Functional Na+/K+ pump in rat dorsal root ganglia neurons

Terpenes in Cannabis sativa Inhibit Capsaicin Responses in Rat DRG Neurons via Na+/K+ ATPase Activation

Terpenes in Cannabis sativa exert analgesic effects, but the mechanisms are uncertain. We examined the effects of 10 terpenes on capsaicin responses in an established model of neuronal hypersensitivity. Adult rat DRG neurons cultured with neurotrophic factors NGF and GDNF were loaded with Fura2AM for calcium imaging, and treated with individual terpenes or vehicle for 5 min, followed by 1 µMol capsaicin. In vehicle treated control experiments, capsaicin elicited immediate and sustained calcium influx. Most neurons treated with terpenes responded to capsaicin after 6-8 min. Few neurons showed immediate capsaicin responses that were transient or normal. The delayed responses were found to be due to calcium released from the endoplasmic reticulum, as they were maintained in calcium/magnesium free media, but not after thapsigargin pre-treatment. Terpene inhibition of calcium influx was reversed after washout of medium, in the absence of terpenes, and in the presence of the Na+/K+ ATPase inhibitor ouabain, but not CB1 or CB2 receptor antagonists. Thus, terpenes inhibit capsaicin evoked calcium influx by Na+/K+ ATPase activation. Immunofluorescence showed TRPV1 co-expression with α1β1 Na+/K+ ATPase in most neurons while others were either TRPV1 or α1β1 Na+/K+ ATPase positive.

Muscarinic regulation of the neuronal Na+ /K+ -ATPase in rat hippocampus

KEY POINTS

Stimulation of postsynaptic muscarinic receptors was shown to excite principal hippocampal neurons by modulating several membrane ion conductances. We show here that activation of postsynaptic muscarinic receptors also causes neuronal excitation by inhibiting Na⁺ /K⁺ -ATPase activity. Muscarinic Na⁺ /K⁺ -ATPase inhibition is mediated by two separate signalling pathways that lead downstream to enhanced Na⁺ /K⁺ -ATPase phosphorylation by activating protein kinase C and protein kinase G. Muscarinic excitation through Na⁺ /K⁺ -ATPase inhibition is probably involved in cholinergic modulation of hippocampal activity and may turn out to be a widespread mechanism of neuronal excitation in the brain.

ABSTRACT

Stimulation of muscarinic cholinergic receptors on principal hippocampal neurons enhances intrinsic neuronal excitability by modulating several membrane ion conductances. The electrogenic Na⁺ /K⁺ -ATPase (NKA; the 'Na⁺ pump') is a ubiquitous regulator of intrinsic neuronal excitability, generating a hyperpolarizing current to thwart excessive neuronal firing. Using electrophysiological and pharmacological methodologies in rat hippocampal slices, we show that neuronal NKA pumping activity is also subjected to cholinergic regulation. Stimulation of postsynaptic muscarinic, but not nicotinic, cholinergic receptors activates membrane-bound phospholipase C and hydrolysis of membrane-integral phosphatidylinositol 4,5-bisphosphate into diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP₃ ). Along one signalling pathway, DAG activates protein kinase C (PKC). Along a second signalling pathway, IP₃ causes Ca²⁺ release from the endoplasmic reticulum, facilitating nitric oxide (NO) production. The rise in NO levels stimulates cGMP synthesis by guanylate-cyclase, activating protein kinase G (PKG). The two pathways converge to cause partial NKA inhibition through enzyme phosphorylation by PKC and PKG, leading to a marked increase in intrinsic neuronal excitability. This novel mechanism of neuronal NKA regulation probably contributes to the cholinergic modulation of hippocampal activity in spatial navigation, learning and memory.

