Energy Metabolism in Mouse Sciatic Nerve A Fibres during Increased Energy Demand.

The ability of sciatic nerve A fibres to conduct action potentials relies on an adequate supply of energy substrate, usually glucose, to maintain necessary ion gradients. Under our ex vivo experimental conditions, the absence of exogenously applied glucose triggers Schwann cell glycogen metabolism to lactate, which is transported to axons to fuel metabolism, with loss of the compound action potential (CAP) signalling glycogen exhaustion. The CAP failure is accelerated if tissue energy demand is increased by high-frequency stimulation (HFS) or by blocking lactate uptake into axons using cinnemate (CIN). Imposing HFS caused CAP failure in nerves perfused with 10 mM glucose, but increasing glucose to 30 mM fully supported the CAP and promoted glycogen storage. A combination of glucose and lactate supported the CAP more fully than either substrate alone, indicating the nerve is capable of simultaneously metabolising each substrate. CAP loss resulting from exposure to glucose-free artificial cerebrospinal fluid (aCSF) could be fully reversed in the absence of glycogen by addition of glucose or lactate when minimally stimulated, but imposing HFS resulted in only partial CAP recovery. The delayed onset of CAP recovery coincided with the release of lactate by Schwann cells, suggesting that functional Schwann cells are a prerequisite for CAP recovery.

Microglial depletion abolishes ischemic preconditioning in white matter.

Ischemic preconditioning (IPC) is a phenomenon whereby a brief, non-injurious ischemic exposure enhances tolerance to a subsequent ischemic challenge. The mechanism of IPC has mainly been studied in rodent stroke models where gray matter (GM) constitutes about 85% of the cerebrum. In humans, white matter (WM) is 50% of cerebral volume and is a critical component of stroke damage. We developed a novel CNS WM IPC model using the mouse optic nerve (MON) and identified the involved immune signaling pathways. Here we tested the hypothesis that microglia are necessary for WM IPC. Microglia were depleted by treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. MONs were exposed to transient ischemia in vivo, acutely isolated 72 h later, and subjected to oxygen-glucose deprivation (OGD) to simulate a severe ischemic injury (i.e., stroke). Functional and structural axonal recovery was assessed by recording compound action potentials (CAPs) and by microscopy using quantitative stereology. Microglia depletion eliminated IPC-mediated protection. In control mice, CAP recovery was improved in preconditioned MONs compared with non-preconditioned MONs, however, in PLX5622-treated mice, we observed no difference in CAP recovery between preconditioned and non-preconditioned MONs. Microgliadepletion also abolished IPC protective effects on axonal integrity and survival of mature (APC+ ) oligodendrocytes after OGD. IPC-mediated protection was independent of retinal injury suggesting it results from mechanistic processes intrinsic to ischemia-exposed WM. We conclude that preconditioned microglia are critical for IPC in WM. The "preconditioned microglia" phenotype might protect against other CNS pathologies and is a neurotherapeutic horizon worth exploring.

Oligodendrocyte lineage cells and depression.

Depression is a common mental illness, affecting more than 300 million people worldwide. Decades of investigation have yielded symptomatic therapies for this disabling condition but have not led to a consensus about its pathogenesis. There are data to support several different theories of causation, including the monoamine hypothesis, hypothalamic-pituitary-adrenal axis changes, inflammation and immune system alterations, abnormalities of neurogenesis and a conducive environmental milieu. Research in these areas and others has greatly advanced the current understanding of depression; however, there are other, less widely known theories of pathogenesis. Oligodendrocyte lineage cells, including oligodendrocyte progenitor cells and mature oligodendrocytes, have numerous important functions, which include forming myelin sheaths that enwrap central nervous system axons, supporting axons metabolically, and mediating certain forms of neuroplasticity. These specialized glial cells have been implicated in psychiatric disorders such as depression. In this review, we summarize recent findings that shed light on how oligodendrocyte lineage cells might participate in the pathogenesis of depression, and we discuss new approaches for targeting these cells as a novel strategy to treat depression.

A method for reducing animal use whilst maintaining statistical power in electrophysiological recordings from rodent nerves.

