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Chapter 2 Physiology and function. ACTA ACUST UNITED AC 2006. [DOI: 10.1016/s1567-4231(09)70063-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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52
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Chapter 17 Assessment of nerve excitability properties in peripheral nerve disease. ACTA ACUST UNITED AC 2006. [DOI: 10.1016/s1567-4231(09)70078-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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53
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Maruyama H, Yamamoto M, Matsutomi T, Zheng T, Nakata Y, Wood JN, Ogata N. Electrophysiological characterization of the tetrodotoxin-resistant Na+ channel, Na(v)1.9, in mouse dorsal root ganglion neurons. Pflugers Arch 2005; 449:76-87. [PMID: 15290301 DOI: 10.1007/s00424-004-1315-0] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Small dorsal root ganglion neurons express preferentially the Na+ channel isoform Na(v)1.9 that mediates a tetrodotoxin-resistant (TTX-R) Na+ current. We investigated properties of the Na+ current mediated by Na(v)1.9 (I(NaN)) using the whole-cell, patch-clamp recording technique. To isolate I(NaN) from heterogeneous TTX-R Na+ currents that also contain another type of TTX-R Na+ current mediated by Na(v)1.8, we used Na(v)1.8-null mutant mice. When F- was used as an internal anion in the patch pipette solution, both the activation and inactivation kinetics for I(NaN) shifted in the hyperpolarizing direction with time. Such a time-dependent shift of the kinetics was not observed when Cl- was used as an internal anion. Functional expression of I(NaN) declined with time after cell dissociation and recovered during culture, implying that Na(v)1.9 may be regulated dynamically by trophic factors or depend on subtle environmental factors for its survival. During whole-cell recordings, the peak amplitude of I(NaN) increased dramatically after a variable delay, as if inactive or silent channels had been "kindled". Such an unusual increase of the amplitude could be prevented by adding ATP to the pipette solution or by recording with the nystatin-perforated patch-clamp technique, suggesting that the rupture of patch membrane affected the behaviour of Na(v)1.9. These peculiar properties of I(NaN) may provide an insight into the plasticity of Na+ channels that are related to pathological functions of Na+ channels accompanying abnormal pain states.
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Affiliation(s)
- Hiroshi Maruyama
- Department of Neurophysiology, Graduate School of Biomedical Sciences, Hiroshima University, 734-8551 Hiroshima, Japan
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Lai B, Zhang L, Dong LY, Zhu YH, Sun FY, Zheng P. Impact of inhibition of Qo site of mitochondrial complex III with myxothiazol on persistent sodium currents via superoxide and protein kinase C in rat hippocampal CA1 cells. Neurobiol Dis 2005; 21:206-16. [PMID: 16081299 DOI: 10.1016/j.nbd.2005.07.001] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2004] [Revised: 06/16/2005] [Accepted: 07/06/2005] [Indexed: 11/16/2022] Open
Abstract
Inhibition of Qo site of mitochondrial complex III under hypoxia has received attention, but its downstream pathways remain unclear. We used Qo site inhibitor myxothiazol to mimic the inhibition of the Qo site of complex III and studied the effects of the inhibition of this site on persistent and transient sodium currents and neuron excitability in rat hippocampal CA1 cells. The results showed myxothiazol apparently increased persistent sodium currents but with a weak effect on transient sodium currents; the effect of myxothiazol on persistent sodium currents was blocked by protein kinase C inhibitor and superoxide scavengers, but not by hydrogen peroxide scavenger and hydroxyl radical formation inhibitor; myxothiazol could increase the activity of protein kinase C and neuron excitability. These results suggest that the inhibition of Qo site of mitochondrial complex III increases persistent sodium currents via superoxide production and protein kinase C activation.
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Affiliation(s)
- Bin Lai
- State Key Laboratory of Medical Neurobiology, Fudan University Shanghai Medical College, Shanghai 200032, People's Republic of China
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55
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Baker MD. Protein kinase C mediates up-regulation of tetrodotoxin-resistant, persistent Na+ current in rat and mouse sensory neurones. J Physiol 2005; 567:851-67. [PMID: 16002450 PMCID: PMC1474230 DOI: 10.1113/jphysiol.2005.089771] [Citation(s) in RCA: 86] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
The tetrodotoxin-resistant (TTX-r) persistent Na(+) current, attributed to Na(V)1.9, was recorded in small (< 25 mum apparent diameter) dorsal root ganglion (DRG) neurones cultured from P21 rats and from adult wild-type and Na(V)1.8 null mice. In conventional whole-cell recordings intracellular GTP-gamma-S caused current up-regulation, an effect inhibited by the PKC pseudosubstrate inhibitor, PKC19-36. The current amplitude was also up-regulated by 25 microM intracellular 1-oleoyl-2-acetyl-sn-glycerol (OAG) consistent with PKC involvement. In perforated-patch recordings, phorbol 12-myristate 13-acetate (PMA) up-regulated the current, whereas membrane-permeant activators of protein kinase A (PKA) were without effect. PGE(2) did not acutely up-regulate the current. Conversely, both PGE(2) and PKA activation up-regulated the major TTX-r Na(+) current, Na(V)1.8. Extracellular ATP up-regulated the persistent current with an average apparent K(d) near 13 microM, possibly consistent with P2Y receptor activation. Numerical simulation of the up-regulation qualitatively reproduced changes in sensory neurone firing properties. The activation of PKC appears to be a necessary step in the GTP-dependent up-regulation of persistent Na(+) current.
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Affiliation(s)
- Mark D Baker
- Molecular Nociception Group, Department of Biology, Medawar Building, University College London, UK.
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56
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Wu N, Enomoto A, Tanaka S, Hsiao CF, Nykamp DQ, Izhikevich E, Chandler SH. Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability. J Neurophysiol 2005; 93:2710-22. [PMID: 15625100 DOI: 10.1152/jn.00636.2004] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The functional and biophysical properties of a persistent sodium current ( INaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. INaPactivated around −76 mV and peaked at −48 mV, with V1/2of −58.7 mV. Ramp voltage-clamp protocols showed that INaPundergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (τonset= 2.04 s, τrecov= 2.21 s). Riluzole (≤5 μM) substantially reduced INaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+current ( INaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the INaTwere obtained and revealed that the amplitude and voltage dependence of the resulting “window current” were not consistent with those of the observed INaPrecorded in the same neurons. This suggests an additional mechanism for the origin of INaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+and K+currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model gNaPparameters indicate that INaPis critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.
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Affiliation(s)
- Nanping Wu
- Department of Physiological Science, UCLA, 2859 Slichter Hall, Los Angeles, CA 90095, USA
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57
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Abstract
Marine poisoning results from the ingestion of marine animals that contain toxic substances and causes substantial illness in coastal regions. Three main clinical syndromes of marine poisoning have important neurological symptoms-ciguatera, tetrodotoxin poisoning, and paralytic shellfish poisoning. Ciguatera is the commonest syndrome of marine poisoning and is characterised by moderate to severe gastrointestinal effects (vomiting, diarrhoea, and abdominal cramps) and neurological effects (myalgia, paraesthesia, cold allodynia, and ataxia), but is rarely lethal. Tetrodotoxin poisoning and paralytic shellfish poisoning are less common but have a higher fatality rate than ciguatera. Mild gastrointestinal effects and a descending paralysis are characteristic of these types of poisoning. In severe poisoning, paralysis rapidly progresses to respiratory failure. Diagnosis of all types of marine poisoning is made from the circumstances of ingestion (type of fish and location) and the clinical effects. Because there are no antidotes, supportive care, including mechanical ventilation in patients with severe paralysis, is the mainstay of treatment.
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Affiliation(s)
- Geoffrey K Isbister
- Tropical Toxicology Unit, Menzies School of Health Research, Charles Darwin University, NT, and Department of Clinical Toxicology and Pharmacology, Newcastle Mater Misericordiae Hospital, NSW, Australia.
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58
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Hennings K, Arendt-Nielsen L, Andersen OK. Breakdown of accommodation in nerve: a possible role for persistent sodium current. Theor Biol Med Model 2005; 2:16. [PMID: 15826303 PMCID: PMC1090618 DOI: 10.1186/1742-4682-2-16] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2004] [Accepted: 04/12/2005] [Indexed: 11/10/2022] Open
Abstract
Background Accommodation and breakdown of accommodation are important elements of information processing in nerve fibers, as they determine how nerve fibers react to natural slowly changing stimuli or electrical stimulation. The aim of the present study was to elucidate the biophysical mechanism of breakdown of accommodation, which at present is unknown. Results A model of a space-clamped motor nerve fiber was developed. It was found that this new model could reproduce breakdown of accommodation when it included a low-threshold, rapidly activating, persistent sodium current. However, the phenomenon was not reproduced when the persistent sodium current did not have fast activation kinetics or a low activation threshold. Conclusion The present modeling study suggests that persistent, low-threshold, rapidly activating sodium currents have a key role in breakdown of accommodation, and that breakdown of accommodation can be used as a tool for studying persistent sodium current under normal and pathological conditions.
