The Role of Nervous System Glycogen during Hypoglycemia

Review Article

J Endocr Disord. 2016; 3(1): 1020.

The Role of Nervous System Glycogen during Hypoglycemia

Rockliffe AM¹, Ebling FJP and Brown AM1,2*

¹School of Life Sciences, University of Nottingham, UK

²Department of Neurology, University of Washington, USA

*Corresponding author: Brown AM, School of Life Sciences, University of Nottingham, Queens Medical Centre, Nottingham, NG7 2UH, UK

Received: December 18, 2015; Accepted: April 18, 2016; Published: April 21, 2016


The mammalian brain contains glycogen but in concentrations much lower than in the liver and skeletal muscle, thus a role as a functional energy reserve has been dismissed. Glycogen in the central nervous system is located in astrocytes in the adult, and in the peripheral nervous system is expressed in myelinating Schwann cells. During periods of experimental aglycemia in rodent optic nerve, a model of central white matter, the stimulus evoked Compound Action Potential (CAP) is sustained for up to 30 minutes, and thereafter rapidly falls to zero. Optic nerve glycogen decreases during aglycemia and is exhausted after about 30 minutes. This temporal correlation between glycogen content and maintenance of the CAP suggests that in the face of aglycemia, glycogen supports conduction, but once the limited glycogen stores are exhausted the CAP fails. The glycogen is metabolised to lactate, which is shuttled from the astrocyte to the axon to serve as a transportable energy substrate. In the peripheral nervous system a similar scheme occurs in which Schwann cell glycogen supports conduction of large myelinated A fibres during aglycemia via. transfer of the glycogen-derived conduit lactate. The smaller unmyelinated C fibres do not benefit from the presence of glycogen. However inhibiting glycogen metabolism with DAB during aglycemia abolishes any benefit the A fibres derive from glycogen, and their latency to failure resembles that of the C fibres.

Keywords: Glycogen; Astrocyte; Hypoglycemia; Schwann cell; Isofagomine; DAB; D-lactate


The human brain has an absolute reliance on blood borne glucose in order to function, although the actual energy substrate that the individual brain cells use may not be glucose, but a glucosederived substrate such as lactate. Endocrine functions are well developed to maintain systemic blood glucose levels such that the blood delivers glucose to the brain well in excess of demand. However certain pathological conditions such as insulinomas, and the iatrogenic consequences of mismatch between insulin delivery and prevailing systemic blood glucose levels in patients suffering from type 1 diabetes, can result in insufficient glucose delivery to the brain to support normal function. Under such conditions the brain suffers pathological consequences, which can range in severity from autonomic warning signals, such as trembling and hunger pangs for limited periods of hypoglycemia, but ultimately to death for extended periods of hypoglycemia. There are currently no clinically relevant neuroprotective therapies available to preserve brain tissue in the event of hypoglycemia.

Iatrogenic hypoglycemia

The normal blood glucose concentration varies between 4 and 7.2 mmol l-1 with complex endocrine functions responsible for maintaining this narrow normoglycemic range [1]. This regulation requires the actions of the complementary hormones insulin and glucagon, both released from the pancreas [1]. In times of plenty when glucose is abundant, insulin is released from the pancreatic beta cells and acts upon surface bound glucose transporters (Glut 4) to facilitate the transmembrane movement of glucose into cells [2], with the glucose incorporated into the glycogen macromolecule [2]. Glucagon acts in response to falling glucose levels and liberates glucose from glycogen storage in order to elevate blood glucose levels [3]. This intricate balance between glucose levels and hormone release is disrupted in type 1 diabetes, where autoimmune destruction of insulin secreting cells renders the sufferer unable to regulate glucose levels resulting in uncontrolled hyperglycemia [1]. Prior to the advent of insulin therapy in 1922 [4] a diagnosis of type 1 diabetes was an almost certain death sentence, but the clinical intervention of applying exogenous insulin is a very effective therapy, which if rigidly adhered to, allows patients to lead a relatively normal life [1]. However this therapy has one major drawback, namely the systemic hypoglycemia that starves the brain of glucose when insulin administration is mismatched to prevailing glucose levels [5]. The fear of such hypoglycemic episodes is the primary reasons sufferers of type 1 diabetes do not adhere strictly to such therapies, and is indirectly the cause of numerous pathologies, such a retinopathy, neuropathy and nephropathy, that result from persistent hyperglycemia [6].