Regulation of Neuronal Na+/K+-ATPase by Specific Protein Kinases and Protein Phosphatases

The Na⁺/K⁺-ATPase (NKA) is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasmalemma of living cells. Numerous studies in non-neuronal tissues have shown that this transport mechanism is reversibly regulated by phosphorylation/dephosphorylation of the catalytic α subunit and/or associated proteins. In neurons, Na⁺/K⁺ transport by NKA is essential for almost all neuronal operations, consuming up to two-thirds of the neuron's energy expenditure. However, little is known about its cellular regulatory mechanisms. Here we have used an electrophysiological approach to monitor NKA transport activity in male rat hippocampal neurons in situ We report that this activity is regulated by a balance between serine/threonine phosphorylation and dephosphorylation. Phosphorylation by the protein kinases PKG and PKC inhibits NKA activity, whereas dephosphorylation by the protein phosphatases PP-1 and PP-2B (calcineurin) reverses this effect. Given that these kinases and phosphatases serve as downstream effectors in key neuronal signaling pathways, they may mediate the coupling of primary messengers, such as neurotransmitters, hormones, and growth factors, to the NKAs, through which multiple brain functions can be regulated or dysregulated.SIGNIFICANCE STATEMENT The Na⁺/K⁺-ATPase (NKA), known as the "Na⁺ pump," is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasma membrane of living cells. In neurons, as in most types of cells, the NKA generates the negative resting membrane potential, which is the basis for almost all aspects of cellular function. Here we used an electrophysiological approach to monitor physiological NKA transport activity in single hippocampal pyramidal cells in situ We have found that neuronal NKA activity is oppositely regulated by phosphorylation and dephosphorylation, and we have identified the main protein kinases and phosphatases mediating this regulation. This fundamental form of NKA regulation likely plays a role in multiple brain functions.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Differential contributions of Ca2+ -activated K+ channels and Na+ /K+ -ATPases to the generation of the slow afterhyperpolarization in CA1 pyramidal cells

In many types of CNS neurons, repetitive spiking produces a slow afterhyperpolarization (sAHP), providing sustained, intrinsically generated negative feedback to neuronal excitation. Changes in the sAHP have been implicated in learning behaviors, in cognitive decline in aging, and in epileptogenesis. Despite its importance in brain function, the mechanisms generating the sAHP are still controversial. Here we have addressed the roles of M-type K+ current (IM ), Ca2+ -gated K+ currents (ICa(K) 's) and Na+ /K+ -ATPases (NKAs) current to sAHP generation in adult rat CA1 pyramidal cells maintained at near-physiological temperature (35 °C). No evidence for IM contribution to the sAHP was found in these neurons. Both ICa(K) 's and NKA current contributed to sAHP generation, the latter being the predominant generator of the sAHP, particularly when evoked with short trains of spikes. Of the different NKA isoenzymes, α1 -NKA played the key role, endowing the sAHP a steep voltage-dependence. Thus normal and pathological changes in α1 -NKA expression or function may affect cognitive processes by modulating the inhibitory efficacy of the sAHP.

Presynaptic inhibition of nociceptive neurotransmission by somatosensory neuron-secreted suppressors

Noxious stimuli cause pain by activating cutaneous nociceptors. The Aδ- and C-fibers of dorsal root ganglion (DRG) neurons convey the nociceptive signals to the laminae I-II of spinal cord. In the dorsal horn of spinal cord, the excitatory afferent synaptic transmission is regulated by the inhibitory neurotransmitter γ-aminobutyric acid and modulators such as opioid peptides released from the spinal interneurons, and by serotonin, norepinepherine and dopamine from the descending inhibitory system. In contrast to the accumulated evidence for these central inhibitors and their neural circuits in the dorsal spinal cord, the knowledge about the endogenous suppressive mechanisms in nociceptive DRG neurons remains very limited. In this review, we summarize our recent findings of the presynaptic suppressive mechanisms in nociceptive neurons, the BNP/NPR-A/PKG/BKCa channel pathway, the FSTL1/α1Na⁺-K⁺ ATPase pathway and the activin C/ERK pathway. These endogenous suppressive systems in the mechanoheat nociceptors may also contribute differentially to the mechanisms of nerve injury-induced neuropathic pain or inflammation-induced pain.

FXYD2, a γ subunit of Na⁺, K⁺-ATPase, maintains persistent mechanical allodynia induced by inflammation

Na⁺, K⁺-ATPase (NKA) is required to generate the resting membrane potential in neurons. Nociceptive afferent neurons express not only the α and β subunits of NKA but also the γ subunit FXYD2. However, the neural function of FXYD2 is unknown. The present study shows that FXYD2 in nociceptive neurons is necessary for maintaining the mechanical allodynia induced by peripheral inflammation. FXYD2 interacted with α1NKA and negatively regulated the NKA activity, depolarizing the membrane potential of nociceptive neurons. Mechanical allodynia initiated in FXYD2-deficient mice was abolished 4 days after inflammation, whereas it persisted for at least 3 weeks in wild-type mice. Importantly, the FXYD2/α1NKA interaction gradually increased after inflammation and peaked on day 4 post inflammation, resulting in reduction of NKA activity, depolarization of neuron membrane and facilitation of excitatory afferent neurotransmission. Thus, the increased FXYD2 activity may be a fundamental mechanism underlying the persistent hypersensitivity to pain induced by inflammation.

