Rescue of a-SNS Sodium Channel Expression in Small Dorsal Root Ganglion Neurons After Axotomy by Nerve Growth Factor In Vivo

Dib-Hajj, S. D., J. A. Black, T. R. Cummins, A. M. Kenney, at least six sodium channel a-subunits (Black et al. 1996) . J. D. Kocsis, and S. G. Waxman. Rescue of a-SNS sodium Unmyelinated C-type and myelinated cutaneous afferent channel expression in small dorsal root ganglion neurons after DRG neurons produce relatively high levels of TTX-R axotomy by nerve growth factor in vivo. J. Neurophysiol. 79: sodium current (Cummins and Waxman 1997; Honmou et 2668–2676, 1998. Small (18–25 mm diam) dorsal root ganglion al. 1994; Rizzo et al. 1994) . The expression of TTX-R (DRG) neurons are known to express high levels of tetrodotoxinsodium currents in C-type DRG neurons, in particular, is resistant (TTX-R) sodium current and the mRNA for the aassociated with high levels of mRNA for the a-SNS soSNS sodium channel, which encodes a TTX-R channel when dium channel (Black et al. 1996; Dib-Hajj et al. 1996) , expressed in oocytes. These neurons also preferentially express the high affinity receptor for nerve growth factor (NGF) , TrkA. which is expressed preferentially in these cells and encodes Levels of TTX-R sodium current and of a-SNS mRNA are rea TTX-R sodium current when expressed in oocytes duced in these cells after axotomy. To determine whether NGF (Akopian et al. 1996; Sangameswaran et al. 1996) . After participates in the regulation of TTX-R current and a-SNS axotomy, the density of TTX-R sodium current decreases mRNA in small DRG neurons in vivo, we axotomized small in myelinated cutaneous afferent (Rizzo et al. 1995) and lumbar DRG neurons by sciatic nerve transection and adminisC-type DRG neurons (Cummins and Waxman 1997) and, tered NGF or Ringer solution to the proximal nerve stump using concomitantly, levels of a-SNS mRNA are reduced in osmotic pumps. Ten to 12 days after pump implant, whole cell these cells (Dib-Hajj et al. 1996) . patch-clamp recording demonstrated that TTX-R current density was decreased in Ringer-treated axotomized neurons (154 { The regulatory mechanisms that trigger and maintain the 45 pA/pF; mean { SE) compared with nonaxotomized control expression of the multiple sodium channel isoforms that are neurons (865 { 123 pA/pF) and was restored partially toward present in DRG neurons (Black et al. 1996) are not fully control levels in NGF-treated axotomized neurons (465 { 78 understood. Nevertheless, there is evidence indicating that pA/pF) . The V1/2 for steady-state activation and inactivation of nerve growth factor (NGF) may be involved in this process, TTX-R currents were similar in control, Ringerand NGF-treated at least for several of the isoforms expressed in these cells. axotomized neurons. Reverse transcription polymerase chain reThe electrophysiological phenotype of nociceptive and cutaaction revealed an upregulation of a-SNS mRNA levels in NGFneous afferent DRG neurons is regulated by NGF during treated compared with Ringer-treated axotomized DRG. In situ development (Ritter and Mendell 1992; Ritter et al. 1991). hybridization showed that a-SNS mRNA levels were decreased significantly in small Ringer-treated axotomized DRG neurons NGF induces increases in sodium channel expression in sevin vivo and also in small DRG neurons that were dissociated eral types of cells (D’Arcangelo et al. 1993; Fanger et al. and maintained in vitro, so as to correspond to the patch-clamp 1995; Kalman et al. 1990; Mandel et al. 1988; Toledo-Aral conditions. NGF-treated axotomized neurons had a significant et al. 1995), including increases in the expression of TTX-R increase in a-SNS mRNA expression, compared with Ringersodium current in PC-12 cells (Rudy et al. 1987) and in treated axotomized cells. These results show that the administraDRG neurons (Aguayo and White 1992). NGF also has tion of exogenous NGF in vivo, to the proximal nerve stump of been shown to upregulate the expression of mRNA for sothe transected sciatic nerve, results in an upregulation of TTXdium channel a-subunits II (Mandel et al. 1988) and PN1 R sodium current and of a-SNS mRNA levels in small DRG neurons. Retrogradely transported NGF thus appears to partici(D’Arcangelo et al. 1993; Toledo-Aral et al. 1995) in PCpate in the control of excitability in these cells via actions that 12 cells and a-subunit II in embryonic DRG neurons (Zur include the regulation of sodium channel gene expression in vivo. et al. 1995). The decrease in TTX-R sodium current expression that occurs after axotomy in myelinated cutaneous afferent DRG I N T R O D U C T I O N neurons (Rizzo et al. 1995) is reversed partially by administration of NGF to the injured nerve stump in vivo (Oyelese Dorsal root ganglion (DRG) neurons, with axons that et al. 1997). However, the effect of NGF on sodium channel project from the periphery to the spinal cord, exhibit a gene expression in these cells has not been studied, and the variety of electrophysiological phenotypes related to their contribution of changes in a-SNS mRNA expression to the function. Two broad classes of sodium currents, tetrodoeffects of NGF on sodium currents in DRG neurons is not toxin-sensitive (TTX-S) and TTX-resistant (TTX-R) known. A similar decrease in TTX-R sodium current density (Caffrey et al. 1993; Elliott and Elliott 1993; Kostyuk et occurs after axotomy in C-type DRG neurons (Cummins al. 1981; Roy and Narahashi 1992) , have been recorded in DRG neurons, and these cells express the mRNAs for and Waxman 1997) and is associated with a decrease in a-