Stretch receptor-associated expression of alpha 3 isoform of the Na+, K+-ATPase in rat peripheral nervous system

The Cell Autonomous and Non-Cell Autonomous Aspects of Neuronal Vulnerability and Resilience in Amyotrophic Lateral Sclerosis

Simple Summary

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by a progressive paralysis due to the loss of particular neurons in our nervous system called motor neurons, that exert voluntary control of all our skeletal muscles. It is not entirely understood why motor neurons are particularly vulnerable in ALS, neither is it completely clear why certain groups of motor neurons, including those that regulate eye movement, are rather resilient to this disease. However, both vulnerability and resilience to ALS likely reflect cell intrinsic properties of different motor neuron subpopulations as well as non-cell autonomous events regulated by surrounding cell types. In this review we dissect the particular properties of different motor neuron types and their responses to disease that may underlie their respective vulnerabilities and resilience. Disease progression in ALS involves multiple cell types that are closely connected to motor neurons and we here also discuss their contributions to the differential vulnerability of motor neurons.

Abstract

Amyotrophic lateral sclerosis (ALS) is defined by the loss of upper motor neurons (MNs) that project from the cerebral cortex to the brain stem and spinal cord and of lower MNs in the brain stem and spinal cord which innervate skeletal muscles, leading to spasticity, muscle atrophy, and paralysis. ALS involves several disease stages, and multiple cell types show dysfunction and play important roles during distinct phases of disease initiation and progression, subsequently leading to selective MN loss. Why MNs are particularly vulnerable in this lethal disease is still not entirely clear. Neither is it fully understood why certain MNs are more resilient to degeneration in ALS than others. Brain stem MNs of cranial nerves III, IV, and VI, which innervate our eye muscles, are highly resistant and persist until the end-stage of the disease, enabling paralyzed patients to communicate through ocular tracking devices. MNs of the Onuf’s nucleus in the sacral spinal cord, that innervate sphincter muscles and control urogenital functions, are also spared throughout the disease. There is also a differential vulnerability among MNs that are intermingled throughout the spinal cord, that directly relate to their physiological properties. Here, fast-twitch fatigable (FF) MNs, which innervate type IIb muscle fibers, are affected early, before onset of clinical symptoms, while slow-twitch (S) MNs, that innervate type I muscle fibers, remain longer throughout the disease progression. The resilience of particular MN subpopulations has been attributed to intrinsic determinants and multiple studies have demonstrated their unique gene regulation and protein content in health and in response to disease. Identified factors within resilient MNs have been utilized to protect more vulnerable cells. Selective vulnerability may also, in part, be driven by non-cell autonomous processes and the unique surroundings and constantly changing environment close to particular MN groups. In this article, we review in detail the cell intrinsic properties of resilient and vulnerable MN groups, as well as multiple additional cell types involved in disease initiation and progression and explain how these may contribute to the selective MN resilience and vulnerability in ALS.

Alternating Hemiplegia of Childhood: gastrointestinal manifestations and correlation with neurological impairments

Background

Alternating Hemiplegia of Childhood (AHC) is caused by mutations of the ATP1A3 gene which is expressed in brain areas that include structures controling autonomic, gastrointestinal, gut motility and GABAergic functions. We aimed to investigate, in a cohort of 44 consecutive AHC patients, two hypotheses: 1) AHC patients frequently manifest gastrointestinal, particularly motility, problems. 2) These problems are often severe and their severity correlates with neurological impairments.

Results

41/44 (93%) exhibited gastrointestinal symptoms requiring medical attention. For these 41 patients, symptoms included constipation (66%), swallowing problems (63%), vomiting (63%), anorexia (46%), diarrhea (44%), nausea (37%), and abdominal pain (22%). Symptoms indicative of dysmotility occurred in 33 (80%). The most common diagnoses were oropharyngeal dysphagia (63%) and gastroesophageal reflux (63%). 16 (39%) required gastrostomy and two fundoplication. Severity of gastrointestinal symptoms correlated with non-paroxysmal neurological disability index, Gross Motor Function Classification System scores, and with the presence/absence of non-gastrointestinal autonomic dysfunction (p = 0.031, 0.043, Spearman correlations and 0.0166 Cramer’s V, respectively) but not with the paroxysmal disability index (p = 0.408).

Conclusions

Most AHC patients have gastrointestinal problems. These are usually severe, most commonly are indicative of dysmotility, often require surgical therapies, and their severity correlates with that of non-paroxysmal CNS manifestations. Our findings should help in management-anticipatory guidance of AHC patients. Furthermore, they are consistent with current understandings of the pathophysiology of AHC and of gastrointestinal dysmotility, both of which involve autonomic and GABAergic dysfunction.

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'.

Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2

Although mutations in more than 90 genes are known to cause CMT, the underlying genetic cause of CMT remains unknown in more than 50% of affected individuals. The discovery of additional genes that harbor CMT2-causing mutations increasingly depends on sharing sequence data on a global level. In this way-by combining data from seven countries on four continents-we were able to define mutations in ATP1A1, which encodes the alpha1 subunit of the Na⁺,K⁺-ATPase, as a cause of autosomal-dominant CMT2. Seven missense changes were identified that segregated within individual pedigrees: c.143T>G (p.Leu48Arg), c.1775T>C (p.Ile592Thr), c.1789G>A (p.Ala597Thr), c.1801_1802delinsTT (p.Asp601Phe), c.1798C>G (p.Pro600Ala), c.1798C>A (p.Pro600Thr), and c.2432A>C (p.Asp811Ala). Immunostaining peripheral nerve axons localized ATP1A1 to the axolemma of myelinated sensory and motor axons and to Schmidt-Lanterman incisures of myelin sheaths. Two-electrode voltage clamp measurements on Xenopus oocytes demonstrated significant reduction in Na⁺ current activity in some, but not all, ouabain-insensitive ATP1A1 mutants, suggesting a loss-of-function defect of the Na⁺,K⁺ pump. Five mutants fall into a remarkably narrow motif within the helical linker region that couples the nucleotide-binding and phosphorylation domains. These findings identify a CMT pathway and a potential target for therapy development in degenerative diseases of peripheral nerve axons.

Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2

Although mutations in more than 90 genes are known to cause CMT, the underlying genetic cause of CMT remains unknown in more than 50% of affected individuals. The discovery of additional genes that harbor CMT2-causing mutations increasingly depends on sharing sequence data on a global level. In this way-by combining data from seven countries on four continents-we were able to define mutations in ATP1A1, which encodes the alpha1 subunit of the Na⁺,K⁺-ATPase, as a cause of autosomal-dominant CMT2. Seven missense changes were identified that segregated within individual pedigrees: c.143T>G (p.Leu48Arg), c.1775T>C (p.Ile592Thr), c.1789G>A (p.Ala597Thr), c.1801_1802delinsTT (p.Asp601Phe), c.1798C>G (p.Pro600Ala), c.1798C>A (p.Pro600Thr), and c.2432A>C (p.Asp811Ala). Immunostaining peripheral nerve axons localized ATP1A1 to the axolemma of myelinated sensory and motor axons and to Schmidt-Lanterman incisures of myelin sheaths. Two-electrode voltage clamp measurements on Xenopus oocytes demonstrated significant reduction in Na⁺ current activity in some, but not all, ouabain-insensitive ATP1A1 mutants, suggesting a loss-of-function defect of the Na⁺,K⁺ pump. Five mutants fall into a remarkably narrow motif within the helical linker region that couples the nucleotide-binding and phosphorylation domains. These findings identify a CMT pathway and a potential target for therapy development in degenerative diseases of peripheral nerve axons.

Quantitative models of feline lumbosacral dorsal root ganglia neuronal cell density

BACKGROUND

Dorsal root ganglia (DRG) are spinal root components that contain the cell bodies of converging primary sensory neurons. DRG are becoming a therapeutic target for electrical neural interfaces. Our purpose was to establish methods for quantifying the non-random nature and distribution of neuronal cell bodies within DRG.

NEW METHOD

We identified neuronal cell body locations in 26 feline lumbosacral DRG cross-section histological images and used computational tools to quantify spatial trends. We first analyzed spatial randomness using the nearest-neighbor distance method. Next we overlaid a 6×6 grid, modeling neuronal cellular density in each grid square and comparing regions statistically. Finally we transformed DRG onto a polar map and calculated neuronal cellular density in annular sectors. We used a recursive partition model to determine regions of high and low density, and validated the model statistically.

RESULTS

We found that the arrangement of neuronal cell bodies at the widest point of DRG is distinctly non-random with concentration in particular regions. The grid model suggested a radial trend in density, with increasing density toward the outside of the DRG. The polar transformation model showed that the highest neuronal cellular density is in the outer 23.9% radially and the dorsal ±61.4° angularly.

COMPARISON WITH EXISTING METHODS

To our knowledge, DRG neuronal cell distribution has not been previously quantified.

CONCLUSIONS

These results confirm and expand quantitatively on the existing understanding of DRG anatomy. Our methods can be useful for analyzing the distribution of cellular components of other neural structures or expanding to three-dimensional models.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

The Genetic Homogeneity of CAPOS Syndrome: Four New Patients With the c.2452G>A (p.Glu818Lys) Mutation in the ATP1A3 Gene

BACKGROUND

The clinical syndrome of cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) was first described 20 years ago, but it was only recently that whole exome sequencing unveiled the causative mutation in the ATP1A3 gene. We present four patients from the seventh and eighth family identified worldwide, provide a critical review of all patients published thus far, and speculate about the pathophysiologic processes underlying the acute neurological manifestations.

CLINICAL OBSERVATIONS

The individuals presented here experienced one to three paroxysmal, short-lasting episodes in childhood with cerebellar symptoms and signs, hypotonia, ophthalmoparesis, motor weakness, areflexia, and/or lethargy that were consistently associated with febrile illness. An underlying c.2452G>A mutation in the ATP1A3 gene was found in all four individuals. Besides the persisting CAPOS features, other possibly related sequelae included dystonia, myoclonus, and emotional and behavioral changes. After initiation of acetazolamide in two patients, no further episodes occurred.

CONCLUSION

Targeted sequencing of the ATP1A3 gene is recommended in children exhibiting paroxysmal, fever-induced ataxia and in adults with a more or less stationary or slowly progressive cerebellar syndrome since childhood accompanied by mixed combinations of areflexia, pes cavus, profound visual impairment, and/or sensorineural hearing loss. Similar to some other types of episodic ataxia, acetazolamide may be considered in patients with CAPOS syndrome to prevent or attenuate bouts of ataxia, but this requires further study.

Effects of peripheral nerve injury on parvalbumin expression in adult rat dorsal root ganglion neurons

Background

Parvalbumin (PV) is a calcium binding protein that identifies a subpopulation of proprioceptive dorsal root ganglion (DRG) neurons. Calcitonin gene-related peptide (CGRP) is also expressed in a high proportion of muscle afferents but its relationship to PV is unclear. Little is known of the phenotypic responses of muscle afferents to nerve injury. Sciatic nerve axotomy or L5 spinal nerve ligation and section (SNL) lesions were used to explore these issues in adult rats using immunocytochemistry.

Results

In naive animals, the mean PV expression was 25 % of L4 or L5 dorsal root ganglion (DRG) neurons, and this was unchanged 2 weeks after sciatic nerve axotomy. Colocalization studies with the injury marker activating transcription factor 3 (ATF3) showed that approximately 24 % of PV neurons expressed ATF3 after sciatic nerve axotomy suggesting that PV may show a phenotypic switch from injured to uninjured neurons. This possibility was further assessed using the spinal nerve ligation (SNL) injury model where injured and uninjured neurons are located in different DRGs. Two weeks after L5 SNL there was no change in total PV staining and essentially all L5 PV neurons expressed ATF3. Additionally, there was no increase in PV-ir in the adjacent uninjured L4 DRG cells. Co-labelling of DRG neurons revealed that less than 2 % of PV neurons normally expressed CGRP and no colocalization was seen after injury.

Conclusion

These experiments clearly show that axotomy does not produce down regulation of PV protein in the DRG. Moreover, this lack of change is not due to a phenotypic switch in PV immunoreactive (ir) neurons, or de novo expression of PV-ir in uninjured neurons after nerve injury. These results further illustrate differences that occur when muscle afferents are injured as compared to cutaneous afferents.