The stimulus evoked compound action potential, recorded from ex vivo nerve trunks such as the rodent optic and sciatic nerve, is a popular model system used to study aspects of nervous system metabolism. This includes (1) the role of glycogen in supporting axon conduction, (2) the injury mechanisms resulting from metabolic insults, and (3) to test putative benefits of clinically relevant neuroprotective strategies. We demonstrate the benefit of simultaneously recording from pairs of nerves in the same superfusion chamber compared with conventional recordings from single nerves. Experiments carried out on mouse optic and sciatic nerves demonstrate that our new recording configuration decreased the relative standard deviation from samples when compared with recordings from an equivalent number of individually recorded nerves. The new method reduces the number of animals required to produce equivalent Power compared with the existing method, where single nerves are used. Adopting this method leads to increased experimental efficiency and productivity. We demonstrate that reduced animal use and increased Power can be achieved by recording from pairs of rodent nerve trunks simultaneously.

Microglial Hv1 proton channels promote white matter injuries after chronic hypoperfusion in mice.

Microglia are critical in damage/repair processes during ischemic white matter injury (WMI). Voltage-gated proton channel (Hv1) is expressed in microglia and contributes to nicotinamide adenine dinucleotide phosphate oxidase complex-dependent production of reactive oxygen species (ROS). Recent findings have shown that Hv1 is involved in regulating luminal pH of M1-polarized microglial phagosomes and inhibits endocytosis in microglia. We previously reported that Hv1 facilitated production of ROS and pro-inflammatory cytokines in microglia and enhanced damage to oligodendrocyte progenitor cells from oxygen and glucose deprivation. To investigate the role of Hv1 in hypoperfusion-induced WMI, we employed mice that were genetically devoid of Hv1 (Hv1-/- ), as well as a model of subcortical vascular dementia via bilateral common carotid artery stenosis. Integrity of myelin was assessed using immunofluorescent staining and transmission electron microscopy, while cognitive impairment was assessed using an eight-arm radial maze test. Hv1 deficiency was found to attenuate bilateral common carotid artery stenosis-induced disruption of white matter integrity and impairment of working memory. Immunofluorescent staining and western blotting were used to assay changes in oligodendrocytes, OPCs, and microglial polarization. Compared with that in wild-type (WT) mice, Hv1-/- mice exhibited reduced ROS generation, decreased pro-inflammatory cytokines production, and an M2-dominant rather than M1-dominant microglial polarization. Furthermore, Hv1-/- mice exhibited enhanced OPC proliferation and differentiation into oligodendrocytes. Results of mouse-derived microglia-OPC co-cultures suggested that PI3K/Akt signaling was involved in Hv1-deficiency-induced M2-type microglial polarization and concomitant OPC differentiation. These results suggest that microglial Hv1 is a promising therapeutic target for reducing ischemic WMI and cognitive impairment.

Metabolism of Glycogen in Brain White Matter.

Brain glycogen is a specialized energy buffer, rather than a conventional reserve. In the rodent optic nerve, a central white matter tract, it is located in astrocytes, where it is converted to lactate, which is then shuttled intercellularly from the astrocyte to the axon. This basic pathway was elucidated from non-physiological experiments in which the nerve was deprived of exogenous glucose. However, this shuttling also occurs under physiological conditions, when tissue energy demand is increased above baseline levels in the presence of normoglycemic concentrations of glucose. The signaling mechanism by which axons alert astrocytes to their increased energy requirement is likely to be elevated interstitial K+, the inevitable consequence of increased neuronal activity.

Hypothermic neuroprotection during reperfusion following exposure to aglycemia in central white matter is mediated by acidification.

Hypoglycemia is a common iatrogenic consequence of type 1 diabetes therapy that can lead to central nervous system injury and even death if untreated. In the absence of clinically effective neuroprotective drugs we sought to quantify the putative neuroprotective effects of imposing hypothermia during the reperfusion phase following aglycemic exposure to central white matter. Mouse optic nerves (MONs), central white matter tracts, were superfused with oxygenated artificial cerebrospinal fluid (aCSF) containing 10 mmol/L glucose at 37°C. The supramaximal compound action potential (CAP) was evoked and axon conduction was assessed as the CAP area. Extracellular lactate was measured using an enzyme biosensor. Exposure to aglycemia, simulated by omitting glucose from the aCSF, resulted in axon injury, quantified by electrophysiological recordings, electron microscopic analysis confirming axon damage, the extent of which was determined by the duration of aglycemia exposure. Hypothermia attenuated injury. Exposing MONs to hypothermia during reperfusion resulted in improved CAP recovery compared with control recovery measured at 37°C, an effect attenuated in alkaline aCSF. Hypothermia decreases pH implying that the hypothermic neuroprotection derives from interstitial acidification. These results have important clinical implications demonstrating that hypothermic intervention during reperfusion can improve recovery in central white matter following aglycemia.