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Affiliation(s)
- Kristian Hennings
- Center for Sensory-Motor Interaction (SMI), Aalborg University. Frederik Bajers Vej D3-203, 9220 Aalborg Ø, Denmark
| | - Lars Arendt-Nielsen
- Center for Sensory-Motor Interaction (SMI), Aalborg University. Frederik Bajers Vej D3-203, 9220 Aalborg Ø, Denmark
| | - Ole K Andersen
- Center for Sensory-Motor Interaction (SMI), Aalborg University. Frederik Bajers Vej D3-203, 9220 Aalborg Ø, Denmark
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59
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Cummins TR, Dib-Hajj SD, Herzog RI, Waxman SG. Nav
1.6 channels generate resurgent sodium currents in spinal sensory neurons. FEBS Lett 2005; 579:2166-70. [PMID: 15811336 DOI: 10.1016/j.febslet.2005.03.009] [Citation(s) in RCA: 95] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2004] [Revised: 02/15/2005] [Accepted: 03/02/2005] [Indexed: 11/30/2022]
Abstract
The Na(v)1.6 voltage-gated sodium channel has been implicated in the generation of resurgent currents in cerebellar Purkinje neurons. Our data show that resurgent sodium currents are produced by some large diameter dorsal root ganglion (DRG) neurons from wild-type mice, but not from Na(v)1.6-null mice; small DRG neurons do not produce resurgent currents. Many, but not all, DRG neurons transfected with Na(v)1.6 produce resurgent currents. These results demonstrate for the first time the intrinsic ability of Na(v)1.6 to produce a resurgent current, and also show that cell background is critical in permitting the generation of these currents.
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Affiliation(s)
- Theodore R Cummins
- Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
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60
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Rush AM, Dib-Hajj SD, Waxman SG. Electrophysiological properties of two axonal sodium channels, Nav1.2 and Nav1.6, expressed in mouse spinal sensory neurones. J Physiol 2005; 564:803-15. [PMID: 15760941 PMCID: PMC1464456 DOI: 10.1113/jphysiol.2005.083089] [Citation(s) in RCA: 200] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Sodium channels Na(v)1.2 and Na(v)1.6 are both normally expressed along premyelinated and myelinated axons at different stages of maturation and are also expressed in a subset of demyelinated axons, where coexpression of Na(v)1.6 together with the Na(+)/Ca(2+) exchanger is associated with axonal injury. It has been difficult to distinguish the currents produced by Na(v)1.2 and Na(v)1.6 in native neurones, and previous studies have not compared these channels within neuronal expression systems. In this study, we have characterized and directly compared Na(v)1.2 and Na(v)1.6 in a mammalian neuronal cell background and demonstrate differences in their properties that may affect neuronal behaviour. The Na(v)1.2 channel displays more depolarized activation and availability properties that may permit conduction of action potentials, even with depolarization. However, Na(v)1.2 channels show a greater accumulation of inactivation at higher frequencies of stimulation (20-100 Hz) than Na(v)1.6 and thus are likely to generate lower frequencies of firing. Na(v)1.6 channels produce a larger persistent current that may play a role in triggering reverse Na(+)/Ca(2+) exchange, which can injure demyelinated axons where Na(v)1.6 and the Na(+)/Ca(2+) exchanger are colocalized, while selective expression of Na(v)1.2 may support action potential electrogenesis, at least at lower frequencies, while producing a smaller persistent current.
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Affiliation(s)
- Anthony M Rush
- Department of Neurology, Center for Neuroscience and Regeneration Research, Yale School of Medicine, LCI 707, 333 Cedar Street, New Haven, CT 06510, USA
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61
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Heckmann CJ, Gorassini MA, Bennett DJ. Persistent inward currents in motoneuron dendrites: implications for motor output. Muscle Nerve 2005; 31:135-56. [PMID: 15736297 DOI: 10.1002/mus.20261] [Citation(s) in RCA: 312] [Impact Index Per Article: 16.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
The dendrites of motoneurons are not, as once thought, passive conduits for synaptic inputs. Instead they have voltage-dependent channels that provide the capacity to generate a very strong persistent inward current (PIC). The amplitude of the PIC is proportional to the level of neuromodulatory input from the brainstem, which is mediated primarily by the monoamines serotonin and norepinephrine. During normal motor behavior, monoaminergic drive is likely to be moderately strong and the dendritic PIC generates many of the characteristic features of motor unit firing patterns. Most of the PIC activates at or below recruitment threshold and thus motor unit firing patterns exhibit a linear increase just above recruitment. The dendritic PIC allows motor unit derecruitment to occur at a lower input level than recruitment, thus providing sustained tonic firing with little or no synaptic input, especially in low-threshold units. However the dendritic PIC can be readily deactivated by synaptic inhibition. The overall amplification due to the dendritic PIC and other effects of monoamines on motoneurons greatly increases the input-output gain of the motor pool. Thus the brainstem neuromodulatory input provides a mechanism by which the excitability of motoneurons can be varied for different motor behaviors. This control system is lost in spinal cord injury but PICs nonetheless recover near-normal amplitudes in the months following the initial injury. The relationship of these findings to the cause of the spasticity syndrome developing after spinal cord injury is discussed.
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Affiliation(s)
- C J Heckmann
- Department of Physiology, Neuroscience Institute, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
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62
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Kuwabara S, Misawa S, Tamura N, Kanai K, Hiraga A, Ogawara K, Nakata M, Hattori T. The effects of mexiletine on excitability properties of human median motor axons. Clin Neurophysiol 2005; 116:284-9. [PMID: 15661106 DOI: 10.1016/j.clinph.2004.08.014] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/30/2004] [Indexed: 11/29/2022]
Abstract
OBJECTIVE To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo. METHODS Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day. RESULTS After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (P=0.01), increased rheobasic currents (P=0.06), and lower refractoriness (P=0.02), all of which were consistent with reduced nodal Na+ currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent Na+ conductance. The lowered refractoriness after treatment might result from reduced transient Na+ currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis. CONCLUSIONS Oral mexiletine in a dosage of 300 mg daily suppresses persistent Na+ currents in human motor axons. SIGNIFICANCE Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps.
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Affiliation(s)
- Satoshi Kuwabara
- Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan.
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63
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Misawa S, Kuwabara S, Ogawara K, Kitano Y, Hattori T. Strength-duration properties and glycemic control in human diabetic motor nerves. Clin Neurophysiol 2005; 116:254-8. [PMID: 15661101 DOI: 10.1016/j.clinph.2004.08.003] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/04/2004] [Indexed: 12/01/2022]
Abstract
OBJECTIVE To investigate the influences of hyperglycemia on axonal excitability in human diabetic nerves. Hyperglycemia results in decreased Na+-K+ pump function, presumably leading to intra-axonal Na+ accumulation and thereby, reduced Na+ currents. METHODS The strength-duration time constant (tau(SD)), which partly depends on persistent Na+ conductance active at the resting membrane potential, was measured in median motor axons of 79 diabetic patients. The relationship of tau(SD) with the state of glycemic control (hemoglobin A1c [HbA1c] levels) was analyzed. RESULTS The mean tau(SD) was longer for diabetic patients than for normal controls, but the difference was not significant. Among diabetic patients, the subgroup of patients with good glycemic control (HbA1c<7%) had significantly longer tau(SD) than the patient group with poor control (HbA1c>9%; P=0.04). The mean tau(SD) was longest at the HbA1c level of 5-6%, gradually decreasing and reaching a plateau around the HbA1c level of 9%. There was an inverse relationship between HbA1c levels and tau(SD), when the HbA1c levels ranged from 5 to 9% (P=0.04). CONCLUSIONS In diabetic nerves, tau(SD) is generally longer than normal, but hyperglycemia is associated with paradoxically shortened tau(SD), because of a decrease in axonal persistent Na+ conductance, possibly related to reduced membranous Na+ gradient, tissue acidosis, or other metabolic factors. SIGNIFICANCE Measurements of tau(SD) could provide a new insight into changes in ionic conductance in human diabetic nerves.
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Affiliation(s)
- Sonoko Misawa
- Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan.
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Kiernan MC, Isbister GK, Lin CSY, Burke D, Bostock H. Acute tetrodotoxin-induced neurotoxicity after ingestion of puffer fish. Ann Neurol 2005; 57:339-48. [PMID: 15732107 DOI: 10.1002/ana.20395] [Citation(s) in RCA: 141] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
This study documents the effects of puffer-fish poisoning on peripheral nerve. Excitability measurements investigated membrane properties of sensory and motor axons in four patients. The median nerve was stimulated at the wrist, with compound muscle potentials recorded from abductor pollicis brevis and compound sensory potentials from digit 2. Stimulus-responses, strength-duration time constant (tau(SD)), threshold electrotonus, and current-threshold relations were recorded. The urine of each patient tested positive for tetrodotoxin. Compared with controls, axons were of higher threshold, compound muscle action potentials and compound sensory nerve action potentials were reduced in amplitude, latency was prolonged, and tau(SD) was reduced. In recovery cycles, refractoriness, superexcitability, and late subexcitability were decreased. Threshold electrotonus of motor axons exhibited distinctive abnormalities with less threshold decline than normal on depolarization and greater threshold increase on hyperpolarization (p < 0.0005 for each patient). The changes in excitability were reproduced in a mathematical model by reducing sodium (Na(+)) permeabilities by a factor of two. This study confirms that the neurotoxic effects of puffer-fish poisoning can be explained by tetrodotoxin blockade of Na(+) channels. It demonstrates the ability of noninvasive nerve excitability studies to detect Na(+) channel blockade in vivo and also the utility of mathematical modeling to aid interpretation of altered excitability properties in disease.
