Excitability changes of dorsal root axons following nerve injury: implications for injury-induced changes in axonal Na(+) channels

Recovery from nerve injury induced behavioral hypersensitivity in rats parallels resolution of abnormal primary sensory afferent signaling

Pain and hypersensitivity months after peripheral injury reflect abnormal input from peripheral afferents likely in conjunction with central sensitization. We hypothesize that peripheral changes occur in defined sensory afferents and resolve as behavioral response to injury resolves. Male Sprague-Dawley rats underwent sham or partial L5 spinal nerve ligation, and paw withdrawal threshold (PWT) was sequentially measured during recovery. At 2, 4, 8, and 12 weeks after injury, randomized animals underwent electrophysiologic assessment of L4 fast-conducting high- and low-threshold mechanoreceptors, and individual neuronal mechanical thresholds (MTs) were contrasted with PWTs in the same animals. Paw withdrawal thresholds decreased after injury and resolved over time (P < 0.001). Similarly, MTs of fast-conducting high-threshold mechanoreceptors decreased after injury and resolved over time (P < 0.001). By contrast, MTs of low-threshold mechanoreceptors increased after injury and resolved over time (P < 0.001). Distributions of recordings from each afferent subtype were perturbed after injury, and this too resolved over time. After resolution of behavioral changes, several electrical abnormalities persisted in both neuronal subtypes. These data extend previous findings that mechanically sensitive nociceptors are sensitized, whereas tactile, largely Aβ afferents are desensitized after nerve injury by showing that the time course of resolution of these changes mirrors that of behavioral hypersensitivity in a surgical injury including neural damage. These data support a role of abnormal peripheral input, from both nociceptor and tactile afferents, during recovery from peripheral injury and underscore the potential importance of both classes of afferents as potential targets for pain treatment.

Multiple sites for generation of ectopic spontaneous activity in neurons of the chronically compressed dorsal root ganglion

In a chronically compressed dorsal root ganglion (CCD) in the rat, a model of foraminal stenosis and radicular pain in human, a subpopulation of neurons with functional axons exhibits spontaneous activity (SA) that originates within the ganglion. Intracellular electrophysiological recordings were obtained from the somata of neurons of the compressed ganglion both in vitro and in vivo. The SA was classified into two types according to the presence (type I) or absence (type II) of subthreshold membrane potential oscillation. Neurons with type I SA had significantly higher somal excitability than those with type II SA. In most cases, depolarization of the membrane potential by current injection increased the discharge rates of type I--but not type II SA. Both types occasionally coexisted in the same neuron. Several lines of evidence suggested that the origin of SA in the DRG was most likely the soma for type I SA and the axon for type II. Therefore CCD neurons have multiple sites for generation of action potentials other than the terminal endings. In vivo recordings revealed the same two types of SA in a subpopulation of neurons with functionally characterized peripheral receptive fields. Thus, SA might not only produce spurious sensory input to the afferent pathways but also add to or block impulse transmission generated by natural stimulation of peripheral receptors. SA originating in the compressed ganglion is likely to interfere with sensory transmission in nociceptive and non-nociceptive neurons, thereby contributing to radicular pain, paresthesias, hyperalgesia and allodynia.

The actions of lamotrigine and levetiracetam on the conduction properties of isolated rat sciatic nerve

The purpose of this study was to investigate the actions of lamotrigine and levetiracetam on the conduction properties of isolated rat sciatic nerves in-vitro. Compound action potentials from rat sciatic nerves were recorded using a sucrose-gap technique with single and repetitive stimulation. Lamotrigine, at 0.01 to 1 mM, reduced the amplitude of compound action potentials (3.9+/-0.6% to 47.9+/-2.4%) and produced at high frequency dependent (phasic) and independent (tonic) conduction block. Lamotrigine extended the peak time of the compound action potentials significantly without changing the half falling-time (P<0.05). Lamotrigine reduced the amplitude of the delayed depolarization, which was more pronounced than that of the amplitude of the compound action potentials in the presence of 4-aminopyridine. With tonic and phasic stimulation, 0.1 to 10 mM of levetiracetam did not alter the amplitude, peak time and half falling time of the compound action potentials. In addition, levetiracetam did not change the amplitude of the delayed depolarization and the area of the compound action potentials following application of 4-aminopyridine. These results indicate that lamotrigine produces a powerful tonic block with delayed depolarization, whereas it produces a weaker phasic block in rat sciatic nerve. Levetiracetam has no effect on peripheral nerve conduction even at high concentrations. These results may have the relevance to our understanding of the peripheral effects of lamotrigine and levetiracetam.

