Axonal Excitability Research Articles

Novel therapeutic for multiple sclerosis protects white matter function in EAE mouse model.

Multiple sclerosis (MS) is a chronic demyelinating disease with prominent axon dysfunction. Our previous studies in an MS mouse model, experimental autoimmune encephalomyelitis (EAE), demonstrated that major histocompatibility complex Class II constructs can reverse clinical signs of EAE. These constructs block binding and downstream signaling of macrophage migration inhibitory factors (MIF-1/2) through CD74, thereby inhibiting phosphorylation of extracellular signal-regulated kinase (ERK) activation and tissue inflammation and promoting remyelination. To directly assess the effects of a novel third generation construct, DRhQ, on axon integrity in EAE, we compared axon conduction properties using electrophysiology on corpus callosum slices and optic nerves. By using two distinct white matter (WM) tracts, we aimed to assess the impact of the EAE and the benefit of DRhQ on myelinated and unmyelinated axons as well as to test the clinical value of DRhQ on demyelinating lesions in CC and optic myelitis. Our study found that EAE altered axon excitability, delayed axon conduction and slowed spatiotemporal summation correlated with diffuse astrocyte and microglia activation. Because MS predisposes patients to stroke, we also investigated and showed that vulnerability to WM ischemia is increased in the EAE MS mouse model. Treatment with DRhQ after the onset of EAE drastically inhibited microglial and astrocyte activation, improved functional integrity of the myelinated axons and enhanced recovery after ischemia. These results demonstrate that DRhQ administered after the onset of EAE promotes WM integrity and function, and reduces subsequent vulnerability to ischemic injury, suggesting important therapeutic potential for treatment of progressive MS.

Prospective assessment of vincristine-induced peripheral neuropathy in paediatric acute lymphoblastic leukemia

ObjectiveVincristine is a mainstay treatment for paediatric cancers, particularly acute lymphoblastic leukemia (ALL), with common toxicity including vincristine-induced peripheral neuropathy (VIPN). The present study comprehensively assessed VIPN outcomes in patients receiving vincristine treatment for ALL. MethodsChildren diagnosed with ALL commencing vincristine treatment were prospectively evaluated (baseline, post-induction, pre-reinduction, post-reinduction, follow-up). VIPN was examined clinically using the Balis sensory/motor scale, neurophysiologically using axonal excitability techniques and quality-of-life using Pediatric Quality of Life Inventory. ResultsThirty-one patients were recruited to this study (age = 6.8 ± 4.4; 61.3% female). Incidence of motor VIPN (motor Balis grade > 0) symptoms were higher than sensory VIPN (sensory Balis grade > 0) at post-induction (92.0% vs 36.0%) and post-reinduction (81.8% vs 22.7%) vincristine treatment. Neurophysiological assessment also demonstrated greater change in motor axonal excitability parameters compared to sensory parameters including changes in depolarising threshold electrotonus (P < 0.0125), superexcitability and subexcitability parameters (all P < 0.0125). Follow-up assessment demonstrated persisting VIPN symptoms with reduced quality-of-life scores compared to baseline. ConclusionsClinical and neurophysiological evaluation of VIPN suggests vincristine produces a motor-prominent sensorimotor neuropathy in children which persisted at follow-up. SignificanceVIPN signs and symptoms develop early in the treatment course, in line with axonal excitability profiles. Early detection of significant nerve changes may support timely implementation of neuroprotection strategies.

D.2 Cross-sectional axonal excitability and motor unit number index profile in early stages of weakness in ALS

