Effect of glucose concentration on peripheral nerve and its response to anoxia

Amyloid-β Effects on Peripheral Nerve: A New Model System

Although there are many biochemical methods to measure amyloid-β (Aβ)42 concentration, one of the critical issues in the study of the effects of Aβ42 on the nervous system is a simple physiological measurement. The in vitro rat sciatic nerve model is employed and the nerve action potential (NAP) is quantified with different stimuli while exposed to different concentrations of Aβ42. Aβ42 predominantly reduces the NAP amplitude with minimal effects on other parameters except at low stimulus currents and short inter-stimulus intervals. The effects of Aβ42 are significantly concentration-dependent, with a maximum reduction in NAP amplitude at a concentration of 70 nM and smaller effects on the NAP amplitude at higher and lower concentrations. However, even physiologic concentrations in the range of 70 pM did reduce the NAP amplitude. The effects of Aβ42 became maximal 5-8 h after exposure and did not reverse during a 30 min washout period. The in vitro rat sciatic nerve model is sensitive to the effects of physiologic concentrations of Aβ42. These experiments suggest that the effect of Aβ42 is a very complex function of concentration that may be the result of amyloid-related changes in membrane properties or sodium channels.

Parasympathetic tonus in type 2 diabetes and pre-diabetes and its clinical implications

Autonomic imbalance reflected by higher resting heart rate and reduced parasympathetic tone may be driven by low-grade inflammation (LGI) and impaired glycemic control in type 2 diabetes mellitus (T2DM) and pre-diabetes. We examined the interaction of parasympathetic components of heart rate variability (HRV), variables of LGI, and glucose metabolism in people with T2DM, pre-diabetes, and normal glucose metabolism (NGM). We recorded HRV by Holter (48 h) in 633 community-dwelling people of whom T2DM n = 131, pre-diabetes n = 372, and NGM n = 130 and mean HbA1c of 7.2, 6.0 and 5.3%, respectively. Age was 55-75 years and all were without known cardiovascular disease except from hypertension. Fasting plasma glucose, fasting insulin, HOMA-IR, HbA1c and LGI (CRP, Interleukin-18 (IL-18), and white blood cells) were measured. Root-mean-square-of-normal-to-normal-beats (RMSSD), and proportion of normal-to-normal complexes differing by more than 50 ms (pNN50) are accepted measures of parasympathetic activity. In univariate analyses, RMSSD and pNN50 were significantly inversely correlated with level of HbA1c and CRP among people with T2DM and pre-diabetes, but not among NGM. RMSSD and pNN50 remained significantly inversely associated with level of HbA1c after adjusting for age, sex, smoking, and BMI among people with T2DM (β = - 0.22) and pre-diabetes (β = - 0.11); adjustment for LGI, HOMA-IR, and FPG did not attenuate these associations. In backward elimination models, age and level of HbA1c remained associated with RMSSD and pNN50. In people with well controlled diabetes and pre-diabetes, a lower parasympathetic activity was more related to age and HbA1c than to markers of LGI. Thus, this study shows that the driver of parasympathetic tonus may be more the level of glycemic control than inflammation in people with prediabetes and well controlled diabetes.

Finite Element Analysis of Glucose Diffusivity in Cellulose Nanofibril Peripheral Nerve Conduits

Peripheral neuropathy arising from physical trauma is estimated to afflict 20 million people in the United States alone. In one common surgical intervention, neural conduits are placed over the nerve stumps to bridge the gap and create a microenvironment conducive to regeneration. It has been proposed that a biocompatible material such as cellulose nanofiber may serve as a viable conduit material, providing a non-inflammatory and mechanically stable system. Preliminary studies have shown that cellulose nanofiber conduits successfully aid neural regeneration and further, that the dimensions of the conduit relative to the nerve gap have an impact on efficacy in murine models. It has been hypothesized that the reliance of regeneration upon the physical dimensions of the conduit may be related to modified modes of diffusion and/or distances of key cellular nutrients and waste metabolites to/from the injury site. The present work investigates the concentration profile of glucose within the conduit via finite element analysis as a function of the physical dimensions of the conduit. It was determined that the magnitude of glucose diffusion was greater through the conduit walls than through the luminal space between the nerve and the inner wall of the conduit, and that as such radial diffusion is dominant over axial diffusion.

Fibre sub-type specific conduction reveals metabolic function in mouse sciatic nerve

KEY POINTS

We have developed an improved method that enables simultaneous recording of stimulus evoked compound action potentials from large myelinated A fibres and small unmyelinated C fibres in mouse sciatic nerves. Investigations into the ability of fructose to support conduction in sciatic nerve revealed a novel glia-to-axon metabolic pathway in which fructose is converted in Schwann cells to lactate for subsequent shuttling to A fibres. The C fibres most likely directly take up and metabolise fructose. These differences are indicative of fibre sub-type specific metabolic profiles. These results demonstrate that the physiological insights provided by the method can be applied to investigations of peripheral nerve, with a view to understanding the metabolic disruptions that underlie diabetic neuropathy.