Comparative effects of lidocaine and tramadol on injured peripheral nerves

The aim of the present study was to investigate the action of lidocaine and tramadol on the abnormal impulse characteristics of injured peripheral nerves. The ultrastructure of nerves was studied with electron microscopy and the action of lidocaine and tramadol on intact and injured rat sciatic nerves was examined by using the sucrose gap recording technique. Tramadol and lidocaine caused concentration- and frequency-dependent decreases in the amplitude of the compound action potential. Injured nerves were more sensitive to lidocaine than to tramadol. Lidocaine suppressed the delayed depolarization and decreased the hyperpolarizing afterpotentials to a greater extent than did tramadol. A low concentration of lidocaine may restore the abnormal impulse characteristics of injured nerves without changing the normal impulse pattern. The efficacy of lidocaine and inefficacy of tramadol on abnormal impulse characteristics may contribute, at least in part, to our understanding of the mechanisms of action of these drugs in neuropathic pain therapy.

Kv1 Channels in Signal Conduction of Myelinated Nerve Fibers

The myelinated axon can be divided into three domains: the internodal axon, the paranodal axon and the nodal axon. The internodal axolemma contains high concentrations of K+ channels that are enriched in the juxtaparanodal region, whereas Na+ channels cluster in the node. This molecular organization of the myelinated axon membrane is critically important for the rapid and successful transmission of electrical impulses. The juxtaparanodal K+ channels are believed to be electrically inactive in adult peripheral nerves, but experiments with blocking drugs and genetic deletion have shown that they may serve important functions at earlier developmental stages, and during remyelination and regeneration.

Changes in electrophysiological properties of regenerating rat peripheral nerves after crush injury

The conduction of action potential in peripheral nerves requires the coordinated opening and closing of Na(+) and K(+) channels. In the present study, we used the sucrose-gap recording technique to determine the electrophysiological changes of the regenerating nerves after sciatic nerve injury by using 4-aminopyridine (4-AP) and tetraethylammonium (TEA), and lidocaine. 4-AP enhanced the amplitude and duration of the compound action potentials (CAPs) of regenerating sciatic nerve 15 days post crush (15 dpc), and elicited delayed depolarizations (Del-dep) in 38 dpc and intact groups. Hyperpolarizing afterpotentials elicited by 4-AP were completely removed by TEA in both 15 and 38 dpc. Lidocaine effectively blocked the CAP amplitude. This blockage was more pronounced in 15 dpc than 38 dpc. This agent also exhibited a partial blockage on the Del-dep amplitude. These results may indicate that the changes in the activities of 4-AP- and TEA-sensitive K(+) channels and slow Na(+) channels may play critical roles in nerve excitability and conduction.

Factors directly affecting impulse transmission in inflammatory demyelinating disease: recent advances in our understanding

Demyelination and inflammation both contribute to the neurological deficits characteristic of multiple sclerosis and Guillain-Barré syndrome. Conduction deficits attributable to demyelination are well known, but it is becoming clear that factors such as nitric oxide, endocaine, cytokines, and antiganglioside antibodies also play significant roles. Demyelination directly affects conduction and also causes changes in both the distribution and repertoire of expressed axolemmal ion channels, which in turn affect impulse propagation and can promote hyperexcitability. In conducting axons, sustained trains of impulses can produce intermittent conduction failure, and, in the presence of nitric oxide exposure, can also cause axonal degeneration. Other factors impairing impulse transmission include nodal widening, glutamate toxicity, and disturbances of both the blood-brain barrier and synaptic transmission.