Emerging Roles for Glycogen in the CNS.

The ability of glycogen, the depot into which excess glucose is stored in mammals, to act as a source of rapidly available energy substrate, has been exploited by several organs for both general and local advantage. The liver, expressing the highest concentration of glycogen maintains systemic normoglycemia ensuring the brain receives a supply of glucose in excess of demand. However the brain also contains glycogen, although its role is more specialized. Brain glycogen is located exclusively in astrocytes in the adult, with the exception of pathological conditions, thus in order to benefit neurons, and energy conduit (lactate) is trafficked inter-cellularly. Such a complex scheme requires cell type specific expression of a variety of metabolic enzymes and transporters. Glycogen supports neural elements during withdrawal of glucose, but once the limited buffer of glycogen is exhausted neural function fails and irreversible injury ensues. Under physiological conditions glycogen acts to provide supplemental substrates when ambient glucose is unable to support function during increased energy demand. Glycogen also supports learning and memory where it provides lactate to neurons during the conditioning phase of in vitro long-term potentiation (LTP), an experimental correlate of learning. Inhibiting the breakdown of glycogen or intercellular transport of lactate in in vivo rat models inhibits the retention of memory. Our current understanding of the importance of brain glycogen is expanding to encompass roles that are fundamental to higher brain function.

Connexin Hemichannels in Astrocytes: An Assessment of Controversies Regarding Their Functional Characteristics.

Astrocytes in the mammalian central nervous system are interconnected by gap junctions made from connexins of the subtypes Cx30 and Cx43. These proteins may exist as hemichannels in the plasma membrane in the absence of a 'docked' counterpart on the neighboring cell. A variety of stimuli are reported to open the hemichannels and thereby create a permeation pathway through the plasma membrane. Cx30 and Cx43 have, in their hemichannel configuration, been proposed to act as ion channels and membrane pathways for different molecules, such as fluorescent dyes, ATP, prostaglandins, and glutamate. Published studies about astrocyte hemichannel behavior, however, have been highly variable and/or contradictory. The field of connexin hemichannel research has been complicated by great variability in the experimental preparations employed, a lack of highly specific pharmacological inhibitors and by confounding changes associated with genetically modified animal models. This review attempts to critically assess the gating, inhibition and permeability of astrocytic connexin hemichannels and proposes that connexins in their hemichannel configuration act as gated pores with isoform-specific permeant selectivity. We expect that some, or all, of the controversies discussed here will be resolved by future research and sincerely hope that this review serves to motivate such clarifying investigations.

The role of AQP4 in neuromyelitis optica: More answers, more questions.

Neuromyelitis optica (NMO) is a recurrent inflammatory disease that preferentially targets the optic nerves and spinal cord. The presence of antibodies to the water channel protein aquaporin-4 (AQP4), expressed almost exclusively in astrocytes in the central nervous system (CNS), is a reliable biomarker for NMO. These antibodies, NMO-IgG, may be responsible for the sequential cascade of immune events, including IgG/IgM deposition, infiltration of granulocytes and complement-mediated cytotoxicity (i.e. astrocyte loss) and demyelination. This review summarizes current thinking about the role of NMO-IgG in the pathogenesis of this condition. New insights were also generated along with important additional questions.

Ischemic Preconditioning in White Matter: Magnitude and Mechanism.

