Kobayashi N R, Bedard A M, Hincke M T, Tetzlaff W
Department of Physiology, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada.
Eur J Neurosci. 1996 May;8(5):1018-29. doi: 10.1111/j.1460-9568.1996.tb01588.x.
Motoneurons of the adult survive after axotomy even though they are deprived of putative target derived trophic factors. Alternative sources of trophic support may substitute. In this study we test the hypothesis that the immediate environment of the motoneuronal cell body or the cell body itself increases the production of trophic factors after axonal injury. Using in situ hybridization (ISH) and reverse transcription-polymerase chain reaction (RT-PCR), we report that after axotomy, rat facial motoneurons increase the expression of mRNA for brain-derived neurotrophic factor (BDNF) and its receptor trkB. After transection of the facial nerve, we measured a 2- to 4-fold increase in BDNF mRNA expression which had its onset between 3 and 8 h after injury. The BDNF mRNA levels peaked at approximately 1-2 days and gradually declined thereafter to return to contralateral levels within 7 days of injury. Western blotting revealed a several-fold increase in BDNF as early as 24 h, which subsequently reached a maximum in approximately 5-7 days and was still sustained at 2 weeks post-axotomy. Using exon-specific primers, we determined that the increase in BDNF mRNA is largely due to an increased expression from the promoters of exons IV and III, and to a lesser extent from exons I and II. Analysing the mRNA expression for the BDNF receptor, trkB, we found a 2- to 3-fold increase in full-length trkB mRNA expression starting 2 days after axotomy which lasted 2-3 weeks. These findings suggest that BDNF might act locally on axotomized motoneurons in an autocrine fashion, providing support for axotomized motoneurons during the first weeks after axotomy.
成年运动神经元在轴突切断后仍能存活，尽管它们被剥夺了假定的靶源性营养因子。营养支持的替代来源可能会起到替代作用。在本研究中，我们检验了这样一个假设：运动神经元胞体的直接环境或胞体本身在轴突损伤后会增加营养因子的产生。通过原位杂交（ISH）和逆转录-聚合酶链反应（RT-PCR），我们发现轴突切断后，大鼠面部运动神经元会增加脑源性神经营养因子（BDNF）及其受体trkB的mRNA表达。切断面神经后，我们测量到BDNF mRNA表达增加了2至4倍，其起始时间在损伤后3至8小时之间。BDNF mRNA水平在大约1至2天达到峰值，此后逐渐下降，在损伤后7天内恢复到对侧水平。蛋白质免疫印迹显示，早在24小时时BDNF就增加了几倍，随后在大约5至7天达到最大值，并且在轴突切断后2周仍持续存在。使用外显子特异性引物，我们确定BDNF mRNA的增加主要是由于外显子IV和III启动子的表达增加，而外显子I和II的贡献较小。分析BDNF受体trkB的mRNA表达，我们发现全长trkB mRNA表达从轴突切断后2天开始增加2至3倍，并持续2至3周。这些发现表明，BDNF可能以自分泌方式在局部作用于轴突切断的运动神经元，在轴突切断后的最初几周为轴突切断的运动神经元提供支持。
