Differential role of GDNF and NGF in the maintenance of two TTX-resistant sodium channels in adult DRG neurons

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Following sciatic nerve transection, the electrophysiological properties of small dorsal root ganglion (DRG) neurons are markedly altered, with attenuation of TTX-R sodium currents and the appearance of rapidly repriming TTX-S currents. The reduction in TTX-R currents has been attributed to a down-regulation of sodium channels SNS/PN3 and NaN. While infusion of exogenous NGF to the transected nerve restores SNS/PN3 transcripts to near-normal levels in small DRG neurons, TTX-R sodium currents are only partially rescued. Binding of the isolectin IB4 distinguishes two subpopulations of small DRG neurons: IB4 + neurons, which express receptors for the GDNF family of neurotrophins, and IB4 − neurons that predominantly express TrkA. We show here that SNS/PN3 is expressed in approximately one-half of both IB4 + and IB4 − DRG neurons, while NaN is preferentially expressed in IB4 + neurons. Whole-cell patch-clamp studies demonstrate that TTX-R sodium currents in IB4 + neurons have a more hyperpolarized voltage-dependence of activation and inactivation than do IB4 − neurons, suggesting different electrophysiological properties for SNS/PN3 and NaN. We confirm that NGF restores SNS/PN3 mRNA levels in DRG neurons in vitro and demonstrate that the trk antagonist K252a blocks this rescue. The down-regulation of NaN mRNA is, nevertheless, not rescued by NGF-treatment in either IB4 + or IB4 − neurons and NGF-treatment in vitro does not significantly increase the peak amplitude of the TTX-R current in small DRG neurons. In contrast, GDNF-treatment causes a twofold increase in the peak amplitude of TTX-R sodium currents and restores both SNS/PN3 and NaN mRNA to near-normal levels in IB4 + neurons. These observations provide a mechanism for the partial restoration of TTX-R sodium currents by NGF in axotomized DRG neurons, and demonstrate that the neurotrophins NGF and GDNF differentially regulate sodium channels SNS/PN3 and NaN.