Background: What is the number and size of motor units, and axonal excitability profile in the early stages of muscle weakness in ALS compared to controls? Methods: We enrolled ALS patients with APB manual strength testing rated four or four-minus (ALS:4-arm) and four-plus (ALS:4+ arm) and control participants &gt;35 years-old from the University of Toronto, University of Alberta and Universidade Federal de Sao Paulo. Mean±SD, one-way ANOVA and ANCOVA of ALSFRS-R, PUMN Score, MUNIX, MUSIX, and nerve-excitability testing using QTRAC TROND protocol were reported. Results: Twenty-five ALS patients and 63 controls were included. Mean MUNIX was significantly lower (p&lt;0.0001) and MUSIX was significantly higher (p&lt;0.001) in both ALS groups compared to controls. Mean strength-duration time constant in the ALS:4- arm (0.50ms±0.11; p&lt;0.05) and superexcitability in both ALS groups (ALS:4- -29.05%±9.24, ALS:4+ -27.67%±8.03; p&lt;0.05) were relatively increased, supporting axonal hyperexcitability. Conclusions: Significant motor unit loss measured by MUNIX is already present at the earliest detection of muscle weakness in ALS. Increased MUSIX and altered axonal physiology are associated with axonal sprouting and geometry change(1), along with ion channel dysfunction(2). Future trials targeting muscle weakness in ALS should consider the altered neuronal physiology during early disease stages and utilize neurophysiological biomarkers only in normal-to-mildly weak muscles.

Altered flexor carpi radialis motor axon excitability properties after cerebrovascular stroke.

Spinal motoneurons may become hyperexcitable after a stroke. Knowledge about motoneuron hyperexcitability remains clinically important as it may contribute to a number of phenomena including spasticity, flexion synergies, and abnormal limb postures. Hyperexcitability seems to occur more often in muscles that flex the wrist and fingers (forearm flexors) compared to other upper limb muscles. The cause of hyperexcitability remains uncertain but may involve plastic changes in motoneurons and their axons. To characterize intrinsic membrane properties of flexor carpi radialis (FCR) motor axons after stroke using nerve excitability testing. Nerve excitability testing using threshold tracking techniques was applied to characterize FCR motor axon properties in persons who suffered a first-time unilateral cortical/subcortical stroke 23 to 308 days earlier. The median nerve was stimulated at the elbow bilaterally in 16 male stroke subjects (51.4 ± 2.9 y) with compound muscle action potentials recorded from the FCR. Nineteen age-matched males (52.7 ± 2.4 y) were also tested to serve as controls. Axon parameters after stroke were consistent with bilateral hyperpolarization of the resting potential. Nonparetic and paretic side axons were modeled by a 2.6-fold increase in pump currents (IPumpNI) together with an increase (38%-33%) in internodal leak conductance (GLkI) and a decrease (23%-29%) in internodal H conductance (Ih) relative to control axons. A decrease (14%) in Na+ channel inactivation rate (Aah) was also needed to fit the paretic axon recovery cycle. "Fanning out" of threshold electrotonus and the resting I/V slope (stroke limbs combined) correlated with blood potassium [K+] (R = -0.61 to 0.62, p< 0.01) and disability (R = -0.58 to 0.55, p < 0.05), but not with spasticity, grip strength, or maximal FCR activity. In contrast to our expectations, FCR axons were not hyperexcitable after stroke. Rather, FCR axons were found to be hyperpolarized bilaterally post stroke, and this was associated with disability and [K+]. Reduced FCR axon excitability may represent a kind of bilateral trans-synaptic homeostatic mechanism that acts to minimize motoneuron hyperexcitability.

N°166 – Cross-sectional axonal excitability and motor unit number index profile in early stages of weakness in ALS

N°166 – Cross-sectional axonal excitability and motor unit number index profile in early stages of weakness in ALS

N°210 – Can axonal excitability features distinguish between hereditary sensory polyneuropathies

N°210 – Can axonal excitability features distinguish between hereditary sensory polyneuropathies

N°34 – iMAX: A new tool for assessment of motor axon excitability

N°34 – iMAX: A new tool for assessment of motor axon excitability

Strength-duration properties and excitability of motor and sensory axons across different target thresholds.