Age-dependent decline in density of human nerve and spinal ganglia neurons expressing the α3 isoform of Na/K-ATPase

Ambulatory instability and falls are a major source of morbidity in the elderly. Age-related loss of tendon reflexes is a major contributing factor to this morbidity, and deterioration of the afferent limb of the stretch reflex is a potential contributing factor to such age-dependent loss of tendon reflexes. To evaluate this, we assessed the number and distribution of muscle spindle afferent fibers in human sacral spinal ganglia (S1) and tibial nerve samples obtained at autopsy, using immunohistochemical staining for the α3 isoform of Na(+), K(+)-ATPase (α3NKA), a marker of muscle spindle afferents. Across all age groups, an average of 26 ± 4% of myelinated fibers of tibial nerve and 17 ± 2% of ganglion neuronal profiles were α3NKA-positive (n = 8 per group). Subject age explained 85% of the variability in these counts. The relative frequency of α3NKA-labeled fibers/neurons starts to decline during the 5th decade of life, approaching half that of young adult values in 65-year-old subjects. At all ages, α3NKA-positive neurons were among the largest of spinal ganglia neurons. However, as compared to younger subjects, the population of α3NKA-positive neurons from advanced-age subjects showed diminished numbers of large (both moderately and strongly labeled), and medium-sized (strongly labeled) profiles. Considering the critical significance of ion transport by NKA for neuronal activity, our data suggest that functional impairment and, also, most likely atrophy and/or degeneration of muscle spindle afferents, are mechanisms underlying loss of tendon reflexes with age. The larger and more strongly α3NKA-expressing spindle afferents appear to be proportionally more vulnerable.

CAOS-Episodic Cerebellar Ataxia, Areflexia, Optic Atrophy, and Sensorineural Hearing Loss: A Third Allelic Disorder of the ATP1A3 Gene

We describe the molecular basis of a distinctive syndrome characterized by infantile stress-induced episodic weakness, ataxia, and sensorineural hearing loss, with permanent areflexia and optic nerve pallor. Whole exome sequencing identified a deleterious heterozygous c.2452 G>A, p.(E818K) variant in the ATP1A3 gene and structural analysis predicted its protein-destabilizing effect. This variant has not been reported in context with rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood, the 2 main diseases associated with ATP1A3. The clinical presentation in the family described here differs categorically from these diseases in age of onset, clinical course, cerebellar over extrapyramidal movement disorder predominance, and peripheral nervous system involvement. While this paper was in review, a highly resembling phenotype was reported in additional patients carrying the same c.2452 G>A variant. Our findings substantiate this variant as the cause of a unique inherited autosomal dominant neurologic syndrome that constitutes a third allelic disease of the ATP1A3 gene.

De novo expression of Nav1.7 in injured putative proprioceptive afferents: Multiple tetrodotoxin-sensitive sodium channels are retained in the rat dorsal root after spinal nerve ligation

Tetrodotoxin-sensitive (TTX-s) spontaneous activity is recorded from the dorsal roots after peripheral nerve injury. Primary sensory neurons in the dorsal root ganglion (DRG) express multiple TTX-s voltage-gated sodium channel α-subunits (Navs). Since Nav1.3 increases, whereas all other Navs decrease, in the DRG neurons after peripheral nerve lesion, Nav1.3 is proposed to be critical for the generation of these spontaneous discharges and the contributions of other Navs have been ignored. Here, we re-evaluate the changes in expression of three other TTX-s Navs, Nav1.1, Nav1.6 and Nav1.7, in the injured 5th lumbar (L5) primary afferent components following L5 spinal nerve ligation (SNL) using in situ hybridization histochemistry and immunohistochemistry. While the overall signal intensities for these Nav mRNAs decreased, many injured DRG neurons still expressed these transcripts at clearly detectable levels. All these Nav proteins accumulated at the proximal stump of the ligated L5 spinal nerve. The immunostaining patterns of Nav1.6 and Nav1.7 associated with the nodes of Ranvier were maintained in the ipsilateral L5 dorsal root. Interestingly, putative proprioceptive neurons characterized by α3 Na+/K+ ATPase-immunostaining specifically lacked Nav1.7 mRNA in naïve DRG but displayed de novo expression of this transcript following SNL. Nav1.7-immunoreactive fibers were significantly increased in the ipsilateral gracile nucleus where central axonal branches of the injured A-fiber afferents terminated. These data indicate that multiple TTX-s channel subunits could contribute to the generation and propagation of the spontaneous discharges in the injured primary afferents. Specifically, Nav1.7 may cause some functional changes in sensory processing in the gracile nucleus after peripheral nerve injury.

Distinct neurological disorders with ATP1A3 mutations

Genetic research has shown that mutations that modify the protein-coding sequence of ATP1A3, the gene encoding the α3 subunit of Na(+)/K(+)-ATPase, cause both rapid-onset dystonia parkinsonism and alternating hemiplegia of childhood. These discoveries link two clinically distinct neurological diseases to the same gene, however, ATP1A3 mutations are, with one exception, disease-specific. Although the exact mechanism of how these mutations lead to disease is still unknown, much knowledge has been gained about functional consequences of ATP1A3 mutations using a range of in-vitro and animal model systems, and the role of Na(+)/K(+)-ATPases in the brain. Researchers and clinicians are attempting to further characterise neurological manifestations associated with mutations in ATP1A3, and to build on the existing molecular knowledge to understand how specific mutations can lead to different diseases.

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.