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Affiliation(s)
- Matthew C Kiernan
- Prince of Wales Medical Research Institute, University of New South Wales, Barker Street, Randwick, Sydney NSW 2031, Australia.
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65
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Krishnan AV, Lin CSY, Kiernan MC. Excitability differences in lower-limb motor axons during and after ischemia. Muscle Nerve 2005; 31:205-13. [PMID: 15609346 DOI: 10.1002/mus.20258] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Neuropathic diseases typically begin distally and spread proximally. Irrespective of the etiology, pathological investigations often indicate changes consistent with ischemia. In the present study, threshold tracking was used to investigate length-dependent differences in ischemic susceptibility of lower-limb axons in 6 healthy volunteers, with ischemia induced by a sphygmomanometer cuff inflated to 200 mm Hg and maintained for 13 minutes. Following stimulation of the peroneal nerve at the fibula neck, compound muscle action potentials were recorded proximally from tibialis anterior (TA) and distally from extensor digitorum brevis (EDB). During ischemia, excitability changes were consistent with nerve depolarization, with a greater reduction in threshold in EDB than TA. This reduction in threshold was associated with an increase in refractoriness, decrease in superexcitability, and prolongation of strength-duration time constant, consistent with axonal depolarization. With release of ischemia, reversal of these changes was associated with an increase in threshold, greater in EDB than TA, indicating axonal hyperpolarization. The rate of recovery of threshold was similar proximally and distally, arguing against a gradient in Na(+)/K(+) pump function along the peroneal nerve. The greater changes in threshold in EDB during and after ischemia suggest an increased susceptibility of more distal axons to ischemia and are likely to contribute to the length-dependent development of neuropathy.
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Affiliation(s)
- Arun V Krishnan
- Prince of Wales Medical Research Institute and Prince of Wales Clinical School, University of New South Wales, Barker Street, Randwick, Sydney, NSW 2031, Australia
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66
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Schwab Y, Jahke R, Jover E. Expression of tetrodotoxin-sensitive and resistant sodium channels by rat melanotrophs. Neuroreport 2004; 15:1219-23. [PMID: 15129178 DOI: 10.1097/00001756-200405190-00028] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Rat melanotrophs fire Na+ and Ca2(+)-dependent action potentials. Whereas the molecular identity of Ca2+ channels expressed by these cells is well documented, less is known about Na channels. We characterize the expression of seven sodium channel alpha-subunit and the beta1- and beta2-subunit mRNAs. The tetrodotoxin-resistant Nav1.8 and Nav1.9 alpha subunit mRNAs are detected in the newborn intermediate lobe and in cultured melanotrophs. Electrophysiological recordings further demonstrate the expression of both tetrodotoxin-sensitive and tetrodotoxin-resistant currents by dissociated melanotrophs. Moreover, activated sodium channels are able to elicit intracellular calcium waves, both in the absence or in the presence of tetrodotoxin. This work shows that rat melanotrophs express functional tetrodotoxin-resistant sodium channels, whose activation can lead to the generation of intracellular calcium waves.
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Affiliation(s)
- Yannick Schwab
- UMR-CNRS-ULP 7519, Laboratoire de Neurophysiologie Cellulaire et Intégrée, Université Louis Pasteur 21, rue René Descartes F-67084 Strasbourg-Cedex, France.
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67
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Magistretti J, Ragsdale DS, Alonso A. Kinetic diversity of single-channel burst openings underlying persistent Na(+) current in entorhinal cortex neurons. Biophys J 2004; 85:3019-34. [PMID: 14581203 PMCID: PMC1303579 DOI: 10.1016/s0006-3495(03)74721-3] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
The kinetic diversity of burst openings responsible for the persistent Na(+) current (I(NaP)) in entorhinal cortex neurons was examined by separately analyzing single bursts. Although remarkable kinetic variability was observed among bursts in terms of intraburst opening probability and mean open and closed times, the values of time constants describing intraburst open times (tau(o(b))s) and closed times (tau(c(b))s) were distributed around well-defined peaks. At -40 mV, tau(o(b)) peaks were found at approximately 0.34 (tau(o(b))1) and 0.77 (tau(o(b))2) ms, and major tau(c(b)) peaks were found at approximately 0.24 (tau(c(b))1) and 0.54 (tau(c(b))2) ms. In approximately 80% of the bursts two preferential gating modes were found that consisted of a combination of either tau(o(b))1 and tau(c(b))2 ("intraburst mode 1"), or tau(o(b))2 and tau(c(b))1 ("intraburst mode 2"). Individual channels could switch between different gating modalities, but normally tended to maintain a specific gating mode for long periods. Mean burst duration also displayed considerable variability. At least three time constants were found to describe burst duration, and the frequencies at which each of the corresponding "bursting states" occurred varied in different channels. Short-lasting bursting states were preferentially associated with intraburst mode 1, whereas very-long-lasting bursts tended to gate according to mode 2 only or other modes that included considerably longer mean open times. These results show that I(NaP) channels can generate multiple intraburst open and closed states and bursting states, but these different kinetic states tend to combine in definite ways to produce a limited number of prevalent, well-defined gating modalities. Modulation of distinct gating modalities in individual Na(+) channels may be a powerful form of plasticity to influence neuronal excitability and function.
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Affiliation(s)
- Jacopo Magistretti
- Dipartimento di Scienze Fisiologiche-Farmacologiche Cellulari-Molecolari, Università degli Studi di Pavia, Pavia, Italy.
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68
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Kitano Y, Kuwabara S, Misawa S, Ogawara K, Kanai K, Kikkawa Y, Yagui K, Hattori T. The acute effects of glycemic control on axonal excitability in human diabetics. Ann Neurol 2004; 56:462-7. [PMID: 15455400 DOI: 10.1002/ana.20232] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
In diabetic nerves, the activation of the polyol pathway and a resulting decrease in Na(+)-K(+) ATPase activity lead to intra-axonal Na(+) accumulation and a smaller Na(+) gradient across the axolemma than normal. To investigate whether glycemic control is associated with acutely reversible changes in axonal excitability and Na(+) conductance, we measured the multiple excitability indices (strength-duration time constant, rheobase, refractoriness, and refractory period) of the median motor axons of 21 diabetic patients before and after intensive insulin treatment. Within 4 weeks after treatment was begun, there was a significant improvement in nerve conduction velocities, associated with increased strength-duration time constant, decreased rheobase, increased refractoriness, and prolonged refractory periods. Assuming that the strength-duration time constant partly reflects persistent Na(+) conductance, and that refractoriness/refractory periods depend on inactivation of transient Na(+) channels caused by prior depolarization (the influx of Na(+)), the patterns of changes in these indices may reflect a reduced trans-axonal Na(+) gradient during hyperglycemia and its restoration by glycemic control in diabetic patients. Measurement of the excitability indices could provide new insights into the pathophysiology of human diabetic neuropathy.
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Affiliation(s)
- Yukiko Kitano
- Department of Neurology, Chiba University School of Medicine, Chiba, Japan
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69
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Kiernan MC, Baker MD, Bostock H. Characteristics of late Na(+) current in adult rat small sensory neurons. Neuroscience 2003; 119:653-60. [PMID: 12809686 DOI: 10.1016/s0306-4522(03)00194-5] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Na(+) currents were recorded using patch-clamp techniques from small-diameter (<25 micrometers) dorsal root ganglion neurons, cultured from adult rats (>150 g). Late Na(+) currents maintained throughout long-duration voltage-clamp steps (>/=200 ms) were of two types: a low-threshold, tetrodotoxin-sensitive (TTX-s) current that was largely blocked by 200 nM TTX, and a high-threshold, TTX-resistant (TTX-r) current. TTX-s late current was found in approximately 28% (10/36) of small-diameter neurons and was recorded only in neurons exhibiting TTX-s transient current. TTX-s transient current activation/inactivation gating overlap existed over a narrow potential range, centered between -30 and -40 mV, whereas late current operated over a wider range. The kinetics associated with de-inactivation of TTX-s late current were slow (tau approximately 37 ms at -50 mV), strongly suggesting that different subpopulations of TTX-s channel generate transient and late current. High-threshold TTX-r late current was only present in neurons generating TTX-r transient current. TTX-r late current operated over the same potential range as that for TTX-r transient current activation/inactivation gating overlap, and activation/inactivation gating overlap could be measured even after 1.5-s-duration pre-pulses. We suggest that TTX-s late sodium current results from channel openings different from those generating transient current. As in large-diameter sensory neurons, TTX-s channels generating late openings may play a key role in controlling membrane excitability. In contrast, a single population of high-threshold TTX-r channels may account for both transient and late TTX-r currents.
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Affiliation(s)
- M C Kiernan
- Sobell Department of Neurophysiology, Institute of Neurology, Queen Square, WC1N 3BG, London, UK.
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70
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Rybak IA, Ptak K, Shevtsova NA, McCrimmon DR. Sodium currents in neurons from the rostroventrolateral medulla of the rat. J Neurophysiol 2003; 90:1635-42. [PMID: 12761275 DOI: 10.1152/jn.00150.2003] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Rapidly inactivating and persistent sodium currents have been characterized in acutely dissociated neurons from the area of rostroventrolateral medulla that included the pre-Bötzinger Complex. As demonstrated in many studies in vitro, this area can generate endogenous rhythmic bursting activity. Experiments were performed on neonate and young rats (P1-15). Neurons were investigated using the whole cell voltage-clamp technique. Standard activation and inactivation protocols were used to characterize the steady-state and kinetic properties of the rapidly inactivating sodium current. Slow depolarizing ramp protocols were used to characterize the noninactivating sodium current. The "window" component of the rapidly inactivating sodium current was calculated using mathematical modeling. The persistent sodium current was revealed by subtraction of the window current from the total noninactivating sodium current. Our results provide evidence of the presence of persistent sodium currents in neurons of the rat rostroventrolateral medulla and determine voltage-gated characteristics of activation and inactivation of rapidly inactivating and persistent sodium channels in these neurons.
