Abstract
Advances in functional imaging allowed us to visualize brain glucose metabolism in vivo and non-invasively with [18F]fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) imaging. In the past decades, FDG-PET has been instrumental in the understanding of brain function in health and disease. The source of the FDG-PET signal has been attributed to neuronal uptake, with hypometabolism being considered as a direct index of neuronal dysfunction or death. However, other brain cells are also metabolically active, including astrocytes. Based on the astrocyte-neuron lactate shuttle hypothesis, the activation of the glutamate transporter 1 (GLT-1) acts as a trigger for glucose uptake by astrocytes. With this in mind, we investigated glucose utilization changes after pharmacologically downregulating GLT-1 with clozapine (CLO), an anti-psychotic drug. Adult male Wistar rats (control, n = 14; CLO, n = 12) received CLO (25/35mgkg-1) for 6weeks. CLO effects were evaluated in vivo with FDG-PET and cortical tissue was used to evaluate glutamate uptake and GLT-1 and GLAST levels. CLO treatment effects were also assessed in cortical astrocyte cultures (glucose and glutamate uptake, GLT-1 and GLAST levels) and in cortical neuronal cultures (glucose uptake). CLO markedly reduced in vivo brain glucose metabolism in several brain areas, especially in the cortex. Ex vivo analyses demonstrated decreased cortical glutamate transport along with GLT-1 mRNA and protein downregulation. In astrocyte cultures, CLO decreased GLT-1 density as well as glutamate and glucose uptake. By contrast, in cortical neuronal cultures, CLO did not affect glucose uptake. This work provides in vivo demonstration that GLT-1 downregulation induces astrocyte-dependent cortical FDG-PET hypometabolism-mimicking the hypometabolic signature seen in people developing dementia-and adds further evidence that astrocytes are key contributors of the FDG-PET signal.
Highlights
Regulation of brain glucose metabolism is key for supporting high-energy demands during neuronal firing and it involves multiple enzymes and transporters
CLO markedly reduced in vivo brain glucose metabolism in several brain areas, especially in the cortex
Glutamate transport via glutamate transporter 1 (GLT-1) is considered as a triggering signal for glucose uptake by astrocytes [7], this biological process being vital for brain energetics
Summary
Regulation of brain glucose metabolism is key for supporting high-energy demands during neuronal firing and it involves multiple enzymes and transporters. The ANLS model has been corroborated by a series of in vitro and in vivo studies [7,8,9,10,11,12,13,14,15,16]. It was observed a neuronal preference for lactate oxidation and a predominant glycolytic metabolism of glucose by astrocytes in cellular models [1, 2]. It is important to mention that there are reports challenging the ANLS concept [21], which makes this topic controversial and requiring further experimental evidence
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More From: European Journal of Nuclear Medicine and Molecular Imaging
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