Tetrodotoxin-resistant sodium channels in neuropathic pain

Abstract

Injury to the peripheral nervous system can cause neuropathic pain. Abnormal sodium channel activity has been implicated as a source of ectopic firing and changes in nociceptive threshold following nerve injury. Primary sensory neurons exhibit at least two types of sodium currents: rapidly inactivating tetrodotoxin-sensitive (TTX-S) and slowly inactivating TTX-resistant (TTX-R) sodium currents. Two TTX-R sodium channels that are expressed in primary sensory neurons have been cloned, SNS/PN3 and NaN/SNS2. The general aim of this thesis was to describe the expression of the TTX-R sodium channels in small-diameter sensory neurons, most of which are nociceptive in function.

First, the localization, expression and kinetic properties of TTX-R sodium channels in IB4-positive (IB4+) and negative (IB4-) DRG neurons were described. NaN is localized along the axons of many unmyelinated fibers in teased sciatic nerve, most of which are also reactive to IB4, a marker for a subset of unmyelinated sensory neurons. NaN immunoreactivity was also found at the nodes of Ranvier of thinly myelinated sensory neurons, in nerve fibers in the cornea, and in the sensory neuron cell bodies within the dorsal root ganglion (DRG). While SNS mRNA is expressed in both IB4+ and IB4- small DRG neurons, NaN mRNA is predominantly expressed in IB4+ cells.

Second, the effect of chronic constriction injury (CCI) on TTX-R sodium channels was studied. Following CCI, the mRNA levels for SNS and NaN, and TTX-R sodium currents, are reduced in hyperalgesic rats. These findings mimic those seen after axotomy of the sciatic nerve, indicating that TTX-R sodium channels are affected in nerve injury that give rise to neuropathic pain.

Third, the role of NGF in the regulation of SNS and NaN was analyzed. In vivo NGF deprivation led to lower expression of SNS mRNA and diminished TTX-R sodium currents in IB4- but not in IB4+ neurons. Transgenic mice over-expressing NGF in the skin had augmented mRNA levels of SNS and NaN, as well as most TTX-S sodium channels, but no significant changes were seen in either TTX-R or TTX-S sodium currents. NGF added to adult DRG neurons maintained in culture for 7 days led to an increase of SNS mRNA levels in both IB4+ and IB4- neurons, and addition of K252a, a blocker of trkA activity, abolished this effect. In contrast, NGF treatment had no effect on the expression of NaN mRNA or TTX-R sodium currents.

Fourth, the effect of GDNF on TTX-R sodium channels was studied. Adult DRG neurons were treated with GDNF in vitro for 7 days. GDNF treated cultures exhibited higher SNS and NaN mRNA levels and larger TTX-R sodium currents than untreated cultures.

Fifth, the expression of sodium channels in a spontaneous model of demyelination in the central nervous system was studied. Surprisingly, SNS mRNA, as well as SNS antibody staining, was found in cerebellar purkinje cells of Taiep rats.

In summary, both SNS and NaN mRNAs are reduced in neuropathic pain models and NGF and GDNF can regulate their expression in different subgroups of nociceptors. Neurotrophic factors might therefore be used to normalize abnormal sodium channel expression in patients with neuropathic pain following peripheral nerve injury.