Hypoglycemia symptoms

The effects of hypoglycemia can be broadly divided into two, autonomic symptoms and neuroglycopenic symptoms [1]. In the event of a hypoglycemic episode, as the blood glucose falls to levels below 4 mM, there occurs a reaction mediated by the autonomic nervous system whose symptoms include the following: sweating, trembling, difficulty concentrating, tenseness and light headedness, and dizziness [1]. Such symptoms are a warning of an impending hypoglycemic episode, which can be avoided if the patient rapidly ingests concentrated glucose in the form of a gel or high-energy drink. If no such interventions are made hypoglycemia can progress to evoke neuroglycopenic symptoms, which include confusion, drowsiness, unpredictable behaviour, speech difficulty and the loss of co-ordination [1]. Such symptoms may render the patient incapable of the reasoning required to take counter measures and as such are potentially life threatening. The ultimate pathology associated with hypoglycemia is neuronal death, which can occur relatively rapidly and encompass broad regions of the brain [7-9]. The autonomic warning symptoms at the onset of an impending hypoglycemic episode are complicated by the phenomenon of hypoglycemia unawareness. This is a condition in which the warning signs are missed by the patient for reasons as yet unknown. It is proposed that successive hypoglycemic episodes cause a pathological change to the autonomic warning signs, such that the threshold for the onset of these symptoms occurs at glucose concentrations lower than those that trigger the neuroglycopenic symptoms [6]. The pathology associated with hypoglycemia in the brain has been documented and consist of neuron death, with the intriguing aspect that some brain regions are more sensitive to hypoglycemic damage than others. However care must be taken to isolate the effects of hypoglycemia from those long-term effects of type 1 diabetes [8].

Glycogen in the central nervous system

The dogmatic view of brain energy metabolism is that there are no significant energy reserves within the brain [2], in the manner of glycogen in the liver or skeletal muscle, rendering the brain exquisitely sensitive to shortfalls in glucose delivery, and this view is supported by the following compelling evidence. Occlusion of the carotid artery renders human unconscious within 6 to 8 seconds [10], neuroglycopenic symptoms are temporally correlated with hypoglycemia [11], and no significant energy reserves have been located within the brain [2]. However, the brain must be assessed for the unique organ that it is, and assessment of its energy requirements accordingly, rather than comparison with organs such as the liver and skeletal muscle.

The most likely source of an energy supply in the brain, given its absolute reliance on glucose, is glycogen. Glycogen was first identified in the brain by biochemical assay [12], and later by electron microscopy, but interest in it as a functional entity was limited due to the very low concentration in which it occurs relative to the liver and skeletal muscle. The paucity of brain glycogen suggests that it does not play the role of an energy reserve in the manner of liver glycogen. To understand the role of brain glycogen we must carefully analyse the preliminary studies that led to suggestions of it as having an important role in energy metabolism, and these experiments concerned functional aspects of in vitro sections of brain exposed to hypoglycemia.

Effect of systemic hypoglycemia on brain glycogen

The advent of insulin therapy also introduced the spectre of hypoglycemia as a therapeutic treatment to alleviate symptoms of schizophrenia [13]. The widespread uptake of this practice demanded a more detailed knowledge of the effects of systemic hypoglycemia on brain function, and such studies commenced in the 1940s and 1950s. In a study on dogs, hypoglycemia provoked a decrease in glycogen content in areas of the brain recognized for their high metabolic rate, with the revealing detail being the depletion of glycogen depended on the metabolic rate of the region, and not the initial concentration [14]. Comparable studies in the rabbit showed a significant fall in glycogen in the brain after a period of hypoglycemia [15]. Collectively these studies suggested for the first time a link between brain glycogen content and brain function. Such basic biochemical studies have been supplemented with NMR spectroscopic data showing that insulininduced hypoglycemia is rats depletes glycogen content [16], results recently confirmed in human subjects [17,18]. Although a clear correlation between insulin induced hypoglycemia and brain glycogen has been established, the finer details of the brain glycogen metabolism required more invasive techniques, and these subsequent experiments have tended to be carried out in vitro. An initial study, although not intending to study any direct role of glycogen, highlighted the dependent nature between neuron survival and glycogen. Hypothalamic neurons that were co-cultured with astrocytes survived for longer periods than neurons cultured in isolation, linking the fate of neurons to the presence of astrocytes [19]. However it was not merely the presence of the astrocytes that sustained the neurons, as a subsequent study of cultured cortical cells demonstrated. Cortical neurons co-cultured with astrocytes in conditions designed to deplete glycogen showed decreased survival compared to neurons co-cultured with astrocytes with plentiful glycogen [20,21]. Thus the presence of glycogen was the key factor that promoted neuron survival. It is one of the key features of glycogen that it is located almost exclusively in astrocytes in the adult mammalian brain [22]. Such a cellular location dictates certain fundamental aspects of the role of glycogen, the most important of which is that there must be transport of energy substrate from the astrocyte to the neural (axon / neuron) element in order for the neuron to benefit from the presence of glycogen.