Na+/K+ ATPase α1 and α3 isoforms are differentially expressed in α- and γ-motoneurons

The Na(+)/K(+) ATPase (NKA) is an essential membrane protein underlying the membrane potential in excitable cells. Transmembrane ion transport is performed by the catalytic α subunits (α1-4). The predominant subunits in neurons are α1 and α3, which have different affinities for Na(+) and K(+), impacting on transport kinetics. The exchange rate of Na(+)/K(+) markedly influences the activity of the neurons expressing them. We have investigated the distribution and function of the main isoforms of the α subunit expressed in the mouse spinal cord. NKAα1 immunoreactivity (IR) displayed restricted labeling, mainly confined to large ventral horn neurons and ependymal cells. NKAα3 IR was more widespread in the spinal cord, again being observed in large ventral horn neurons, but also in smaller interneurons throughout the dorsal and ventral horns. Within the ventral horn, the α1 and α3 isoforms were mutually exclusive, with the α3 isoform in smaller neurons displaying markers of γ-motoneurons and α1 in α-motoneurons. The α3 isoform was also observed within muscle spindle afferent neurons in dorsal root ganglia with a higher proportion at cervical versus lumbar regions. We confirmed the differential expression of α subunits in motoneurons electrophysiologically in neonatal slices of mouse spinal cord. γ-Motoneurons were excited by bath application of low concentrations of ouabain that selectively inhibit NKAα3 while α-motoneurons were insensitive to these low concentrations. The selective expression of NKAα3 in γ-motoneurons and muscle spindle afferents, which may affect excitability of these neurons, has implications in motor control and disease states associated with NKAα3 dysfunction.

α3Na+/K+-ATPase deficiency causes brain ventricle dilation and abrupt embryonic motility in zebrafish

Na(+)/K(+)-ATPases are transmembrane ion pumps that maintain ion gradients across the basolateral plasma membrane in all animal cells to facilitate essential biological functions. Mutations in the Na(+)/K(+)-ATPase α3 subunit gene (ATP1A3) cause rapid-onset dystonia-parkinsonism, a rare movement disorder characterized by sudden onset of dystonic spasms and slow movements. In the brain, ATP1A3 is principally expressed in neurons. In zebrafish, the transcripts of the two ATP1A3 orthologs, Atp1a3a and Atp1a3b, show distinct expression in the brain. Surprisingly, targeted knockdown of either Atp1a3a or Atp1a3b leads to brain ventricle dilation, a likely consequence of ion imbalances across the plasma membrane that cause accumulation of cerebrospinal fluid in the ventricle. The brain ventricle dilation is accompanied by a depolarization of spinal Rohon-Beard neurons in Atp1a3a knockdown embryos, suggesting impaired neuronal excitability. This is further supported by Atp1a3a or Atp1a3b knockdown results where altered responses to tactile stimuli as well as abnormal motility were observed. Finally, proteomic analysis identified several protein candidates highlighting proteome changes associated with the knockdown of Atp1a3a or Atp1a3b. Our data thus strongly support the role of α3Na(+)/K(+)-ATPase in zebrafish motility and brain development, associating for the first time the α3Na(+)/K(+)-ATPase deficiency with brain ventricle dilation.

Expression and properties of hyperpolarization-activated current in rat dorsal root ganglion neurons with known sensory function

The hyperpolarization-activated current (I(h)) has been implicated in nociception/pain, but its expression levels in nociceptors remained unknown. We recorded I(h) magnitude and properties by voltage clamp from dorsal root ganglion (DRG) neurons in vivo, after classifying them as nociceptive or low-threshold-mechanoreceptors (LTMs) and as having C-, Aδ- or Aα/β-conduction velocities (CVs). For both nociceptors andLTMs, I(h) amplitude and I(h) density (at -100 mV) were significantly positively correlated with CV.Median I(h) magnitudes and I(h) density in neuronal subgroupswere respectively:muscle spindle afferents(MSAs):-4.6 nA,-33 pA pF(-1); cutaneous Aα/β LTMs: -2.2 nA, -20 pA pF(-1); Aβ-nociceptors: -2.6 nA, -21 pA pF(-1); both Aδ-LTMs and nociceptors: -1.3 nA, ∼-14 pA pF(-1); C-LTMs: -0.4 nA, -7.6 pA pF(-1); and C-nociceptors: -0.26 nA, -5 pApF(-1). I(h) activation slow time constants (slow τ values) were strongly correlated with fast τ values; both were shortest in MSAs. Most neurons had τ values consistent with HCN1-related I(h); others had τ values closer to HCN1+HCN2 channels, or HCN2 in the presence of cAMP. In contrast, median half-activation voltages (V(0.5)) of -80 to -86 mV for neuronal subgroups suggest contributions of HCN2 to I(h). τ values were unrelated to CV but were inversely correlated with I(h) and I(h) density for all non-MSA LTMs, and for Aδ-nociceptors. From activation curves ∼2-7% of I(h)would be activated at normal membrane potentials. The high I(h) may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered high I(h) may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered Ih expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.

Motor neuron diversity in development and disease

Although often considered as a group, spinal motor neurons are highly diverse in terms of their morphology, connectivity, and functional properties and differ significantly in their response to disease. Recent studies of motor neuron diversity have clarified developmental mechanisms and provided novel insights into neurodegeneration in amyotrophic lateral sclerosis (ALS). Motor neurons of different classes and subtypes--fast/slow, alpha/gamma--are grouped together into motor pools, each of which innervates a single skeletal muscle. Distinct mechanisms regulate their development. For example, glial cell line-derived neurotrophic factor (GDNF) has effects that are pool-specific on motor neuron connectivity, column-specific on axonal growth, and subtype-specific on survival. In multiple degenerative contexts including ALS, spinal muscular atrophy (SMA), and aging, fast-fatigable (FF) motor units degenerate early, whereas motor neurons innervating slow muscles and those involved in eye movement and pelvic sphincter control are strikingly preserved. Extrinsic and intrinsic mechanisms that confer resistance represent promising therapeutic targets in these currently incurable diseases.

