Part 1:Astrocyte Glycogen And Lactate: New Insights Into Learning And Memory Mechanisms
Mar 14, 2022
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Cristina M. Alberini, Emmanuel Cruz, Giannina Descalzi, Benjamin Bessières, and Virginia Gao
Center for Neural Science, New York University, New York, NY, 10003
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Abstract
Memory, the ability to retain learned information, is necessary for survival. Thus far, molecular and cellular investigations of memory formation and storage have mainly focused on neuronal mechanisms. In addition to neurons, however, the brain comprises other types of cells and systems, including glia and vasculature. Accordingly, recent experimental work has begun to ask questions about the roles of non-neuronal cells in memory formation. These studies provide evidence that all types of glial cells (astrocytes, oligodendrocytes, and microglia) make important contributions to the processing of encoded information and storing memories. In this review, we summarize and discuss recent findings on the critical role of astrocytes as providers of energy for the long-lasting neuronal changes that are necessary for long-term memory formation. We focus on three main findings: first, the role of glucose metabolism and the learning- and activity-dependent metabolic coupling between astrocytes and neurons in the service of long-term memory formation; second, the role of astrocytic glucose metabolism in arousal, a state that contributes to the formation of very long-lasting and detailed memories; and finally, in light of the high energy demands of the brain during early development, we will discuss the possible role of astrocytic and neuronal glucose metabolisms in the formation of early-life memories. We conclude by proposing future directions and discussing the implications of these findings for brain health and disease.
Keywords
glucose; metabolism; glia; glycolysis; glycogenolysis; emotional arousal; development
Long-term memory and its underlying neuron-centric biological mechanisms of their underlying biological mechanisms and circuitry. Although long-term memories generally require denovogene expression, short-term memories rely on post-translational protein modifications (Alberini 2009; Alberini and Kandel 2014; Squire and Dede 2015).
Memories can also be divided into different categories on the basis of the type of information encoded and stored. For example, one major distinction classifies memories as explicit (also known as declarative in humans) or implicit (non-declarative) (Squire 2004). Explicit memories retain information about facts, people, places, and things (also known as memories of what, where, who, and when, or www memories), and include episodic and semantic memories. Implicit memories, which are recalled in an unconscious/automatic manner, retain information about learned automatic responses, and include priming, procedural memories (memories of how to do things), and simple reflexes (Tulving 1972; Squire and Wixted 2011). Explicit and implicit memories recruit distinct systems (network of regions) for their encoding, consolidation, and storage. Both clinical and animal studies have revealed that explicit memories are processed by the medial temporal lobe, within which one critical region is the hippocampus, whereas implicit memories are processed elsewhere and can operate in the absence of an intact explicit system (Eichenbaum 2006; Kim and Fanselow 1992; Scoville and Milner 1957; Squire and Wixted 2011). Thus, explicit memories are also referred to as hippocampus-dependent memories. Although implicit and explicit memory systems can be functionally dissociated, under normal healthy conditions they cooperate to process and store complex information (Kim and Baxter 2001; McDonald et al. 2004).
Studies aimed at elucidating the biological bases of long-term memories have mainly focused on hippocampus-dependent memories. However, most of our understanding of the cellular and molecular mechanisms underlying memory formation and storage initially arose from investigations of simple forms of learning, such as the gill-withdrawal reflex of Aplysia California and olfactory learning in Drosophila melanogaster(Yin et al. 1994; Dubnau and Tully 1998; Davis 2011; Kandel 2012). In Aplysia, these studies uncovered a great deal of information about the molecular and cellular pathways activated and recruited to implement long-term modifications of synaptic strength or long-term synaptic plasticity. These data converged with genetic and behavioral results obtained in Drosophila. Guided by this knowledge from these two invertebrate systems, studies on mammalian memory paradigms revealed that similar molecular pathways are also necessary in the more complex mammalian memory, including hippocampus-dependent memories. Ultimately, numerous studies in the last 30 years on many species converged on the conclusion that evolutionarily conserved biological mechanisms underlie long-term synaptic plasticity and long-term memory formation (Alberini 2009; Kandel 2012; Kandel et al. 2014). One classical example, which has been extensively investigated, is the evolutionarily conserved role ofthe cyclic adenosine monophosphate (cAMP)-a dependent pathway and the functionally linked activation of the cAMP response element-binding protein (CREB)-a dependent cascade of gene expression (Kida and Serita 2014; Lonze and Ginty 2002; Silva et al. 1998) (Figure 1).