Glycogen supports neural function

The dominant experimental manoeuvre to study the role of glycogen during hypoglycaemia is to completely remove glucose from the tissue, i.e. expose the tissue to aglycemic conditions. Although such conditions are extreme and never occur even in the most extreme case of iatrogenic hypoglycemia in humans, it is a very useful protocol as it removes confusion as to whether glucose and / or glycogen is supporting function. In hippocampal slices of rat in which the stimulus evoked Excitatory Post Synaptic Potentials (EPSPs) from the CA1 region were recorded, EPSP slope was maintained in aCSF containing 10 mM glucose, but substitution with aCSF devoid of glucose led to delayed attenuation of the EPSP slope [23]. This delay could be decreased by pre-exposure of the hippocampal slice to conditions that would deplete the tissue of glycogen, prior to introduction of aglycemia [24]. In the rodent optic nerve model, a popular model of central white matter [25], the stimulus evoked Compound Action Potential (CAP) is a useful model of axon conduction, as it allows post insult area of the CAP to be compared to baseline to estimate the degree of injury incurred by the tissue [26]. Exposure of the Rat Optic Nerve (RON) to anoxic conditions resulted in a rapid fall of the CAP area to zero in less than 5 minutes [27]. However exposure of the RON to aglycemia led to a delayed failure of the CAP, up to 30 minutes, after introduction of aglycemia [28]. Such an extended latency to failure suggested that there was an endogenous energy reserve that could sustain function in the absence of glucose, but that the energy reserve was limited and was exhausted within 30 minutes, after which function could not be supported. This energy reserve is glycogen. From these preliminary findings the role of glycogen in the optic nerve was examined over the next decade, and its role under hypoglycemic and normal conditions was unravelled.

Based on the initial findings that in the absence of exogenously applied glucose the CAP was sustained for 30 minutes [28] the following hypothesis were developed, which could be tested experimentally.

These hypotheses were tested as follows in the RON. As previously described withdrawal of glucose from the optic nerve previously incubated in an aCSF containing 10 mM glucose causes the CAP to fail about 30 minutes after aglycemia introduction. A parallel set of experiments were carried out in which the RONs were exposed to aglycemia but harvested every 10 minutes, and the glycogen content assessed by biochemical assay. The basal level of glycogen was about 7 pmol μg protein-1, but this decreased over the period of aglycemia to the extent that glycogen content had fallen to 2 pmol μg protein-1 after 30 minutes of aglycemia, and from this point it fell no further. Some glucosyl molecules remain bound to the glycogenin skeleton, so although they can be measured biochemically they are non functional and thus 2 pmol μg protein-1 is equivalent to zero glycogen. The glycogen fell at a constant rate during aglycemia while the CAP was fully supported. However once the glycogen had reached its nadir, the CAP fell rapidly to zero [29]. Such a scenario suggests that during aglycemia glycogen metabolism is activated and the glycogen is broken down to support CAP function. However once all the glycogen has been exhausted there are no other energy reserves available to support the CAP and it fails rapidly.

Glycogen can be up or down regulated according to the glucose concentration the tissue is bathed in, thus incubating the nerve in high glucose (25 mM) results in elevated glycogen levels compared to control (10 mM) glucose. Glycogen can also be down regulated by incubating with 1 mM nor adrenaline, thus the glycogen content can be varied over a 3 fold range by incubating in 1 mM nor-adrenaline (lowest), 10 mM glucose (intermediate) or 25 mM glucose (highest). Incubating nerves in the above conditions prior to introduction of aglycemia led to differences in the latency to CAP failure, that are consistent with elevated glycogen content increasing the latency to CAP failure during subsequent aglycemia, and down regulating glycogen content leading to the opposite effect [29] (Figure 1).