Immunostaining for the α3 isoform of the Na+/K+-ATPase is selective for functionally identified muscle spindle afferents in vivo

Muscle spindle afferent (MSA) neurons can show rapid and sustained firing. Immunostaining for the α3 isoform of the Na⁺/K⁺-ATPase (α3) in some large dorsal root ganglion (DRG) neurons and large intrafusal fibres suggested α3 expression in MSAs (Dobretsov et al. 2003), but not whether α3-immunoreactive DRG neuronal somata were exclusively MSAs. We found that neuronal somata with high α3 immunointensity were neurofilament-rich, suggesting they have A-fibres; we therefore focussed on A-fibre neurons to determine the sensory properties of α3-immunoreactive neurons. We examined α3 immunointensity in 78 dye-injected DRG neurons whose conduction velocities and hindlimb sensory receptive fields were determined in vivo. A dense perimeter or ring of staining in a subpopulation of neurons was clearly overlying the soma membrane and not within satellite cells. Neurons with clear α3 rings (n = 23) were all MSAs (types I and II); all MSAs had darkly stained α3 rings, that tended to be darker in MSA1 than MSA2 units. Of 52 non-MSA A-fibre neurons including nociceptive and cutaneous low-threshold mechanoreceptive (LTM) neurons, 50 had no discernable ring, while 2 (Aα/β cutaneous LTMs) had weakly stained rings. Three of three C-nociceptors had no rings. MSAs with strong ring immunostaining also showed the strongest cytoplasmic staining. These findings suggest that α3 ring staining is a selective marker for MSAs. The α3 isoform of the Na⁺/K⁺-ATPase has previously been shown to be activated by higher Na⁺ levels and to have greater affinity for ATP than the α1 isoform (in all DRG neurons). The high α3 levels in MSAs may enable the greater dynamic firing range in MSAs.

The nicotinic acetylcholine receptor and the Na,K-ATPase alpha2 isoform interact to regulate membrane electrogenesis in skeletal muscle

The nicotinic acetylcholine receptor (nAChR) and the Na,K-ATPase functionally interact in skeletal muscle (Krivoi, I. I., Drabkina, T. M., Kravtsova, V. V., Vasiliev, A. N., Eaton, M. J., Skatchkov, S. N., and Mandel, F. (2006) Pflugers Arch. 452, 756-765; Krivoi, I., Vasiliev, A., Kravtsova, V., Dobretsov, M., and Mandel, F. (2003) Ann. N.Y. Acad. Sci. 986, 639-641). In this interaction, the specific binding of nanomolar concentrations of nicotinic agonists to the nAChR stimulates electrogenic transport by the Na,K-ATPase alpha2 isozyme, causing membrane hyperpolarization. This study examines the molecular nature and membrane localization of this interaction. Stimulation of Na,K-ATPase activity by the nAChR does not require ion flow through open nAChRs. It can be induced by nAChR desensitization alone, in the absence of nicotinic agonist, and saturates when the nAChR is fully desensitized. It is enhanced by noncompetitive blockers of the nAChR (proadifen, QX-222), which promote non-conducting or desensitized states; and retarded by tetracaine, which stabilizes the resting nAChR conformation. The interaction operates at the neuromuscular junction as well as on extrajunctional sarcolemma. The Na,K-ATPase alpha2 isozyme is enriched at the postsynaptic neuromuscular junction and co-localizes with nAChRs. The nAChR and Na,K-ATPase alpha subunits specifically coimmunoprecipitate with each other, phospholemman, and caveolin-3. In a purified membrane preparation from Torpedo californica enriched in nAChRs and the Na,K-ATPase, a ouabain-induced conformational change of the Na,K-ATPase enhances a conformational transition of the nAChR to a desensitized state. These results suggest a mechanism by which the nAChR in a desensitized state with high apparent affinity for agonist interacts with the Na,K-ATPase to stimulate active transport. The interaction utilizes a membrane-delimited complex involving protein-protein interactions, either directly or through additional protein partners. This interaction is expected to enhance neuromuscular transmission and muscle excitation.

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.

The beta3 subunit of the Na+,K+-ATPase mediates variable nociceptive sensitivity in the formalin test

It is widely appreciated that there is significant inter-individual variability in pain sensitivity, yet only a handful of contributing genetic variants have been identified. Computational genetic mapping and quantitative trait locus analysis suggested that variation within the gene coding for the beta3 subunit of the Na+,K+-ATPase pump (Atp1b3) contributes to inter-strain differences in the early phase formalin pain behavior. Significant strain differences in Atp1b3 gene expression, beta3 protein expression, and biophysical properties of the Na+,K+ pump in dorsal root ganglia neurons from resistant (A/J) and sensitive (C57BL/6J) mouse strains supported the genetic prediction. Furthermore, in vivo siRNA knockdown of the beta3 subunit produced strain-specific changes in the early phase pain response, completely rescuing the strain difference. These findings indicate that the beta3 subunit of the Na+,K+-ATPase is a novel determinant of nociceptive sensitivity and further supports the notion that pain variability genes can have very selective effects on individual pain modalities.

Cough sensors. V. Pharmacological modulation of cough sensors

Several airway afferent nerve subtypes have been implicated in coughing. These include bronchopulmonary C-fibers, rapidly adapting airway mechanoreceptors and touch-sensitive tracheal Adelta-fibers (also called cough receptors). Although the last two afferent nerve subtypes are primarily sensitive to mechanical stimuli, all can be acted upon by one or more different chemical stimuli. In this review we catalogue the chemical agents that stimulate and/or modulate the activity of the airway afferent nerves involved in cough, and describe the specific mechanisms involved in these effects. In addition, we describe the mechanisms of action of a number of chemical inhibitors of these afferent nerve subtypes, and attempt to relate this information to the regulation of coughing in health and disease.

Distribution of the Na,K-ATPase alpha subunit in the rat spiral ganglion and organ of corti

Processing of sound in the cochlea involves both afferent and efferent innervation. The Na,K-ATPase (NKA) is essential for cells that maintain hyperpolarized membrane potentials and sodium and potassium concentration gradients. Heterogeneity of NKA subunit expression is one mechanism that tailors physiology to particular cellular demands. Therefore, to provide insight into molecular differences that distinguish the various innervation pathways in the cochlea, we performed a variety of double labeling experiments with antibodies against three of the alpha isoforms of the NKA (NKA alpha 1-3) and markers identifying particular subsets of neurons or supporting cells in whole mount preparations of the organ of Corti and spiral ganglion. We found that the NKA alpha 3 is abundantly expressed within the membranes of the spiral ganglion somata, the type I afferent terminals contacting the inner hair cells, and the medial efferent terminals contacting the outer hair cells. We also found expression of the NKA alpha 1 in the supporting cells that neighbor the inner hair cells and express the glutamate transporter GLAST. These findings suggest that both the NKA alpha 1 and NKA alpha 3 are poised to play an essential role in the regulation of the type I afferent synapses, the medial efferent synapses, and also glutamate transport from the afferent-inner hair cell synapse.

