The delayed depolarization in rat cutaneous afferent axons is reduced following nerve transection and ligation, but not crush: implications for injury-induced axonal Na+ channel reorganization

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.

Role of Potassium Channels in the Frequency-Dependent Activity of Regenerating Nerves

After a peripheral nerve injury, ion channel organization and the electrical properties of nerve fibers drastically change during the regeneration process. The present study was designed to compare the frequency-dependent characteristics of regenerating nerves in the presence of 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The results showed that increasing the stimulus frequency produced a greater impulse blockade (frequency-dependent block--FDB) and distinct hyperpolarizing afterpotentials (HAPs) in regenerating nerves. In particular, regenerating sciatic nerves 15 days post-crush (dpc) were more sensitive to the frequency-dependent stimulations than 38-dpc and intact nerves in the presence or absence of drugs. The frequency-dependent effects of TEA on the compound action potentials (CAPs) appeared when TEA was applied to 4-AP-treated nerves. This shows that TEA-sensitive channels may not be masked by the myelin. 4-AP was here found to have more pronounced frequency-dependent effects on regenerating nerves than on intact nerves. Delayed depolarization (in 38-dpc: 22.6 +/- 1.3 mV and 47.52 +/- 3.63 ms, in intact: 12.0 +/- 1.9 mV and 88.51 +/- 4.72 ms) elicited by 4-AP resulted in an increase in FDBs and HAP amplitudes. These results suggest that 4-AP-sensitive channels may play important roles in frequency-dependent nerve conduction. Consequently, regenerating or myelin damaged nerves are more sensitive to repetitive firing with or without drug. An understanding of the frequency-dependent properties of regenerating nerves may be of value in the treatment of the nerve diseases.

Elevating pain thresholds in humans using depolarizing prepulses

Electrocutaneous stimulation is a potentially useful communication tool for applications in virtual reality, sensory substitution, and sensory augmentation. Many of these applications require the use of arrays of small electrodes. Stimulation through small electrodes is often painful, however, limiting the practicality of such arrays. The purpose of this study was to test a method for elevating the pain threshold to electrocutaneous stimulation through small (1-mm diameter) electrodes on the fingertip. We hypothesized that long, subthreshold, depolarizing prepulses (PP) would elevate the pain threshold so that a subsequent stimulus pulse (SP) would be less likely to be painful. We used psychophysical methods to measure the probability that an SP would be perceived as painful both by itself and when preceded by a PP that was 2, 4, 6, 8, or 10 dB lower in amplitude than the SP. We found that the PPs significantly increased the pain threshold, reducing the likelihood that the SP was painful (p < .0001). The dose effect of PP amplitude was also highly significant (p < .0001), with larger PPs elevating pain thresholds more. To our knowledge, this is the first report of PPs being used to elevate electrical stimulation thresholds in humans. PPs may be useful for selective inactivation of neural subpopulations in many human neuroprosthetic applications.

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

Electrophysiological recordings were obtained from rat dorsal roots in a sucrose gap chamber to study changes in Na(+) currents following nerve injury. Application of 4-aminopyridine unmasks a prominent and well-characterized depolarization (delayed depolarization) following the action potential. In our previous studies, this potential, which is only present in cutaneous afferent axons, has been shown to correlate with activation of a slow Na(+) current. The delayed depolarization in the dorsal root was reduced 1 week after sciatic nerve ligation, suggesting a reduction in the kinetically slow Na(+) currents on dorsal root axons [control: 44. 2+/-7.3% (n=5); injury: 7.3+/-4.7% (n=5), P<0.001]. The refractory period of the action potential was reduced following nerve injury, in agreement with biophysical studies indicating faster "repriming" of fast Na(+) currents on cutaneous afferent cell bodies. Dorsal root ligation near the spinal cord also results in a reduction in the delayed depolarization. These results indicate that changes in Na(+) channel organization occur on dorsal root axons following either central or peripheral target disconnection, suggesting trophic support can be derived from either the CNS or the PNS.

Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: aberrant innervation of lamina II neurons by Abeta afferents

Intracellular recording and extracellular field potential (FP) recordings were obtained from spinal cord dorsal horn neurons (laminae I-IV) in a rat transverse slice preparation with attached dorsal roots. To study changes in synaptic inputs after neuroma formation, the sciatic nerve was sectioned and ligated 3 weeks before in vitro electrophysiological analysis. Horseradish peroxidase labeling of dorsal root axons indicated that Abeta fibers sprouted into laminae I-II from deeper laminae after sciatic nerve section. FP recordings from dorsal horns of normal spinal cord slices revealed long-latency synaptic responses in lamina II and short-latency responses in lamina III. The latencies of synaptic FPs recorded in lamina II of the dorsal horn after sciatic nerve section were reduced. The majority of monosynaptic EPSPs recorded with intracellular microelectrodes from lamina II neurons in control slices were elicited by high-threshold nerve stimulation, whereas the majority of monosynaptic EPSPs recorded in lamina III were elicited by low-threshold nerve stimulation. After sciatic nerve section, 31 of 57 (54%) EPSPs recorded in lamina II were elicited by low-threshold stimulation. The majority of low-threshold EPSPs in lamina II neurons after axotomy displayed properties similar to low-threshold EPSPs in lamina III of control slices. These results indicate that reoccupation of lamina II synapses by sprouting Abeta fibers normally terminating in lamina III occurs after sciatic nerve neuroma formation. Furthermore, these observations indicate that the lamina II neurons receive inappropriate sensory information from low-threshold mechanoreceptor after sciatic nerve neuroma formation.