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon whereby brief ischemic exposure confers tolerance to a subsequent ischemic challenge. IPC has not been studied selectively in CNS white matter (WM), although stroke frequently involves WM. We determined whether IPC is present in WM and, if so, its mechanism. We delivered a brief in vivo preconditioning ischemic insult (unilateral common carotid artery ligation) to 12- to 14-week-old mice and determined WM ischemic vulnerability [oxygen-glucose deprivation (OGD)] 72 h later, using acutely isolated optic nerves (CNS WM tracts) from the preconditioned (ipsilateral) and control (contralateral) hemispheres. Functional and structural recovery was assessed by quantitative measurement of compound action potentials (CAPs) and immunofluorescent microscopy. Preconditioned mouse optic nerves (MONs) showed better functional recovery after OGD than the non-preconditioned MONs (31 ± 3 vs 17 ± 3% normalized CAP area, p < 0.01). Preconditioned MONs also showed improved axon integrity and reduced oligodendrocyte injury compared with non-preconditioned MONs. Toll-like receptor-4 (TLR4) and type 1 interferon receptor (IFNAR1), key receptors in innate immune response, are implicated in gray matter preconditioning. Strikingly, IPC-mediated WM protection was abolished in both TLR4(-/-) and IFNAR1(-/-) mice. In addition, IPC-mediated protection in WM was also abolished in IFNAR1(fl/fl) LysM(cre), but not in IFNAR1(fl/fl) control, mice. These findings demonstrated for the first time that IPC was robust in WM, the phenomenon being intrinsic to WM itself. Furthermore, WM IPC was dependent on innate immune cell signaling pathways. Finally, these data demonstrated that microglial-specific expression of IFNAR1 plays an indispensable role in WM IPC. SIGNIFICANCE STATEMENT: Ischemic preconditioning (IPC) has been studied predominantly in gray matter, but stroke in humans frequently involves white matter (WM) as well. Here we describe a novel, combined in vivo/ex vivo mouse model to determine whether IPC occurs in WM. It does. Using genetically altered mice, we identified two innate immune cell receptors, Toll-like receptor 4 and type 1 interferon receptor (IFNAR1), that are required for IPC-mediated protection in WM. Furthermore, using microglia-targeted IFNAR1 knockdown, we demonstrate that interferon signaling specifically in microglia is essential for this protection. The discovery of IPC as an intrinsic capability of WM is novel and important. This is also the first in vivo demonstration that cell-type-specific expression of an individual gene plays an indispensable role in IPC-mediated protection.

Dual pathways mediate ß-amyloid stimulated glutathione release from astrocytes.

Oxidative stress plays an important role in the progression of Alzheimer's disease (AD) and other neurodegenerative conditions. Glutathione (GSH), the major antioxidant in the central nervous system, is primarily synthesized and released by astrocytes. We determined if ß-amyloid (Aß42), crucially involved in Alzheimer's disease, affected GSH release. Monomeric Aß (mAß) stimulated GSH release from cultured cortical astrocytes more effectively than oligomeric Aß (oAß) or fibrillary Aß (fAß). Monomeric Aß increased the expression of the transporter ABCC1 (also referred to as MRP1) that is the main pathway for GSH release. GSH release from astrocytes, with or without mAß stimulation, was reduced by pharmacological inhibition of ABCC1. Astrocytes robustly express connexin proteins, especially connexin43 (Cx43), and mAß also stimulated Cx43 hemichannel-mediated glutamate and GSH release. Aß-stimulation facilitated hemichannel opening in the presence of normal extracellular calcium by reducing astrocyte cholesterol level. Aß treatment did not alter the intracellular concentration of reduced or oxidized glutathione. Using a mouse model of AD with early onset Aß deposition (5xFAD), we found that cortical ABCC1 was significantly increased in temporal register with the surge of Aß levels in these mice. ABCC1 levels remained elevated from 1.5 to 3.5 months of age in 5xFAD mice, before plunging to subcontrol levels when amyloid plaques appeared. Similarly, in cultured astrocytes, prolonged incubation with aggregated Aß, but not mAß, reduced induction of ABCC1 expression. These results support the hypothesis that in the early stage of AD pathogenesis, less aggregated Aß increases GSH release from astrocytes (via ABCC1 transporters and Cx43 hemichannels) providing temporary protection from oxidative stress which promotes AD development.

Astrocyte glycogen as an emergency fuel under conditions of glucose deprivation or intense neural activity.

Energy metabolism in the brain is a complex process that is incompletely understood. Although glucose is agreed as the main energy support of the brain, the role of glucose is not clear, which has led to controversies that can be summarized as follows: the fate of glucose, once it enters the brain is unclear. It is not known the form in which glucose enters the cells (neurons and glia) within the brain, nor the degree of metabolic shuttling of glucose derived metabolites between cells, with a key limitation in our knowledge being the extent of oxidative metabolism, and how increased tissue activity alters this. Glycogen is present within the brain and is derived from glucose. Glycogen is stored in astrocytes and acts to provide short-term delivery of substrates to neural elements, although it may also contribute an important component to astrocyte metabolism. The roles played by glycogen awaits further study, but to date its most important role is in supporting neural elements during increased firing activity, where signaling molecules, proposed to be elevated interstitial K(+), indicative of elevated neural firing rates, activate glycogen phosphorylase leading to increased production of glycogen derived substrate.

