Dib-Hajj S D, Black J A, Cummins T R, Kenney A M, Kocsis J D, Waxman S G
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
J Neurophysiol. 1998 May;79(5):2668-76. doi: 10.1152/jn.1998.79.5.2668.
Small (18-25 microm diam) dorsal root ganglion (DRG) neurons are known to express high levels of tetrodotoxin-resistant (TTX-R) sodium current and the mRNA for the alpha-SNS sodium channel, which encodes a TTX-R channel when expressed in oocytes. These neurons also preferentially express the high affinity receptor for nerve growth factor (NGF), TrkA. Levels of TTX-R sodium current and of alpha-SNS mRNA are reduced in these cells after axotomy. To determine whether NGF participates in the regulation of TTX-R current and alpha-SNS mRNA in small DRG neurons in vivo, we axotomized small lumbar DRG neurons by sciatic nerve transection and administered NGF or Ringer solution to the proximal nerve stump using osmotic pumps. Ten to 12 days after pump implant, whole cell patch-clamp recording demonstrated that TTX-R current density was decreased in Ringer-treated axotomized neurons (154 +/- 45 pA/pF; mean +/- SE) compared with nonaxotomized control neurons (865 +/- 123 pA/pF) and was restored partially toward control levels in NGF-treated axotomized neurons (465 +/- 78 pA/pF). The V1/2 for steady-state activation and inactivation of TTX-R currents were similar in control, Ringer- and NGF-treated axotomized neurons. Reverse transcription polymerase chain reaction revealed an upregulation of alpha-SNS mRNA levels in NGF-treated compared with Ringer-treated axotomized DRG. In situ hybridization showed that alpha-SNS mRNA levels were decreased significantly in small Ringer-treated axotomized DRG neurons in vivo and also in small DRG neurons that were dissociated and maintained in vitro, so as to correspond to the patch-clamp conditions. NGF-treated axotomized neurons had a significant increase in alpha-SNS mRNA expression, compared with Ringer-treated axotomized cells. These results show that the administration of exogenous NGF in vivo, to the proximal nerve stump of the transected sciatic nerve, results in an upregulation of TTX-R sodium current and of alpha-SNS mRNA levels in small DRG neurons. Retrogradely transported NGF thus appears to participate in the control of excitability in these cells via actions that include the regulation of sodium channel gene expression in vivo.
已知小型（直径18 - 25微米）背根神经节（DRG）神经元表达高水平的河豚毒素抗性（TTX - R）钠电流以及α - SNS钠通道的mRNA，该通道在卵母细胞中表达时编码一种TTX - R通道。这些神经元还优先表达神经生长因子（NGF）的高亲和力受体TrkA。轴突切断后，这些细胞中TTX - R钠电流和α - SNS mRNA的水平会降低。为了确定NGF是否参与体内小型DRG神经元中TTX - R电流和α - SNS mRNA的调节，我们通过坐骨神经横断术切断小型腰段DRG神经元，并使用渗透泵向近端神经残端给予NGF或林格液。泵植入后10至12天，全细胞膜片钳记录显示，与未切断轴突的对照神经元（865±123 pA/pF）相比，林格液处理的切断轴突神经元的TTX - R电流密度降低（154±45 pA/pF），而NGF处理的切断轴突神经元的TTX - R电流密度部分恢复至对照水平（465±78 pA/pF）。对照、林格液处理和NGF处理的切断轴突神经元中TTX - R电流稳态激活和失活的半值电压（V1/2）相似。逆转录聚合酶链反应显示，与林格液处理的切断轴突的DRG相比，NGF处理的DRG中α - SNS mRNA水平上调。原位杂交表明，在体内林格液处理的小型切断轴突的DRG神经元以及解离并在体外培养的小型DRG神经元中，α - SNS mRNA水平显著降低，这与膜片钳记录条件一致。与林格液处理的切断轴突细胞相比，NGF处理的切断轴突神经元的α - SNS mRNA表达显著增加。这些结果表明，在体内向横断坐骨神经的近端神经残端给予外源性NGF，会导致小型DRG神经元中TTX - R钠电流和α - SNS mRNA水平上调。因此，逆行运输的NGF似乎通过包括体内钠通道基因表达调节在内的作用，参与了这些细胞兴奋性的控制。