Numerous mammalian models of different types of short- and long-term memory, particularly in rodents, have been employed to investigate the complexity of mammalian memory processing in a variety of brain regions. These studies revealed that the expression and post-translational regulation of many classes of genes, RNAs, and proteins are required for long-term memory formation and storage; these include immediate-early genes (e.g., c- Fos, Zif268, NPAS4 and Arc/Arg3.1) (Bramham et al. 2008; Guzowski 2002; Loebrich and Nedivi 2009; Sun and Lin 2016; Veyrac et al. 2014), metabotropic and ionotropic receptors
for various neurotransmitters (e.g., AMPA, NMDA, Kainate, GABA, and metabotropic glutamate receptors) and neuromodulators (e.g., dopaminergic and serotoninergic receptors), neurotrophic factors (e.g. tyrosine receptor kinase) (Fanselow et al. 1994; Gonzalez-Burgos and Feria-Velasco 2008; Kandel 2001; Makkar et al. 2010; Morris 2013; Purcell and Carew 2003; Riedel 1996; Riedel et al. 2003), kinases (e.g., ERK, CamKIIα, PKA, PKC, PKMζ, and MAPK) (Bejar et al. 2002; Kandel 2012; Lisman et al. 2002; Mayford 2007; Pastalkova et al. 2006; Rahn et al. 2013), transcription factors (e.g., CREB, C/EBP, NFkB, AP1, NPAS4, Zif268, NR4a, and SRF) (Alberini 2009; Alberini and Kandel 2014; Jones et al. 2001; Sun and Lin 2016), epigenetic regulators (e.g., MSK1, RSK2, NFkB, DNMT, HATs, and HDACs) (Day and Sweatt 2011; de la Fuente et al. 2015; Franklin and Mansuy 2010; Rudenko and Tsai 2014), microRNAs (e.g., miR-124, miR-132, miR-128b, and miR-134) (Bredy et al. 2011; Nudelman et al. 2010; Saab and Mansuy 2014), and a number of effector proteins engaged in structural changes, such as cell-adhesion molecules (e.g., neurexin and neuroligin) (Murase and Schuman 1999; Rose 1996; Ye et al. 2017; Bailey et al. 2015) (Figure 1).
These molecular investigations have been paralleled by electrophysiological studies, which showed that the cellular mechanisms underlying long-term memory involve long-term synaptic functional changes, and in particular long-term increases or decreases in synaptic transmission known as long-term potentiation (LTP) and long-term depression (LTD), respectively (Bliss and Collingridge 1993; Malenka and Bear 2004). Additional electrophysiological changes in the brain that have been implicated in long-term memory formation include electroencephalogram (EEG) coherence, i.e., phase synchronization of field potential oscillations, which coordinates the timing of neuronal spiking to promote synaptic plasticity across distributed brain regions (Corcoran et al. 2016; Zanto et al. 2011). Notably, this system-level communication among brain regions is controlled by sharp wave ripples (SPW-Rs) (Buzsáki 2015), asynchronous population pattern in the hippocampus that engages in crosstalk with a wide area of the cortex and several subcortical nuclei. SPW-Rs occur in “off-line” states of the brain during waking and in non-REM sleep and are believed to consolidate episodic memories across the hippocampal-cortical system (Buzsáki 2015; Inostroza and Born 2013). These system-wide activities provide a possible mechanistic explanation for why hippocampus-dependent memories, which are fragile during the initial period when they are engaging a network of both hippocampal and cortical regions, become more stable and exclusively hippocampus-independent over time. This redistribution of memory representations and storage is known as system-level consolidation(Dudai et al. 2015; Squire et al. 2015; Frankland and Bontempi 2005).