Activation, permeability, and inhibition of astrocytic and neuronal large pore (hemi)channels.

Astrocytes and neurons express several large pore (hemi)channels that may open in response to various stimuli, allowing fluorescent dyes, ions, and cytoplasmic molecules such as ATP and glutamate to permeate. Several of these large pore (hemi)channels have similar characteristics with regard to activation, permeability, and inhibitor sensitivity. Consequently, their behaviors and roles in astrocytic and neuronal (patho)physiology remain undefined. We took advantage of the Xenopus laevis expression system to determine the individual characteristics of several large pore channels in isolation. Expression of connexins Cx26, Cx30, Cx36, or Cx43, the pannexins Px1 or Px2, or the purinergic receptor P2X7 yielded functional (hemi)channels with isoform-specific characteristics. Connexin hemichannels had distinct sensitivity to alterations of extracellular Ca(2+) and their permeability to dyes and small atomic ions (conductance) were not proportional. Px1 and Px2 exhibited conductance at positive membrane potentials, but only Px1 displayed detectable fluorescent dye uptake. P2X7, in the absence of Px1, was permeable to fluorescent dyes in an agonist-dependent manner. The large pore channels displayed overlapping sensitivity to the inhibitors Brilliant Blue, gadolinium, and carbenoxolone. These results demonstrated isoform-specific characteristics among the large pore membrane channels; an open (hemi)channel is not a nonselective channel. With these isoform-specific properties in mind, we characterized the divalent cation-sensitive permeation pathway in primary cultured astrocytes. We observed no activation of membrane conductance or Cx43-mediated dye uptake in astrocytes nor in Cx43-expressing C6 cells. Our data underscore that although Cx43-mediated transport is observed in overexpressing cell systems, such transport may not be detectable in native cells under comparable experimental conditions.

Novel hypoglycemic injury mechanism: N-methyl-D-aspartate receptor-mediated white matter damage.

OBJECTIVE: Hypoglycemia is a common adverse event and can injure central nervous system (CNS) white matter (WM). We determined whether glutamate receptors were involved in hypoglycemic WM injury. METHODS: Mouse optic nerves (MON), CNS WM tracts, were maintained at 37°C with oxygenated artificial cerebrospinal fluid (ACSF) containing 10mM glucose. Aglycemia was produced by switching to 0 glucose ACSF. Supramaximal compound action potentials (CAPs) were elicited using suction electrodes, and axon function was quantified as the area under the CAP. Amino acid release was measured using high-performance liquid chromatography. Extracellular lactate concentration ([lactate(-)]o) was measured using an enzyme electrode. RESULTS: About 50% of MON axons were injured after 60 minutes of aglycemia (90% after 90 minutes); injury extent was not affected by animal age. Blockade of N-methyl-D-aspartate (NMDA)-type glutamate receptors improved recovery after 90 minutes of aglycemia by 250%. Aglycemic injury was increased by reducing [Mg(2+)]o or increasing [glycine]o , and decreased by lowering pHo , expected results for NMDA receptor-mediated injury. pHo increased during aglycemia due to a drop in [lactate(-)]o. Aglycemic injury was dramatically reduced in the absence of [Ca(2+)]o. Extracellular aspartate, a selective NMDA receptor agonist, increased during aglycemia ([glutamate]o fell). INTERPRETATION: Aglycemia injured WM by a unique excitotoxic mechanism involving NMDA receptors (located primarily on oligodendrocytes). During WM aglycemia, the selective NMDA agonist aspartate is released, probably from astrocytes. Injury is mediated by Ca(2+) influx through aspartate-activated NMDA receptors made permeable by an accompanying alkaline shift in pHo caused by a fall in [lactate(-)]o. These insights have important clinical implications.

Glycogen function in adult central and peripheral nerves.