Electrical stimulation of transplanted motoneurons improves motor unit formation

Motoneurons die following spinal cord trauma and with neurological disease. Intact axons reinnervate nearby muscle fibers to compensate for the death of motoneurons, but when an entire motoneuron pool dies, there is complete denervation. To reduce denervation atrophy, we have reinnervated muscles in Fisher rats from local transplants of embryonic motoneurons in peripheral nerve. Since growth of axons from embryonic neurons is activity dependent, our aim was to test whether brief electrical stimulation of the neurons immediately after transplantation altered motor unit numbers and muscle properties 10 wk later. All surgical procedures and recordings were done in anesthetized animals. The muscle consequences of motoneuron death were mimicked by unilateral sciatic nerve section. One week later, 200,000 embryonic day 14 and 15 ventral spinal cord cells, purified for motoneurons, were injected into the tibial nerve 10-15 mm from the gastrocnemii muscles as the only neuron source for muscle reinnervation. The cells were stimulated immediately after transplantation for up to 1 h using protocols designed to examine differential effects due to pulse number, stimulation frequency, pattern, and duration. Electrical stimulation that included short rests and lasted for 1 h resulted in higher motor unit counts. Muscles with higher motor unit counts had more reinnervated fibers and were stronger. Denervated muscles had to be stimulated directly to evoke contractions. These results show that brief electrical stimulation of embryonic neurons, in vivo, has long-term effects on motor unit formation and muscle force. This muscle reinnervation provides the opportunity to use patterned electrical stimulation to produce functional movements.

Antitussive drugs--past, present, and future

Cough remains a serious unmet clinical problem, both as a symptom of a range of other conditions such as asthma, chronic obstructive pulmonary disease, gastroesophageal reflux, and as a problem in its own right in patients with chronic cough of unknown origin. This article reviews our current understanding of the pathogenesis of cough and the hypertussive state characterizing a number of diseases as well as reviewing the evidence for the different classes of antitussive drug currently in clinical use. For completeness, the review also discusses a number of major drug classes often clinically used to treat cough but that are not generally classified as antitussive drugs. We also reviewed a number of drug classes in various stages of development as antitussive drugs. Perhaps surprising for drugs used to treat such a common symptom, there is a paucity of well-controlled clinical studies documenting evidence for the use of many of the drug classes in use today, particularly those available over the counter. Nonetheless, there has been a considerable increase in our understanding of the cough reflex over the last decade that has led to a number of promising new targets for antitussive drugs being identified and thus giving some hope of new drugs being available in the not too distant future for the treatment of this often debilitating symptom.

Regulation and pharmacological blockade of sodium-potassium ATPase: a novel pathway to neuropathy

Inflammation causes upregulation of NaV1.7 sodium channels in the associated dorsal root ganglia (DRG). The resultant increase in sodium influx must be countered to maintain osmotic homeostasis. The primary mechanism to pump sodium out of neurons is Na(+), K(+)-ATPase. To test whether there is a compensatory upregulation of Na(+), K(+)-ATPase after inflammation, rats received an injection of complete Freund's adjuvant (CFA) into one hindpaw and saline into the contralateral hindpaw. Three days later, L4-L6 DRGs were extracted and analyzed using gel electrophoresis and immunohistochemistry. Immunoreactivity for both the α-1 and α-3 subunits were increased in DRG associated with CFA-treatment, compared to saline-treatment. To test whether dysregulation of Na(+), K(+)-ATPase may cause cell death after inflammation, we produced a pharmacological blockade with ouabain (10mg/kg, s.c.) three days after CFA injection and paws were stimulated or not. Twenty-four hours later, DRG were removed and stained with cresyl violet. Greater cell death was seen in DRG from ouabain-treated animals on the CFA treated side than the saline-treated side. Paw stimulation doubled this difference. Control DRG showed little neuronal death. These results are evidence that regulation of Na(+), K(+)-ATPase during major inflammatory disease states is critical for homeostatic protection of primary afferent neurons.