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Affiliation(s)
- Ilya A Rybak
- School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania 19104, USA.
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71
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Tokuno HA, Kocsis JD, Waxman SG. Noninactivating, tetrodotoxin-sensitive Na+ conductance in peripheral axons. Muscle Nerve 2003; 28:212-7. [PMID: 12872326 DOI: 10.1002/mus.10421] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
A noninactivating, persistent sodium current has been demonstrated previously in dorsal root ganglia neurons and in rat optic nerve. We report here that Na(+) channel blockade with tetrodotoxin (TTX) in isolated dorsal and ventral roots elicits membrane hyperpolarization, suggesting the presence of a persistent Na(+) current in peripheral axons. We used a modified sucrose-gap chamber to monitor resting and action potentials and observed a hyperpolarizing shift in the nerve potential of rat dorsal and ventral roots by TTX. The block of transient inward Na(+) currents was confirmed by the abolition of compound action potentials (CAPs). Moreover, depolarization of nerve roots by elevating extracellular K(+) concentrations to 40 mM eliminated CAPs but did not significantly alter TTX-induced hyperpolarizations, indicating that the persistent Na(+) currents in nerve roots are not voltage-dependent. Tetrodotoxin-sensitive persistent inward Na(+) currents are present in both dorsal and ventral root axons at rest and may contribute to axonal excitability.
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Affiliation(s)
- Hajime A Tokuno
- Rehabilitation Research Center, VA Hospital, West Haven, Connecticut 06516, USA
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72
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Baker MD, Chandra SY, Ding Y, Waxman SG, Wood JN. GTP-induced tetrodotoxin-resistant Na+ current regulates excitability in mouse and rat small diameter sensory neurones. J Physiol 2003; 548:373-82. [PMID: 12651922 PMCID: PMC2342858 DOI: 10.1113/jphysiol.2003.039131] [Citation(s) in RCA: 149] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
Peripheral pain thresholds are regulated by the actions of inflammatory mediators. Some act through G-protein-coupled receptors on voltage-gated sodium channels. We have found that a low-threshold, persistent tetrodotoxin-resistant Na+ current, attributed to NaV1.9, is upregulated by GTP and its non-hydrolysable analogue GTP-gamma-S, but not by GDP. Inclusion of GTP-gamma-S (500 microM) in the internal solution led to an increase in maximal current amplitude of > 300 % within 5 min. In current clamp, upregulation of persistent current was associated with a more negative threshold for action potential induction (by 15-16 mV) assessed from a holding potential of -90 mV. This was not seen in neurones without the low-threshold current or with internal GDP (P < 0.001). In addition, persistent current upregulation depolarized neurones. At -60 mV, internal GTP-gamma-S led to the generation of spontaneous activity in initially silent neurones only when persistent current was upregulated. These findings suggest that regulation of the persistent current has important consequences for nociceptor excitability.
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Affiliation(s)
- Mark D Baker
- Molecular Nociception Group, Department of Biology, Medawar Building, University College London, Gower Street, UK.
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73
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Scholz A, Appel N, Vogel W. Two types of TTX-resistant and one TTX-sensitive Na+channel in rat dorsal root ganglion neurons and their blockade by halothane. Eur J Neurosci 2003. [DOI: 10.1046/j.1460-9568.1998.00268.x] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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74
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Buitenweg JR, Rutten WLC, Marani E. Extracellular stimulation window explained by a geometry-based model of the neuron-electrode contact. IEEE Trans Biomed Eng 2002; 49:1591-9. [PMID: 12549741 DOI: 10.1109/tbme.2002.804504] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Extracellular stimulation of single cultured neurons which are completely sealing a microelectrode is usually performed using anodic or biphasic currents of at least 200 nA. However, recently obtained experimental data demonstrate the possibility to stimulate a neuron using cathodic current pulses with less amplitude. Also, a stimulation window is observed. These findings can be explained by a finite-element model which permits geometry-based electrical representation of the neuron-electrode interface and can be used to explore the required conditions for extracellular stimulation in detail. Modulation of the voltage sensitive channels in the sealing part of the membrane appears to be the key to successful cathodic stimulation. Furthermore, the upper limit of the stimulation window can be explained as a normal consequence of the neuronal membrane electrophysiology.
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Affiliation(s)
- Jan Reinoud Buitenweg
- Institute for Biomedical Technology, Signals and Systems Group, Faculty of Electrical Engineering, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands.
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75
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Buitenweg JR, Rutten WLC, Marani E. Modeled channel distributions explain extracellular recordings from cultured neurons sealed to microelectrodes. IEEE Trans Biomed Eng 2002; 49:1580-90. [PMID: 12549740 DOI: 10.1109/tbme.2002.805555] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Amplitudes and shapes of extracellular recordings from single neurons cultured on a substrate embedded microelectrode depend not only on the volume conducting properties of the neuron-electrode interface, but might also depend on the distribution of voltage-sensitive channels over the neuronal membrane. In this paper, finite-element modeling is used to quantify the effect of these channel distributions on the neuron-electrode contact. Slight accumulation or depletion of voltage-sensitive channels in the sealing membrane of the neuron results in various shapes and amplitudes of simulated extracellular recordings. However, estimation of channel-specific accumulation factors from extracellular recordings can be obstructed by co-occuring ion currents and defect sealing. Experimental data from cultured neuron-electrode interfaces suggest depletion of sodium channels and accumulation of potassium channels.
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Affiliation(s)
- Jan Reinoud Buitenweg
- Institute for Biomedical Technology, Signals and Systems Group, Faculty of Electrical Engineering, University of Twente. PO Box 217, 7500 AE Enschede, The Netherlands.
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76
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Lin CSY, Chan JHL, Pierrot-Deseilligny E, Burke D. Excitability of human muscle afferents studied using threshold tracking of the H reflex. J Physiol 2002; 545:661-9. [PMID: 12456841 PMCID: PMC2290676 DOI: 10.1113/jphysiol.2002.026526] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
In human peripheral nerves, physiological evidence has been presented for a number of biophysical differences between cutaneous afferents and alpha motor axons. The differences in strength-duration properties for cutaneous afferents and motor axons in the median nerve have been attributed to greater expression of a persistent Na(+) conductance (I(Na,P)) on cutaneous afferents. However, it is unclear whether the biophysical properties of human group Ia afferents differ from those of cutaneous afferents. The present studies were undertaken to determine whether the properties of human group Ia afferents can be studied indirectly using 'threshold tracking' to measure the excitability changes in the H reflex, and to determine whether the excitability of group Ia afferents differs from that of cutaneous afferents. The strength-duration properties of the soleus H reflex and soleus motor axons were measured at rest and during sustained voluntary contractions. Similar experiments were performed on the median nerve at the wrist to study the strength-duration properties of cutaneous afferents, alpha motor axons and H reflex of the thenar muscles. In addition, the technique of 'latent addition' was used to determine whether there was a difference in a low-threshold conductance on soleus Ia afferent and motor axons. The present findings indicate that the strength-duration time constant (tau(SD)) for the H reflex is longer than that for alpha motor axons, but similar to that for cutaneous afferents. There were no differences in tau(SD) for the soleus H reflex at rest and during contractions, suggesting that tau(SD) for the H reflex is largely unaffected by changes in synaptic or motoneurone properties. Finally, the difference in latent addition suggests that the longer tau(SD) of the soleus H reflex may indeed be due to greater activity of a persistent Na(+) conductance on Ia afferents than on soleus alpha motor axons.
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Affiliation(s)
- Cindy S-Y Lin
- Prince of Wales Medical Research Institute, University of New South Wales, and College of Health Sciences, University of Sydney, Sydney, Australia
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77
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Ogata N, Ohishi Y. Molecular diversity of structure and function of the voltage-gated Na+ channels. JAPANESE JOURNAL OF PHARMACOLOGY 2002; 88:365-77. [PMID: 12046980 DOI: 10.1254/jjp.88.365] [Citation(s) in RCA: 123] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
A variety of different isoforms of voltage-sensitive Na+ channels have now been identified. The recent three-dimensional analysis of Na+ channels has unveiled a unique and unexpected structure of the Na+ channel protein. Na+ channels can be classified into two categories on the basis of their amino acid sequence, Nav1 isoforms currently comprising nine highly homologous clones and Nax that possesses structure diverging from Nav1, especially in several critical functional motifs. Although the functional role of Nav1 isoforms is primarily to form an action potential upstroke in excitable cells, recent biophysical studies indicate that some of the Nav1 isoforms can also influence subthreshold electrical activity through persistent or resurgent Na+ currents. Nav1.8 and Nav1.9 contain an amino acid sequence common to tetrodotoxin resistant Na+ channels and are localized in peripheral nociceptors. Recent patch-clamp experiments on dorsal root ganglion neurons from Nav1.8-knock-out mice unveiled an additional tetrodotoxin-resistant Na+ current. The demonstration of its dependence on Nav1.9 provides evidence for a specialized role of Nav1.9, together with Nav1.8, in pain sensation. Although Nax has not been successfully expressed in an exogenous system, recent investigations using relevant native tissues combined with gene-targeting have disclosed their unique "concentration"-sensitive but not voltage-sensitive roles. In this context, these emerging views of novel functions mediated by different types of Na+ channels are reviewed, to give a perspective for future research on the expanding family of Na+ channel clones.