We studied the roles of glycogen in axonal pathways of the central nervous system (CNS) and peripheral nervous system (PNS). By using electrophysiological recordings, in combination with biochemical glycogen assay, it was possible to determine whether glycogen was crucial to axon function under different conditions. Glycogen was present both in mouse optic nerve (MON) and in mouse sciatic nerve (MSN). Aglycemia caused loss of the compound action potential (CAP) in both pathways after a latency of 15 min (MON) and 120 min for myelinated axons (A fibers) in the MSN. With the exception of unmyelinated axons (C fibers) in the MSN, CAP decline began when usable glycogen was exhausted. Glycogen was located in astrocytes in the MON and in myelinating Schwann cells in the MSN; it was absent from the Schwann cells surrounding unmyelinated C fibers. In MON, astrocytic glycogen is metabolized to lactate and "shuttled" to axons to support metabolism. The ability of lactate to support A fiber conduction in the absence of glucose suggests a common pathway in both the CNS and the PNS. Lactate is released from MON and MSN in substantial quantities. That lactate levels fall in MSN in the presence of diaminobenzidine, which inhibits glycogen phosphorylase, strongly suggests that glycogen metabolism contributes to lactate release under resting conditions. Glycogen is a "backup" energy substrate in both the CNS and the PNS and, beyond sustaining excitability during glucose deprivation, has the capacity to subsidize the axonal energy demands during times of intense activity in the presence of glucose.

High titers of autoantibodies to glutamate decarboxylase in type 1 diabetes patients: epitope analysis and inhibition of enzyme activity.

OBJECTIVE: Autoantibodies to glutamate decarboxylase (GAD65Ab) are found in patients with autoimmune neurological disorders or type 1 diabetes. The correct diagnosis of GAD65Ab-associated neurological disorders is often delayed by the variability of symptoms and a lack of diagnostic markers. We hypothesized that the frequency of neurological disorders with high GAD65Ab titers is significantly higher than currently recognized. METHODS: We analyzed GAD65Ab titer, GAD65 enzyme activity inhibition, and GAD65Ab epitope pattern in a cohort of type 1 diabetes patients (n = 100) and correlated our findings with neurological symptoms and diseases. RESULTS: Overall, 43% (43/100) of patients had detectable GAD65Ab titers (median = 400 U/mL, range: 142-250,000 U/mL). The GAD65Ab titers in 10 type 1 diabetes patients exceeded the 90th percentile of the cohort (2,000-250,000 U/mL). Sera of these 10 patients were analyzed for their GAD65Ab epitope specificity and their ability to inhibit GAD65 enzyme activity in vitro. GAD65Ab of 5 patients inhibited the enzyme activity significantly (by 34-55%). Three patients complained of muscle stiffness and pain, which was documented in 2 of these patients. CONCLUSIONS: Based on our findings, we suggest that neurological disorders with high GAD65Ab titers are more frequent in type 1 diabetes patients than currently recognized.

Schwann cell glycogen selectively supports myelinated axon function.

OBJECTIVE: Interruption of energy supply to peripheral axons is a cause of axon loss. We determined whether glycogen was present in mammalian peripheral nerve, and whether it supported axon conduction during aglycemia. METHODS: We used biochemical assay and electron microscopy to determine the presence of glycogen, and electrophysiology to monitor axon function. RESULTS: Glycogen was present in sciatic nerve, its concentration varying directly with ambient glucose. Electron microscopy detected glycogen granules primarily in myelinating Schwann cell cytoplasm, and these diminished after exposure to aglycemia. During aglycemia, conduction failure in large myelinated axons (A fibers) mirrored the time course of glycogen loss. Latency to compound action potential (CAP) failure was directly related to nerve glycogen content at aglycemia onset. Glycogen did not benefit the function of slow-conducting, small-diameter unmyelinated axons (C fibers) during aglycemia. Blocking glycogen breakdown pharmacologically accelerated CAP failure during aglycemia in A fibers, but not in C fibers. Lactate was as effective as glucose in supporting sciatic nerve function, and was continuously released into the extracellular space in the presence of glucose and fell rapidly during aglycemia. INTERPRETATION: Our findings indicated that glycogen is present in peripheral nerve, primarily in myelinating Schwann cells, and exclusively supports large-diameter, myelinated axon conduction during aglycemia. Available evidence suggests that peripheral nerve glycogen breaks down during aglycemia and is passed, probably as lactate, to myelinated axons to support function. Unmyelinated axons are not protected by glycogen and are more vulnerable to dysfunction during periods of hypoglycemia. .