Although these studies provided a great deal of information about the biological bases of learning and memory, they focused on neuronal mechanisms and consequently generated conclusions mostly limited to neurons and neuronal functions. However, in addition to neurons, the brain comprises many types of cells and systems, including glia and vascular
systems. Recent investigations have begun to assess the role of non-neuronal cells in long-term memory and provided clear evidence that all glial cell types (i.e. astrocytes, oligodendrocytes, and microglia) play critical roles in-memory processing (Adamsky and Goshen 2017; Fields 2008; Gibbs et al. 2008; Lee et al. 2014; Moraga-Amaro et al. 2014; Parkhurst et al. 2013; Suzuki et al. 2011).
Astrocytes are particularly well equipped to influence neuronal functions involved in memory formation (Haydon and Nedergaard 2014; Moraga-Amaro et al. 2014): they are excitable through calcium fluctuations and respond to neurotransmitters released at synapses; they synchronize via calcium waves and release their own gliotransmitters, which are essential for synaptic plasticity; they communicate with blood vessels thus coupling circulation (blood flow) to local brain activity; and finally, they regulate energy metabolism in support of neuronal functions, including those required for memory formation (Henneberger et al. 2010; Pannasch and Rouach 2013; Perea et al. 2009; Bazargani and Attwell 2016). In regard to this metabolic role, astrocytes are perfectly positioned to balance the metabolism of glucose in the brain: on one side, the astrocytic endfeet directly contact the layers of the blood vessel that import glucose from the blood via the selective glucose transporter GLUT1, and on the other side, these cells extend processes that wrap around the pre-and post-synaptic compartments of neurons (Falkowska et al. 2015; Morgello et al.
1995) (Figure 2).
In this review, we will specifically discuss the critical contribution of astrocytes, acting as regulators of glucose metabolism, to memory formation and storage.
Glycogen and glucose metabolisms play critical roles in memory formation
Studies by Paul Gold and colleagues identified systemic glucose as an intermediary of the memory-enhancing effect of norepinephrine (Gold and Korol 2012). Memories encoded in arousal states are remembered better (i.e., for longer periods and with greater detail), and arousal is well known to regulate the release of epinephrine from the adrenal glands. Epinephrine binds adrenergic receptors (ARs) on hepatocytes and initiates the breakdown of glycogen, a polymer of glucose stored in the liver (Sutherland and Rall 1960), leading to the release of glucose into the bloodstream. Systemic glucose injections at doses comparable to those found in the blood after epinephrine treatment are sufficient to enhance memory, whereas low liver glycogen storage, as in food-deprived or aged rats, is associated with a lack of memory enhancement following epinephrine treatment (Morris et al. 2010; Talley et al. 2000). Conversely, peripherally blocking adrenergic receptors blocks the ability of epinephrine to enhance memory and increase blood glucose. Collectively, these studies support the conclusion that a major mechanism underlying the actions of epinephrine released by arousal is the increase in blood glucose.
The effect of glucose as a memory enhancer has been observed with both systemic and intracerebral injections, and it has been linked to the regulation of either norepinephrine or acetylcholine release. Ragozzino and colleagues showed that both systemic and intra- hippocampal injections of glucose, like injections of epinephrine, enhance spontaneous alternation, a form of spatial working memory, and increase the release of acetylcholine in the hippocampus (Ragozzino et al. 1998; Ragozzino et al. 1996).
The understanding of the role of glucose on memory modulation was considerably advanced by the observation that when rats are tested on a spontaneous alternation task, the levels of extracellular glucose in the hippocampus decrease significantly. Hence, it was suggested that learning and memory consume glucose, presumably to support the energy demands ofthe brain as it processes the new experience and stores the important information (McNay et al.
2000; McNay et al. 2001; McNay and Sherwin 2004).