The present series of studies aimed to investigate the biophysical basis underlying differences in behavior between motor and sensory axons at different target response levels. In 24 healthy individuals, axonal excitability protocols measured strength-duration properties and latent addition across several axonal populations, with target amplitudes set at 10%, 20%, 40%, and 60%. Strength-duration time constants (SDTCs) were typically longer at lower target levels for both motor and sensory axons. Threshold change at 0.2 ms during assessment of latent addition, representing a persistent Na+ current (Nap), was higher in sensory axons. Passive membrane properties were not different across target levels. Significant relationships were evident between the threshold change at 0.2 ms and SDTC across all target levels for motor and sensory axons. These differences were explored using mathematical modeling of excitability data. With decreasing target size, as the internodal leak conductance increased in sensory axons, the Barrett-Barrett conductance decreased, whereas the hyperpolarization-activated cation current (Ih) channels became more depolarized. A similar pattern was observed in motor axons. As such, it was concluded that Nap was not responsible for the differences observed in SDTC between different target levels, although within specific target levels, Nap changes contributed to the variability of SDTC. This study provides a comprehensive assessment of Nap current, SDTC, and outlines key factors operating at different target levels in motor and sensory axons. Findings from the present study may point to the contributing factors of symptom development in human neuropathy.NEW & NOTEWORTHY This study provides a comprehensive assessment concerning the strength-duration behavior of motor and sensory axons at differing target levels of the compound nerve response. Strength-duration time constant was increased at lower target response levels particularly for sensory axons, whereas threshold change at 0.2 ms and passive membrane properties were not different. The results have established templates for axonal behavior in normal human axons, demonstrating altered adaptive responses, presumably secondary to different patterns of nerve activation.

Structure and Function of Neuronal Circuits Linking Ventrolateral Preoptic Nucleus and Lateral Hypothalamic Area.

To understand how sleep-wakefulness cycles are regulated, it is essential to disentangle structural and functional relationships between the preoptic area (POA) and lateral hypothalamic area (LHA), since these regions play important yet opposing roles in the sleep-wakefulness regulation. GABA- and galanin (GAL)-producing neurons in the ventrolateral preoptic nucleus (VLPO) of the POA (VLPOGABA and VLPOGAL neurons) are responsible for the maintenance of sleep, while the LHA contains orexin-producing neurons (orexin neurons) that are crucial for maintenance of wakefulness. Through the use of rabies virus-mediated neural tracing combined with in situ hybridization (ISH) in male and female orexin-iCre mice, we revealed that the vesicular GABA transporter (Vgat, Slc32a1)- and galanin (Gal)-expressing neurons in the VLPO directly synapse with orexin neurons in the LHA. A majority (56.3 ± 8.1%) of all VLPO input neurons connecting to orexin neurons were double-positive for Vgat and Gal Using projection-specific rabies virus-mediated tracing in male and female Vgat-ires-Cre and Gal-Cre mice, we discovered that VLPOGABA and VLPOGAL neurons that send projections to the LHA received innervations from similarly distributed input neurons in many brain regions, with the POA and LHA being among the main upstream areas. Additionally, we found that acute optogenetic excitation of axons of VLPOGABA neurons, but not VLPOGAL neurons, in the LHA of male Vgat-ires-Cre mice induced wakefulness. This study deciphers the connectivity between the VLPO and LHA, provides a large-scale map of upstream neuronal populations of VLPO→LHA neurons, and reveals a previously uncovered function of the VLPOGABA→LHA pathway in the regulation of sleep and wakefulness.SIGNIFICANCE STATEMENT We identified neurons in the ventrolateral preoptic nucleus (VLPO) that are positive for vesicular GABA transporter (Vgat) and/or galanin (Gal) and serve as presynaptic partners of orexin-producing neurons in the lateral hypothalamic area (LHA). We depicted monosynaptic input neurons of GABA- and galanin-producing neurons in the VLPO that send projections to the LHA throughout the entire brain. Their input neurons largely overlap, suggesting that they comprise a common neuronal population. However, acute excitatory optogenetic manipulation of the VLPOGABA→LHA pathway, but not the VLPOGAL→LHA pathway, evoked wakefulness. This study shows the connectivity of major components of the sleep/wake circuitry in the hypothalamus and unveils a previously unrecognized function of the VLPOGABA→LHA pathway in sleep-wakefulness regulation. Furthermore, we suggest the existence of subpopulations of VLPOGABA neurons that innervate LHA.

Simulation test for impartment of use-dependent plasticity by inactivation of axonal potassium channels on hippocampal mossy fibers.

