Increased Expression of BDNF and trkB mRNA in Rat Facial Motoneurons after Axotomy

Abstract

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.