rats were subjected to exhaustive incremental load treadmill exercise under either normal pressure and normoxic conditions or simulated high-altitude, low-pressure and hypoxic conditions, with subsequent evaluation of the average exhaustive time as well as the expression of MCT2 and MCT4 in the cerebral motor cortex, the average neuronal density in the hippocampus, and the brain lactate content. The results illustrate that the average exhaustive time, neuronal density, MCT expression and brain lactate content were positively correlated with the altitude acclimatization time. These findings demonstrate that an MCT-dependent mechanism is involved in the adaptability of the body to central fatigue and provide a potential basis for medical intervention for exercise-induced fatigue in a high-altitude hypoxic environment.