Changes in the expression and current of the Na+/K+ pump in the snail nervous system after exposure to a static magnetic field

Compelling evidence supports the use of a moderate static magnetic field (SMF) for therapeutic purposes. In order to provide insight into the mechanisms underlying SMF treatment, it is essential to examine the cellular responses elicited by therapeutically applied SMF, especially in the nervous system. The Na(+)/K(+) pump, by creating and maintaining the gradient of Na(+) and K(+) ions across the plasma membrane, regulates the physiological properties of neurons. In this study, we examined the expression of the Na(+)/K(+) pump in the isolated brain-subesophageal ganglion complex of the garden snail Helix pomatia, along with the immunoreactivity and current of the Na(+)/K(+) pump in isolated snail neurons after 15 min exposure to a moderate (10 mT) SMF. Western blot and immunofluorescence analysis revealed that 10 mT SMF did not significantly change the expression of the Na(+)/K(+) pump α-subunit in the snail brain and the neuronal cell body. However, our immunofluorescence data showed that SMF treatment induced a significant increase in the Na(+)/K(+) pump α-subunit expression in the neuronal plasma membrane area. This change in Na(+)/K(+) pump expression was reflected in pump activity as demonstrated by the pump current measurements. Whole-cell patch-clamp recordings from isolated snail neurons revealed that Na(+)/K(+) pump current density was significantly increased after the 10 mT SMF treatment. The SMF-induced increase was different in the two groups of control snail neurons, as defined by the pump current level. The results obtained could represent a physiologically important response of neurons to 10 mT SMF comparable in strength to therapeutic applications.

Sodium MRI in a rat migraine model and a NEURON simulation study support a role for sodium in migraine

INTRODUCTION

Increased lumbar cerebrospinal fluid (CSF) sodium has been reported during migraine. We used ultra-high field MRI to investigate cranial sodium in a rat migraine model, and simulated the effects of extracellular sodium on neuronal excitability.

METHODS

Behavioral changes in the nitroglycerin (NTG) rat migraine model were determined from von Frey hair withdrawal response and photography. Central sensitization was measured by counting cFos-immunoreactive cells in the trigeminal nucleus caudalis (TNC). Sodium was quantified in vivo by ultra-high field sodium MRI at 21 Tesla. Effects of extracellular sodium on neuronal excitability were modeled using NEURON software.

RESULTS

NTG decreased von Frey withdrawal threshold (p=0.0003), decreased eyelid vertical height:width ratio (p<0.0001), increased TNC cFos stain (p<0.0001), and increased sodium between 7.5 and 17% in brain, intracranial CSF, and vitreous humor (p<0.05). Simulated neurons exposed to higher sodium have more frequent and earlier spontaneous action potentials, and corresponding earlier sodium and potassium currents.

CONCLUSIONS

In the rat migraine model, sodium rises to levels that increase neuronal excitability. We propose that rising sodium in CSF surrounding trigeminal nociceptors increases their excitability and causes pain and that rising sodium in vitreous humor increases retinal neuronal excitability and causes photosensitivity.

Direct inhibition of the transient voltage-gated K(+) currents mediates the excitability of tetrodotoxin-resistant neonatal rat nodose ganglion neurons after ouabain application

The purpose of the present study was to determine the relationship between the responses of transient and sustained K(+) currents, and action potentials to ouabain, and to compare the immunoreactive expression of alpha Na(+)-K(+)-ATPase isoforms (α(1), α(2) and α(3)) in neonatal rat small-diameter nodose ganglion neurons. We used perforated patch-clamp techniques. We first confirmed that the neurons (n=20) were insensitive to 0.5 μM tetrodotoxin (TTX). Application of 1 μM ouabain 1) decreased the transient K(+) currents in 60% of neurons and the sustained K(+) currents in 20%, 2) increased voltage-gated transient and sustained K(+) currents in 20% of neurons, and 3) had no effect on transient K(+) currents in 20% of neurons and on sustained K(+) currents in 60%. Thirteen of the neurons were of a rapidly adapting type, and the remaining 7 were of a slowly adapting type. In 6 rapidly adapting type neurons (46%), their activity was not significantly altered by ouabain application, but in 4 rapidly adapting type neurons, the activity increased. In the remaining 3 rapidly adapting type neurons, ouabain application hyperpolarized the resting membrane potential. The slowly adapting type 7 neurons each showed increased activity after 1 μM ouabain application. The α(1) isoform of Na(+)-K(+)-ATPase was identified as the predominant immunoreactive isoforms in small-diameter nodose ganglion neurons. These results suggest that the increased activity of small-diameter nodose ganglion neurons seen after application of 1 μM ouabain is mediated by direct inhibition of the transient K(+) current.