Indeed, the brain consumes high levels of energy: the adult brain uses on average about 20% of total body energy, despite accounting for only 2% of total body weight. Glucose, the major source of energy entering the brain from the circulation, can either be directly metabolized or stored in the form of glycogen. In the mature brain, glycogen is stored mostly in astrocytes (Brown et al. 2004; Brunet et al. 2010; Cali et al. 2016; Cataldo and Broadwell, 1986; Maxwell and Kruger 1965; Petersen 1969; Pfeiffer-Guglielmi et al. 2003; reviewed in Waitt et al. 2017), and, under conditions of high energy demand such as glucose deprivation or intense neural activity, can be catabolized to rapidly deliver metabolic substrates (i.e., pyruvate and lactate) (Brown and Ransom 2015). Although neurons possess the enzymatic machinery to store and break down glycogen, under physiological conditions, they suppress glycogen storage through a series of mechanisms. In fact, glycogen storage in neurons is observed only in severe neurological diseases such as progressive myoclonus epilepsy or Lafora disease, a brain disorder characterized by recurrent seizures (epilepsy) and a decline in intellectual function (Vilchez et al. 2007). Thus, glucose, either directly metabolized via glycolysis or supplied by astrocytic glycogenolysis, may fuel the high energy demands associated with the cellular changes underlying learning, memory formation, and memory storage.
One long-debated question is whether neurons directly import glucose entering the brain from the blood and use it immediately to provide the energy required to support their functions. An alternative model, suggested by Pellerin and Magistretti (Pellerin and Magistretti 1994), proposes that the high energy demands of stimulated neurons are supported by astrocytes, which supply the neurons with lactate produced via aerobic glycolysis, thereby providing the energy required for the activity-induced neuronal functions; hence, in the case of learning, for the changes involved in the processing and storing memories. It is also possible that both mechanisms are utilized, perhaps in response to specific conditions.
The model proposed by Magistretti and Pellerin has been highly debated. These debates are complex and likely reflect the intricacy of metabolic regulations in different conditions. Given the variety ofthese conditions and systems, we will not be able discuss the points of the debate in this manuscript, thus we refer to several reviews reporting them (Chih et al., 2001; Chih and Roberts, 2003; Dienel and Hertz, 2001; Pellerin and Magistretti, 2003, 2012; Aubert et al., 2005; Dienel, 2010, 2017; DiNuzzo et al., 2010; Steinman et al. 2016). We will, however, discuss the literature important for the findings of the roles of glycogen, glucose, and lactate in learning and memory as well as in brain plasticity.
Several studies reported that stimulation of brain areas increases glycogenolysis and glycolysis, as well as glucose uptake, in astrocytes, consistent with the idea that astrocytic glycogen and glucose metabolism are needed to sustain activity-dependent processes. For example, NMR spectroscopy, which allows measurement of lactate invivo, revealed an elevation of lactate in the human visual cortex during physiologic photic stimulation (Prichard et al. 1991), and microsensor-based measures revealed an increase in extracellular lactate concentration in the dentate gyrus of the rat hippocampus after electrical stimulation of the perforant pathway (Hu and Wilson 1997). Moreover, whisker stimulation in the awake rat leads to rapid glycogen breakdown in layer IV of the somatosensory cortex (Swanson et al. 1992) and results in a preferential increase in glucose uptake into astrocytes in comparison with neurons in the somatosensory cortex invivo(Chuquet et al., 2010), although more mechanistic details need to be understood (Dienel and Cruz 2015). The physical position of astrocytes, between the blood flow on one side and neurons on the other, further supports the idea that astrocytic regulation of glucose metabolism subsidizes the energy requirements of activity, plasticity, learning, and memory formation.
In accordance with this view, metabolic profiling of astrocytes and neurons revealed distinct features indicating that glycolysis occurs mainly in astrocytes. For example, cultured neurons produce CO2 at a much higher rate than astrocytes, and their respective enzymatic profiles are consistent with the relative predominance of glycolysis in glial cells and oxidation in neurons (Bélanger et al. 2011; Hamberger and Hydén 1963; Hydén and Lange 1962). In addition, acutely isolated, FACS-purified astrocytes exhibit a primarily glycolytic profile (Lovatt et al. 2007; Zhang et al. 2014). Finally, the enzyme 6-phosphofructo-2- kinase/fructose-2,6-bisphosphatase 3 (Pfkfb3), which promotes glycolysis, is active in astrocytes but constantly subjected to proteasomal degradation in neurons (Bolaños et al. 2010; Herrero-Mendez et al. 2009), once again supporting the idea that astrocytes are the primary sites of glycolysis. Thus, a large body of evidence converges on the conclusion that astrocytes are predominantly glycolytic cells, whereas neurons are not, and instead exhibit high oxidative activity.