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.

The neurotrophic effects of glial cell line-derived neurotrophic factor on spinal motoneurons are restricted to fusimotor subtypes

Glial cell line-derived neurotrophic factor (GDNF) regulates multiple aspects of spinal motoneuron (MN) development, including gene expression, target selection, survival, and synapse elimination, and mice lacking either GDNF or its receptors GDNF family receptor alpha1 (GFRalpha1) and Ret exhibit a 25% reduction of lumbar MNs at postnatal day 0 (P0). Whether this loss reflects a generic trophic role for GDNF and thus a reduction of all MN subpopulations, or a more restricted role affecting only specific MN subpopulations, such as those innervating individual muscles, remains unclear. We therefore examined MN number and innervation in mice in which Ret, GFRalpha1, or GDNF was deleted and replaced by reporter alleles. Whereas nearly all hindlimb muscles exhibited normal gross innervation, intrafusal muscle spindles displayed a significant loss of innervation in most but not all muscles at P0. Furthermore, we observed a dramatic and restricted loss of small myelinated axons in the lumbar ventral roots of adult mice in which the function of either Ret or GFRalpha1 was inactivated in MNs early in development. Finally, we demonstrated that the period during which spindle-innervating MNs require GDNF for survival is restricted to early neonatal development, because mice in which the function of Ret or GFRalpha1 was inactivated after P5 failed to exhibit denervation of muscle spindles or MN loss. Therefore, although GDNF influences several aspects of MN development, the survival-promoting effects of GDNF during programmed cell death are mostly confined to spindle-innervating MNs.

Differential expression of Na+/K+-ATPase alpha-subunits in mouse hippocampal interneurones and pyramidal cells

The sodium pump (Na+/K+-ATPase), maintains intracellular and extracellular concentrations of sodium and potassium by catalysing ATP. Three sodium pump alpha subunits, ATP1A1, ATP1A2 and ATP1A3, are expressed in brain. We compared their role in pyramidal cells and a subset of interneurones in the subiculum. Interneurones were identified by their expression of GFP under the GAD-65 promoter. We used the sensitivity to the cardiac glycoside, ouabain, to discriminate between different alpha subunit isoforms. GFP-positive interneurones were depolarized by nanomolar doses of ouabain, but higher concentrations were needed to depolarize pyramidal cells. Comparison of pump currents in these cells revealed a current sensitive to low doses of ouabain in interneurones, while micromolar doses of ouabain were needed to suppress the pump current in subicular pyramidal cells. As predicted, nanomolar doses of ouabain increased the frequency but not the amplitudes of IPSPs in pyramidal cells. Immunostaining confirmed a differential distribution of alpha-subunits of the Na+/K+-ATPase in subicular interneurones and pyramidal cells. In conclusion, these data suggest that while ATP1A3-isoforms regulate sodium and potassium homeostasis in subicular interneurones, ATP1A1-isoforms assume this function in pyramidal cells. This differential expression of sodium pump isoforms may contribute to differences in resting membrane potential of subicular interneurones and pyramidal cells.

Sensory neural targets for the treatment of cough

1. Cough is a primary defensive reflex that protects the airways from potentially harmful stimuli. 2. During many respiratory diseases, the cough reflex threshold is lowered and coughing becomes excessive. 3. Currently available therapeutics are mostly ineffective at suppressing excessive coughing. 4. In the present review, we describe the sensory neural pathways involved in cough, how these pathways may become dysfunctional in airway disease and the most recent advances that have been made in identifying future targets for cough suppression.

Localization and irregular distribution of Na,K-ATPase in myelin sheath from rat sciatic nerve

Sodium, potassium adenosine triphosphatase (Na,K-ATPase) is a membrane-bound enzyme that maintains the Na(+) and K(+) gradients used in the nervous system for generation and transmission of bioelectricity. Recently, its activity has also been demonstrated during nerve regeneration. The present study was undertaken to investigate the ultrastructural localization and distribution of Na,K-ATPase in peripheral nerve fibers. Small blocks of the sciatic nerves of male Wistar rats weighing 250-300g were excised, divided into two groups, and incubated with and without substrate, the para-nitrophenyl phosphate (pNPP). The material was processed for transmission electron microscopy, and the ultra-thin sections were examined in a Philips CM 100 electron microscope. The deposits of reaction product were localized mainly on the axolemma, on axoplasmic profiles, and irregularly dispersed on the myelin sheath, but not in the unmyelinated axons. In the axonal membrane, the precipitates were regularly distributed on the cytoplasmic side. These results together with published data warrant further studies for the diagnosis and treatment of neuropathies with compromised Na,K-ATPase activity.

Sensory Receptors in the Visceral Pleura

Today, diagnosis and treatment of chest pain related to pathologic changes in the visceral pleura are often difficult. Data in the literature on the sensory innervation of the visceral pleura are sparse. The present study aimed at identifying sensory end-organs in the visceral pleura, and at obtaining more information about neurochemical coding. The immunocytochemcial data are mainly based on whole mounts of the visceral pleura of control and vagally denervated rats. It was shown that innervation of the rat visceral pleura is characterized by nerve bundles that enter in the hilus region and gradually split into slender bundles with a few nerve fibers. Separate nerve fibers regularly give rise to characteristic laminar terminals. Because of their unique association with the elastic fibers of the visceral pleura, we decided to refer to them as "visceral pleura receptors" (VPRs). Cryostat sections of rat lungs confirmed a predominant location on mediastinal and interlobar lung surfaces. VPRs can specifically be visualized by protein gene product 9.5 immunostaining, and were shown to express vesicular glutamate transporters, calbindin D28K, Na+/K+-ATPase, and P2X3 ATP-receptors. The sensory nerve fibers giving rise to VPRs appeared to be myelinated and to have a spinal origin. Because several of the investigated proteins have been reported as markers for sensory terminals in other organs, the present study revealed that VPRs display the neurochemical characteristics of mechanosensory and/or nociceptive terminals. The development of a live staining method, using AM1-43, showed that VPRs can be visualized in living tissue, offering an interesting model for future physiologic studies.