Follistatin-like 1 suppresses sensory afferent transmission by activating Na+,K+-ATPase

Excitatory synaptic transmission is modulated by inhibitory neurotransmitters and neuromodulators. We found that the synaptic transmission of somatic sensory afferents can be rapidly regulated by a presynaptically secreted protein, follistatin-like 1 (FSTL1), which serves as a direct activator of Na(+),K(+)-ATPase (NKA). The FSTL1 protein is highly expressed in small-diameter neurons of the dorsal root ganglion (DRG). It is transported to axon terminals via small translucent vesicles and secreted in both spontaneous and depolarization-induced manners. Biochemical assays showed that FSTL1 binds to the α1 subunit of NKA and elevates NKA activity. Extracellular FSTL1 induced membrane hyperpolarization in cultured cells and inhibited afferent synaptic transmission in spinal cord slices by activating NKA. Genetic deletion of FSTL1 in small DRG neurons of mice resulted in enhanced afferent synaptic transmission and sensory hypersensitivity, which could be reduced by intrathecally applied FSTL1 protein. Thus, FSTL1-dependent activation of NKA regulates the threshold of somatic sensation.

Selective expression of a sodium pump isozyme by cough receptors and evidence for its essential role in regulating cough

We have identified a distinct subtype of airway vagal afferent nerve that plays an essential role in regulating the cough reflex. These afferents are exquisitely sensitive to punctate mechanical stimuli, acid, and decreases in extracellular chloride concentrations, but are insensitive to capsaicin, bradykinin, histamine, adenosine, serotonin, or changes in airway intraluminal pressures. In this study we used intravital imaging, retrograde neuronal tracing, and electrophysiological analyses to characterize the structural basis for their peculiar mechanical sensitivity and to further characterize the regulation of their excitability. In completing these experiments, we uncovered evidence for an essential role of an isozyme of Na(+)-K(+) ATPase in regulating cough. These vagal sensory neurons arise bilaterally from the nodose ganglia and are selectively and brilliantly stained intravitally with the styryl dye FM2-10. Cough receptor terminations are confined and adherent to the extracellular matrix separating the airway epithelium and smooth muscle layers, a site of extensive remodeling in asthma and chronic obstructive pulmonary disease. The cough receptor terminals uniquely express the alpha(3) subunit of Na(+)-K(+) ATPase. Intravital staining of cough receptors by FM2-10, cough receptor excitability in vitro, and coughing in vivo are potently and selectively inhibited by the sodium pump inhibitor ouabain. These data provide the first detailed morphological description of the peripheral terminals of the sensory nerves regulating cough and identify a selective molecular target for their modulation.

Immunohistochemical characterization of nodose cough receptor neurons projecting to the trachea of guinea pigs

Background

Cough in guinea pigs is mediated in part by capsaicin-insensitive low threshold mechanoreceptors (cough receptors). Functional studies suggest that cough receptors represent a homogeneous population of nodose ganglia-derived sensory neurons. In the present study we set out to characterize the neurochemical profile of cough receptor neurons in the nodose ganglia.

Methods

Nodose neurons projecting to the guinea pig trachea were retrogradely labeled with fluorogold and processed immunohistochemically for the expression of a variety of transporters (Na⁺/K⁺/2C1^- co-transporter (NKCC1), α1 and α3 Na⁺/K⁺ ATPase, vesicular glutamate transporters (vGlut)1 and vGlut2), neurotransmitters (substance P, calcitonin gene-related peptide (CGRP), somatostatin, neuronal nitric oxide synthase (nNOS)) and cytosolic proteins (neurofilament, calretinin, calbindin, parvalbumin).