Modification of axonal excitability directly impacts information transfer through the neuronal networks in the brain. However, the functional significance of modulation of axonal excitability by the preceding neuronal activity largely remains elusive. One remarkable exception is the activity-dependent broadening of action potential (AP) propagating along the hippocampal mossy fibers. The duration of AP is progressively prolonged during repetitive stimuli and facilitated presynaptic Ca2+ entry and subsequent transmitter release. As an underlying mechanism, accumulated inactivation of axonal K+ channels during AP train has been postulated. As the inactivation of axonal K+ channels proceeds on a timescale of several tens of milliseconds slower than the millisecond scale of AP, the contribution of K+ channel inactivation in AP broadening needs to be tested and evaluated quantitatively. Using the computer simulation approach, this study aimed to explore the effects of the removal of the inactivation process of axonal K+ channels in the simple but sufficiently realistic model of hippocampal mossy fibers and found that the use-dependent AP broadening was completely abolished in the model replaced with non-inactivating K+ channels. The results demonstrated the critical roles of K+ channel inactivation in the activity-dependent regulation of axonal excitability during repetitive action potentials, which critically imparts additional mechanisms for robust use-dependent short-term plasticity characteristics for this particular synapse.

Heparan Sulfates Regulate Axonal Excitability and Context Generalization through Ca2+/Calmodulin-Dependent Protein Kinase II.

Our previous studies demonstrated that enzymatic removal of highly sulfated heparan sulfates with heparinase 1 impaired axonal excitability and reduced expression of ankyrin G at the axon initial segments in the CA1 region of the hippocampus ex vivo, impaired context discrimination in vivo, and increased Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity in vitro. Here, we show that in vivo delivery of heparinase 1 in the CA1 region of the hippocampus elevated autophosphorylation of CaMKII 24 h after injection in mice. Patch clamp recording in CA1 neurons revealed no significant heparinase effects on the amplitude or frequency of miniature excitatory and inhibitory postsynaptic currents, while the threshold for action potential generation was increased and fewer spikes were generated in response to current injection. Delivery of heparinase on the next day after contextual fear conditioning induced context overgeneralization 24 h after injection. Co-administration of heparinase with the CaMKII inhibitor (autocamtide-2-related inhibitory peptide) rescued neuronal excitability and expression of ankyrin G at the axon initial segment. It also restored context discrimination, suggesting the key role of CaMKII in neuronal signaling downstream of heparan sulfate proteoglycans and highlighting a link between impaired CA1 pyramidal cell excitability and context generalization during recall of contextual memories.

Similar Presynaptic Action Potential-Calcium Influx Coupling in Two Types of Large Mossy Fiber Terminals Innervating CA3 Pyramidal Cells and Hilar Mossy Cells.

Morphologically similar axon boutons form synaptic contacts with diverse types of postsynaptic cells. However, it is less known to what extent the local axonal excitability, presynaptic action potentials (APs), and AP-evoked calcium influx contribute to the functional diversity of synapses and neuronal activity. This is particularly interesting in synapses that contact cell types that show only subtle cellular differences but fulfill completely different physiological functions. Here, we tested these questions in two synapses that are formed by rat hippocampal granule cells (GCs) onto hilar mossy cells (MCs) and CA3 pyramidal cells, which albeit share several morphologic and synaptic properties but contribute to distinct physiological functions. We were interested in the deterministic steps of the action potential-calcium ion influx coupling as these complex modules may underlie the functional segregation between and within the two cell types. Our systematic comparison using direct axonal recordings showed that AP shapes, Ca2+ currents and their plasticity are indistinguishable in synapses onto these two cell types. These suggest that the complete module that couples granule cell activity to synaptic release is shared by hilar mossy cells and CA3 pyramidal cells. Thus, our findings present an outstanding example for the modular composition of distinct cell types, by which cells employ different components only for those functions that are deterministic for their specialized functions, while many of their main properties are shared.

Does the Severity of Carpal Tunnel Syndrome Symptoms Vary Due to Vitamin and Mineral Deficiencies and Anemia?