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Affiliation(s)
- Nobukuni Ogata
- Department of Physiology, Hiroshima University School of Medicine, Japan.
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78
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Lin CSY, Grosskreutz J, Burke D. Sodium channel function and the excitability of human cutaneous afferents during ischaemia. J Physiol 2002; 538:435-46. [PMID: 11790811 PMCID: PMC2290064 DOI: 10.1113/jphysiol.2001.012478] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The changes in excitability of cutaneous afferents in the median nerve of healthy subjects were compared during 13 min of ischaemia and during 13 min continuous depolarizing DC. In addition, intermittent polarizing currents were used to compensate for or to accentuate the threshold change produced by ischaemia. Measurements were made alternately of the ischaemic (or current-induced) changes in threshold, refractoriness and, in some experiments, supernormality. The strength-duration time constant (tau(SD)) was calculated from the thresholds to test stimuli of different duration. During ischaemia for 13 min, the threshold decreased steadily by 34 % over the initial 8 min, reached a plateau and increased slightly over the final few minutes. However, with continuous depolarizing DC, the threshold decreased linearly with the applied current, by 55 % with strong current ramps. Intermittent injection of hyperpolarizing DC was used to compensate for the ischaemic threshold change, but the compensating current increased progressively and did not reach a plateau as had occurred with the ischaemic threshold change. During ischaemia, tau(SD) increased to a plateau, following the threshold more closely than the current required to compensate for threshold. Refractoriness, on the other hand, increased more steeply than the applied compensating current. There were similar discrepancies in the relationships of tau(SD) and refractoriness to supernormality. The smaller-than-expected threshold change during ischaemia could result from limitations on the change in excitability produced by ischaemic metabolites acting on the gating and/or permeability of Na(+) channels. Intermittent depolarizing DC was applied during the ischaemic depolarization to determine whether it would reduce or accentuate the discrepancies noted during ischaemia alone. The extent of the threshold change was greater than with ischaemia alone, and there was a greater change in tau(SD) and a proportionately smaller change in refractoriness. It is concluded that ischaemia produces factors that can block Na(+) channels and/or alter their gating. Without these processes, the ischaemic change in threshold would be much greater than that actually recorded, probably sufficient to produce prominent ectopic impulse activity.
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Affiliation(s)
- Cindy S-Y Lin
- Prince of Wales Medical Research Institute, University of New South Wales and Department of Neurology, The Prince of Wales Hospital, Sydney, Australia
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79
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Abstract
The excitability of human axons can be studied reliably using the technique of threshold tracking, which allows the strength of a test stimulus to be adjusted by computer to activate a defined fraction of the maximal nerve or muscle action potential. The stimulus current that just evokes the target response is considered the "threshold" for that response. More useful than the resting threshold are other indices of axonal excitability derived from pairs of threshold measurements, such as refractoriness, supernormality, strength-duration time constant and "threshold electrotonus" (i.e. the changes in threshold produced by long-lasting depolarizing or hyperpolarizing current pulses). Each of these measurements depends on membrane potential and on other biophysical properties of the axons. Together they can provide new information about the pathophysiology underlying abnormalities in excitability in neuropathy.
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Affiliation(s)
- D Burke
- Department of Neurology, Prince of Wales Hospital and Medical Research Institute, University of New South Wales, Barker Street, Randwick, Sydney, N.S.W., Australia.
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80
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Thoby-Brisson M, Ramirez JM. Identification of two types of inspiratory pacemaker neurons in the isolated respiratory neural network of mice. J Neurophysiol 2001; 86:104-12. [PMID: 11431492 DOI: 10.1152/jn.2001.86.1.104] [Citation(s) in RCA: 148] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
In the respiratory network of mice, we characterized with the whole cell patch-clamp technique pacemaker properties in neurons discharging in phase with inspiration. The respiratory network was isolated in a transverse brain stem slice containing the pre-Bötzinger complex (PBC), the presumed site for respiratory rhythm generation. After blockade of respiratory network activity with 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX), 18 of 52 inspiratory neurons exhibited endogenous pacemaker activity, which was voltage dependent, could be reset by brief current injections and could be entrained by repetitive stimuli. In the pacemaker group (n = 18), eight neurons generated brief bursts (0.43 +/- 0.03 s) at a relatively high frequency (1.05 +/- 0.12 Hz) in CNQX. These bursts resembled the bursts that these neurons generated in the intact network during the interval between two inspiratory bursts. Cadmium (200 microM) altered but did not eliminate this bursting activity, while 0.5 microM tetrodotoxin suppressed bursting activity. Another set of pacemaker neurons (10 of 18) generated in CNQX longer bursts (1.57 +/- 0.07 s) at a lower frequency (0.35 +/- 0.01 Hz). These bursts resembled the inspiratory bursts generated in the intact network in phase with the population activity. This bursting activity was blocked by 50-100 microM cadmium or 0.5 microM tetrodotoxin. We conclude that the respiratory neural network contains pacemaker neurons with two types of bursting properties. The two types of pacemaker activities might have different functions within the respiratory network.
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Affiliation(s)
- M Thoby-Brisson
- Department of Organismal Biology and Anatomy, Committee on Neurobiology, The University of Chicago, Chicago, Illinois 60637, USA
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81
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Torkkeli PH, Sekizawa S, French AS. Inactivation of voltage-activated Na(+) currents contributes to different adaptation properties of paired mechanosensory neurons. J Neurophysiol 2001; 85:1595-602. [PMID: 11287483 DOI: 10.1152/jn.2001.85.4.1595] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Voltage-activated sodium current (I(Na)) is primarily responsible for the leading edge of the action potential in many neurons. While I(Na) generally activates rapidly when a neuron is depolarized, its inactivation properties differ significantly between different neurons and even within one neuron, where I(Na) often has slowly and rapidly inactivating components. I(Na) inactivation has been suggested to regulate action potential firing frequency in some cells, but no clear picture of this relationship has emerged. We studied I(Na) in both members of the paired mechanosensory neurons of a spider slit-sense organ, where one neuron adapts rapidly (type A) and the other slowly (type B) in response to a step depolarization. In both neuron types I(Na) activated and inactivated with single time constants of 2--3 ms and 5--10 ms, respectively, varying with the stimulus intensity. However, there was a clear difference in the steady-state inactivation properties of the two neuron types, with the half-maximal inactivation (V(50)) being -40.1 mV in type A neurons and -58.1 mV in type B neurons. Therefore I(Na) inactivated closer to the resting potential in the more slowly adapting neurons. I(Na) also recovered from inactivation significantly faster in type B than type A neurons, and the recovery was dependent on conditioning voltage. These results suggest that while the rate of I(Na) inactivation is not responsible for the difference in the adaptation behavior of these two neuron types, the rate of recovery from inactivation may play a major role. Inactivation at lower potentials could therefore be crucial for more rapid recovery.
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Affiliation(s)
- P H Torkkeli
- Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada.
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82
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Mazzocchio R, Scarfò GB, Cartolari R, Bolognini A, Mariottini A, Muzii VF, Palma L. Abnormalities of the soleus H-reflex in lumbar spondylolisthesis: a possible early sign of bilateral S1 root dysfunction. JOURNAL OF SPINAL DISORDERS 2000; 13:487-95. [PMID: 11132979 DOI: 10.1097/00002517-200012000-00004] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Using routine electrodiagnostic procedures, the authors searched for physiologic evidence of nerve root compromise in patients with chronic mechanical perturbation to the lumbar spine. They examined 37 patients with spondylolisthesis and various degrees of degenerative changes in the lumbar canal. Clinical and neurophysiologic findings were compared with data obtained from 36 healthy persons. The soleus H-reflex appeared to be a sensitive indicator of sensory fiber compromise at the S1 root level, because changes correlated well with the focal sensory signs and preceded clinical and electromyographic signs of motor root involvement. When these occurred, the clinical findings were consistent with a more severe nerve root deficit and with radiographic evidence of neural compression. The greater sensitivity of the soleus H-reflex may be related to the pathophysiologic events that occur at the lesion site.
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Affiliation(s)
- R Mazzocchio
- Institute of Neurosurgery, University of Siena, Italy.
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83
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Mogyoros I, Lin CS, Kuwabara S, Cappelen-Smith C, Burke D. Strength-duration properties and their voltage dependence as measures of a threshold conductance at the node of Ranvier of single motor axons. Muscle Nerve 2000; 23:1719-26. [PMID: 11054751 DOI: 10.1002/1097-4598(200011)23:11<1719::aid-mus8>3.0.co;2-w] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
In a number of clinical studies, measurement of axonal strength-duration properties has been used to provide indirect insight into conductances at the node of Ranvier, particularly persistent Na(+) conductance. However, the specificity of any changes is limited because other factors can affect strength-duration behavior. The present study was undertaken to define the relationship between different strength-duration measures at rest and at different membrane potentials, and also to determine the limits within which strength-duration behavior can be used as a measure of nodal conductances. The strength-duration time constant (tau(SD)) and rheobase of 20 single motor units in the flexor carpi ulnaris were calculated from thresholds defined using threshold tracking. "True" rheobase and rheobasic latencies were measured using test stimuli of 100-ms duration. For ten units, the technique of latent addition was used to measure threshold changes directly attributable to nodal conductances, and for six units these were compared with strength-duration properties at different membrane potentials. The data indicate that measurements of tau(SD) and rheobase can provide sensitive indicators of conductances present at the node of Ranvier when membrane potential changes. There is a reciprocal relationship between tau(SD) and rheobase for single motor units at different membrane potentials, and this relationship may allow changes in tau(SD) due to depolarization and demyelination to be differentiated.
