THE EXPRESSION OF TRKB AND P75 AND THE ROLE OF BDNF IN THE DEVELOPINGNEUROMUSCULAR SYSTEM OF THE CHICK-EMBRYO

The neurotrophin, brain-derived neurotrophic factor, prevents motoneur on cell death during the normal development of the chick embryo. Brain -derived neurotrophic factor is a ligand for the low-affinity NGF rece ptor, p75, and for the high-affinity neurotrophin receptor, trkB. If m otoneurons respond directly to brain-derived neurotrophic factor then they must possess at least one, and possibly both, of these receptors during the period of naturally occurring cell death. Histological sect ions from the lumbar region of chick embryos were probed for the prese nce of trkB and p75 mRNA using digoxigenin-labeled anti-sense RNA prob es. p75 mRNA was present in spinal cord motoneurons at stages of devel opment that correlate with motoneuron cell death. Immunohistochemical localization also revealed that p75 protein was present in motoneurons , primarily along the ventral roots and developing intramuscular nerve s. In contrast, trkB mRNA was not present in chick motoneurons until a fter the process of cell death was underway. The timing of trkB expres sion suggested that some motoneurons, i.e., those that die prior to th e onset of tukB expression, may be insensitive to brain-derived neurot rophic factor. This was confirmed by comparing the number of surviving motoneurons following different in vivo treatment paradigms. The evid ence indicates that motoneurons undergo a temporal shift in sensitivit y to brain-derived neurotrophic factor.