Results

Fluorogold labeled ~3 per cent of neurons in the nodose ganglia with an average somal perimeter of 137 ± 6.2 μm (range 90–200 μm). All traced neurons (and seemingly all nodose neurons) were immunoreactive for NKCC1. Many (> 90 per cent) were also immunoreactive for vGlut2 and neurofilament and between 50 and 85 per cent expressed α1 ATPase, α3 ATPase or vGlut1. Cough receptor neurons that did not express the above markers could not be differentiated based on somal size, with the exception of neurofilament negative neurons which were significantly smaller (P < 0.05). Less than 10 per cent of fluorogold labeled neurons expressed substance P or CGRP (and these had somal perimeters less than 110 μm) and none expressed somatostatin, calretinin, calbindin or parvalbumin. Two distinct patterns of nNOS labeling was observed in the general population of nodose neurons: most neurons contained cytosolic clusters of moderately intense immunoreactivity whereas less than 10 per cent of neurons displayed uniform intensely fluorescent somal labeling. Less than 3 per cent of the retrogradely traced neurons were intensely fluorescent for nNOS (most showed clusters of nNOS immunoreactivity) and nNOS immunoreactivity was not expressed by cough receptor nerve terminals in the tracheal wall.

Conclusion

These data provide further insights into the neurochemistry of nodose cough receptors and suggest that despite their high degree of functional homogeneity, nodose cough receptors subtypes may eventually be distinguished based on neurochemical profile.

Conduction velocity is regulated by sodium channel inactivation in unmyelinated axons innervating the rat cranial meninges

Axonal conduction velocity varies according to the level of preceding impulse activity. In unmyelinated axons this typically results in a slowing of conduction velocity and a parallel increase in threshold. It is currently held that Na(+)-K(+)-ATPase-dependent axonal hyperpolarization is responsible for this slowing but this has long been equivocal. We therefore examined conduction velocity changes during repetitive activation of single unmyelinated axons innervating the rat cranial meninges. In direct contradiction to the currently accepted postulate, Na(+)-K(+)-ATPase blockade actually enhanced activity-induced conduction velocity slowing, while the degree of velocity slowing was curtailed in the presence of lidocaine (10-300 microm) and carbamazepine (30-500 microm) but not tetrodotoxin (TTX, 10-80 nm). This suggests that a change in the number of available sodium channels is the most prominent factor responsible for activity-induced changes in conduction velocity in unmyelinated axons. At moderate stimulus frequencies, axonal conduction velocity is determined by an interaction between residual sodium channel inactivation following each impulse and the retrieval of channels from inactivation by a concomitant Na(+)-K(+)-ATPase-mediated hyperpolarization. Since the process is primarily dependent upon sodium channel availability, tracking conduction velocity provides a means of accessing relative changes in the excitability of nociceptive neurons.

Phylogenetic preservation of alpha3 Na+,K+-ATPase distribution in vertebrate peripheral nervous systems

The alpha(3) isoform of Na(+),K(+)-ATPase is uniquely expressed in afferent and efferent neurons innervating muscle spindles in the peripheral nervous system (PNS) of adult rats, but the distribution pattern of this isoform in other species has not been investigated. We compared expression of alpha(3) Na(+),K(+)-ATPase in lumbar dorsal root ganglia (DRG), spinal roots, and skeletal muscle samples of amphibian (frog), reptilian (turtle), avian (pigeon and chicken), and mammalian (mouse and human) species. In all species studied, the alpha(3) Na(+),K(+)-ATPase isoform was nonuniformly expressed in peripheral ganglia and nerves. In spinal ganglia, only 5-20% of neurons expressed this isoform, and, in avian and mammalian species, these alpha(3) Na(+),K(+)-ATPase-expressing neurons belonged to a subpopulation of large DRG neurons. In ventral root fibers of pigeons, mice, and humans, the alpha(3) Na(+),K(+)-ATPase was abundantly expressed predominantly in small myelinated axons. In skeletal muscle samples from turtles, pigeons, mice, and humans, alpha(3) Na(+),K(+)-ATPase was detected in intramuscular myelinated axons and in profiles of nerve terminals associated with the equatorial and polar regions of muscle spindle intrafusal fibers. These results show that the expression profiles for alpha(3) Na(+),K(+)-ATPase in the peripheral nervous system of a wide variety of vertebrate species are similar to the profile of rats and suggest that stretch receptor-associated expression of alpha(3) Na(+),K(+)-ATPase is preserved through vertebrate evolution.

Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the alpha3 Na(+)/K(+)-ATPase

The excitability of CNS presynaptic terminals after a tetanic burst of action potentials is important for synaptic plasticity. The mechanisms that regulate excitability, however, are not well understood. Using direct recordings from the rat calyx of Held terminal, we found that a fast Na(+)/K(+)-ATPase (NKA)-mediated post-tetanic hyperpolarization (PTH) controls the probability and precision of subsequent firing. Notably, increasing the concentration of internal Ca(2+) buffers or decreasing Ca(2+) influx led to larger PTH amplitudes, indicating that an increase in [Ca(2+)](i) regulates PTH via inhibition of NKAs. The characterization for the first time of a presynaptic NKA pump current, combined with immunofluorescence staining, identified the alpha3-NKA isoform on calyx terminals. Accordingly, the increased ability of the calyx to faithfully fire during a high-frequency train as it matures is paralleled by a larger expression of alpha3-NKA during development. We propose that this newly discovered Ca(2+) dependence of PTH is important in the post-burst excitability of nerve terminals.

"Clock-scan" protocol for image analysis

Comparative analysis of extra- and intracellular distributions of protein markers in immunohistochemical and immunofluorescent studies relies on techniques of image analysis. Line or region of interest pixel intensity scans are methods routinely used. However, although having good spatial resolution, linear pixel intensity scans fail to produce integral image of the cellular distribution of the label. On the other hand, the regions of interest scans have good integrative capacity but low spatial resolution. In this work, we describe a "clock-scan" protocol that, when applied to convex objects (such as neuronal cell bodies and the majority of cells in culture), combines advantages and circumnavigates limitations of the above-mentioned techniques. The protocol 1) collects multiple radial pixel intensity profiles scanned from the cell center to the periphery, 2) scales these profiles according to the cell radius measured in the direction of the scan, and finally, 3) averages these individual profiles into one integral radial pixel intensity profile. Because of scaling, the mean pixel intensity profiles produced by the clock-scan protocol depend on neither the cell size nor, within reasonable limits, the cell shape. This allows direct comparison or, if required, averaging or subtraction of profiles of different cells. We have successfully tested the clock-scan protocol in experiments with immunostained dorsal root ganglion neurons. In addition, the protocol seems to be equally applicable for studies in a variety of other preparations.

Regeneration influences expression of the Na+, K+-atpase subunit isoforms in the rat peripheral nervous system

Neural injury triggers changes in the expression of a large number of gene families. Particularly interesting are those encoding proteins involved in the generation, propagation or restoration of electric potentials. The expression of the Na+, K+-ATPase subunit isoforms (alpha, beta and gamma) was studied in dorsal root ganglion (DRG) and sciatic nerve of the rat in normal conditions, after axotomy and during regeneration. In normal DRG, alpha1 and alpha2 are expressed in the plasma membrane of all cell types, while there is no detectable signal for alpha3 in most DRG cells. After axotomy, alpha1 and alpha2 expression decreases evenly in all cells, while there is a remarkable onset in alpha3 expression, with a peak about day 3, which gradually disappears throughout regeneration (day 7). beta1 Is restricted to the nuclear envelope and plasma membrane of neurons and satellite cells. Immediately after injury, beta1 shows a homogeneous distribution in the soma of neurons. No beta2 expression was found. Beta3 Specific immunofluorescence appears in all neurons, although it is brightest in the smallest, diminishing progressively after injury until day 3 and, thereafter, increasing in intensity, until it reaches normal levels. FXYD7 is expressed weakly in a few DRG neurons (less than 2%) and Schwann cells. It increases intensely in satellite cells immediately after axotomy, and in all cell types at day 3. Transient switching of members of the Na+, K+-ATPase isoform family elicited by axotomy suggests variations in the sodium pump isozymes with different affinities for Na+, K+ and ATP from those in intact nerve. This adaptation may be important for regeneration.