The first demonstration that astrocytic glycolysis is critical for learning and memory came from studies performed by Leif Hertz, Marie Gibbs, and colleagues, who showed that glycogenolysis is necessary for memory formation. Using taste avoidance training in a day- old chick, they showed that intracranial injection of an inhibitor of glycogen phosphorylase, 1,4-Dideoxy-1,4-imino-d-arabinitol (DAB), impaired memory in a dose-dependent manner, and concluded that glycogenolysis is a critical requirement for long-term memory storage (Gibbs et al. 2006). In agreement with this conclusion, breakdown of glycogen in the brain increases significantly during sensory activation in rats (Cruz and Dienel 2002; Swanson et al. 1992), and later studies detailed below demonstrated that glycogen contributes to several types of memory formation in rats and mice. In addition to glycogenolysis, aerobic glycolysis may also be necessary for memory formation, as revealed by experiments in which the glycolysis inhibitor 2-deoxyglucose was injected into the brains of 1 day-old chicks at training, resulting in long-term memory impairment (Gibbs et al. 2007). Thus, several studies have converged on the conclusion that glycogenolysis and aerobic glycolysis, resulting in the production of lactate, are critically linked to memory formation. This raises several questions: How exactly does this regulation occur? How are astrocytes functionally coupled to neurons? What are the target mechanisms that consume high levels of energy upon learning and allow memory consolidation to occur?
Astrocytic glycogenolysis, aerobic glycolysis, and lactate are critical for long-term memory formation in several brain regions
A model proposed by Pellerin and Magistretti (Pellerin and Magistretti 1994), known as the astrocyte-neuron lactate shuttle (ANLS), suggests that astrocyte glycolysis and neuronal oxidation play coordinated roles in long-term memory formation via transport of lactate. This model predicts that excitation, and hence glutamate release, stimulates the uptake of glutamate by astrocytes, which is converted into glutamine (glutamate-glutamine cycle), eventually sustaining synaptic release of glutamate. This cycle requires energy from astrocytes, which would therefore activate glucose uptake from the blood and metabolize it into lactate. Lactate, released by astrocytes via monocarboxylate transporters (MCTs), can enter other types of cells using similar transporters, which operate on the basis of concentration gradients of protons and monocarboxylate across the plasma membrane (Halestrap 2013; Pierre and Pellerin 2005). MCTs are proton-linked plasma membrane transporters that carry molecules containing one carboxylate group (hence the term monocarboxylates), such as lactate, pyruvate, and ketone bodies, across plasma membranes. MCT1 is expressed in astrocytes, ependymocytes, oligodendrocytes, and endothelial cells of blood vessels, whereas MCT4 is selectively expressed by astrocytes and enriched at synaptic sites (Pierre and Pellerin 2005; Rinholm et al. 2011; Suzuki et al. 2011). MCT2, on the other hand, is selectively expressed by neurons (Debernardi et al. 2003).
Thus, lactate, released by astrocytes via MCT4 and MCT1 is transported by MCT2 into neurons, where it is converted to pyruvate that is subsequently metabolized through oxidative phosphorylation in mitochondria to produce 14–17 ATPs per lactate molecule (Figure 2). This lactate supply from astrocytes to neurons provides an explanation for how neurons might handle the high-energy requirements evoked by active processes in response to stimuli.