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Affiliation(s)
- I Mogyoros
- Department of Neurology, Prince of Wales Hospital and Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia.
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84
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Abstract
The effects of 0.1-100 microM riluzole, a neuroprotective agent with anticonvulsant properties, were studied on neurons from rat brain cortex. Patch-clamp whole-cell recordings in voltage-clamp mode were performed on thin slices to examine the effects of the drug on a noninactivating (persistent) Na+ current (INa,p). INa,p was selected because it enhances neuronal excitability near firing threshold, which makes it a potential target for anticonvulsant drugs. When added to the external solution, riluzole dose-dependently inhibited INa,p up to a complete blocking of the current (EC50 2 microM), showing a significant effect at therapeutic drug concentrations. A comparative dose-effect study was carried out in the same cells for the other main known action of riluzole, the inhibitory effect on the fast transient sodium current. This effect was confirmed in our experiments, but we found that it was achieved at levels much higher than putative therapeutic concentrations. Only the effect on INa,p, and not that on fast sodium current, can account for the reduction in neuronal excitability observed in cortical neurons following riluzole treatment at therapeutic concentrations, and this might represent a novel mechanism accounting for the anticonvulsant and neuroprotective properties of riluzole.
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Affiliation(s)
- A Urbani
- Dipartimento di Scienze Biomediche & Terapie Avanzate, Sezione di Fisiologia Umana, Università di Ferrara, Via Fossato di Mortara 17/19, 44100 Ferrara, Italy
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85
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Baker MD. Selective block of late Na(+) current by local anaesthetics in rat large sensory neurones. Br J Pharmacol 2000; 129:1617-26. [PMID: 10780966 PMCID: PMC1572015 DOI: 10.1038/sj.bjp.0703261] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The actions of lignocaine and benzocaine on transient and late Na(+) current generated by large diameter (> or =50 microm) adult rat dorsal root ganglion neurones, were studied using patch-clamp techniques. Both drugs blocked whole-cell late Na(+) current in a concentration-dependent manner. At 200 ms following the onset of a clamp step from -110 to -40 mV, the apparent K for block of late Na(+) current by lignocaine was 57.8+/-15 microM (mean+/-s.e.mean, n = 4). The value for benzocaine was 24.9+/-3.3 microM, (mean+/-s.e. mean, n = 3). The effect of lignocaine on transient current, in randomly selected neurones, appeared variable (n = 8, half-block from approximately 50 to 400 microM). Half-block by benzocaine was not attained, but both whole-cell (n = 11) and patch data suggested a high apparent K,>250 microM. Transient current always remained after late current was blocked. The voltage-dependence of residual late current steady-state inactivation was not shifted by 20 microM benzocaine (n = 3), whereas 200 microM benzocaine shifted the voltage-dependence of transient current steady-state inactivation by -18.7+/-5.9 mV (mean+/-s.e.mean, n = 4). In current-clamp, benzocaine (250 microM) could block subthreshold, voltage-dependent inward current, increasing the threshold for eliciting action potentials, without preventing their generation (n = 2). Block of late Na(+) current by systemic local anaesthetic may play a part in preventing ectopic impulse generation in sensory neurones.
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Affiliation(s)
- Mark D Baker
- Sobell department of Neurophysiology, Institute of Neurology, Queen Square, London WC1N 3BG
- Author for correspondence:
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86
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Grosskreutz J, Lin CS, Mogyoros I, Burke D. Ischaemic changes in refractoriness of human cutaneous afferents under threshold-clamp conditions. J Physiol 2000; 523 Pt 3:807-15. [PMID: 10718757 PMCID: PMC2269819 DOI: 10.1111/j.1469-7793.2000.t01-1-00807.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
1. A technique was developed to counteract the changes in threshold to electrical stimuli of large myelinated cutaneous afferents in the human median nerve induced by ischaemia for 13 min. Intermittent application of polarizing currents was used in five subjects, in whom refractoriness, supernormality and the strength-duration time constant (tauSD) were tracked to determine whether compensating for the ischaemia-induced changes in threshold also controlled the ischaemic changes in these excitability parameters. 2. The threshold compensation prevented the ischaemic changes in tauSD, an excitability parameter dependent on nodal Na+ channels. Threshold compensation did not prevent the changes in refractoriness and supernormality, whether the compensation began 10, 100 or 200 ms prior to the test stimuli. 3. In three subjects, continuous polarizing current was injected for 13 min to compensate for the ischaemic change in threshold, thus clamping threshold at the pre-ischaemic level. Again, tauSD was effectively controlled, but there were still ischaemic changes in refractoriness and supernormality. 4. The effective control of tauSD suggests that both the intermittent threshold compensation and the continuous threshold clamp effectively controlled membrane potential at the node of Ranvier. 5. The ischaemic increase in refractoriness when threshold was kept constant could be due to interference with the processes responsible for refractoriness by a metabolic product of ischaemia. The ischaemic change in supernormality during effective compensation probably results from the intrusion of refractoriness into the conditioning-test intervals normally associated with maximal supernormality. 6. The present results indicate that ischaemia has effects on axonal excitability that cannot be readily explained by changes in membrane potential. Specifically, it is suggested that ischaemic metabolites interfere with the recovery of Na+ channels from inactivation.
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Affiliation(s)
- J Grosskreutz
- Department of Neurology, The Prince Henry and Prince of Wales Hospitals, and Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia
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87
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Ectopic mechanosensitivity in injured sensory axons arises from the site of spontaneous electrogenesis. Eur J Pain 2000; 2:165-178. [PMID: 10700312 DOI: 10.1016/s1090-3801(98)90009-x] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Injured sensory axons trapped in a neuroma or freely regenerating in the distal nerve stump, frequently display ectopic mechanosensitivity, spontaneous impulse discharge or both. This abnormal neural activity is thought to contribute to spontaneous and movement-evoked neuropathic paraesthesias, dysaesthesias and pain, as well as to allodynia and hyperalgesia. The present paper examines the relationship between mechanosensitivity and spontaneous discharge in three distinct sciatic nerve injury models in the rat: nerve transection (neuroma), nerve crush and chronic nerve constriction injury (CCI). Impulse pattern analysis was used to determine that the sites of mechanosensitivity and of spontaneous electrogenesis are either identical or very close to one another. This suggests that mechanosensitivity and spontaneous firing are aspects of a single underlying pathophysiological process. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.
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88
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Abstract
Paresthesias are common manifestations of central and peripheral pathological processes and are due to ectopic impulse activity in cutaneous afferents or their central projections. Cutaneous afferents are more excitable than motor axons, due to differences in their biophysical properties. These differences probably include more persistent Na(+) conductance and inward rectification on cutaneous afferents, properties which probably confer greater protection from impulse-dependent conduction failure but create a greater tendency to ectopic activity. Ectopic discharges can be induced in normal afferents by four maneuvers: hyperventilation, ischemia, release of ischemia, and prolonged tetanization. The alkaline shift produced by hyperventilation selectively increases the persistent Na(+) conductance, while the membrane depolarization produced by ischemia affects both transient and persistent Na(+) channels. Postischemic and posttetanic paresthesias occur when hyperpolarization by the Na(+)/K(+) pump is transiently prevented by raised extracellular K(+). The electrochemical gradient for K(+) is reversed, and inward K(+) currents trigger regenerative depolarization. These mechanisms of paresthesia generation can account for paresthesias in normal subjects and may be relevant in some peripheral nerve disorders.
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Affiliation(s)
- I Mogyoros
- Department of Neurology, Prince of Wales Hospital, Prince of Wales Medical Research Institute, University of New South Wales, Randwick, NSW 2031, Australia.
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89
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Waxman SG. The neuron as a dynamic electrogenic machine: modulation of sodium-channel expression as a basis for functional plasticity in neurons. Philos Trans R Soc Lond B Biol Sci 2000; 355:199-213. [PMID: 10724456 PMCID: PMC1692729 DOI: 10.1098/rstb.2000.0559] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Neurons signal each other via regenerative electrical impulses (action potentials) and thus can be thought of as electrogenic machines. Voltage-gated sodium channels produce the depolarizations necessary for action potential activity in most neurons and, in this respect, lie close to the heart of the electrogenic machinery. Although classical neurophysiological doctrine accorded 'the' sodium channel a crucial role in electrogenesis, it is now clear that nearly a dozen genes encode distinct sodium channels with different molecular structures and functional properties, and the majority of these channels are expressed within the mammalian nervous system. The transcription of these sodium-channel genes, and the deployment of the channels that they encode, can change significantly within neurons following various injuries. Moreover, the transcription of these genes and the deployment of various types of sodium channels within neurons of the normal nervous system can change markedly as neurons respond to changing milieus or physiological inputs. As a result of these changes in sodium-channel expression, the membranes of neurons may be retuned so as to alter their transductive and/or encoding properties. Neurons within the normal and injured nervous system can thus function as dynamic electrogenic machines with electroresponsive properties that change not only in response to pathological insults, but also in response to shifting functional needs.