ABSTRACT

The stimulus evoked compound action potential (CAP), recorded using suction electrodes, provides an index of the relative number of conducting axons within a nerve trunk. As such the CAP has been used to elucidate the diverse mechanisms of injury resulting from a variety of metabolic insults to central nervous white matter, whilst also providing a model with which to assess the benefits of clinically relevant neuroprotective strategies. In addition the technique lends itself to the study of metabolic cell-to-cell signalling that occurs between glial cells and neurones, and to exploring the ability of non-glucose substrates to support axon conduction. Although peripheral nerves are sensitive to metabolic insult and are susceptible to diabetic neuropathy, there is a lack of fundamental information regarding peripheral nerve metabolism. A confounding factor in such studies is the extended duration demanded by the experimental protocol, requiring stable recording for periods of many hours. We describe a method that allows us to record simultaneously the stimulus evoked CAPs from A and C fibres from mouse sciatic nerve, and demonstrate its utility as applied to investigations into fibre sub-type substrate use. Our results suggest that C fibres directly take up and metabolise fructose, whereas A fibre conduction is supported by fructose-derived lactate, implying there exist unique metabolic profiles in neighbouring fibre sub-types present within the same nerve trunk.

Modulating peripheral nerve damage from hyperglycemia/anoxia

INTRODUCTION

Combined hyperglycemia and anoxia have a marked negative effect on peripheral nerve not seen with either alone. We studied whether nerve injury is related to the presence of hyperglycemia in the oxygenated state, anoxic state, or both.

METHODS

We performed recordings of the nerve action potential (NAP) from rat sciatic nerve in a perfusion apparatus with different metabolic substrates during anoxia and oxygenated state.

RESULTS

The NAP was best preserved when one of the perfusates contained no glucose. Transient improvements were seen with normoglycemic perfusates or perfusates that contained substrates that were poorly used in glycolysis/gluconeogenesis. Hyperglycemia has important negative effects in both the oxygenated and anoxic states.

CONCLUSIONS

Deleterious effects of hyperglycemia on the peripheral nerve may relate to metabolic products produced during the oxygenated state, such as glycogen and the effects of reperfusion after prolonged periods of anoxic metabolism. Muscle Nerve, 2016 Muscle Nerve 55: 735-740, 2017.

Glycolysis selectively shapes the presynaptic action potential waveform

Mitochondria are major suppliers of cellular energy in neurons; however, utilization of energy from glycolysis vs. mitochondrial oxidative phosphorylation (OxPhos) in the presynaptic compartment during neurotransmission is largely unknown. Using presynaptic and postsynaptic recordings from the mouse calyx of Held, we examined the effect of acute selective pharmacological inhibition of glycolysis or mitochondrial OxPhos on multiple mechanisms regulating presynaptic function. Inhibition of glycolysis via glucose depletion and iodoacetic acid (1 mM) treatment, but not mitochondrial OxPhos, rapidly altered transmission, resulting in highly variable, oscillating responses. At reduced temperature, this same treatment attenuated synaptic transmission because of a smaller and broader presynaptic action potential (AP) waveform. We show via experimental manipulation and ion channel modeling that the altered AP waveform results in smaller Ca²⁺ influx, resulting in attenuated excitatory postsynaptic currents (EPSCs). In contrast, inhibition of mitochondria-derived ATP production via extracellular pyruvate depletion and bath-applied oligomycin (1 μM) had no significant effect on Ca²⁺ influx and did not alter the AP waveform within the same time frame (up to 30 min), and the resultant EPSC remained unaffected. Glycolysis, but not mitochondrial OxPhos, is thus required to maintain basal synaptic transmission at the presynaptic terminal. We propose that glycolytic enzymes are closely apposed to ATP-dependent ion pumps on the presynaptic membrane. Our results indicate a novel mechanism for the effect of hypoglycemia on neurotransmission. Attenuated transmission likely results from a single presynaptic mechanism at reduced temperature: a slower, smaller AP, before and independent of any effect on synaptic vesicle release or receptor activity.

Effect of hypoxia on heart rate variability and baroreflex sensitivity during hypoglycemia in type 1 diabetes mellitus

PURPOSE

Patients with type 1 diabetes mellitus exhibit impairments in autonomic and cardiovascular control which are worsened with acute hypoglycemia--thus increasing the risk of adverse cardiovascular events. Hypoxia, as seen with the common comorbidity of sleep apnea, may lead to further autonomic dysfunction and an increased risk of ventricular arrhythmias. Therefore, we hypothesized that heart rate variability (HRV) and baroreflex sensitivity (BRS) would be reduced during hypoglycemia in adults with type 1 diabetes, with a further decline when combined with hypoxia.