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.

Neuromuscular development in the absence of programmed cell death: phenotypic alteration of motoneurons and muscle

The widespread, massive loss of developing neurons in the central and peripheral nervous system of birds and mammals is generally considered to be an evolutionary adaptation. However, until recently, models for testing both the immediate and long-term consequences of preventing this normal cell loss have not been available. We have taken advantage of several methods for preventing neuronal death in vivo to ask whether rescued neurons [e.g., motoneurons (MNs)] differentiate normally and become functionally incorporated into the nervous system. Although many aspects of MN differentiation occurred normally after the prevention of cell death (including the expression of several motoneuron-specific markers, axon projections into the ventral root and peripheral nerves, ultrastructure, dendritic arborization, and afferent axosomatic synapses), other features of the neuromuscular system (MNs and muscle) were abnormal. The cell bodies and axons of MNs were smaller than normal, many MN axons failed to become myelinated or to form functional synaptic contacts with target muscles, and a subpopulation of rescued cells were transformed from alpha- to gamma-like MNs. Additionally, after the rescue of MNs in myogenin glial cell line-derived neurotrophic factor (MyoGDNF) transgenic mice, myofiber differentiation of extrafusal skeletal muscle was transformed and muscle physiology and motor behaviors were abnormal. In contrast, extrafusal myofiber phenotype, muscle physiology, and (except for muscle strength tests) motor behaviors were all normal after the rescue of MNs by genetic deletion of the proapoptotic gene Bax. However, there was an increase in intrafusal muscle fibers (spindles) in Bax knock-out versus both wild-type and MyoGDNF mice. Together, these data indicate that after the prevention of MN death, the neuromuscular system becomes transformed in novel ways to compensate for the presence of the thousands of excess cells.

Vagal afferent nerves regulating the cough reflex

Coughing is initiated by activation of mechanically and chemically sensitive vagal afferent nerves innervating the airways. All afferent nerve subtypes innervating the airways can modulate the cough reflex. Rapidly adapting and slowly adapting stretch receptors (RARs and SARs, respectively) innervating the intrapulmonary airways and lung may enhance and facilitate coughing. Activation of intrapulmonary C-fibers has been shown to inhibit coughing in anesthetized animals. Extrapulmonary C-fibers and RARs can initiate coughing upon activation. C-fiber-dependent coughing is uniquely sensitive to anesthesia. Tracheal and bronchial C-fibers may also interact with other afferents to enhance coughing. Recent studies in anesthetized guinea pigs have identified a myelinated afferent nerve subtype that can be differentiated from intrapulmonary RARs and SARs and play an essential role in initiating cough. Whether these "cough receptors" are the guinea pig equivalent of the irritant receptors described in the extrapulmonary airways of other species is unclear.

Evidence for a role of neuroepithelial bodies as complex airway sensors: comparison with smooth muscle-associated airway receptors

The epithelium of intrapulmonary airways in many species harbors diffusely spread innervated groups of neuroendocrine cells, called neuroepithelial bodies (NEBs). Data on the location, morphology, and chemical coding of NEBs in mammalian lungs are abundant, but none of the proposed functions has so far been fully established. Besides C-fiber afferents, slowly adapting stretch receptors, and rapidly adapting stretch receptors, recent reviews have added NEBs to the list of presumed sensory receptors in intrapulmonary airways. Physiologically, the innervation of NEBs, however, remains enigmatic. This short overview summarizes our present understanding of the chemical coding and exact location of the receptor end organs of myelinated vagal airway afferents in intrapulmonary airways. The profuse populations that selectively contact complex pulmonary NEB receptors are compared with the much smaller group of smooth muscle-associated airway receptors. The main objective of our contribution was to stimulate the idea that the different populations of myelinated vagal afferents that selectively innervate intraepithelial pulmonary NEBs may represent subpopulations of the extensive group of known electrophysiologically characterized myelinated vagal airway receptors. Future efforts should be directed toward finding out which airway receptor groups are selectively coupled to the complex NEB receptors.

"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.

On the functional interaction between nicotinic acetylcholine receptor and Na+,K+-ATPase

Previous studies have shown that nanomolar acetylcholine (ACh) produces a 2 to 4-mV hyperpolarization of skeletal muscle fibers putatively due to Na(+),K(+)-ATPase activation. The present study elucidates the involvement of the nicotinic ACh receptor (nAChR) and of Na(+),K(+)-ATPase isoform(s) in ACh-induced hyperpolarization of rat diaphragm muscle fibers. A variety of ligands of specific binding sites of nAChR and Na(+),K(+)-ATPase were used. Dose-response curves for ouabain, a specific Na(+),K(+)-ATPase inhibitor, were obtained to ascertain which Na(+),K(+)-ATPase isoform(s) is involved. The ACh dose-response relationship for the hyperpolarization was also determined. The functional relationship between these two proteins was also studied in a less complex system, a membrane preparation from Torpedo electric organ. The possibility of a direct ACh effect on Na(+),K(+)-ATPase was studied in purified lamb kidney Na(+),K(+)-ATPase and in rat red blood cells, systems where no nAChR is present. The results indicate that binding of nAChR agonists to their specific sites results in modulation of ouabain-sensitive (most probably alpha2) isoform of Na(+),K(+)-ATPase, leading to muscle membrane hyperpolarization. In the Torpedo preparation, ouabain modulates dansyl-C6-choline binding to nAChR, and vice versa. These results provide the first evidence of a functional interaction between nAChR and Na(+),K(+)-ATPase. Possible interaction mechanisms are discussed.