ABSTRACT Purpose: The purpose of the study is to investigate the effect of serum vitamin, electrolyte levels, and anemia on carpal tunnel syndrome (CTS) symptoms and findings. Materials and Methods: The study included 166 patients with CTS and 80 healthy controls. The patients with CTS were further divided into two groups: those with positive (EP+) and negative (EP−) electrophysiology findings. The participants completed the Boston Carpal Tunnel Questionnaire (BCTQ) to assess their symptoms, and electromyography investigations were performed to confirm the diagnosis. Results: The mean age of individuals in the EP+ group was older, and the body mass index scores were greater than those of other groups. The hemoglobin (Hb) level was significantly lower in the EP+ group than in the EP− group (P = 0.001). Ferritin levels were substantially lower in both the EP+ and the EP− groups compared with the control group (P = 0.045 and P = 0.017, respectively). However, no correlation was found between Hb and ferritin levels and BCTQ scores. The groups had no significant differences in Vitamin D, B12, calcium, and magnesium levels. No relationship was found between the severity of CTS symptoms and deficiencies in Vitamins D and B12. Conclusions: Based on this study, anemia and lower levels of ferritin were common in patients with CTS, but they did not directly contribute to the severity of symptoms; the significance of this finding concerning CTS is unclear. However, considering the limited number of studies suggesting that iron deficiency reduces axonal excitability and leads to the slowing of nerve conduction velocities, iron deficiency may facilitate the development of CTS.

Chronic Hypoxemia Triggers a Neuropathic Process in Chronic Obstructive Pulmonary Disease: Insight From In Vivo Neurophysiological Assessments.

Peripheral neuropathies (PNs) are a common but poorly understood complication of chronic obstructive pulmonary disease (COPD). To clarify the initial trigger of a PN in COPD, we investigated the excitability of peripheral nerves in patients with COPD. The automated nerve excitability test (NET) using the threshold-tracking paradigm was applied to 20 COPD patients. The recording protocol calculated the strength-duration time constant, threshold electrotonus (TE), current-threshold relationship, and recovery cycle (RC). Each NET parameter was compared with two control groups: normal controls group (NC group) and smokers without COPD group (smoker group). In the motor NETs, the change in the threshold in the mid-depolarizing phase of TE (40-60 ms) was smaller in the COPD group (50.7%±1.2%, mean±SEM; n=20) than in the NC group (54.5%±0.7%, n=25; p<0.01), as was the prominence of superexcitability in the RC (-22.6%±1.5% and -26.4%±1.1%, respectively; p=0.04). There were no significant differences in the sensory NETs. Comparisons between the COPD and smoker groups (n=25) also showed no differences in either the motor or sensory NETs. The pattern of excitability in COPD revealed a membrane depolarization attributable to Na+-K+-ATPase failure in the axolemma of distal motor nerves. This finding suggests that chronic hypoxemia and adaptative process can alter axonal excitability and trigger a resultant neuropathic process that is antecedent to PN in COPD.

Parvalbumin basket cell myelination accumulates axonal mitochondria to internodes

Parvalbumin-expressing (PV+) basket cells are fast-spiking inhibitory interneurons that exert critical control over local circuit activity and oscillations. PV+ axons are often myelinated, but the electrical and metabolic roles of interneuron myelination remain poorly understood. Here, we developed viral constructs allowing cell type-specific investigation of mitochondria with genetically encoded fluorescent probes. Single-cell reconstructions revealed that mitochondria selectively cluster to myelinated segments of PV+ basket cells, confirmed by analyses of a high-resolution electron microscopy dataset. In contrast to the increased mitochondrial densities in excitatory axons cuprizone-induced demyelination abolished mitochondrial clustering in PV+ axons. Furthermore, with genetic deletion of myelin basic protein the mitochondrial clustering was still observed at internodes wrapped by noncompacted myelin, indicating that compaction is dispensable. Finally, two-photon imaging of action potential-evoked calcium (Ca2+) responses showed that interneuron myelination attenuates both the cytosolic and mitochondrial Ca2+ transients. These findings suggest that oligodendrocyte ensheathment of PV+ axons assembles mitochondria to branch selectively fine-tune metabolic demands.

Difference in axon diameter and myelin thickness between excitatory and inhibitory callosally projecting axons in mice.