The first studies that described the ANLS were performed in-vitro,and questions were raised about whether these mechanisms occurred invivo(Chih and Roberts 2003; Dienel and Cruz 2004; Gjedde et al. 2002). However, studies by Hertz and Gibbs in the chick described above suggested that glycogenolysis is involved in memory formation (for review see Gibbs 2016). In these studies, the chicks were exposed to two beads, one red and one blue, and trained to avoid pecking the red bead by association with an aversive taste. During the retention test, the ratio between the number of pecks of red and blue beads was measured, revealing an increase in avoidance of pecking red beads; the change in the discrimination ratio was indicative of memory (Hertz et al. 1996). The initial results showed that glycogen levels in the forebrain decreased 30 minutes after learning, concomitant with an elevation of glutamate, suggesting denovosynthesis of glutamate from glycogen to support memory consolidation (Hertz et al. 2003; O’Dowd et al. 1994). A few years later, the same group showed that DAB impairs taste aversion memory in day-old chicks when infused into the multimodal forebrain association region, the intermediate medial mesopallium (IMM), a brain region required for memory consolidation (Gibbs et al. 2006; Gibbs and Hertz 2008). They then found that glutamine was sufficient to rescue memory, and hence proposed that glycogenolysis was critical for the glutamate/glutamine shuttle, which may also be affected by DAB. A subsequent study from the same authors demonstrated that L-lactate is also sufficient to rescue chick taste aversion memory after treatment with an inhibitor of either glycogenolysis (DAB) or glycolysis (2-deoxyglucose) (Gibbs et al. 2007). Furthermore, administration of D-lactate, the competitive non-biologically active form of lactate, impaired chick taste aversion memory with a time delay that suggested it was inhibiting L-lactate metabolism and not uptake, leading the authors to conclude that astrocytic metabolism through glycogenolysis and lactate metabolism is critical for memory formation (Gibbs and Hertz 2008). These findings supported the idea that learning in the neonatal chick relies on the breakdown of glycogen for glutamate synthesis in astrocytes (Gibbs et al. 2007).
However, an additional interpretation is that lactate produced by glycogenolysis is transported into neurons for their use, thus contributing to support neuronal modifications critical for memory formation. We tested this hypothesis invivoin mammalian brains, focusing specifically on whether mechanisms of glycogenolysis, astrocytic lactate release and transport into neurons are involved in memory consolidation, the process that stabilizes a newly formed, initially fragile memory into a long-lasting stable representation (Alberini 2009, Dudai 2004).
Using adult rats trained in an inhibitory avoidance (IA) task, in which the animals learn to avoid a context previously paired with a foot-shock (a contextual response to threat), we demonstrated that lactate transported from astrocytes to neurons in the hippocampus plays a critical role in long-term memory consolidation (Suzuki et al. 2011). Specifically, we found that hippocampal astrocytic glycogenolysis is required for memory consolidation, invivo hippocampal long-term potentiation, and learning-induced increases in synaptic and cellular macromolecular changes, including expression of the immediate early gene (IEG) activity-regulated cytoskeleton-associated protein (Arc or Arg3.1) and phosphorylation ofthe transcription factor CREB and of the actin-severing protein cofilin, all of which are markers of long-term synaptic plasticity. In fact, DAB injected bilaterally in the dorsal hippocampus before or immediately after IA training persistently disrupted memory retention, and this disruption was prevented by co-injection of L-lactate, but not equicaloric concentrations of glucose. In addition, after IA training the hippocampal extracellular concentration of lactate, measured by invivomicrodialysis, significantly increased and remained elevated for more than 1 hour, returning to baseline by approximately 90 minutes post-training. This increase in lactate was completely abolished by bilateral DAB injection into the hippocampus, suggesting that it was the result of astrocytic glycogenolysis.
Furthermore, we found that hippocampal injection of the inactive isomer D-lactate before training also blocks long-term memory retention, suggesting that lactate metabolism is critical for long-term memory formation. Similar effects on memory retention were observed following the knockdown of the lactate transporters (MCTs). Notably, although the memory impairments induced by the knockdown of lactate transporters expressed in astrocytes (MCT1 and MCT4) was rescued by the addition of L-lactate, the impairment induced by knockdown of the transporter expressed in neurons (MCT2) was not, consistent with the idea that the transport of lactate out of astrocytes and into neurons is critical for memory formation. In accordance with this interpretation, a lactate gradient between astrocytes and neurons was recently observed and characterized at high resolution invivousing two-photon microscopy (Machler et al. 2016). Therefore, we concluded that glycogenolysis and astrocyte-neuron lactate transport critically supports neuronal functions required for long-term memory formation. A more recent investigation further supported the role of astrocytic lactate in memory formation by showing that IA training induces hippocampal expression of molecules involved in astrocytic-neuronal transport, such as MCTs and the expression of lactate dehydrogenase (LDH) A and B, the enzymes that catalyze the interconversion of lactate and pyruvate (Tadi et al. 2015).