Sensory receptors in the airways: neurochemical coding of smooth muscle-associated airway receptors and pulmonary neuroepithelial body innervation

Mainly due to the lack of conclusive morphological data, correlation between functionally and morphologically defined lung receptors has so far been unsatisfactory. In the present study, multiple immunocytochemical stainings with a panel of markers for (mechanso)sensory nerve fibres were performed in order to visualise putative receptor terminals in rat intrapulmonary airways. We first focussed on determining the location, morphology and neurochemical coding of subepithelial receptor-like structures that have been sporadically reported in the wall of large diameter airways. Immunostaining with antibodies against Na+/K+-ATPase alpha3, vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 revealed branching laminar subepithelial receptor endings associated with airway smooth muscle. The latter nerve terminals appeared to further express calbindin D28k (CB), and the ATP receptor P2X3, but were calcitonin gene-related peptide (CGRP)-negative. The nerve fibres that give rise to these terminals were shown to be myelinated and have a vagal sensory origin. Because of the close association between the laminar terminals of this receptor-like structures and airway smooth muscle, we will further refer to these clearly morphologically identifiable sensory end organs as 'smooth muscle-associated airway receptors (SMARs)'. Secondly, we further explored the sensory innervation of pulmonary neuroepithelial bodies (NEBs). NEBs are intraepithelial groups of neuroendocrine cells, contacted by several nerve fibre populations, at least three of which are sensory. The spinal sensory innervation of NEBs expresses CGRP and substance P, contacts NEBs at their basal pole, and is capsaicin-sensitive. The intraepithelial vagal sensory innervation of NEBs, on the other hand, appears to be myelinated and could be labelled by antibodies against VGLUT1, VGLUT2, CB and P2X3 receptors. Na+/K+-ATPase alpha3 immunostaining additionally labelled part of the vagal sensory innervation of rat pulmonary NEBs. The neurochemical coding and receptor-like appearance of SMARs and of the complex vagal sensory innervation of NEBs appeared to be almost identical and reminiscent of mechanosensors. Both SMARs and vagal nodose nerve terminals in NEBs therefore likely represent the morphological counterparts of subgroups of the extensive population of physiologically characterised myelinated vagal airway receptors, the majority of which are mechanosensitive. Electrophysiological data based on 'local' stimuli should be interpreted with caution, because of the regular close apposition of SMARs and NEBs and the very similar characteristics of their nerve terminals.

Anatomy and Neurophysiology of the Cough Reflex

OBJECTIVES

To describe the anatomy and neurophysiology of the cough reflex.

METHODS

A review of the literature was carried out using PubMed and the ISI Web of Knowledge from 1951 to 2004. Most of the referenced studies were carried out in animals

CONCLUSIONS

Studies carried out in animals provide suggestive but inconclusive evidence that C-fibers and rapidly adapting receptors (RARs) arising from the vagus nerves mediate coughing. Recent studies also have suggested that a vagal afferent nerve subtype that is not readily classified as a RAR or a C-fiber may play an important role in regulating cough. Afferent nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature also likely play a less essential but important accessory role in regulating cough. The responsiveness and morphology of the airway vagal afferent nerve subtypes and the extrapulmonary afferent nerves that regulate coughing are described.

Neurochemical characterisation of sensory receptors in airway smooth muscle: comparison with pulmonary neuroepithelial bodies

Descriptions of morphologically well-defined sensory airway receptors are sparse, in contrast to the multiplicity of airway receptors that have been identified electrophysiologically. The present study aimed at further determining the location, morphology and neurochemical coding of subepithelial receptor-like structures that have been sporadically reported in the wall of large diameter airways. The results were compared with those obtained for pulmonary neuroepithelial bodies (NEBs), which are complex intraepithelial sensory airway receptors. Multiple immunocytochemical staining showed branching laminar subepithelial receptor-like endings, which were found to intercalate in the smooth muscle layer of intrapulmonary conducting airways in rats. Because of the consistent intimate association with the airway smooth muscle, the laminar terminals will further be referred to as 'smooth muscle-associated airway receptors (SMARs)'. SMARs were characterised by their Na(+)/K(+)-ATPase alpha3, vesicular glutamate transporter 1 (VGLUT1) and VGLUT2-immunoreactivity, expression of the ATP receptor P2X(3), and the presence of calcium-binding proteins. Nerve fibres giving rise to SMARs were shown to be myelinated and to have a vagal origin. Interestingly, the neurochemical coding and receptor-like appearance of SMARs appeared to be almost identical to at least part of the complex vagal sensory terminals in NEBs. Intraepithelial nerve endings in pulmonary NEBs were indeed also shown to originate from myelinated vagal afferent nerve fibres, and to express Na(+)/K(+)-ATPase alpha3, VGLUT1, VGLUT2, P2X(3) and calcium-binding proteins. Since several of the latter proteins have been reported as markers for mechanoreceptor terminals in other organs, both SMARs and the vagal nodose nerve terminals in NEBs seem good candidates to represent the morphological counterparts of at least subsets of the extensive population of physiologically characterised myelinated vagal airway mechanoreceptors. The observation that SMARs and NEBs are regularly found in each other's immediate neighbourhood, and the very similar characteristics of their nerve terminals, point out that the interpretation of electrophysiological data based on 'local' stimuli should be made with great caution.

Transcriptional regulation of myotube fate specification and intrafusal muscle fiber morphogenesis

Vertebrate muscle spindle stretch receptors are important for limb position sensation (proprioception) and stretch reflexes. The structurally complex stretch receptor arises from a single myotube, which is transformed into multiple intrafusal muscle fibers by sensory axon–dependent signal transduction that alters gene expression in the contacted myotubes. The sensory-derived signal transduction pathways that specify the fate of myotubes are very poorly understood. The zinc finger transcription factor, early growth response gene 3 (Egr3), is selectively expressed in sensory axon–contacted myotubes, and it is required for normal intrafusal muscle fiber differentiation and spindle development. Here, we show that overexpression of Egr3 in primary myotubes in vitro leads to the expression of a particular repertoire of genes, some of which we demonstrate are also regulated by Egr3 in developing intrafusal muscle fibers within spindles. Thus, our results identify a network of genes that are regulated by Egr3 and are involved in intrafusal muscle fiber differentiation. Moreover, we show that Egr3 mediates myotube fate specification that is induced by sensory innervation because skeletal myotubes that express Egr3 independent of other sensory axon regulation are transformed into muscle fibers with structural and molecular similarities to intrafusal muscle fibers. Hence, Egr3 is a target gene that is regulated by sensory innervation and that mediates gene expression involved in myotube fate specification and intrafusal muscle fiber morphogenesis.

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.