Synchronization of network oscillation in spatially distant cortical areas is essential for normal brain activity. Precision in synchronization between hemispheres depends on the axonal conduction velocity, which is determined by physical parameters of the axons involved, including diameter, and extent of myelination. To compare these parameters in long-projecting excitatory and inhibitory axons in the corpus callosum, we used genetically modified mice and virus tracing to separately label CaMKIIα expressing excitatory and GABAergic inhibitory axons. Using electron microscopy analysis, we revealed that (i) the axon diameters of excitatory fibers (myelinated axons) are significantly larger than those of nonmyelinated excitatory axons; (ii) the diameters of bare axons of excitatory myelinated fibers are significantly larger than those of their inhibitory counterparts; and (iii) myelinated excitatory fibers are significantly larger than myelinated inhibitory fibers. Also, the thickness of myelin ensheathing inhibitory axons is significantly greater than for excitatory axons, with the ultrastructure of the myelin around excitatory and inhibitory fibers also differing. We generated a computational model to investigate the functional consequences of these parameter divergences. Our simulations indicate that impulses through inhibitory and excitatory myelinated fibers reach the targetalmost simultaneously, whereas action potentials conducted by nonmyelinated axons reach target cells with considerable delay.

Neuroplasticity of peripheral axonal properties after ischemic stroke.

ObjectiveThis study investigated how peripheral axonal excitability changes in ischemic stroke patients with hemiparesis or hemiplegia, reflecting the plasticity of motor axons due to corticospinal tract alterations along the poststroke stage.MethodsEach subject received a clinical evaluation, nerve conduction study, and nerve excitability test. Nerve excitability tests were performed on motor median nerves in paretic and non-paretic limbs in the acute stage of stroke. Control nerve excitability test data were obtained from age-matched control subjects. Some patients underwent excitability examinations several times in subacute or chronic stages.ResultsA total of thirty patients with acute ischemic stroke were enrolled. Eight patients were excluded due to severe entrapment neuropathy in the median nerve. The threshold current for 50% compound muscle action potential (CMAP) was higher in paretic limbs than in control subjects. Furthermore, in the cohort with severe patients (muscle power ≤ 3/5 in affected hands), increased threshold current for 50% CMAP and reduced subexcitability were noted in affected limbs than in unaffected limbs. In addition, in the subsequent study of those severe patients, threshold electrotonus increased in the hyperpolarization direction: TEh (100–109 ms), and the minimum I/V slope decreased. The above findings suggest the less excitable and less accommodation in lower motor axons in the paretic limb caused by ischemic stroke.ConclusionUpper motor neuron injury after stroke can alter nerve excitability in lower motor neurons, and the changes are more obvious in severely paretic limbs. The accommodative changes of axons progress from the subacute to the chronic stage after stroke. Further investigation is necessary to explore the downstream effects of an upper motor neuron insult in the peripheral nerve system.

WE-149. Axonal excitability changes in children with spinal muscular atrophy treated with nusinersen

WE-149. Axonal excitability changes in children with spinal muscular atrophy treated with nusinersen

TH-115. Axonal and cortical excitability findings in hand and face transplant patients

TH-115. Axonal and cortical excitability findings in hand and face transplant patients

The periodic axon membrane skeleton leads to Na nanodomains but does not impact action potentials

Recent work has established that axons have a periodic skeleton structure comprising of azimuthal actin rings connected via longitudinal spectrin tetramer filaments. This structure endows the axon with structural integrity and mechanical stability. Additionally, voltage-gated sodium channels follow the periodicity of the active-spectrin arrangement, spaced ∼190 nm segments apart. The impact of this periodic arrangement of sodium channels on the generation and propagation of action potentials is unknown. To address this question, we simulated an action potential using the Hodgkin-Huxley formalism in a cylindrical compartment, but instead of using a homogeneous distribution of voltage-gated sodium channels in the membrane, we applied the experimentally determined periodic arrangement. We found that the periodic distribution of voltage-gated sodium channels does not significantly affect the generation or propagation of action potentials but instead leads to large, localized sodium action currents caused by high-density sodium nanodomains. Additionally, our simulations show that the distance between periodic sodium channel strips could control axonal excitability, suggesting a previously underappreciated mechanism to regulate neuronal firing properties. Together, this work provides a critical new insight into the role of the periodic arrangement of sodium channels in axons, providing a foundation for future experimental studies.