Similar conclusions were reached by Newman et al. (2011), who employed sensitive bioprobes to measure brain glucose and lactate levels in the hippocampus of rats while they underwent a spatial working memory task. They found that while extracellular glucose decreased, lactate levels increased during task performance, and intrahippocampal infusions ofL-lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis with DAB impaired memory, and this impairment was reversed by either L-lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. In this study, as in ours, blockade of the MCTs responsible for lactate uptake into neurons impaired memory, and this impairment was not reversed by either glucose or L- lactate, again supporting the idea that lactate uptake by neurons is necessary to support memory formation. The authors concluded, as we did, that astrocytes regulate memory formation by controlling the provision of lactate to sustain neuronal functions.
Additional studies based on genetic approaches support these conclusions. Delgado-Garcia and colleagues found that knockout of glycogen synthase in the nervous system of mice impairs both hippocampal LTP and associative learning (Duran et al. 2013). In addition, Boury-Jamot et al. (2016) and Zhang et al. (2016) reported that the consolidation and reconsolidation of appetitive conditioning using drugs of abuse (i.e., cocaine-conditioned place preference or self-administration) are also dependent on glycogenolysis and the directional transport of lactate from astrocytes to neurons via MCTs in the basolateral amygdala (BLA) of rats. Furthermore, extracellular lactate, as measured by invivo microdialysis, is elevated in the BLA after IA training and retrieval (Sandusky et al. 2013).
Consistent with the results of these studies, we found that BLA glycogenolysis is critical for IA memory formation, as demonstrated by the fact that bilateral injection ofDAB into the BLA 15 minutes prior to IA training severely and persistently disrupted memory retention in rats. This impairment was not rescued by a reminder shock delivered in a different context, a protocol that re-instates extinguished memories (Inda et al. 2011), suggesting that blocking glycogenolysis in the amygdala before training disrupts the consolidation process. Co-administration of L-lactate with DAB in the amygdala rescued the memory impairment, confirming the importance of the roles of glycogenolysis and lactate in diverse brain areas for IA memory consolidation (Figure 3).
The target functions fueled by lactate and/or glucose metabolism are still largely unknown. Brain energy is needed to support the electrical pulses required for neuronal communication, and for many housekeeping activities, including protein synthesis, phospholipid metabolism, neurotransmitter cycling, and transport of ions across cellular membranes (Du et al. 2008). As shown by the studies described above, lactate metabolism supports long-term memory formation and the training-dependent increase in expression of several molecules related to activity and plasticity, including Arc, cFos, and Zif268 (Gao et al. 2016; Suzuki et al. 2011;
Yang et al. 2014). These effects are NMDA receptor–dependent, implying that lactate- dependent changes are linked to activity and/or plasticity (Yang et al. 2014). Invivo, lactate is sufficient to maintain neuronal activity (Wyss et al. 2011) and recent data showed that interstitial K+ elevations can activate a channel on the astrocyte membrane through which astrocytic lactate can flow into the interstitium, in parallel with the established transport via MCTs (Sotelo-Hitschfeld et al., 2015). This route for astrocytic lactate release is coupled to the membrane potential and allows lactate release against a concentration gradient, whereas the MCT is electro-neutral and net flux is governed by the trans-membrane concentrations of H+ and lactate. Furthermore, an astrocytic mechanism via bicarbonate-responsive soluble adenylyl cyclase leading to glycogen breakdown, enhanced glycolysis, and the release of lactate into the extracellular space, which is subsequently taken up by neurons for use as an energy substrate has been demonstrated (Choi et al. 2012). Collectively these studies support the conclusion that lactate delivery by astrocytes to neurons can be regulated in many ways in response to activity and studies are needed to understand whether parallel or selective mechanisms occur invivoupon learning. Nevertheless it emerges that lactate is needed to support not only ionic membrane homeostasis after depolarization, but also numerous other neuronal functions required for long-term modifications associated with memory formation and storage.