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Affiliation(s)
- S G Waxman
- Department of Neurology, Yale University School of Medicine, New Haven, CT 06510, USA.
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90
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Smith GT, Zakon HH. Pharmacological characterization of ionic currents that regulate the pacemaker rhythm in a weakly electric fish. JOURNAL OF NEUROBIOLOGY 2000; 42:270-86. [PMID: 10640333 DOI: 10.1002/(sici)1097-4695(20000205)42:2<270::aid-neu10>3.0.co;2-v] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Electric organ discharge (EOD) frequency in the brown ghost knifefish (Apteronotus leptorhynchus) is sexually dimorphic, steroid-regulated, and determined by the discharge rates of neurons in the medullary pacemaker nucleus (Pn). We pharmacologically characterized ionic currents that regulate the firing frequency of Pn neurons to determine which currents contribute to spontaneous oscillations of these neurons and to identify putative targets of steroid action in regulating sexually dimorphic EOD frequency. Tetrodotoxin (TTX) initially reduced spike frequency, and then reduced spike amplitude and stopped pacemaker activity. The sodium channel blocker muO-conotoxin MrVIA also reduced spike frequency, but did not affect spike amplitude or production. Two potassium channel blockers, 4-aminopyridine (4AP) and kappaA-conotoxin SIVA, increased pacemaker firing rates by approximately 20% and then stopped pacemaker firing. Other potassium channel blockers (tetraethylammonium, cesium, alpha-dendrotoxin, and agitoxin-2) did not affect the pacemaker rhythm. The nonspecific calcium channel blockers nickel and cadmium reduced pacemaker firing rates by approximately 15-20%. Specific blockers of L-, N-, P-, and Q-type calcium currents, however, were ineffective. These results indicate that at least three ionic currents-a TTX- and muO-conotoxin MrVIA-sensitive sodium current; a 4AP- and kappaA-conotoxin SIVA-sensitive potassium current; and a T- or R-type calcium current-contribute to the pacemaker rhythm. The pharmacological profiles of these currents are similar to those of currents that are known to regulate firing rates in other spontaneously oscillating neural circuits.
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Affiliation(s)
- G T Smith
- Section of Neurobiology, School of Biological Sciences, Patterson Laboratories (C0920), University of Texas, Austin, USA
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91
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Membrane potential oscillations in dorsal root ganglion neurons: role in normal electrogenesis and neuropathic pain. J Neurosci 1999. [PMID: 10493758 DOI: 10.1523/jneurosci.19-19-08589.1999] [Citation(s) in RCA: 207] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Abnormal afferent discharge originating at ectopic sites in injured primary sensory neurons is thought to be an important generator of paraesthesias, dysaesthesias, and chronic neuropathic pain. We report here that the ability of these neurons to sustain repetitive discharge depends on intrinsic resonant properties of the cell membrane and that the prevalence of this characteristic increases after nerve injury. Recording from primary sensory neurons in excised rat dorsal root ganglia, we found that some cells show subthreshold oscillations in their membrane potential. The amplitude, frequency, and coherence of these oscillations were voltage sensitive. Oscillations gave rise to action potentials when they reached threshold. Indeed, the presence of oscillations proved to be a necessary condition for sustained spiking both at resting membrane potential and on depolarization; neurons without them were incapable of sustained discharge even on deep depolarization. Previous nerve injury increased the proportion of neurons sampled that had subthreshold oscillations, and hence the proportion that generated ectopic spike discharge. Oscillatory behavior and ectopic spiking were eliminated by [Na(+)](o) substitution or bath application of lidocaine or tetrodotoxin (TTX), under conditions that preserved axonal spike propagation. This suggests that a TTX-sensitive Na(+) conductance contributes to the oscillations. Selective pharmacological suppression of subthreshold oscillations may offer a means of controlling neuropathic paraesthesias and pain without blocking afferent nerve conduction.
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92
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Magistretti J, Alonso A. Biophysical properties and slow voltage-dependent inactivation of a sustained sodium current in entorhinal cortex layer-II principal neurons: a whole-cell and single-channel study. J Gen Physiol 1999; 114:491-509. [PMID: 10498669 PMCID: PMC2229464 DOI: 10.1085/jgp.114.4.491] [Citation(s) in RCA: 125] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The functional and biophysical properties of a sustained, or "persistent," Na(+) current (I(NaP)) responsible for the generation of subthreshold oscillatory activity in entorhinal cortex layer-II principal neurons (the "stellate cells") were investigated with whole-cell, patch-clamp experiments. Both acutely dissociated cells and slices derived from adult rat entorhinal cortex were used. I(NaP), activated by either slow voltage ramps or long-lasting depolarizing pulses, was prominent in both isolated and, especially, in situ neurons. The analysis of the gating properties of the transient Na(+) current (I(NaT)) in the same neurons revealed that the resulting time-independent "window" current (I(NaTW)) had both amplitude and voltage dependence not compatible with those of the observed I(NaP), thus implying the existence of an alternative mechanism of persistent Na(+)-current generation. The tetrodotoxin-sensitive Na(+) currents evoked by slow voltage ramps decreased in amplitude with decreasing ramp slopes, thus suggesting that a time-dependent inactivation was taking place during ramp depolarizations. When ramps were preceded by increasingly positive, long-lasting voltage prepulses, I(NaP) was progressively, and eventually completely, inactivated. The V(1/2) of I(NaP) steady state inactivation was approximately -49 mV. The time dependence of the development of the inactivation was also studied by varying the duration of the inactivating prepulse: time constants ranging from approximately 6.8 to approximately 2.6 s, depending on the voltage level, were revealed. Moreover, the activation and inactivation properties of I(NaP) were such as to generate, within a relatively broad membrane-voltage range, a really persistent window current (I(NaPW)). Significantly, I(NaPW) was maximal at about the same voltage level at which subthreshold oscillations are expressed by the stellate cells. Indeed, at -50 mV, the I(NaPW) was shown to contribute to >80% of the persistent Na(+) current that sustains the subthreshold oscillations, whereas only the remaining part can be attributed to a classical Hodgkin-Huxley I(NaTW). Finally, the single-channel bases of I(NaP) slow inactivation and I(NaPW) generation were investigated in cell-attached experiments. Both phenomena were found to be underlain by repetitive, relatively prolonged late channel openings that appeared to undergo inactivation in a nearly irreversible manner at high depolarization levels (-10 mV), but not at more negative potentials (-40 mV).
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Affiliation(s)
- Jacopo Magistretti
- From the Department of Neurology and Neurosurgery, McGill University and Montreal Neurological Institute, Montréal, Québec, H3A 2B4 Canada
- Dipartimento di Neurofisiologia Sperimentale, Istituto Nazionale Neurologico “Carlo Besta”, 20133 Milano, Italy
| | - Angel Alonso
- From the Department of Neurology and Neurosurgery, McGill University and Montreal Neurological Institute, Montréal, Québec, H3A 2B4 Canada
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93
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Safronov BV. Spatial distribution of NA+ and K+ channels in spinal dorsal horn neurones: role of the soma, axon and dendrites in spike generation. Prog Neurobiol 1999; 59:217-41. [PMID: 10465379 DOI: 10.1016/s0301-0082(98)00051-3] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Spinal dorsal horn neurones play an important role in processing sensory information received from primary afferent fibers. The application of the patch-clamp technique to thin slices of rat spinal cord has enabled the study of ionic channels in visually identified dorsal horn neurones. The small soma of these neurones isolated from the slice by means of a novel method of 'entire soma isolation' has become a convenient model for investigating the properties and distributions of ionic channels. The present review summarizes results of recent experiments studying different types of voltage-gated Na+ and K+ channels expressed in dorsal horn neurones. Uneven distribution of the channels between the soma. axon and dendrites appears to play a major role in determining the neuronal excitability. The contribution of the soma, axon and dendrites to generation and propagation of the action potentials in central neurones is discussed.
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Affiliation(s)
- B V Safronov
- Physiologisches Institut, Justus-Liebig-Universität Giessen, Germany. boris.safronov@physiologie,med.uni-giessen.de
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94
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Mogyoros I, Lin C, Dowla S, Grosskreutz J, Burke D. Strength-duration properties and their voltage dependence at different sites along the median nerve. Clin Neurophysiol 1999; 110:1618-24. [PMID: 10479029 DOI: 10.1016/s1388-2457(99)00087-5] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
OBJECTIVES There is some evidence that the ease with which ectopic activity can be induced varies systematically along the course of a nerve and is greater at more proximal sites. Recent studies have implicated a non-inactivating threshold conductance, possibly due to persistent Na+ channels, in ectopic activity associated with ischaemia and hyperventilation. This conductance is largely responsible for the voltage dependence of strength-duration time constant (tauSD), and changes in it can explain the time constant changes that occur during hyperventilation and ischaemia. METHODS To determine whether the strength-duration properties of motor axons of the median nerve vary along the course of the nerve, tauSD and rheobase were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC polarizing current to change axonal excitability. RESULTS tauSD was similar at the 3 stimulating sites but increased less at the axilla with depolarizing current. CONCLUSIONS These data indicate that the greater tendency for ectopic activity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to tauSD and underlie its dependence on axonal excitability. The findings provide further support for the view that the precise relationship of the stimulating electrodes to the nerve has little effect on tauSD, at least when it is measured in the forearm.