Target-determined expression of ?3 isoform of the Na+,K+-ATPase in the somatic nervous system of rat

Factors that determine the differential expression of isoforms of Na(+),K(+)-ATPase in the nervous system of vertebrates are not understood. To address this question we studied the expression of alpha(3) Na(+),K(+)-ATPase in the L5 dorsal root ganglia (DRG) of developing rat, the normal adult rat, and the adult rat after peripheral axotomy. During development, the first alpha(3) Na(+),K(+)-ATPase-positive DRG neurons appear by embryonic day 21. At birth, the L5 DRG have a full complement (14 +/- 2%) of these neurons. By 15 days after sciatic nerve transection in adult rat, the number of alpha(3) Na(+),K(+)-ATPase-positive DRG neurons and small myelinated L5 ventral root axons decreases to about 35% of control counts. These results combined with data from the literature suggest that the expression of alpha(3) Na(+),K(+)-ATPase by rat somatic neurons is determined by target-muscle spindle-derived factors.

Role of the electrogenic Na/K pump in disinhibition-induced bursting in cultured spinal networks

Disinhibition-induced bursting activity in cultures of fetal rat spinal cord is mainly controlled by intrinsic spiking with subsequent recurrent excitation of the network through glutamate synaptic transmission, and by autoregulation of neuronal excitability. Here we investigated the contribution of the electrogenic Na/K pump to the autoregulation of excitability using extracellular recordings by multielectrode arrays (MEAs) and intracellular whole cell recordings from spinal interneurons. The blockade of the electrogenic Na/K pump by strophanthidin led to an immediate and transient increase in the burst rate together with an increase in the asynchronous background activity. Later, the burst rate decreased to initial values and the bursts became shorter and smaller. In single neurons, we observed an immediate depolarization of the membrane during the interburst intervals concomitant with the rise in burst rate. This depolarization was more pronounced during disinhibition than during control, suggesting that the pump was more active. Later a decrease in burst rate was observed and, in some neurons, a complete cessation of firing. Most of the effects of strophanthidin could be reproduced by high K+-induced depolarization. During prolonged current injections, spinal interneurons exhibited spike frequency adaptation, which remained unaffected by strophanthidin. These results suggest that the electrogenic Na/K pump is responsible for the hyperpolarization and thus for the changes in excitability during the interburst intervals, although not for the spike frequency adaptation during the bursts.

Properties of the Na+/K+ pump current in small neurons from adult rat dorsal root ganglia

1 The present investigation was undertaken to characterize the Na(+)/K(+) pump current in small ( Collapse

Key Words

• na⁺/k⁺ pump current
• dorsal root ganglion
• neuron
• ouabain
• α1 isoform
• α3 isoform

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MESH Headings

• Animals
• Cell Size/drug effects
• Cell Size/physiology
• Cells, Cultured
• Dose-Response Relationship, Drug
• Ganglia, Spinal/cytology
• Ganglia, Spinal/drug effects
• Ganglia, Spinal/physiology
• Male
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Neurons/cytology
• Neurons/drug effects
• Neurons/physiology
• Ouabain/pharmacology
• Rats
• Rats, Sprague-Dawley
• Sodium-Potassium-Exchanging ATPase/physiology

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Affiliation(s)

Kanako Hamada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Hiroshi Matsuura Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mitsuru Sanada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Futoshi Toyoda Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mariko Omatsu-Kanbe Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Atsunori Kashiwagi Division of Endocrinology and Metabolism, Department of Medicine, Otsu, Shiga University of Medical Science, Shiga 520-2192, Japan
Hitoshi Yasuda Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan

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Cold transduction by inhibition of a background potassium conductance in rat primary sensory neurones

Transduction in cutaneous cold receptors is poorly understood at present. We have studied this question using dorsal root ganglion (DRG) neurones in primary culture as a model of the otherwise inaccessible receptor terminal. Whole-cell recordings during cooling from 32 to 20 degrees C revealed a large depolarization (>8mV) in 22 of 88 DRG neurones (25%), sometimes accompanied by action potentials. In cold-sensitive neurones cooling inhibited a time-independent background K+ current (Icold) which was resistant to tetraethylammonium and 4-aminopyridine. Ouabain elicited a substantially smaller depolarization than cooling, and no action potentials. We conclude that excitation by cooling in this model is primarily due to inhibition of Icold and that the previously suggested role of the Na+/K+ adenosine triphosphatase is secondary. We suggest that Icold may underlie cold transduction in cutaneous thermoreceptors.