METHODS

Subjects with type 1 diabetes (n = 13; HbA1c = 7.5 ± 0.3 %, duration of diabetes = 17 ± 5 yrs) completed two 180 min hyperinsulinemic (2 mU/kg TBW/min), hypoglycemic (~3.3 µmol/mL) clamps separated by a minimum of 1 week and randomized to normoxia (SpO2 ~98 %) or hypoxia (SpO2 ~85 %). Heart rate (electrocardiogram) and blood pressure (finger photoplethysmography) were analyzed at baseline and during the hypoglycemic clamp for measures of HRV and spontaneous cardiac BRS (sCBRS).

RESULTS

Hypoglycemia resulted in significant reductions in HRV and sCBRS when compared with baseline levels (main effect of hypoglycemia: p < 0.05). HRV and sCBRS were further impaired during hypoxia (main effect of hypoxia: p < 0.05).

CONCLUSIONS

Acute hypoxia worsens hypoglycemia-mediated impairments in autonomic and cardiovascular control in patients with type 1 diabetes and may increase the risk of cardiovascular mortality. These results highlight the potential cumulative dangers of hypoglycemia and hypoxia in this vulnerable population.

Acute anoxic changes in peripheral nerve: anatomic and physiologic correlations

INTRODUCTION

The response of the peripheral nerve to anoxia is modulated by many factors including glucose and temperature. The purposes of this article are to demonstrate the effects of these factors on the pathological changes induced by anoxia and to compare the electrophysiologic changes and pathological changes in the same nerves.

METHODS

Sciatic nerves were harvested from rats and placed in a perfusion apparatus where neurophysiologic responses could be recorded continuously during a 16 h experiment. After the experiment, light microscopy and electron microscopy were performed.

RESULTS

Light microscopic images showed mild changes from anoxia at normoglycemia. Hypoglycemic anoxia produced massive axonal swelling while hyperglycemic anoxia produced apparent changes in the myelin. Anoxic changes were not uniform in all axons. Electron microscopy showed only minor disruptions of the cytoskeleton with anoxia during normoglycemia. At the extremes of glucose concentration especially with hyperglycemia, there was a more severe disruption of intermediate filaments and loss of axonal structure with anoxia. Hypothermia protected axons from the effect of anoxia and produced peak axonal swelling in the 17-30°C range.

CONCLUSIONS

The combination of hyperglycemia or hypoglycemia and anoxia produces extremely severe axonal disruption. Changes in axonal diameter are complex and are influenced by many factors.

Anoxia-induced changes in optimal substrate for peripheral nerve

Hyperglycemia accentuates the injury produced by anoxia both in the central and peripheral nervous system. To understand whether this is a consequence of changes in metabolic pathways produced by anoxia, the effect of the metabolic substrate used by the rat peripheral nerve on the nerve action potential (NAP) was studied in the presence and absence of anoxia. In the continuously oxygenated state, the NAP was well preserved with glucose, lactate, as well as with high concentrations of sorbitol and fructose but not β-hydroxybutyrate, acetate or galactose. With intermittent anoxia, the pattern of substrate effects on the NAP changed markedly so that low concentrations of fructose became able to support neurophysiologic activity but not high concentrations of glucose. These alterations occurred gradually with repeated episodes of anoxia as reflected by the progressive increase in the time needed for the NAP to disappear during anoxia when using glucose as substrate. This "preconditioning" effect was not seen with other substrates and an opposite effect was seen with lactate. In fact, the rate at which the NAP disappeared during anoxia was not simply related to degree of recovery after anoxia. These are distinct phenomena. For example, the NAP persisted longest during anoxia in the setting of hyperglycemia but this was the state in which the anoxic damage was most severe. Correlating the results with existing literature on the metabolic functions of Schwann cells and axons generates testable hypotheses for the mechanism of hyperglycemic damage during anoxia and lead to discussions of the role for a metabolic shuttle between Schwann cells and axons as well as a potential important role of glycogen.

Effects of pH on the response of peripheral nerve to anoxia

PURPOSE/AIM

Changes in pH are not infrequently encountered in clinical situations and can be associated with significant effects on ion channels, mitochondria and axon function. The purpose of this paper is to study the modulatory effects of pH on the anoxic response in peripheral nerve.

MATERIALS AND METHODS

A total of 48 rat sciatic nerves were studied in vitro in a perfusion apparatus. Experiments were carried out at 6 pH levels from 6.0 to 7.8.

RESULTS

The amplitude of the nerve action potential (NAP) drops more dramatically with repetitive periods of anoxia when the pH is reduced below 6.5. In addition, velocity decreases and duration increases more with each cycle of anoxia at low pH values. Despite these effects of pH on recovery after anoxia, there was no significant effect of pH on the time course of changes during anoxia. During recovery from anoxia, the NAP recovered more slowly when the pH was lowered.

CONCLUSIONS

The pattern of changes in amplitude, velocity and duration suggest that they may be due to interference of high hydrogen ion concentrations with sodium and potassium channel function.