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Affiliation(s)
- I Mogyoros
- Department of Neurology, Prince of Wales Hospital, Prince of Wales Medical Research Institute, Randwick, N.S.W., Australia.
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95
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Abstract
Steady-state Na+/K+ pump current (Ip) in isolated adult rat dorsal root ganglia neurons was studied to determine if the plasma membrane Na+/K+ pump activity is uniform across the population of dorsal root ganglia neurons. Cells were voltage-clamped at -40 mV and holding current (Ih) was recorded using whole-cell patch-clamp techniques under conditions that stimulate the Na+/K+ pump (60 mM intracellular Na+ and 5.4 mM extracellular K+). Ip was defined as the 1 mM ouabain-sensitive fraction of Ih. Data suggest the existence of three subpopulations of dorsal root ganglia neurons having mean steady-state Ip densities of 1.6+/-0.1, 3.8+/-0.2 and 7.5+/-0.4 pA/pF. Neurons with small Ip had an average soma perimeter of 95+/-3 microm, while neurons with medium and large Ip density had significantly larger soma sizes (131+/-8 and 141+/-7 microm, respectively). Neurons with a large Ip density had a significantly lower specific membrane resistance (Rm; mean 4.0+/-0.3 kohm x cm2) than neurons with medium or small Ip density (19+/-6 and 31+/-6 kohm x cm2, respectively). Regardless of these differences, in all groups of neurons Ip had a low sensitivity to ouabain (Ip half inhibition by ouabain was observed at 80-110 microM). These data suggest that the Na+/K+ pump site density and/or its activity is not uniform throughout the dorsal root ganglia neuron population; however, this non-uniformity does not appear to relate to the functional expression of the different alpha isoforms of the Na+/K+ pump. The major functional Na+/K+ pump in the dorsal root ganglia neuron plasma membrane appeared to be the low ouabain affinity (alpha1) isoform.
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Affiliation(s)
- M Dobretsov
- Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock 72205, USA
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96
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Abstract
The effects of altering extracellular pH on late Na+ currents were investigated in large dorsal root ganglion neurons from rats (100-300 g), using patch-clamp techniques. The late current amplitude was steeply dependent upon pH over a range which included normal physiological values: raising the pH from 7.3 to 8.3 approximately doubled the amplitude. Whole-cell late currents 60 ms after depolarization to - 30 mV were blocked with an apparent pKa of 6.96. The pH-dependent changes in current amplitude could not be accounted for by the effects of altered surface charge. In recordings of unitary Na+ currents from outside-out membrane patches, acidification promoted channel opening to a reduced conductance level, near one-half of its maximal value. Acidification to pH < 6.0 also changed the kinetics of the current recruited with the lowest threshold from non-inactivating to inactivating, with the elimination of late openings. We conclude that lowering pH from an initial alkaline or neutral value blocks late Na+ current by reducing the number of contributing channels while also reducing the single channel conductance. The pH dependence of late Na+ current helps to explain clinically relevant changes in neuronal excitability in response to small (i.e. < 1 unit) perturbations in extracellular pH.
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Affiliation(s)
- M D Baker
- Sobell Department of Neurophysiology, Institute of Neurology, London, UK
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97
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Abstract
The temperature dependence of different indices of axonal excitability (threshold, latency, refractoriness, supernormality, strength-duration time constant, and rheobase) was studied for cutaneous afferents of 8 healthy human volunteers using threshold tracking. Cooling from approximately 32 - approximately 22 degrees C dramatically increased the threshold for a conditioned potential evoked during the relatively refractory period (average increase 573%) but had little effect on the threshold for unconditioned potentials (increased by 4% with 0.1-ms test stimuli), strength-duration time constant (increased by 18%), or rheobase (decreased by 12%). Cooling increased the latency of the unconditioned test potential by 41%, but this slowing was small compared with the effect of cooling on the latency slowing attributable to refractoriness. This measure of refractoriness was initially 0.17 ms at a conditioning-test interval of 2 ms, and increased with cooling to 1.30 ms at the same interval. With cooling, refractoriness was both greater at any one conditioning-test interval and longer in duration, extending into intervals normally associated with supernormality. It is concluded that, although cooling affects all excitability indices to some extent, the most prominent feature is the increase in refractoriness. By contrast, strength-duration time constant is influenced little by temperature.
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Affiliation(s)
- D Burke
- Department of Neurology, Prince of Wales Hospital and Prince of Wales Medical Research Institute, and University of New South Wales, Randwick, Sydney, Australia
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98
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Slow closed-state inactivation: a novel mechanism underlying ramp currents in cells expressing the hNE/PN1 sodium channel. J Neurosci 1998. [PMID: 9822722 DOI: 10.1523/jneurosci.18-23-09607.1998] [Citation(s) in RCA: 280] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
To better understand why sensory neurons express voltage-gated Na+ channel isoforms that are different from those expressed in other types of excitable cells, we compared the properties of the hNE sodium channel [a human homolog of PN1, which is selectively expressed in dorsal root ganglion (DRG) neurons] with that of the skeletal muscle Na+ channel (hSkM1) [both expressed in human embryonic kidney (HEK293) cells]. Although the voltage dependence of activation was similar, the inactivation properties were different. The V1/2 for steady-state inactivation was slightly more negative, and the rate of open-state inactivation was approximately 50% slower for hNE. However, the greatest difference was that closed-state inactivation and recovery from inactivation were up to fivefold slower for hNE than for hSkM1 channels. TTX-sensitive (TTX-S) currents in small DRG neurons also have slow closed-state inactivation, suggesting that hNE/PN1 contributes to this TTX-S current. Slow ramp depolarizations (0.25 mV/msec) elicited TTX-S persistent currents in cells expressing hNE channels, and in DRG neurons, but not in cells expressing hSkM1 channels. We propose that slow closed-state inactivation underlies these ramp currents. This conclusion is supported by data showing that divalent cations such as Cd2+ and Zn2+ (50-200 microM) slowed closed-state inactivation and also dramatically increased the ramp currents for DRG TTX-S currents and hNE channels but not for hSkM1 channels. The hNE and DRG TTX-S ramp currents activated near -65 mV and therefore could play an important role in boosting stimulus depolarizations in sensory neurons. These results suggest that differences in the kinetics of closed-state inactivation may confer distinct integrative properties on different Na+ channel isoforms.
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99
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Baker MD, Bostock H. Inactivation of macroscopic late Na+ current and characteristics of unitary late Na+ currents in sensory neurons. J Neurophysiol 1998; 80:2538-49. [PMID: 9819261 DOI: 10.1152/jn.1998.80.5.2538] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Na+ currents in adult rat large dorsal root ganglion neurons were recorded during long duration voltage-clamp steps by patch clamping whole cells and outside-out membrane patches. Na+ current present >60 ms after the onset of a depolarizing pulse (late Na+ current) underwent partial inactivation; it behaved as the sum of three kinetically distinct components, each of which was blocked by nanomolar concentrations of tetrodotoxin. Inactivation of one component (late-1) of the whole cell current reached equilibrium during the first 60 ms; repolarizing to -40 or -50 mV from potentials of -30 mV or more positive gave rise to a characteristic increase in current (tau >/= 5 ms), attributed to removal of inactivation. A second component (late-2) underwent slower inactivation (tau > 80 ms) at potentials more positive than -80 mV, and steady-state inactivation appeared complete at -30 mV. In small membrane patches, bursts of brief openings (gamma = 13-18 pS) were usually recorded. The distribution of burst durations indicated that two populations of channel were present with inactivation rates corresponding to late-1 and late-2 macroscopic currents. The persistent Na+ current in the whole cell that extended to potentials more positive than -30 mV appeared to correspond to sporadic, brief openings that were recorded in patches (mean open time approximately 0.1 ms) over a wide potential range. None of the three types of gating described corresponded to activation/inactivation gating overlap of fast transient currents.
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Affiliation(s)
- M D Baker
- Sobell Department of Neurophysiology, Institute of Neurology, London WC1N 3BG, United Kingdom
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100
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Bouskila Y, Bostock H. Modulation of voltage-activated calcium currents by mechanical stimulation in rat sensory neurons. J Neurophysiol 1998; 80:1647-52. [PMID: 9772228 DOI: 10.1152/jn.1998.80.4.1647] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We examined the effects of mechanical stress, induced by a stream of bath solution, on evoked action potentials, electrical excitability, and Ca2+ currents in rat dorsal root ganglion neurons in culture with the use of the whole cell patch-clamp technique. Action-potential duration was altered reversibly by flow in 39% of the 51 neurons tested, but membrane potential and excitability were unaffected. The flow-induced increases and decreases in action-potential duration were consistent with the different effects of flow on two types of Ca2+ channel, determined by voltage-clamp recordings of Ba2+ currents. Current through omega-conotoxin-sensitive (N-type) Ca2+ channels increased by an estimated 74% with flow, corresponding to 23% increase in the total high voltage-activated current, whereas current through low-threshold voltage-activated (T-type) channels decreased by 14%. We conclude that modulation of voltage-activated Ca2+ currents constitutes a route by which mechanical events can regulate Ca2+ influx in sensory neurons.
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Affiliation(s)
- Y Bouskila
- Sobell Department of Neurophysiology, Institute of Neurology, London WC2N 3BG, United Kingdom
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