Research reportIon channels associated with the ectopic discharges generated after segmental spinal nerve injury in the rat
Introduction
Injured sensory neurons often produce spontaneous discharges at an abnormal site — ectopic discharges. The repetitive and prolonged firing of such abnormal discharges for days and weeks may have important consequences. These include plastic changes in the central nervous system and production of abnormal sensations such as pain [18], [22], [51], [52]. Therefore, studying the mechanism of ectopic discharge generation is important.
The mechanisms of ectopic discharge generation are not clear. One likely mechanism is the up-regulation or mobilization of one or more ion channels after a peripheral nerve injury. Supporting this hypothesis is the fact that applications of various channel blockers to the injury site have been shown to inhibit ectopic discharges. These include blockers for sodium channels [15], [36], [39], [40], [54], potassium channels [27], [36], [54], and various subtypes of calcium channels [53], [54]. The information on sodium channels is of particular interest not only because their blockers inhibit the ectopic discharges, but also because the density of the sodium channels has been shown to be increased [13], [35], [36] and because it has been demonstrated that a new type of sodium channel appears at the injury site [42], [43], [50].
Functionally, sodium channels have been classified by their sensitivity to tetrodotoxin (TTX). Most of the cloned subtypes of sodium channels are referred to as TTX-sensitive since they are blocked by a low dose of TTX (<100 nM). On the other hand, a few (e.g. SNS/PN3 and NaN) subtypes of sodium channels are referred to as TTX-resistant because they require a much higher dose (>1 μM) of TTX to be blocked [26]. Although there seems to be general agreement that changes in sodium channels play an important role in the generation of ectopic discharges after a peripheral nerve injury, it is not clear which specific subtype is critically important nor is it clear whether it belongs to a family of TTX-sensitive or TTX-resistant subtypes.
Using an in-vitro electrophysiological set-up, the present study examined ectopic discharges generated from injured sensory neurons after spinal nerve ligation in the rat. Various ion channel blockers were added to the perfusion solution of the DRG, commonly known as the site where practically all ectopic discharges originate after spinal nerve ligation [33]. The present study examined the sensitivity of ectopic discharges to TTX in an attempt to identify the critically important subtype(s) of sodium channels involved in ectopic discharge generation. In addition, the effects of potassium and calcium channel blockers were also tested to confirm the results of many previous studies.
Section snippets
Experimental animals and surgical operation
Male Sprague–Dawley rats weighing 125–150 g were used in this study. The experimental protocol was approved by the Animal Care and Use Committee of the University of Texas Medical Branch. Under gaseous anesthesia with a mixture of halothane (2% for induction and 0.8% for maintenance) and a 2:1 flow ratio of N2O and O2, spinal nerves were tightly ligated at a site distal to the DRG following a similar approach to the operation used in the spinal nerve ligation (SNL) model of neuropathic pain [28]
The origin of ectopic discharges
The originating site of ectopic discharges was verified on some units. Since several units were frequently recorded from a single preparation, a destructive lesion experiment could not be done in all units. At the end of the experiment, successive resections were made while recording the discharges from the last unit of some preparations. We concluded that the discharges recorded in the dorsal root were originated from the DRG when a unit discharged continuously after a resection at the
Discussion
The present study examined the effects of various ion channel blockers on spontaneous activity originating from an abnormal impulse generating site in injured sensory neurons (i.e. ectopic discharges). Using an in-vitro recording set-up, the ectopic discharges were recorded from single units isolated from the DR of the segments that had been injured by tightly ligating the spinal nerves 7–14 days prior to the recording session. We applied ion channel blockers to the DRG, and the outcome was
Summary
After segmental spinal nerve injury in the rat, ectopic discharges were recorded from the DR fascicles using an in-vitro recording set-up. Various ion channel blockers were applied to the DRG, where most ectopic discharges are known to originate in this preparation. The main focus of this study was to test the sensitivity of ectopic discharges to TTX in order to identify the types of sodium channels that are important for the generation of ectopic discharges. Ectopic discharges were inhibited
References (54)
- et al.
Spinal sensory neurons express multiple sodium channel α-subunit mRNAs
Mol. Brain Res.
(1996) - et al.
Three types of sodium channels in adult rat dorsal root ganglion neurons
Brain Res.
(1992) Potassium channels moderate ectopic excitability of nerve end neuromas in rats
Neurosci. Lett.
(1983)- et al.
Systemic lidocaine silences ectopic neuroma and DRG discharge without blocking nerve conduction
Pain
(1992) - et al.
Activity-dependent neuronal plasticity following tissue injury and inflammation
Trends Neurosci.
(1992) - et al.
Painful neuropathy: altered central processing maintained dynamically by peripheral input
Pain
(1992) - et al.
Spontaneous discharge originates in the dorsal root ganglion at the onset of a painful peripheral neuropathy in the rat
Neurosci. Lett.
(1992) - et al.
Ionic currents in the somatic membrane of rat dorsal root ganglion neurons – I. Sodium currents
Neuroscience
(1981) - et al.
Alteration of Na+ currents in dorsal root ganglion neurons from rats with a painful neuropathy
Pain
(1999) - et al.
Tactile allodynia in the absence of C-fiber activation: altered firing properties of DRG neurons following spinal nerve injury
Pain
(2000)
Ectopic discharges and adrenergic sensitivity of sensory neurons after spinal nerve injury
Brain Res.
Spontaneous activity of axotomized afferent neurons after L5 spinal nerve injury in rats
Pain
Na+ conductance and the threshold for repetitive neuronal firing
Brain Res.
QX-314 inhibits ectopic nerve activity associated with neuropathic pain
Brain Res.
Tetrodotoxin inhibits neuropathic ectopic activity in neuromas, dorsal root ganglia and dorsal horn neurons
Pain
Selective loss of slow and enhancement of fast Na+ currents in cutaneous afferent dorsal root ganglion neurons following axotomy
Neurobiol. Dis.
Structure and function of a novel voltage-gated, tetrodotoxin-resistant sodium channel specific to sensory neurons
J. Biol. Chem.
Purinergic cotransmission: sympathetic nerves
Semin. Neurosci.
Electrophysiological properties of subpopulations of rat dorsal root ganglion neurons in vitro
Neuroscience
A tetrodotoxin-resistant voltage-gated sodium channel expressed by sensory neurons
Nature
Classes of calcium channels in vertebrate cells
Annu. Rev. Physiol.
Upregulation of a silent sodium channel after peripheral, but not central, nerve injury in DRG neurons
J. Neurophysiol.
ω-Conotoxin block of N-type calcium channels in and from rat sympathetic neurons
J. Neurosci.
A low voltage-activated, fully inactivating Ca channel in vertebrate sensory neurones
Nature
Role of voltage-dependent calcium channel subtypes in experimental tactile allodynia
J. Pharmacol. Exp. Ther.
Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens
J. Neurophysiol.
A novel persistent tetrodotoxin-resistant sodium current in SNS-null and wild-type small primary sensory neurons
J. Neurosci.
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